ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2023 18 remanence resulting in low maximum energy product, thus limiting their viability in electric motors[7]. Therefore, the manufacture of high performance, 3D-printed permanent magnets has yet to be demonstrated[6]. Materials development is a crucial step toward improving the performance of AM electric motors. Commercial powder grades are a suitable option for AM in some instances, but often higher performance materials are required or the commercially available materials are not suitable for the AM process. Figure 2 illustrates the materials development of Fe-Si alloys as conducted in an EU project called SOMA (Lightweight Solutions for E-Mobility by AM of Soft Magnetic Alloys)[9]. In this example, an alloy with higher electrical resistivity was required to mitigate the losses induced by eddy currents. However, the unmodified high-silicon steel suffered from cold cracking; hence a more ductile alloy was needed. A combination of computational analysis and experimental tests resulted in development of an alloy with sufficient ductility for the AM process as well as high resistivity to reduce losses. The as-built material does not yet have the desired magnetic properties and requires heat treatment to improve the properties, which is achieved by relaxation of stresses, grain coarsening, and in some cases the formation of a texture[10,11]. Motor Design: The design freedom of AM enables electrical machinery application specialists to investigate and exploit new design methodologies and approaches, whereas conventional methods are tuned to assume 2D flux patterns. Numerical optimization methods such as topology and shape optimization allow a greater variety of geometrical details to form and thus can support development of electrical machinery structures for AM. For example, with the help of topology optimization, it is possible to find the ideal shapes of ferromagnetic parts to reduce overall motor mass or optimize the flux path. The main development goals for design, materials development, and manufacturing for AM of electric motors are summarized in Fig. 3. With regard to design, two distinct yet connected paths can be identified. First, the design methodologies and algorithms themselves are being researched, namely different computational approaches such as topology optimization that could help capitalize on the possibility of true 3D flux patterns, for example. Second, the use of these techniques to create new 3D designs is in its early stages and the first func- tional prototypes are now being introduced. Desired benefits of design optimization range from weight reduction to energy efficiency improvements. One of the main challenges regarding additive manufacturing of electric motors is the high losses in AM ferromagnetic cores induced by eddy currents in medium to high-frequency applications. Therefore, loss mitigation is a crucial step in improving the performance of AM motors. Several approaches are available and can be implemented. Aside from reducing losses by increasing the resistivity of the core material, a loss mitigating topology can be introduced such as slits or gaps[5,12]. Figure 4 shows different AM topologies and their influence on total core losses at 1.5 T, 50 Hz[12]. A topology- optimized loss mitigation structure (TO DfAM) resulted in significant loss reduction compared to solid material and was comparable to a laminated reference sample. Losses can also be reduced by multi-material AM where magnetic and insulating material layers are deposited sequentially[13]. Despite all the developments pertaining to loss reduction, a clear framework for eddy current mitigation is still needed. SUMMARY With demand for electric motors on the rise, methods for manufacturing lighter and more efficient versions are being intensively researched. Additive Fig. 2 – Example of development of soft magnetic materials for PBF-LB process. Fig. 3 – Development goals for AM of electric motors with illustration of a 3D structure for guiding the magnetic field in a claw pole transverse flux electric motor.
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