The effect of Mg on the microstructure and mechanical behavior of Al-Si-Mg casting alloys

摘要

The microstructure and tensile behavior of two Al-7pet Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7pet, have been studied. Different microstructures were produced byvarying the solidification rate and by modification with strontium.An extraction technique was used to determine the maximum sizeof the eutectic silicon flakes and particles. The eutectic Si particlesin the unmodified alloys and, to a lesser extent, in the Sr-modifiedalloys are larger in the alloys with higher Mg content. Large Fe-rich π-phase (Al9FeMg3Si5) particles are formed in the 0.7 petMg alloys together with some smaller β-phase (Al5FeSi) plates; incontrast, only β-phase plates are observed in the 0.4 pet Mg alloys.The yield stress increases with the Mg content, although, at 0.7 petMg, it is less than expected, possibly because some of the Mg islost to π-phase intermetallics. The tensile ductility is less in thehigher Mg alloys, especially in the Sr-modified alloys, comparedwith the lower Mg alloys. The loss of ductility of the unmodifiedalloy seems to be caused by the larger Si particles, while thepresence of large π-phase intermetallic particles accounts for theloss in ductility of the Sr-modified alloy.