Effects of Steel Coatings on Electrode Life in Resistance Spot Welding of Galvannealed Steel Sheets

The effects of different galvannealed (GA) coatings, containing Fe varying from 7.0 to 11.4 mass%, on steel sheets on the electrode life in resistance spot welding (RSW) have been investigated with metallurgical analysis of the coating microstructures and properties, and the surfaces and cross-sections of failed electrodes. The results showed that the electrode life in RSW of GA steel with 11.4 mass% Fe in coating was 110% higher than that with coatings containing 7.0 or 9.6 mass% Fe. The improvement was believed to be caused by the build-up of a Fe-rich alloy layer on the electrode surface, which could serve as a barrier to prevent copper loss from the electrode surfaces to the steel sheets, thus reducing the growth rate of the electrode tip face diameters. In addition, higher Fe content in the coating resulted in increased contact resistance and hence a lower welding current needed in RSW.