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Mechanical behavior of aluminum matrix composite during extrusion in the semisolid state

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Title: Mechanical behavior of aluminum matrix composite during extrusion in the semisolid state


1
Mechanical behavior of aluminum matrix composite
during extrusion in the semisolid state
S Turenne N Legros S Laplante F
Ajersch Metallurgical and Materials Transactions
Apr 1999 30A, 4 Research Library pg. 1137
Brad Smith ME 462-1
14 September 2009
2
Objective
The objective of this study is to compare the
behavior of rheocast with as-cast metal matrix
composites during semisolid extrusion.
As Cast Referring to metal which has not received
finishing (beyond gate removal or sandblasting)
or treatment of any kind including heat treatment
after casting.
Semi-solid metal casting (SSM), also known as
thixocasting, rheocasting, thixoforming or
thixomolding, is a near net shape process in the
production of parts out of non-ferrous metals.
Simply, thixotropic fluids shear when the
material flows, but thicken when standing.
3
Importance
Semisolid processing appears to be an interesting
alternative for forming metal matrix composite
materials because it does not result in fracture,
lead to severe wear of the tools, or require high
forming pressure as solid-state processing. It
also greatly reduces processing costs. By
comparing the two materials (as-cast and
rheoprocessed) during semi-solid extrusion, the
authors sought to determine the differences in
properties and create a predictive model for the
extrusion forces required.
4
References
5
Models and Design Principles
A356 Aluminum alloy (6.5 to 7.5 pct Si and 0.25
to 0.45 pct Mg) reinforced with 15 vol pct of
10- to 15 µm SiC particles. Stirring in argon
atmosphere resulted in 20, 30, 40 pct solid
volume fractions.
6
Models and Design Principles
Extrusion pressure is sum of die pressure and
friction contribution Between the container and
the billet
Extrusion pressure as a function of the
characteristics of the material and geometric
parameters
7
Results
8
Results
9
Results
Flow stress F assumed to be 1 due to
sticking. Other values determined
experimentally and through material
properties. The authors referenced other sources
for some values. Constant extrusion conditions
(i.e. temperature at 580 C)
10
Results (analysis)
  • Process studied using finite element code DEFORM
    (Scientific Forming
  • Technologies Corporation)
  • Conditions
  • The stress state must be analyzed and should lead
    to the distribution
  • of stresses, strains, strain rates in the
    deformed material.
  • A relevant flow stress must be used.
  • The effect of friction forces must be assessed.
  • Solid volume fraction correction for liquid
    expulsion.

11
Results (FEM analysis)
12
Results
Semi-solid state extrusion and as-cast extrusion
forces can be accurately predicted Using FEM
analysis. Semi-solid state extrusion requires
less pressures and less severe die components.
13
Conclusions
  • Behavior of semi-solid composite only slightly
    influenced by rheocasting
  • processing temperature.
  • Extrusion forces follow similar pattern but far
    less in semi-solid extrusion.
  • Friction stresses result in intense deformation
    of primary phases near the walls.
  • Multiple linear regression leads to a
    constitutive equation that can be applied
  • to most of the experimental data.
  • 5. Very good agreement obtained between predicted
    values and experimental
  • measurements of the force.

14
Conclusions
Semi-solid extrusion more cost effective. Study
shows that it is possible to create a predictive
model of forces on aluminum. Practical
Industrial Use Absolutely, for A356 Aluminum
under these conditions. Models proprietary, so
not globally beneficial. Technical advancement
showing that FEM can be used for semi-solid
extrusion including taking into account liquid
expulsion. This paper affects industries where
extrusion is done in high volume. Semi-solid
state extrusion greatly reduces costs and can
predict forces.
15
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