Achieving columnar-to-equiaxed transition to reduce hot cracking susceptibility in TIG-welded AZ31 alloy through MgO modification

The fusion zone (FZ) of AZ31 alloy typically exhibits undesirable coarse columnar grains, often accompanied by solidification cracks, resulting in a decrease in the mechanical properties of the welded joint. In this work, a novel welding wire containing MgO particles has been developed to promote the columnar-to-equiaxed transition (CET) in TIG-welded AZ31 alloy. The results indicate that the liquid film caused by Al and Zn segregation, along with the high thermal stress induced by columnar grains, is responsible for the formation of solidification cracks. Moreover, cracks are prone to propagate along high-angle grain boundaries exceeding 30°. A fully equiaxed grain structure in the FZ is obtained through MgO modification, with a significant 65.8% reduction in grain size to 70.5 μm from the original coarse columnar dendrites. The high number density of MgO particles and good lattice matching between MgO and the α-Mg matrix, with the orientation relationship of \([11\overline{2}0]_{{\alpha - {\text{Mg}}}}\) ${[11\overline{2}0]}_{\alpha -\text{Mg}}$ // \([01\overline{1}]_{{{\text{MgO}}}}\) ${[01\overline{1}]}_{\text{MgO}}$ and \((0002)_{{\alpha - {\text{Mg}}}}\) ${(0002)}_{\alpha -\text{Mg}}$ // \((111)_{{{\text{MgO}}}}\) ${(111)}_{\text{MgO}}$ , are crucial factors contributing to grain refinement, thereby reducing the susceptibility to hot cracking of the welded joint. This study presents an innovative method for promoting CET and suppressing crack formation in the FZ of Mg alloys.