• Author / Uploaded
• Aravindhan Anbalagan

Citation preview

MDB3073 Manufacturing Technology 2

Metal Forming- Extrusion and Drawing of Metals

Open

Extrusion: Cylindrical billet is forced through die

Open

Schematic illustration of the direct-extrusion process.

Extrusions and examples of products made by sectioning off extrusions. Open

Types of extrusion: (a) indirect; (also called reverse/ inverted / backward extrusion) The die moves toward the extruded billet

(b) hydrostatic; The pressure is transmitted to the fluid by a ram

(c) Lateral or side extrusion

Open

Process variables in direct extrusion: the die angle, reduction in cross-section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure.

Extrusion ration R = Ao/ Af Open

Extrusion Force The force required for extrusion depends on • Strength of the billet material, extrusion ratio, friction and process variables • For small die angle α, the extrusion pressure can be approximated as p = Y(1+ tan α /μ)(Rμ cot α -1) Where μ coeff. Friction, Y is yield stress

The extrusion force F = Ao k ln(Ao/Af) Where k is extrusion constant (experimental) Open

Example • Calculate the extrusion force for a round billet 250 mm in diameter, made of 304 stainless steel, and extruded at 10000 C to a diameter of 70 mm. From Fig., k for stainless steel at 1000 C is approximately 400 MPa. The extrusion ratio is we have F = Ao k ln (Ao/Af) = ……….. ……

F = 50 MN

Open

Rod, Wire, and Tube Drawing • • •

Open

Drawing: the cross-section of a rod or wire is reduced or changed in shape by pulling through a die. Rod and wire drawing are finishing processes and is further processed into other shapes. Applications such as electrical wiring, electronic equipment, cables and springs,

Drawing Dies

Terminology of a typical die used for drawing a round rod or wire.

Open

Tungsten-carbide die insert in a steel casing. Diamond dies used in drawing thin wire are encased in a similar manner.

Mechanics of rod and wire drawing 1. Ideal and frictionless condition  A0  • The drawing force is given by F  YA f ln A f    Where Y⁻ is average true stress of the material in die gap • The drawing force increases with increasing friction. 2. The Drawing force which includes frictional force is Ao 2   F  Y avg A f 1   Ln( )   Af 3   • Where α is die angle in radians

Open

Example : Power required and die pressure in rod drawing A round rod of annealed 302 stainless steel is being drawn from a diameter of 10 mm to 8 mm at a speed of 0.5 m/s. Assume that the frictional and redundant work together constitute 40% of the ideal work of deformation. (a) Calculate the power required in this operation, and (b) calculate the die pressure at the exit of the die.

Solution 2  10 True strain is 1  ln    0.446  2  8 

With K=1300 MPa and n=0.30 (For SS 302):

K1n Y   785 MPa n 1

a) Thus, F  YAf lnA0 / Af   0.0175 MN and Power  FV f  0.01750.5  8.75 kW Actual power will be 40% higher  Actual  1.48.75  12.25 kW Open

Power required and die pressure in rod drawing Solution Flow stress of the material, Y f  K1n  13000.446

0.30

Drawing stress, d 

 1020 MPa

F 1.40.0175   490 MPa Af 0.00005

b) Thus for die pressure, p  Y f   d  1020  490  530 MPa

Open

Mechanics of rod and wire drawing Optimum die angle • Optimum die angle depends on ideal, friction, and redundant work. Maximum reduction per pass • As reduction increases, the drawing stress increases. The max. reduction per pass (without friction) Max reduction per pass 

Open

A0  A f A0

 1

1  0.63  63% e

Defects in drawing Major defects: Center cracking

Tendency for center cracking increases with: 1. Increasing die angle 2. Decreasing reduction per pass 3. Friction 4. Presence of inclusions in the material.

Open

Chevron cracking (central burst) in extruded round steel bars; unless the products are inspected, such internal defects may remain undetected and later cause failure of the part in service; this defect can also develop in the drawing of rod, wire, and tubes

Open

Multistage Wire-drawing Machine

A multistage wire-drawing machine that typically is used in the making of copper wire for electrical wiring. Source: After H. Auerswald Open