Effects of annealing under fixed temperature and cyclic temperature on strength and microstructure of Al–Mg–Mn-Sc-Zr alloy

Fractures sometimes are located at heat affected zone rather than weld zone in weldments. This phenomenon may be related with the thermal shock which impacts the heat affected zone. In order to study this mechanism, the Al–Mg–Mn-Sc-Zr alloy plates were processed by different heat treatment methods. One method is persevering the samples at 550 °C and the other is heating the samples to 550 °C then cooling to room temperature and recycling this process to simulate welding thermal shock. The results show that tensile properties of the samples under fixed temperature (FT) decrease only a little at first. However, the strength of the samples under cyclic temperature (CT) decrease dramatically after just one cycle. This is different from traditional perception. Generally, dislocations would move and be polygonised at high temperature, which accords to the microstructure of FT sample. But in CT sample, the phenomenon of polygonization is not uniform. The dislocations tend to aggregate at the place where particle number is high, like tiny defects in grain cutting the continuity of fiber texture. Therefore, the strength decreases a lot.