Abstract The gas-tungsten arc (GTA) welding behaviors of the commercial AZ91 magnesium alloy were examined in terms of process efficiencies and microstructure characteristics. This study focused on the effects of GTA welding process parameters (like welding current in the range of 100/300 A and welding speed in the range of 3.33/13.33 mm/s) on energy absorption by the substrate material. The dependences of arc and welding efficiency on the used process parameters were presented. The measurements revealed that the arc efficiency values ranged from 0.63 to 0.88. Melting efficiency was found to rise with both increasing welding current and speed. The analyses revealed a strong influence of the GTA welding process on the width and depth of the fusion zone and also on the refinement of the microstructure in the fusion zone. The results of dendrite arm size (DAS) measurements were presented. Additionally, the presence of a partially melted zone (PMZ) was disclosed.