Effects of Sc and Zr on Microstructure in A356 Aluminum Alloy under Various Solidification Cooling Rates

The effects of scandium (Sc) and zirconium (Zr) additions on the solidification microstructure and mechanical properties of A356 alloy at different cooling rates have been systematically studied. The macrostructure and microstructure were examined by optical microscopy and scanning electron microscopy. The mechanical properties were investigated by analyzing hardness in both the as-cast and heat-treated states. The results demonstrated that the addition of Sc-Zr reduces the primary α-Al dendrite of A356 at both low (1 °C/s) and high (10 °C/s) solidification cooling rates. The addition of Sc also impacts the eutectic Si structure, transforming the coarse plate-like formation into a finer fibrous structure at high cooling rate. However, at low cooling rate, Sc refines the eutectic Si but Sc, in this particular case, does not cause a plate to fibrous transition. Furthermore, the addition of Sc-Zr enhances the as-cast hardness, which is a result of the microstructure refinement. More importantly, the addition of Sc-Zr refiners reduces the sensitivity of the alloy’s hardness to cooling rate changes in the castings. Artificial aging treatment resulted in significant improvement in the hardness of both A356 and Sc-Zr added alloys.