Microstructure and tensile properties of squeeze cast SiC particulate reinforced AI-7Si alloy

Abstract

Casting alloy A356 (AI-7Si-0.3Mg) and A356 + 10%SiCp and A356 + 20%SiCp metal matrix composite (MMC) billets can be readily fabricated by ingot remelting and squeeze casting. Squeeze cast A356 billets are free from porosity with a fine primary Al dendritic structure and a secondary dendrite arm spacing (SDAS) of 10–40 μm, although macrosegregation occurs towards the centre of the billet as a result of high cooling rates and temperature gradients. Squeeze cast A356 + 10%SiCp and A356 + 20%SiCp also exhibit afine microstructure, although macrosegregation is less severe thanfor A356. Particle clustering occurs in the eutectic constituent and is more severe for the A356 + 10%SiCp MMC. In addition, thin oxide films are sometimes entrained in the MMCs during melt stirring or pouring. The MMCs are probably more susceptible than the A356 alloy to oxide entrapment because of their relatively high melt viscosity. Squeeze cast A356, A356 + 10%SiCp and A356 + 20%SiCp have superior mechanical properties in the as cast condition compared with equivalent gravity cast material because of the low porosity and fine SDAS obtained by the squeeze casting process. After solution treatment for 12 h at 540°C and quenching in water, the ductility of the A356 alloy and the MMCs increases. In general, decreasing the water quench temperature increases the 0.2% proof stress, ultimate tensile strength, and ductility. In the peak aged condition, the squeeze cast A356 alloy and MMCs have better mechanical properties, especially ductility, than gravity cast and twin roll cast material.