Effect of internal oxidation on the weldability of CMnSi steels

Surface oxide formation from the high alloying contents in advanced high strength steels is detrimental to their Zn coating quality. Selective oxidation by annealing is used in steel production to improve the wettability of Zn to steel surfaces in the continuous hot-dip galvanizing process. Internal (sub-surface) oxides formed in the annealing process suppress surface oxide formation. However, the effect of internal oxides on resistance spot welding has not been thoroughly investigated. In this study, internal oxides were found to cause the weld lobe to shift towards higher welding current by promoting shallow surface melting early in the weld cycle. Molten metal at the contact surface reduces contact resistance and heat input, delaying nugget growth. Dynamic resistance profile and heat input analysis results further confirm the surface melting mechanism in annealed samples.