Effect of Ca modification on the elemental composition, microstructure and tensile properties of Al-7Si-0.3Mg alloy

The effect of Ca addition on the elemental composition, microstructure, Brinell hardness and tensile properties of Al-7Si-0.3Mg alloy were investigated. The residual content of Ca in the alloy linearly increased with the amount of Ca added to the melt. The optimal microstructure and properties were obtained by adding 0.06wt% Ca to Al-7Si-0.3Mg alloy. The secondary dendrite arm spacing (SDAS) of the primary alpha phase decreased from 44.41 mu m to 19.4 mu m, and the eutectic Si changed from coarse plates to fine coral. The length of the Fe-rich phase (beta-Al5FeSi) decreased from 30.2 mu m to 3.8 um, and the Brinell hardness can reach to 66.9. The ultimate tensile strength, yield strength, and elongation of the resulting alloy increased from 159.5 MPa, 79 MPa, and 2.5% to 212 MPa, 86.5 MPa, and 4.5%, respectively. The addition of Ca can effectively refine the primary alpha phase and modify the eutectic Si phase, likely because Ca enrichment at the front of the solid-liquid interface led to undercooling of the alloy, reduced the growth rate of the primary alpha phase, and refined the grain size. Also, it could increase the latent heat of crystallization, undercooling, and the nucleation rate of eutectic Si, which was beneficial to the improvement of the morphology of eutectic Si.