Pulsed Nd:YAG laser spot welding of an AZ31 magnesium alloy

The relationships between the processing parameters and the welding modes, dimensions, microstructures, and defects are studied for pulsed laser spot welding of an AZ31 alloy. Conduction mode occurs at the welds with peak power densities between 2.5 and 4.5 GW/m2, and when the peak power density exceeds 4.5 GW/m2, the welds of keyhole mode are generated. In conduction welding, the laser beam radiates on the pool surface, and the melt flow is driven by the buoyancy convection and Marangoni convection, resulting in a wide and shallow cross section and the welding diameter and penetration are not affected by processing parameters. For keyhole welding, the laser beam passes through the keyhole and directly radiates on the base material at the bottom of the molten pool under the influence of recoil pressure, which increases the absorption rate and then greatly increases the welding penetration and aspect ratio. In the keyhole, a large bubble is formed, which is mainly composed of magnesium vapor, causing porosity and overfill after welding. The cooling rate in the deep of the molten pool is higher, which is helpful to the formation of solidification cracks. The area percent of the porosity and overfill and the solidification crack sensitivity increase with peak power density. As the energy density increases, the temperature gradient becomes larger, and the constitutional supercooling decreases, leading to the transformation of microstructures near the fusion boundary of the welding surface from columnar crystals to cellular crystals, and the area of the cellular crystals gradually widens.