Effects of Heat Treatment on Microstructure and Mechanical Properties of Variable-Cavity Liquid Forging Mg–Zn–Y–Zr Alloy

The influences of different heat treatment processes on the microstructure and mechanical properties of Mg-4.8Zn-1Y-0.4Zr magnesium alloy by variable-cavity liquid forging were studied. The results show that the grain and second phase of the part changed obviously during heat treatment. The T4 solution treatment experiments reveal that the microstructure of the alloy undergoes a remarkable transformation during solution treatment. While the grain size of the alloy increases significantly, the plasticity of the material is enhanced considerably. This improvement can be attributed to the decomposition of massive fishbone W phase into fine particle W phase, which reduces the grain boundary pinning effect. Recrystallization occurs in the T4 alloy. The microstructure is no longer dendritic crystal stretched along the deformation direction, but equiaxed crystal, and the grain is obviously refined. The Mg3Zn6Y in the test part is completely dissolved, and most of the W phase changes from coarse blocks to fine particles. On the whole, after T4 treatment, the tensile strength of the alloy decreases and the elongation increases significantly. After a long time of heat preservation, there is almost no W phase remained. The W phase changes to Mg-Zn and Mg-Y binary phase. The binary phases dispersively distributed in the grain and obviously improve the strength. The ultimate tensile strength increases from 266.1 MPa in T4 alloy to 331.6 MPa. Although the strength of the T6 treatment part is slightly lower than that of the variable-cavity liquid forging part, the elongation is significantly higher than that of the variable-cavity liquid forging part. Considering the comprehensive properties, the specimens treated with T6 have the best mechanical properties. On the whole, the best heat treatment scheme in the paper for Mg-4.8Zn-1Y-0.4Zr magnesium alloy is (475 °C × 3.5h + 200 °C × 48h).