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In order to understand the mechanism of pore formation and accurately simulate the dynamic process of keyhole-induced pore in laser deep penetration welding, a three-phase mathematical model of laser keyhole welding is established to reflect the formation process of bubble, and a ray tracing method based on Particle level-set method is taken to track free surfaces of keyhole and pore. The results shows that the depth and shape of keyhole have an obvious characteristic of periodic changes and a phenomenon of high-frequency oscillations in the process of laser deep penetration welding, and the high-frequency oscillations of the keyhole are the main factors of laser welding instability and induced the collapse of keyhole and bubbles forming in the molten pool. These bubbles moved following the fluid flow in the molten pool, where some bubbles could escape out of molten pool under the competition of flow and solidification speed. But some bubbles captured by a solidified wall during the migration process in the molten pool would evolve into porosities. A good agreement between simulation and experimental results proved the reliability of this mathematical model, while the mechanism of pore formation can better illustrate with the model.

Numerical simulation of formation process of keyhole-induced pore for laser deep penetration welding