Growth of Si particles and Al dendrites during solidification of a hypereutectic Al-Si-Cu alloy

Abstract

Hypereutectic Al-Si alloys containing up to 16-20 wt% Si and often several wt% Cu are widely applied in automobile and aerospace industry, because of their high wear resistance and high-temperature resistance. The mechanical and physical properties of these alloys are strongly influenced by the size and morphology of the primary silicon particles. Here, rod-like bulk samples of hypereutectic Al-18wt%Si-10wt%Cu alloys were solidified in a Bridgman-Stockbarger furnace with different solidification parameters. Microstructure analysis of longitudinal and radial cross-sections demonstrated nearly equiaxed growth of Silicon particles in the 3D samples. At low cooling rate a few but larger Si particles exist compared to many and smaller Si particles for solidification with higher cooling rate. Also the structure of the Al dendrites is much finer at higher cooling rates. Additionally, sheet-like samples were solidified under isothermal cooling conditions. A real-time observation method using X-ray radiography allowed for the in-situ observation. During cooling down below the liquidus temperature, facetted Si particles are the first to nucleate. Reaching the nucleation temperature of Al, growth of non-facetted Al dendrites was observed simultaneously in the whole melt, generally not starting from the surface of existing Si particles. Some Al dendrites growing in the plane of the sheet-like samples develop rather rapidly in one direction, whereas other Al dendrites grow more equiaxed, which is attributed to different grain orientations of the Al-rich dendrites in quasi-2D solidification.