Method to systematically investigate the effect of tramp elements in resistance spot welding

Abstract With the increasing utilisation of scrap in metallurgical processes due to recycling to reduce CO 2 footprint, the content of tramp elements in the base material is increasing. In order to quickly investigate the influence of tramp elements on the weld nugget microstructure and mechanical properties during resistance spot welding (RSW), a method has been developed to introduce tramp elements into the weld nugget. By making an indentation in one of the two sheets to be welded and inserting the desired quantity of tramp element before welding, the weld nugget can specifically be alloyed. This allows to quickly analyse the influence of individual tramp elements on the microstructure and its influence on the resulting mechanical properties. The applicability of the method was investigated using Cu as tramp element material. As part of the investigations, a targeted Cu content of 0.4 wt% was set, which was confirmed using energy-dispersive X-ray spectroscopy (EDS). The spot welds alloyed according to the method were compared with spot welds without the Cu addition and spot welds of an already Cu-alloyed material. The weld nugget microstructure of all steels analysed by light optical microscopy (LOM) and scanning electron microscopy (SEM) was martensitic with similar grain size and morphology, regardless of the Cu content. In tensile shear (TS) and cross tension (CT), testing plug failure occurred in all samples. The results from the TS tests with peak forces of 11.4–12.2 kN and absorption energies between 14.3 and 18.5 J were very close to each other. The CT results with peak forces of 6.7–8.5 kN and energies between 46 and 69 J showed a similar picture. The average weld nugget hardness of all three weld configurations was in between 383 and 398 HV1. The microstructural and mechanical results showed no significant differences. The presented method for a targeted weld nugget alloying during resistance spot welding allows a repeatable, easy and quick investigation of the influence of alloy modification by tramp elements such as Cu on the weld nugget properties and offers a practical approach to assess material changes due to an increased tramp element content.