Two soft-magnetic materials, a ferro-silicon alloy and a ferro-cobalt alloy, were processed with selective laser melting (SLM). Both alloys are used in the electrical industry for different applications due to their increased specific resistance, in particular regarding the FeSi alloys. The conventional production methods of electrical steel sheets have been extensively researched and optimized in terms of cost-effectiveness. Therefore, new production techniques have to be taken into account to increase the efficiency of motor components, e.g., rotors.
In this study, two soft-magnetic materials, a FeSi alloy with a Si content of 2.9 wt% and a FeCo alloy having a Co content of 50 wt%, were selected for processing with Selective Laser Melting (SLM). In the electrical industry, both alloys are widely used as sheet material due to their increased specific resistance, in particular in the FeSi alloys. The conventional production methods of electrical steel sheets have been extensively researched and optimized for cost-effectiveness. Therefore, new production techniques have to be taken into account to increase the efficiency of motor components, e.g., rotors. Both alloys were gas-atomized in a nitrogen atmosphere. The particles produced were spherical in shape and showed the desired particle size distribution of 10 μm - 45 μm. In this investigation, the samples and rotors were generated using a SLM 280HL machine, supplied by SLM Solutions Group (Lübeck, Germany), equipped with a 400 W ytterbium fiber laser. Furthermore, the targeted variation and adaptation of the laser parameters during the SLM process enabled a suitable parameter window for the processing of both selected materials. In particular, the FeCo alloy was characterized by a relative density above 99.9% as compared to FeSi alloy, which had a relative density of 99.5%. The microstructural evolution of both alloys is typical for SLM-processed iron-based materials without a phase transformation in the solid state. The EBSD micrograph in Figure 1 shows an epitaxial grain growth in building direction as well as the structure of melt pools along the scan direction of the laser. The high temperature gradient, as well as the high solidification velocity of the melt pool, evokes a columnar epitaxial grain morphology along the building direction. The resulting mechanical properties directly after the manufacturing process are slightly anisotropic. For reducing the residual stresses, and to improve the magnetic properties, a heat-treatment (900 °C, 2 h) was conducted. The mechanical characteristics for both materials are above the required yield strength of 400 MPa (FeSi: YS > 450 MPa; FeCo: YS > 500 MPa). In particular, the FeSi alloy is characterized by a lower anisotropy. Due to the selected heat-treatment a significant increase in the maximum permeability (μmax = 3,000) for the FeSi alloy in comparison to the initial, as-built condition (μmax = 950) was achieved. A reduction in the re-magnetization losses (eddy current and hysteresis losses) was also observed.
In summary, the application of a soft magnetic ferro-silicon alloy in an additively manufactured PMSM rotor active part and shaft could be proven successfully. Therefore, the additively manufactured rotor was tested and compared to a conventionally laminated rotor. Inserting lightweight lattice structures into the shaft region, the rotor mass could be reduced by 21% unaffecting mechanical strength. Besides that, additional rotor coils were added to improve the self-sensing performance of the machine, which is an alternative to a meandering wire directly positioned in the q-axis. The experimental results show that the magnetic anisotropy could be improved by adding short-circuited wires underneath the magnets considering the aforementioned boundary conditions. The resulting deviations between the conventionally produced rotor, the additively manufactured rotor and the simulation results could be explained by the increased air-gap, the production tolerances and the rotor coils. Using the present tool chain, a fast determination of the machine characteristics was possible if three-dimensional effects or production tolerances could be neglected.