Effects of pulse shaping on Nd:YAG laser spot welds in an AZ31 magnesium alloy

The effects of pulse shaping on the initial coupling, welding defects, and microstructures are studied to improve the processing adaptability of pulsed Nd:YAG laser spot welding with an AZ31 magnesium alloy. Defects within laser spot welding with rectangular pulses are very serious, and step-down (SD) pulse shaping can effectively reduce the appearance of defects such as porosity, overfills, undercuts, and solidification cracks. With prolongation of the slow cooling period in an SD pulse, the area percentages of porosity and overfill and the undercut depth decrease. Increasing peak power of the slow cooling period can reduce the sensitivity to solidification cracking. If the peak power during the slow cooling period is too high (40% or 50% of the peak power during fusion welding), cracking sensitivity will increase again. The coupling effect of the ramp-up pulse is stronger than that of the rectangular pulse and leading-spike pulse at the same pulse energy. The microstructure at the upper centre of the spot-welding cross-section changes from equiaxial crystals to columnar crystals to cellular crystals with increasing peak power of the slow cooling period (10 ms) in the SD pulse.