title: Additive Manufacturing of Bonded NdFeB Magnets by Selective Laser Sintering
Creators: name: Mapley, Martin C
Publisher: name: Griffith University
Year: 2020
Subjects: subject: Additive Manufacturing
subject: 3D Printing
subject: Selective Laser Sintering
subject: Permanent Magnets
subject: NdFeB

description: Permanent magnets are used in many everyday devices to assist in the conversion of electronic energy into mechanical energy. Traditional manufacturing pathways are well established, however, they inherently suffer from long lead times and high costs for low volume production runs. A technique known as additive manufacturing (colloquially 3D printing) has been applied to many industries using a multitude of materials with success in reducing both product development times and cost for prototypes and low volume production runs. Due to this reported success, several additive manufacturing techniques have been investigated in order to rapidly produce magnets of custom geometries using isotropic magnetic material (specifically neodymium-iron-boron). Although some of these techniques have shown promise, many require pre-processing of the raw powders into a filament or fluid suspension, thus increasing the number of steps in the process. As permanent magnet raw material is commercially available in powdered form, it was hypothesised that a powder-based additive manufacturing technique known as selective laser sintering could be used to produce permanent magnets by mechanical alloying of magnetic powder with a powdered polymer binder. It was further hypothesised that by combining the use of high energy anisotropic magnetic powder with an in-situ alignment fixture, as used in the industrial manufacturing process, permanent magnets with higher performance than those produced using additive manufacturing from isotropic powders. Accordingly, there were three key aims in this research project: ii Additive Manufacturing of Bonded NdFeB Magnets by Selective Laser Sintering  Develop a selective laser sintering machine from off-the-shelf components and compare the geometrical and mechanical properties of the produced polyamide-12 parts with that of a commercial machine.  Using two commercially available isotropic magnetic powders with flake and sphere particle morphologies mixed with polyamide-12 powder, determine the processing parameters and mixing ratios which lead to magnets with the most accurate geometrical and strongest mechanical and magnetic properties.  Develop and test an in-situ particle alignment fixture and examine its effect on the geometrical and magnetic properties of magnets produced from anisotropic magnetic powder mixed with polyamide-12. The first experimental chapter (Chapter 3) covers designing, building and validating the tool required to conduct the research which is a selective laser sintering machine produced solely from off-the-shelf parts. Over the next two chapters (Chapters 4 and 5), the influence of machine processing parameters (laser power = 0.5 – 1.17 W, scan spacing = 0.11, 0.22, 0.31 mm) and powder loading fraction (0 – 90%/vol) on the properties of magnets produced from two commercially available neodymium-iron-boron powders was examined. By examining the geometrical, mechanical and magnetic properties, preferred processing parameters and mixing ratios could be identified based on their deviation from model data and highest mechanical and magnetic strength. Next (Chapter 6), an in-situ alignment fixture based upon a Helmholtz coil was proposed and a prototype developed. The ability of the coil to impart torque to anisotropic particles while minimising translational forces was examined using images captured with an optical microscope then evaluated using image processing techniques to characterise the percentage of loose particles it could align. Finally (Chapter 7), the Helmholtz coil-based alignment fixture was added to the selective laser sintering machine where it was used to provide an alignment field to each layer of anisotropic powder prior to consolidation by the laser. The geometry, density and magnetic properties of the permanent magnets were examined in the presence and absence of the alignment field. The constructed selective laser sintering machine was demonstrated to produce parts from polyamide-12 reaching densities of 918 ± 9 kg/m3 and achieving an elastic Additive Manufacturing of Bonded NdFeB Magnets by Selective Laser Sintering iii modulus of 358.36 ± 3.04 MPa and elongation at break of 11.13 ± 0.02%. Permanent magnets produced from the mechanically alloyed commercial powders demonstrated the best properties with a supplied energy density of 0.255 J/mm2 possessing comparable magnetic characteristics to those produced with other AM methods (311 ± 9 and 363 ± 6 mT for the flake and spherical powders respectively). It was also demonstrated that magnetic powder loading fractions above 30%/vol for the spheres and 50%/vol for the flakes showed no significant increases in magnetic performance while the mechanical performance deteriorated significantly. An in-situ alignment fixture, based upon a Helmholtz coil, demonstrated 30% alignment of a selection of particles according to optical image analysis of randomly orientated flake particles. When used during the processing of magnets, the magnetic performance, when using anisotropic powder, was shown to increase by 28%. However, the low loose packing density of the starting powder (2880 kg/m3) compared to the isotropic powders (3023 kg/m3 for the flakes and 4017 kg/m3 for the spheres) limited the maximum density of the magnets and thus suffered from poorer than expected magnetic performance. The content of this thesis demonstrates the development of an open source-based selective laser sintering machine which was then used to explore the production of permanent magnets from commercially available isotropic and anisotropic powders with the addition of an in-situ alignment fixture.