The role of liquid feeding in nugget-edge cracking in resistance spot welding of dissimilar magnesium alloys

In fusion welding, cast magnesium alloys typically present higher sensitivity to liquation cracking than wrought alloys. In this study, a fluid-mechanics-based analytical model was established to investigate an unusual phenomenon: a lack of cracking in cast magnesium alloy while cracking took place in wrought magnesium alloy. This was observed in resistance spot welding, in which the pressure within the molten nugget allows the liquid to feed through the interface between the fusion zone and the partial melting zone, an aspect which is not generally considered in the typical criteria for liquation cracking. The analytical model shows that the liquid is easy to feed into the inter-grain zone of a cast coarse-grained metal but difficult to feed into wrought metal. The liquid feeding effect changes the composition of the inter-grain region, causing the semi-solid grains to bond to each other tightly in the cast magnesium alloy. Electro-probe microanalysis illustrates the mass transfer pattern of liquid feeding at the interface between the fusion zone and the partial melting zone. Magnesium alloys AZ31, AZ91, and ZK61 were selected to carry out resistance spot welding tests of which results match the theory raised in this work well.