Analysis of Bubble Flow in the Deep-Penetration Molten Pool of Vacuum Electron Beam Welding

Based on the vacuum electron beam welding with deep-penetration process, the convection phenomenon of the bubble flow in partially penetrated and fully penetrated molten pool of AZ91D magnesium alloy was simulated under the unsteady-state conditions. At the same time, the distributions of the cavity-type defects in deep-penetration weld were studied. The results showed that the cavity-type defects are more prone to distribute at the bottom of the weld and accumulate along the axis of the weld for the partially penetrated weld seam; there is a high incidence of cavity-type defects in the middle of the weld for the fully penetrated weld seam. As a smooth escape channel for the gas phase is formed in the fully penetrated molten pool, the possibility of gas escaping is much higher than that in the partially penetrated molten pool. A high liquid convection velocity is more conducive to the escape of the gas in molten pool. The liquid convection velocity in the fully penetrated molten pool is higher than that in the partially penetrated molten pool. So, the final gas fraction in the fully penetrated molten pool is low. Therefore, the appearance of cavity-type defects in the fully penetrated weld seam is less than that in the partially penetrated weld seam.