Effect of post weld heat treatment on mechanical properties and microstructures of friction stir welded AZ31B magnesium alloy

The effects of post weld heat treatment (PWHT) on microstructures and mechanical properties of friction stir welded AZ31B magnesium alloy were investigated. Average grain sizes at the upper region and at the bottom region in the stir zone (SZ) of as welded joint were 9.5 μm and 14.4 μm, respectively. When heat treatment temperature at 673 K, a few relatively large grains were formed at the retreating side in upper region, subsequently the region extended in the stir zone as heat treatment time increased. It is believed that the abnormal grain growth is caused by inhomogeneous distribution of dislocation. The elongation to failure of joint was improved by the heat treatment at 573 K for 7.2 ks. However, the formation of abnormal large grains caused degrading elongation. The yield stress at upper side in SZ of as welded joint was larger than that at bottom side and was decreased gradually as the joint was annealed. Because the additional strength at the upper side was due to the work hardning that was added by the shoulder of tool.