• Author / Uploaded
• sarvesh

• Categories
• Soldadura
• Corriente eléctrica
• Materiales
• Física Aplicada e Interdisciplinaria
• Química

Citation preview

Savitribai Phule Pune University

1. INTRODUCTION Resistance welding is a thermo-electric process in which heat is generated at the interface of the parts to be joined by passing an electrical current through the parts for a precisely controlled time and under a controlled pressure (also called force). The name “resistance” welding derives from the fact that the resistance of the work pieces and electrodes are used in combination or contrast to generate the heat at their interface. Resistance welding is a fairly simple heat generation process; the passage of current through a resistance generates heat [9]. This is the same principle used in the operation of heating coils. In addition to the bulk resistances, the contact resistances also play a major role. The contact resistances are influenced by the surface condition (surface roughness, cleanliness, oxidation, and platings).

Bulk resistance is a function of temperature. All metals exhibit a Positive Temperature Coefficient (PTC), which means that their bulk resistance increases with temperature. Bulk resistance becomes a factor in longer welds. Contact resistance is a function of the extent to which two surfaces mate intimately or come in contact. Contact resistance is an important factor in the first few milliseconds of a weld. The surfaces of metal are quite rough if they are examined on a molecular scale. When the metals are forced together with a relatively small amount of force, some of the peaks make contact. On those peaks where the contact pressure is sufficiently high, the oxide layer breaks, forming a limited number of metal-to-metal bridges. The weld current is distributed over a large area as it passes through the bulk metal. However, as it approaches the interface, the current is forced to flow through these metallic bridges. This “necking down” increases the current density, generating enough heat to cause melting. As the first of these bridges melt and collapse, new peaks come into contact, forming new bridges and additional current paths. The resistance of the molten metal is higher than that of the new bridges so that the current flow transfers from bridge-tobridge. This process continues until the entire interface is molten. When the current stops, the molten metal cools rapidly thus forming a weld [9].

Key advantages of the resistance welding process include: 

Very short process time.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 1

Savitribai Phule Pune University



No consumables, such as brazing materials, solder, or welding rods.



Operator safety because of low voltage.



Clean and environmentally friendly.



A reliable electro-mechanical joint is formed.

Fig. 1.1 Resistance Welding

1.1 Types of Resistance Welding: Depending on the shape of the work pieces and the form of the electrodes, resistance welding processes can be classified into several variants as described below: 

Resistance Spot Welding (RSW)



Resistance Seam Welding



Resistance Projection Welding



Resistance Butt Welding



Percussion Welding



High Frequency Resistance Welding



Low Frequency Resistance Welding



Resistance Weld Bonding

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 2

Savitribai Phule Pune University

1.2 Resistance Spot Welding: Spot welding is one form of resistance welding, which is a method of welding two or more metal sheets together without using any filler material by applying pressure and heat to the area to be welded. The process is used for joining sheet materials and uses shaped copper alloy electrodes to apply pressure and convey the electrical current through the work pieces. In all forms of resistance welding, the parts are locally heated. The material between the electrodes yields and is squeezed together. It then melts, destroying the interface between the parts. The current is switched off and the "nugget" of molten materials solidifies forming the joint [2].

Fig. 1.2.1 Resistance Spot welding

The principle of resistance welding is based on Joule heating. The work pieces are clamped between the electrodes by applying an electrode force, then an electric current passes through the top and bottom electrodes and heats the work pieces by Joule heating. When the temperature at the interface reaches the melting point of the material, a molten nugget begins to form and grow. When the welding current is switched off, this nugget will solidify to form a weld that joins the work pieces together. Figure 1.2.2 shows the general setup and procedure of the resistance spot welding with the two electrodes. The process is used extensively for joining low and mild carbon steel sheet metal components for automobiles, cabinets, furniture and similar products. Stainless steel, aluminium and copper alloys are also spot welded commercially. Excessive heating in resistance welding results in metal expulsion

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 3

Savitribai Phule Pune University

during the welding operation. Since accurate method for selection of welding variables i.e. welding current, welding time and electrode force, thickness of sheet, electrode type, electrode tip diameter, gap in the electrodes, shape of electrode tip, electrode material etc. are lacking [1].

There are various factors involves resistance spot welding process which are responsible for the quality of weld. Depending on the thickness and type of the metal, welding conditions such as weld current, weld time, electrode type and electrode force should be adjusted [10].

Fig. 1.2.2 General Setup of Resistance Spot Welding

Generally, a weld cycle can be divided into a number of stages: 

Pre-squeeze stage (1 to 2)



Squeeze stage (2 to 3)

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 4

Savitribai Phule Pune University



Weld stage (3 to 4)



Hold stage (4 to 5)



Release stage (5 to 6)

In the Pre-squeeze stage, the moving electrode closes towards the work pieces, resulting in contact between the electrodes and work pieces. The velocity of the electrodes at this point of impact is very important considering that too high impact energy will results in excessive electrode wear [8].

In the squeeze stage, the electrodes are forced on the work piece surfaces by means of the force actuating system and this offers the possibility to deform the work piece to ensure sound contact between them.

Fig. 1.2.3 Welding cycle of RSW process

In the weld stage, while force remains on the work pieces, current will be flowing through the electrodes and the work piece, heating up every part in the secondary weld circuit proportional to the effective resistance present at each point.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 5

Savitribai Phule Pune University

In the hold stage, the current is cut off and produced weld is allowed to solidify and cool down under maintain pressurizing force. After sufficient holding time, the electrodes release the work piece and the cycle is complete.

The major advantages of the resistance spot welding processes over any other welding process is the feature that heat necessary for weld formation is generated at the exact location where the joints needs to appear. The possibility to highly reduce the time to complete a weld resulting in cycle times being competitive over other welding process. The another advantage is the absence of a molten weld pool penetrating from one side through a work piece, resulting in less aesthetical damage to the work piece surfaces [8].

1.2.1

Resistance Spot Welding Parameters:

a. Electrode Force: The purpose of the electrode force is to squeeze the metal sheets to be joined together. This requires a large electrode force because else the weld quality will not be good enough. However, the force must not be too large as it might cause other problems. When the electrode force is increased the heat energy will decrease. This means that the higher electrode force requires a higher weld current. When weld current becomes too high spatter will occur between electrodes and sheets. This will cause the electrodes to get stuck to the sheet. An adequate target value for the electrode force is 90 N/mm2. One problem, though, is that the size of the contact surface will increase during welding. To keep the same conditions during the whole welding process, the electrode force needs to be gradually increased. As it is rather difficult to change the electrode force in the same rate as the electrodes are "mushroomed", usually an average value is chosen [9]. b. Diameter of the electrode contact surface: One general criterion of resistance spot-welding is that the weld shall have a nugget diameter of 5*t1/2, “t” being the thickness of the steel sheet. Thus, a spot weld made in

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 6

Savitribai Phule Pune University

two sheets, each 1 mm in thickness, would generate a nugget 5 mm in diameter according to the 5*t½-rule. Diameter of the electrode contact surface should be slightly larger than the nugget diameter. For example, spot welding two sheets of 1 mm thickness would require an electrode with a contact diameter of 6 mm. In practice, an electrode with a contact diameter of 6 mm is standard for sheet thickness of 0.5 to 1.25 mm. This contact diameter of 6 mm conforms to the ISO standard for new electrodes [6]. c. Squeeze time: Squeeze Time is the time interval between the initial application of the electrode force on the work and the first application of current. Squeeze time is necessary to delay the weld current until the electrode force has attained the desired level [8]. d. Weld time: The rate of heat generation must be such that welds with adequate strength will be produced without excessive electrode heating and rapid deterioration. The total heat developed is proportional to weld time. During a spot welding operation, some minimum time is required to reach melting temperature at some suitable current density.

Fig. 1.2.1.1 Tensile-shear strength as a function of weld time

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 7

Savitribai Phule Pune University

Excessively long weld time will have the same effect as excessive amperage on the base metal and electrodes. Furthermore, the weld heat-affected zone will extend farther into the base metal. The relationship between weld time and spot weld shear strength is shown in Fig. 1.2.1.1 assuming all other conditions remain constant. To a certain extent, weld time and amperage may be complementary. The total heat may be changed by adjusting either the amperage or the weld time. Heat transfer is a function of time and the development of the proper nugget size requires a minimum length of time, regardless of amperage [4]. e. Hold time (cooling-time): Hold time is necessary to allow the weld nugget to solidify before releasing the welded parts, but it must not be too long as this may cause the heat in the weld spot to spread to the electrode and heat it. The electrode will then get more exposed to wear. Further, if the hold time is too long and the carbon content of the material is high (more than 0.1%), there is a risk the weld will become brittle [9]. f. Weld current: The weld current is the current in the welding circuit during the making of a weld. The amount of weld current is controlled by two things; first, the setting of the transformer tap switch determines the maximum amount of weld current available; second the percent of current control determines the percent of the available current to be used for making the weld. Low percent current settings are not normally recommended as this may impair the quality of the weld. Tensile shear strength increases rapidly with increasing current density. Excessive current density will cause molten metal expulsion (resulting in internal voids), weld cracking, and lower mechanical strength properties.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 8

Savitribai Phule Pune University

Fig. 1.2.1.2 Effect of welding current on spot weld shear strength Typical variations in shear strength of spot welds as a function of current magnitude are shown in Fig. 1.2.1.2. In the case of spot welding excessive current will overheat the base metal and result in deep indentations in the parts and, it will cause overheating and rapid deterioration of the electrodes [3]. g. Contact Resistance: The contact resistance at the weld interface is the most influential parameter related to materials. It however has highly dynamic interaction with the process parameters. The figure below shows the measured contact resistance of mild steel at different temperatures and different pressures. It is noticed that the contact resistance generally decreases with increasing temperature but has a local ridge around 300°C, and it decreases almost proportionally with increasing pressure. All metals have rough surfaces in micro scale. When the welding force increases, the contact pressure increases thereby the real contact area at the interface increases due to deformation of the rough surface asperities. Therefore the contact resistance at the interface decreases which reduces the heat generation and the size of weld nugget. On the metal surfaces, there are also oxides, water vapour, oil, dirt and other contaminants. When the temperature increases, some of the surface contaminants (mainly water and oil based ones) will be

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 9

Savitribai Phule Pune University

burned off in the first couple of cycles, and the metals will also be softened at high temperatures. Thus the contact resistance generally decreases with increasing temperature. Even though the contact resistance has most significant influence only in the first couple of cycles, it has a decisive influence on the heat distribution due to the initial heat generation and distribution. h. Surface coatings: Most surface coatings are applied for protection of corrosion or as a substrate for further surface treatment. These surface coatings often complicate the welding process. Special process parameter adjustments have to be made according to individual types of the surface coatings. Some surface coatings are introduced for facilitating the welding of difficult material combinations. These surface coatings are strategically selected to bring the heat balance to the weld interface. Most of the surface coatings will be squeezed out during welding, some will remain at the weld interface as a braze metal [9].

Fig. 1.2.1.3 Cause and effect diagram of main welding parameters

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 10

Savitribai Phule Pune University

2. LITERATURE REVIEW B.D. Gurav & S.D. Ambekar presented a paper “Optimization of the Welding Parameters in Resistance Spot Welding” International Journal of Mechanical Engineering and Technology (IJMET), Volume 4, September - October (2013) [2]. They studied on the optimization and the effect of the welding parameters (welding current, welding time, & electrode force) on the tensile shear strength of the resistance spot welded joints for using CRCA (close rolled close annealing) steel sheets with 2mm thickness. For using orthogonal array of Taguchi method, the signal-to-noise ratio, the analysis of variance (ANOVA) employed to find the optimal process parameters levels and to analyse the effect of these parameters on tensile shear strength values. At last for the experimental setup and results to show that the input parameters are high current, medium electrode force and high weld time. The welding current is the most effective factor in spot welding process. The contribution of welding current, weld time and electrode force towards tensile strength is 49.72%, 42.19%, and 7.85% respectively as determined by the ANOVA method. Norasiah Muhammad and Yupiter HP Manurung presented a paper “Design parameters selection and Optimization of weld zone development in Resistance Spot Welding” World Academy of Science, Engineering and Technology 71 2012 [4]. They have been investigates the development of weld zone in Resistance Spot Welding. The work piece material used in this study was 1.21mm thick coated low carbon steel as the base metal. A general 24 factorial design augmented by 5 centre points was used to study the effect of factors namely weld current, weld time, electrode force and hold time on the development of weld zone. Optimization of the significant parameters affecting the development of weld zone obtained from the factorial design was carried out using CCD (Central Composite Design) in Response Surface Method. An experimental design was used to determine the effects of welding parameters (weld current, weld time, electrode force and hold time) on the development of the weld zone. On the results define that all the selected factors expect hold time affected the radius of weld nugget and HAZ (Heat Affected Zone) significantly and optimized using the Central Composite Design by RSM. A quadratic model of for radius of weld nugget and radius of HAZ were developed. The experimental results obtained under

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 11

Savitribai Phule Pune University

optimum operating conditions were compared with the predicted values and found to agree satisfactorily with each other. D.S.Sahota, Ramandeep Singh, Rajesh Sharma published a paper “Study of Effect of Parameters on Resistance Spot Weld of ASS316 Material” ISSN: 2320-2491 Vol. 2, No. 2, February-March 2013 [1]. The objective of this work was to study the effect of parameters on resistance spot weld of ASS316 material. In order to study the significance of the process parameters i.e. current, electrode force and weld cycles, towards the percentage improvement in material hardness. It was clear from the results that parameters significantly affect both the mean and the variation in the percentage improvement in Hardness values of the ASS316 material. The S/N ratio analysis suggests third level of weld current, third level of weld cycle and third level of electrode force as the best levels for maximum percentage improvement in hardness of ASS316 work-piece in spot welding operation. Aravinthan Arumugam, Mohd. Amizi Nor presented a paper “Spot Welding Parameter Optimization to Improve Weld Characteristics for Dissimilar Metals” International journal of scientific & technology research volume 4, issue 01, January 2015 [3]. This work concentrates on the parameter optimization when spot welding steels with dissimilar thickness and type using Grey Based Taguchi Method. The experimentation in this work used a L9 orthogonal array with three factors with each factor having three levels. The three factors used are welding current, weld time and electrode force. The three weld characteristics that were optimized are weld strength, weld nugget diameter and weld indentation. The analysis of variance (ANOVA) that was carried out showed that welding current gave the most significant contribution in the optimum welding schedule. The comparison test that was carried out to compare the current welding schedule and the optimum welding schedule showed distinct improvement in the increase of weld diameter and weld strength as well as decrease in electrode indentation. J.B. Shamsul and M.M. Hisyam presented a paper “Study of Spot Welding of Austenitic Stainless Steel Type 304” School of Materials Engineering, University Malaysia Perlis, Journal of Applied Sciences Research, 2007, INSI net Publication [5].

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 12

Savitribai Phule Pune University

On this study paper, austenitic stainless steel type 304 was welded by resistance spot welding. It was investigated the relationship between of the nugget diameter and welding current and also Hardness distribution along welding zone. They used Austenitic SS304 of 3mm thickness. The experimental result showed that the changes of the current, the nugget diameter were changed. So it is proven that the weld nugget increases with the increasing of welding current. Micro hardness testing was used to carried out on the cross section samples across the horizontal & vertical of the nugget the graphs were studied to show that the weld nuggets due to the varied welding current. Austenitic stainless steel AISI-304 is an extremely important commercial alloy due to its excellent corrosion resistance, high strength, good ductility and toughness. The results showed that increasing welding current increased the nugget size. The nugget size did not influence the hardness distribution. In addition, increasing welding current does not increase the hardness distribution. K. Pandey, M. I. Khan, K. M. Moeed published a paper “Optimization of Resistance Spot Welding Parameters Using Taguchi Method” Integral University, Lucknow, India. International Journal of Engineering Science and Technology (IJEST), ISSN: 0975-5462 Vol. 5 No.02 February 2013 [6]. In this research they have represented the optimization of various process parameters of resistance spot welding process. The material used is low carbon cold rolled 0.9mm mild steel sheets (AISI 1008/ASTM A366). For the experimental setup and investigation of varying process parameters (welding current, welding pressure, and welding time) to effect of the quality characteristic (tensile strength) using Taguchi Method. An experimental result showed that, S/N ratio to tensile strength indicates the welding current to be the most significant parameter that controls the weld tensile strength where the holding time and pressure are less. The contribution of Welding current, holding time and Pressure towards tensile strength was 61%, 28.7%, and 4% determined by the ANNOVA method. Manoj Raut and Vishal Achwal presented a paper “Optimization of Spot Welding Process Parameters for Maximum Tensile Strength” International Journal of Mechanical Engineering & Robotics Research ISSN 2278 – 0149 Vol. 3, No. 4, October 2014 [7]. This experimental study was based on an investigation of the effect

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 13

Savitribai Phule Pune University

and optimization of welding parameters on the tensile shear strength in the Resistance Spot Welding (RSW) process. The experimental studies were conducted under varying electrode forces, welding currents, and welding times. The settings of welding parameters were determined by using the Taguchi experimental design of L18 Orthogonal array method. The combination of the optimum welding parameters have determined by using the analysis of Signal-to-Noise (S/N) ratio. The confirmation test performed clearly showed that it is possible to increase the tensile shear strength of the joint by the combination of the suitable welding parameters. Hence, the experimental results confirmed the validity of the used Taguchi method for enhancing the welding performance and optimizing the welding parameters in resistance spot welding operations.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 14

Savitribai Phule Pune University

3. PROBLEM STATEMENT In electrical industries Resistance Spot Welding process plays an important role in joining the sheet metals. There is a great demand for obtaining good quality weld joint in order to have defect free product. Distribution box is made up of different sheet metal thickness, which requires process parameters to be varied. Therefore following are the problems associated with the optimization of spot welding:

Tendency of alloying with the electrode resulting in increased tool wear, and subsequent deterioration of weld quality.



More current and time lead to expulsion and overheating of the electrode affecting the weld quality and less value result in insufficient weld strength.



Weld time should be as short as possible.



The weld current should give the best weld quality as possible.



The weld parameters should be chosen to give as little wearing of the electrodes as possible.



The weld time shall cause the nugget diameter to be big when welding thick sheets.



Diameter of the electrode contact surface should be slightly larger than the nugget diameter.



Interfacial mode (or nugget fracture) is the fracture of the weld nugget through the plane of the weld; the dominant failure mode for small diameter spot welds.



Nugget pull-out mode (or sheet fracture) is the fracture of the sheet around the weld; the nugget remains intact; dominant for large diameter spot welds.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 15

Savitribai Phule Pune University



Excessive heat in the electrodes reduces the electrode cap life and deteriorates the weld quality.

Fig. 3.1 Problem of Weld open in VTPN DB

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 16

Savitribai Phule Pune University

4. OBJECTIVE & SCOPE OF THE PROJECT 4.1 Objectives: This project work is intended for the following objectives: i.

To study the basics of Resistance Welding Processes.

ii.

To Study Heat Generation & Control.

iii.

To Study Spot Welding Process and its Application.

iv.

To study the optimization technique by using Minitab.

v.

Requirement for quality assurance.

vi.

Validation of experiment using optimization to standardize process parameter.

vii.

To enhance the weld strength in VTPN DB.

4.2 Scope: The basic aim of the project work is to optimize resistance spot welding process parameters to avoid mechanical failure in VTPN DB. The Scope of this work is to: i.

Find the optimum parameters to optimize the size of weld nugget and Heat Affected Zone.

ii.

Study; understand the theory and controlling of mechanical response of resistance spot dissimilar welds.

iii.

Apply Design of Experiment (DoE) methodologies to develop mathematical models for prediction of responses and optimization.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 17

Savitribai Phule Pune University

iv.

Predict, compare the developed model for optimization of various process parameters.

v.

Develop a mathematical relationship between the process input parameters and the output variables in order to optimize the input parameters that lead to the desired quality of weld.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 18

Savitribai Phule Pune University

5. METHODOLOGY

To identify the stages where failure occurs

Study existing contact spot welding process

To create a mathematical model using process parameters

Solving the equation using optimization solver

Validation of Optimization Result

Optimization

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 19

Savitribai Phule Pune University

6. CONCLUSION i.

Resistance Spot welding is quick and easy process, no need to use any fluxes or filler metal to create a join and also there is no dangerous open flame.

ii.

Resistance Spot welding process mainly depends on process parameters (e.g. welding current, weld time, electrode force), material parameters (e.g. types, thickness).

iii.

The most of the parametric study has been done by considering the three process parameters such as weld time, applied force and squeeze time and weld quality has been evaluated with respect to the individual properties such as tensile shear strength, tensile peel strength etc.

iv.

The literature review infers that there is a need to study the effects of all process parameters on the weld quality and weld strength in detail. At the some of the research papers concluded that current is the major factor to affect of the weld quality and weld strength by increasing/decreasing others parameters.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 20

Savitribai Phule Pune University

REFERENCES 1. D.S.Sahota, Ramandeep Singh, Rajesh Sharma, “Study of Effect of Parameters on Resistance Spot Weld of ASS316 Material”, ISSN: 2320-2491, Vol. 2, No. 2, pp.68- 77, February-March 2013. 2. B.D.Gurav & S.D.Ambekar “Optimization of the Welding Parameters in Resistance Spot Welding” International Journal of Mechanical Engineering and Technology (IJMET), Volume 4, September - October (2013). 3. Aravinthan Arumugam, Mohd. Amizi Nor “Spot Welding Parameter Optimization to Improve Weld Characteristics for Dissimilar Metals” International journal of scientific & technology research volume 4, issue 01, January 2015. 4. Norasiah Muhammad and Yupiter HP Manurung “Design parameters selection and Optimization of weld zone development in Resistance Spot Welding” World Academy of Science, Engineering and Technology 71 2012. 5. J.B. Shamsul and M.M. Hisyam “Study of Spot Welding of Austenitic Stainless Steel Type 304” School of Materials Engineering, University Malaysia Perlis, Journal of Applied Sciences Research, 2007, INSI net Publication. 6. K. Pandey, M. I. Khan, K. M. Moeed “Optimization of Resistance Spot Welding Parameters Using Taguchi Method” Integral University, Lucknow, India. International Journal of Engineering Science and Technology (IJEST), ISSN: 09755462 Vol. 5 No.02 February 2013. 7. Manoj Raut and Vishal Achwal presented a paper “Optimization of Spot Welding Process Parameters for Maximum Tensile Strength” International Journal of Mechanical Engineering & Robotics Research ISSN 2278 – 0149 Vol. 3, No. 4, October 2014. 8. B. S. Gawai and Dr. C. M. Sedani “Optimization of Process Parameters for Resistance Spot Welding Process of HR E-34 Using Response Surface Method” International Journal of Science and Research (IJSR) Volume 5 Issue 3, March 2016.

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 21

Savitribai Phule Pune University

9. Fundamentals of Small Parts Resistance Welding- AMADA Miyachi Co., Ltd. 10.A.S.Panchakshari, Dr.M.S.Kadam “Comparative Study of Responses of Resistance Spot Welding Obtained From Genetic Algorithm, Response Surface and DOptimal Method” International Journal of Engineering Science and Innovative Technology (IJESIT) Volume 2, Issue4, July 2013, ISSN: 2319-5967).

N.D.M.V.P’s K.B.T COE, B.E (Mechanical) 22