Hot Tearing Behavior of $$\text{Mg}{-}4\text{Zn}{-}x\text{Sn}{-}0.6\text{Zr}$$ Alloys

Abstract

The effects of different Sn additions (0, 0.5, 1.0, 2.0 and 3.0 wt%) on hot tearing susceptibility (HTS) of Mg–Zn–Zr alloy were studied using a “T-shaped” hot tearing mold in a pouring temperature of 700 °C and a mold temperature of 270 °C. The “Clyne–Davies” hot tearing prediction model and commercial simulation software, Procast, were used to characterize the HTS of alloys. The dendrite coherency temperature was obtained by means of differential thermal analysis. The phases evolution, microstructures and morphology of the crack zone of Mg–4Zn–xSn–0.6Zr alloys were investigated by using X-ray diffraction and scanning electron microscope. The experimental results show that the HTS of Mg–4Zn–xSn–0.6Zr alloys decreases with Sn additions up to 1.0 wt%, and then exhibits a slight increase with further Sn additions up to 3.0 wt%. The Sn additions into Mg–4Zn–0.6Zr alloy can form Mg2Sn phase with high melting point. Appropriate addition of Sn can refine grain size, decrease the dendrite coherency temperature, increase the thickness of liquid film and the feeding ability at the end of solidification, which reduce the HTS of the alloy. However, excessive Sn addition will make Mg2Sn aggregate and grow up, and hinder the residual liquid-phase feeding in the late solidification stage, resulting in an increase in HTS of the alloy. The order of HTS of studied alloys is: CSC(Mg–4Zn–0.6Zr) > CSC(Mg–4Zn–0.5Sn–0.6Zr) > CSC(Mg–4Zn–3Sn–0.6Zr) > CSC(Mg–4Zn–2Sn–0.6Zr) > CSC(Mg–4Zn–1Sn–0.6Zr).