Numerical simulation of gravity effects on keyhole behaviors in laser mirror welding of 2219 aluminum alloy

Due to the mirror image symmetry of laser mirror welding heat sources, the post-weld deformation is observably lower than that of conventional welding methods. However, the mechanism of gravitational effects on the coupled keyhole molten pool at different welding positions is still unclear, which results in the tough control of welding defects, hindering the application of laser mirror welding in precision manufacturing of aerospace structural parts. In this study, the gravity effects on keyhole behaviors in laser mirror welding of 2219 aluminum alloy were investigated by establishing a computational fluid dynamics (CFD) model. The keyhole morphology, coupling characteristics, and its dynamic evolution as well as force conditions were analyzed at length by employing the calibrated model. The results showed that gravity prominently affects the fluid velocity in the molten pool, which determines the keyhole characteristics. The keyhole surface flow, which enlarges the keyhole diameter, is impeded by gravity, resulting in weak keyhole coupling. The keyhole area on the weld cross-section before coupling is less than 2 mm² and is negatively correlated with the gravitational component on this section. The keyhole area after coupling is tightly relevant to the keyhole diameter. The increase or decrease in the keyhole diameter is contingent on the resultant direction of gravity, fluid pressure, and vapor recoil pressure. Additionally, the keyhole coupling and dynamic behaviors in the welding direction are primarily affected by the surface tension gradient, while slightly affected by gravity.