Microstructure and mechanical properties of probeless friction stir spot welded Al-Li alloy joints via fast electric pulse treatment: A comparative study

To improve the performance of probeless friction stir spot welded (P-FSSW) joints, a fast electric pulse treatment (EPT) method was proposed firstly, and the microstructure evolution and mechanical properties of the joints after EPT was investigated in this study. Results show that the tensile and shear strength decreases slightly by about 10%, but the joint plasticity increases by about 23.7% after three electric pulse treatments. As the number of electric pulse treatments increases, the joint lap interface undergoes dynamic recrystallization and atomic diffusion, so microcracks are closed and interface bonding is improved. However, due to the different recrystallization degree in different characteristic zones of welded joints after electric pulse treatment, the plastic deformation gradient in the transition zone of welded joints increases. The incongruity of plastic deformation compatibility in the transition region would cause stress concentration. Therefore, the interface on both sides of the joint is favorable for crack propagation, which makes the shear fracture mode change into the plug fracture mode.