Quality characteristics and analysis of input parameters on laser beam welding of hairpin windings in electric drives

Abstract

The relevance of electric drives in individual mobility is continuously increasing. This requires the use of new manufacturing processes in automotive production. In modern stators, the so-called hairpin winding is used. The production process entails rectangular copper conductors (hairpin) to be contacted by laser beam welding. To avoid temperature peaks due to resistance heating in the current-carrying wire, a high cross-sectional area in the welding seam is required. Therefore, the pore volume must be considered. This is proven by thermographic investigations of electrically stressed hairpin connections. To optimize the welding process, effects of the previous manufacturing steps and the welding parameters themselves must be taken into account. This paper analyses systematically the impact of all translational and rotational deviations on the corresponding welding quality. The tests are performed by using a disk laser. A laser spot with a focus diameter of 170 μm is compared to a statically shaped beam, capable of distributing power in core and ring beam. As an additional parameter, the influence of oxygen content in the copper wire is investigated by using electrolytic tough pitch copper (Cu-ETP) and oxygen-free copper (Cu-OF). Furthermore, the interaction between an additional cutting step before contacting and the welding path is considered. X-ray computed tomography (CT) is used to analyse the welding quality. The main finding of the studies is the significant influence of the gap and height offset between the wire ends on the resulting cross-sectional area. In addition, a low oxygen content of the copper and the use of static beam shaping reduce weld spatters, porosity and increase process stability. Three geometries in the welding path perform better in gap bridging and cause less spatters.