INFLUENCE OF CASTING AND HEAT-TREATMENT PARAMETERS IN CONTROLLING THEPROPERTIES 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 det ermining 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 s tudied. For the composite, melt-temperature, hydrogen level, and the c leanliness and stirring procedure, control, respectively, the harmful melt reactions of the SiC reinforcement with the alloy matrix, gas por osity, inclusion and oxide-film contamination, whereas casting conditi ons are mainly controlled through the use of a proper mold temperature and appropriate mold coating materials that enhance the feedability a nd reduce or eliminate the effects of shrinkage. The beneficial effect of the SiC reinforcement particles is two-fold: 1. they act as prefer ential sites for the nucleation of the eutectic silicon particles, lea ding to an overall refinement of the latter and lowering the amount of strontium modifier required from 150 to 90 ppm to achieve the same le vel of refinement in the as-cast microstructures of both composite and base alloy; 2. their presence also results in a more uniform redistri bution of the silicon particles in the as-cast structure (cf. the larg e, irregular interdendritic regions of eutectic silicon observed in th e base alloy). Both composite and base alloy exhibit a similar heat tr eatment response with respect to tensile properties for the various he at 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 duct ility by almost 80%.