Investigation on the keyhole/molten pool dynamic behavior during adjustable ring-mode laser welding of medium-thick aluminum alloy

Adjustable ring-mode laser welding (ARM) provides a new solution for the pore defects of aluminum alloy joints. However, the inhibition mechanism of ARM for porosity defects in the aluminum alloy welds with medium thickness is still lacking. In this work, the dynamic behavior of the keyhole/molten pool and the formation/ transport process of bubbles during adjustable ring-mode laser welding of medium-thick aluminum alloy are investigated by experiments and simulations. The results display that the laser power ratio of central-ring beam of 6:4, the porosity is 4.19 +/- 2.32 %, which is reduced by 52.2 % compared with the weld by single laser welding (SLW) of the same penetration depth. The addition of ring beam helps to enlarge the tilt angle of the keyhole rear wall, which increases the ability of the keyhole wall to resist the impact of the molten pool. Besides, it also lowers the radial pressure gradient and the local variation of the keyhole wall. These reasons reduce the formation of pores. Furthermore, the main eddy current direction of the molten pool changes less frequently, and the melt flow velocity impacting the keyhole wall only changes in the range of 0-0.5 m/s (SLW: 0.25-0.75 m/s). This diminishes the fluid dynamic pressure which is the primary driving force of the bubbles. The velocity of the bubble moving away from the bottom of the keyhole is decreased and it is more likely to remelt the keyhole and escape under the action of the laser beam at the next moment. This work provides engineering application and theoretical reference value for the suppression of pore defects in laser welding of aluminum alloy in medium thick plates.