This paper presents a comparative study of the AC and MFDC resistance spot welding process with consideration of sheet thickness. The previous studies have confirmed that there is difference in the optimum welding current and expulsion current with AC and MFDC. The aim of this study was revealing the effect of sheet thickness on weldable current range and expulsion behavior for AC and MFDC welding processes. The optimum welding current of AC was lower (1.6 kA) than MFDC welding process in 0.8 mm sheet thickness. Early nugget growth being caused by the peak current of AC developed weld interface deformation, which resulted in suppressing the growth of corona bond and occurrence of low current expulsion. The resistance spot welding for thicker sheet (1.4 mm) required lower current of 0.6 kA for the expulsion on the MFDC welding process. The growth of contact diameter (size of corona bond) and button diameter was linear up to the expulsion current with MFDC welding process. Therefore, more attention is required when the AC and MFDC resistance spot welding process is applied for different thickness of steel sheet combination for automotive application.

In this study, the effect of Al-Si coating melting behavior and weldability during the resistance spot welding of hot stamped boron steels was investigated. In the case of the Al-Si coated hot-stamped boron steel, high heat was generated on the surface and the oxide layer by the intermetallic coating, and the molten coating layer pushed toward the outer edge of the weld like a film of liquid. The liquid coating layer accumulated at the periphery, preventing current flow and extension of the contact area, and this influenced the height of the nugget growth. As a result of the limited nugget growth, weld strength decreased. However, in the case of bare hot stamped boron steel, relatively low heat was generated due to the large contact area, and the nugget grew in the longitudinal direction. Compared to the nugget diameter of Al-Si coated hot-stamped boron steel, this can result in superior weldability.

In dissimilar overlap joints that are welded using gas metal arc welding, the shape of the weld bead changes due to changes in welding variables such as current, voltage, speed, and teaching position. The bead shape is a major factor that influences the evaluation of the tensile strength. Thus, it is crucial to control the shape of the bead and produce a bead with an optimal shape. In particular, in the case of a dissimilar combination with a thickness ratio of 4:1, it is difficult to weld all the joint surfaces of the upper plates. The presence of such a non-jointed area can change the tensile strength according to the bonded area. It is expected that a particular value of the critical jointed area satisfies the tensile strength constraints. In this study, various bead shapes were fabricated according to the welding parameters of dissimilar combinations with a thickness ratio of 4:1, and the corresponding tensile properties were evaluated. In addition, regression analysis was used to investigate the correlation between the bead shape and tensile strength.