Effect of process parameters on microstructure and microhardness of resistance spot welding in Zr-1%Nb alloy

In this study, the effects of process parameters including weld current and electrode force on the microstructure of resistance spot welding on Zr-1%Nb alloy sheets of 0.3mm thickness were investigated. In the microhardness investigation of the weld nugget cross section, the Vickers hardness measurement was performed in the direction of the weld nugget diameter on a base metal, HAZ, and weld nugget for the welded specimens with different parameters. Applying more electrode force in the same welding time yielded the grain growth which was found to be an important observation in this study. In the metallographic images, it was observed that the grains were deformed to be longer in the heat flow direction when more electrode force was applied. The hardness measurement of the weld zones and base metal showed that, the weld nugget had a maximum amount of hardness where it decreased in HAZ and subsequently in the base metal. This difference is due to transformation of the α+β and β phases. Transformation from β to α phase, which was carried out by a massive diffusion process and formation of the Widmanstatten α structure was achieved by slow cooling. The mentioned structure was found to be harder and stronger than the equiaxed α microstructure of the weld metal.