Contact resistance is a complicated phenomenon and resistance welding significantly escalates its complexity. Interference of various parameters in resistance spot welding, despite extensive research in this field, still makes this process puzzling to the welding researchers. Various models have been developed to deal with the contact resistance, but more or less all of them end up with trial and error to yield an acceptable outcome. Quantifying the contact resistance is a critical problem in the modeling of the resistance spot welding. In this paper, the published models for the electrical contact resistance in the resistance spot welding process are reviewed. Since the total or dynamic electrical resistance is composed of bulk resistance and contact resistance, it is realized that contact resistance is the most essential factor in heat generation. The contact resistance, by itself, is composed of two parts, constriction resistance and film resistance. Some models have been proposed to explain the relationship for constriction resistance of a single point, while some of the other models have dealt with a number of points. In contrast with constriction resistance, few formulations have been suggested for film resistance. Meanwhile, in some models, the combined effects of both constriction and film are considered as general models. One key observation can be drawn from these studies, and it is that reported measurements have shown obvious discrepancies. Contact resistance modeling for simulation of resistance spot welding depends on the modeling of contact resistance after film breakdown. Therefore, mathematical modeling of the constriction resistance is the key to a comprehensive model of the contact resistance. Despite valuable studies that mainly are experimental, generally recognized model of contact resistance that can take into account all of the important influencing parameters has not been developed yet. This might be resulted from the complexity of contact resistance. In the previous studies, analysis of contact resistance is mostly qualitative rather than quantitative, because the performance of the films differs extensively. The major way to study the influence of diverse films has been experimental investigations. The electrical contact involves problematical interaction between contact members of complex geometry and it is not easy to model. Only for the constriction resistance of some simplified cases analytical models are available. In resistance spot welding, understanding the surface conditions, such as size, quantity, and distribution of contacting asperities, is very hard to find. Moreover, it is difficult to explain and describe the surface films precisely. Although the theoretical investigations have been provided for a long time, a useful and systematic model of dynamic resistance has not been presented yet. To provide the researchers in resistance spot welding and other related fields with a comprehensive account of contact resistance, this article reviews and analyzes the published works in this area. Also, an inclusive comparison of the models is presented.
