Keyhole-induced pore formation mechanism in laser-MIG hybrid welding of aluminum alloy based on experiment and multiphase numerical model

An innovative 3D multi-phase numerical model considering droplet transfer, metallic vapor and molten pool dynamics was developed to study the melt flow and keyhole-induced pore formation mechanism in laser-metal inert gas (MIG) hybrid welding process. It mimics the whole process of bubble generation, movement, merger, shrinkage, and captured by the solidification front of bubbles in the molten pool for the first time. The results show that a counterclockwise eddy (0–0.8 m·s−1) and a clockwise eddy (0.4–2.2 m·s−1) generally occur in the upper and lower zones of the molten pool respectively driven by the metallic vapor and recoil pressure. A trap zone with near-zero flow velocity appears between those two eddies. The clockwise eddy in the lower zone tends to make the keyhole collapse and wrap up some vapor from the keyhole tip to form bubbles. Those two eddies could merge into one at a relative lower welding speed and the melt flow will bring some of the bubbles into the trap zone and to form pores. At a high welding speed, the clockwise eddy in the lower molten zone has a lower flow velocity (0.4–1.2 m·s−1), which helps transfer the bubbles back into the keyhole and thus results in fewer pores. Methods such as swinging arc or laser that can destabilize the trap zone are suggested to reduce porosity. •An innovative 3D multi-phase numerical model considering droplet transfer, metallic vapor and molten pool dynamic was developed firstly to simulate the formation process of keyhole-induced pore in laser-MIG hybrid welding of aluminum alloy.•Showing that the process of formation, movement, merger, shrinkage and captured by the solidification front of metallic vapor bubbles in the molten pool in detail in laser-MIG hybrid welding of aluminum alloy by numerical model.•The reason why the porosity is lower at high welding speed than at low welding speed is explained from the new perspective of molten pool flow mode in laser-MIG hybrid welding of aluminum alloy by experimental results and numerical model.