CASTING AND HIPPING OF AL-BASED METAL-MATRIX COMPOSITES (MMCS)

The commercially available Al (trade mark A-356 Al) alloy, with approx imately 7 wt%Si, 0.4 wt%Mg and other small amount of elements, is the most common type of Al foundry alloy due to its excellent castability. The needle-like Si phase embedded in the Al matrix is the most typica l structural feature of this alloy system and the morphology of the Si phase has been shown to be important in determining the mechanical pr operties of this alloy. Much work has been done to modify the Si phase morphology by heat treatment or addition of grain refiners such as So dium and Strontium. The present work describes the microstructural cha nges by the addition of alumina particles into molten Al, focusing on the effect of the alumina particle on the location, amount and morphol ogy of the Si or eutectic phase in the matrix. Since the alumina parti cles are not wettable by molten Al, small amount of reactive agent suc h as Li was added to the Al. Reasonable alumina particle distribution within the matrix Al was achieved by low temperature mixing and gravit y casting technique under open atmosphere. The cast MMC bars were proc essed by Hot Isostatic Pressing (HIPping) in order to densify the MMCs . The cast and HIPped MMCs were examined and mechanically tested. Resu lts show that the 0.2% Yield Strength was improved over the unreinforc ed Al alloy.