Microstructure evolution and mechanical characteristics of interface migration zone in friction stir lap welded superlight duplex Mg–Li alloy LZ91

Abstract

The superlight duplex Mg–Li alloy LZ91 was successfully friction stir lap welded (FSLW) using a high rotational speed of 2000 rpm and the welding speed was 800 mm/min with a traditional H13 tool. The tensile-shear load of the joint was about 2.01 kN with a width of 12 mm, and the microhardness of nugget zone (about 52 HV) was significantly higher than that of the base material (about 47.8 HV). The fracture occurred along the interface distortion zone, showing the characteristic of shear fracture, indicating that this is the weak location of the joint. In addition, it is found that the β phase grains grow up at the interface distortion zone in the FSLW joint. However, the α phase grain is refined with continuously strengthened mechanical action, and distributed in the boundaries of β phase grains. The dislocation density varies with the deformation degree and the difference of the heat input, the highest at the nugget zone interface, the second at the cold interface near the original interface, and the lowest at the hook interface. Moreover, the dislocation density of α phase grains is always higher than that of β phase grains.