Influence of intermetallics and element concentrations on the performance of the heat treated HVDC AlSiMgMnCu alloys at nano-scale

Abstract

A method to calculate element concentrations in Al alloy castings, based on the serial block-face scanning electron microscopy (SBFSEM) measurement at nano-scale, was proposed. For the high vacuum die casting (HVDC) Al-10Si-0.4MgMn-0.6Cu alloy, it reveals that the solute fractions of the Si, Mg and Cu in the Al matrix reaches about 0.39 wt%, 0.15 wt%, and 0.10 wt%, respectively, under the as-cast condition. After it was heat treated via T5 process, larger nano-scale precipitation phases with low density and non-uniform distribution were formed in the α-Al matrix. After T6-treated, the narrow neck regions of the seaweed-like α-Fe phase particles were fragmented as well, but could not dissolve into the α-Al matrix. The element concentrations of the Si, Mg and Cu in the α-Al matrix increases to 0.75 wt%, 0.23 wt%, and 0.43 wt%, respectively. Hence, finer nano-scale precipitation phases with high density and uniform distribution are formed in the Al matrix. Consequently, the ultimate tensile strength, yield strength and elongation of the T6-treated alloy simultaneously increase of about 15%, 52% and 63%, respectively. It presents that the solute solution during high pressure die casting solidification has decisive effect on the mechanical properties before and after heat treatment.