Mass balance characterisation of Al-7Si-Mg alloy microstructures as a function of solution treatment time

Abstract

Mass balance calculations were performed to model the effect of solution treatment time on A356 and A357 alloy microstructures. Image analysis and electron probe microanalysis were used to characterise microstructures and confirm model predictions. In as-cast microstructures, up to 8 times more Mg is tied up in the π-phase than in Mg2Si. The dissolution of π is accompanied by a corresponding increase in the amount of β-phase. This causes the rate of π dissolution to be limited by the rate of β formation. It is predicted that solution treatments of the order of tens of minutes at 540°C produce near-maximum T6 yield strengths, and that Mg contents in excess of 0.52 wt% have no advantage.