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 MgZn2·5YxZr0·5 (x = 0·5, 1, 2, 4, 6) alloys were collected by thermal analysis method. The solidification temperature and shrinkage stress during the solidification of MgZn2·5YxZr0·5 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 Tn1-cc and Tn2-cc decrease with increasing yttrium content except MgZn2·5Y2Zr0·5 alloy, and the dendritic coherency temperature Tcoh had the same trend, while the temperature difference of the first and the second characteristic temperature of primary crystal nucleation (Tn1-cc−Tn2-cc) increases slowly with yttrium content. The solid fraction of dendritic coherent, fscoh, of MgZn2·5YxZr0·5 alloys are from 0·38 to 0·74, and the fscoh of MgZn2·5Y2Zr0·5 and MgZn2·5Y6Zr0·5 alloys are relatively lower, while the MgZn2·5Y4Zr0·5 alloy has the highest fscoh. By analysing the effect factors of hot tearing susceptibility of MgZn2·5YxZr0·5 alloys, such as mushy zone properties, the morphology of hot tearing regions and the solidification shrinkage stress curve, the hot tearing mechanism of MgZn2·5YxZr0·5 alloys can be described as follows: with the lower Y content, the main mechanism of hot tearing of MgZn2·5YxZr0·5 alloys is dendritic bridging; with the higher Y content, the main mechanism of hot tearing of MgZn2·5YxZr0·5 alloys is liquid film combined with solidification shrinkage repairing.