extrustion

8/13/2019 EXTRUSTION

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Extrusion and wire drawing of

metals

December 8-15, 2010

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Extrusion

A plastic deformation process in which a billet of certain

material is formed by forcing it to flow through a die of

the required geometrical cross-section.

Advantages

Wide variety of shapes

High production rates Close tolerances are possible

Economical

Design flexibility

Extrusion process can be carried out hot and cold.

Complicated cross-sectional shape can be produced by

extrusion.

In general, extrusion is used to produce long parts of

uniform cross sections


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Types of Extrusion

Direct extrusion

Metal is pushed through the die.


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Types of Extrusion Indirect extrusion: Die is pushed into the metal

Impact extrusion: is indirect extrusion but used for hollow shapes

ydrostatic

Impact


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Hydrostatic Extrusion Hydrostatic extrusion: chamber is filled with incopressible fluid to transmit the pressure to the

billet, there is no friction along the container walls, and the fluid provides triaxial compressive

stresses on the workpiece providing improved formability

Usually carried at room temperature, typically using vegetable oils as the fluid

Brittle materials are extruded generally by this method

It increases ductility of the material

It has complex nature of the tooling

General view of a 9-MN (1000-ton) hydraulic-extrusion press.5


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a. The most homogeneous flow pattern is obtained when there is no friction at the

billet-container-die interfaces. This happens if the lubricant is very effective or with

indirect extrusion.

b. When friction along all interfaces is high, a dead-metal zone develops . Note thehigh-shear area as the material flows into the die exit, somewhat like a funnel. This

configuration may indicate that the billet surfaces (with their oxide layer and

lubricant) could enter this high-shear zone and be extruded, causing defects in the

extruded product.

c. The high-shear zone extends farther back. This extension can result from highcontainer-wall friction, which retards the flow of the billet, or materials in which

the flow stress drops rapidly with increasing temperature. In hot working, the

material near the container walls cools rapidly and hence increases in strength.

Thus the material in the central regions flows toward the die more easily than that

at the outer regions. As a result, a large dead-metal zone forms and the flow is

inhomogeneous. This flow pattern leads to a defect known as a pipe or extrusion

defect.

Metal flow in extrusion


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Metal flow in extrusion

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P h f P h di l


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Punch force Punch displacement

1 2

Punch displacement

C

B

A

Point A: Elastic Limit.

Point B: Maximum Extrusion Force.

Point C: Start of piping

Note: Pont C > Elastic Limit (Point A)

Start of extrusion

Punch

load

(F) Phase I: Coining phase

The plastic deformation (flow) of thework piece to fill the cavity in thecontainer.

Phase II: Steady state phase

Due to reduction of the billet length, thefriction force between the billet and

the container is decreased, thus, theforce is reduced

Phase III: Non steady state phase

- Region 1: Non steady state withoutpipe defect.

- Region 2: Non steady state with pipedefect.


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Factors affecting extrusion pressure

Extrusion ratio is defined as the ratio of billet (initial) area to

final area. If redundant work is neglected, the absolute valueof true strain is = ln(Ao/Af ). Thus, the extrusion ratio affects

the extrusion force directly in an ideal situation.

Die geometry has an effect related to material flow and, thus,

contributes to the redundant work of deformation.

Extrusion speed has an effect because at elevated

temperatures the flow stress will increase with increasing

strain rate, depending on the strain-rate sensitivity of the

workpiece material.

Higher temperatures lower the yield stress and thus, reduce

forces.

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Extrusion Defects

1. Surface cracking.

2. Pipe.

3. Internal Cracking

Fig : (a) Chevron cracking (central burst) in extruded round steel bars. Unless the products are inspected, such internaldefects may remain undetected, and later cause failure of the part in service. This defect can also develop in thedrawing of rod, of wire, and of tubes. (b) Schematic illustration of rigid and plastic zones in extrusion. Thetendency toward chevron cracking increases if the two plastic zones do not meet. Note that the plastic zone can bemade larger either by decreasing the die angel or by increasing the reduction in cross-section (or both).

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Mechanics of Extrusion:

Ideal deformation and Frictionless

Extrusion ratio R = A0/Af

Total strain el = Ln(A0/Af) = Ln (lf/l0) = Ln(R)

Strain energy, u = Y el - (Perfectly Plastic

Material)

Work = u.Volume = u.A0l0

Work = Fl0 = P.A0l0 , P is extrusion pressure

P = u=Y ln(R) (Siebel Formula)

For strain hardening material Y should be

replaced by the average flow stressYf


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Example 1:

Calculate the force required in direct extrusion of 1100-O

aluminum from a diameter of 6 in. to 2 in. Assume that

the redundant work is 30% of the ideal work of

deformation, and the friction work is 25% of the totalwork of deformation.

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Example 2:

A planned extrusion operation involves steel at 800C, with

an initial diameter of 100 mm and a final diameter of 20

mm. Two presses, one with a capacity of 20 MN and the

other of 10 MN, are available for this operation.

the larger press requires greater care and more expensive

tooling. Is the smaller press sufficient for this operation? If

not, what recommendations would you make to allow the

use of the smaller press?

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