Study on mechanism of the effect of Al element on the arc shape and molten pool fluctuation pattern in flux bands constricting arc welding (FBCA)

Herein, to investigate dynamic penetration control in ultra-narrow gap grooves, the effect of Al addition at a fraction of 0.2 %–1 % in flux bands on welding arc shape changes and molten-pool fluctuations in flux bands constricting arc (FBCA) welding is examined via a high-speed camera and side glass observation system. The influence mechanism of Al on weld formation is analyzed based on the weld depth-to-width ratio. Results reveal that as the Al content increases, the profile of the welding arc shifts from a divergent spherical shape to a contracted ellipsoidal shape, and the arc-burning position changes from the middle to the lower section of the groove. In addition, Al changes the side flow morphology of the molten pool from a shallow concave to a deep single-V shape, thus accelerating the molten pool flow and resulting in a humped molten pool. As Al in the flux bands changes the molten-pool Marangoni convection modes, the weld formation is affected significantly. When the Al addition is at 0.80 %, the weld depth-to-width ratio reaches a maximum of 1.32, and the joint presents the best welding quality. Meanwhile, at 0.92 % Al addition, fluctuations in the weld depth-to-width ratio increase, porous slag inclusions are observed, and the welding quality degrades. This occurs because Al3+ vapor is preferentially ionized, deoxidation within the arc induces further contraction of the arc, and the arc force impact on the bottom of the molten pool increases. In addition, Al2O3 is regenerated in the molten pool and accumulates on the surface, thus resulting in the highest levels of flow and solidification in the molten pool. However, when excess Al is added to intensify the molten pool reaction, the weld deteriorates.