Improving performance of friction stir welded AZ31/AM60 dissimilar joint by adjusting texture distribution and microstructure

Abstract

Inhomogeneous deformation, which was usually found during the deformation of friction stir welding (FSW) Mg alloys joints, could deteriorate the joint performance. This work aims to understand the effect of microstructure and texture distribution in joint on mechanical properties and deformation behavior of the FSW dissimilar Mg alloys joints. Two kinds of AZ31/AM60 dissimilar Mg alloys joints were obtained via FSW at two sets of welding parameters (higher rotation rate and welding speed (HRW), lower rotation rate and welding speed (LRW)). Electron backscatter diffraction measurements were conducted to examine the microstructure and texture distribution of these two joints. The results showed that the joint with finer grains, weaker texture and higher nugget zone (NZ) interface angle was obtained with HRW. The weaker texture and finer microstructure were beneficial to the homogeneous deformation of the joint. The larger angle between the NZ interface and the transverse direction (about 75°) made it away from the maximum shear force direction (45°) to avoid premature fracture. With these advantages, the mechanical properties of the joint with HRW were better than that with LRW. The yield strength, ultimate strength and elongation were improved 14.4%, 19.7% and 63.7%, respectively. In addition, the grain size of AZ31 of NZ in two joints was smaller than that of AM60. All the joints fractured at the AM60 side near the NZ boundary during the tension tests. This was mainly attributed to the larger grain size and higher Schmid factor of basal slip at the AM60 side near the NZ interface. This study provided some insights in understanding the improvement of the dissimilar FSW Mg joint performance by controlling texture distribution and microstructure.