Investigation on pore formation in pulsed laser spot welding of AZ31 magnesium alloy

Micropores exist in the conduction-mode welds (depth/width < 0.4), while the keyhole-mode welds (depth/width ≥ 0.4) contain macropores. The equation of critical nucleus radius of the micropores is derived, which is related to the recoil pressure, atmospheric pressure, metal pressure and surface tension. After calculation, the critical nucleus radius is 3.51 μm, which is consistent with the measured radius in practice. The equation of the shortest floating time of the macropores with radius as variable is proposed, and the shortest time of bubble floating (tmin = 11.35 ms) is calculated, which is larger than the longest solidification time (tmax = 9.5 ms) of the welding pool, thus quantitatively demonstrates the formation mechanism of the macropores. The influence of pulse processing parameters on micropores is not significant, while the area percent of the macropores increases with the increase of pulse peak power density.