Effect of Cu on the Hot Tearing Susceptibility of Al–6Zn–2.5Mg–xCu Alloy

Abstract

Based on the modified Clyne–Davies model and the “T”-type hot tearing test system, the hot tearing susceptibility of Al–6Zn–2.5Mg–xCu (x = 0, 1, 2, 3 wt%) alloy was predicted and verified. The results showed that the hot tearing susceptibility first decreased and then increased with the increase in the content of Cu, and the smallest was at the content of 2 wt%. It was found that dendrite coherence temperature (Tcoh) measured by double-couple test and hot tearing initiation temperature (Thi) measured by cooling stress–strain curve of T-type specimen are important parameters to characterize hot tearing susceptibility of alloys. The lower Tcoh and Thi, the lower the hot tearing susceptibility of alloys. The effect of different Cu contents on the crystallization of the parent phase and the precipitation of the second phase was analyzed by optical microscopy, scanning electron microscopy and X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The results show that the cooling structure of Al–6Zn–2.5Mg–2Cu alloy with the lowest hot tearing susceptibility corresponds to the finest grains, and a suitable amount of S (Al2CuMg) and T (Al2Mg3Zn3) phases is distributed on the grain boundary.