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.

Effects of Zn Content on Hot Tearing Susceptibility of Mg–7Gd–5Y–0.5Zr Alloy

“…Figure 1 shows a T-shaped hot tearing acquisition device used to capture the shrinkage force and temperature of hot tearing initiation and propagation of the experimental alloys. When the molten alloys were poured into the T-shaped mold and cooled in air, the intersection between the sprue and the connecting rod was prone to stress concentration [26,27]. Generally, hot tearing may occur both inside and outside, and it is more difficult to evaluate internal tears than external tears.…”

“…It can be seen that when Zn content was 3 wt%, 5 wt% and 7 wt%, HTS can be reduced by 27%, 83% and 100%, respectively, that is to say, Zn can significantly reduce HTS of Mg-7Gd-5Y-0.5Zr alloy, which almost linearly decreased with the Zn content. When the alloy solidifies in the T-shaped mold, hot tearing was most likely to occur at the corner of the mold where the thermal stress was the most concentrated [26,27]. Figure 10 shows the macrotears at the T-shaped hot junction of Mg-7Gd-5Y-xZn-0.5Zr alloy castings.…”

Hot tearing susceptibility of MgZn4.5YxZr0.5 alloys and mechanism

“…Only W-phase is formed when the ratio of Zn/Y ratio is 1.125. It is known that W-phase gradually exists with grid-like structure at the grain boundaries when Zn/Y ratio is closed to 1 [17] . According to previous studies [10][11][12] , W-phase could result in the difficulties Research & Development CHINA FOUNDRY The morphology of fracture for MgZn 4.5 Y x Zr 0.5 alloys is shown in Fig.…”

“…5. The crack initiates from the surface of the casting [17] . The internal dendrites separate under solidification shrinkage stress.…”

“…Formed bridges can hinder the propagation of cracks [17] . Figures 5(c) [4] , when the dendritic growth reaches the interference point, as the temperature decreases, the feeding mechanism in the solidification process is changed gradually from the overall feeding to the interdendritic feeding [12] .…”