Fabrication of hypereutectic Al–Si alloy with improved mechanical and thermal properties by hot extrusion

Abstract

Silicon phase in hypereutectic AlSi powders appears as primary Si and eutectic Si. Due to the poor thermal stability of eutectic Si, the material suffers from pores, stress concentration and dislocation packing during hot extrusion process, resulting in low density and poor thermal properties. In order to eliminate the eutectic Si, a heat treatment is conducted on Al20Si powders before hot extrusion. Because of the Ostwald ripening effect, the eutectic Si and dislocations in the powders are replaced by isolated Si lumps and a clear Al matrix. As a result, the formability and thermal properties of the material are greatly improved. The extruded samples made from the treated powder present high relative densities 99.7–100%, ultimate tensile strength (UTS) 167–170 MPa, elongations 8.8–10.8% and thermal conductivities (TCs) 191–195 Wm−1 K−1 and low coefficient of thermal expansions (CTEs) 13.8–14.5 × 10−6/K. The excellent mechanical and thermal properties are attributed to the elimination of eutectic Si from the treated powders and as-extruded alloys. In addition, hot extrusion introduces in-situ Al2O3 in the Si surface, Al grain boundary and Al grains, which is beneficial to reducing and stabilizing the CTE of the material. Moreover, it hinders Ostwald ripening and thus growth of the Si particles during the service of the material.