MICROSTRUCTURE AND TENSILE PROPERTIES OF SQUEEZE EAST SIC PARTICULATE-REINFORCED AL-7SI ALLOY

Casting alloy A356 (Al-7Si-0.3Mg) and A356 + 10%SiCp and A356 + 20%SiC p metal matrix composite (MMC) billets can be readily fabricated by in got remelting and squeeze casting. Squeeze cast A356 billets are free from porosity with a fine primary Al dendritic structure and a seconda ry dendrite arm spacing (SDAS) of 10-40 mu m, although macrosegregatio n occurs towards the centre of the billet as a result of high cooling rates and temperature gradients. Squeeze cast A356 + 10%SiC, and A356 + 20%SiCp also exhibit a fine microstructure, although macrosegregatio n is less severe than for A356. Particle clustering occurs in the eute ctic constituent and is more severe for the A356 + 10%SiCp MMC. In add ition, thin oxide films are sometimes entrained in the MMCs during mel t 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 s uperior mechanical properties in the as cast condition compared with e quivalent gravity cast material because of the low porosity and fine S DAS obtained by the squeeze casting process. After solution treatment for 12 h at 540 degrees C and quenching in water, the ductility of the A356 alloy and the MMCs increases. II? general, decreasing the water quench temperature increases the 0.2% proof stress, ultimate tensile s trength, and ductility. In the peak aged condition, the squeeze east A 356 alloy and MMCs have better mechanical properties especially ductil ity, than gravity cast and twin roll cast material.