Microstructural Evaluation and Influence of Welding Parameters on Electrode Plunge Depth in Resistance Spot Welded Dissimilar DP800HF/1200M Steel Joints

Abstract

Advanced high strength steels (AHSS) are newly developed steels that has versatile mechanical properties. These steels enables to design low weight cars with high safety standards. Also, weight reduction in vehicles plays a significant role for saving fossil fuels which is limited and causes carbon emissions. Dual phase (DP) and Martensitic steels are prominent in AHSS family because they are inexpensive and has vast application areas. DP steels are used for general purpose applications and Martensitic steels are used for reinforcement parts in vehicles. In this study, high formable grade Dual phase steel with 800 MPa tensile strength and Martensitic steel with 1200 MPa tensile strength were welded with resistance spot welding technique which is the most widely practiced joining method in the industry. Electrode indentation depths, its effect on tensile-shear loads and microstructural characterizations were investigated. According to the results, the lowest tensile shear loads were acquired between both between 0-0.2 mm and between 0.85-1mm electrode plunge depths. Medial electrode plunge depths showed high tensile shear loads. Some welding defects were encountered including secondary phase formations, shrinkage voids, intergranular shrinkage gaps and vertical cracks in the weld nugget. It is find out that the weld defects were formed due to cooling gradient while solidifying, electrode force, and improper weld parameters.