Effects of the reduction of the copper wire cross-section on the welding process in the manufacturing of hairpin stators for electric traction motors

The radical transformation of the automotive industry away from the combustion engine and towards the electric drive requires a constant development and improvement of the related components and production processes. One of these processes is the partial removal of the insulation from copper wires in the production of hairpin stators. Currently, the deinsulation is done by mechanical or laser-based methods. The laser-based process is considered to be a comparatively cost-intensive method, whereby stripping rates can be achieved with almost no copper removal independent of variations within the insulation layer thickness of the original copper wire. Mechanical processes, on the other hand, are sometimes seen critically, since a mechanical process always involves a certain amount of removal of the copper raw material to ensure that the insulation layer is completely removed even if the layer thickness varies due to material and manufacturing based tolerances. A critical limit at which copper removal has a negative influence on product functionality has not yet been identified. Therefore, this work focuses on the effects to which the critical process of contacting the hairpins is influenced by the reduction of the copper cross section. For this purpose, hairpins of the same batch are mechanically stripped to different depths, resulting in different reductions of the copper cross-section. The samples are then welded with a constant set of parameters. The varying stripping depths result in both a variation of the copper cross-section and a variation of the gap width between the pins during welding. All other process parameters are identical for all the samples. The welding samples are then metallurgically analyzed and the mechanical, thermal and electrical properties are measured. These results are then analyzed regarding possible interdependencies and evaluated in the context of possible influences on the functionality of the electric drive.