Thermal Management of Metal Matrix Composite Solidification

Metal matrix composite (MMC) has been well known for its superior material properties com- pared with traditional composite. A new method is introduced to improve the properties of MMC in the sense that the ends of the reinforcement phase of the composite are allowed to extend out of the mold and cooled by a heat sink in order to promote the rate of heat transfer through the fibers and promote the for- mation of primary alpha phase around the reinforcement. This paper presents the experimental results ob- tained from the foundry in the University of Wisconsin - Milwaukee and some numerical simulation re- sults of the solidification process in the cast mold. Heat transfer analysis of solidification of metal matrix composites has been carried out to understand the forma- tion of solidification microstructure. It is of prime interest in casting industry because composites are widely used in many applications such as in the manufacturing of engine components of a vehicle and brake rotors of selected ve- hicles. It is highly desirable to produce composite cast- ings with superior material properties, which can be ob- tained through improved solidified microstructure that directly influence several properties. The traditional way of producing a composite compo- nent is to fill the casting mold with liquid melt containing the reinforcement and let the setup to cool below its melt- ing temperature as a result of heat transfer primarily through the mold and cooled by convection. The micro- structures of composites containing fibers or particles so- lidified in this conventional manner have been studied by Rohatgi (1), who observed that the nucleation of liquid metal frequently occurs away from the surface of the re- inforcement. Composites produced in this manner are be- lieved to have inferior material properties. One possible way to improve the microstructure is to extend the ends of the reinforcing phase outside the casting mold and cool the ends of the fibers by a heat sink. Since a faster heat extraction is introduced, the solidification time can be shortened. This process of cooling the fiber ends during processing of composites is referred to as thermal man- agement of fibers. Rohatgi et al. (2) investigated the solidification of aluminum alloys which had graphite cylinders extending outside the solidifying melt, and were put in a lower tem- perature environment to promote heat transfer through the graphite rod. Solidification microstructure of MMC has demonstrated that upon solidification, dispersoids gener- ally tend to segregate in the boundaries of pro-eutectic - Al dendrites (3-5), thereby diminishing the properties of these materials. Generally, one would expect that during solidification, the dispersed particles should act as prefer- ential sites for heterogeneous nucleation of the solidifying primary phase. A large number of observations by differ- ent research groups confirm that in aluminum base alloys, aluminum dendrites do not nucleate on the surfaces of dispersoids like graphite and silicon carbide. However, if nucleation of different phases on these reinforcements can be controlled, then the properties will be enhanced due to avoidance of particle segregation and a variety of interfaces between the dispersoids and the matrix can be generated, leading to tailor-made properties. Experiments in which the ends of the cylindrical rods are cooled, indicate altered microstructures at the inter- faces between matrix and the cylindrical rods as well as in the matrix. The alterations in the interfacial structure due to cooling of the ends of cylinders are attributed to changes in the local temperature differences at the inter- face between cylinders and the matrix during solidifica- tion. These observations suggest that thermal manage- ment can significantly change the nature of the interface and the matrix and therefore, the properties of cast com- posites.