Microstructure and mechanical properties of matching and non-matching resistance spot welds of DP450 and DP800 steels

Abstract In this study, resistance spot welding (RSW) joints of DP450 and DP800 steels were produced using three different electrode forces (2.1 daN, 2.8 daN, 3.5 daN) and three different welding currents (9 kA, 10 kA, 11 kA). The effects of different welding parameters on microstructure, tensile shear force (TSF), failure mode, and microhardness were investigated. The focus was on optimizing the welding parameters. As a result, the RSW process led to the formation of three distinct regions in the microstructure: the base metal (BM), the heat-affected zone (HAZ), and the weld metal (WM). Generally, the TSF increased with higher heat input resulting from decreased electrode force and increased welding current. However, in similar joints, an increase in welding current from 10 kA to 11 kA caused expulsion, negatively affecting the welding quality. The maximum TSF value (15.59 kN) was obtained in DP800-DP800 joints welded with 2.1 daN-10 kA welding parameters. After the tensile shear tests, three different failure modes were observed: Interfacial failure (IF), partial interfacial failure (PIF), and pull-out failure (PF). Hardness values exhibited a linear correlation with TSF results. Furthermore, a significant increase in hardness values was observed from the BM to the WM across all welding parameters.