Effect of pulse shaping on coupling efficiency of laser spot welding in AZ31 magnesium alloy

Energy coupling efficiency has an important effect on welding quality in pulsed laser spot welding. In this paper, laser spot welding experiments are conducted on 3 mm AZ31 magnesium alloy using an AC-500 W Nd:YAG pulsed laser welder. Results show that the rectangular pulse (R) energy coupling is the most efficient when peak power is less than or equal to 3 kW, but the energy coupling of ramp-up pulse (R-U) is the most efficient when peak power is greater than 3 kW. When the peak power is 4 kW, the energy coupling efficiency of laser spot welding under R-U pulse is the highest. At this time, the cross-sectional area of welding spot reaches 1.3 mm², peak temperature is 1550 ℃, the metal loss of melting pool is 4.4 mg, the depth-to-width ratio of keyhole is 1.06, and the laser absorptance is 0.9. In addition, a numerical model of velocity field of transient keyhole laser spot welding is established by using ANSYS, and the shape and size of keyhole are calculated during laser spot welding. The mechanism of pulse shaping to improve the energy coupling efficiency of deep-melt laser welding of magnesium alloys is revealed.