Effects of Y on hot tearing formation mechanism of Mg–Zn–Y–Zr alloys

Abstract

The hot tearing behaviour of magnesium alloys is one of the very important parameters to estimate the alloys actual application. The hot tearing mechanism of different Y content of Mg–Zn–Y–Zr alloys was studied in the present paper. The several important parameters of the mushy zone of MgZn_2·5Y_xZr_0·5 (x = 0·5, 1, 2, 4, 6) alloys were collected by thermal analysis method. The solidification temperature and shrinkage stress during the solidification of MgZn_2·5Y_xZr_0·5 alloys were acquired by using the ‘T’ type permanent mould hot tearing testing instrument and the attached computer. The fracture morphology and the cross section of hot tearing regions were observed by scanning electron microscopy. The results shown that the first and the second characteristic temperature of primary crystal nucleation T_n1-cc and T_n2-cc decrease with increasing yttrium content except MgZn_2·5Y₂Zr_0·5 alloy, and the dendritic coherency temperature T_coh had the same trend, while the temperature difference of the first and the second characteristic temperature of primary crystal nucleation (T_n1-cc−T_n2-cc) increases slowly with yttrium content. The solid fraction of dendritic coherent, f_s^coh, of MgZn_2·5Y_xZr_0·5 alloys are from 0·38 to 0·74, and the f_s^coh of MgZn_2·5Y₂Zr_0·5 and MgZn_2·5Y₆Zr_0·5 alloys are relatively lower, while the MgZn_2·5Y₄Zr_0·5 alloy has the highest f_s^coh. By analysing the effect factors of hot tearing susceptibility of MgZn_2·5Y_xZr_0·5 alloys, such as mushy zone properties, the morphology of hot tearing regions and the solidification shrinkage stress curve, the hot tearing mechanism of MgZn_2·5Y_xZr_0·5 alloys can be described as follows: with the lower Y content, the main mechanism of hot tearing of MgZn_2·5Y_xZr_0·5 alloys is dendritic bridging; with the higher Y content, the main mechanism of hot tearing of MgZn_2·5Y_xZr_0·5 alloys is liquid film combined with solidification shrinkage repairing.