Modeling of solidification of metal-matrix particulate composites with convection
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I.
INTRODUCTION
METAL-MATRIX particulate composites (MMPCs) are an important class of materials that exhibit the desirable properties of composites such as low density, damping properties, increased fatigue resistance, and high specific strength, but also are among the least expensive composites to produce via molten metal routes.[1,2] General reviews on MMPC solidification can be found in a number of publications.[3,4,5] The macroscopic (i.e., system scale) redistribution of particles during solidification of MMPCs is an important factor influencing final cast properties,[3,6,7] although properties are also affected by the particles’ influence on such phenomena as heat transfer, melt convection, species macrosegregation, and microstructure formation. Control of the above phenomena during casting enables the elimination of defects as well as the design of novel materials with spatially varying properties in response to performance requirements. The present study is aimed at modeling the macroscopic particle redistribution and multiphase transport phenomena during solidification of MMPCs. The casting of MMPCs via dispersion processing[8] involves the addition of a reinforcing particle phase (e.g., SiC or graphite particles) to a liquid metal matrix (typically Al alloys) that is subsequently solidified. A wealth of complex physical phenomena may occur in this process, including particle incorporation into the liquid melt, chemical reactions, fluid motion induced by the buoyant motion of the particles, clustering, thermosolutal convection, and multiR.J. FELLER, formerly Graduate Research Assistant, Department of Mechanical Engineering, The University of Iowa, is Senior Research Engineer, Caterpillar, Inc., Peoria, IL 61656. C. BECKERMANN, Professor, is with the Department of Mechanical Engineering, The University of Iowa, Iowa City, IA 52242. Manuscript submitted July 8, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
component solidification with and without the presence of particles. The reinforcing phase in MMPC melts is generally mobile, and may be segregated to different regions of the casting due to settling/flotation as well as due to interactions with the advancing semisolid region (i.e., mushy zone). Han and Hunt[9,10] found that for a sufficiently small spacing between the primary solid dendrites of the mush, the flow conditions may be such that the particles are prevented from passing through the interface between the fully liquid and mushy regions (i.e., the macroscopic growth front) and instead flow along this ‘‘rough’’ interface. On the other hand, for a relatively ‘‘open’’ columnar or an equiaxed dendritic mush morphology (the latter being most common in Al alloys), the particles are not rejected by the front and can penetrate into the mushy zone. Once inside the mush, macroscopic particle motion is largely prevented by the presence of the microscopically complex solid/liquid interfaces (i.e., dendrites) which entrap the particles. Any particle pushing (review provided by Asthana and Tewari[11]) by
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