Importance of surface roughness on the magnetic properties of additively manufactured FeSi thin walls

Thin-walled structures are being used in soft magnetic components manufactured by additive manufacturing to limit eddy current losses in AC machines. Fe-6.5wt%Si has been shown to be a promising material in such components, however most characterisation has taken place using thicker bulk material. Thermal conditions and microstructure have been shown to differ within thin-walled structures, hence magnetic properties may also differ. This study characterises the magnetic properties of thin-walled structures, showing that the <001> texture usually apparent in laser powder-bed fusion does not persist in thin-walled samples built at an angle to the build platform. Surface roughness (Sa) is shown to increase with build angle from 28 µm when perpendicular to the build platform, to 80 µm when parallel, causing a deterioration in magnetic properties such as susceptibility which is reduced by up to 25 %. Improvements in magnetic properties are demonstrated for samples with lower surface roughness due to improved laser parameters, with even larger improvements available when using polishing as a post-process finishing operation which was shown to improve susceptibility by over 10 %. This study enables the designers of soft magnetic components made by additive manufacturing, the freedom to design magnetic flux paths at any angle in the build chamber and gives surface roughness as a key parameter to improve magnetic properties.