Role of ambient pressure in keyhole dynamics based on beam transmission path method for laser welding on Al alloy

The influences of reduced ambient pressure on weld formation and pore defects were investigated by establishing a three-dimensional mathematical model for laser welding of aluminum alloy. The model was based on beam transmission with multiple reflection in the keyhole. The attenuation of the laser energy and the refraction of the laser beam by metal vapor plume above the keyhole were considered as a part of the heat source. The keyhole dynamics and molten pool fluid flow under varied ambient pressures were studied in comparison way. The lower ambient pressure plays an important role in deepening the keyhole by lower evaporation temperature of the material, higher laser energy density, and smaller laser beam refractive angle. The necking is easier to be reopened. Moreover, the laser energy distributes more uniformly on the keyhole wall at lower ambient pressure, giving rise to a smaller possibility for bubble formation. The ambient pressure also affects the shape and size of the molten pool as well as the distance and the viscosity of the zone that the bubble travels, which further influence the escape of bubbles. The modelling results of weld formation and pore defects are in the same trend as previous experimental data.