Microstructural Studies of A357 Alloy Cast By RSF Process

Abstract

We studied rheocasting of an A357 ( Al-7Si-0.6Mg ) alloy using the Rapid Slurry Formation (RSF) technology developed by Rheometal. The technology is based on a so-called solid enthalpy exchange material (EEM). A cold piece of the same material is immersed into a melt bath and stirred in the melt which subsequently melts during stirring operation. This leads to a temperature reduction of the melt bath within a minute to a preset value between solidus and liquidus temperature at an also preset amount of fraction solid. Semi-solid slurries of A357 with around 30% solid fraction were produced in this way. The slurry formation was modified by adding several grain refiners ( Al-5Ti-1B, Al-3Ti-0.15C and Si-1B master alloys). Grain refiners were added individually a few minutes before start of the RSF process. Stirring speed was varied to study its effect on microstructure. The slurry so produced was quenched in water or cast into a heated stainless steel ring and compacted by a mild squeezing into a disc. The microstructures are analyzed with respect to grain size, grain density, aspect ratio and homogeneity, revealing an additional effect of grain refiners. The results are discussed with models of nucleation, growth and flow assisted coarsening.