Electromagnetic evaluation and quantification of welding processes for the packaging of electrical steel

• Applicants: Professor Dr.-Ing. Kay Hameyer; Professor Dr.-Ing. Uwe Reisgen
• Subject Area: Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
• Term: from 2019 to 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 432930813

• Final Report Year: 2023

Final Report Abstract

The objective of the project was to quantify the influence of the laser spot welding process at different working pressures on the magnetic properties and consequential on the degradation of magnetizability and iron loss of welded electrical steel stacks by application of novel welding technologies. Both with the multi-mode laser and the single-mode laser used, a weld could be produced at a working pressure of 1 mbar, which is similar in its grain structure to that of the base material. This was not possible at higher working pressures. The multi-mode laser connected three to four lamellae with each spot weld, which tends to increase eddy current losses slightly. Nevertheless, the required magnetization field strength for toroidal cores with single-mode welds is higher and the total losses are greater, even though only two lamellae were connected. Although the grain structure of the multi-mode welds equals the base material at low working pressures, the magnetic properties are slightly improved with increasing working pressure. The results indicate that, in addition to the grain structure and the eddy current paths, another influencing factor affects the change in magnetic properties. Both the observed polarization range and the frequencydependent behaviors, as well as the work on the mechanical residual stresses carried out in the project, suggest that these are also influenced by the working pressure. The stability of the multi-mode process outweighs the larger short-circuit paths in comparison to the single-mode process and enables the production of toroidal cores with optimized magnetic properties. With linear welds, the insulation coating of the electrical steel sheets lead to severe grain refinement and hardening of the weld. In addition, long continuous short-circuit paths are formed. Using single weld spots the grain structure of the weld can solidify almost identically to the base material. In addition, the unavoidable short-circuit paths are evenly distributed over the toroidal core, thus reducing the global eddy current losses. With the help of dynamic beam shaping, which is made possible by certain modern laser beam sources, the knowledge gained in this project can be used as a basis for further developments to minimize the total losses. Here the focus here is on solidifying the structure of the weld similarly to the base material and, at the same time, to create small joining cross-sections with high process stability to reduce the short-circuit paths.

Publications

• Electromagnetic assessment of welding processes for packaging of electrical sheets. 2020 10th International Electric Drives Production Conference (EDPC), 1-6. IEEE.
  Ukwungwu, David; Krichel, Thomas; Schauerte, Benedikt; Leuning, Nora; Olschok, Simon; Reisgen, Uwe & Hameyer, Kay
  
• Laser in Vacuum spot welding of electrical steel sheets with 3.7% Si-content. In: LiM - Lasers in Manufacturing Conference, June 21 - 24, 2021.
  Krichel, T. & S. Olschok, U. Reisgen
  
• Parameter Study on the Effects of Spot-welding on the Electromagnetic Properties of Magnetic Cores Constructed from Electrical Steel. 2021 11th International Electric Drives Production Conference (EDPC), 1-9. IEEE.
  Ukwungwu, David; Krichel, Thomas; Schauerte, Benedikt; Leuning, Nora; Olschok, Simon; Reisgen, Uwe & Hameyer, Kay
  
• Pulsed electron beam spot welding of non-oriented electrical steel sheets. 6th IEBW - International Electron Beam Welding Conference. In: DVS-Berichte Band 368. ISBN: 978-3-96144-133-4.
  Krichel, T. & S. Olschok, U. Reisgen
  
• Efficiency improvement of a high-speed permanent magnet excited synchronous machine by the use of spot-welding as lamination packaging technology. International Journal of Applied Electromagnetics and Mechanics, 71(1_suppl), S95-S106.
  Ukwungwu, David; Leuning, Nora & Hameyer, Kay
  
• Influence of laser beam welding in vacuum on the residual stress of non-grain oriented electrical steel sheets. In: LiM - Lasers in Manufacturing Conference, June 26 - 29, 2023.
  Krichel, T. & S. Olschok, U. Reisgen
  
• Influence of pulsed laser beam welding in vacuum on the mechanical properties of non-grain oriented electrical steel sheets. Materials Testing, 65(1), 3-9.
  Reisgen, Uwe; Olschok, Simon; Rasbach, Erik & Krichel, Thomas
  
• Influence of working pressure on laser beam welding in vacuum of electrical steel sheets. Vacuum, 207, 111659.
  Krichel, T.; Olschok, S. & Reisgen, U.