Contact conditions on nugget development during resistance spot welding of Zn coated steel sheets using rounded tip electrodes

► Nugget formation in resistance spot welding process strongly depends on interfacial conditions. ► The global heat produced in the assembly is mainly governed by the evolution of the contact surfaces. ► Zinc coating on the steel sheet influences the interfacial conditions, particularly the contact resistances. ► Ejection of molten zinc at periphery of the contact zones does not radically change the nugget formation and development. ► The nugget growth and maximum size are predicted using finite element simulation. This paper investigates the influence of coating on the nugget development during resistance spot welding of Zn coated steel sheets using rounded tip electrodes. The study relies on an experimental observation of zinc behaviour coupled with a numerical Electro-Thermo-Metallurgical–Mechanical analysis. The electro-thermal contact surface enlargement due to zinc ejection at sheet/sheet and electrode/sheet interfaces is found to have a negligible effect on the nugget development. The numerical analysis shows that the dispersion on the efficient weld size, attributable to the zinc effect, is below 10%.