Numerical analysis of coupling dynamics of keyhole and molten pool in vertical-up laser mirror welding of 2219 aluminum alloy

In vertical-up laser mirror welding, the keyhole dynamics and flow behavior of molten pool play crucial roles in weld quality, yet the mechanisms of these coupling dynamics have not been well clarified. In this work, a three-dimensional transient model for simulation of vertical-up laser mirror welding process was carried out to account for the flow behavior of molten pool and dynamic evolution characteristics of keyhole. Two laser heat sources with regard to laser mirror welding were also taken into consideration in the combined heat source model. It is proved that the simulation results are basically consistent with the experimental data. The simulation results indicate that the morphology and flow field distribution of upper and lower molten pool both exhibit a high degree of consistency on the X-Y section, while significant difference is visible on the X-Z section. Before keyhole coupling, the high velocity fluid exists on the surface of molten pool and tends to increase the diameter of keyhole, of which the maximum velocity reaches 5.26 m s-1. During the keyhole coupling process, the coupled keyhole in the center area is found to fall, which results in deteriorated coupling process. Force analysis of keyhole illustrates that the resultant force on the X-Y section points to the direction of increasing the depth and diameter of keyhole before and during the keyhole coupling process. In contrast, the resultant force on the X-Z section is conductive to keyhole coupling occurring at the lower position. After keyhole coupling, the diameter of coupled keyhole in the center zone is increased and the fluid velocity is reduced to 0.109 m s-1, contributing to a firm connection.