EXTRUSION SHAPES

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EXTRUSION SHAPES
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DIRECT INDIRECT EXTRUSION METHODS
Direct Extrusion
Indirect Extrusion
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MECHANICS OF EXTRUSION
Reduction, R Ao/Af
Ideal work (without friction or redundant work)
is obtained by considering a simple tension test,
for which seff sapplied, and eeff e lnR.
For a non work-hardening material, with s Y
(constant), we then have
With work hardening, use an average Yavg, which
gives the same ideal work of deformation with
elnR.
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MECHANICS OF EXTRUSION
The specific work can also be calculated from the
ram pressure (p), using u (
p.Ao).Lo/(AoLo) i.e., the work done in deforming
the entire billet.
a
Equating this work to the ideal work in the
previous page, we obtain the simple result
pideal Y lnR
With friction, one obtains
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REDUNDANT WORK AND FRICTION EFFECTS
In a simple tension test, we have the following
change in grid spacing
Ideal conditions
After
Before
In actuality, we have the following grid shapes
The effect of modified grid shape requires
additional work. This additional work was
necessary in order to keep the material primarily
under compression loading. The additional work
is known as Redundant Work, and results in
additional pressure requirement.
Friction effect also increases the pressure
required for extrusion.
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MECHANICS OF EXTRUSION
Often, the die pressure is expressed through the
empirical relation
Where L and D are the length and diameter of the
billet in the die, respectively. When
lubrication is good, the last term is much
smaller.
Where a and b are estimated as follows
Because of dead zones, angles above 60 degrees
are typically approximated with a of 45 degrees.
Efficiency of deformation
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EFFECTS OF DIE ANGLE ON THE VARIOUS FORCE
PARAMETERS
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STRAIN RATE DURING EXTRUSION
At high temperature, the flow stress is largely a
function of the strain rate. Recall
Hence, strain rate is important in most extrusion
operations.
For high extrusion ratios and a 45 degrees,
this reduces to
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EXAMPLE PROBLEM
Consider extrusion of pure copper in a square die
(a90 degrees) at 1500 F. The initial diameter
and length of the billet are 5" and 10"
respectively. Assuming poor lubrication,
determine the force required to reduce the
diameter to 2" at a ram speed of 10"/sec.
RAo/Af52/226.25
Get, p 170 ksi. Therefore, force, Fp.Ao
3,338 kip
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DEFECTS IN EXTRUSION
1. Surface Cracking High friction and
stick-slip lead to fir-tree cracking. At high
temperature and high strain rates, surface
sticking occurs, and intergranular failures are
observed. At low temperatures, high friction
with stick slip can result in metal coming out in
bursts. The appearance is like bamboo tree, and
is often known as bamboo defect.
2. Oxide inclusion drawn in from surface, also
called fish-tailing
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DEFECTS IN EXTRUSION
3. Center-Burst or Chevron Cracking Caused by
accumulation of a mean tensile stress in the
center region of the billet.
High h/L may result in plastic zones not meeting.
In order to attain the same length everywhere,
the central region is put in tension. This
results in cracking.
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LUBRICATION IN EXTRUSION
Lubrication is critical during extrusion. Graphit
e and molybdenum disulfide are often used. At
high temperatures, glass is used. A circular
glass pad is used as the starting material, and
this softens up at the extrusion
temperature. For materials with a tendency to
stick, softer metal jackets of Al or Cu are used.
These jackets also prevent contamination.