Laser Hairpin Stripping With a New Generation CO2 Laser at Lower Wavelength for E-Drive Manufacturing

Electric drives are conventionally produced with copper hairpin windings. Rather than the flexible winding of thin wires, Cu bars with typically rectangular cross section are separately bent into the desired form and inserted to the stator to form a continuous winding. The open ends to be connected generate the so-called hairpin couples. These regions should be free of the insulating polymer coating in order to apply the successive laser welding operation successfully. The presence of polymeric material in the welded area generates material bursts, spatter, and pores due to the very low vaporization temperature compared to the welded metal. Laser hairpin stripping removes the electrically insulating enamel layer and ensures a clean surface for the successive laser welding operation that produces the contacts. Several types of pulsed wave lasers can be operated producing different types of material removal mechanisms from surface evaporation to material expulsion. CO2 lasers have been long employed in the industry for processing polymeric materials thanks to the high optical absorptivity at IR region. High power CO2 lasers sources can be an appealing solution for the hairpin stripping process combining the high absorption by the polymeric enamel and the low absorptivity of underlying copper. New generation of CO2 lasers at lower wavelengths (i.e. 9.4 µm) can provide relatively smaller spot sizes and possibly enhanced processing. This work proposes a systematic analysis of the process development starting from single pulsed material removal towards areal scanning. The processed material was Polyester (Amide)(Imide) overcoated with Polyamideimide enamel material with 95 µm thickness on ETP Cu (electrolytic tough pitch copper) bars. A CO2 laser operating at 9.4 µm wavelength with 400 W maximum average power was employed. The laser produced µs-long pulses by regulating the pulse repetition rate in the kHz level along with the duty cycle. First the material removal was evaluated at maximum output power and duty cycle varying the pulse duration and the scan speed. The conditions were analyzed categorically assessing partial and complete material removal. Multiple pass scan strategies were developed to remove material in larger areas. The most productive solutions as well as the one providing the surface with the least amount of material remaining on the surface were chosen for further evaluation. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and focal variation microscopy (FVM) were employed to analyze the surface topography, chemistry, and roughness. Using the chosen conditions hairpin couples were stripped and laser welded using a high-power active fiber laser. The quality of the hairpin stripping operation was therefore evaluated also through the outcome of the welding operation in terms of bead geometry and its mechanical strength.