CO₂ lasers combined with UV solid-state lasers take electric vehicle motor welding to new heights
Electric vehicle manufacturers are under increasing pressure to optimize operations, cut costs, and improve product quality. These efforts are critical to the necessary expansion of electric vehicle market share, especially in a high-volume market that is highly sensitive to total cost of ownership (TCO) and where fuel-powered vehicles are still prevalent.
One of the key opportunities for optimizing electric vehicle motor production is the welding of hairpin windings, a process that is critical to motor reliability and performance. Traditional methods of mechanical insulation stripping often fall short, leading to process inefficiencies such as surface damage, tool wear, and production delays that increase costs and affect process reliability.
Laser stripping and cleaning offers a transformative alternative that overcomes the limitations of mechanical stripping while achieving superior results at a lower overall cost. This article describes Coherent's innovative two-step process that combines CO2 lasers with ultraviolet (UV) lasers to produce clean, weld-ready hairpin windings. The data presented show that this method provides a new, cost-effective way for electric vehicle manufacturers to achieve higher quality hairpin motor welds.
The windings in electric vehicle motors are typically made from a single copper wire. These copper wires are bent into a "U" shape (hence the name "hairpin winding") and then placed into an assembly. Next, the ends of adjacent hairpin windings need to be welded together to achieve an electrical connection and form a single continuous winding. In addition, some alternative designs use a continuous flat wire that is formed into a wavy pattern (called a wave winding or S-shaped winding) and then inserted into the stator slots before welding.
All electric motors have windings with insulation. In hairpin winding motors, the insulation needs to be thicker and stronger to accommodate their compact design and the high voltage requirements typically required in electric vehicles.
Before welding, a small amount of insulation must be removed from both ends of each hairpin winding. This is critical to ensure a high-quality electrical and mechanical connection.
Traditionally, this task has been mainly accomplished using mechanical methods, but there are also single-step laser processes using infrared (1μm) pulsed lasers. Mechanical stripping involves using a cutting tool to directly contact the surface of the wire to scrape off the insulation. While these methods have been standard practice for many years, they present significant challenges in the fast-paced production of electric vehicles.
For example, the physical contact required for mechanical stripping can scrape a layer of copper from the hairpin winding, leaving a textured surface that can cause gaps between windings and component fitment issues, compromising the integrity and consistency of the weld. In addition, mechanical tools can wear out, leading to inconsistent processes, frequent maintenance, unplanned downtime, and potential production interruptions. These issues are exacerbated by the slow speed of the process, which makes it difficult to meet the high-volume production needs of large-scale electric vehicle production.
Lasers can be used for a wide range of material removal, including insulation stripping. In insulation stripping applications, laser stripping provides the following advantages:
- : Ensures a clean surface for optimal weld quality while completely removing the insulation without damaging the copper wire.
- No tool wear and any possibility of machine jamming is eliminated, ensuring an uninterrupted production process.
- Continuous stripping while feeding the wire improves production efficiency.
- With no wear or contact, laser machining is a stable and repeatable process.
Clearly, laser machining can lead to process improvements. But the real question is, "Which laser is best to use?" In other words, of the many possible laser sources and implementations that could be used to perform this process, which one offers the best balance of high quality, high speed, and low cost for high-volume EV motor production applications?
With a broad portfolio of industrial lasers, Coherent was able to objectively investigate this application using a variety of lasers without any inherent preference for one technology over another.
In fact, rather than finding one laser that was best for hairpin stripping insulation, Coherent developed a dual-laser process to achieve optimized stripping results. This approach provides customers with the best pre-weld surface quality available today in the most cost-effective manner. The primary motivation for investigating the dual-laser process was to overcome the issues with the single-step laser stripping process.
Single-step laser stripping is a compromise between coating absorption and underlying copper absorption. The fiber laser's infrared wavelength near 1μm is not easily absorbed by the coating, resulting in heat generation at the interface, coating flaking and the formation of airborne particles. These particles interfere with the incident laser beam, affecting the cleanliness of the hairpin winding, and may intrude into the system, causing frequent downtime for cleaning. In addition, the fiber laser's penetrating infrared beam cannot completely remove polymer residue from the copper surface. It may melt the copper wire surface and expose the copper surface to insulating polymer components such as hydrogen and carbon. This contaminates the copper surface, which in turn affects the subsequent weld quality.
Coherent's dual-laser based hairpin winding preparation two-step process includes:
A medium-power CO2 laser is used to quickly remove the majority of the insulation layer. This type of laser is well suited for high-throughput non-metallic material removal.
A pulsed low-power nanosecond UV solid-state laser is then used to remove any insulating residue that is present. This provides a clean surface for welding. The short wavelength of UV solid-state lasers excels at high-precision material processing, and is particularly effective at removing a variety of materials including polymers, other organic materials, and copper.
For the first step of the bulk stripping process, Coherent's J-5-10.6μm laser proved to be an ideal source. Although a variety of different materials are used in the hairpin winding insulation layer (including polyimide, polyetherimide, polyester, polyesterimide, polyamide, polyetheretherketone, epoxy resin, and various fluoropolymers), all of these materials strongly absorb the laser's 10.6μm output wavelength. Moreover, for all of these materials, the absorption rate at 10.6μm is higher than that of Coherent's other available CO2 laser wavelengths, such as 9.4μm and 10.2μm.
Coherent's J-5-10.6μm laser also has ideal practical characteristics. It is a completely sealed, pulsed CO2 laser with an average power of more than 400W, which means it can perform high-throughput stripping operations. Additionally, its self-contained and compact package makes it ideal for integration into automated equipment.
For the second step, final cleaning process, Coherent's Avia LX 355-30-60 HE demonstrated just the right combination of output parameters. It is a frequency-tripled, diode-pumped, solid-state laser that outputs 30W of average power at 355nm. Most importantly, it supports operation at repetition rates up to 300kHz and pulse energies up to 500μJ. This enables it to perform high-precision ablations at the speeds required for this application.
The Avia LX 355-30-60 HE is also designed for easy integration. And, it integrates Coherent's PureUV™ active laser cleaning engine for exceptionally long life and maintenance-free operation.
The Coherent Applications Lab investigated many different processes before settling on the two-step combination described above. Various lasers were investigated individually and in combination, including CO2 lasers, nanosecond pulsed infrared fiber lasers, and nanosecond UV lasers. The results of the treated insulated hairpin windings were analyzed in detail and once the most promising laser sources were identified, specific process parameters were optimized.
As part of the testing, X-ray Photoelectron Spectroscopy (XPS) was used to analyze the surface chemistry in order to characterize and quantify contaminants. It was at this stage that Coherent found that a two-step laser stripping process (CO2 laser combined with UV laser) was the most effective way to remove all residues from the copper surface before laser welding.
The series of photos in Figure 3 compares the various stripping techniques using CO2 lasers, nanosecond infrared fiber (FL) lasers, and nanosecond UV solid-state lasers. In the upper row of photos, the insulation material is polyamide (PA) and in the lower row, polyetheretherketone (PEEK). Even by visual inspection alone, it can be seen that in both cases, the combination of CO2 laser and UV laser gave the best results. This was also confirmed by actual welding trials.
Of course, the most important metric is the weld quality obtained after the insulation stripping process. Figure 4 shows a series of X-ray images of the hairpin windings after welding. This indicates that the two-step (CO2 laser + UV laser) process can ultimately achieve better quality welds. The two-step CO2 laser + UV laser stripping process minimizes weld porosity by removing polymer residues on the surface before welding. It is well known that polymer residues contain hydrogen, carbon, and other organic elements, which may enter the molten pool when liquid and then form pores during the weld solidification process, which can affect the mechanical and electrical properties of the weld.
