Effects of rheocasting and thixoforming on the microstructure and mechanical properties of A356 aluminium alloy

Abstract

This paper presents the changes in mechanical properties and microstructure of aluminium alloy A356 that undergoes cooling slope casting. The alloy was cut into small cubes with an estimated weight of 400g before it was heated in a crucible induction casting machine. The temperature was set to 880 °C with a heating rate of 15 °C per min. Then, the metal was cooled to 620 °C until it turns to a semisolid, before pouring into a stainless steel mould through a 250 mm long and 60° cooling slope before it was cooled to room temperature. For the thixoforming experiments, the liquid fraction was between 30% and 50%, with various semisolid temperatures (583 °C-585 °C). The ram speed and die temperature were 85 mm/s and 200 °C, respectively. The microstructure and mechanical properties of rheocast feedstock in T6 condition were determined and compared with the metal without any heat treatment, rheocast and thixoformed alloy. It was found that thixoformed metal had the highest ultimate tensile and yield strength with reduced ductility. The microstructures are rosette, near globular and spherical, and were obtained in rheocast and thixoformed alloys, respectively. The α-Al grains were larger at higher semisolid temperatures.