Quantitative study of pore evolution and escape mechanisms induced by gap factor in laser-arc hybrid welding of aluminum alloys

Porosity is one of the most critical defects in aluminum alloy welding. However, the influence of gap fluctuations on pore formation in aluminum alloy laser-arc hybrid welding (LAHW) remains unclear. In this study, a multiphysics-coupled numerical model of LAHW incorporating an optimized free-surface tracking algorithm was developed to investigate the mechanisms of pore formation and escape under different gaps. By analyzing the evolution of the temperature and flow fields, the effects of molten flow on the formation, fusion, fission, and escape processes of pores under varying gaps were elucidated. The results demonstrate that for the 5 mm thick aluminum alloy plates investigated, keyhole-induced porosity is effectively eliminated when the gap size reaches a critical threshold of 0.6 mm. The elimination is primarily attributable to three factors: First, as the gap increases from 0 to 0.6 mm, the depth-to-width ratio of the keyhole decreases by 19.4%, reducing the tendency for keyhole closure and thereby lowering the frequency of pores formation. Second, at a gap of 0.6 mm, a narrow recirculation zone forms at the root of the molten pool, increasing the distance between the solidification front and the pore, significantly reducing the probability of pores being trapped by the solidification front. Lastly, compared to no gap, the presence of a gap facilitates the escape of pores in the welding direction, and the flow field results show that a larger gap increases the driving force for pore escape along the welding direction. These findings provide insight into the complex interplay between gap factor and pore dynamics in aluminum alloy LAHW, offering valuable guidance for broadening process window and improving weld quality.