Expulsion investigation and fracture characteristics in spot welded advanced high strength auto steels

Increased use of advanced high strength steels in resistance spot welding (RSW) is necessary for manufacturing safe and affordable vehicles. Unfortunately, the investigation of expulsion monitoring and control of advanced high strength steels (AHSS) is limited. A data acquisition system was designed for monitoring weld expulsion via the measurement of voltage, current, dynamic resistance, electrode force and displacement. Three control strategies were proposed on the basis of the rate of change in the dynamic resistance, the electrode force and the electrode displacement. Micro-hardness tests, tensile shear tests and fatigue tests were carried out. The expulsion effects on the mechanical properties, fracture modes, and metallurgical features were investigated using optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Fatigue crack initiation locations were also observed, which were verified by the theorectical stress analysis. In addition, the causes of interfacial fracture were discussed in relation to weld parameters.