Rising to the challenge of large-scale e-mobility production
Electric cars are no longer an exotic sight, with the required charging infrastructure also steadily growing. All large-scale manufacturers have now added a number of electrified models to their portfolios. In Norway – a trendsetter in e-mobility – almost two-thirds of the newly registered passenger vehicles in the past year have been electric. In three years, no new combustion engines will be permitted in the country.
The transition to e-mobility presents challenges not only for battery technology and the charging infrastructure, but also for the adaptation of production technologies.
In addition to the frequently discussed topic of battery technology, which is the decisive factor for vehicle range, the production of drive motors is also presenting manufacturers with a number of challenges. Most importantly, these motors must be suitable for economical large-scale production. Laser-based hairpin welding is therefore becoming increasingly common in the production of traction motors.
Hairpins: A key technology
The stator accounts for the largest proportion of production costs in an electric motor. Not only is the copper used in the part quite expensive, but the winding of the coils is also a relatively complex process. When producing traction motors for electric vehicles, most manufacturers use an alternative to coil winding, in which the coils consist of individual copper pins bent in the shape of a hairpin. These hairpin coils are inserted into the stator's laminations during assembly and then laser welded. The result is a coil, similar to those produced by traditional winding, which generates the necessary magnetic field. This method permits a compact motor design and is highly efficient.
Laser-based hairpin welding is becoming increasingly common in the production of traction motors (Credit: Scansonic)
An automated and safe process is particularly important for large-scale production. Depending upon the motor design, several hundred hairpins must be welded with one another. A single defective weld can render the entire stator unusable. The stators are therefore produced using laser welding technology, which is already successfully employed in a wide variety of areas within the automotive industry. This method permits short cycle times, can be easily automated, and is very flexible. The hairpins can be welded with a targeted and focused application of energy, without damage to the insulation layers. Unlike electron-beam welding, which also enables a targeted application of energy, no vacuum is required. The greatest challenge is the material, since copper requires innovative solutions to achieve a safe and stable laser welding process.
Absorption depends upon temperature
Economical laser sources with scalable power ranges emit beams in the infrared wavelength range at 1,030 or 1,070nm. At these wavelengths, copper absorbs only around 5 per cent of laser light at room temperature. The degree of absorption then increases to around 15 per cent shortly before the melting temperature is reached, and ultimately reaches nearly 100 per cent when a vapour capillary, or keyhole, has formed. However, this process is extremely dynamic – for example, the keyhole can briefly close and the vapour pressure can cause the ejection of molten material. This spattering must be prevented because the ejected material can lead to short-circuits within the stator or to other defects.
Spattering can be effectively avoided if the welding speed is higher than 20m/min. Achieving this for the start of the process requires a specific adjustment of the relevant process parameters, such as laser power, speed, and focus size. This stabilises the keyhole and practically eliminates spattering.
Image processing for optimal results
The geometry and positioning of the hairpin ends can have an enormous effect on the result of the welding process. The hairpins have a rectangular cross-section of only a few square millimetres. Before being inserted in the stator laminations, they are cut to length, bent, and insulation is stripped from the ends. Burrs on the cut surfaces, remnants of insulation, and imprecisely bent hairpins can influence the welding process and affect quality. To achieve an optimal weld seam, the control of the automated manufacturing process must be adjusted, for example, if the ends of the hairpins have a vertical offset or there is a gap between the two ends being welded.
RLW-S laser optics have been successfully implemented in the production of electric motors (Credit: Scansonic)
Image processing is generally used to account for these parameters in the process. Berlin-based Scansonic, which develops processing optics for autobody construction, has adopted an innovative approach to the laser welding of copper hairpins with its line of RLW-S optics. High-performance scanner drives are used to deflect the laser beam in order to maximise shape accuracy of the oscillation function, even at high frequencies. The result is a reproducible and highly reliable process. Basically, two functional modules integrated into the optics work together to precisely determine the position of the hairpins.
The optics use conventional camera-based image processing. Because the surfaces of the individual hairpins reflect varying degrees of light, they appear in the grayscale image with widely varying levels of brightness and may not be reliably recognised. Illuminating the surfaces from different angles significantly improves recognition reliability. Because the camera system provides images only from the front side, gaps between the ends can be detected, but the system cannot identify a vertical offset. Laser line triangulation is useful for detecting an offset between the two ends being welded. In this process, a sensor projects a straight laser line onto the surface of the two hairpins. The light is reflected and measured by the receiving element in the sensor. Using an angular offset between the projection unit and the receiving element, the sensor can measure the object's height profile. The height offset is then precisely identified and taken into account in the subsequent welding process.
Successful demonstration on the production line
The reliable and high-quality welding of hairpins offered by such optics is only half of the equation for large-scale production, however. In addition to process reliability, the method must also be compatible with the 60-second cycle time customary in the automotive industry. To increase speed, a large scanning field could be used, which would allow all hairpin pairs to be welded without moving the stator. In practice, however, this approach leads to quality problems caused by the angular offset in the various positions of the stator.
As a material, copper places high demands on the control of the welding process – pore formation and spattering must be avoided through optimised parameter selection (Credit: Scansonic)
With this in mind, Scansonic focused on process quality and stability when developing its RLW-S system. The company’s developers used a smaller scanning field and adapted the camera's field of view for optimal effect. This ensures optimal positioning of the laser beam processing point on the component, as well as the position measurement of this point via the camera. To weld a complete stator, the unit must be rotated. The desired cycle time can be achieved by using a powerful rotary axis. Standardised processing optics can then take full advantage of their technological capabilities. Using this approach, Scansonic RLW-S laser processing optics have already demonstrated flawless operation on the production lines of a major automotive manufacturer.
Pravin Sievi is a product owner at Scansonic MI GmbH in Berlin and is responsible for remote welding solutions.