INFLUENCE OF CASTING AND HEAT-TREATMENT PARAMETERS IN CONTROLLING THE PROPERTIES OF AN AL-10 WT-PERCENT SI-0.6 WT-PERCENT MG/SIC/20P COMPOSITE

The influence of melting, casting and heat treatment parameters in determining the quality and tensile properties of an Al-10 wt% Si-0.6 wt% Mg/SiC/20p composite in comparison to its base alloy (359) has been studied. For the composite, melt-temperature, hydrogen level, and the cleanliness and stirring procedure, control, respectively, the harmful melt reactions of the SiC reinforcement with the alloy matrix, gas porosity, inclusion and oxide-film contamination, whereas casting conditions are mainly controlled through the use of a proper mold temperature and appropriate mold coating materials that enhance the feedability and reduce or eliminate the effects of shrinkage. The beneficial effect of the SiC reinforcement particles is two-fold: 1. they act as preferential sites for the nucleation of the eutectic silicon particles, leading to an overall refinement of the latter and lowering the amount of strontium modifier required from 150 to 90 ppm to achieve the same level of refinement in the as-cast microstructures of both composite and base alloy; 2. their presence also results in a more uniform redistribution of the silicon particles in the as-cast structure (cf. the large, irregular interdendritic regions of eutectic silicon observed in the base alloy). Both composite and base alloy exhibit a similar heat treatment response with respect to tensile properties for the various heat treatments applied. Addition of 20 vol % SiC to the base alloy (359) is seen to increase the Young's modulus and yield strength by 30-40%, marginally affect the ultimate tensile strength, but reduce the ductility by almost 80%.