Degradation Mechanism of Electrode Tip during Resistance Spot Welding of Zinc-coated Galvannealed Steel Sheets

This study investigated the electrode life and the electrode degradation characteristics during resistance spot welding of zinc-coated galvannealed steel sheets. As a result, it was found that the electrode life in alternate resistance spot welding of galvannealed steel sheets and bare steel sheets was extremely short in comparison with welding of only galvannealed steel sheets. During alternate resistance spot welding, the ring-shape nuggets were formed easily. The reasons for this is explained by the sticking of the alloy layers formed on the electrode top during previous welding of the galvannealed steel, to the bare steel sheet surface at the following welding of bare steel, so that notable wear took place and the surface profile of electrode tip became flat. Additionally an accumulation of highly resistant carbonaceous substance layer at the center of the electrode top caused an annular flow of the welding current to hinder the nugget formation.