Investigation of molten pool flow behavior and porosity formation mechanisms in laser welding under different oscillation modes

This study systematically analyses the melt pool flow behaviour and porosity formation mechanisms during laser welding of aluminium alloy plates. A three-dimensional transient numerical model coupling thermal, fluid flow, and interface phenomena was established to investigate the melt pool flow characteristics, keyhole dynamic evolution, and porosity generation processes under different laser oscillation modes. A composite heat source model, consisting of a Gaussian surface heat source and a rotating Gaussian volumetric heat source, was employed to accurately characterise the laser energy deposition under non-oscillating, vertical oscillating, and circular oscillating conditions. Numerical results indicate that laser oscillation effectively homogenizes the heat input distribution within the molten pool, significantly reduces surface temperature gradients, and enhances flow stability. Compared to non-oscillating welding, vertical oscillation attenuates the upward recirculation flow within the molten pool, mitigating localised deformation of the keyhole wall. Circumferential oscillation induces stable circumferential flow patterns, significantly suppressing keyhole contraction and collapse, thereby effectively reducing fluctuations in keyhole depth. Under high-amplitude, high-frequency oscillation conditions, the periodic motion of the keyhole promotes the dynamic entrapment and escape of bubbles, substantially reducing the likelihood of porosity and inclusion formation during solidification. This study elucidates the intrinsic mechanism of porosity suppression in oscillating laser welding from a fluid dynamics perspective, providing a theoretical basis for optimising oscillation parameters in aluminium alloy laser welding.