An approach to studying the hot tearing mechanism of alloying elements in ternary Mg-Zn-Al alloys

An approach to studying the mechanism of each alloying elements in ternary alloys is proposed based on theoretical and experimental verification of Mg-(5-7)Zn-(2-3)Al alloys. The hot tearing susceptibility (HTS) measurement is carried out based on a "T" shape constraint rod casting mold with a temperature and load acquisition system. It shows that the Mg-6/7Zn-3Al alloys have similar HTS to that of AZ91 alloy and the liquid fraction (f(l)) of 0.3-0.5 is the critical f(l) range for hot tearing crack initiating in Mg-Zn-Al alloys. The theoretical calculation of the shrinkage stress/strain concentration and liquid phase properties during solidification effectively explain the mechanism of alloying elements Zn and Al on the HTS. It shows that the shrinkage stress/strain concentration from the critical f(l) range to the end of solidification decrease with increasing the Zn and Al content, which alleviates the cracks formation or propagation. Meanwhile, alloys with higher Zn and Al content maintain a higher f(l) before the last stage of solidification, which can improve liquid feeding. However, excessive f(l) in the last stage of solidification is found to be detrimental to the HTS. Therefore, increasing Al content can further alleviate the HTS by controlling f(l) in the last stage of solidification. The idea of studying the shrinkage stress/ strain concentration and liquid phase properties during solidification could hopefully help clarify the role of alloying elements in HTS and establish a reasonable relation between alloying elements and HTS for multi -element alloys, which contributes to designing hot-crack resistant alloy.