Effect of Zr content on microstructure and tensile properties of cast and T6 heat-treated A356-0.3Yb alloy

In this paper,the influence of different Zr addition amounts on the microstructure and tensile properties of A356 alloy modified by 0.3 wt%Yb in cast state and T6 heat-treated state were systematically investigated. The results show that adding Zr on the basis of 0.3 wt%Yb can further refine alpha-Al and eutectic Si branches. When the content of Zr is 0.25 wt%, alpha-Al has the smallest grain size of 133 mu m, and the aspect ratio of eutectic Si is 1.6. However, the addition of excessive Zr will weaken the modified effect of Yb and make the eutectic Si start coarsening. The addition of Zr forms the (Al,Si)(3)(Zr,Ti) phase with Ti in the alloy, provides the heterogeneous nucleation base, increases the number of alpha-Al grains, and refines the alpha-Al matrix. After T6 heat treatment, the eutectic Si is further spheroidized and the aspect ratio decreases to 1.3. The tensile strength and elongation of A356 alloy with Yb and Zr under cast condition are 187.4 MPa and 5.6% respectively, which are 10.4% and 36.6% higher than those without Zr. The tensile strength and elongation of the alloy after T6 heat treatment are 296.3 MPa and 9.2% respectively, which are 8.2% and 27.7% higher than those of the alloy without Zr after the same heat treatment.