A novel technology has been developed for the resistance spot welding (RSW) of magnesium to steel producing joint strength as high as 95 pct of that of Mg to a Mg joint. The mechanisms of the dissimilar joining have been investigated using mechanical testing and metallurgical examination employing scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The results show that the mechanisms of joining during RSW of a magnesium alloy to Zn-coated steel involve braze welding, solid-state bonding, and soldering. The joint formation in comparison of RSW of Zn-coated steel with steel, Au-plated Ni, and bare Ni sheets is discussed. A possible expansion of this technology also is suggested.

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 Al 8 Mn 5 second-phase particles. Submicron Al 8 Mn 5 particles of 0.09 to 0.4 lm in length occured in both SA and SB alloys; however, larger Al 8 Mn 5 particles of 4 to 10 lm in length were observed only in the SA alloy. The welding process did not have a great effect on the populations of Al 8 Mn 5 particles in these AZ31 welds. The earlier columnar-equiaxed transition (CET) is believed to be related to the pre-existence of the coarse Al 8 Mn 5 intermetallic phases in the SA alloy as an inoculant of a-Mg heterogeneous nucleation. This was revealed by the presence of Al 8 Mn 5 particles at the origin of some equiaxed dendrites. Finally, the columnar grains of the SB alloy, which did not contain coarse secondphase particles, were efficiently restrained and equiaxed grains were found to be promoted by adding 10 lm-long Mn particles into the fusion zone during resistance spot welding (RSW).

No abstract

No abstract

No abstract