Enabling 3D magnetic circuits by the additive manufacturing of soft magnetic material

Additive manufacturing has been revolutionary in enabling complex structural components such as lattice structures and topologically optimised parts, however the utilisation of functional materials such as soft magnetic materials is only just being realised. 3D magnetic flux pathways in electrical machines have been illusive due to the high eddy current losses caused by thick cross-sections, and the inability to process electrical steel laminations into 3D structures. By processing soft magnetic materials with additive manufacturing, geometry can be tailored to avoid large bulk cross sections and reduce eddy currents whilst maintaining a 3D flux pathway, enabling the creation of new electrical machine architectures in the pursuit of higher power density and efficiency, which may enable the decarbonisation of the transport sectors including commercial aircraft. This thesis demonstrates the processing of high silicon electrical steel (Fe-6.5 wt%Si) using laser powder bed fusion and characterises the magnetic properties of this material. The importance of surface roughness on the magnetic susceptibility is investigated, showing that contours may be used to improve the as-built surface finish, but post-processing methods such as polishing are required to obtain the best magnetic properties. The samples in this study exhibited a weak crystallographic texture and the orientation of the samples in the build chamber displayed little impact on the magnetic susceptibility. Two methods are used in this study to reduce eddy currents and enable components with 3D magnetic flux pathways to be manufactured. The first is by designing thin-walled cross-sections, which use air as an insulating medium to reduce the thick cross section of the material. This method is demonstrated in lab experiments showing a reduction of the eddy current loss coefficient to 0.0005 using a novel hexagonal cross section. A Hilbert pattern was implemented into an axial flux electrical machine, demonstrating loss performance comparable to thick electrical steel laminations below 500 Hz, increasing torque density by 13% by achieving a reduction in volume of magnetic material of 33%. The second method uses process control to create stochastic cracking within the material, demonstrating excellent loss behaviour of 2.2 W/kg (50 Hz, 1T) with stacking factors >97%. The mechanical integrity was confirmed to be adequate for implementation into the axial flux machine tested with a UTS of 25 MPa when embedded with epoxy resin. These methods can be implemented into electrical machines enabling the creation of new architectures, with the hope to increase power density and efficiency. This is the first time that additively manufactured soft magnetic material has been characterised in an electrical machine, overcoming the issues of large cross sections. Although the soft magnetic material has not displayed loss behaviour as good as electrical steel laminations, it does enable 3D magnetic circuits within electrical machines which may be exploited to improve the performance of the machine. Further optimisation of the stochastic cracking method of eddy current management by aligning the cracks with the flux direction will yield further improvements, and may compete with the thinnest laminations. Due to the current cost and limitations of additive manufacturing, this technology is only likely to be implemented into the highest value electrical machines, such as those in top end automotive and aerospace, where benefits in performance are of upmost importance. This development in processing of soft magnetic material is the missing piece to enable fully additively manufactured motors, which could revolutionise electrical machine architecture.