Effects of zinc layer thickness on resistance spot welding of galvanized mild steel

This study investigated the resistance spot welding (RSW) parameters for an uncoated mild steel sample and two hot-dip galvanized steel samples with different zinc layer thicknesses. Two RSW schemes were considered: a one-step scheme with no pre-heating stage and a two-step scheme comprising a pre-heating stage followed by a main welding stage. The strengths and weldabilities of the weldments were evaluated by peeling tests, cross-tension tests and weld lobe diagrams with welding currents ranging from 5 to 12 kA and welding times of 8-24 cycles. The optimal welding time was found to be 16 cycles. The weldability window shifted to a higher welding current region as the zinc layer thickness increased. Compared to the one-step RSW scheme, the two-step scheme suppressed interfacial failure and achieved a higher peeling load at a lower welding current. For a given welding current, the weldments produced using the two-step scheme had a larger nugget diameter and higher cross tension load. Overall, the two-step RSW scheme achieved a better welding quality than the one-step scheme irrespective of the zinc layer thickness.