The use of conventional resistance spot welding in the car body shop for joining an aluminium part to a steel part is possible with the process-invention of the resistance element welding (REW) with upset auxiliary joining elements. Within this process, the challenges arise from the novel approach of combining forming process steps in the insertion of the auxiliary joining elements. Thus, the process understanding is to be compiled to the combination of the individual process steps. For this reason, the study deals with the influence of the forming process steps on joint strength of REW with upset auxiliary joining elements. The focus is set to the improvement of the positive locking and force fit considering the results of previous strength analysis. As the key result, the initial relative position of the carrier sheet and the auxiliary joining element is highlighted as the most significant factor on joint strength. The material flow, illustrated in metallographic microsections, is used to explain this effect. Based on the elaborated cause-and-effect-chains, a geometrical model is derived and validated to produce high-quality joints with high joint strength.