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

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.

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.

References

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.

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:

Results

Results

Results Flow stress Φ 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)

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.

Results (FEM analysis)

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.

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.

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.