Comparison of the Mechanical, Electrical, and Microstructural Properties of Copper Wire Hairpins Welded by Infrared and Combined Infrared and Blue Diode Laser Techniques

With the rise in demand for electric vehicles (EV), hairpin welding is gaining popularity for its efficient manufacturing of critical EV motor components. Due to its high electrical and thermal conductivity and relative affordability, copper is commonly used. The tip of the hairpin can be joined by laser welding, micro TIG, and resistance brazing. Of these techniques, infrared (IR) laser welding is commonly used for its high dimensional accuracy and non-contact joining. However, copper is highly reflective to IR wavelengths, limiting the speed of this technique in joining copper hairpin couples. Considering the high manufacturing volume of the EV motor industry combined with copper hairpins being a high-volume component in each EV motor, copper’s high reflectivity to IR wavelengths presents a significant challenge to EV manufacturing efficiency. To accommodate this challenge, many researchers have examined the feasibility of using blue diode lasers to produce copper hairpin welds to leverage copper’s higher absorptivity to blue light wavelengths. Alternatively, this paper investigates a hybrid approach in which both IR and blue diode lasers (BDL) are used simultaneously to benefit from the advantages of both IR and blue wavelengths. To investigate this technique, hairpins were laser spot welded using IR, and hybrid technology was also analyzed and presented. Successful welds were produced in 0.6 s with both IR-only and hybrid techniques. Using IR-only and a power of 1000 W, a shallow weld joint with a fusion zone depth ranging from 0.2 to 2.8 mm was produced. A satisfactory weld joint (weld bead) was achieved when the IR power was increased to 1300 W exhibiting a fusion zone depth of 3.0 to 3.1 mm. When the hybrid method (1000 W IR with 750 W BDL) was employed, a satisfactory weld joint was also achieved with a fusion zone depth of 2.9 to 3.0 mm. Electrical resistivity measurements of the 1300 W IR-only and hybrid methods were on the same order of magnitude as the unwelded copper reference of 1.9 × 10−4 Ω.cm. Hairpins welded with 1000 W IR resulted in higher electrical resistivities ranging from 1.2 × 10−3 to 6.2 × 10−3 Ω.cm. Peel force tests demonstrated the highest max peel force of 770 ± 15 N under the 1300 W IR condition, whereas the 1000 W IR and hybrid methods resulted in max peel forces of 410 ± 15 and 720 ± 20 N, respectively. Regarding peel test elongation at failure, the hybrid method was highest at 13 ± 3%, followed by IR with 1300 W at 12 ± 3% and IR with 1000 W at 11 ± 2%. The results of this paper demonstrate comparable microstructural, electrical, and mechanical properties under hybrid welding to higher power IR welding, simultaneously showing hybrid laser welding as a suitable alternative in copper hairpin joining for EV motor application.