Conduction mode welding of Cu hairpins with a 3 kW blue laser with external features correlated to quality attributes

Hairpin winding technology is a crucial part of the contemporary electric drives used for traction applications. The contacting of the hairpin couples requires a fast process with high quality and cleanliness in the processing environment. High-brilliance NIR sources provide industrially accepted solutions with high productivity, while spatter, porosity, and material burst defects remain open issues to be resolved. The use of conduction mode welding may provide a more stable process in the absence of keyhole fluctuations. The blue wavelength at high power can provide the means for a conduction mode weld with high absorptivity despite low irradiance, sufficiently deep and wide as required by the hairpin welding applications. This work investigates the use of a 3 kW blue diode laser with 720 µm spot size for the welding of pure Cu hairpin couples. The work systematically investigates the melting capacity of the blue laser beam from bead-on-plate experiments to the hairpin welds with elliptical trajectories and an increasing number of scans. The results show that the blue laser is capable of producing the weld beads with sufficient size using scan speeds between 50 and 150 mm/s and a number of rotations between 2 and 7, characterized by low porosity and reduced material ejection. The process was observed to be relatively slower compared to keyhole welding but proved to be favorable for reducing spatter generation. The conduction-based welding is also provided to correlate more easily the external dimensions of the weld bead to weld strength and electrical resistance. Such features are highly appealing for an easier process of quality assessment.