Effects of plunge depth on fracture behaviors of refill friction stir spot welding

Effects of welding parameters on fracture behaviors of refill friction stir spot welding (RFSSW) welds are investigated. RFSSW is used to weld LY12 aluminum alloys with different thicknesses by changing the sleeve plunge depth. The fracture mechanisms of the welds are investigated from the aspects of material flow, atomic diffusion and softening, and are verified by the cross section, microstructure and hardness of the weld. Results show that the crack propagates along the interface of lap weld with lower joining strength and forms a shear fracture mode when the plunge depth is smaller than the thickness of upper plate, the hook defect is flat and thicknesses of bonding ligament in the weld are almost the same, and the crack is more easily to propagate along the withdrawing path with shorter atom diffusion time and results in the shear-plug fracture. When the sleeve penetrates into the lower plate thickness of 15%, the hook defect bends upwards and the crack defect appears at the sleeve withdrawing path. When the sleeve further plunges into the lower plate thickness of 35% and the hook defects present along a downward bending direction, the crack grows downwards along the withdrawing path of sleeve. When the crack reaches the bottom of lower plate, the plug fracture of RFSSW weld completed. These results show that the fracture morphologies are in agreement with analytical results of the fracture features. ©, 5015, Xi'an Jiaotong University. All right reserved.