Investigation effect of resistance spot welding parameters on dissimilar DP1000HF/CP800 steel joints

Recently, advanced high strength steels (AHSS) have gained importance especially in automobile industry due to excellent tensile and elongation properties. Dual phase and complex steels are among prominent members of the AHSS family. DP steels contains martensite within the ferrite matrix while the CP steels contains bainite, retained austenite, martensite and ferrite. The tensile strength of these steels can reach up to 1200 MPa. Also, new grades of the DP steels offer high formability with high tensile strength. In this work, resistance spot weldability of dissimilar CP800 and high formability grade of DP1000HF steel joint were investigated. Tensile-shear tests, weld nugget diameters and electrode indentation depths were evaluated with microstructural examinations. A weld lobe was determined in the light of obtained findings and standards. According to the results, SCHAZ of the DP1000HF is the weakest region of the joint affecting the tensile-shear strength. Maximum tensile-shear strength was 26380 N at 25 cycle welding time and 9.44 kA welding current where diameter of the weld nugget was measured as 9780 µm. Interfacial fracture separation mode was generally obtained below weld nugget diameter of 7500 µm. CP800 exhibited less electrode indentation depth compared to DP1000HF. The difference in electrode indentation depths between CP800 and DP1000HF was increased by raising the heat input.