Grain refining can improve the solidification-cracking resistance and mechanical properties of welds. Mg alloys are increasingly used for vehicle weight reduction, AZ91 Mg being the most widely used Mg casting alloy. The present study was conducted to study grain refining in gas–tungsten arc welding of AZ91 Mg, focusing on identifying the mechanism of grain refining. Significant grain refining was obtained by transverse arc oscillation. Dendrite fragmentation was identified as the grain refining mechanism, instead of heterogeneous nucleation or grain detachment. This was done by using an overlap welding procedure in which the weld made with transverse arc oscillation (test weld) overlapped with another weld made previously without arc oscillation (preweld). It was found that repeated prewelding followed by welding with transverse arc oscillation resulted in very fine grains.
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