Hybrid PBF-LB/M of Pure Copper for Hairpin Winding Heads of Electric Traction Drives

Due to the product-side advantages of hairpin technology in the copper winding of stators in electric traction machines, such as the increased copper fill factor due to the rectangular wire cross-section, many OEMs and suppliers rely on hairpin stators. Production also has a more deterministic character compared to the production of wound stators with round wire. Depending on the definition, the production process chain of hairpin stators can be divided into 12–15 individual process steps, which are shown in Fig. 1:

First, the flat copper wire is formed into its name-giving hairpin shape, and then a large number of the individual hairpins are assembled into the stator lamination stack. After this, the individual ends of the assembled hairpins are brought into the appropriate positions for the following contacting processes by means of a twist process, the forming of all hairpin ends on the welding side in the tangential direction. Subsequently, adjacent hairpin ends are welded together in a contacting process to produce continuous coils of the individual phases along the entire stator circumference. Busbar elements such as phase connections or neutral points are also welded to the hairpin winding, and then all welded ends and the busbar elements are insulated and the stator slots impregnated using a resin before the entire stator can be subjected to an end-of-line test. The total machine and equipment investment for a complete hairpin stator production is approximately € 3 million (for prototyping lines) to € 10 million (for series production), depending on the requirements regarding quantity scenarios, cycle times, automation level and equipment configuration like parallel paths of machines. Efficiency in terms of the costs to be invested in hairpin stator production is achieved in the automotive industry at a production volume of around 200,000 stators [1]. It should be noted here that the individual production process steps have low flexibility with regard to different wire and stator variants. Due to dimension- and geometry-dependent product restrictions and tools along the production process chain, often only a small range of product variants of a stator can be produced. However, development cycles of new electric machine and thus also stator generations are becoming shorter and shorter, and dimensions of wire and stator lamination stacks are changing. Increasing the variant flexibility of hairpin stator production therefore involves further investment in tooling and retooling. The publicly funded HaPiPro² project is therefore investigating variant-flexible product and production concepts. In particular, additive manufacturing (AM) is being considered as a technology alternative. New concepts for variant-flexible tools, for example for clamping hairpin ends for the contacting process, have already been developed and prototypically implemented using powder bed fusion technology with a laser beam for metal materials (PBF-LB/M) [2]. In general, additive manufacturing also offers the possibility to directly fabricate electrical copper conductors. Through innovative design methods such as algorithmic design and the potential to manufacture even complex three-dimensional structures with additive manufacturing, a wide variety of approaches to reducing electromagnetic losses or improving the thermal budget through additively manufactured copper conductors have been investigated [3‐6]. This contribution investigates the hypothesis of whether PBF-LB/M processes can be used to substitute conventional process steps in hairpin stator production and increase variant flexibility through tool-free production. The focus here is on printing entire winding heads of a hairpin stator, in this case the winding head on the twisting side of a hairpin stator, from pure copper. A special feature is that direct printing on conventional conductors is being researched.

To integrate additive manufacturing and substitute conventional process steps, the production process chain introduced is adapted. Since the aim is to manufacture the entire winding head of the hairpin stator on the twist side additively using PBF-LB/M (see Fig. 2 ), production process steps 1 to 7 remain in place. First, hairpins are formed from flat copper wire (PS 1–5) and assembled in the stator lamination stack (PS 5–7). It should be noted here that the conventionally manufactured hairpins to be imprinted must be prepared accordingly.

Thus, when cutting (PS 3) the copper wire, the length must be reduced accordingly by the original conductor length of a hairpin on the twisting side. The hairpin ends should still protrude a few millimeters beyond the stator lamination stack on the twisting side, since the slot insulation paper of the stator slots (PS 5) also protrudes and there must be sufficient buffer for the heat-affected zone for the AM process. Here, there is also the requirement that the height offset of the hairpin ends to be imprinted on should be a maximum of 1‑to-3-layer thicknesses in the axial direction of the stator axis of the layer thickness set in the PBF-LB/M process. The shortened ends of the hairpins must also be freed from the insulating varnish beforehand (PS 2), as in the conventional process chain, so that the transition zone between conventional and additive conductors remains as free as possible from contamination. Furthermore, for the assembled hairpins, clearances must be created between the individual hairpin ends of a stator slot on the twisting side in radial direction. Here, a spacer is required that can withstand the heat influence during the AM process and can be removed afterwards without leaving any residue. To ensure complete electrical separation after the AM process, the distance in this area should be at least two times the focus diameter of the laser used in the focal plane [8]. The adapted production process chain is shown in Fig. 3:

A digital assistance system is implemented in the algorithmic design software Rhinoceros3D® and Grasshopper® for the design generation of the hairpin winding heads. The development and implementation of such digital assistance systems for the automation and digitization of product development processes are based on a generally developed methodology, which is not explained further here and reference is made to the relevant literature [7].

The digital assistance system is to be used to generate the three-dimensional design of the hairpin winding heads automatically under specification of defined boundary conditions and input parameters. Compared to direct modeling in CAD software, the designers do not create the product design themselves, but implement design algorithms that are combined into a holistic design configurator. The design configurator is to be used to perform design iterations within a very short time and to generate different product variants. This is mainly due to the numerous interfaces between conventional conductors and the additively manufactured winding head as well as the preparation of distances between neighboring conductors and thus the modification of the radial center planes. Here, mathematical, logical and geometric operators are used to process input data into output data. To generate the winding head design of hairpin stators, there are some boundary conditions and design restrictions to be considered with respect to the PBF-LB/M process. In radial direction, the generated hairpin designs should not extend beyond the inner or outer diameter of the stator lamination stack. Minimal spacing should be maintained between adjacent conductors in three-dimensional space. Furthermore, critical overhang angles for the additive manufacturing process should be considered for support structure avoidance. Design guidelines for PBF-LB/M processes can also be used as a reference, but the possible build-up angles are strongly dependent on the geometry and the process parameters of the PBF-LB/M process [8]. The build-up angles of the hairpins are in direct conflict with the goal of achieving the lowest possible winding head heights in order to reduce electromagnetic losses of the stator. The winding head height describes the extent to which the entire winding protrudes beyond the stator lamination stack in axial direction of the stator axis. Finally, further importance must be attached to not falling below minimum wall thicknesses of the hairpins, whereby usual wire dimensions exceed the minimum limits of the PBF-LB/M process many times over. The methodical procedure for implementing the design configurator of the PBF-LB/M hairpin winding head is shown in Fig. 4.

By specifying general stator and wire parameters known from conventional stator design, key geometrical points (KGP) of a single hairpin in the winding head area are first calculated, taking into account the explained boundary conditions and design restrictions. Subsequently, additional points are calculated between the KGP in the areas where a high level of detail is required. The calculated points are then projected onto the circular surfaces of the stator and then connected by a centerline (CL). Along the centerline, cross sectional areas (CS) are then shifted and rotated to the calculated points using matrix transformation. The centerline and the cross sectional areas are then used to generate the individual hairpin design in the winding head area using a sweep function [9]. Since the interconnection of the winding on the twist side of hairpin stators always runs uniformly along the entire stator circumference, the individual hairpin design is finally multiplied by a polar array function according to the required number of hairpins to the entire hairpin winding head.

To prepare for hybrid printing, the conventional stator part must be inserted into the system and aligned. To protect the conventional winding head on the bending side and to prevent the mounted conventional hairpins from shifting due to the self-weight along the stator slots, additional support elements are attached to the face of the stator lamination stack. The conventional stator including support elements is then aligned and fastened centrally on the building platform of the PBF-LB/M system. The build platform is then lowered by the appropriate depth so that the end faces of the hairpins are flush with the plane of the powder coating or the focal plane of the optics. The entire platform, including the stator, is then filled with powder and a target/actual comparison is made of the positions of the conductor ends on which the powder is to be printed. For this purpose, low-power exposure is applied to the focal plane corresponding to the geometry to be produced. Using a high-resolution coaxial camera to the laser beam, the positioning is then determined in software via the exposure data. The deviation between the conventional stator and the winding head to be produced additively can then be reduced or completely avoided by shifting the processed data for printing.

A conventional hairpin stator with 48 slots and 6 conductors per slot is prepared for the hybrid PBF-LB/M process. The wire dimensions of the hairpins are 4.05 mm (width) and 2.03 mm (depth). In accordance with the adapted process chain (see Fig. 3), the copper wire is cut to the shortened length and bent into the hairpin shape using a CNC bending process. To remove the insulation varnish on the wire, a non-contact stripping process was carried out using laser radiation. It is important to ensure that the cut surface is as planar as possible in order to create a flat surface on which printing can be carried out. Therefore, after the stripping process, the cut surface was machined again. A total of 144 hairpins are bent and mounted in the stator lamination stack together with the slot insulation paper. The length of the hairpins is calculated and cut so that 4 mm still protrude above the laminations on the twisting side. A metal wire is then inserted between adjacent conductors of a slot to create spacing. The prepared, conventional part of the hairpin stator is shown in Fig. 5.

The diameters of the individual radial layers on which the individual conductor centers are located after space generation are measured and transferred to the design configurator (see Fig. 6) as input parameters. The measurement is carried out both analogously by means of a measuring instrument and digitally by means of an image evaluation with the ImageJ software. In addition to the diameters of the individual radial layers, the number of stator slots, the number of conductors per stator slot, the conductor cross-section dimensions, the start and end points of an individual electrical connection and the winding head height also serve as input data. The winding head height is then used to calculate the overhang angle and the distance between adjacent conductors.

To specify the winding head height, the process parameters were optimized by preliminary tests so that support structure-free overhang angles of more than 65 ° can be realized. Based on this, the winding head height is specified as 20.5 mm.

For the hybrid PBF-LB/M process, the conventionally prepared hairpin stator is finally aligned and fixed in the system. For better positioning of the individual conductor ends to be imprinted, a template for tangential positioning is added in addition to spacing in the radial direction by the wire. The stator is then lowered together with the build platform until the conductor ends are in the focal plane of the laser beam. A blade/squeegee is helpful here for checking the correct distance. The pre-processing of the stator in the system is shown in Fig. 7.

After the entire platform including the stator has been filled with copper powder, the winding head to be printed can then be aligned with the conventional conductors. The position of the conventional conductor ends is recorded via the coaxial camera by means of image generation. Here, the overall image is generated in software from 20 times 20 individual images, each with 24 megapixels. By exposing the scan vectors of the first layer to be printed at low power, the positioning of the hairpin winding to be printed can then also be detected. The deviation between the position of the conventional conductors and the conductors to be printed can then be corrected in the software by shifting the scan vectors (see Fig. 8).

The process parameters used for the hybrid PBF-LB/M process on the Aconity Midi+ System are shown in Table 1.

Before hybrid PBF-LB/M printing, the hairpin winding head was first printed alone on a building plate (see Fig. 9). Here, overhang angles below 25° with distances between adjacent conductors of 0.6 mm could be realized and printed collision-free.

With a relative density of up to 99.8% and na  electrical conductivity of 57.4–58 MS/m (99–100% IACS), measured by eddy current measuring method, it was also possible to achieve surface roughnesses (Sa) of 14, 85 µm in the upskin area, 32.09 µm in the downskin area and 6.71 µm (Sa) on the side surfaces. The winding head height is 20.5 mm as specified.

With hybrid printing, a winding head height of 24.5 mm could be achieved (see Fig. 10). By correcting the position deviation, an accuracy of 0.1 mm could be achieved between conventional and additive conductors. The production time was 12 h and 18 min (77.7% exposure time, 8% interization, 8% setup, 6% powder coating) using a system configuration with a single laser. In a quad laser configuration, a theoretical fabrication time of 5 h and 8 min is possible (46.6% exposure time, 19.5% interization, 19.5% preparation, 14.4% powder coating). The fabrication cost is about 1075 € (about 668 € in the quad laser configuration). Post processing effort was about 10 minutes of depowdering, removal of alignment and fixation aids about 15 minutes and optional sandblasting of 30 minutes.

By integrating the PBF-LB/M process for manufacturing the hairpin winding head on the twisting side of the stator, a total of up to five process steps in the conventional production process chain can be substituted. Separating, twisting, cutting to length, contacting the hairpin ends and the busbars with system investment costs of € 1.0–4 million are realized in one process with the additive manufacturing method. Since the highest investment costs for equipment and tools are predominant in the twisting and contacting process (€ 0.8–2.3 million), investment costs can be reduced here while at the same time significantly increasing variant flexibility. With a necessary investment of €470k for the used Aconity Midi+ PBF-LB/M system (€ 775k for quad laser configuration), € 0.2–3.5 million in investment costs could be saved. However, the productivity in the PBF-LB/M process is significantly lower by a multiple compared to the substituted, conventional production steps. The adapted production process chain with the printed winding head on the twisting side is therefore suitable for small batches, research purposes, or prototype construction at the current state of system technology. However, it should be emphasized that tool-free production using the PBF-LB/M process can significantly increase variant flexibility. With regard to the product parameters number of stator slots, number of conductors per slot, conductor cross-section geometry as well as start and end points of a single conductor, the input data can be continuously realized in the value ranges typical for e‑machines in the automotive sector. Only the outer diameter and the total axial length of the stator (winding head heights + active length) define the limiting factors with regard to the build volume of PBF-LB/M systems. Here, small and medium-sized companies as well as companies previously outside the market have the opportunity to open up a new market in the field of e‑mobility alongside established, large supplier companies in the automotive industry and to produce small batches of hairpin stators in various product variants. Also for developing and validating new e‑machine generations or prototypes, the costs of approximately € 1075 for a printed winding head are comparatively low.

Advantages can also be achieved on the product side through the additively manufactured winding head. Compared with the conventionally manufactured winding head on the twisting side, which has a winding head height of 37 mm, an axial length of 12.5 mm could thus be saved. This corresponds to around one third of the conventional winding head height. A reduced winding head height allows electromagnetic losses of the e‑machine to be reduced, thus increasing efficiency. With the smaller winding head, a copper volume of approx. 63 cm³ or a copper mass of approx.0.564 kg can be saved in the present hybrid stator compared with the conventional hairpin stator. Based on an exemplary small series of 8000 stators of the type presented and a current copper price of € 7.71/kg, this amounts to a projected € 34,787.52. It was also possible to produce the transition zone and the connection in accordance with the requirements and with a positive fit. Here, the heat-affected zone did not damage the insulation of the conventional wire. This is due to the short exposure times for the dimensions of a single conductor. With regard to the accuracy in the alignment of conventional conductors and additive conductors, the positioning and fixing of the conventional conductors is the limiting factor.

After additive manufacturing of the hairpin winding head from pure copper onto the conventional conductors, the individual conductors must be electrically insulated from each other. At this point, the conventional production process chain is continued. The surface roughness achieved for the printed hairpin connections plays a decisive role here, since typical insulation thicknesses for the conductor dimensions used are around 100–300 µm. With a surface roughness of 32 µm (Sa) in the downskin area, the printed conductors are significantly rougher compared to conventionally manufactured hairpins. Nevertheless, complete encapsulation of the bare pure copper with an insulating layer is possible. However, additional tests must be carried out here in further research. It may also be considered to post process the surfaces of the additively manufactured winding head before the insulation process in order to increase the surface quality, e.g. by chemical etching.

In this contribution, methods for a hybrid PBF-LB/M process for the direct additive manufacturing of hairpin winding heads from pure copper onto conventional copper conductors were presented. The adapted production process chain in the context of hairpin stator production was defined, a method for design automation of hairpin winding heads and the preparation and execution of the PBF-LB/M process were explained and implemented. The results show that tool-less production of hairpin winding heads can significantly increase variant flexibility in production. Equipment and tooling investment costs can also be reduced. However, productivity is significantly lower. On the producside, the winding head height on the twisting side could be reduced around 33% compared to the conventional design due to the achieved overhang angles. This cannot only save copper material, but also reduce electromagnetic losses of the stator.

In further research, the design configurator will be extended with the camera sensor technology of the system technology. During image or feature recognition in the manufacturing system, the diameters of the radial layers of the conductor centers are to be determined automatically by an algorithm and transferred to the design configurator. In this way, the digital design will be automatically adapted to the real, conventional prepared stator design and automatically generated. In addition, research will be carried out into how the surface roughness in the downskin areas affects the subsequent process steps in hairpin stator production, in particular the insulation process of the bare pure copper. Breakdown voltage tests must be performed out here.