Evaluation of weldability for resistance spot welded single-lap joint between GA780DP and hot-stamped 22MnB5 steel sheets

Recently, in the automotive industry, Al-coated boron steel sheets (22MnB5) have been used for hot stamping, and the use of these sheets makes it possible to achieve a tensile strength of over 1,500 MPa, since a metallurgical transformation from austenite to martensite occurs during the process. In this study, resistance spot welding (RSW) experiments were performed in order to evaluate the weldability of single-lap joints between GA780DP and 22MnB5. The effect of the weld current on the nugget diameter and load-carrying capacity was evaluated by observing the nugget diameter and performing a tensile-shear test. Furthermore, the fracture behavior was evaluated by carrying out optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) observations. Ductile regions were observed on the interfacially fractured surface of the weld, and this implies that a high load-carrying capacity can be obtained even when interfacial fracture (IF) occurs. IF is caused by the stress concentration resulting from the presence of the sharp notch at the boundary of the nugget as well as by the high hardness and the brittle microstructure of the weld; the microstructure is brittle because of the high carbon equivalent (C-eq) and the penetration of Al in the weld.