Numerical analysis of the influence of molten pool instability on the weld formation during the high speed fiber laser welding

•A numerical simulation method for molten pool instability analysis is proposed.•The relationship between molten pool profile and Rayleigh instability is revealed.•The weld formation is greatly affected by the molten pool behaviors.•The simulated results show good agreement with the experimental results. The dynamic behaviors of the molten pool play a critical role in determining the weld bead formation process and the welded joints quality. A numerical simulation modeling method is proposed to investigate the instability of the molten pool and understand its influence on the formation process of the weld and the related welding defects during the high speed fiber laser welding. In the modeling, the molten metal flowing dynamic behaviors and free surface evolution of molten pool are coupling taken into account. The instable phenomena including the profiles variation of the molten pool surface in the vertical direction and edges oscillation in the horizontal direction are demonstrated and discussed in the simulation results in details. The Rayleigh instability of the molten pool is numerically approved and the relationship between the molten pool profile and Rayleigh instability is revealed, which is kept consistent with the experiment and literature results. It is found that the welding defects formation process in the weld is greatly affected by the instable phenomena in the molten pool. The oscillation and fluctuation of the molten pool surface in the horizontal direction and vertical direction during the cooling process can directly result in the defects formation in the weld. [Display omitted]