Non-uniform deformation behavior of dissimilar friction stir welded AM60/ AZ31 joint and its influence on fracture

In this work, the defect-free dissimilar AM60/AZ31 Mg joint was obtained by friction stir welding (FSW). Tensile tests were employed under different stress states to investigate the tensile behavior of the joint. The joint microstructures before and after deformation were characterized via electron backscatter diffraction to reveal the relationship between the microstructure and the deformation and fracture behaviors. The results indicated that significant and complex evolution of grain orientation generated in the joint after FSW. The grain size of the weld bottom was smaller than that of the upper-middle part. However, compared with the base materials, the grains in upper-middle part were not refined obviously for the high welding heat input. In addition, there was a triple junction region (TJR) with significant texture fluctuations in the upper area of the weld. Further, the reasons for the fracture of joint approximately along the centerline of nugget zone (NZ) in the back face and both the NZ and the TJR interfaces in the front face were explained. During the tensile process, cracks initiated at the bottom of NZ-center in the back face due to the appearance of compression twins and double twins. Cracks developed towards the AM60 side owing to its larger grain size. Further, near the NZ boundary was a favorable orientation for basal slip and extension twinning, which was more prone to deformation and thereby made cracks terminate here. Meanwhile, there were significant differences in texture distribution and grain size in the different parts of TJR. This resulted in an incompatible deformation between the micro-regions, making cracks easily develop along the interface of these micro-regions.