Effects of Y and Addition of Refiners on Hot Tearing Susceptibility of MgZn-Based Alloy

Abstract

On the basis of studying the effect of rare earth elements on hot tearing susceptibility (HTS) of Mg–Zn alloys, further exploring effective ways to reduce HTS of Mg–Zn–Y alloys are very important for the development and application of magnesium alloys. In this paper, the effects of 2 at% Y on HTS of MgZn1 alloy and several grain refiners on HTS of MgZn1Y2 alloy were studied. The modified Clyne–Davies model is used to predict the HTS of the investigated alloys, which are found in the descending order: MgZn1 > MgZn1Y2 > MgZn1Y2–0.5 wt% Na2CO3 > MgZn1Y2–0.5 wt% Ti > MgZn1Y2–0.5 wt% Zr. By means of XRD, SEM, TEM and EBSD, the evolution of the microstructure during solidification is revealed. The results show that the addition of 2 at% Y can not refine α-Mg grains, but it can change the precipitation type of the second phase. When the precipitation type of the second phase changes from Mg7Zn3 to Mg12ZnY (LPSO phase) and Mg3Zn3Y2 (W-phase), the solidification end temperature of the alloy increases by more than 100 °C, and the coherent lattice of LPSO and α-Mg has a better bridging effect on the grain boundary, which reduces the HTS by 28.4%. Moreover, the addition of 0.5 wt% Na2CO3, 0.5 wt% Ti or 0.5 wt% Zr is an effective method for α-Mg grain refinement, equiaxation and dendrite coherent delay of MgZn1Y2 alloy. In particular, the addition of 0.5 wt% Zr can reduce the α-Mg grain size of MgZn1Y2 alloy by 89.1% and increase the solid fraction by 45% when dendrites are coherent, which results in a 77.1% reduction in HTS.