The Influence of Microstructural Heterogeneity on Mechanical Properties of Friction Stir Welded Joints of T6-Treated Al-Zn-Mg Alloy 7A52

A high strength Al-Zn-Mg alloy 7A52 with T6 treatment was successfully friction stir welded. The grain structure, dislocations and precipitates in typical regions of the weld joint, including the weld nugget zone (WNZ), thermos-mechanically affected zones (TMAZ) and heat affected zones (HAZ) were investigated to understand the mechanical properties of each zone and the weld joint. In WNZ, a relatively higher density of dislocations is observed on the advancing side, caused by vacancy collapse induced by severe plastic deformation during stirring. However, in the center and on the retreating side, the dislocation density is very low. The strength of the WNZ is influenced by grain refinement, solution strengthening, or natural ageing hardening. In TMAZs, different mechanical properties on each side are due to different grain structures and precipitates introduced by the asymmetrical thermo-mechanical cycle. In HAZs, the mechanical properties are a strong function of the ratio of eta' to eta phase. Compared to the micro-tensile results, premature failure of the weld joint occurs in HAZs on the advancing side, resulting from stress concentration near the area with the lowest hardness.