Effects of β-Mg₂Si Precipitates on Semi-Solid Microstructures of Wrought Al–Mg–Si Based Alloys Produced by Deformation-Semi-Solid-Forming Process

Abstract

The Deformation-Semi-Solid-Forming (D-SSF) process has been developed to produce superior semi-solid microstructures of fine α-Al grains and refined intermetallic compounds. The wrought Al–Mg–Si (6xxx alloys) alloys are generally considered to be complicated in controlling the semi-solid forming process because of the high temperature sensitivity to the liquid fraction. Additional alloying elements to the 6xxx series aluminum alloys are usually required to produce second-phase particles in order to refine the spheroidized α-Al grains during the reheating process. Generally, the β-Mg₂Si phase is precipitated in the supersaturated Al matrix with Mg and Si atoms at various heat treatments in the manufacturing process of Al–Mg–Si alloys. In this study, the annealing process was performed for the precipitation of the equilibrium β-Mg₂Si phase. By applying mechanical deformation, the stored energy is much increased by the presence of the rod-like β-Mg₂Si precipitates, which accelerates recrystallization during heating to the semi-solid state. Fine α-Al grains with the average size of 104 µm were achieved by annealing and 60% cold-rolling in the alloy without additional alloying elements. The coarsened harmful Fe-intermetallic compounds were fragmented and became effective particles to refine the α-Al grains. Moreover, the combination of the fragmented Fe-intermetallic compounds and β-Mg₂Si precipitates can further refine the α-Al grains with the average size of 79 µm by annealing and 40% cold-rolling.