Effect of initial/tail of the pulse waveform on surface morphology of AZ31 magnesium alloy laser spot welding

Effects of initial/tail of the pulse waveform on coupling efficiency and surface morphology of AZ31 magnesium alloy laser spot welding is investigated. Research shows that initial ramp-up waveform can improve the laser spot welding absorptance, but with limited effect (0.88 → 0.9), and melting time of ramp-up waveform is longer than rectangular for equal pulsed energies, making the welding energy coupling efficient and diameter larger. The ramp-down waveform can extend duration (9 ms → 12 ms) of molten pool above melting point temperature (630 °C), increase Marangoni number (R1: 29.29, R2: 32.11, R3: 23.70, R4: 26.62), slow down cooling rate (3 × 106 °C/s → 1 °C/s) and recoil pressure (R3: 52.90 P0 → 12.16 P0, R4: 95.95 P0 → 20.58 P0) of instantaneous closure of rectangular waveform, reduce the sensitivity of welding defects such as overfill, undercut and spatter, and improve the surface morphology of laser welding spots. •Initial/closing pulse shaping affects the welding morphology and coupling efficiency.•Coupling efficiency and welding diameter can be improved by ramp-up pulse.•Sensitivity of laser welding defects of Mg can be reduced by ramp-down pulse.•Morphology of welding spots are optimal with Ramp-up + Rectangle + Ramp-Down.