The dominant way of welding a car body is by using resistance spot welding (RSW). It is a low-cost process with high efficiency. The process can be automated, does not require the addition of materials, and can use inexpensive equipment when compared to arc welding or laser welding. Within a single car, there can be more than 4000 spot welds. To determine if a weld is sufficient, the size of the weld nugget is measured, with a larger weld nugget correlating to higher mechanical strength of the weld. The weld nuggets can be calculated using X-ray or ultrasonic testing. However, the most used method is destructive testing performed on spot-welded test pieces. To predict and control the quality and strength during production, the automotive industry utilises standards for its processes. With the electrification of the automotive sector, the demand for new advanced steel is increasing. Advanced steel offers higher yield strength, contributing to weight reductions in vehicles while maintaining excellent crash characteristics and formability. These new steels are more difficult to weld with resistance spot welding and demand a tailored process window. This thesis summarises the difficulties encountered when welding advanced steels using resistance spot welding. It compares the procedure to establish a weldability lobe in accordance with the ISO 18278-2:2016 and SEP 1220-2:2011 standards. The focus of this thesis is on the process parameters, weld schedules, materials, and coatings that affect the quality of the weld. This thesis aims to identify the differences between the standards and whether there are production-based parameters that are not accounted for by the standards, which could impact the welds. Ideas on how to improve the standards that would benefit the automotive industry will be discussed. The results of this thesis show that for mild steel, the current step for expulsion limit and the minimum nugget size are the same for both ISO and SEP. For most of the advanced high-strength steel materials, SEP achieves a more conservative welding window, which can result in a lower risk of expulsion during production. ISO uses more material to create a weldability lobe and is more time-consuming. For coated materials, a pre-pulse can give a wider process window. Both ISO and SEP are designed for a single-pulse schedule, which raises a question about their relevance within the automotive industry, which today often uses multi-pulse schedules.
Johannes Lundvik (Wed,) studied this question.