Microstructure and Fracture Behaviors of Oscillating Laser Welded 5A06 Aluminum Alloy Lock Butt Joint

Abstract

Oscillating laser welding is potential to improve the quality of aluminum alloy joints, but has been seldom addressed on lock butt joint. In this paper, the effects of beam oscillation frequencies (f) on the properties of laser-welded 5A06 aluminum alloy lock butt joints were investigated, especially those at the lock step. In the microstructure, the columnar grain zone (CGZ) near the fusion line narrowed, the porosity was reduced, and the angle between lock step and fusion line increased with the increase of f. Correspondingly, the fracture changed from equiaxed grain zone to heat affected zone (HAZ), and the fracture angle between lock step and crack propagation line from 90° to 45°. The maximum ultimate tensile strength and elongation of oscillating weld reached 308 MPa and 18.2%, respectively, 36.3% and 203.3% higher than non-oscillating weld. The fracture behaviors indicated that the crack always initiated at the lock step, and then preferably propagated to the pores, followed closely by the weaker CGZ, and then the stronger HAZ when CGZ was narrowed enough. Notably, when the pore size was small (<0.39 mm) and located below the lock step, the pore was not on the crack propagation path. The crack tended to propagate towards the weaker CGZ. Finally, the fracture mechanism was discussed. The results clarify the fracture mechanism of oscillating laser-welded lock butt joints and contribute to the development of oscillating laser welding.