Magnesium Effects on the Microstructure of Unmodified and Modified Al-Si Alloys

Abstract

Small amounts of magnesium are often used to improve the mechanical properties of Al-Si foundry alloys. The effect of this magnesium on the eutectic microstructure has been investigated at magnesium levels up to 1 wt%. Experiments were performed on AI-7% Si, AI-13% Si and 413.2 alloys. Assessment of the eutectic microstructure was undertaken using thermal analysis and metallography. The silicon phase was quantified in terms of its perimeter, shape factor and aspect ratio. The results reveal that in hypoeutectic and eutectic alloys, magnesium decreases the homogeneity of the microsructure. At about 1 wt% Mg, however, magnesium slightly refines the silicon phase. It was found that magnesium has a negative effect on strontium modification, that is, it changes the microstructure from well modified to partially modified. This is believed due to the presence of a complex intermetallic compound (Mg₂SrAl₄Si₃) formed prior to the eutectic transformation.