Plates of AZ91 cast magnesium alloy with a thickness of 3.5 mm were butt-welded using a laser power of 2000 W and helium as the shielding gas. The effect of the welding speed on the weld cross-sectional geometry and porosity was determined by microscopic analysis. It was found that to avoid the formation of macropores, welding should be carried out at a speed of 3.4 m/min or higher. Non-equilibrium solidification of the laser-melted metal causes fragmentation of the weld microstructure. Joints that were welded at optimal laser processing parameters were subjected to structural observations using optical and scanning microscopy and to mechanical tests. The mechanical properties were determined through Vickers hardness measurements in the joint cross-section and through tensile testing. The results indicate that the hardness in the fusion zone was about 20 HV (30%) higher than that of the base material. The weld proved to be a mechanically stable part of the joint; all the tensile-tested specimens fractured outside the fusion zone.

Plates of AZ91 magnesium alloy were butt-welded using a CO2 laser. The non-equilibrium solidification of the laser-melted metal caused fragmentation of the weld microstructure as well as the supersaturation of a solid solution of aluminium in magnesium, which enabled the T5 ageing of the weld. The weld proved to be a mechanically stable part of the joint; all the tensile-tested specimens, both as-welded and post-weld T5 aged, fractured outside it. During the ageing of the supersaturated joint, which involved heat treating it to the T6 condition, the weld was the region where discontinuous precipitation was observed and this was the location of fracture in the tensile specimens. Thus, the strength properties of welded, supersaturated and aged AZ91 were much worse than when the non-welded material was T6 tempered.