The production of complex/intricate shaped NdFeB magnets can be challenging when using conventional manufacturing techniques due to their inherent brittle structure. The additive manufacturing technique selective laser melting (SLM) may offer an opportunity to produce NdFeB magnets with high geometric complexity and expand the range of applications and magnet capabilities. Nevertheless, some SLM processing challenges exist for the processing of this material, including the generation of the internal porosity due to the intermetallic structure of NdFeB, the creation of residual thermal stresses and crack formation due to the fast-cooling rate of the SLM process. The main objective of this work is to explore SLM process parameter effects on the formation of NdFeB, with the aim of reducing part porosity, internal cracks using process parameter optimization. This study for the first time will also explore the effect of using a heated powder during processing. In addition, this thesis is the first study which investigates the effect of heat treatment, Ni infiltration and HIPping on magnetic properties.
During this investigation, it was found that low laser energy densities, between 50 to 100 J/mm3 enabled the fabrication of samples with up to 95.72 % density. Pores and crack defects were observed mainly on the edge of the samples. It was found that density is improved by lowering hatch distance and layer thickness, while the former has higher impact on sample density. Regarding the magnetic properties, magnets with a maximum energy product of 81 kJ/m3 could be produced without any post-treatment. Furthermore, there is no entirely linear relationship observed between the remanence and the density of the samples. Magnetic properties increase with the increasing volume fraction of the strong magnetic phase, Nd2Fe14B, in the samples.
The density of the samples and remanence, Br can be improved with use of the heated bed while the coercivity, Hc decreases. The maximum energy product improved up to 84 kJ/m3. The Br improvement is related to density improvement, and Hc drops related to the bigger /or dendritic grains as a result of slowing cooling rates in heated bed. However, the integrity of the samples is less better than at room temperature fabrication. However, there is an improvement in the sample density, Br ,and Hc until 580 °C annealing temperature, then Hc starts to decrease while Br and density increases. The Br increment in heat treatment was not as expected according to the literature ofthe sintered magnets. It was found that Ni infiltration and HIPping did not improve the magnetic properties of the samples as the former was applied at very high temperatures (1400 °C), resulting in larger grains, reducing the coercivity. In addition, the amount of hard magnetic phase reduced after infiltration causing a reduction in Br.HIPping, causes loss of Nd rich phases with the application of heat and inert gas pressure on the samples during the process. Therefore, with the lack of isolating Nd rich grain boundaries, coercivity drops significantly.
In summary, this study found that processing of NdFeB by SLM is very challenging. The brittle structure of the material with the combination of high cooling rates of the SLM, and the low strength of the material, makes it difficult to process without cracks and porosities. Additionally, the high oxygen sensitivity of the material compromises the phases for high magnetic properties, and the oxide layers on top of some samples make it difficult to get high-quality SEM images. However, the contributions of this study to literature are valuable.
This study is the first which examines the effect of the heated bed. It was found that the heated bed has a potential to improve both the magnetic properties (remanence Br and maximum energy product BHmax) and the density of the magnets, in addition to improvement in surface quality. Also, it is the first study that examines the effect of heat treatment on magnetic properties. In addition, the relationship between the magnetic properties and printing parameters are valuable findings for literature. No other study in the literature discusses the Ф phase- a-Fe phase and printing parameter relationships. The SEM images and discussions are also valuable in an academic context since they clearly show the grain formation and morphology in the melt pool, and no discussion is found in the literature about them. Finally, the most important contribution is that the magnetic properties, Br and BHmax, are the highest properties that have improved so far, by parameter optimisation, without any post-processing or element doping, 0.72 T and 81 kJ/mm3, respectively.
