Research articlesAdditive manufacturing of anisotropic hybrid NdFeB-SmFeN nylon composite bonded magnets
Introduction
Complex near net-shaped Nd2Fe14B (Nd-Fe-B) permanent magnets are desirable in numerous applications because they enable device miniaturization and weight reduction. Traditionally, bonded magnets are fabricated by either compression molding with a thermoset binder or injection molding with a thermoplastic binder [1]. Also tooling dies used for conventional molding are expensive and capable of producing only a limited number of sizes and shapes. This challenge can be addressed by additive manufacturing techniques which recently have attracted great interest due to their advantages in printing complex shapes without any tooling and minimizing loss of critical materials. Baldissera et al. showed a novel approach utilizing a laser beam to melt the mixed powder of Nd-Fe-B and polyamide 12 [2]. Recently, Huber et al. used the conventional end-user 3D printer to print polymer-bonded rare-earth magnets with a variable magnetic compound fraction for a predefined stray field [3].
Hybrid magnetic composite bonded magnets have been explored recently for the improvement of dynamic mechanical, thermal and magnetic properties, as well as corrosion resistance. Many studies of Nd-Fe-B composite bonded magnets with the addition of Sm2Fe17Nx (Sm-Fe-N) powders have been made [4], [5], [6], [7]. Sm-Fe-N powders with a particle size of 1–5 μm, show good hard magnetic properties and high oxidation resistance [8], [9], [10], [11]. Also, Sm-Fe-N exhibits fairly high saturation magnetization (1.54–1.57 T), high anisotropy field (14 T) and Curie temperature (750 K), which are comparable or superior to Nd-Fe-B [10], [12], [13]. In NdFeB/SmFeN hybrid magnets prepared by warm compaction method, fine Sm-Fe-N particles fill in the voids between the larger NdFeB particles and hence improve the density of the magnets. In addition, this hybrid-magnet shows excellent magnetic properties and improves the magnetic particle alignment resulting in a higher energy product [6], [7]. Therefore, hybrid materials development and utilization are economically motivated, due to the possibility of tuning their final properties while maintaining low manufacturing costs.
Anisotropic composite magnetic powders can be used in bonded magnets to obtain higher Br and (BH)max than isotropic bonded magnets. However, the degree of alignment of anisotropic magnetic powders in the binder matrix majorly determines how large the remanence and (BH)max can be. Nlebedim et al. [14] identified the optimum alignment temperature and magnetic fields for magnequench anisotropic powder, MQA NdFeB – Ethylene-vinyl acetate (EVA) flexible bonded magnets produced by extrusion and discussed the effect of alignment and thermo-rheological conditions on magnetic properties. We have previously demonstrated 3D printing of isotropic NdFeB bonded magnets via both the binder jetting [15], [16] and the extrusion-based [17], [18], [19] techniques. It is known that anisotropic Nd-Fe-B bonded magnets offer higher energy product if the particles are aligned in a state in which the energy supplied by the alignment magnetic field is dominant [14]. In this article, we report the fabrication of anisotropic Nd-Fe-B + Sm-Fe-N composite magnets via the extrusion-based Big Area Additive Manufacturing system. However, an efficient system for magnetic field alignment of the anisotropic powder during the printing process needs to be designed and implemented. To overcome such challenges for alignment during printing, we determined the optimum temperature and magnetic field for post-printing alignment using a Vibrating Sample Magnetometer, VSM. Optimum conditions from post alignment in a VSM was used as a guide for post-printing alignment of printed magnets using an electromagnet. Post-printing alignment resulted in a significant enhancement in both remanence and energy product. It is possible to achieve anisotropic printed magnets without deforming and still maintaining the original shape during the post-printing alignment process.
Section snippets
Experimental
Commercial composite pellets for this work were supplied by Aichi Steel Inc. [20]. It consists of 65 vol% magfine anisotropic composite Dy-free Nd-Fe-B + Sm-Fe-N powders and 35 vol% Nylon 12. These pellets are typically used for conventional injection molding of anisotropic magnets. The pellets were used as feedstock materials for 3D printing in the Big Area Additive Manufacturing system as well. Magnetic properties were determined by using a SQUID magnetometer. The magnetic hysteresis loop of
Results and discussion
Fig. 2 shows the magnetization data of the starting pellet, and the printed magnet measured parallel and perpendicular to the in-printing alignment direction. The printing resulted in a slight increase in remanence and decrease in coercivity. The decrease in coercivity is likely related to the possible effects of magnet degradation during printing. It can be seen that the printing did not result in any observable alignment of the magnetic particles, comparing the data in both parallel and
Conclusion
The results of a study on post-printing alignment of 3D printed anisotropic bonded magnets have been presented. Optimum conditions from the VSM alignment experiment served as a guide for additional post-printing alignment using an electromagnet. 3D printed anisotropic magfine Dy-free Nd-Fe-B + Sm-Fe-N magnet powder bonded in Nylon copolymer were aligned at different magnetic field strengths and temperatures to determine the optimum conditions for alignment. In addition to the alignment magnetic
Acknowledgments
This research was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office.
Additional information
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will
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