| Résumé | Contaminated surfaces of the feedstock materials in aluminum alloy casting processes often produce various types of defects which can affect the tensile properties of the final products as well as their fatigue reliabilities. Semi-solid processing takes advantage of a much higher apparent viscosity of the die cast materials by limiting the risk of oxides formed at the free surfaces to become incorporated into the casting when the material is injected into the die. Most of existing semi-solid processes that use billets as feedstock material are however tied up with a different type of contaminated surface. During the injection phase, the external-skin on the periphery of the billet, which has been in contact with air and lubricant during the transfer in the shot sleeve, can be incorporated into the casting. When subjected to a heat treatment, the lubricant is decomposed and produces lens shape porosities. This might be a cause of reject for most structural parts. To avoid this kind of defects, the paths along which the billet skin evolves must be controlled during filling. In order to investigate the possibility of skin inclusion into cast parts during injection of the billet, a two-phase finite element mixture model is employed to model the metal flow. The formation of a skin on the periphery of the billet is modeled by setting an initial solid phase concentration profile in the radial direction. Microscopic observations of the real castings show that the approach is able to model the shear layers and to predict the paths along which the “lens porosity” defects could be formed. An Arbitrary Eulerian-Lagangian (ALE) method is also investigated and appears to be very promising to follow the skin movement in the casting. |
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