Effects of the Addition of Mg on the Microstructure and Mechanical Properties of Hypoeutectic Al-7%Si Alloy

It is well known that the mechanical properties of hypoeutectic Al-Si alloys are influenced by the size, morphology and distribution of primary alpha-Al dendrites, eutectic Si crystals and other intermetallics. In the present work, the effect of Mg addition on the size and morphology of the primary alpha-Al dendrites and eutectic Si phases in Al-7%Si alloy has been investigated. The microstructure of as-cast samples with different Mg content was characterized by scanning electron microscopy. Microstructural analyses demonstrated that primary alpha-Al was significantly refined from coarse dendrites to fine equiaxed grain with increasing the amounts of Mg. In addition, the eutectic Si structure was modified from coarse needle-like/platelet-like structure to a fine coral-like fibrous structure. The mechanical properties were investigated by tensile test with various concentrations of Mg. It was found that the ultimate tensile strength improved by 61% with an increase in Mg content. However, the ductility initially increased by 80% with an increase in Mg content from 0.5 to 1.5%, then it decreased with further increasing Mg content up to 2.0 and 2.5%. It is due to precipitation of coarse Chinese script Mg2Si phase that leads to the increase in strength and the decrease in ductility.