Laser Powder Bed Fusion of Nd-Fe-B Permanent Magnets: Solving the Optimization Problem using Design of Experiments and Statistics

Additive manufacturing of Nd-Fe-B permanent magnets by means of Laser powder bed fusions (LPBF) offers the possibility to overcome geometrical restrictions of the traditional powder metallurgical fabrication route for sintered magnets. LPBF is a layer-by-layer additive manufacturing technique, which utilizes lasers to selectively melt metallic powders, which then fuse during solidification. The final product of this manufacturing technique is a dense metallic part. The unique manufacturing process, which includes local melting of a powder material, rapid cooling, directional heat transfer and several re-heating and re-melting, creates special microstructural features and can improve magnetic properties of the final magnet significantly. For example, the coercivity of a LPBF-processed magnet can be increased by more than 30% - from 700 kA/m to more than 900 kA/m - compared to an Nd-lean powder, which was originally optimized for polymer bonded magnets. However, the right choice of the processing parameters is of critical importance for the successful application of LPBF to permanent magnets. The interaction of the major influencing variables, such as laser powder or scan velocity, is difficult to treat in physically based models. An alternative approach for the creation of a suited process model is a statistically based Design of Experiment, which can be used to find an optimal parameter set in a given experimental space. In our current work we use the DoE approach for the comparison of different LPBF machines and for prediction of optimal magnetic properties within a predefined manufacturing parameter range. Furthermore, the impact of preheating temperature and layer thickness on the processability and resulting properties of additively manufactured Nd-Fe-B magnets will be discussed.