Aluminium alloy-silica sand composites: preparation and properties

Abstract

Ceramic particulate composites containing up to 25 wt% silica sand in commercially pure aluminium (LM-O) and its eutectic silicon alloy (LM-6) were prepared by liquid metallurgy techniques. Pre-treated sand particles of sizes ranging from −180 to +90μm were added to the alloy melts, followed by pouring the resulting mix into permanent moulds. Quantitative metallographic examination revealed that sand particles were uniformly distributed in both types of cast composite. Scanning electron microscopic examination of the composites showed voids around the sand particles. Tensile specimens, when fractured in an Instron machine, showed an interfacial mode of failure of the composite without affecting the sand particles, indicating poor bonding with the matrices. The hardness of LM-O alloyed with magnesium increased from 52 to 78 BHN, whereas the ultimate tensile stress (UTS) decreased from 92 to 62 MPa as a result of the addition of 20 wt% sand particles. In the case of LM-6-sand composites, the hardness remained almost constant but the UTS decreased from 184 to 112 MPa with the addition of 20 wt% sand particles. The compressive strength of both types of composite also decreased as a result of sand additions. However, a favourable effect of magnesium alloying on the strength of the cast composite was also observed.