Fatigue properties of continuous wave and pulsed wave laser cold-wire welding of thick section AA6005-T6 aluminum alloys

The effect of laser wave modes on the fatigue behavior of laser cold-wire welding made of 4.8 mm thick AA6005-T6 aluminum alloy was investigated using continuous and pulsed wave lasers. Due to the inherent differences in these two wave laser modes, different welding parameters were used, while keeping the interaction time constant. The mechanical properties of welded joints were measured using tensile tests, while their fatigue performances were quantified using a constant amplitude force-controlled technique to obtain S-N curves. The pulsed wave laser mode produced higher fatigue resistance as compared to the continuous wave mode. The fatigue strength corresponding to the run-out condition (i.e., 107 cycles, in this study) was about 28% higher for the pulsed wave mode than for the continuous wave laser mode. At a high stress amplitude (30 MPa), the lifespan of the pulsed wave joints was about twice as high as that with continuous wave joints. These fatigue results were cross-referenced with a 3D topographic map, a 2D microhardness map, a metallographic study, and a fractographic analysis to better understand the crack nucleation and crack propagation mechanisms. A microhardness analysis performed along the cross-section of the joints did not reveal any significant difference between pulsed wave and continuous wave modes. A fractographic analysis confirmed crack propagation within the fusion zone and that 83 to 90% of cracks nucleated from the root undercuts. Topographic maps of the joints before fracture revealed that the continuous wave laser mode produces deeper and narrower (i.e., more acute) undercut defects than does the pulsed wave mode. A Weibull approach based on the biggest defects found on the fracture surfaces also confirmed that the continuous wave process produces larger defects at the root. Top defects are significantly small (55%), but they have a larger size dispersion than do root defects. Since the root undercuts act as the main stress concentrators, they are mainly responsible for the lower fatigue performance of the joints, and their sizes and shapes should be minimized during further process development of the welding process.