Numerical study of keyhole dynamics and keyhole-induced porosity formation in remote laser welding of Al alloys

Keyhole-induced porosity in remote laser welded Al joints leads to weakened joint strength. In the study, the remote laser welding processes are numerically simulated to reveal mechanism of keyhole and keyhole-induced porosity formation. It is found that porosity formation takes three steps: bubble formation, bubble floating to the back of molten pool and bubble being captured by solidification front. The porosity prevention can be achieved by interrupting one of these three steps. The process simulation shows that violent melt flow behind the keyhole is the root cause of pore formation. It leads to keyhole collapsing and resulting in large fluctuation of keyhole depth and bubble formation. The vortex-type melt flow behind the keyhole is also the main cause of the bubbles floating from the keyhole's bottom into the molten pool; and Al's high thermal conductivity and strong melt flow make the bubbles difficult to escape. Various porosity prevention approaches are simulated in the study to check their effectiveness in terms of interrupting the three steps. Also, the corresponding experimental test are carried out as verification. The amounts of porosity predicted by the simulations agree very well with what being observed in the experimental test. The study suggests that high welding speed is helpful in keeping the keyhole open and not creating strong melt flow; large forward inclination angle also creates quiescent molten pool flow and hence makes the bubbles difficult to float into the rear molten pool. The findings from the study provides fundamental insights into the mechanism of porosity formation during laser welding of Al alloys and guidance in keyhole-induced porosity prevention.