Effects of reinforcement distribution on low and high temperature tensile properties of Al356/SiCp cast composites produced by a novel reinforcement dispersion technique

Abstract

A major challenge for full utilization of the potentials of SiCp reinforced metal matrix composites is the uniform dispersion of very fine SiC particles in the matrix alloys. In this study, a novel method for gradual in situ release of properly wetted SiC particles with average size of less than 3 μm in the liquid metal was employed which greatly overcame this challenge. SiC particles were injected into the melt in three different forms, i.e., untreated SiCp, milled particulate Al–SiCp composite powder, and milled particulate Al–SiCp–Mg composite powder. The resultant composite slurries were then cast in either a fully liquid state (stir casting) or semisolid state (compocasting). Subsequently, the effects of the type of the injected powder and the casting method on the microstructural and mechanical characteristics of the cast composites at room temperature and 300 °C were investigated. The results demonstrated that distribution of SiC particles in the matrix were greatly improved by injecting milled composite powders instead of untreated SiC particles into the melt. Also casting the composite slurries in a semisolid state instead of fully liquid state slightly improved the distribution. The ultimate tensile strength, yield strength and elongation at room temperature of Al356/5 vol.% SiCp composite manufactured by compocasting of the (Al–SiCp–Mg)cp-injected melt were increased by 113%, 90% and 135%, respectively, compared to those of the composite manufactured by stir casting of the untreated-SiCp injected melt. The improvements in these properties at 300 °C were about 100%, 103% and 129%, respectively. Almost all the composite samples retained more than 90% of their strengths at 300 °C, whereas the monolithic samples lost more than 25% of their strength at this temperature. The composites manufactured by compocasting of (Al–SiCp–Mg)cp-injected melts exhibited a typical ductile fracture surface with equiaxed dimples at both room temperature and 300 °C.