P 642 – Improvement of the notched geometry to increase the fatigue strength of high strength and stainless steels by process control and innovative treatment for laser and arc welding and brazing
The aim of the project was to achieve higher fatigue strength (ideally up to that of the base material) by improving the seam geometry of fillet welds at lap joints (sheet thickness s = 2mm). For this purpose, the process control of already known and established joining methods (laser beam welding and soldering, MSG welding, MSG soldering, laser-MSG hybrid welding) was first optimised for two steels, a high-strength ferritic-martensitic dualphase steel (DP-W600/1.0936) and a stainless austenitic steel (H400/1.4376). The influence of the energy intensity and its distribution during welding, the influence of the welding or brazing filler metals and the shielding gas on the weld transition radius and weld rise were investigated. Furthermore, in the MSG brazing processes, an attempt was made to reduce the influence of the seam geometry by thermal post-treatment using the combined MIG-WIG process. In order to ensure the implementation of the results in industrial production, the transferability of the processes to a demonstrator component was also be examined.
For a first assessment of the fatigue strength of both materials, Wöhler tests were first carried out on flat samples with overlapping seams. In the next step, Gassner tests (variable load amplitude) were carried out on flat specimens with overlapping seams.
Laser-MSG hybrid welding, laser beam welding and MIG-WIG soldering were used as welding methods for both materials. The latter represents an improvement of the MSG brazing with regard to the seam geometry due to the thermal post-treatment by the TIG process and, according to the results of the Wöhler tests, was used with the filler material AlBz5Ni2 for the DP-W600 and with the filler material CuAl8 for the H400. In addition, the corresponding Wöhler tests were also carried out for these variants for reasons of comparability.
The feasibility of the optimisation of the joining process was verified on a real component at the end of the project. For this purpose, instrument panel carriers with a welded steering column attachment were manufactured by an industrial partner of the user committee according to the findings gained in the project.
Dr. J. Laakmann, Dr.-Ing. M. Küppers, Prof. Dr.-Ing. C. M. Sonsino, Dipl.-Ing. A. Sevim, Dipl.-Ing. St. Longerich, Dipl.-Ing. D. Piontek, Dr.-Ing. J. Gollnik, Dr.-Ing. A. Zwick, Dipl.-Ing. R. Imhoff