Abstract The conventional gas metal arc welding (GMAW) of aluminium alloys often risks the occurrence of hydrogen porosity, which must be controlled. Intensifying the flow of liquid metal in the molten pool may be an effective strategy. The solidification time of the metal is prolonged, so that bubbles can escape from the molten pool more easily. In this study, a longitudinal external magnetic field was applied, which drove the rotation and geometry change of the arc. The arc expanded as the excitation frequency increased. This was caused by the rotation of charged particles in the arc being accelerated by the Lorentz force. The expanded and rotated arc drives the flow of molten pool. This results in a reduction in the temperature gradient close to the molten pool boundary, on the one hand. Eventually, an equiaxed grain zone was formed at the boundary. On the other hand, the high fluidity of molten pool facilitated the floating of bubbles, which contributed to the reduction in porosity.
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