Mechanical properties and nugget evolution in resistance spot welding of Zn–Al–Mg galvanized DC51D steel

Zn–Al–Mg coating galvanized steel in resistance spot welded (RSW) in different configurations of DC51D was investigated to illustrate the nugget evolution process and mechanical properties of the joints. Results show that the microstructure of welded joints can be divided into nugget zone (FZ), heat-affected zone (HAZ), and base metal zone (BM). FZ was composed of lath martensite. The average hardness value of the weld joint was 110 HV0.2 while the FZ was up to 300 HV0.2 due to the formation of lath martensite. The failure modes can be divided into interface fracture (IF) and pull-out fracture occurred (PF) under different welding parameters, in which shear dimples showed had a typical plastic fracture morphology. The best range for welding parameters was found to be 12–18 cycles in which the nugget diameter reached 5.5 mm. The process of nugget evolution in HAZ and FZ was discussed.