Weld growth mechanism of resistance spot welds in zinc coated steel

Whilst resistance welding has been the primary joining technique for automotive body-in-white (B-I-W) assembly since the 1930s, far more welds than necessary are used to fabricate the structure to achieve the required structural performance. This can be attributed due to lack of confidence in weld quality, particularly when joining three or more sheet combinations. To achieve the necessary confidence, there is a need to understand the mechanism of heat development and weld formation, and develop appropriate welding conditions to ensure acceptable weld quality. With the high proportion of zinc coated steels in B-I-W assembly, one of the key issues is to understand the role of the zinc coating in weld nugget development. Consequently, in this study, the mechanisms of weld nugget growth for three thickness joints of both uncoated low carbon and hot dip zinc (GI) coated steels have been investigated using similar welding conditions. The welding conditions selected were based on those recommended for the uncoated low carbon steel. Since these are unlikely to be optimal in the case of the coated steel, further studies were undertaken to establish the effect of electrode force on the mechanism of weld growth. The results obtained indicated that the rate of heat development was reduced in the case of the coated steel compared to the uncoated steel variant. This can be attributed to the lower surface resistance in the presence of the zinc coating. Variations in electrode force indicated flexibility in the selection of welding conditions to produce an acceptable weld quality. Where, an increase in the level of electrode force was used, a significant change in the location of heat development and weld nugget formation was noted compared to that observed for the low force variant. This was attributed to the change in the dominant resistive heating mechanism that took place.