Characterization of FeSi2.9 and SS 316L Produced by Directed Energy Deposition for Bimetallic High-Speed Rotors

Abstract The high-speed electric machinery sector has experienced significant growth over the last decade. Permanent magnet motors dominate this sector thanks to their superior torque volumetric density and efficiency compared to other AC magnet-free electrical machines, but their reliance on rare earth elements raises cost, supply chain issues, and environmental concerns. To address this, Synchronous Reluctance (SynRel) motors, which do not employ permanent magnets, are being explored. However, their mechanical limitations restrict rotor speed when produced with the internal flux barriers cuts. Additive manufacturing (AM), particularly Directed Energy Deposition (DED), enables the production of multi-material structures, allowing to overcome these limitations. For high-speed rotors, FeSi alloys are excellent soft magnetic materials, thanks to their high magnetic permeability. AISI 316L austenitic stainless steel has a magnetic permeability similar to that of air; therefore, it can substitute the structural cuts and act as a flux barrier while providing mechanical strength. Although both materials have been studied in AM processes, no comprehensive research has focused on DED’s potential for multi-material printing. This work aims to fill that gap by assessing DED’s ability to produce high quality FeSi2.9 and 316L structures for bimetallic electric rotors. The experimental campaign involved the fabrication of specimens for microstructural, mechanical and magnetic characterization, starting from gas-atomized powders. Both materials were characterized in terms of particle size distribution and chemical composition prior to printing. The printed specimens underwent the same annealing process (850°C for 1 hour, followed by air cooling). Although the treatment aimed to improve the magnetic properties of FeSi, it was also applied to the 316L stainless steel to evaluate its effect, with a view toward subsequent bimetallic printing. The microstructures were examined before and after heat treatment through optical and scanning electron microscopy, focusing on chemical composition, grain size and morphology. Vickers microhardness tests were performed on both as-printed and annealed samples, showing a consistent trend along the deposition height for all specimens. Additionally, magnetic tests were conducted on toroidal samples to evaluate the BH curves and specific loss trends at different excitation frequencies. Finally, tensile tests were used to measure the mechanical properties of both materials, highlighting the ductility of 316L and the brittle behavior of the FeSi2.9. The results indicated that both materials exhibited excellent printability, achieving crack-free depositions with relative densities greater than 99%. The combination of FeSi2.9 and AISI 316L offers promising prospects for developing DED-printed synchronous reluctance rotors with enhanced performance and operational speed capabilities.