Interactions between Fe-rich intermetallics and Mg-Si phase in Al-7Si-xMg alloys

Abstract

This paper investigated the influences of Mg level (0.3 wt%-0.6 wt%) and heat treatment process on the phase transformation of Mg-Si phase and Fe-rich intermetallics of low pressure die-cast Al-7Si-xMg alloys. Their corresponding mechanical properties were also studied. The results indicated that at as-cast state, the addition of Mg element accelerated the formation of π-Fe phase while the mechanical properties decreased correspondingly. During the heat treatment process, the transformation of Mg-Si phase and π-Fe intermetallics were determined by the Mg content. In the alloys with low level of 0.3 wt%∼0.45 wt% Mg, the diffusion of Mg element from π-Fe phase into α-Al matrix prompted the formation of more Mg2Si strengthening phase, which was beneficial to the improvement of tensile properties. However, in the alloys with high level of 0.6 wt% Mg, the diffusion kinetic of Mg element from π-Fe phase into α-Al matrix was repressed due to the saturated Mg in the matrix. The π-Fe phase consumed large amount of Mg element and facilitated crack initiation which definitely weakened the strengthening effect. The mechanical properties, especially the ductility, were strongly deteriorated. In this study, the best comprehensive mechanical properties were obtained from the Al-7Si-0.45 Mg alloy, which was 267.3 MPa and 333.7 MPa, 9% for YS, UTS and Elongation, respectively.