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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