Control of the Martensitic Transformation During Resistance Spot Welding of High Strength S700MC Steel

Resistance spot welding (RSW) is an extensively used joining method especially in automotive industry. In this method, Joule heating resulting from electrical current application is employed to melt and join sheet metals by squeezing sheets together with water cooled electrodes. For steels, due to heating and fast cooling during RSW process, martensitic transformation occurs at weld section. A specified holding time after the current application is used in RSW so that joint can be cooled by contact between electrodes and sheets. Therefore, holding time in RSW is a major factor for cooling behavior and hence, martensitic transformation. In this study, effects of holding time on mechanical properties and microstructures of RSW joints were investigated. It was observed that increasing holding time decreased weld nugget size due to evacuation of heat from sheets. On the other hand, mechanical properties, namely tensile-shear strengths and failure energies were increased by increasing holding time from 1 to 20 cycles for 1.2 mm thick S700MC steel sheets. However, holding time of 30 cycles had adverse effects on mechanical properties. These differences were mainly the results of more favorable martensitic transformation when holding time of 20 cycles was employed compared to other cases.