Double enhancement in porosity suppression and penetration depth during laser welding Al–Mg alloy via high-frequency outward-spiral scanning mode

Abstract

Achieving substantial penetration while suppressing keyhole-induced porosity remains challenging during laser welding of Al–Mg alloys. Current studies primarily address keyhole porosity by utilizing the stirring effect of the scanning laser beam on the molten pool, which reduces energy density. Herein, we developed a high-frequency outward-spiral scanning laser mode, where the laser beam within a single keyhole moves reciprocally, eliminating wall convexity. Results demonstrated that this mode effectively suppressed keyhole collapse and pore formation while maintaining higher central energy density. Specifically, the keyhole collapse frequency decreased dramatically from 137.8 Hz with a conventional laser to 8.5 Hz using the high-frequency outward-spiral scanning laser. Under scanning laser welding condition, the keyhole walls remained flat and smooth, without swelling capillary waves and humps. Concurrently, the homogenized laser energy deposition on the keyhole wall and effectively mitigated the impact of locally vaporized Mg, preventing keyhole closure and pore formation. Additionally, the elevated central energy density ensured sufficient penetration, increasing the penetration depth by approximately 30 % compared to stirring modes. In contrast with conventional laser liner welded joint, the optimal outward-spiral scanning laser welded joint exhibited superior strength-ductility synergy, and the tensile strength reaching 371 ± 2 MPa (21.0 % enhancement) with an elongation of 9.8 ± 0.3 % (69.0 % increase).