Increase of electrode lifetime in resistance spot welding of aluminum alloys by means of applied diffusion barrier coatings
Project leader: Prof. Dr.-Ing. Volker Wesling; Prof. Dr. rer. nat. Harald Schmidt
Funding period: 11/2021 - 10/2024
Funding agency: DFG
Researchers: Sascha Brechelt, M. Sc.; Jochen Junge, M. Sc.
The use of aluminum alloys in the field of car body production holds great potential for reducing the overall vehicle mass. Thus, the specific fuel consumption and the generated exhaust emissions can be reduced and the effective range of BEV (Battery Electric Vehicle) can be further increased. However, the processing of aluminum alloys by resistance spot welding is subject to significantly accelerated thermal and metallurgical degradation of the electrode caps. While up to 6,000 welds are possible with steel materials until the quality limits are not reached, only 1,200 welds are possible with zinc-coated steel materials and 50 with aluminum alloys. The causal diffusion-based mechanisms require frequent renewal or repair of used electrodes, which reduces the cost efficiency of resistance spot welding. Within the DFG project, diffusion barrier coatings in the µm range are to be developed by means of PVD (physical vapor deposition), which withstand the stresses occurring in the welding process and significantly reduce the degradation processes that occur.
The degradation process that causes premature failure of electrode caps depends, among other things, on the level of electrical contact resistance and the local current density in the corresponding contact area. Targeted structuring of the electrode surfaces is intended to overcome the natural oxide layers of the aluminum substrates and achieve a homogeneous metallic contact. Together with the development of composite thin-film layers of metallic and highly conductive ceramic components, effective diffusion barriers are also to be developed to protect the electrodes. Qualified layer systems are investigated with regard to their temperature-dependent properties within diffusion experiments. The generated coating systems will be further evaluated by electron microscopy and nanoindentation, and the achieved coating adhesion strength will be assessed by scratch tests. The aim is to fully elucidate all mechanisms that contribute to the degradation process in the copper-aluminum material system. In addition, the measures developed to prevent degradation are to be tested on a laboratory and user scale.