Effect of sheet-thickness ratio on fluid flow and keyhole dynamics in laser wobble lap welding of AA1050 aluminum

The integrity of the laser welding joints at the interface between battery tabs and busbars is crucial for ensuring the safety and efficiency of electric vehicles (EVs). Despite advancements in laser welding techniques, the effect of varying interface position on melt pool morphology, fluid flow, and keyhole stability remains unexplored. This study examined how different lap-joint interface positions affected fluid flow, melt pool depth and keyhole dynamics in AA1050 aluminum sheets, using a combination of experiments and numerical simulations. A wobbling pattern was employed to achieve the desired weld joint width, with a ring spot beam introduced to minimize spatter in the melt pool. The results reveal that the lap-joint interface position significantly alters heat distribution, melt pool convection patterns, and keyhole behavior. It was found in a lap-joint of 1 mm upper sheet and 3 mm lower sheet (1-3 mm) that early interaction with the interface promotes unstable fluid flow and keyhole fluctuations. In a 1.5-2.5 mm configuration, balanced heat transfer conditions were observed, resulting in deep penetration. In a 2-2 mm configuration, the penetration efficiency is reduced and susceptibility to porosity at the interface is increased. To gain further insight into the mechanisms leading to these observations, a 3D multiphysics simulation of laser wobbling with a ring spot beam was implemented. The model was validated against single-track experiments, and the results demonstrated that the interface position strongly influenced keyhole morphology and stability.