Abstract
Mg–7Gd–5Y–0.5Zr alloy has excellent mechanical properties but poor hot tearing resistance. The latter makes it difficult to cast billets, which limits the size of subsequently processed parts. Therefore, the hot tearing susceptibility of Mg–7Gd–5Y–xZn–0.5Zr (x = 0, 3, 5, 7 wt%) alloys was studied. It was found that Zn can significantly reduce hot tearing susceptibility of Mg–7Gd–5Y–0.5Zr alloy, which almost linearly decreased with Zn content. When Zn content was 3 wt%, 5 wt% and 7 wt%, hot tearing susceptibility will be reduced by 27%, 83% and 100%, respectively. It was further revealed that the solid solubility of Gd and Y in α-Mg decreased with the increase of Zn content, and the nucleation temperature decreased accordingly, which resulted in the increase of nucleation rate and the refinement of final grains. On the macro level, it showed that the dendrite coherency temperature decreased, the solidification shrinkage stress of α-Mg slowed down, and the residual liquid channel became shorter and hot tearing susceptibility decreased. It was also found that with the increase of Zn content, the content of Zn, Gd and Y enriched on the grain boundary increased, the content of residual liquid phase between dendrites increased after α-Mg crystallization, and the solidified precipitated second phase also changed from Mg5RE phase to long-period stacking ordered phase + W-phase (a little), long-period stacking ordered phase + W-phase (much) and finally to W-phase only. The feeding effect of sufficient intergranular residual liquid on the shrinkage of α-Mg dendrite and the bridging effect of the precipitated phase at the grain boundary (especially long-period stacking ordered phase which is coherent with the parent phase) also led to the decrease of hot tearing susceptibility.
Highlights
Hot tearing is often a major casting defect in magnesium alloys and has a significant impact on the quality of their casting products
[34], which effected of the alloy solidification was determined by the ratio of Zn/RE [34], which effected hot tearing susceptibility (HTS) of the alloy [35]
The solidification path of the above-mentioned finalfinal structure can be clearly displayed by thermal thermal analysis method, so that the change of liquid fraction with time can be further calculated by analysis method, so that the change of liquid fraction with time can be further calculated by Newton
Summary
Hot tearing is often a major casting defect in magnesium alloys and has a significant impact on the quality of their casting products. Mg–7Gd–5Y–0.5Zr alloy is a new type of magnesium alloy with high strength, high toughness and high thermal stability developed in recent years [20,21,22,23] It shows a high hot tearing tendency during ingot casting. Gunde et al [24] found that Y can reduce the HTS of Mg–Zn alloy by shortening the solidification path, whereas the effect of Zn on the hot tearing of Mg–RE alloy and the micro mechanism are not clear. For this reason, the authors are inspired by the results that. The micro-mechanism of hot tearing formation was discussed based on the measured solidification characteristic parameters, as well as the observation of the microstructure and crack morphology of the alloys
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