Effects of in-situ γ-Al2O3 particles and heat treatment on the microstructure and mechanical properties of A356 aluminium alloy

It is well known that the mechanical properties of hypoeutectic Al-Si alloys are influenced by the size, morphology and distribution of primary α-Al and eutectic Si crystals. In the present study, the effect of in-situ γ-Al2O3 particles and heat treatment on the size and morphology of the primary α-Al dendrites and eutectic Si phases in A356 alloy has been investigated. The as-cast and solution treatment samples were characterized by scanning electron microscopy (SEM). Microstructural analyses demonstrated that primary α-Al was significantly refined by in-situ γ-Al2O3 particles. The secondary dendrite arm space (SDAS) of unrefined α-Al is about 25 μm. The SDAS decreases to 11 μm when the content of in-situ γ-Al2O3 particles is 0.6%. In addition, the eutectic Si structure was modified from coarse needle-like/platelet-like structure to short plate-like structure and some discrete particles. The mechanical properties were investigated by tensile test with various concentrations of γ-Al2O3 particles. It was found that the ultimate tensile strength (UTS) and elongation (El) increased by 41% and 137%, respectively, due to decreasing SDAS of primary α-Al dendrites and modifying eutectic Si crystals. Moreover, the change of mechanical properties corresponds to the evolution of microstructure.