Laser contacting of hairpin windings in electrical vehicle motors using ring-mode laser beams based on angularly-uniform Lissajous trajectories

Abstract

The contacting process of hairpin windings is deemed the most important and challenging process in manufacturing electric vehicle motors. This paper proposes a ring-mode laser contacting method for copper hairpin windings based on Lissajous-shaped cyclic scanning with a uniform angular speed. Two key variables, trajectory cycle n and angular multiplier k, which represent the laser energy input and distribution respectively, were investigated via single-factor experiments. Morphology, microstructure, mechanical and electrical properties were analyzed consequentially to show the influence on the contact quality. The results show that, as n increases, contact appearance is criticized for being under-contacted, well-contacted, or over-contacted. Molten pool behaviors affected by n variations determine morphology formation. Larger k facilitates molten pool evolution and improves laser contacting efficiency. Fusion zone (FZ) has directional columnar grains, while heat-affected zone (HAZ) has overgrown equiaxed grains coarser than those of base metal (BM). The microstructure of FZ becomes coarser if n increases, or finer if k increases. The highest tensile force of 712.5 N was reached when k = 4, n = 6. Ductile dimple fractures in FZ indicate the contacts’ excellent strength and toughness. FZ and HAZ show lower microhardness than BM due to thermal softening. FZ is slightly harder than HAZ due to its densely-interlaced microstructure. Poor contacts cause reduction in their electrical conductivity and increase in their resistance.