Resistance-Spot-Welded AZ31 Magnesium Alloys: Part I. Dependence of Fusion Zone Microstructures on Second-Phase Particles

A comparison of microstructural features in resistance spot welds of two AZ31 magnesium (Mg) alloys, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), with the same sheet thickness and welding conditions, was performed via optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). These alloys have similar chemical composition but different sizes of second-phase particles due to manufacturing process differences. Both columnar and equiaxed dendritic structures were observed in the weld fusion zones of these AZ31 SA and SB alloys. However, columnar dendritic grains were well developed and the width of the columnar dendritic zone (CDZ) was much larger in the SB alloy. In contrast, columnar grains were restricted within narrow strip regions, and equiaxed grains were promoted in the SA alloy. Microstructural examination showed that the as-received Mg alloys contained two sizes of Al8Mn5 second-phase particles. Submicron Al8Mn5 particles of 0.09 to 0.4 mu m in length occured in both SA and SB alloys; however, larger Al8Mn5 particles of 4 to 10 mu m in length were observed only in the SA alloy. The welding process did not have a great effect on the populations of Al8Mn5 particles in these AZ31 welds. The earlier columnar-equiaxed transition (CET) is believed to be related to the pre-existence of the coarse Al8Mn5 intermetallic phases in the SA alloy as an inoculant of alpha-Mg heterogeneous nucleation. This was revealed by the presence of Al8Mn5 particles at the origin of some equiaxed dendrites. Finally, the columnar grains of the SB alloy, which did not contain coarse second-phase particles, were efficiently restrained and equiaxed grains were found to be promoted by adding 10 mu m-long Mn particles into the fusion zone during resistance spot welding (RSW).