Effect of pulse gas tungsten arc welding on the metallurgical characteristics & microstructural evolution in AZ31B magnesium alloy welds

This work assessed the effects of frequency & the pulse duration ratios on the weldability & structure-property relationships for the magnesium alloy AZ31B. In addition, AC/DC pulse welding with different pulse duration ratios was explored, to assess controlled repair welding on magnesium alloy castings. Complete penetration welds can be produced with conventional GTA welding using a heat input of ∼500 J/mm without the need for an active flux. However, the weld metal has a coarse grain structure and micro cracks, suggesting the need for an even better control of heat input for achieving defect free welds. Pulse GTA welding with at lower frequencies give relatively deeper penetration joints at different pulse duration ratios (1:1, 1:2 and 1:4). However, the full penetration welds are possible only with pulse duration ratio of 1:1. All the welds produced under pulsed GTA welding conditions were defect free. The grain sizes of the weld metals were much finer than for conventional GTA welding conditions and have a higher hardness compared to conventional GTA weld & also the parent alloy. Use of AC/DC mix for pulse welding, with the peak current being AC and the base current as DC, yields shallow penetration welds with no defects. The resultant weld metal microstructures are of finer grain size, and with finely distributed β-phase particles. This welding technique can be exploited for repair welding of magnesium alloy castings that have near-surface defects.