Multi-stage keyhole evolution in fiber laser welding: An experimental study and theoretical analysis

Laser welding with high beam intensity generates a metal vapor, which partially displaces the metal melt, forming a narrow keyhole-shaped fusion zone. This study conducted theoretical analysis of the keyhole formation processes within fiber laser welding, based on three types of experimental observation. During the welding mode transition process, the keyhole formation was abrupt due to the jump-like absorptivity variation in the concave molten pool. In the primary stage of the laser keyhole welding process (when the laser was turned on), the keyhole depth grew rapidly and then slowed down before reaching a steady state. The keyhole formation time was related to the welding characteristic time (WCT), determined as the ratio of laser spot diameter to the welding speed. In the steady stage of fiber laser keyhole welding, the keyhole formation process was a periodic motion of the laser beam on the front keyhole wall (FKW). The periodic drilling occurred because the laser beam was irradiated on the FKW, with the inclination angle fluctuating around the Brewster angle.