- Abstract Gas tungsten arc (GTA) welds were performed on 4-mm thick plates, machined from as-cast magnesium AZ91D ingots. The microstructure and defect formation was investigated by optical and scanning electron microscopy. Mechanical properties were determined by standard tensile tests on small-scale specimens. A wide heat affected zone (HAZ) (>3 mm) was created adjacent to the fusion line that consisted of two regions: (1) a partially melted zone (PMZ), created near the fusion line; and (2) a wide region, which was heat affected without melting. It was found that, after resolidification, a continuous Al 12 Mg 17 phase existed along the PMZ grain boundaries that markedly reduced the joint strength to below that of fully annealed Mg-AZ91D alloys. In contrast, specimens machined from welded metal exhibited improved strength and ductility that resulted from the microstructural refinement caused by the rapid cooling during resolidification of the fusion zone. When the creation of a large PMZ was prevented, as in electron beam welding, the joint strength was comparable to that of the base metal.