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AWS C5.5/C5.5M:2003 An American National Standard

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Copyright American Welding Society Provided by IHS under license with AWS No reproduction or networking permitted without license from IHS

Recommended Practices for Gas Tungsten Arc Welding

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AWS C5.5/C5.5M:2003 An American National Standard

Key Words —Gas tungsten arc welding, GTAW, TIG, training, process, qualification, equipment, quality, safe practices, WIG, Heliarc®

Approved by American National Standards Institute June 4, 2003

Recommended Practices for Gas Tungsten Arc Welding Supersedes AWS C5.5-80

Prepared by AWS C5 Committee on Arc Welding and Arc Cutting Under the Direction of AWS Technical Activities Committee Approved by AWS Board of Directors

Abstract This document is designed to assist anyone who is associated with gas tungsten arc welding (GTAW). This includes welders, welding technicians, welding engineers, quality control personnel, welding supervisors, purchasing personnel, educators, and students. This document discusses welding principles, equipment, gas shielding, and techniques for manual and automatic GTAW. Welding safety, troubleshooting, and related items are included for understanding by all types of personnel in establishing better production welding operations. Educators will find this publication a handy reference for teaching all aspects of gas tungsten arc welding. It can become a quick reference for students after their graduation or during their employment.

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Statement on Use of AWS American National Standards All standards (codes, specifications, recommended practices, methods, classifications, and guides) of the American Welding Society (AWS) are voluntary consensus standards that have been developed in accordance with the rules of the American National Standards Institute (ANSI). When AWS standards are either incorporated in, or made part of, documents that are included in federal or state laws and regulations, or the regulations of other governmental bodies, their provisions carry the full legal authority of the statute. In such cases, any changes in those AWS standards must be approved by the governmental body having statutory jurisdiction before they can become a part of those laws and regulations. In all cases, these standards carry the full legal authority of the contract or other document that invokes the AWS standards. Where this contractual relationship exists, changes in or deviations from requirements of an AWS standard must be by agreement between the contracting parties. International Standard Book Number: 0-87171-715-8 American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126 © 2003 by American Welding Society. All rights reserved Printed in the United States of America AWS American National Standards are developed through a consensus standards development process that brings together volunteers representing varied viewpoints and interests to achieve consensus. While AWS administers the process and establishes rules to promote fairness in the development of consensus, it does not independently test, evaluate, or verify the accuracy of any information or the soundness of any judgments contained in its standards. AWS disclaims liability for any injury to persons or to property, or other damages of any nature whatsoever, whether special, indirect, consequential or compensatory, directly or indirectly resulting from the publication, use of, or reliance on this standard. AWS also makes no guaranty or warranty as to the accuracy or completeness of any information published herein. In issuing and making this standard available, AWS is not undertaking to render professional or other services for or on behalf of any person or entity. Nor is AWS undertaking to perform any duty owed by any person or entity to someone else. Anyone using these documents should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. This standard may be superseded by the issuance of new editions. Users should ensure that they have the latest edition. Publication of this standard does not authorize infringement of any patent. AWS disclaims liability for the infringement of any patent resulting from the use or reliance on this standard. Finally, AWS does not monitor, police, or enforce compliance with this standard, nor does it have the power to do so. On occasion, text, tables, or figures are printed incorrectly, constituting errata. Such errata, when discovered, are posted on the AWS web page (www.aws.org). Official interpretations of any of the technical requirements of this standard may be obtained by sending a request, in writing, to the Managing Director, Technical Services Division, American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126 (see Annex A). With regard to technical inquiries made concerning AWS standards, oral opinions on AWS standards may be rendered. However, such opinions represent only the personal opinions of the particular individuals giving them. These individuals do not speak on behalf of AWS, nor do these oral opinions constitute official or unofficial opinions or interpretations of AWS. In addition, oral opinions are informal and should not be used as a substitute for an official interpretation. This standard is subject to revision at any time by the AWS C5 Committee on Arc Welding and Arc Cutting. It must be reviewed every five years, and if not revised, it must be either reapproved or withdrawn. Comments (recommendations, additions, or deletions) and any pertinent data that may be of use in improving this standard are required and should be addressed to AWS Headquarters. Such comments will receive careful consideration by the AWS C5 Committee on Arc Welding and Arc Cutting and the author of the comments will be informed of the Committee’s response to the comments. Guests are invited to attend all meetings of the AWS C5 Committee on Arc Welding and Arc Cutting to express their comments verbally. Procedures for appeal of an adverse decision concerning all such comments are provided in the Rules of Operation of the Technical Activities Committee. A copy of these Rules can be obtained from the American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126.

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Photocopy Rights Authorization to photocopy items for internal, personal, or educational classroom use only, or the internal, personal, or educational classroom use only of specific clients, is granted by the American Welding Society (AWS) provided that the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, Tel: 978-750-8400; online: http://www.copyright.com.

Personnel AWS C5 Committee on Arc Welding and Arc Cutting J. R. Hannahs, Chair N. A. Sanders, 1st Vice Chair D. B. Holliday, 2nd Vice Chair P. Howe, Secretary *E. R. Bohnart H. A. Chambers C. Connelly J. DeVito R. M. Dull D. A. Fink I. D. Harris *R. T. Hemzacek G. K. Hicken K. Y. Lee R. P. Munz S. R. Potter *B. L. Shultz R. L. Strohl *E. G. Yevick L. Yost

Edison Community College Hypertherm, Incorporated Northrop Grumman Corporation American Welding Society Welding Education and Consulting TRW Nelson Stud Welding Division Poly-Weld, Incorporated ESAB Welding and Cutting Products Edison Welding Institute The Lincoln Electric Company Edison Welding Institute Consultant Sandia National Laboratory, Retired The Lincoln Electric Company The Lincoln Electric Company SRP Consulting Services The Taylor-Winfield Corporation Tweco-Arcair Corporation Weld-Met International Group The Lincoln Electric Company

AWS C5C Subcommittee on Gas Tungsten Arc Welding G. K. Hicken, Chair R. D. Campbell, Vice Chair P. Howe, Secretary E. A. Benway *E. R. Bohnart *C. Connelly D. E. Destefan R. W. Diesner J. C. Downey T. W. Edwards *J. R. Hannahs *L. M. Hellemann E. J. LaCoursiere P. C. McClay D. E. Spragg *J. S. Thrower D. A. Wright *B. Young

Sandia National Laboratories, Retired Purity Systems, Incorporated American Welding Society Swagelok Company Welding and Education Consulting Poly-Weld, Incorporated High Current Technologies, Incorporated Retired Retired American Welding and Engineering, Incorporated Edison Community College General Electric Aircraft Engines EJL & Associates Pratt and Whitney Pratt and Whitney Optimum Engineering Manufacturing, Incorporated Zephyr Products, Incorporated Westinghouse Savannah River Company, Retired

*Advisor

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Foreword (This Foreword is not a part of AWS C5.5/C5.5M:2003, Recommended Practices for Gas Tungsten Arc Welding, but is included for informational purposes only.)

Gas tungsten arc welding (GTAW) was introduced as a practical fabricating process in the 1940s. In the decades since then, advances have been made in the equipment and in the development of techniques for automatic applications. GTAW is now accepted as the only practical joining method in some metal joining applications. These recommended practices were first prepared by the AWS C5 Committee on Arc Welding and Arc Cutting and the AWS C5C Subcommittee on Gas Tungsten Arc Welding in 1980. The 1980 edition was reaffirmed in 1989. The current AWS C5 Committee on Arc Welding and Arc Cutting and the current C5C Subcommittee on GTAW have prepared these recommended practices to present the basic practices and methods of GTAW and to expand the document to include the latest advancements in this process. These recommended practices are based on present uses of GTAW in the metal fabricating industry, along with research and development and new applications of the process. We should all encourage our younger generation to consider welding and welding related fields as places to become involved with the high rewards and challenges to be encountered in the future. We should also be willing and pleased to share our prior experiences and knowledge that could help new members excel in this occupation. The description of GTAW and its features are presented here as clearly and concisely as possible. The Committee has developed these guidelines in the hope that they would lead to further development of the GTAW process and, thus, to higher quality and performance standards. Comments and suggestions for the improvement of this standard are welcomed. They should be sent to the Secretary, AWS C5 Committee on Arc Welding and Arc Cutting, American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126. Official interpretations of any of the technical requirements of this standard may be obtained by sending a request, in writing, to the Managing Director, Technical Services Division, American Welding Society. A formal reply will be issued after it has been reviewed by the appropriate personnel following established procedures. Guidelines for technical inquiries regarding AWS standards are shown in Annex A. This document will be reviewed periodically to assure its success in serving all parties concerned with its provisions. Revisions will be issued when warranted.

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Table of Contents Page No.

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Personnel .................................................................................................................................................................... iii Foreword.......................................................................................................................................................................v List of Tables.................................................................................................................................................................x List of Figures..............................................................................................................................................................xi 1. Scope and Introduction ........................................................................................................................................1 1.1 Scope .........................................................................................................................................................1 1.2 Introduction to the Gas Tungsten Arc Welding (GTAW) Process.............................................................1 1.3 History.......................................................................................................................................................1 2. Normative References ..........................................................................................................................................4 2.1 American Conference of Governmental Industrial Hygienists Standards ................................................5 2.2 AWS Standards..........................................................................................................................................5 2.3 ISO Standards............................................................................................................................................6 2.4 OSHA Standards .......................................................................................................................................6 3. Definitions .............................................................................................................................................................6 4. GTAW Principles .................................................................................................................................................9 4.1 Process Description ...................................................................................................................................9 4.2 Process Advantages...................................................................................................................................9 4.3 Process Limitations .................................................................................................................................11 4.4 Process Variables.....................................................................................................................................12 4.5 Related Variables.....................................................................................................................................17 5. Equipment and Supplies.....................................................................................................................................18 5.1 Welding Power Sources (Used for GTAW)—Introduction .....................................................................18 5.2 Controllers...............................................................................................................................................20 5.3 Pulse Controllers .....................................................................................................................................22 5.4 Weld Sequence Controllers .....................................................................................................................22 5.5 Arc Welding Torches...............................................................................................................................23 5.6 Wire Feeders............................................................................................................................................30 5.7 Arc and Torch Oscillators........................................................................................................................31 5.8 Arc Initiation Equipment.........................................................................................................................32 6. Tungsten Electrodes ...........................................................................................................................................33 6.1 General ....................................................................................................................................................33 6.2 Classifications of Electrodes ...................................................................................................................34 6.3 Surface Finishes ......................................................................................................................................37 6.4 Electrode Sizes and Current Capacities ..................................................................................................37 6.5 Electrode Tip Configurations ..................................................................................................................38 6.6 Electrode Cutting.....................................................................................................................................41 6.7 Factors Affecting Electrode Life .............................................................................................................41 6.8 Removing Contamination .......................................................................................................................42 6.9 Grinding Dust..........................................................................................................................................42 6.10 Storage.....................................................................................................................................................43 7. Gas Shielding, Purging, and Backing ................................................................................................................43 7.1 Torch Shielding Gas ................................................................................................................................43

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Page No. 7.2 7.3 7.4 7.5 7.6

Purging ....................................................................................................................................................50 Shielding and Purging Gas Purity ...........................................................................................................62 Shielding and Purging Gas Economics ...................................................................................................65 Purifiers ...................................................................................................................................................65 Purging Gas Safety..................................................................................................................................67

8. Fixturing and Tooling.........................................................................................................................................68 8.1 Material Selection ...................................................................................................................................68 8.2 Tooling/Fixturing Considerations............................................................................................................68 8.3 Temporary (Soft)/Permanent (Hard) Tooling..........................................................................................69 9. Welding Techniques ...........................................................................................................................................71 9.1 General ....................................................................................................................................................71 9.2 Manual and Semiautomatic Welding ......................................................................................................71 9.3 Mechanized Welding...............................................................................................................................80 9.4 Automated Welding.................................................................................................................................83 10. Joint Design, Preparation, and Welding Positions .............................................................................................85 10.1 Introduction .............................................................................................................................................85 10.2 Basic Joint Configurations and Welding Positions .................................................................................85 10.3 Edge Preparation and Surface Cleaning..................................................................................................85

12. Qualification of Procedures, Welders, and Welding Operators..........................................................................94 12.1 Introduction .............................................................................................................................................94 12.2 Welding Program.....................................................................................................................................95 12.3 Establishing Welding Requirements .......................................................................................................95 12.4 Welding Procedure Specifications (WPS)...............................................................................................95 12.5 Procedure Qualification Records (PQR) .................................................................................................95 12.6 Welder and Welding Operator Qualification Tests..................................................................................95 13. Quality Control ..................................................................................................................................................96 13.1 Introduction .............................................................................................................................................96 13.2 Weldment Quality....................................................................................................................................96 13.3 Specifications ..........................................................................................................................................96 14. Troubleshooting .................................................................................................................................................97 14.1 General ....................................................................................................................................................97 14.2 Electrical..................................................................................................................................................97 14.3 Inert Shielding Gas Troubleshooting ....................................................................................................102 14.4 Water Cooling Systems .........................................................................................................................105 14.5 Tools and Fixtures .................................................................................................................................106

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11. Welding Characteristics of Selected Alloys .......................................................................................................85 11.1 Introduction .............................................................................................................................................85 11.2 Carbon and Alloy Steels..........................................................................................................................89 11.3 Stainless Steels and Iron-Based Superalloys...........................................................................................89 11.4 Aluminum Alloys ....................................................................................................................................90 11.5 Magnesium Alloys ..................................................................................................................................91 11.6 Beryllium.................................................................................................................................................91 11.7 Copper Alloys .........................................................................................................................................91 11.8 Nickel Alloys...........................................................................................................................................92 11.9 Cobalt Alloys...........................................................................................................................................92 11.10 Refractory and Reactive Metals ..............................................................................................................92 11.11 Cast Irons.................................................................................................................................................92 11.12 Welding Dissimilar Materials..................................................................................................................92 11.13 Filler Metals ............................................................................................................................................92

Page No. 14.6 Filler Material........................................................................................................................................106 14.7 Design of Welded Assemblies...............................................................................................................106 14.8 Weld Joint Fit-up ...................................................................................................................................108 15. Safety ...............................................................................................................................................................108 15.1 Hazards..................................................................................................................................................108 15.2 Electrical Shock.....................................................................................................................................108 15.3 Arc Radiation and Burns .......................................................................................................................108 15.4 Welding Environment............................................................................................................................109 15.5 Oxygen Deficiency................................................................................................................................110 15.6 Noise......................................................................................................................................................110 15.7 Safe Handling of Cylinders ...................................................................................................................110 15.8 Fires And Explosions ............................................................................................................................110 15.9 Common Sense......................................................................................................................................110 15.10 Grinding Dust........................................................................................................................................110 Nonmandatory Annexes............................................................................................................................................113 Annex A—Guidelines for Preparation of Technical Inquiries for AWS Technical Committees................................113 Annex B—Suggested Reading List and Other References ........................................................................................115 List of AWS Documents on Arc Welding and Arc Cutting ........................................................................................117

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List of Tables

Table 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Page No. Welding Process Comparison Based on Quality and Economics ................................................................11 Comparison of Typical Current Ratings for Gas-Cooled and Water-Cooled GTAW Torches.....................25 Typical Welding Cable Capacities ...............................................................................................................29 Guide for Selecting the Size of Cable Based on the Welding Current.........................................................30 Chemical Composition Requirements for Tungsten Electrodes ..................................................................34 Typical Current Ranges for Tungsten Electrodes and Recommended Gas Cup Sizes ................................35 Comparison of Surface Finish Designations................................................................................................37 Recommended Types of Current, Tungsten Electrodes, and Shielding Gases for Welding of Various Metals and Alloys ...........................................................................................................................39 Tungsten Electrode Tip Shapes and Examples of Current Ranges ..............................................................39 General Properties of Gases .........................................................................................................................44 Thermodynamic Properties of Gases ...........................................................................................................44 Dew Point Conversions ................................................................................................................................45 Advantages of Shielding Gases....................................................................................................................46 Typical Argon Flow Rates............................................................................................................................50 Gas Purity Specification by Industrial Grade...............................................................................................62 Purity Requirements for Gaseous Argon .....................................................................................................62 Purity Requirements for Gaseous Helium ...................................................................................................63 Purity Requirements for Gaseous Hydrogen ...............................................................................................63 Welding Equipment or Components ............................................................................................................72 AWS Specifications Related to Gas Tungsten Arc Welding........................................................................88 Troubleshooting ...........................................................................................................................................98 Guide for Shade Numbers..........................................................................................................................109

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List of Figures Figure 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 --``,``-`-`,,`,,`,`,,`---

29 30 31 32 33 34 35 36 37

Page No. Early Gas Tungsten Arc Welding Torches and Accessories, Circa 1943, with a Torch Body and an Early Flowmeter .................................................................................................................................2 Early Gas Tungsten Arc Welding Torches .....................................................................................................2 SMAW Power Source Used for Early Gas Tungsten Arc Welding ...............................................................3 Motor-Generator SMAW Power Source Used for Early Gas Tungsten Arc Welding ...................................3 Gas Tungsten Arc Welding Power Source—Pulsed ......................................................................................4 Gas Tungsten Arc Welding Power Source .....................................................................................................4 Stylized Representation of the Gas Tungsten Arc Welding Process..............................................................9 Gas-Cooled GTAW Torch and Stylized Representation of Typical Gas Tungsten Arc Welding Equipment ....................................................................................................................................................10 Clean Weld Beads Typical of Properly Shielded GTAW Welds ..................................................................11 Characteristics of Current Types Used for Gas Tungsten Arc Welding.......................................................13 Relationship Between Arc Pressure and Pulse Frequency...........................................................................14 Programmed (Modulated) Current without Polarity Reversal .....................................................................15 Characteristics of Variable Polarity (Programmed) Weld Current...............................................................16 Arc Shape and Fusion Zone Profiles as a Function of Electrode Tip Geometry in Pure Argon Shielding Gas ...............................................................................................................................................17 Typical Volt-Amp Characteristic Curves for GTAW Power Sources...........................................................18 Typical Inverter Power Source Components................................................................................................20 Typical Wire Feeder .....................................................................................................................................22 Typical GTAW Sequence for Non-Pulsed DC Welding...............................................................................23 Typical GTAW Sequence for Pulsed DC Current and Pulsed Wire Feed ....................................................24 Typical Water-Cooled GTAW Torch (Cross-Sectional View)......................................................................25 Examples of GTAW Torches........................................................................................................................26 Orbital Weld Head with Wire Feeder...........................................................................................................27 Components of a Typical GTAW Torch, Including Gas Nozzle/Cup, Gas Lens, Collet Body, Torch Body, Collet, and Electrode ...............................................................................................................27 GTAW Torch Without a Gas Lens (Left) and with a Gas Lens (Right) .......................................................28 GTAW Torch with Cold Wire Feed..............................................................................................................31 Schematic of GTAW with Hot Wire Feed....................................................................................................31 Magnetically Deflected Arc Laying a Stringer Bead in a Deep Groove Weld ............................................32 Cross Sections of Welds Made in 1/2 in. [13 mm] Thick Stainless Steel; (A) with Magnetic Arc Oscillation, and (B) without Magnetic Arc Oscillation ...............................................................................32 High-Frequency Arc Starting.......................................................................................................................33 Balled Tip on the End of a Pure Tungsten Electrode Used for AC Welding ...............................................36 Ground Tapered Tip on End of Doped Tungsten Electrodes .......................................................................38 Ground Electrode Tip Geometry..................................................................................................................39 Typical Preparation Method of Tungsten Electrodes Used for GTA Welding, Including Tip Truncation, Grinding, and Cutting.........................................................................................................40 The Desired Surface Finish of a Ground Electrode .....................................................................................41 Proper Cutting of Tungsten Electrodes with a Diamond Cut-Off Blade .....................................................42 GTA Weld Bead Shape as a Function of Shielding Gas Composition and Electrode Tip Geometry (on 304 Stainless Steel) ........................................................................................................47 GTA Voltage—Current Relationships with Argon and Helium Shielding Gases for Different Arc Lengths ..................................................................................................................................48

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Figure 38 39 --``,``-`-`,,`,,`,`,,`---

40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79

Page No. Monel®1

Improved Surface Cleanliness on Welds Produced with the 5% Hydrogen Mixture in Argon Shielding Gas with GTAW ...........................................................................................................49 The Effects of Shielding Gas Contamination on Titanium Weldments (Color Chart for Titanium Welding Acceptance)....................................................................................................................51 GTAW Weld Underbead Contamination with Various Levels of Oxygen Contents in the Purging Gas.........52 Purging Times for Various Pipe Sizes..........................................................................................................54 Purging a Piping System with Open Ends Blanked .....................................................................................55 Purging of a Piping System with Appropriate Venting to Eliminate Dead Air Pockets ..............................55 Purging with Removable Plugs ....................................................................................................................56 Purging with Removable Chamber ..............................................................................................................56 Purge Distributor Ring .................................................................................................................................56 Purging with Water Soluble Paper Dams .....................................................................................................57 Purging with a Backing Channel..................................................................................................................57 Purging with a Gas Distributor (Diffuser)....................................................................................................58 Typical Inert Gas Glove Box Chamber ........................................................................................................58 Flexible Plastic Purge Bag ...........................................................................................................................59 Trailing Shields—Bottom View of Inert Gas Trailing Shield Fabricated Using Stainless Porous (100 micron) Tubing (Shown with High Temperature Tape)......................................................................60 Trailing Shields—Inert Gas Trailing Shield Fabricated Using Stainless Porous (100 micron) Tubing (Shown with High Temperature Tape) .........................................................................................................60 Trailing Shields—Inert Gas Trailing Shield Fabricated Using Stainless Porous (40–100 micron) Sheet Metal ..................................................................................................................................................61 Shielding with the Use of a Backing Tape ...................................................................................................61 Point-of-Use (POU) Purifiers (Waferpure®2 Reactive Resin Type) Below a Welding Fixture....................66 Point-of-Use Gas Purifier (Heated Metal Getter Type) ...............................................................................67 Cylinder Status Tag (The Use of a Simple Tagging System Can Be Very Helpful) ....................................68 Weld Distortion in Ti-6Al-V Bead-on-Plate (Sheet) Weld ..........................................................................69 Tooling for GTAW of Fuel Cell Components to Control Distortion of Weldment......................................70 Run-On/Run-Off Tabs Used for Welding Ends of Strip Material................................................................70 Walking-the-Cup Technique ........................................................................................................................74 Dragging-the-Finger Technique ...................................................................................................................74 Folding Fingerstall Technique......................................................................................................................75 Brace Technique Showing Wrist in Contact with Workpiece to Stabilize the Torch...................................75 Small Rotary Positioner Used for Workpiece Manipulation........................................................................76 Mechanical Manipulation in a Mechanized Welder.....................................................................................76 Gas Tungsten Arc Welding Torch with Wire Feeders [(A) and (B)] for Spooled Wire...............................78 Cross Sections of Typical Consumable Inserts ............................................................................................79 Lathe-Type Welding Setup...........................................................................................................................81 Orbital GTAW Weld Head ...........................................................................................................................84 Basic Joint Types..........................................................................................................................................86 Weld Joint Edge Preparation (U-Groove, J-Groove, and V-Groove) ...........................................................87 Effects of Sulfur Content on Bead-on-Plate Weld Bead Shape in 304L Made with the Same Parameters ..........................................................................................................................................90 GTAW in 6061-0 Aluminum Showing the Surface Contours with Pulsed Direct Current Straight Polarity (DCEN).............................................................................................................................91 High Quality Welds in Inconel®1 718, Original Scale 5X ...........................................................................93 High Quality Welds in Cobalt Alloy HS188, Original Scale 5X .................................................................93 Manual GTAW of Titanium in an Inert Gas Chamber (Glove Box) ............................................................94 Criteria for Acceptable GTAW in Titanium via Tack Welds Only ............................................................104

1. Monel and Inconel are registered trademarks of Special Metals Corporation. 2. Waferpure is a registered trademark of Mykrolis Corporation.

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AWS C5.5/C5.5M:2003

Recommended Practices for Gas Tungsten Arc Welding

1. Scope and Introduction

ples of this would be certain welded metal sculptures and/or a “perfectly” welded part or assembly. The “science” end of the spectrum would include recent developments such as fully automated robotic welding cells that could include through-the-torch vision that allows real-time viewing of the weld as well as real-time weld joint tracking. Also, weld parameter data acquisition and feedback control are routinely accomplished in real-time.

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1.1 Scope. This document presents recommended practices for the gas tungsten arc welding (GTAW) process.1 Its purpose is to provide a fundamental explanation of the process, describe basic practices and concepts, and outline some advanced methods and applications of GTAW. These should enable welding personnel to determine the best applications of this process and evaluate its use compared with other joining processes. The section covering principles of operation will help the reader understand how the process works, the general types of equipment needed, and the advantages and limitations of the gas tungsten arc welding process. The basic concepts and practices include both general and specific recommendations and technical data for equipment, consumables, procedures, variables, applications, and safety considerations. This standard makes use of U.S. Customary Units. Approximate mathematical equivalents in the International System of Units (SI) are provided for comparison in brackets [ ] or in appropriate columns in tables and figures.

1.3 History. Although arc welding was first developed in the 1880s, its commercial use in the United States did not commence until the first decade of the 1900s. The years of the First World War brought the initial large-scale commercial use of arc welding, when shielded metal arc welding (SMAW) began to replace riveting as the means of joining in the manufacture of ships. During the 1920s, H. M. Hobart and P. K. Devers performed preliminary work on using inert gases to shield the carbon or metallic electrode’s welding arc and molten weld pool. In 1926 they applied for patents2 on the use of an electric welding arc in which an inert gas was independently supplied around the arc, thus replacing flux as the shielding method. Other investigators experimented with both helium and argon as shielding gases, but because of the high costs associated with these inert gases, very little commercial use was made of them at that time.

1.2 Introduction to the Gas Tungsten Arc Welding (GTAW) Process. Welding as an occupation and a career is a very “special” and rewarding choice to pursue. It is one of the most interesting manufacturing disciplines as it involves both art and science. This is illustrated by manual gas tungsten arc welding (GTAW) because a person’s manual dexterity, hand-eye coordination, and selfdiscipline in combination with the correct welding procedure(s) are paramount to its success. The “art” portion is most evident when an individual welder expresses their unique signature to the manually applied welds. Exam-

By the onset of the Second World War, shielded metal arc welding had become the dominant welding process. However, there was a need within the aircraft industry for welds made with better shielding than that provided by SMAW when joining reactive metals such as aluminum and magnesium. Also, in the aircraft industry there was a need to develop an acceptable welding process to replace riveting for joining of thin gage materials. These needs led to the first commercial development of gas tungsten arc welding equipment.

1. Gas tungsten arc welding is defined as an arc welding process that uses an arc between a tungsten electrode (nonconsumable) and the weld pool. The process is used with shielding gas and without the application of pressure. (Ref. AWS A3.0, Standard Welding Terms and Definitions.)

2. H. M. Hobart, U.S. Patent 1,746,081, 2/4/1930 and P. K. Devers, U.S. Patent 1,746,191, 2/4/1930.

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AWS C5.5/C5.5M:2003

In 1941, R. Meredith and V. H. Pavlecka developed the first practical electrode holders (torches) using a nonconsumable electrode made from tungsten. These first torches were simply shielded metal arc welding electrode holders that had been modified to provide the shielding gas flow. A 1/8 in. [3 mm] diameter tungsten electrode was held in a copper tube through which the inert helium gas flowed to protect the electrode, weld pool, and adjacent heated areas of the workpiece. Helium was elected to provide the necessary shield because, at the time, it was the only readily available inert gas. Tungsten inert gas torches and accessories typical of that period are shown in Figures 1 and 2.3 A patent was issued for this process in 1942.4 This arc welding process was initially named “Heliarc®,” 5, 6 welding because helium was used as the shielding gas. It has also been called nonconsumable electrode welding, tungsten inert gas (TIG) welding, wolfram inert gas (WIG) welding7 and tungsten-arc welding. However, the proper AWS terminology for this process is gas tungsten arc welding (GTAW), because shielding gas mixtures containing inert gases other than helium, or gases which are not inert, are sometimes used. Using a tungsten electrode and direct current power source, a stable, efficient heat source (the arc) was used

to produce acceptable welds. The inert shielding gas provided full protection of the arc and weld pool, which was imperative in welding of aluminum and magnesium, because even a small amount of air could contaminate the weld. The process also allows for better control over the heat input, thus making it easier to weld thin materials.

3. Reprinted with permission from “Modern Welding Technology,” H. B. Cary, 2nd Edition, Prentice Hall, NJ 1989, Figure 1-9, p. 8. 4. R. Meredith, U.S. Patent 2,274,631. 2/24/1942. 5. Registered trademark of ESAB. 6. “Heliweld” was the Airco description of the process. 7. Wolfram is the German name for tungsten.

Figure 2—Early Gas Tungsten Arc Welding Torches

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Figure 1—Early Gas Tungsten Arc Welding Torches and Accessories, Circa 1943, with a Torch Body and an Early Flowmeter

AWS C5.5/C5.5M:2003

ever, because it did not provide the cleaning action like DCEP, it was not acceptable for welding of aluminum or magnesium, which have tenacious surface oxides that must be removed before acceptable welds can be produced. To obtain the cleaning action of DCEP along with the improved penetration characteristics and lower electrode heating of DCEN, alternating current (AC) welding power sources were developed. A high-frequency, highvoltage current was superimposed over the basic welding current to stabilize the arc during current reversals. This method was successfully applied to GTAW of aluminum and magnesium. By the early 1950s, GTAW had gained acceptance in the welding industry. Argon was the most widely accepted shielding gas, followed by helium. However, because of the high cost of argon and helium gases, carbon dioxide and nitrogen were investigated as shielding gases. Since that time, numerous other gases and mixtures of gases have been used with this welding process to provide improved welding performance for some metals. These include argon-helium mixtures, argonhydrogen mixtures, and argon-nitrogen mixtures. As a tool for increasing deposition rates beyond that of the commonly used cold wire feed during gas tungsten arc welding, the hot wire feed method8 of filler metal addition was introduced. This allowed the high quality

Figure 3—SMAW Power Source Used for Early Gas Tungsten Arc Welding

When the GTAW process was first developed, SMAW was being performed utilizing direct current with electrode positive (DCEP or reverse polarity). The same power sources (DC generators of the rotating type) were thus used for the early gas tungsten arc welding process. Photographs of early GTAW/SMAW power sources are shown in Figures 3 and 4. One of the major benefits of DCEP was the tremendous cathodic cleaning action of the workpiece surface for aluminum and magnesium. However, overheating of the electrode and subsequent splitting, melting, and transfer of tungsten particles into the weld limited the useful current range. Since these early torches were air-cooled, they had only a 75 A current capacity. As a result, it soon became apparent that DCEP was not the best polarity to use for this process. By making the electrode negative, overheating was avoided and weld penetration was improved. Use of DC electrode negative (DCEN or straight polarity) proved acceptable for welding of stainless steels. How-

8. A. F. Manz, U.S. Patent 3,122,629, File 2562, issued 2/25/1964.

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Figure 4—Motor-Generator SMAW Power Source Used for Early Gas Tungsten Arc Welding

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Figure 5—Gas Tungsten Arc Welding Power Source —Pulsed

Figure 6—Gas Tungsten Arc Welding Power Source

welds produced by the GTAW process without incurring the spatter produced by the consumable electrode processes.

compositions have been developed for improved arc starting, arc stability, and electrode life. GTAW systems are available with several tungsten electrodes positioned around the part to be welded, or impinging on a single weld pool.

Over the last several decades, numerous improvements have been made to the GTAW equipment, process and controls. Welding power sources have been developed specifically for the GTAW process (see Figures 5 and 6). Some provide pulsed direct current and others produce variable polarity alternating welding current. Automatic arc starting systems, automatic arc length/ voltage controls, vision and penetration sensors, and positioning equipment are all commercially available. Computer controls, automatic sequence controls, and data acquisition systems are readily available to allow data recording and statistical process control. Watercooled torches were developed, which allow welding currents of up to 1500 A. Different tungsten electrode

2. Normative References If a code or other standard is cited without a date of publication, it is understood that the latest edition of the document referred to applies. As codes and other standards undergo frequent revision, the reader is encouraged to consult the most recent edition. If a code or other standard is cited with the date of publication, the citation refers to that edition only, and it is understood that any future revisions or amendments to the code or standard are not included.

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2.1 American Conference of Governmental Industrial Hygienists Standards9 (1) ACGIH, Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices

(20) AWS B2.1, Specification for Welding Procedure and Performance Qualification (21) AWS B2.1-1-002, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding of Carbon Steel, (M-1/P-1, Group 1 or 2), 3/16 through 7/8 inch, in the As-Welded Condition, With or Without Backing (22) AWS B2.1-1-007, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding of Galvanized Steel (M-1), 18 through 10 Gauge, in the AsWelded Condition, with or without Backing (23) AWS B2.1-1-008, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding of Carbon Steel (M-1, P-1, or S-1), 18 through 10 Gauge, in the As-Welded Condition, with or without Backing (24) AWS B2.1-1-009, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding of Austenitic Stainless Steel (M-8, P-8, or S-8), 18 through 10 Gauge, in the As-Welded Condition, with or without Backing (25) AWS B2.1-1/8-010, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding of Carbon Steel to Austenitic Stainless Steel (M-1, P-1, or S-1 to M-8, P-8, or S-8), 18 through 10 Gauge, in the AsWelded Condition, with or without Backing (26) AWS B2.1-22-015, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding of Aluminum (M/P/S-22 to M/P/S-22), 18 through 10 Gauge, in the As-Welded Condition, with or without Backing (27) AWS B2.1-1-021, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding Followed by Shielded Metal Arc Welding of Carbon Steel (M-1/P-1/S-1, Group 1 or 2), 1/8 through 1-1/2 inch Thick, ER70S-2 and E7018, As-Welded or PWHT Condition (primarily for pipe applications) (28) AWS B2.1-8-024, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding Followed by Shielded Metal Arc Welding of Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/8 through 1-1/2 inch Thick, ER3XX and E3XX-XX, As-Welded Condition, Primarily Plate and Structural Applications. (29) AWS B2.1-8-025, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding of Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/16 through 1-1/2 inch Thick, ER3XX, As-Welded Condition, Primarily Plate and Structural Applications. (30) ANSI/AWS B2.1-1-207, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding of Carbon Steel (M-1/P-1/S-1, Group 1 or 2), 1/8 through 1-1/2 inch Thick, ER70S-2, As-Welded or PWHT Condition, Primarily Pipe Applications (31) ANSI/AWS B2.1-1-209, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding Followed by Shielded Metal Arc Welding of Carbon Steel (M-1/P-1/S-1, Group 1 or 2), 1/8 through 1-1/2 inch Thick, ER70S-2 and E7018, As-Welded or PWHT Condition, Primarily Pipe Applications

2.2 AWS Standards10 (1) ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes (2) AWS A1.1, Metric Practice Guide for the Welding Industry (3) AWS A3.0, Standard Welding Terms and Definitions (4) AWS A5.7, Specification for Copper and Copper Alloy Bare Welding Rods and Electrodes (5) AWS A5.9, Specification for Bare Stainless Steel Welding Electrodes and Rods (6) AWS A5.10/A5.10M, Specification for Bare Aluminum and Aluminum Alloy Welding Electrodes and Rods (7) AWS A5.12/A5.12M, Specification for Tungsten and Tungsten Alloy Electrodes for Arc Welding and Cutting (8) AWS A5.13, Specification for Surfacing Welding Electrodes for Shielded Metal Arc Welding (9) AWS A5.14/A5.14M, Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods (10) AWS A5.15, Specification for Welding Electrodes and Rods for Cast Iron (11) AWS A5.16, Specification for Titanium and Titanium Alloy Welding Electrodes and Rods (12) AWS A5.18/A5.18M, Specification for Carbon Steel Electrodes and Rods for Gas Shielded Arc Welding (13) AWS A5.19, Specification for Magnesium-Alloy Welding Electrodes and Rods (14) AWS A5.21, Specification for Bare Electrodes and Rods for Surfacing (15) AWS A5.22, Specification for Stainless Steel Electrodes for Flux Cored Arc Welding and Stainless Steel Flux Cored Rods for Gas Tungsten Arc Welding (16) AWS A5.24, Specification for Zirconium and Zirconium Alloy Welding Electrodes and Rods (17) AWS A5.28, Specification for Low Alloy Steel Electrodes and Rods for Gas Shielded Arc Welding (18) AWS A5.30, Specification for Consumable Inserts (19) AWS A5.32, Specification for Welding Shielding Gases 9. Available through American Conference of Governmental Industrial Hygienists, 1330 Kemper Meadow Drive, Cincinnati, OH 45240-1634. 10. Available through Global Engineering Documents, Handling Services Group, 15 Inverness Way East, Englewood, CO 80112-5776, (800) 854-7179 (303) 397-7956, Fax (303) 3972740, Internet: www.global.ihs.com.

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(41) AWS B2.1-4-221, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding (Consumable Insert Root) followed by Shielded Metal Arc Welding of Chromium-Molybdenum Steel (M-4/P-4, Group 1 or 2), 1/8 through 1/2 inch Thick, As-Welded Condition, 1/8 through 1-1/2 inch Thick, PWHT Condition, IN515 and ER80S-B2, and E8018-B2, Primarily Pipe Applications (42) AWS B2.1-5A-222, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding of Chromium-Molybdenum Steel (M-5A/P-5A), ER90S-B3, 1/8 through 1/2 inch Thick, As-Welded Condition, 1/8 through 3/4 inch Thick, PWHT Condition, Primarily Pipe Applications (43) AWS B2.1-5A-224, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding followed by Shielded Metal Arc Welding of ChromiumMolybdenum Steel (M-5A/P-5A), 1/8 through 1/2 inch Thick, As-Welded Condition, 1/8 through 1-1/2 inch Thick, PWHT Condition, ER90S-B3 and E9018-B3, Primarily Pipe Applications (44) AWS B2.1-5A-225, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding (Consumable Insert Root) of Chromium-Molybdenum Steel (M-5A/P-5A), 1/8 through 1/2 inch Thick, As-Welded Condition, 1/8 through 3/4 inch Thick, PWHT Condition, IN521 and ER90S-B3, Primarily Pipe Applications (45) AWS B2.1-5A-226, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding (Consumable Insert Root) followed by Shielded Metal Arc Welding of Chromium-Molybdenum Steel (M-5A/P-5A), 1/8 through 1/2 inch Thick, As-Welded Condition, 1/8 through 1-1/2 inch Thick, PWHT Condition, IN521 and ER90S-B3, and E9018-B3, Primarily Pipe Applications (46) AWS B2.1-1/8-227, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding of Carbon Steel (M-1/P-1/S-1, Groups 1 or 2) to Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/16 through 1-1/2 inch Thick, ER309(L), As-Welded Condition, Primarily Pipe Applications (47) AWS B2.1-1/8-229, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding followed by Shielded Metal Arc Welding of Carbon Steel (M-1/P-1/S-1, Groups 1 or 2) to Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/8 through 1-1/2 inch Thick, ER309(L) And ER309(L)-15, -16, or -17, AsWelded Condition, Primarily Pipe Applications (48) AWS B2.1-1/8-230, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding with Consumable Insert Root of Carbon Steel (M-1/P-1/ S-1, Groups 1 or 2) to Austenitic Stainless Steel (M-8/P-8/ S-8, Group 1), 1/16 through 1-1/2 inch Thick, IN309 and ER309(L), As-Welded Condition, Primarily Pipe Applications

(32) AWS B2.1-1-210, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding with Consumable Insert Root of Carbon Steel (M-1/P-1/S-1, Group 1 or 2), 1/8 through 1-1/2 inch Thick, INMs-1 and ER70S-2, As-Welded Condition, Primarily Pipe Applications (33) AWS B2.1-1-211, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding with Consumable Insert Root followed by Shielded Metal Arc Welding of Carbon Steel (M-1/P-1/S-1, Group 1 or 2), 1/8 through 1-1/2 inch Thick, INMs-1, ER70S-2, and E7018, AsWelded or PWHT Condition, Primarily Pipe Applications (34) AWS B2.1-8-212, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding of Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/16 through 1-1/2 inch Thick, ER3XX, As-Welded Condition, Primarily Pipe Applications (35) AWS B2.1-8-214, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding followed by Shielded Metal Arc Welding of Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/8 through 1-1/2 inch Thick, ER3XX and E3XX-XX, As-Welded Condition, Primarily Pipe Applications (36) AWS B2.1-8-215, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding with Consumable Inserts of Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/8 through 1-1/2 inch Thick, IN3XX and ER3XX, As-Welded Condition, Primarily Pipe Applications (37) AWS B2.1-8-216, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding with Consumable Insert Followed by Shielded Metal Arc Welding of Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/8 through 1-1/2 inch Thick, IN3XX, ER3XX and ER3XXXX, As-Welded Condition, Primarily Pipe Applications (38) AWS B2.1-4-217, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding of Chromium-Molybdenum Steel (M-4/P-4, Group 1 or 2), ER80S-B2, 1/8 through 1/2 inch Thick, As-Welded Condition, 1/8 through 1-1/2 inch Thick, PWHT Condition, Primarily Pipe Applications (39) AWS B2.1-4-219, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding followed by Shielded Metal Arc Welding of ChromiumMolybdenum Steel (M-4/P-4, Group 1 or 2), 1/8 through 1/2 inch Thick, As-Welded Condition, 1/8 through 1-1/2 inch Thick, PWHT Condition, ER80S-B2 and E8018-B2, Primarily Pipe Applications (40) AWS B2.1-4-220, Standard Welding Procedure Specification (WPS) for Gas Tungsten Arc Welding (Consumable Insert Root) of Chromium-Molybdenum Steel (M-4/P-4, Group 1 or 2), 1/8 through 1/2 inch Thick, AsWelded Condition, 1/8 through 3/4 inch Thick, PWHT Condition, IN515 and ER80S-B2, Primarily Pipe Applications

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(49) AWS B2.1-1/8-231, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding with Consumable Insert Root followed byShielded Metal Arc Welding of Carbon Steel (M-1/P-1/S-1, Groups 1 or 2) to Austenitic Stainless Steel (M-8/P-8/S-8, Group 1), 1/8 through 1-1/2 inch Thick, IN309 and ER309, and E309-15, -16, or -17, or IN309, ER309(L), and ER309(L)-15, -16, or -17, As-Welded Condition, Primarily Pipe Applications (50) AWS C5.10, Recommended Practices for Shielding Gases for Welding and Plasma Arc Cutting (51) AWS D1.1, Structural Welding Code—Steel (52) AWS D1.2, Structural Welding Code—Aluminum (53) AWS D1.2, Structural Welding Code—Stainless Steel (54) AWS D10.11, Recommended Practices for Root Pass Welding of Pipe Without Backing

arc preheating. An arc that does not melt the base metals is sometimes used to remove the chill or significantly raise the temperature of the weld joint. This can be used to remove moisture or light residues of oils. This can be accomplished by a first pass or a second leading arc. arc welding torch. A device used to transfer current to a fixed welding electrode, position the electrode, and direct the flow of shielding gas. automatic welding. Welding with equipment that requires only occasional or no observation of the welding, and no manual adjustment of the equipment controls. backing or back-up bars. Plates or bars that are in contact with the workpieces on either the root or the face side of the welds should be located so as not to contaminate the weld or base metal or cause gas or flux (if used) entrapment. These accessories are usually made of copper or stainless steel.

2.3 ISO Standards11 (1) ISO6848, Tungsten Electrodes for Inert Gas Shielded Arc Welding, and for Plasma Cutting and Welding.

brace-technique. A technique used in manual welding of vessels, sheet metal and structures. In this welding technique, the welder stabilizes the torch by resting his wrist, arm or elbow as far away from the weld zone as possible and sliding the wrist in the direction of welding.

2.4 OSHA Standards12 (1) Code of Federal Regulations (OSHA), Title 29 Labor, Parts 1910.1 to 1910.1450

direct current electrode negative (DCEN). The arrangement of direct current arc welding leads in which the electrode is the negative pole and the workpiece is the positive pole of the welding arc.

3. Definitions

direct current electrode positive (DCEP). The arrangement of direct current arc welding leads in which the electrode is the positive pole and the workpiece is the negative pole of the welding arc.

The terms and definitions are divided into two categories: (1) general welding terms (highlighted by bold print) compiled by the AWS Committee on Definitions and Symbols (which can be found in the latest AWS A3.0, Standard Welding Terms and Definitions); and, (2) terms or definitions (highlighted by bold italics) defined by the AWS C5C Subcommittee on Gas Tungsten Arc Welding, which are defined as they relate to this Recommended Practice, or are additions or modifications to AWS A3.0 terms or definitions.

dragging-the-finger. A technique used in manual welding of all types of joint configurations. It can be used when welding all diameters of pipe and tubing. In this welding technique a fingerstall is made by overlapping a “sleeve” of fiberglass material approximately 8 in. [200 mm] long. The fingerstall is placed over the middle finger and should extend approximately 1 in. [25 mm] beyond the end of the finger. The fingerstall is used to stabilize the hand holding the torch as it is slid along during welding.

air cooled torch. A nonstandard term for a gas-cooled torch. arc postheating. The intense heat of an arc can be used to reduce the cooling rate of a weld after a fusion weld has been made. This is primarily a nonmelting pass.

dressing pass. The weld bead shape can be changed or improved by the use of a cosmetic pass after the weld has been completed. This is primarily accomplished by remelting of the solidified weld bead surface. This is frequently accompanied with transverse oscillation of the arc or torch. Sometimes a small amount of filler metal is added.

11. Available through ISO Central Secretariat, International Organization for Standardization (ISO) 1, rue de Varembé, Case postale 56, CH-1211 Geneva 20, Switzerland; Online: http://www.iso.ch/iso/en/ISOOnline.frontpage. 12. Available through U.S. Government Printing Office, Superintendent of Documents, P.O. Box 371954, Pittsburgh, PA 15250-7954.

electrode holder. A device used for mechanically holding and conducting current to an electrode during welding. See arc welding torch.

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run-off weld tab. Additional material that extends beyond the end of the joint, on which the weld is terminated.

fixture. A device designed to hold and maintain parts to be joined in proper relation to each other. freehand technique. The free hand technique is a method where welding is accomplished without the benefit of stabilization of hand and welding torch. It is the most difficult way to effect a weld and is not recommended unless no other option is available.

run-on/off tabs. Additional material (devices) used mostly on butt and lap joints to facilitate starting and extinguishing the arc. semiautomatic welding. Manual welding with equipment that automatically controls one or more of the welding conditions.

gas cup. A nonstandard term for gas nozzle. gas nozzle. A device at the exit end of the torch that directs shielding gas.

straight polarity. A nonstandard term for direct current electrode negative.

gas tungsten arc welding (GTAW). An arc welding process that uses an arc between a tungsten electrode (nonconsumable) and the weld pool. The process is used with shielding gas and without the application of pressure. Filler metal may or may not be used. Heliarc® welding. A nonstandard term for gas tungsten arc welding. It refers to the earliest use of the process when only the inert gas helium was utilized as the shielding method.

tooling. Weld tooling can be a variety of devices (fixtures) that are associated with the welding operation and their use assists in producing a weldment that meets all specified requirements. Weld joint backing and/or chill bars are examples of weld tooling.

hold down and backing bars. Fixtures which provide the means to clamp and position the weldment, and supply backing support and/or shielding gas to the underside of the weld bead. Backing bars usually contain passages to facilitate inert gas purging. When grooved backing bars are used, the grooves should be shallow to minimize melt through and to limit the height of root reinforcement. Grooves in backing bars should have rounded corners to be elliptical in shape to prevent entrapments and to minimize stress raisers relative to underbead shape. These devices usually extend the full length of the weld.

torch. See arc-welding torch. tungsten-arc welding. A nonstandard term for gas tungsten arc welding. tungsten electrode. A non-filler metal electrode used in arc welding, arc cutting, and plasma spraying, made principally of tungsten. tungsten inert gas (TIG) welding. A nonstandard term for gas tungsten arc welding. It refers to the early use of the process when only the inert gases argon and helium were utilized as the shielding method.

machine welding. A nonstandard term when used for mechanized welding.

walking-the-cup. A technique for manipulating the torch when manually welding groove and fillet welds. With this technique, the electrode extension is adjusted to allow the proper arc length while the edge of the cup rests on the side of the joint. The torch is manipulated in a manner to swing the tip of the tungsten back and forth across the side of the joint by “walking-the-cup” on each edge of the joint. The left or right edge of the gas nozzle/cup is in constant contact with the members giving the cup a walking motion on the weld joint.

manual welding. Welding with the torch held and manipulated by hand. Accessory equipment, such as part motion devices and manually controlled fillermaterial feeders may be used. mechanized welding. Welding with equipment that requires manual adjustment of the equipment controls in response to visual observation of the welding, with the torch held by a mechanical device. opposing arcs. This is a welding process employing two arcs. Each arc is located on opposite sides of the weld joint. The purpose is to equalize stresses on plates to reduce the tendency for warping or distortion.

welding fixture (also see fixture). A device designed and built to hold parts to be joined in proper relationship with each other. Chill bars are often used to help cool the weld area rapidly and usually result in controlling weld shrinkage and workpiece distortion.

reverse polarity. A nonstandard term for direct current electrode positive.

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tandem arcs. To increase the speed or increase penetration in single pass welds, closely spaced arcs can be employed. Two to five arcs in tandem can be employed with three arcs common. This multiple arc process is primarily employed for high-speed tube or pipe welding. It can also be employed for sheet and plate splicing.

AWS C5.5/C5.5M:2003

(2) The other method involves touch or contact starting where the tungsten electrode is brought in contact with the base metal and then withdrawn to initiate the arc. The electrode is then retracted to the proper arc gap.

4. GTAW Principles 4.1 Process Description. The GTAW process and equipment are illustrated in Figures 7 and 8. GTAW produces fusion (coalescence caused by melting) of base metals from the heat generated by an electric arc. The arc is established between the tip of a nonconsumable tungsten or tungsten alloy electrode and the workpiece (also called base metal). The electrode is held in an arc welding torch, either water-cooled or gas-cooled, through which shielding gas is fed. The shielding gas provides the required arc characteristics by becoming a plasma (ionized), and also shields the electrode, filler metal end, and the molten weld pool from contamination by the atmosphere. Once the arc and weld pool are established, the torch is moved along the joint and the arc progressively melts the faying surfaces (surfaces to be joined). Filler wire, when used, is added to the weld pool to fill the joint. Filler wire is usually added to the leading edge of the weld pool, but may be added to the trailing edge, or both. To avoid weld contamination, the nonconsumable electrode requires special arc initiation techniques. There are two main methods utilized. (1) One method involves arc starters such as highfrequency generators or capacitor discharge starters, which provide a high voltage across the arc gap. This causes the shielding gas to ionize and thus makes it conductive, allowing the arc to initiate.

Note: It is considered good practice, when the touch starting system is used, to initiate the arc in a location that will be consumed in the subsequent weld metal. The initiation point is termed an arc strike or arc burn and, if visible after welding, is cause for rejection by a number of specifications. The GTAW process is adaptable to manual, semiautomatic, mechanized, and automatic applications. It can be used to produce continuous welds, intermittent welds, and spot welds. Unlike other arc welding processes, the GTAW process can be used to produce autogenous (no filler wire) welds because the electrode is nonconsumable. For many applications, the addition of filler wire is desirable or necessary. The process can be used to produce welds on essentially all joint designs and geometries. It can be used to weld a wide range of thicknesses and sizes of plate, sheet, pipe, tube, and other structural shapes. The GTAW process can be successfully used in any welding position. 4.2 Process Advantages. GTAW has become indispensable as a tool for many industries. If properly utilized, the GTAW process will produce the high quality welds required for the aerospace, nuclear, pharmaceutical, semiconductor and other industries. As illustrated in Figure 9, weld beads produced with the GTAW process typically are cleaner than with any other arc welding process because no slag or spatter is present. Visual and nondestructive inspecting of GTAW welds is thus easier. The combination of GTAW for root pass welding with either shielded metal arc welding (SMAW) or gas metal arc welding (GMAW) for the fill passes is particularly advantageous for welding pipe. The GTAW process produces a smooth, uniform root pass weld while the fill passes are made with a higher deposition rate (more economical) process. Table 1 presents an overview of GTAW, GMAW, and SMAW processes for different base materials. The following are some advantages of the GTAW process: (1) capable of producing superior quality welds, generally free of defects (2) is free of the spatter which occurs with most other arc welding processes (3) can be used with or without filler metal as required for the specific application (4) allows excellent control of root pass weld penetration (5) can produce inexpensive autogenous welds at high speeds

Figure 7—Stylized Representation of the Gas Tungsten Arc Welding Process

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Figure 8—Gas-Cooled GTAW Torch and Stylized Representation of Typical Gas Tungsten Arc Welding Equipment

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(6) can use inexpensive power sources (7) allows precise control of the welding variables (8) is very good for joining thin base metals because of the excellent control of heat input (9) can be used to weld almost all metals, including dissimilar metal joints (10) is especially useful for joining aluminum and magnesium, which form refractory oxides (11) is probably the most used process for reactive metals like titanium (12) allows the heat source and filler metal additions to be controlled independently 4.3 Process Limitations. The GTAW process requires continuous and efficient weld metal shielding. Backup shielding or enclosed shielding is also required in many applications. This basic requirement tends to limit the process to indoor types of applications. However, with proper shielding techniques, field (outdoor) welding is also readily accomplished.

Figure 9—Clean Weld Beads Typical of Properly Shielded GTAW Welds

Table 1 Welding Process Comparison Based on Quality and Economics Welding Processes(1) and Ratings(2) (All Positions) Applications Carbon steel plate >3/16 in. [5 mm] Carbon steel sheet ≤3/16 in. [5 mm] Carbon steel structural Carbon steel pipe ≤3 in. [75 mm] IPS Carbon steel pipe >3 in. [75 mm] IPS Stainless steel plate >3/16 in. [5 mm] Stainless steel sheet ≤3/16 in. [5 mm] Stainless steel pipe ≤3 in. [75 mm] IPS Stainless steel pipe >3 in. [75 mm] IPS Aluminum plate >3/16 in. [5 mm] Aluminum sheet ≤3/16 in. [5 mm] Aluminum structural Aluminum pipe ≤3 in. [75 mm] IPS Aluminum pipe >3 in. [75 mm] IPS Nickel and nickel alloy sheet Nickel and nickel alloy tubing Nickel and nickel alloy pipe ≤3 in. [75 mm] IPS Nickel and nickel alloy pipe >3 in. [75 mm] IPS Reactive metals—titanium—sheet, tubing, pipe Refractory metals—Ta and Cb—sheet, tubing

GTAW

GMAW

SMAW

G E F E G G E E G G E E E E E E E E E E

E E F F G E G F G E G G NR F F NR F F NR NR

E G E F G G F F F NR NR NR NR NR F NR NR NR NR NR

Notes: (1) GTAW = Gas Tungsten Arc Welding (TIG). GMAW = Gas Metal Arc Welding (MIG). SMAW = Shielded Metal Arc Welding (Stick). (2) E = Excellent, G = Good, F = Fair, NR = Not Recommended on basis of cost, usability, or quality.

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change weld pool fluid flow. If present in low levels (less than 30 parts per million [ppm]) in these materials, increasing arc current can produce a dramatic increase in weld width without much increase in penetration. If sulfur content is greater than 50 ppm, an increase in arc current can produce a dramatic increase in penetration.13, 14

4.3.1 Limitations. The following are some limitations of the GTAW process: (1) Deposition rates are lower than the rates possible with the consumable electrode arc welding processes unless used with hot-wire additions. Therefore, it can be less economical than the consumable electrode arc welding processes for thicker sections (greater than 3/8 in. [10 mm]). (2) For manual welding, there is a need for slightly more dexterity and welder coordination than with gas metal arc welding or shielded metal arc welding. (3) Shielding the weld zone properly in drafty environments is difficult.

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4.4.1.1 Current Polarity. Three basic categories of welding current are available: direct current (DC), alternating current (AC), and programmed current. Programmed current is a category combining some of the features of AC and DC. Variable polarity (VP) can be considered as a subset of programmed current and is replacing AC and balanced AC in many applications. There are two polarities or directions in which the current can flow in the welding circuit; direct current electrode negative [DCEN, so-called straight polarity, see Figure 10(A)] and direct current electrode positive [DCEP, so-called reverse polarity, see Figure 10(B)]. When using direct current, the tungsten electrode may be connected to either the negative or positive terminal of the power source. When the electrode is negative (cathode), the electrons flow from the electrode to the work and the ions move from the work to the electrode. When the electrode is positive (anode), the electrons flow from the work to electrode and the ions move from the electrode to the work. The choice of whether to use DC, AC, VP, or programmed current depends largely on the material to be welded.

4.3.2 Potential Problems. Potential problems with the process include: (1) Tungsten inclusions can occur if the electrode is allowed to contact the weld pool or if the electrode is overheated. (2) Contamination of the weld metal can occur if proper shielding of the filler metal by the gas stream is not maintained or if the torch shielding gas does not properly protect the weld pool. (3) There is low tolerance for contaminants on filler or base metals. (4) Possible contamination or porosity can be caused by coolant leakage from water-cooled torches, as with other processes. (5) Arc blow or arc deflection can be a concern, as with other arc welding processes.

4.4.1.2 Direct Current with Respect to Polarity. The two types of direct current GTAW to be considered are direct current electrode negative (DCEN), and direct current electrode positive (DCEP). Each type has its own characteristics and areas of application. Direct current electrode negative (DCEN) when combined with a thermionic electrode (e.g., tungsten) generates approximately 70% of the heat at the anode (work) and approximately 30% of the heat at the cathode (electrode). Since DCEN produces the greatest amount of heat at the workpiece, this offers the advantage of deep penetration and fast welding speeds. DCEN is the most common configuration used in GTAW, and is used with argon, helium, or a mixture of gases to weld most metals. Helium and helium mixes are frequently the gases of choice for mechanized welding because of helium’s higher thermal conductivity. Additions of helium or hydrogen to argon are also frequently used for mechanized welding because these produce a “hotter” arc.

4.4 Process Variables. The primary variables in GTAW are welding current, arc voltage (arc length), travel speed, electrode (condition, shape, and alloy), and shielding gas. Additional input parameters include wire feed rate, electrode orientation, travel angle, arc deflections (magnetic and mechanical arc oscillations), and pulsing parameters. The main issue that these variables affect is the energy transferred from the power source through the arc and into the work, and the rate and control of that energy transfer. Variation in chemical composition of the base metal and filler metals as well as condition of the joint design, joint preparation (including fixturing and tooling), and welding position must also be considered. 4.4.1 Arc Current. Weld penetration is directly proportional to arc current, i.e., as the current is increased, the weld penetration increases. Arc current also effects the voltage as a function of the power source characteristic and from the forces (pressure, Lorentz, buoyancy) acting on the weld pool to change the pool’s position under the arc. For this reason, it is necessary to change the voltage setting when the current is adjusted to keep a fixed arc length in automatic welding. For some materials, such as stainless steels and nickel alloys, certain trace elements (such as sulfur and oxygen) alter the surface tension and

13. Burgardt, P. and Campbell, R., 1992, Chemistry Effects on Stainless Steel Weld Penetration, Ferrous Alloy Weldments, Trans Tech Publications, Switzerland. 14. Burgardt, P., and Heiple, C., Interaction Between Impurities and Welding Variables in Determining GTA Weld Shape, Welding Journal, Vol. 65, No. 6, June 1986, pp. 150-s–155-s.

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CURRENT TYPE

DCEN (A)

DCEP (B)

AC Balanced/Variable Polarity (Programmed) (C)

ELECTRODE POLARITY

Negative

Positive

Negative and Positive

No

Yes

Yes, Once Every Half Cycle

70% at Work 30% at Electrode

30% at Work 70% at Electrode

50% at Work, 50% at Electrode (Varies with Program Setting)

Deep, Narrow

Shallow, Wide

Medium

Excellent, e.g., 1/8 in. [3 mm]: 400 A

Poor, e.g., 1/4 in. [6 mm]: 120 A

Good, e.g., 1/8 in. [3 mm]: 225 A

OXIDE CLEANING ACTION HEAT BALANCE IN THE ARC (APPROXIMATE) PENETRATION ELECTRODE CAPACITY

Figure 10—Characteristics of Current Types Used for Gas Tungsten Arc Welding

resistance heating and increase thermal conduction into the electrode collet, a larger diameter electrode is required for a given welding current when using DCEP (reverse polarity) to reduce electrode tip melting. DCEP is sometimes used for thin cross sections to reduce heat input.

With direct current electrode positive (DCEP), a cathodic cleaning action is created at the surface of the workpiece. This cathodic cleaning at the work surface can be beneficial in removing oxides present on the surface of some metals. In practice, this method is not used as frequently as DCEN, AC, or Variable Polarity (VP) because of the overheating of the electrode that occurs, but it does have a particular advantage of surface cleaning on metals whose oxides cause problems to the welding operation. For example, this cleaning action is beneficial for the welding of aluminum and magnesium. This same action occurs in the reverse polarity half cycle of AC and VP welding. In practice, alternating current (AC) or variable polarity (VP) schemes are usually used when cathodic cleaning action is needed. Unlike DCEN, in which the electrode tip is cooled by the evaporation of electrons, when the electrode is used as the positive pole (i.e., DCEP), its tip is heated by the bombardment of electrons as well as by its resistance to their passage through the electrode. Therefore, to reduce

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4.4.1.3 Alternating Current with Respect to Polarity. The condition existing when the welding current polarity is periodically alternated from electrode positive to electrode negative is called alternating current [AC, see Figure 10(C)]. AC combines the work cleaning action of electrode positive (reverse polarity) with the deep penetration characteristic of electrode negative (straight polarity). Conventional AC welding power sources produce a sinusoidal open circuit voltage output. When changing from positive to negative flow, the current must pass through zero, which extinguishes the arc. The arc must then be reignited or it will remain extinguished.

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Figure 11—Relationship Between Arc Pressure and Pulse Frequency

In pulsed DC welding, the peak current level is typically set at 2 to 10 times the background current level. This condition provides the benefits of the driving, forceful arc characteristics of high current with the low heat input of low current. The pulse current is used to obtain the good fusion and penetration while the background current maintains the arc and allows the weld area to cool and/or solidify. Low-frequency DC pulsing of the weld current can be thought of as a moving series of overlapping spot welds.

Some means of maintaining the arc during the voltage reversal is required with conventional sinusoidal welding power sources. This has been done by using high open circuit voltage power sources; by discharging capacitors at the appropriate time in the cycle; by using highvoltage high-frequency generators in parallel with the arc; and by using power sources with a square wave output. 4.4.1.4 Pulsed DC Welding. Pulsed GTAW is characterized by a repetitive switching between a peak (high) current and a background (low) current. Alternate melting and solidification are obtained by switching between high and low currents. The high (peak) current (5 A–200 A) is normally higher than continuous DC welding to ensure rapid full penetration, after which switching to a low (background) current (1 A–15 A) allows the molten pool to solidify but maintains a pilot arc for application of the next pulse.15 There are three broad categories of pulsing: low frequency (0.5 Hz–20 Hz), sometimes called “thermal pulsing”; intermediate frequency (>20 Hz–500 Hz); and, high frequency (>500 Hz up to 16 kHz or more).

There are several advantages of pulsed current. The effect of high-frequency switching is to produce a “stiff” welding arc. Arc pressure is a measure of arc stiffness. As shown in Figure 11,16 as the switching frequency nears 10 kHz, arc pressure increases to nearly four times that of a steady DC arc. As arc pressure increases, there is a reduced effect by magnetic fields (such as arc blow), shielding gas movement (wind), etc. For a given average current level, greater penetration can be obtained by pulsing the current compared with continuous current. This is useful on metals sensitive to 16. Refer to AWS Welding Handbook, 8th Edition, Volume 2: Welding Processes, Chapter 3, “Gas Tungsten Arc Welding,” Figure 3.12, and p. 86 for more information.

15. Pulsed Arc Welding. J. A. Street, Abington Publishing, p. 11, (1990).

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Figure 12—Programmed (Modulated) Current without Polarity Reversal

heat input and for distortion control. Also, because there is insufficient time for significant heat flow during the current pulse, metals of vastly dissimilar thickness respond nearly equally and thus nearly equal penetration can be achieved. For a similar reason, very thin metals can be joined with pulsed DC. In addition, one set of welding parameters can be used on a joint in all positions, such as a circumferential weld in a horizontal pipe. Pulsed DC is also useful for bridging gaps in open root joints and for welding with consumable inserts. A disadvantage of pulsed DC current is that the welding power

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source and controller are more sophisticated and relatively more expensive. 4.4.1.5 Programmed Current. Welding power sources have also been developed that are capable of providing various types of modulated current with or without actual polarity reversal. See Figure 1217 for an example. These current pulsation techniques have been 17. References: AWS Welding Handbook, 8th Edition, Volume 2: Figure 3.10, p. 85; Muncaster, Figure 4.2, p. 37.

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Figure 13—Characteristics of Variable Polarity (Programmed) Weld Current

directly proportional to the arc length, while the pool depth is inversely proportional to the arc length. Therefore, in most applications the desired arc length is as short as possible.

developed to increase weld penetration, to reduce heat input, and to control or improve weld root and bead contour, grain size, and out-of-position welding capabilities. Two such programmed current techniques are square wave AC pulsation and pulsating DC. Variable polarity can be thought of as a subset of programmed current.

4.4.3 Travel Speed. Travel speed affects both the width and penetration of a gas tungsten arc weld. However, its effect on width is more pronounced than on penetration. Travel speed is important because of its effects on cost and weld quality. High travel speeds minimize distortions caused by thermal expansions and contractions during welding. On the other hand, alloys prone to cold cracking are usually not welded at high speeds, since the associated steep thermal gradients and rapid cooling rates would contribute to crack formation. Lower welding speeds are applied to circumvent this cracking problem (often used in combination with preheating the base metal to further reduce the possibility of cold cracks and decrease the cooling rates).

4.4.1.6 Variable Polarity. The term variable polarity describes an alternating welding current that is asymmetrical about the zero current level. This is shown in Figure 13. The symmetry of the current above and below zero is controlled by the current balance adjustment on the power source. The variable polarity power source was developed to meet the criteria established for effective plasma “keyhole” welding of aluminum, especially in heavy sections. While primarily used for plasma arc welding (PAW) it also is used for certain GTAW applications. This direct current (DC) machine produces cyclic changes of DCEN and DCEP polarities with independent control of the current amplitude and the duration.

4.4.4 Electrodes. The shape of the tungsten electrode tip is an important process variable in GTAW. The tungsten electrodes may be used with a variety of tip configurations and preparation methods. Electrode tip configuration is a welding variable that should be studied during the welding procedure development. It is important that consistent electrode geometry be used once a welding procedure has been established. Changes in electrode geometry can significantly influence the weld bead shape and size as shown in Figure 14.18

4.4.2 Arc Voltage. The voltage measured between the tungsten electrode and the work is commonly referred to as the arc voltage. The arc voltage is changed by the effects of the other variables, and is used in describing welding procedures only because it is easy to measure. Since the other variables such as the shielding gas, electrode, and current have been predetermined, the arc voltage becomes a way to control the arc length. The arc length is a critical variable that is difficult to monitor without appropriate equipment. Arc length is important with this process because it affects the width and depth of the weld pool. The arc voltage is a function of gas composition and arc gap length. The weld width is

18. Reference: Key, J., “Anode/Cathode Geometry and Shielding Gas Interrelationships in GTAW,” Welding Journal, Vol. 59, No. 12, Dec. 1980, Figure 1, p. 265-s.

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Figure 14—Arc Shape and Fusion Zone Profiles as a Function of Electrode Tip Geometry in Pure Argon Shielding Gas

In mechanized or automated welding the electrode (torch) angle can be an important parameter to counter the effects of arc drag or to force changes to the shape of the pool (and thus change the resultant solidification) by redirecting the arc force either into or away from the direction of travel.

4.4.5 Shielding Gas. As mentioned previously, the shielding gas has several influences on the GTAW process. Attention should be paid to the gas quality (purity), to its voltage-current characteristic, and to its thermal conductivity. Shielding gases will be covered in detail in Section 7. 4.5 Related Variables. A number of variables can have a pronounced effect upon GTAW quality and speed. These include filler metal (type, size, feed mechanism), joint design, material composition, and tooling. In addition, electrode orientation and arc deflections must be considered.

4.5.2 Arc Motion and Deflections. Several variations of arc deflection have been applied in attempts to modify the resultant fusion zone, including grain refinement. These techniques include weld pool stirring, arc oscillations and arc modulations/pulsations. One of the more popular techniques is magnetic arc deflections or oscillations. The application of one or more magnetic poles is used to stabilize the arc column and minimize the effects of local magnetic disturbances that result in arc blow. Fixed and/or variable electromagnetic poles have been applied in a similar manner to stabilize or to oscillate the arc column. Stable arc deflections can be used to counter the effects of arc drag during higher speed welding.19

4.5.1 Electrode to Work Angle and Position. The forces exerted on the weld pool vary with the angle of the torch (electrode) with respect to the vertical and the direction of travel. In manual welding the welder may use this to: (1) Properly distribute weld deposit relative to the joint being welded, e.g., fillet or groove weld. (2) Affect multipass or stringer beads that may overlap one another. (3) Control the amount of weld penetration or drop through (root reinforcement) and/or weld bead shape.

19. Hicken, G. K. and Jackson, C. E., 1966, “The Effects of Applied Magnetic Fields on Welding Arcs,” Welding Journal, Vol. 45, No. 11, Nov. 1966, pp. 515-s–524-s.

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17

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source are a compromise of cost, weight, size, type of input or primary power, performance, and the type of current to be used in the welding process.

Another variation of arc deflection is the weaving or mechanical motion of the torch (electrode) side to side, to be sure that a full fusion weld is achieved.

Selecting the correct power source, of course, depends upon process requirements. The first step is to determine the electrical requirements of the welding process with which it will be used. Other factors to consider include such things as future requirements, maintenance, economic considerations, portability, environment, available skills, safety, manufacturer’s support, code compliance, and standardization. Most electronically controlled power sources offer rapid dynamic response; i.e., can change from one level to another. As a result, these power sources can be used to provide pulsed welding current. Series linear regulator and switched secondary designs provide DC welding current from rectified single or three-phase input power. Silicon controlled rectifier (SCR) designs can provide AC and DC current from single-phase power and DC current from three-phase power. Depending on the design, inverter power sources can provide AC and DC output from single or three-phase input power. Inverter power sources are the most versatile, with many offering multi-process capabilities and variable welding current waveform output. Inverters are also lighter and more compact than standard 60 Hz transformer-rectifier power sources of equivalent current rating.

5. Equipment and Supplies

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5.1 Welding Power Sources (Used for GTAW)—Introduction. Constant-current type power sources are normally used for GTAW. Power required for both AC (alternating current) and DC (direct current) GTAW can be supplied by inverter, transistor, or transformerrectifier power sources or from rotating AC or DC generators. A description of the types of currents is found in Section 4. Advances in semiconductor electronics have made transformer-rectifier, and more recently, transistor and inverter type power sources popular for both shop and field GTAW. Engine driven power sources continue to be widely used in the field. In constant current power sources the weld current remains nearly constant with variations in output voltage, as shown in Figure 15. It is the goal of the power source manufacturer to provide power sources that produce the desired type of current (e.g., DC, pulsed DC, AC, etc.) and maintain the current at the desired level regardless of external influences such as power line voltage fluctuations. The components used for each type of power

Figure 15—Typical Volt-Amp Characteristic Curves for GTAW Power Sources

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It is important to select a GTAW power source based on the type of welding current required for a particular application. The types of welding current include AC sine wave, AC square-wave, DC, and pulsed DC. The following sections have more information on the types of power sources. Many power sources are available with a variety of additional controls and functions such as water and shielding gas control, wire feeder and travel mechanism sequencing, current up-slope and down-slope, and multiple-current sequences.20, 21

currents are the major determining factors in the voltages and currents that each of these devices generate.22 5.1.3 Transistorized Power Sources. The transistorized power source derives its name from the fact that the output of the welding power source utilizes transistors. The transistorized source takes the DC created by a transformer and diodes and uses the transistors to control the power source output current. The use of transistors allows for very fast and precise control of the output current, which is especially advantageous in pulsed GTAW applications. The two common types of transistorized designs are referred to as series linear regulators and secondary switchers. The transistorized power source has a very stable output and is fast to respond to control signals. Advanced design methods make it possible to obtain a modulated signal whereby the output current is proportional to the input control voltage. The fast response of the power source when combined with the modulation capability makes it possible to generate sine wave, thermal pulsed DC, and high-frequency pulsed DC all from the same power source by simply changing the input control voltage or modulation signal.

5.1.1 Transformer Rectifier Power Sources. One of the most common types of GTAW power sources utilizes transformers and some form of rectification. The rectification can be provided by simple inexpensive diodes or SCRs. The purpose of the transformer in this power source design is to reduce the incoming line voltage and increase the current to a level that is more closely needed for the welding process. For example, a welding power source using 480 V supply voltage needs to be reduced to a voltage in the range of 30 V–80 V, and the current is increased proportionally to the reduction in voltage. Single rectifiers, also known as diodes, can be connected to create a single polarity of voltage, i.e., direct current. The SCRs are simply controllable diodes that improve the ability of the electronics to control current. Prior to the use of SCRs this type of control had been accomplished using mechanically driven magnetic control techniques and saturable reactor magnetic controls. The magnetic type of control was relatively slow and had many mechanically related deficiencies.

5.1.4 Inverter/Converter Power Sources. The inverter power source was originally developed for the military to provide significant reduction in weight and size of the power source relative to the other types of sources described thus far in this text. The welding inverter power source provides a weight reduction factor of 4 to 5. The inverter design is also electrically more efficient and the output more stable than SCR power source design technologies. This is primarily accomplished by the reduction of the transformer size. This reduction in transformer size is possible by increasing the frequency used in the transformer section. The transformer section of the inverter power source is utilized in the many thousands of Hertz as opposed to 60 Hz in other power source designs. The inverter power source is a type of GTAW power source that takes AC input power, typically at 60/50 Hz, and changes it to DC, which is converted to higher frequency AC and then rectifies the higher frequency AC to obtain the desired output current. Several variations of the inverter design are available. An example of the design schematic is shown in Figure 16. This higher frequency is in the range of 1 kHz–125 kHz. The size of a transformer is indirectly proportional to the frequency of the voltage. That is, as the frequency increases the size

5.1.2 Generator Power Sources. Generators are most commonly used for field applications or in industrial settings where sufficient power is not available from the existing power system (mains). The generator-type power sources convert mechanical energy into electrical power suitable for arc welding. The mechanical power can be obtained from an internal combustion engine, an electric motor, or from a power take-off from other equipment. For welding, two basic types of rotating power sources are used: the generator and the alternator. Both have a rotating member normally called a rotor or an armature, and a stationary member, called a stator. A system of excitation is needed for both types. Alternators are used to produce alternating currents while generators are used to produce direct currents. The speed of rotation, the winding configuration and external excitation 20. Refer to AWS Welding Handbook, 8th Edition, Volume 2: Welding Processes, Chapter 1, “Arc Welding Power Sources,” for more detailed information. 21. Grist, F. J., Farrell, W. and Lawrence, G. S. 1993. Power Sources. ASM Handbook, 9th Edition, Volume 6, Welding, Brazing and Soldering, pp. 36–44.

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22. For additional information, see AWS Welding Handbook, 8th Edition, Volume 2: Welding Processes. Also see ASM Handbook, 9th Edition, Volume 6: Welding, Brazing, and Soldering.

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measured and controlled, but rather the major process variables that affect heat input and torch position are measured. Control of the process variables improves process consistency, but does not ensure consistent welds under all conditions. Although the actual weld characteristics cannot typically be controlled, it is possible to precisely control a majority of the process variables that drastically affect weld quality and consistency (repeatability). These variables include arc voltage, weld current, travel speed, wire feed speed, shielding gas flow, and multiple axes of position. There are two categories of controls used in most GTAW processes. The first is the open loop control system. In the open loop control system the process variable is generated and no corrections are made to this setting to maintain the desired set point, even if the output value is not correct. A closed loop control assures the set point value is maintained regardless of the outside influences. A sensor measures the process variable and sends this signal back to the comparison circuitry, i.e., feedback. The circuitry compares the actual variable value to the desired set point (reference) value. If the actual value is lower than the reference value then the signal to correct is increased. This process continues until the reference value is achieved. The ability to maintain a process variable at the desired or set point value, however, is greatly affected by whether or not the system is governed by open or closed loop control methods. Besides the open loop and closed loop control methods, either of these types of control can be based on analog or digital principles. The analog controller typically

and weight of a transformer decreases. Since the voltage being transformed is very high in frequency, the size of a transformer is significantly less, as noted above, when compared to a transformer for the same application used at 60 Hz. Thus significant size and weight reduction is possible. The inverter power source also has a very fast response time compared to many other types of power sources, e.g., transformer-rectifiers. The fast response time is useful in pulsed welding applications. 5.2 Controllers. The term controller is used throughout industry and has many meanings. In the welding field there are two common types of controllers. The first is the controller used to control a single process variable. The second type of controller is the weld sequence controller that orchestrates all of the individual process variables so that each weld variable is set to the proper level at the proper time during the weld cycle. The most common weld sequence controller is time based. That is, the control is programmed in time relative to the arc start or the pre-flow of electrode shielding gas. It is also possible to have a position based weld sequence controller. In this type of controller the sequencing of the process variables is determined or programmed using the relative position or location of the torch with respect to the part, e.g., a change in speed. 5.2.1 General Control Concepts. Each process variable in the GTAW process is set to an appropriate level to produce desired weld characteristics (e.g., the desired weld width, penetration). The variables that are commonly measured and/or controlled are called process variables. The desired weld characteristics are not being

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Figure 16—Typical Inverter Power Source Components

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uses “circuitry” to perform the necessary set-point and output comparisons. Such functions are, in many cases, easily performed using transistor or integrated circuit designs. In other situations it is more desirable to implement the control functions using digital circuits, microprocessors, or some other digital computation system such as a personal computer. In all of the following process variable control discussions it must be remembered that the open loop and closed loop implementations are possible, and, the control may be implemented using digital or analog techniques or a combination of digital and analog methods. The following provides a brief discussion of typical individual process variable controls.

The simplest and most common travel speed implementations are the simple linear torch movement or the constant rotation speed of a part. These are examples of a single axis of motion. On the other extreme of complexity is the robotic movement of a torch that may involve many axes of rotation and linear motion to weld a complex part. 5.2.1.4 Wire Feed Speed. For mechanized and automatic gas tungsten arc welding, wire feeders (see Figure 17) are often used to deposit filler metal into a weld joint. The wire must be deposited at a constant volumetric rate so as to not overfill or underfill the weld joint. The typical wire being fed into the joint by the wire feeder comes from a supply spool of wire. In most cases, the wire that is fed to the weld is stiff and the amount of force needed to uncoil or feed the wire is not constant during the feeding process. Friction forces inside the conduit, that guide the wire to the arc, vary considerably as the conduit is manipulated during the weld process. Since the force required to feed the wire is not constant, there must be provisions made in the design of wire feeders to assure constant wire feed rates. In some of the less expensive designs the speed of the electric motor used to feed the wire is reduced or the motor speed is geared such as to achieve mechanical advantage to reduce the effect of the varying force on the speed of the motor. Feedback control methods can also be used to assure the constant wire feed rate. The latter provides the best assurance of constant wire feed rate. As with other controllers, the wire feed rate may be set using front panel control knobs, signals from computers, or weld sequence controllers. The motors used most often for this application are DC motors, but digital stepper motors or AC motors can also be used. For many welding processes the wire feed rate is continuous throughout the welding sequence. That is, the wire feed rate is maintained at one level throughout the portion of the welding sequence that requires weld wire. In thermal pulsed GTAW wire may be fed only during the peak pulse current time. This requires synchronization of the wire feed command signal to the power source current control signal. This places additional demands on the controllers and wire feed motor systems to accomplish this task. In addition to the speed and synchronization of the wire feed there are two additional variations for feeding wire in the GTAW process. These variations are hot and cold wire feeders as described in more detail in 5.6.

5.2.1.1 Current. The desired current from the power source is set either using front panel adjustments, hand or foot controller, or through an external signal that may come from a computerized weld controller or a dedicated weld sequence controller. If closed loop control is implemented, a current sensor is used to monitor the actual current that is produced by the power supply.

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5.2.1.2 General Motion Control. Motion control is a general term that refers to control of linear or rotational speed or position, or a combination of these. In general, for motion to occur there must be some drive source, such as a motor. If there is feedback involved then there must be some type of sensor to complete the feedback loop and appropriate electronics for the feedback. There are three major types of motors used for motion in GTAW. The first and most common is the DC motor. The second type sometimes used is the stepper motor. The stepper motor is a digitally controlled motor and can be used with or without feedback. The third, and less common is the AC motor, which is also used for larger applications. 5.2.1.3 Travel Speed. Travel speed is simply an implementation of a general motion control, but instead of using position feedback sensors, a speed or velocity sensor is used with appropriate modification to the controls to assure that speed is controlled instead of position. Travel speed in GTAW applications normally refers to the relative motion between the part and the torch.23 In many GTAW applications, the part is moved with the torch remaining in a fixed position. In other cases it is easier to move the torch with the part remaining fixed. 23. LaCoursiere, E. J., A. H. Farnham, D. G. Howden, L. Zhang. 1993. “Requirements for High-Speed (GTA) Welding, Controller Response Time and Speed Resolution,” in the “International Conference Proceedings on Modeling and Control of Joining Processes,” edited by T. Zacharia, Oak Ridge National Laboratory. American Welding Society, December 8– 10, 1993, pp. 500–509.

5.2.1.5 Gas. The torch gas flow rate is most often manually preset using variable area flowmeters, also commonly known as rotameters. This is the floating ball in a tube-type indicator commonly seen on welding equipment. Gas flow is then started and stopped using

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Figure 17—Typical Wire Feeder

AVC a hunting or continual adjustment of the arc length will occur. Some manufacturers provide an additional option for AVC to prevent this unwanted variation, unless some electronic filtration is used. The option allows the controller to measure the arc voltage only during the high or peak current times. It must be remembered that it is the relationship between arc length and arc voltage that makes the use of AVC possible. Once a weld is developed, it is assumed that this relationship will remain constant over time for production runs. However, many variables can change the arc voltage vs. arc length relationship. For example, use of helium provides greater sensitivity than argon gas.

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electrically controlled solenoid valves. The solenoid valves are most often activated at the appropriate time by the weld sequence controller. The flow rate is adjusted manually on most variable area flowmeters. Flowmeters that can be preset are available. In cases where it is desired to actually measure or control gas flow, controllers are available for this purpose. The most common of these is the mass flow meter and controller. For reasons of cost, it is not common to use mass flow meters and controllers except for cases where flow rates must be remotely controlled or for the most accurate control of flow. 5.2.1.6 Arc Voltage and Arc Length Controllers. Arc voltage controllers (AVC) can be used in mechanized and automated GTAW welding to maintain arc length. In this case, the arc itself is the sensor, since its voltage is a function of its length. A comparison is made between the actual arc voltage and the desired arc voltage. The difference in voltage is used by the AVC to determine which direction (i.e., closer to or further away from) and at what speed the welding electrode will be moved.

5.3 Pulse Controllers. There are numerous types of pulse controllers available. These may be added to appropriate power sources as an option or integrated as part of a power source. In either of these cases, the pulse control options may be set and controlled via computer or by manual selector settings. These controllers provide the necessary additional controls required to set pulse time, pulse current, background current and background time variables.

In pulsed welding the current varies between the peak and background values. This causes corresponding increases and decreases in arc voltage for a given arc length. If this fluctuating arc voltage is presented to the

5.4 Weld Sequence Controllers. The weld sequence controller provides the capability to synchronize the operation of many individual process variable controls so that the proper weld sequence may occur. For instance a

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5.5 Arc Welding Torches. GTAW torches hold the tungsten electrode, which conducts welding current to the arc, and provide a means for conveying shielding gas to the weld zone. Torches are rated in accordance with the maximum welding current and duty cycle that can be used without overheating. Most torches are designed to accommodate a range of electrode sizes and different types and sizes of gas cups/nozzles. The heat generated in the torch during welding is removed by shielding gas cooling and/or water-cooling. Reflected heat can reduce the duty cycle rating.

weld sequence controller may execute the following steps: (1) pre-flow torch gas for 5 seconds, (2) start the welding arc, (3) after the arc is successfully started slope the current from 10 amps to 100 amps in ten seconds, etc. The weld sequence controller can have many forms. The simplest form is a multi-channel timer that simply starts the various individual process variable controllers at the required time. For example, the pre-flow of torch gas may be started by opening a solenoid valve, the timer then signals the arc starter, and when the arc is initiated it simply maintains a current for a predetermined length of time. In the simplest form, relative torch motion may be provided manually by the operator. On the other extreme of capability is the sophisticated computerized weld controller. This may be a dedicated microprocessor or computer system. All process variable control values and functions are handled by this one weld controller. Figure 18 shows a nonpulsed weld sequence and Figure 19 shows a pulsed weld sequenced schematic.

5.5.1 Types of Torches. Several types of welding torches are available and will be briefly described. 5.5.1.1 Gas-Cooled Torches. Gas-cooled torches (a nonstandard term is air-cooled torches) provide cooling by the flow of a relatively cool shielding gas through the torch. Gas-cooled torches are limited to a maximum current of about 200 A at 60% duty cycle. Gas-cooled torches may be used at high current settings for low-duty cycles, short weld lengths with frequent starts and stops,

Figure 18—Typical GTAW Sequence for Non-Pulsed DC Welding

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Figure 19— Typical GTAW Sequence for Pulsed DC Current and Pulsed Wire Feed

and tacking of heavy materials. A gas-cooled torch is illustrated in Figure 8.

Water-cooled torches typically use a water-propylene glycol mixture in a closed system with a recirculator (see 5.5.5 on Water Coolers). This is especially needed for systems employing high-frequency arc stabilization and high-frequency arc starts. This reduces mineral build-up in the torch from tap water, as well as conserving water. However, tap water is sometimes used when it does not interfere with the high-frequency system. A comparison of typical current ratings for gas-cooled and water-cooled torches is shown in Table 2.25

5.5.1.2 Water-Cooled Torches. Water-cooled torches are cooled by the continuous flow of water through passageways in the holder. As illustrated in Figure 20,24 cooling water enters the torch through the inlet hose, circulates through the torch, and exits through an outlet hose. The power cable from the power source to the torch is typically enclosed within the cooling water outlet hose. Water-cooled torches are designed for use at higher welding currents and duty cycles than gas-cooled torches. Typical welding currents of 300 A to 500 A can be used, although some torches have been built to handle welding currents up to 1500 A. Most machine or automatic welding applications use water-cooled torches.

5.5.1.3 Manual Torches. Torches for manual applications have a head angle (angle between the electrode and handle) of 90°, 100°, or 120°. Torches are also available with adjustable angle heads and straight-line (pencil type) heads. Manual torches often have auxiliary switches, valves, and controls built into their handles for controlling current and shielding gas flow.

24. AWS Welding Handbook, 8th Edition, Volume 2: Welding Processes, Chapter 3: Gas Tungsten Arc Welding. Figure 3.4, p. 78.

25. AWS Welding Handbook, 8th Edition, Volume 2: Welding Processes, Chapter 3: Gas Tungsten Arc Welding. Table 3.1, p. 77.

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Figure 20—Typical Water-Cooled GTAW Torch (Cross-Sectional View)

Table 2 Comparison of Typical Current Ratings for Gas-Cooled and Water-Cooled GTAW Torches(1) Torch Size Torch Characteristics

Small

Maximum Current (100% duty cycle), A 200 Cooling Method Gas Electrode Diameters accommodated, in. [mm] 0.020 to 3/32 [0.5 to 2.4] Gas Cup Diameters accommodated, in. [mm] 1/4 to 5/8 [6 to 16]

Medium

Large

200–300 Water 0.040 to 5/32 [1.0 to 4] 1/4 to 3/4 [6 to 19]

500 Water 0.040 to 1/4 [1.0 to 6] 3/8 to 3/4 [10 to 19]

Note: (1) AWS Welding Handbook, 8th Edition, Volume 2: Welding Processes, Chapter 3, Gas Tungsten Arc Welding, Table 3.1, p. 77.

The welder prefers the lightest torch possible for any application because of the tiring effects of positioning and manipulating the torch with relationship to the weld pool. The angle of the torch head is usually dependent on the type of welding to be performed. A 120° torch head is preferred when making fillet welds and a 100° torch head is preferred when making butt welds. Figure 2126 shows typical GTAW torches.

5.5.1.4 Mechanized and Automated Torches. The basic mechanized or automatic torch has a straightline (pencil type) head (see Figure 21). The torch is either rotated mechanically around a joint to be welded, mounted on a tracking device for longitudinal welds or on a robot for all configuration welds. 5.5.1.5 Orbital Weld Heads. Integral electrode holders are often used in orbital weld heads. With closedhead types, the electrode is held in a planetary gear housing that rotates around the stationary workpiece (see

26. Photo courtesy of CK Worldwide.

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Figure 21—Examples of GTAW Torches

ramic, fused quartz, lava, or other materials. Ceramic cups are the least expensive and most popular, but are brittle and must be replaced often. Fused quartz cups are transparent and allow better vision of the arc and electrode. However, contamination from metal vapors from the weld can cause them to become opaque, and they are also brittle. Water-cooled metal nozzles have longer life and are used mostly for machine and automatic welding applications where welding currents exceed 250 A.

9.4.1.2 for further details). Open head types feature the torch mounted on a gear that rotates around the workpiece (see Figure 2227). Shielding gases are contained within the weld head for closed head types and are provided through the torch in open head types. 5.5.2 Collets. Electrodes of various diameters are secured in the torch by appropriately sized collets or chucks. Collets are typically made of a copper alloy. The electrode is gripped by the collet when the torch cap is tightened in place. Good contact between the electrode and the inside diameter of the collet is essential for proper current transfer and electrode cooling.

5.5.3.2 Sizes and Shapes of Gas Nozzles (Cups). The gas nozzle (cup) must be large enough to provide shielding gas coverage of the weld pool area and surrounding hot base metal. The gas nozzle diameter must be appropriate for the volume of shield gas needed to provide protection and the stiffness needed to sustain coverage in drafts. A delicate balance exists between the gas nozzle diameter and the flow rate. If the flow rate for a given diameter is excessive, the effectiveness of the shield is destroyed because of turbulence. High flow rates without turbulence require large diameters; these are essential conditions at high currents. Size selection depends on electrode size, type of weld joint, weld area to be effectively shielded, and access to the weld joint.

5.5.3 Gas Nozzles/Cups. Shielding gases are directed to the weld zone by gas nozzles, which fit onto the head of the torch as illustrated in Figure 23. Nozzles are more often called by the nonstandard term gas cup. Also incorporated in the torch body are diffusers or carefully patterned jets or holes that feed the shield gas to the gas nozzle (cup). Their purpose is to assist in producing a laminar flow of the exiting gas shield (see 5.5.3.3 for gas lenses). Gas nozzles are made of various heat-resistant materials in different shapes, diameters, and lengths. These gas nozzles are either threaded to the torch or held by friction fit.

Suggested gas nozzle sizes for various electrode diameters are covered in Section 6. Use of the smallest gas nozzle permits welding in more restricted areas, and offers a better view of the weld. However, use of too small a gas nozzle may cause shielding gas turbulence and jetting, as well as melting of the lip of the gas nozzle.

5.5.3.1 Gas Nozzle/Cup Materials. Shielding gas nozzles are made of ceramic, metal, metal jacketed ce27. Photo courtesy of Arc Machines Incorporated.

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Figure 22—Orbital Weld Head with Wire Feeder

O-RINGS (2) (3/16 in. [5 mm])

O-RING (5/16 in. [8 mm])

HIGH IMPACT CERAMIC NOZZLE

COLLET

COLLET BODY

ELECTRODE

GASKET

SHORT CAP MEDIUM LONG

TORCH BODY INSULATOR

GAS LENS HIGH IMPACT NOZZLE

GAS LENS COLLET BODY

TORCH BODY

Figure 23—Components of a Typical GTAW Torch, Including Gas Nozzle/Cup, Gas Lens, Collet Body, Torch Body, Collet, and Electrode

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Larger gas nozzles provide better shielding gas coverage, especially for welding reactive metals such as titanium. Gas nozzles (cups) are available in a variety of lengths to accommodate various joint geometries and the required clearance between the gas nozzle and the work. Longer straight gas nozzles generally produce stiffer, less turbulent shielding; however, they require longer electrode extensions. The majority of gas nozzles (cups) are cylindrical in shape with either straight or tapered ends. To minimize shielding gas turbulence, nozzles with internal streamlining are available. Nozzles are also available with elongated trailing sections or flared ends which provide better shielding for welding metals such as titanium, which is highly susceptible to contamination at elevated temperatures. 5.5.3.3 Gas Lenses. One device used for assuring a laminar flow of shielding gas is an attachment called a gas lens. Gas lenses contain a porous barrier diffuser and are designed to fit around the electrode or collet. Gas lenses produce a longer, undisturbed flow of shielding gas (see Figure 2428) by streamlining the flow of gas as it exits the torch. They enable operators to weld with the gas nozzle (cup) 1 in. [25 mm] or more from the work, improving the welder’s ability to see the weld pool and allowing them to reach places with limited access such as inside corners.

Figure 24—GTAW Torch Without a Gas Lens (Left) and with a Gas Lens (Right)

5.5.4 Cables, Connectors, and Hoses. Gas-cooled torches usually have a composite power and shielding gas cable that goes to the torch body. Water-cooled torches usually have a water inlet hose, a composite power input and water outlet cable, and a shielding gas inlet hose. A protective sheathing over the cable and hoses helps to reduce abrasive wear. If the extra weight can be tolerated by the welder or in machine applications, the cables and hoses going to the torch can be wrapped in this protective cover. It is usually easiest to buy this as an assembly from the manufacturer. Nylon braid or glass fiber zip covers are also available.

carefully prior to installation or use. See Table 3 for typical welding cable capacities. Table 4 provides a guide for selecting the size of the cable based on the welding current to be used. Neoprene jacket cables have good resistance to wear and usually remain flexible over a wide range of operating temperatures. Polyvinyl Chloride (PVC) and other jacket materials are available but are more susceptible to damage from heat or abrasive wear. Stranded copper cables are probably best for the power return (worklead). In certain applications where lower RF noise is desired or the most flexibility is required (e.g., a robot) then braided cable may have to be used. The braided cables are usually larger than stranded cables for the same current rating. Another consideration is the inductance of the cable with respect to the signal going through the cable. In the use of pulsed arc GTAW the cables should be kept as short as possible in order to minimize the attenuation and rounding off of the pulse wave form that occurs because of the inductance.

5.5.4.1 Cables. Good cables that are large enough to handle the maximum weld current should be used. Some cables usually have the current rating and other related information marked on the outside of the jacket at periodic intervals. Tables 3 and 4 are provided “for reference only” as guides to nominal values for current capacity in a portable welding cable. Actual ratings are established by the product manufacturer and by NEMA.29 Actual cable properties should be checked 28. Photo courtesy of CK Worldwide. 29. National Electrical Manufacturers Association.

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AWG or MCM Size(1)

Approximate Net Weight per 100 ft [30.5 m], lb [kg]

Typical OD Minimum, in. [mm]

Welding Current Capacity @104°F [40°C] Ambient, Amperes(2)

Typical Voltage Drop per 100 ft [30.5 m] @ Maximum Current, Volts

8 6 5 4

10 [4.5]. 13 [5.9]. 17 [7.7]. 20 [9.1].

0.31 [7.9]0 0.37 [9.4]0 0.41 [10.4] 0.43 [10.9]

0 60 0 90 0105 0120

4.00 3.76 3.49 3.17

3 2 1

23 [10]0 29 [13]0 36 [16]0

0.46 [11.7] 0.52 [13.2] 0.57 [14.5]

0180 0240 0300

3.76 3.97 3.95

1/0 2/0 3/0 4/0

43 [20]0 54 [24]0 67 [30]0 83 [38]0

0.63 [16.0] 0.69 [17.5] 0.76 [19.3] 0.83 [21.0]

0360 0450 0540 0640

3.74 3.72 3.54 3.32

250 300 350 400

96 [44]0 115 [52]0 131 [59]0 153 [69]0

0.91 [23.1] 0.96 [24.4] 1.02 [25.9] 1.08 [27.4]

0710 0780 0860 0940

3.12 2.86 2.70 2.59

450 500 600 700

171 [78]0 189 [86]0 226 [103] 260 [118]

1.13 [28.7] 1.18 [29.2] 1.30 [33.0] 1.40 [35.6]

1020 1090 1220 1340

2.50 2.40 2.24 2.10

750 800 900 10000

277 [126] 295 [134] 329 [149] 364 [165]

1.43 [36.3] 1.47 [37.3] 1.53 [38.9] 1.61 [40.9]

1400 1450 1540 1630

2.06 2.00 1.89 1.80

Notes: (1) The sizes of cable recommended by the Machine Group of the NEMA Electric Welding Section for standard hand-welding equipment based on total lengths up to 90 ft [27.4 m], that is, 45 ft [13.7 m] of welding cable and 45 ft [13.7 m] of return cable, are as follows: 100 A welding machine—No. 2 cable 200 A welding machine—No. 2 cable 300 A welding machine—No. 1/0 cable 400 A welding machine—No. 2/0 cable 600 A welding machine—No. 3/0 cable (2) Ampacities are based on a copper temperature of 167°F [75°C] and an ambient temperature of 104°F [40°C] and yield load factors of from approximately 32% for the No. 2 AWG cable to approximately 23% for the No. 3/0 AWG cable and higher for the smaller sizes.

5.5.4.2 Connectors. The termination of the cables and hoses (usually called connectors) can vary widely from one manufacturer to another or from one country to another. This applies to both the torch end and the power source end connectors for the cable/hoses. This detail must not be overlooked at the time of purchasing equipment for connections to present equipment or for future compatibility. There are many adapter fittings available to connect from one style to another.

make the hose. Gas hoses should be chosen to reduce the amount of moisture diffusion through the hose wall into the gas stream. This will help to keep the gas as pure as possible. A variety of “plastic” gas hoses are becoming more readily available; all of these usually perform better than the traditional “rubber” hoses because there is less permeation of ambient moisture to the gas.31, 32, 33 31. Bhadha, P. 1995. “Control of Moisture and Contaminants in Shielding Gases.” Welding Journal, Vol. 73, No. 5, pp. 57–63. 32. Bhadha, P. 1999. “How Welding Hose Material Affects Shielding Gas Quality.” Welding Journal, Vol. 78, No. 7, pp. 35–40. 33. Farish, E. 1994. “Atmospheric Contamination of TIG Welding Hoses—Causes and Cures,” Welding & Metal Fabrication, Vol. 62, No. 7, pp. 300, 301.

5.5.4.3 Hoses. Gas hoses and water hoses/power cables may be made from different materials.30 Consideration should be given as to the type of material used to 30. “Practical TIG (GTA) Welding,” Peter W. Muncaster, 1991, Abington Publishing, ISBN 1855730200.

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Table 3 Typical Welding Cable Capacities

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Table 4 Guide for Selecting the Size of Cable Based on the Welding Current(1),(2) Distance from Welding Machine, ft [m] 50 [15.2]

75 [22.9] 100 [30.5] 125 [38.1] 150 [45.7] 200 [61.0] 225 [68.6] 250 [76.2] 300 [91.4] 350 [106.7]

Welding Current, A

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100 150 200 250 300 350 400 450 500 550 600 750 900

Cable Size 4 3 2 1 1 1/0 2/0 2/0 3/0 4/0 250 MCM 350 MCM 500 MCM

4 2 1 1/0 2/0 3/0 4/0 4/0 — — — — —

2 1 1/0 2/0 3/0 4/0 4/0 — — — — — —

2 1/0 2/0 3/0 4/0 — — — — — — — —

1/0 3/0 4/0 — — — — — — — — — —

1 2/0 3/0 4/0 — — — — — — — — —

2/0 4/0 — — — — — — — — — — —

2/0 4/0 — — — — — — — — — — —

3/0 — — — — — — — — — — —

4/0 — — — — — — — — — — — —

Notes: (1) Neoprene welding cable, 600 volt, Class K (30 AWG), flexible stranding copper. Figures above represent half the length needed for a welding installation based on voltage drop of 4 volts per IPCEA and NEMA specifications. These figures should be doubled to obtain total cable length (i.e., welding lead plus the work lead length). (2) Based on information provided by: (a) Nehring Electrical Works Company, P.O. Box 965, 813 East Locust Street, DeKalb, IL 60115 (Tel. 815-756-2741), and (b) ExCel Wire and Cable Co., 108 Elm Avenue, P.O. Box D, Tiffin, OH 44883 (Tel. 419-448-0434).

5.6 Wire Feeders. Wire feed systems are made from a number of components and vary from simple to complex. The basic system consists of a means of gripping the wire sufficiently to pull the wire from the spool and push it through a conduit to a guide tube to the point of welding. Electronic switches and controls are necessary for the electric drive motor. The wire can be either ambient temperature (cold) or preheated (hot). (Note: The hot end of the wire must be maintained in the shielding gas stream.)

The flexibility of the material at various temperatures also must be considered. (See Section 7 for an in-depth discussion on gas purity.) Composite water hoses and power cables are usually made from materials that have a trade off between the dielectric insulation factors and the resistance to other factors such as corrosion, permeation of moisture through the hose, and flexibility. 5.5.5 Water Coolers and Re-Circulators. The coolant in water-cooled torches is usually maintained in a closed system including a reservoir, pump, and radiator or water chiller to disperse heat from the system. The capacity of these systems ranges from one to fifty gallons. The coolant is typically propylene glycol/water mixtures in approximately 50-50 proportions. This helps to prevent freezing, minimizes corrosion and provides lubrication for the water pump. These systems can also be used for cooling some tooling. Deionized water is preferred over standard tap water because of minerals and contaminants in tap water that can clog the torch. If deionized water is used, the supply should be changed periodically. Some manufacturers recommend every three months.

5.6.1 Cold Wire. The system for feeding cold wire has three components: (1) A wire drive mechanism that either pushes or pulls the wire off the wire spool. (2) A speed control to regulate the speed at which the wire is delivered into the weld pool. (3) A wire guide attachment that ensures proper delivery and placement of the wire into the weld pool. The filler wire is normally fed into the leading edge of the weld during cold wire welding. Cold wire feeders usually include controls for wire start delay and automatic retract prior to extinguishing the arc. Cold wire

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Figure 26—Schematic of GTAW with Hot Wire Feed

holder from which inert gas flows to protect the hot wire from oxidation. The hot wire is generally fed into the weld pool following the arc. This system is schematically illustrated in Figure 26.34 Normally, a mixture of 75% helium and 25% argon is used to shield the tungsten electrode and the molten weld pool. Sometimes the interaction of the electrical current used to heat the wire and the arc can result in unwanted deflection of the arc or “arc blow.” In this situation a low voltage AC current is utilized to correct the “arc blow.”

Figure 25—GTAW Torch with Cold Wire Feed

5.7 Arc and Torch Oscillators. Oscillation can be used in manual, mechanized, and automatic welding. For manual welding, the welder moves the torch side to side or back and forth along the length of weld. In mechanized welding the oscillation is typically produced by moving the entire welding torch by mechanical means or by moving the arc plasma with the aid of an externally applied magnetic field. The control variables in arc oscillation are oscillation speed, displacement direction, end dwell times, amplitude, and frequency.

feeders can be used in semiautomatic, mechanized, and automatic GTAW applications. They are available in 2 in.–12 in. [50 mm–300 mm] spool versions. Wires ranging from 0.015 in.–3/32 in. [0.4 mm–2.4 mm] in diameter are used. Special wire feeders are available to provide continuous, pulsed or intermittent wire feed. Figure 25 shows a GTAW torch with cold wire feed. Aluminum wire may require other special features built into the wire feeder.

Oscillation allows placing the welding heat at precise locations. Figure 27 shows a deep groove weld where magnetic arc deflection is used to place stringer beads with limited torch movement. This is advantageous when welding irregular shaped parts. The number of welding passes and total heat input can be reduced when arc

5.6.2 Hot Wire. The process for hot wire addition is similar to that for cold wire, except that the wire is either resistance or induction heated to a temperature close to its melting point just before it contacts the molten weld pool. When using a preheated (hot) wire in machine and automatic gas tungsten arc welding in the flat position, the wire is fed mechanically to the weld pool through a

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34. AWS Welding Handbook, 8th Edition, Volume 2: Welding Processes, Chapter 3: Gas Tungsten Arc Welding. Figure 3.6, p. 83.

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1/2 in. [13 mm]

(A)

Figure 27—Magnetically Deflected Arc Laying a Stringer Bead in a Deep Groove Weld

1/2 in. [13 mm]

oscillation is used which reduces the cost by as much as 50% as well as reducing the weld shrinkage and upsetting. Figure 28 illustrates the differences in distortion by using magnetic oscillation.35 Arc oscillation is suited to a wide range of materials including stainless steel, carbon manganese steels, and nonferrous alloys. Some high strength low alloy steels should be welded using the stringer bead technique to meet weld toughness requirements, for which arc oscillation may be detrimental. Weld performance testing should be completed prior to using arc oscillation.

(B)

Figure 28—Cross Sections of Welds Made in 1/2 in. [13 mm] Thick Stainless Steel; (A) with Magnetic Arc Oscillation, and (B) without Magnetic Arc Oscillation

5.8 Arc Initiation Equipment. There are several common methods used to initiate the welding arc between the electrode and the work. These methods are necessary since an arc cannot be initiated using a standard power source voltage when the electrode to work distance is at normal welding arc lengths. Arc initiation requires a much shorter gap between the electrode and work, higher voltage, or a means of making the gas conductive. The

following methods are the most commonly used to help initiate an arc at the beginning of the weld cycle. 5.8.1 High-Frequency Starters. High-frequency starting can be used with DC or AC power sources for both manual and automatic applications. High-frequency generators usually consist of a spark-gap oscillator that superimposes a high voltage AC output at radio frequencies in series with the welding circuit. The generator output is transferred from the spark-gap circuit to the electrode leg of the welding circuit by means of an air

35. Hicken, G. K., Stucki, N. D., and Randall, H. W., “Application of Magnetically Controlled Welding Arcs,” Welding Journal, Vol. 55, No. 4, April 1976, pp. 264–267.

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tance to establish the arc. The advantage of this method of arc initiation is its simplicity of operation for manual welding. Depending on the instantaneous short circuit current capability of the welding power source, the current through the welding electrode upon arc initiation may cause damage to the weldment or the electrode. This is corrected by limiting the current upon arc start. The disadvantage of touch starting is the tendency for the electrode to stick to the workpiece, causing electrode contamination and transfer of tungsten to the workpiece.

AIR CORE TRANSFORMER

ELECTRODE SPARK GAP OSCILLATOR

WELDING POWER SUPPLY

GAS NOZZLE HIGH FREQUENCY GENERATOR

5.8.4 Touch And Retract Starters. The touch and retract starters are similar to contact starting in that the tip of the electrode is lowered to make contact with the workpiece. However, the controls in this starter do not allow current to flow until the electrode begins to be withdrawn, thus striking the arc. This eliminates the tendency for electrode contamination. Another variation involves a low current to be passed through the electrode, which heats the electrode tip and allows the arc to more easily be initiated upon retraction.

WORKPIECE

Figure 29—High-Frequency Arc Starting

core transformer, as illustrated in Figure 29.36 The highfrequency generator produces a series of closely spaced bursts of high voltage energy. This high voltage ionizes the gas between the electrode and the work. The ionized gas will then conduct welding current that initiates the welding arc.

6. Tungsten Electrodes 6.1 General. In GTAW the word “tungsten” refers to the pure element tungsten and its various composites used as welding electrodes. Tungsten electrodes are called “nonconsumable” (if the process is properly used), because they do not melt or transfer to the weld as a filler metal. In other welding processes, such as SMAW, GMAW, and SAW, the electrode is the filler metal. The function of a tungsten electrode is to serve as one of the electrical terminals in the welding circuit. An electric arc is formed at the electrode tip and this arc supplies the heat required for welding. Pure tungsten has a melting point of 6170°F [3410°C] and a boiling point of 10220°F [5660°C]37; the melting and boiling points of tungsten electrodes vary with the dopant composition. Approaching the high melting temperature, tungsten becomes increasingly thermionic, which means that the tungsten can more easily emit or give off electrons. It reaches this temperature by cathodic or anodic spot heating and resistance heating. Were it not for the significant cooling effect of electrons emitting from its tip, during DCEN (straight polarity) operation, the heating would cause the tip to melt. Provided the electrode is used within the current-carrying capacity range for its specific type, diameter and polarity, it is virtually impossible to melt and/or vaporize a tungsten electrode during welding in an inert shielding gas

Because radiation from a high-frequency generator may disturb radio, electronic, and computer equipment, the use of this type of arc starting equipment is governed by regulations of the Federal Communications Commission. The user should follow the instructions of the manufacturer for the proper installation and use of high-frequency arc starting equipment. 5.8.2 Pulse/Capacitor Discharge Starters. Application of a high-voltage pulse between the tungsten electrode and the work will ionize the shielding gas and establish the welding arc. This method is generally used with DC power sources in machine welding applications. This technique utilizes capacitors and/or inductors to store energy. The stored energy is typically discharged in the form of a high voltage impulse to ionize the gas surrounding the welding electrode, thus initiating the arc. If the arc start is unsuccessful, the process is repeated. 5.8.3 Contact Starting (Scratch or Touch Start). With the power source energized, and the shielding gas flowing from the cup, the torch is lowered toward the workpiece until the tungsten electrode contacts with the workpiece. The torch is quickly withdrawn a short dis-

37. CRC Handbook of Chemistry and Physics, 54th Edition, p. B-35, 1973-74. Robert C. Weast, Editor. CRC Press, Cleveland, OH.

36. AWS Welding Handbook, Eighth Edition, Volume 2: Figure 3.21, p. 93.

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33

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Table 5 Chemical Composition Requirements for Tungsten Electrodes(1) Weight Percent (wt-%)

AWS Classification EWP(5) EWCe-2(5) EWLa-1(5) EWLa-1.5 EWLa-2 EWTh-1(5) EWTh-2(5) EWZr-1(5) EWG(4)

Color Code(6),(7)

UNS Number(2)

W Min. (Difference)(3)

Green Orange(8) Black Gold Blue Yellow Red Brown Gray

R07900 R07932 R07941 R07942 R07943 R07911 R07912 R07920 —

99.5 97.3 98.3 97.8 97.3 98.3 97.3 99.1 94.5

CeO2

La2O3

ThO2

ZrO2

— 1.8–2.2 — — — — — — Not Specified

— — 0.8–1.2 1.3–1.7 1.8–2.2 — — — Not Specified

— — — — — 0.8–1.2 1.7–2.2 — Not Specified

— — — — — — — 0.15–0.40 Not Specified

Other Oxides or Elements Total 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Notes: (1) The electrode shall be analyzed for the specific oxides for which values are shown in this table. If the presence of other elements or oxides is indicated in the course of the work, the amount of those elements or oxides shall be determined to ensure that their total does not exceed the limit specified for “Other Oxides or Elements, Total” in the last column of the table. (2) SAE/ASTM Unified Numbering System for Metals and Alloys. (3) Tungsten content shall be determined by subtracting the total of all specified oxides and other oxides and elements from 100%. (4) Classification EWG must contain some compound or element additive and the manufacturer must identify the type and minimal content of the additive on the packaging. (5) See AWS A5.12/A5.12M for closely matching grades in ISO 6848. (6) The actual color may be applied in the form of bands, dots, etc. at any point on the surface of the electrode. (7) The method of color coding used shall not change the diameter of the electrode beyond the tolerances permitted. (8) Color varies in ISO 6848, Tungsten Electrodes for Inert Gas Shielded Arc Welding and for Plasma Cutting and Welding.

atmosphere. A number of desirable performance characteristics result from additives to tungsten which lower the electronic work function.

coniated, Th is for thoriated, Ce is for ceriated, La is for lanthanated and G stands for general or unspecified oxide additions. Finally, the numbers40 specify the nominal alloying composition (in wt-%). For instance, EWTh-1 is a thoriated tungsten electrode that contains a nominal 1 wt-% thoria.

6.2 Classifications of Electrodes. Tungsten electrodes are classified on the basis of their chemical compositions, as summarized in Table 5. Requirements for tungsten electrodes are given in the latest edition of AWS A5.12, Specification for Tungsten and Tungsten Alloy Electrodes for Arc Welding and Cutting.38 In the AWS classification system, E stands for an electrode, which is used as one terminal of the arc welding circuit.39 The W stands for the chemical symbol for tungsten (also called Wolfram). The final letters indicate the alloying element or oxide additions. P designates a pure tungsten electrode without intentional alloying elements, while all other designations are for certain oxide additions. Zr is for zir-

A marking system was developed to provide identification of the various types of electrodes, and each tungsten electrode classification has a color code, as shown in Table 5. Individual electrodes are marked, typically with a band (or dot) of the appropriate color, on one end of the electrode. See the latest revision of AWS A5.12 for more information. The doped (alloyed) tungsten electrodes contain oxide additions, which should be evenly dispersed throughout the entire length of the electrode. These additions lower the temperature at which the electrodes emit electrons, to a temperature below the melting point of the tungsten. These doped tungsten electrodes are able to handle

38. Additional information can be found in various ISO documents, including ISO6848, Tungsten Electrodes for Inert Gas Shielded Arc Welding, and for Plasma Cutting and Welding. 39. AWS A3.0, Standard Welding Terms and Definitions, American Welding Society, Miami, FL.

40. An exception is EWZr-1 which is not 1% ZrO2, but rather 0.15%–0.40%.

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General Note: See AWS A5.12/A5.12M-98, Specification for Tungsten and Tungsten-Alloy Electrodes for Arc Welding and Cutting, Table 1.

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Table 6 Typical Current Ranges For Tungsten Electrodes(1) and Recommended Gas Cup Sizes --``,``-`-`,,`,,`,`,,`---

Direct Current, A

Electrode Diameter

Gas Cup I.D.

(3)DCEN(3)

(3)DCEP(3)

(DCSP)

(DCRP)

Alternating Current, A

Unbalanced Wave AC(3)

Balanced Wave AC(3)

in.

mm

in.

mm

EWX-X(2)

EWX-X

EWP

EWX-X

EWP

EWX-X

0.010 0.020 0.040 (4)0.060(4) 0.093 (3/32) 0.125 (1/8) 0.156 (5/32) 0.187 (3/16) 0.250 (1/4)

0.30 0.50 1.00 1.60 2.40 3.20 4.00 4.80 6.40

1/4 1/4 3/8 3/8 1/2 1/2 1/2 5/8 3/4

6 6 10 10 13 13 13 16 19

Up to 15 5–20 15–80 70–150 150–250 250–400 400–500 500–750 750–1000

N/A N/A N/A 10–20 15–30 25–40 40–55 55–80 80–125

Up to 15 5–15 10–60 50–100 100–160 150–200 200–275 250–350 325–450

Up to 15 5–20 15–80 70–150 140–235 225–325 300–400 400–500 500–630

Up to 15 10–20 20–30 30–80 60–130 100–180 160–240 190–300 250–400

Up to 15 5–20 20–60 60–120 100–180 160–250 200–320 290–390 340–525

Notes: (1) All values are based on the use of argon as a shielding gas. Other current values may be employed depending on the shielding gas, type of equipment, and application. (2) EWX-X refers to the X-X dopant and percentage of dopant in the tungsten electrode. See the latest AWS A5.12. Ce-2, La-1, La-1.5, La-2, Th-1, Th-2, Zr-1 and others may be included in this category. (3) DCEN = Direct current electrode negative (straight polarity); DCEP = Direct current electrode positive (reverse polarity); AC = Alternating current. (4) Although the metric size 1.6 mm (0.063 in.) is closer to 1/16 in. (0.0625 in.), it has been common industry practice to refer to the U.S. customary size 0.060 in. as 1/16 in.

higher welding currents,41 and they typically provide improved arc starting characteristics, help stabilize the arc, and increase the life of the electrodes. However, each has distinct electrical and arc characteristics, which may result in different welding performances and weld bead shapes.

The pure tungsten electrodes are more prone to contamination of the weld than the other types of tungsten electrodes. They are generally only used for AC welding, because DC welding with pure tungsten electrodes typically produces small amounts of tungsten inclusions (discontinuities) in the weld.

6.2.1 Pure Tungsten Electrodes (EWP). Pure tungsten electrodes (EWP) contain a minimum of 99.5% tungsten, with no intentional alloying elements, as summarized in Table 5. The current-carrying capacity of pure tungsten electrodes is lower than that of the doped (alloyed) tungsten electrodes (see Table 6). Pure tungsten electrodes are used mainly with AC for welding aluminum and magnesium alloys. The tip of the EWP electrode maintains a clean, balled end (see Figure 3042), which provides good arc stability. The pure tungsten electrodes also may be used with DC but they do not provide the arc initiation and arc stability characteristics of thoriated, ceriated, or lanthanated electrodes. DCEP (reverse polarity) welding causes splitting and melting of pure tungsten electrodes.

6.2.2 Zirconiated Tungsten Electrodes (EWZr-1). Zirconiated tungsten electrodes (EWZr-1) contain a small amount of zirconium oxide (ZrO2, referred to as zirconia), as listed in Table 5. Zirconiated tungsten electrodes have welding characteristics that generally fall between those of pure tungsten and thoriated tungsten. They are normally the electrode of choice for AC welding of aluminum and magnesium alloys because they combine the balled end typical of pure tungsten along with the higher current capacity (see Table 6), desirable arc stability and better arc starting characteristics of thoriated tungsten electrodes. They are more resistant to tungsten contamination of the weld pool than pure tungsten and are preferred for radiographic-quality aluminum welding applications where tungsten contamination of the weld must be minimized.

41. Ushio, M. and Matsuda, F., 1986, “Study on Gas-TungstenArc Electrodes: Comparative Study on Characteristics of Tungsten-Oxide Cathode.” IIW Doc. #212-648-86. 42. Photo courtesy of Pratt & Whitney.

6.2.3 Thoriated Tungsten Electrodes (EWTh-X). Thorium oxide (ThO2, called thoria) is the present industrystandard additive to tungsten electrodes. Two types of

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The EWTh-1 and EWTh-2 tungsten electrodes were designed for DCEN applications. They maintain a sharpened tip configuration during welding, which is desirable for the welding of steel, nickel alloys, and most alloys other than aluminum or magnesium. Thoriated tungsten electrodes are not normally used with AC welding because it is difficult to maintain the balled end, which is desirable with AC welding, without splitting the electrode. EWTh-3 is a discontinued classification of tungsten electrode, which had a blue color code. Note: the new EWLa-2 now uses a blue color code. This tungsten electrode had an integral longitudinal or axial segment throughout its length which contained 1.0 wt-%–2.0 wt% thoria and the average thoria content of the electrode was 0.35 wt-%–0.55 wt-%. Advances in powder metallurgy and other processing developments have caused this electrode classification to be discontinued, as it has limited commercial applicability.

Thoria is a very low-level radioactive oxide. However, if welding is to be performed in confined spaces for prolonged periods of time or if electrode grinding dust might be ingested, special precautions relative to ventilation should be considered. The user should consult appropriate safety personnel. The level of contamination/radiation has not been found to represent a health hazard during welding, but rather the grinding dust from the electrodes may be a concern.43, 44, 45, 46 However, in other nations, especially European countries, tungsten electrodes containing greater than 2% thoria are used less now because of concerns with radiation exposure to the welder. Alternative rare earth doped tungsten electrodes are available and are discussed below.

Figure 30—Balled Tip on the End of a Pure Tungsten Electrode Used for AC Welding

thoriated tungsten electrodes are readily available, as shown in Table 5. The EWTh-1 electrodes contain a nominal 1% thoria and the EWTh-2 electrodes contain a nominal 2% thoria, evenly dispersed through their entire lengths.

6.2.4 Ceriated Tungsten Electrodes (EWCe-2). The EWCe-2 electrodes are tungsten alloy electrodes

Thoriated tungsten electrodes are superior to pure tungsten electrodes in several respects. The thoria is responsible for increasing the usable life of these electrodes compared with the pure tungsten electrodes. This is because the thoria provides higher electron emissivity, typically 20% higher current carrying capacity (see Table 6), generally longer life, lower electrode tip temperatures, and greater resistance to contamination of the weld. With these electrodes, arc starting is easier, and the arc is more stable than with pure tungsten or zirconiated tungsten electrodes for DC welding.

43. “Thoriated Tungsten Electrodes Studied for Effects on Welders’ Health,” Welding Journal, Vol. 73, No. 5, May 1994, pp. 88–89. 44. Vinzents, P., Poulsen, O. M., et al., 1994. “Cancer Risk and Thoriated Welding Electrodes,” Occupational Hygiene, Vol. 1, no. 1, 1994 pp. 27–33. 45. McElearny, N. and Irvine, D., 1993. “A Study of Thorium Exposure During Tungsten Inert Gas (TIG) Welding in an Airline Engineering Population,” Journal of Occupational Medicine, Vol. 35, No. 7, July 1993, pp. 707–711. 46. British Health and Safety Executive Information Document HSE 564/56, June 1991.

*ANSI/AWS A5.12/A5.12M-98, Specification for Tungsten and Tungsten-Alloy Electrodes for Arc Welding and Cutting, p. 8.

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SAFETY NOTE*

AWS C5.5/C5.5M:2003

containing a nominal 2% cerium oxide (CeO2—referred to as ceria), as shown in Table 5. These electrodes were developed as possible replacements for thoriated tungsten electrodes because ceria, unlike thoria, is not a radioactive material. Compared with pure tungsten, the ceriated tungsten electrodes provide similar current levels (Table 6) and the improved arc starting and arc stability characteristics of thoriated tungsten electrodes. EWCe-2 electrodes are recommended for DCEN welding but can operate successfully with AC. Because these electrodes contain a different oxide than the thoriated tungsten electrodes, the electrical characteristics are slightly different. The ceriated tungsten electrodes operate at slightly different arc voltages/arc lengths than the thoriated tungsten electrodes. For mechanized and automated welding slight changes to the welding parameters and procedures may be required.

Table 7 Comparison of Surface Finish Designations Grit 500 320 240 180 120

4–16 10–32 15–63 70–90 100

0.1–0.4 0.2–0.8 0.4–1.6 1.8–2.3 2.5

Ra (µin.)

Ra (µm)

3.6–14.4 9.0–28.8 13.5–56.8 63.1–81.1 90

0.1–0.4 0.2–0.7 0.3–1.4 1.6–2.1 2.3

Note: (1) RMS (µin.) = 1.1 Ra (µin.).

6.3 Surface Finishes. Electrodes with a ground finish are recommended. Electrodes with a ground finish are processed to produce a uniform size (usually by centerless grinding which is done to remove surface imperfections). They are then chemically cleaned to remove any surface impurities. The ground-finish electrodes typically provide better electrical current conduction from the collet to the electrode and behave more predictably and more uniformly over time and from electrode to electrode. The ground-finish electrodes are supplied with a bright, polished surface with a maximum surface roughness of 32 µin. AARH [0.8 µm Ra]. See Table 7 for a comparison of surface finishes. Clean-finished electrodes (produced by chemically cleaning the surface to remove only surface impurities after the forming operation) are no longer part of AWS A5.12. Electrodes with a clean (only) finish may be available but are not recommended.

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6.2.5 Lanthanated Tungsten Electrodes (EWLa-X). The EWLa-X categories of tungsten electrodes were developed as nonradioactive alternatives to thoria. These electrodes contain lanthanum oxide (La2O3, referred to as lanthana) in various contents from 1 wt-% to 2 wt-%, as shown in Table 5. The electrodes with higher concentrations of dopants provide greater electron emission efficiencies. The current levels (see Table 6), advantages, and operating characteristics of these electrodes are very similar to the ceriated tungsten electrodes (although these electrodes operate at slightly different arc voltages than thoriated or ceriated tungsten electrodes). 6.2.6 Other Tungsten Electrodes (EWG). The EWG electrode classification was assigned for electrode compositions not included in the above classes. These electrodes contain an unspecified addition of some oxide or combination of oxides (rare earth or others). The purpose of the addition is to improve the nature or characteristics of the arc, or the life of the electrode. Several EWG electrodes are either commercially available or are being developed. These include additions of yttrium oxide or magnesium oxide, or ceriated and lanthanated tungsten electrodes that contain these oxides in amounts other than as listed in Table 5 or in combination with each other or with other oxides. AWS A5.12 requires that the specific type and nominal content of the alloy (dopant) addition shall be marked on the packages. Potential increase in electrode life and improved operating characteristics may result from oxide additions of several rare earths in combination.47

6.4 Electrode Sizes and Current Capacities. Tungsten electrodes are available in a variety of standard diameters (sizes), as listed in Table 6. These tungsten electrodes are commercially available in standard lengths of 2, 3, 6, 7, 12, 18, and 24 in. [50, 75, 150, 175, 305, 455, and 610 mm48] with blunt ends (i.e., no end preparation). Shorter electrodes for applications such as orbital welding equipment are usually available from the equipment manufacturers or other suppliers. Lengths from 1/4 in.–2 in. [6 mm–50 mm] are typically used and can be purchased cut to exact lengths and pre-ground with exact tip configurations (see Figure 3149). The choices of an electrode classification, size, and welding current are influenced by the type and thickness

47. Ushio, M., Matsuda, F. and Sadek, A. A. 1992. “GTA Welding Electrodes,” International Conference on Trends in Welding Science and Technology, June 1992, p. 405–409; ASM Conference Proceedings. Edited by S. A. David and J. M. Vitek.

48. Standard sizes and lengths in ISO6848 “Tungsten electrodes for inert gas shielded arc welding, and for plasma cutting and welding,” although tolerances differ in some cases. 49. Figure 2, Welding Journal, Vol. 74, No. 1, Jan. 1995, p. 41.

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RMS(1) (µin.) RMS (µm)

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ing. For DCEN welding, tapering of the electrode tip is usually done for all but the smallest electrodes. Various electrode tip geometries affect the weld bead shape and size. In general, as the included angle increases, weld penetration increases and the width of the weld bead decreases (see Figure 1450). Electrode tip configuration is a welding variable that should be studied during welding procedure development and adhered to during production, especially for mechanized and automated welding. Regardless of the electrode tip geometry selected, it is important that a consistent tip configuration be used once a welding procedure is established. Changes in electrode geometry can have a significant influence not only on the weld bead width, depth of penetration, and resultant quality, but also on the electrical characteristics of the arc (e.g., arc voltage), arc stability, and electrode life. If the electrode geometry is not selected correctly, tungsten inclusions may result.

THE ELECTRODES ARE PRECUT TO LENGTHS FOR AUTOMATIC ORBITAL WELDING MACHINES. THE ELECTRODES SHOWN HAVE NOT BEEN TRUNCATED.

Figure 31—Ground Tapered Tip on End of Doped Tungsten Electrodes

6.5.1 Balling. With AC welding, a hemispherical tip, or “ball,” is most desirable. Before use in welding, the electrode tip is balled by striking an arc on a copper block or other suitable material using AC or DCEP. Arc current is increased until the end of the electrode turns white hot and the tungsten begins to melt, causing a small ball to form at the end of the electrode, as illustrated in Figure 30. The current is downsloped (decreased gradually) and extinguished, leaving a hemispherical ball on the end of the tungsten electrode (shielding gas flow must be maintained while the tip is cooling). The size of the hemisphere should be between 1 and 1-1/2 times the electrode diameter.

of base metals being welded, as listed in Table 8. The current carrying capacities of all types of tungsten electrodes are affected by the type of welding torch, the type of power source (DCEN, DCEP, AC, or variable polarity), the electrode extension beyond the collet, and the shielding gas. Tables 6 and 9 list suggested current ranges for each electrode type and size using argon shielding gas, along with gas cup diameters recommended for use with different types of welding power. Since the maximum current capacity of an electrode depends on many factors (see 4.4.1.2 and 4.4.1.3), the typical current ranges should be used only as a guide. In general, all of the electrodes may be used with argon or helium, or with a combination of these shielding gases.

6.5.2 Grinding. The most common method of preparing the tip is by grinding. Figure 33 illustrates typical grinding, cutting, and truncating methods in preparing tungsten electrodes for DCEN welding. To produce optimum arc stability, grinding of thoriated, ceriated, and lanthanated tungsten electrodes should be done with the axis of the electrode perpendicular to the axis of the grinding wheel and ground on the flat face of the wheel (i.e., the electrode should be ground in the longitudinal direction) as shown in Figure 33.51 A polished surface condition of better than 10 µin. RMS [0.2 µm] will help to extend the electrode life. Circumferential grinding of the electrode tip is not recommended because it can result in arc instability if the

6.5 Electrode Tip Configurations. The shape of the tungsten electrode tip is an important process variable in GTAW. Tungsten electrodes may be used with a variety of tip preparations. With AC welding, pure or zirconiated tungsten electrodes form a hemispherical balled end (see Figure 30). Thoriated, ceriated and lanthanated tungsten electrodes do not ball as readily as pure or zirconiated tungsten electrodes, and are typically used for DC welding. For these, the end is typically beveled to a conical shape with a specific included angle, usually with a truncated end (see Figure 32). A small flat at the tip of the electrode is important to avoid the tip breaking off. These doped electrodes maintain a ground tip shape much better than the pure tungsten electrodes. Table 9 is a guide for electrode tip preparation for a range of sizes with recommended current ranges.

50. J. F. Key, “Anode/Cathode Geometry and Shielding Gas Interrelationships in GTAW,” Welding Journal, Vol. 59, No. 12, Dec. 1980, p. 365-s. 51. “A Guide to Tungsten Electrode Geometry and Preparation,” Practical Welding Today, Vol. 2, No. 2, March/April 1998.

Tungsten electrode tip configurations are prepared by balling or grinding. For AC welding the square end of the electrode is sometimes slightly chamfered prior to ball-

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Table 8 Recommended Types of Current, Tungsten Electrodes, and Shielding Gases for Welding of Various Metals and Alloys Thickness

Type of Current(1),(2)

Electrode(3)

Shielding Gas

Aluminum

All > 1/8 in. [3 mm] ≤ 1/8 in. (3 mm)

AC DCEN DCEP

Pure or zirconiated Thoriated Thoriated or zirconiated

Argon or argon-helium Argon-helium or argon Argon

Copper, copper alloys

All < 1/8 in. [3 mm]

DCEN AC

Thoriated Pure or zirconiated

Helium or argon-helium Argon

Magnesium alloys

All < 1/8 in. [3 mm]

AC DCEP

Pure or zirconiated Zirconiated or thoriated

Argon Argon

All

DCEN

Thoriated

Argon

Plain carbon, low-alloy steels

All < 1/8 in. [3 mm]

DCEN AC

Pure or zirconiated Thoriated

Argon or argon-helium Argon

Stainless steel

All < 1/8 in. [3 mm]

DCEN AC

Thoriated Pure or zirconiated

Argon or argon-helium Argon

All

DCEN

Thoriated

Argon

Type of Alloy

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Nickel, nickel alloys

Titanium

Notes: (1) Where AC is listed, variable polarity or pulsed current could be used. (2) AC = Alternating current; DCEP = Direct current electrode positive; DCEN = Direct current electrode negative. (3) Where thoriated electrodes are recommended, ceriated or lanthanated electrodes may also be used.

Table 9 Tungsten Electrode Tip Shapes and Examples of Current Ranges DCEN (Electrode Negative/Straight Polarity)(1),(2) Electrode Diameter

Diameter at Tip

in.

mm

in.

mm

Included Angle, Degrees

0.040 0.040 0.060 0.060 0.093 0.093 0.125 0.125

1.00 1.00 1.60 1.60 2.40 2.40 3.20 3.20

0.005 0.010 0.020 0.030 0.030 0.045 0.045 0.060

0.125 0.250 0.50 0.80 0.80 1.10 1.10 1.50

12 20 25 30 35 45 60 90

Constant Current Range, A

Pulsed Current Range, A

2–15 5–30 8–50 10–70 12–90 15–150 20–200 25–250

2–25 5–60 8–100 10–140 12–180 15–250 20–300 25–350

Notes: (1) All values are based on the use of argon as the shielding gas. Other current values may be used, depending upon the shielding gas, type of equipment and application. (2) These values will vary depending on duty cycle, pulse frequency, peak/background ranges, etc.

Figure 32—Ground Electrode Tip Geometry

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Figure 33—Typical Preparation Method of Tungsten Electrodes Used for GTA Welding, Including Tip Truncation, Grinding, and Cutting 40 Copyright American Welding Society Provided by IHS under license with AWS No reproduction or networking permitted without license from IHS

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surface finish is not smooth enough. The desired surface finish of a ground electrode is illustrated in Figure 34.52 The grinding wheel should be reserved for the grinding of only tungsten to eliminate possible contamination of the tungsten tip with foreign matter during the grinding operation. Grinding on diamond wheels is preferred over silicon carbide or aluminum oxide wheels. Equipment designed specifically for grinding and cutting tungsten electrodes is available. Exhaust hoods and personal protective equipment (PPE) should be used when grinding tungsten and tungsten alloy electrodes to remove the grinding dust from the work area.53, 54, 55, 56, 57, 58, 59, 60

POLISHED FINISH: • Maximum electrode life • Ideal for high purity welding • Extremely consistent finish and angle • Diamond ground and polished longitudinally • Excellent arc starting and stability

STANDARD FINISH: • Consistent finish and angle • Diamond ground in longitudinal direction • Good to excellent electrode life • Exceeds all standards set by automatic welding equipment manufacturers

6.6 Electrode Cutting. One of the most overlooked areas of tungsten electrode preparation is the removal of the contaminated and/or oxidized tip. A contaminated electrode produces an erratic arc and a dirty, contaminated weld. Although many welders will simply regrind contaminated electrodes, the recommended practice is to cut off the contaminated portion prior to regrinding. The way to cut tungsten electrodes or remove a contaminated tip is noted in Figure 35.52 The tungsten should be rigidly secured on either side of the cut. The cutting wheel preferably should be diamond to provide a clean, contamination free, smooth separation. This will insure the electrode is not fractured or splintered during the cut off operation. The use of a silicon carbide type wheel, which contaminates the tungsten, is not recommended. Improper methods of removing the electrode tips include breaking with wire cutters, by hand, with a hammer, on a grinding wheel, or other similar means. These produce splintering or shattering of the electrode tip, and also create safety concerns.

HAND GROUND FINISH: • Inconsistent finish • Incorrectly ground angle • Poor arc stability and starting • Reduced electrode life • Risk of tungsten inclusions and potential X-ray defect from tungsten erosion • Involves potentially hazardous hand grinding

6.7 Factors Affecting Electrode Life. The life of any tungsten electrode is shortened by excessive electrode extension from the collet, excessive welding current, or electrode contamination, all of which can adversely affect the arc characteristics. The electrode extension beyond the collet should be kept to a minimum to ensure protection of the electrode by the inert shielding gas. Too long of an extension can cause overheating and possible melting or cracking of the electrode.

52. Photo courtesy of Diamond Ground Products. 53. “Thoriated Tungsten Electrodes Studied for Effects on Welders’ Health,” Welding Journal, Vol. 73, No. 5, May 1994, pp. 88–89. 54. Vinzents, P., Poulsen, O. M., et al., 1994. “Cancer Risk and Thoriated Welding Electrodes,” Occupational Hygiene, Vol. 1, No. 1, 1994, pp. 27–33. 55. McElearny, N. and Irvine, D., 1993. “A Study of Thorium Exposure During Tungsten Inert Gas (TIG) Welding in an Airline Engineering Population,” Journal of Occupational Medicine, Vol. 35, No. 7, July 1993, pp. 707–711. 56. British Health and Safety Executive Information Document HSE 564/56, June 1991. 57. MSDS sheets, tungsten mfg. 58. “Practical TIG (GTA) Welding,” Muncaster, P. W., 1991, pp. 25, Abington Publishing, ISBN 1855730200. 59. Campbell, R. D. and LaCoursiere, E. J., 1995. “A Guide to the Use of Tungsten Electrodes for GTA Welding.” Welding Journal, Vol. 74, No. 1, Jan. 1995, pp. 39–45. 60. AWS A5.12/A5.12M-98, Specification for Tungsten and Tungsten-Alloy Electrodes for Arc Welding and Cutting, p. 8.

6.7.1 Improper Current Levels. Excessively high current levels for the specific tungsten electrode size can cause the electrode to quickly erode, melt or split. In many cases of excessive current, tungsten particles will be deposited into the weld pool and become a discontinuity in the weld joint. In certain high amperage applications, a cold electrode can exhibit “tungsten spitting” if it is brought rapidly or instantaneously up to the welding current. This will usually result in high-density tungsten inclusions in the weld. This condition can be reduced or eliminated by starting the arc at a low current and slowly

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Figure 34—The Desired Surface Finish of a Ground Electrode

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Figure 35—Proper Cutting of Tungsten Electrodes with a Diamond Cut-Off Blade

6.9 Grinding Dust.61 The grinding (sharpening) of tungsten electrodes generates tungsten metal dust. This dust may also contain small amounts of oxides like zirconia, ceria, lanthana and/or thoria. Dust generated by grinding can be considered a health hazard and most health authorities recommend that all metal grinding machines provide transparent eye shields, dust extractors and filters. No dust produced by tungsten grinding should be inhaled by the person doing the grinding or by adjacent personnel. Metal dust can cause pulmonary illness over long periods of inhalation, so it is common sense to protect oneself. Good shop practice and common sense should be used for grinding anything. Wear a full-face shield and safety glasses fitted with side shields, and do not wear loose fitting clothing that could get caught in the machine. If electrode-grinding dust might be inhaled, special precautions relative to ventilation should be considered. A vacuum or exhaust system should be used when grinding electrode tips. Grinding dust from electrodes must not be inhaled. The user should consult the appropriate safety procedures, the manufacturer’s suggested procedures, and follow all company internal safety requirements.

increasing it to the welding current. Current that is too low for a specific electrode diameter may cause arc wandering and arc starting problems. 6.7.2 Contamination of the Electrode. Metal contamination of the tungsten electrode is most likely to occur when a welder accidentally dips the tungsten into the molten weld pool or touches the tungsten with the filler metal. These often produce tungsten inclusions in the weld. The tungsten electrode may also become oxidized by use of an improper shielding gas, insufficient gas flow, loose/leaking gas fittings or gas nozzle (cup), wind or high air movements, or prematurely shutting off the shielding gas flow. Other sources of contamination include: metal vapors from the welding arc, weld pool eruptions or spatter caused by gas entrapment, and evaporated surface impurities. 6.8 Removing Contamination. When the tungsten electrode becomes contaminated, the welding operation should be stopped and the contaminated portion of the electrode removed. The contaminated portion of the electrode should be cut off and then properly redressed to the appropriate tip configuration (see 6.5.2).

61. See footnotes from 6.5.2.

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6.10 Storage. No firm guidelines have been established for the storage of thoriated tungsten electrodes by the American Welding Society62 or the IIW Commission VII. Guidelines may be incorporated in the Material Safety Data Sheets (MSDS) provided by the manufacturers.63 Thorium is a naturally occurring radioactive element that is used in a wide range of industrial processes and applications. The main hazard is associated with inhalation of dust particles generated during tip grinding. Thoriated tungsten electrodes, when not in use, should be stored together in a steel cabinet or similar metal container. The number of electrodes in storage should be kept to a minimum, taking into account the expected usage and availability of further stock from the manufacturer or supplier. As discussed in an earlier section, local exhaust ventilation should be provided at the grinding operation. The spent tips and grinding dust from the collection unit should be disposed of in a sealed container to the appropriate location as may be required by any local, state and federal laws and any company policies. A manager or supervisor should be given the responsibility to ensure that a safe system is in place and followed.

7.1 Torch Shielding Gas 7.1.1 Purpose of Torch Shielding Gas. The primary purpose of the torch shielding gas is to protect the electrode, the molten weld metal and the “hot” end of any filler metal from atmospheric contamination. This contamination is caused mainly by oxygen and water vapor present in the atmosphere. Note: Many of the examples in this section are, in detail, related to the welding of pipe or tubing. The basic principles that these examples are meant to amplify, extend and apply to all GTAW applications. 7.1.2 Types and Selection of Shielding Gases. Argon and helium or mixtures of the two are the most common types of inert gases used for shielding. Argon is preferred for most applications, except where helium’s higher heat input is required for welding thick sections of metals and alloys with high heat conductivity, such as aluminum and copper. In addition, the lower unit cost and the lower flow rate requirements of argon make argon preferable from an economic point of view. Argon-hydrogen mixtures and sometimes argon-nitrogen mixes are used for special applications. The different gas mixtures are used to tailor the weld penetration geometry and to improve deposition rates or travel speeds. Later sections will provide guidelines for using shielding gas mixtures. Tables 8 and 13 provide a summary of some common shielding gases and shielding gas mixtures used for gas tungsten arc welding of different base metals.

7. Gas Shielding, Purging, and Backing64 Shielding gas is directed through the torch to the arc and the weld pool to protect the electrode, the filler metal and the molten weld metal from atmospheric contamination. Backup purge gas can also be used to protect the underside of the weld and its adjacent base metal surfaces from oxidation during welding. Uniformity of root bead (underbead) contour, freedom from undercutting, and the desired amount of root bead reinforcement are more likely to be achieved when using gas backup under controlled conditions. In some materials, gas backup reduces root cracking and porosity in the weld. Most alloys when welded require shielding gas backing; two exceptions may be aluminum and carbon steel. Some general and thermodynamic properties of gases are summarized in Tables 10 and 11 for reference. See Table 12 for dew point conversions. See 7.6 and 15.7 for additional information on gas safety.

Depending on the volume of usage, these gases may be supplied in compressed gas cylinders or as a liquid in insulated tanks. The liquid is vaporized and piped to points within the plant, thus eliminating cylinder handling. Leaking manifold systems are quite common; thus care must be taken to keep the system leak tight. 7.1.2.1 Argon (AWS classification SG-A). The most commonly used shielding gas is argon. Argon is a heavier-than-air monatomic gas with an atomic weight of 40 and a density of 1.7837 g/L at standard temperature and pressure (STP).65 Argon is available in various grades (see 7.3). It is a chemically inert, colorless, odorless, tasteless nontoxic gas. It is obtained from the atmosphere by the separation of liquid air. AWS classification SG-A is defined as having a minimum purity of 99.997% in the gaseous state.

62. “Thoriated Tungsten Electrodes Studied for Effects on Welders’ Health,” 1994, Welding Journal, Vol. 73, No. 5, May 1994, pp. 88-89. 63. Other countries may have specific recommendations; for example, the British Health and Safety Executive Information Document HSE 564/6 (June 1991). 64. Further information on this subject is available in AWS A5.32 and AWS C5.10.

65. Standard conditions for a gas: measured volumes of gases are generally recalculated to 32°F [0°C] temperature and 760 mm Hg (approximately 1 atm) pressure, which have been arbitrarily chosen as standard conditions. These conditions are referred to as standard temperature and pressure or STP.

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Table 10 General Properties of Gases(1),(2) Description

Argon

Helium

Hydrogen

Nitrogen

Oxygen

Ar

He

H2

N2

O2

Formula State

Compressed Gas Compressed Gas Compressed Gas Compressed Gas Compressed Gas

Ionization Potential (eV) Molecular Weight

15.7 39.95

24.5 4.003

15.6 2.02

15.5 28.01

12.5 32.00

Boiling Point, °C Boiling Point, °F Boiling Point, K

–185.7 –302.5 87.3

–268.93 –452.08 4.07

–252.87 –423.17 20.13

–195.8 –320.4 77.2

–182.96 –297.33 90.04

Melting Point, °C Melting Point, °F Melting Point, K

–189.2 –308.6 83.8

–272.2 –458.0 0.8

–259.14 –434.45 13.86

–209.86 –345.75 63.14

–218.4 –361.1 54.6

Density (gm/liter) @21.1°C Density (lb/ft3) @70°F Specific Volume (ft3/lb) @70°F, 1 Atm Specific Gravity (air = 1) @70°F Critical Temp, °C Critical Temp, °F Critical Temp, K Critical Pressure (psia) Critical Pressure (Atm) Critical Density (lbs/ft3)

0.00165 0.103 9.67 1.380 –122.4 –188.4 150.6 705.4 48.8 33.07

0.00016 0.010 96.06 0.138 –267.95 –450.30 5.05 33.2 2.336 4.32

0.00008 0.005 192.31 0.0695 –240.0 –399.8 33 188.1 13.0 1.93

0.00115 0.072 13.8 0.967 –146.9 –232.4 126.1

0.00133 0.083 12.1 1.105 –118.5 –181.1 154.5

492.9 34.0 19.38

731.4 50.5 27.17

Notes: References: (1) CRC Handbook of Chemistry and Physics, 54th Edition, 1973–1974. ISBN: 087819-454-1. (2) Compressed Gas Association, Pamphlet P-9-92.

Table 11 Thermodynamic Properties of Gases(1) Description

Argon

Helium

Hydrogen

Nitrogen

Oxygen

Heat Capacity (BTU/lb–Mole °F) (Cal/gm–°C)

4.97 4.97

4.98 4.98

6.89 6.89

6.90 6.97

7.03 7.03

Thermal Conductivity (BTU/hr–sq ft–°F/ft) (Watts/sq meter–°C)

0.0093 0.0530

0.0823 0.4670

0.096 0.545

0.0146 0.0830

0.0142 0.0806

Specific Heat @1 Atm (J/Kg–K) (BTU/lb–°F)

521.3 0.1246

5192.0 1.241

1490.0 3.561

1041.0 0.2487

917.0 0.219

1739.9 7.131953 54.8–148.1

27.5 4.178969 1.3–5.0

250.6 5.581833 9.7–31.2

1489.8 7.05041 46.9–124.7

1726.1 7.039904 53.9–148.9

Vapor Pressure(2),(3) where A = where B = Valid Range K

Notes: (1) References: (a) CRC Handbook of Chemistry and Physics, 54th Edition, 1973–1974. ISBN: 087819-454-1. (b) Compressed Gas Association, Pamphlet P-9-92. (2) Vapor Pressure P in Torr is given by the following: Log10P = (–0.2185 A/K) + B where: K = Kelvin A = Molar heat of vaporization in cal/gm-mole (3) Definition of Vapor Pressure: The pressure exerted when a solid or liquid is in equilibrium with its own vapor. The vapor pressure is a function of the substance and of the temperature.

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Table 12 Dew Point Conversions(2) Dew Point(1) °F

Moisture °C

Dew Point

Moisture

ppm (v/v)

°F

°C

ppm (v/v)

–59 –59 –58 –58 –57

11.4 12.3 13.3 14.3 15.4

–130 –120 –110

–90 –84 –79

0.10 0.25 0.63

–75 –74 –73 –72 –71

–105 –104 –103 –102 –101

–76 –76 –75 –74 –74

1.00 1.08 1.18 1.29 1.40

–70 –69 –68 –67 –66

–57 –56 –56 –55 –54

16.6 17.9 19.2 20.6 22.1

–100 –99 –98 –97 –96

–73 –73 –72 –72 –71

1.53 1.66 1.81 1.96 2.15

–65 –64 –63 –62 –61

–54 –53 –53 –52 –52

23.6 25.6 27.5 29.4 31.7

–95 –94 –93 –92 –91

–71 –70 –69 –69 –68

2.35 2.54 2.76 3.00 3.28

–60 –59 –58 –57 –56

–51 –51 –50 –49 –49

34.0 36.5 39.0 41.8 44.6

–90 –89 –88 –87 –86

–68 –67 –67 –66 –66

3.53 3.84 4.15 4.50 4.78

–55 –54 –53 –52 –51

–48 –48 –47 –47 –46

48.0 51.0 55.0 59.0 62.0

–85 –84 –83 –82 –81

–65 –64 –64 –63 –63

5.30 5.70 6.20 6.60 7.20

–50 –49 –48 –47 –46

–46 –45 –44 –44 –43

67.0 72.0 76.0 82.0 87.0

–80 –79 –78 –77 –76

–62 –62 –61 –61 –60

7.80 8.40 9.10 9.80 10.5

–45 –44 –43 –42 –41

–43 –42 –42 –41 –41

92.0 98.0 105.0.0 113.0.0 119.0.0

Notes: (1) Dew Point: The temperature at which, in a given mixture of gas and water vapor, the water vapor will condense out of the gas. (2) Conversion of ppm to %: 1 ppm = 0.0001% 10 ppm = 0.001% 100 ppm = 0.01% 1000 ppm = 0.1%

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Table 13 Advantages of Shielding Gases Welding Type

Shielding Gas

Manual

Argon

Aluminum and Magnesium

Machine

Carbon Steel

Stainless Steel

Better arc starting, cleaning action and weld quality; lower gas consumption.

Argon-helium

High welding speeds possible.

Argon-helium

Better weld quality, lower gas flow than required with straight helium.

Helium (DCSP)

Deeper penetration and higher weld speeds than can be obtained with argon-helium.

Spot

Argon

Generally preferred for longer electrode life. Better weld nugget contour, ease-of-starting, and lower gas flows than helium.

Manual

Argon

Better pool control; especially for position welding.

Machine

Helium

Higher speeds obtained than with argon.

Manual

Argon

Excellent control of penetration on light gage materials.

Argon-helium Machine

Argon-hydrogen (up to 35%-H2) Argon-helium

Copper, Nickel, and Cu-Ni Alloys

Advantages

Higher heat input, higher welding speeds possible on heavier gages. Prevents undercutting, produces desirable weld contour at low current levels, requires lower gas flows. An excellent selection for high speed tube mill operation.

Helium

Provides highest heat input and deepest penetration.

Argon

Ease of obtaining pool control, penetration, and bead contour on thin gage metal.

Argon-helium

Higher heat input to offset high heat conductivity of heavier gages.

Helium

Highest heat input for welding speed on heavy metal sections.

Argon

Low gas flow rate minimizes turbulence and air contamination of weld; improved heat-affected zone.

Helium

Better penetration for manual welding of thick sections (inert gas backing required to shield back of weld against contamination).

Silicon Bronze

Argon

Reduces cracking of this “hot short” metal.

Aluminum Bronze

Argon

Less penetration of base metal.

Titanium

(2) Reduced penetration (e.g., for thin materials) (3) Cleaning action when welding materials such as aluminum and magnesium with AC or DCEP current (4) Lower cost and greater availability (5) Lower flow rates for good shielding (6) Better cross-draft resistance (i.e., denser) (7) Easier arc starting (8) Better control of the weld pool The bead profile of an argon-shielded weld is particularly helpful when manual welding thin material because the tendency for excessive melt-through is lessened (see

Although argon itself is chemically inert it is readily ionized to form a plasma. Impurities such as moisture and oxygen can cause variable arc behavior, and in the case of sensitive materials such as the reactive and refractory metals, a reduction in weld metal properties. Oxygen, even at relatively low levels of 50 ppm or lower, can also cause the oxidation of the tungsten electrode. Argon is used more extensively than helium because of the following advantages: (1) Smoother, quieter arc action

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Metal

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VERTEX ANGLE (TRUNCATION, [mm])

AWS C5.5/C5.5M:2003

GAS MIXTURE 100 Ar

75 Ar–25 He

50 Ar–50 He

25 Ar–75 He

100 He

95 Ar–5 H2

39° [0.125]

60° [0.125]

90° [0.500]

180°

Figure 36—GTA Weld Bead Shape as a Function of Shielding Gas Composition and Electrode Tip Geometry (on 304 Stainless Steel)

Figure 36 66 for an example of the penetration characteristics). The weld profile characteristic is advantageous in vertical or overhead welding since the tendency for the base metal to sag or run is decreased. Argon is used for welding a wide range of materials, including mild steel, aluminum, copper, stainless steel, nickel alloys and the reactive metals, titanium and magnesium. --``,``-`-`,,`,,`,`,,`---

lium is a “mined” resource obtained by the separation of natural gas. The higher ionization potential of helium, approximately 25eV compared to 16eV for argon, produces a significantly higher arc voltage at the same arc length and therefore a hotter arc. For given values of welding current and arc length, helium transfers more heat into the work than argon. Because helium has a higher thermal conductivity, it can often promote higher welding speeds and improve the weld bead penetration. The greater heating power of the helium arc can be advantageous for joining metals of high thermal conductivity and for high-speed mechanized applications. Also, helium is used more often than argon for welding heavy plate. Mixtures of argon and helium are useful when some balance between the characteristics of both is desired (see Figure 36 for an example of weld penetration characteristic). Although helium offers definite advantages for some applications, it produces a less stable arc and less desirable arc starting characteristics than argon. As little as 5% argon in helium dramatically increases the ability to

7.1.2.2 Helium (AWS classification SG-He). Helium is the lightest monatomic gas, approximately ten times lighter than argon with an atomic weight of four (4). The density of helium is 0.166 g/L at room temperature. Its cost is significantly higher than that of argon. AWS classification SG-He is defined as having a minimum purity of 99.995% in the gaseous state. A high purity grade of 99.998% is readily available. Helium is a chemically inert, colorless, odorless, tasteless gas. He66. J. F. Key, 1980, “Anode/Cathode Geometry and Shielding Gas Interrelationships in GTAW,” Welding Journal, Vol. 59, No. 12, Dec. 1980, Figure 3, p. 366-s.

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ARC VOLTAGE, V

7.1.2.3 Argon–Helium Mixtures (AWS classifications SG-AHe and HeA). Helium is added to argon to take advantage of the best operating characteristics of each gas. The superior arc starting and stable arc characteristics of argon, with helium’s higher thermal conductivity, produce high quality GTAW welding of aluminum using AC. Increased travel speeds and depth of fusion, for both manual and mechanized welding, can be expected as helium content increases (see Figure 36). Helium content usually ranges between 25% and 75%. The chief factor influencing shielding effectiveness is the gas density. Argon is approximately one and onethird times as heavy as air and ten times heavier than helium. Argon, after leaving the torch gas nozzle (cup), forms a blanket over the weld area. Helium, because it is lighter, tends to rise around the gas nozzle (cup). Experimental work has consistently shown that to produce equivalent shielding effectiveness, the flow volume67 of helium must be two to three times that of argon. The same general relationship is true for mixtures of argon and helium, particularly those high in helium content. The important characteristics of these gases are the voltage-current relationships of the tungsten arc in argon and in helium that are illustrated in Figure 37.68 At all current levels, for equivalent arc lengths, the arc voltage obtained with helium is appreciably higher than that with argon. Since heat in the arc is roughly measured by the product of current and voltage (arc power), helium offers more available heat than argon. The higher available heat favors its selection when welding thick materials and

25 20

HELIUM

15 ARGON 10 0

0

50

100

150

200

250

300

350

400

ARC CURRENT, A

Figure 37—GTA Voltage—Current Relationships with Argon and Helium Shielding Gases for Different Arc Lengths

metals having high thermal conductivity or relatively high melting temperatures. However, it should be noted that at lower currents, the volt-current curves pass through a minimum voltage, after which the voltage increases as the current decreases. For helium, this increase in voltage occurs in the range of 50 A–150 A where much of the welding of thin materials is done. Since the voltage increase for argon occurs below 50 A, the use of argon in the 50 A–150 A range provides the operator with more latitude in arc length to control the welding operation. It is apparent that to obtain equal arc power, appreciably higher current must be used with argon than with helium. Since undercutting with either gas will occur at about equal current levels, helium will produce satisfactory welds at much higher speeds. Another influential characteristic is arc stability. Both gases provide excellent stability with direct current power. With alternating current power, which is used extensively for welding aluminum and magnesium, argon yields much better arc stability and the highly desirable cleaning action, which makes argon superior to helium in this respect.

67. Weight Percent. This is the percentage composition by weight. This is contrasted with atomic percent, which is the number of atoms of an element in a total of 100 representative atoms of a substance. Volume Percent. This is the fraction of the volume of one substance with respect to the total volume of all the constituents (then multiplied by 100). Weight percent gives the absolute quantity of one substance with respect to the total, whereas volume percent gives the relative amount of one substance versus the others where the density of each of the constituents vary individually with conditions of temperature and pressure. This is why, for comparison, gases are usually normalized referring to conditions of Standard Pressure and Temperature (STP). Notwithstanding, volume percent is most practical for metering gas flow. 68. Reference: AWS Welding Handbook, Eighth Edition, Volume 2: Figure 3.15, p. 89.

7.1.2.4 Argon-Hydrogen Mixtures (AWS classifications SG-AH).69 Hydrogen is the lightest and most abundant of all the elements in the universe. It is a flammable, colorless, odorless, nontoxic gas. Hydrogen’s gas 69. See AWS A5.32, Specifications for Welding Shielding Gases, Section A8, General Safety Considerations, and A9, Safety References.

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ARC LENGTH 0.08 in. [2 mm] 0.16 in. [4 mm]

TUNGSTEN ARC, ALUMINUM

30

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initiate the arc. Helium usually requires higher shielding flow rates (typically 2 to 3 times greater) than argon because helium is lighter than argon. Helium is more expensive and is usually provided in high-pressure cylinders that tend to contain more impurities (e.g., oxygen and moisture) than argon.

AWS C5.5/C5.5M:2003

density is 0.08988 g/L at STP. As with the other gases used for GTAW, several grades are available. Argon-hydrogen mixtures are employed in special cases, such as mechanized welding of light gage stainless steel and nickel based alloys, where the hydrogen does not cause adverse metallurgical effects such as porosity and hydrogen-induced cracking. Excessive hydrogen can cause porosity. Increased welding speeds can be achieved in almost direct proportion to the amount of hydrogen added to argon because of the increased arc voltage and thermal conductivity. However, the amount of hydrogen that can be added varies with the metal thickness and type of joint for each particular application. Hydrogen can be added to argon or helium to increase the temperature of the arc and to provide a slightly reducing atmosphere. Argon-hydrogen mixtures are typically 1%–7% hydrogen in argon, although higher hydrogen concentrations have been used. Up to 10% hydrogen addition to either argon or nitrogen is common. Hydrogen is most commonly used for welding in combination with argon or nitrogen to improve the wetting characteristics of the weld root by reducing the surface tension of the weld metal. In high-speed tube welding this helps to reduce the undercut. Mixtures of hydrogen and argon and mixtures of nitrogen and argon have been used on specialty tube mill applications. The higher heat input is derived from the dissociation of the hydrogen in the arc to form atomic hydrogen, which then recombines to the molecular form in the cooler regions of the arc and at the workpiece surface releasing energy to the weld pool. The arc voltage of a hydrogen mixture is correspondingly higher compared to pure argon or helium for identical arc length and current settings. The actual voltage will be determined by the arc length and the welding current level (as a function of the power source volt-current [V-I] characteristic). The arc itself in an argon (or helium)-hydrogen gas mixture is more constricted which improves the weld penetration, i.e., it produces a greater depth to width ratio (see Figure 36) and enables higher welding speeds to be achieved. Additionally, the slightly reducing atmosphere produces a cleaner weld bead surface and, in multipass welds, reduces the risk of oxide/slag build up. Figure 38 shows improved surface cleanliness with the 5% hydrogen mixture. It should be noted, however, that the use of hydrogen may cause cracking in carbon and alloy steels, and excessive hydrogen may produce weld metal porosity in ferritic steels, aluminum, copper and in multi-pass welds in nickel and austenitic stainless steels. Argon-hydrogen mixtures are normally limited to use on austenitic stainless steel, nickel-copper, and nickel-base alloys. An argon-hydrogen mixture containing 5% hydrogen is commonly used for mechanized welding of tight butt

joints in austenitic stainless steel up to 0.154 in. [3.9 mm] thick at speeds comparable to helium (50% faster than argon). It is also used for welding stainless steel barrels, and tube-to-tube sheet joints in a variety of stainless steels and nickel alloys. WARNING All hydrogen mixtures are potentially flammable and explosive. Mixtures above 5% may require special procedures and equipment. 7.1.2.5 Nitrogen (AWS classification SG-N). Nitrogen is nontoxic and for all practical purposes it can be considered chemically inert at room temperature. Nitrogen is a colorless, odorless and tasteless gas. The density is 1.2506 g/L. AWS classification SG-N is defined as having a minimum purity of 99.9% in the gaseous state. Much higher arc temperatures are available when using nitrogen, particularly for automatic welding. Copper, duplex stainless steels and other alloys are sometimes welded with nitrogen or nitrogen mixtures with either or both argon and helium. Nitrogen/argon mixtures provide higher arc stability and easier arc starting. To overcome arc-starting problems in nitrogen, the arc is sometimes started in argon and then switched to nitrogen or nitrogen mixtures. In practice, the tungsten electrodes erode faster in nitrogen.

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Figure 38—Improved Surface Cleanliness on Monel Welds Produced with the 5% Hydrogen Mixture in Argon Shielding Gas with GTAW

AWS C5.5/C5.5M:2003

Small amounts (1%–5%) of nitrogen are sometimes mixed with argon and other inert gases to improve welding speeds or penetration. Excessive nitrogen can cause porosity and change the properties of the weldments. Therefore, user testing should be performed to assure an acceptable weld is made when nitrogen mixtures are applied. The most extensive use of nitrogen has been in Europe.

Table 14 Typical Argon Flow Rates Gas Flow Rate Range

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Welding of austenitic stainless steels with nitrogencontaining shielding gas may reduce weld ferrite content and increase susceptibility to hot cracking. This is because nitrogen enhances formation of austenite. Care should always be exercised when using nitrogencontaining gases to ensure that detrimental metallurgical effects do not occur.

Current Range (A)

(SCFH)

[SLPM]

5–50 50–150 150–350 350–450

7–10 10–20 15–25 20–30

3–5 5–10 7–12 9–14

screens to prevent interference by the wind or draft are preferred to an increase in the flow of the shielding gas.

7.1.3 Selection of Shielding Gas. No set rule governs the choice of shielding gas for any particular application. Either argon or helium, or a mixture of argon and helium may be used successfully for most applications, with the possible exception of manual welding on extremely thin material, for which argon is essential. Argon generally provides an arc that operates more smoothly and quietly, is handled more easily, and is less penetrating than an arc shielded by helium. A guide to the selection of gases is provided in Tables 8 and 13.

Turbulence in the gas flow system can cause instabilities in the welding arc, which can introduce defects into the weld. Sharp bends, sharp edges and massive volume changes in the gas supply system may cause turbulence in the gas flow. A good rule of thumb is that the diameters of the hoses or pipes should provide approximately a 5:1 reduction ratio (to the torch inside diameter) to assure a laminar (nonturbulent) flow. Simply increasing flow rates may cause more turbulence.

7.1.4 Shielding Gas Flow Rates. Shielding gas flow rates are based on the type of gas, gas nozzle (cup) size, weld pool size, and ambient air movement. In general, the flow rate increases in proportion to the crosssectional area of the gas nozzle. The gas nozzle diameter is selected to suit the size of the weld pool and the reactivity of the metal to be welded. The minimum flow rate is determined by the need for a stiff stream of shielding gas to overcome the heating effects of the arc and local cross drafts. With the more commonly used manual torches, typical shielding gas flow rates are 10 SCFH– 25 SCFH70 [5 SLPM–12 SLPM] for argon and 25 SCFH–45 SCFH [12 SLPM–21 SLPM] for helium. Excessive flow rates cause turbulence in the gas stream, which may aspirate atmospheric contamination into the weld pool. Typical argon flow rates for various current levels are listed in Table 14. A cross wind or draft of approximately 5 mph [8 km/hr] and sometimes less can disrupt the shielding gas coverage. The stiffest, nonturbulent gas streams (with high flow velocities) are obtained by incorporating gas lenses in the nozzle (cup) and by using argon as the shielding gas. Figure 24 illustrates similar gas flow rates in torches without a gas lens (left) and with a gas lens (right). Windbreaks or protective

7.1.5 Lack of Shielding. Discontinuities related to the loss of inert gas shielding are tungsten inclusions,71 porosity, oxide films and inclusions, incomplete fusion, and cracking. The extent to which they occur is strongly related to the characteristic of the metal being welded. In addition, the mechanical properties of titanium, aluminum, nickel, and high-strength alloys can be seriously impaired with loss of inert gas shielding. Gas shielding effectiveness can often be evaluated prior to production welding by making a spot weld and continuing gas flow until the weld has cooled to a low temperature. A bright, silvery spot will be evident if shielding is effective. Figure 39 shows effects of shielding gas contamination on titanium weldments. 7.2 Purging 7.2.1 Purpose of Purging. The purpose of purging is to replace unwanted air and other vapor contaminants from the backside of the weld root by a gas that prevents oxidation during welding (see Figure 40).72 Oxidation can produce a variety of problems such as root oxidation (sugaring), incomplete fusion, porosity and changes in 71. AWS Welding Handbook, 8th Edition, Volume 2: p. 102. 72. See AWS D18.2:1999, Guide to Weld Discoloration Levels on Inside of Austenitic Stainless Steel Tube.

70. SCFH = standard cubic feet per hour; SLPM = standard liters per minute; SCFM = standard cubic feet per minute.

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AWS C5.5/C5.5M:2003

AWS C5.5/C5.5M:2003

DISCOLORATION FROM CHILL BLOCKS IS EXPECTED AND IS ACCEPTABLE

1. ACCEPTABLE—NO DISCOLORATION IN WELD OR HEAT-AFFECTED ZONE

2. NOT ACCEPTABLE—WHITE DEPOSITS IN WELDS (ARROWS)

3. NOT ACCEPTABLE—BLUE COLOR IN WELD AND HEAT-AFFECTED ZONE (ARROW)

4. NOT ACCEPTABLE—BLUE COLOR THROUGHOUT HEAT-AFFECTED ZONE

5. NOT ACCEPTABLE—SEVERE DISCOLORATION

Figure 39—The Effects of Shielding Gas Contamination on Titanium Weldments (Color Chart for Titanium Welding Acceptance)

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AWS C5.5/C5.5M:2003

Combinations of argon and helium are used in selected applications. Nitrogen is not a chemically inert gas but has been used with success in purging austenitic stainless steel, carbon steel, copper and low alloy steel. Nitrogen should not be used to purge reactive alloys.

weld metal chemical composition, which can affect the weld metal’s mechanical and corrosion properties. Purging is recommended when welding stainless steel, nickel alloys, and most nonferrous base metals. Purging is usually not required when welding aluminum, carbon steels and some low alloy steels. Purging is beneficial in reducing scaling when welding carbon steel hydraulic lines. The tube shown in Figure 40 was prepared by making 10 full-penetration autogenous welds on the outside diameter of a 2 in. [50 mm] 316L stainless steel tube. Welds on 304L tubing showed no significant difference in heat tint from 316L. The torch shielding gas was 95% argon, 5% hydrogen (with