Improvement in formability of semi-solid cast hypoeutectic Al-Si alloys by equal-channel angular pressing

Abstract

Microstructure of Al-Si alloys are characterized by brittle eutectic Si particles. It is known that morphological change of Si particles from coarse acicular flake to fine fiber improves strength and ductility at once. To control the morphology without alloying, combined effects of semi-solid casting and severe plastic deformation were investigated in this study. The authors prepared a semi-solid JIS AC4CH cast alloy (Al-7.14%Si-0.41%Mg) with fine Si particles and a conventional cast alloy with coarse Si particles. The both alloys were processed through warm equal-channel angular pressing (ECAP) under several thermo-mechanical histories. Microstructure evolution by ECAP was evaluated in relation to deformation behavior.ECAP process was not able to refine Si particles of the conventional cast alloy as fine as those of the semi-solid cast alloy. Submicron Si particles in the semi-solid cast AC4CH slightly coarsened by warm ECAP. Combination of semi-solid casting and ECAP process provided refinement of Si particles, grain refinement of primary phase, and decrease of hardness difference between primary phase and eutectic. Controlling ECAP processing conditions improved strength and/or ductility and enabled flexible design of mechanical properties of semi-solid AC4CH.