Rapid consolidation of Nd-Fe-B-type powders as a useful tool for the manufacture of permanent magnets with additional functionalities

Abstract

Spark-plasma sintering (SPS) is a contemporary, fast sintering technique that has drawn much attention in recent years due to its potential for the waste-free manufacture of the complex- and net-shaped sintered bodies from different types of materials, including ceramics, polymers, alloys, and composites [1]. SPS can be adopted to consolidate melt-spun and HDDR-type Nd-Fe-B powders to full bulk density while preserving their excellent coercivity. Coercivity-boosting elements such as Dy and Tb can be directly mixed with the magnetic powder before sintering to ensure even higher Hci values [2]. For melt-spun powders, the grain coarsening related to the inherent microstructural inhomogeneities of as-spun ribbons (i.e., wheel side and free side) is prevented by carefully tuning the spark-plasma sintering parameters, most notably the temperature. Optimally-prepared isotropic, nanostructured, metallic magnets outperform bonded magnets in terms of their energy products and can be subsequently hot-deformed to induce crystallographic texture and ensure good overall hard-magnetic performance, suitable for motor applications. The SPS approach was also considered for processing rare-earth-rich (RE ≈ 33 wt.%) gas-atomized Nd-Fe-B powders. Regardless of their favorable spherical morphology, such powders are unsuited for use in bonded magnets due to the critical loss of their hard-magnetic properties if exposed to temperatures above 150 °C. The high-temperature instability of the powder is overcome in spark-plasma-sintered magnets [3]. The fast powder consolidation performed at ≈ 700 °C minimizes the grain growth during sintering to ensure Hci values above 1000 kA/m for an HRE-free composition. Furthermore, SPS is an excellent tool for synthesizing nanostructured, Nd-Fe-B magnets composed of distinct regions characterized by either a high intrinsic coercivity or a high remanent magnetization. Such magnets can be prepared in a one-step SPS approach from several melt-spun Nd-Fe-B-type powders with different chemical compositions or a two-step approach from pre-sintered precursor magnets. Simulation results using a multicomponent geometry's magnetic characteristics predict its benefit on many electrical devices' functionality [4]. Finally, we will show that the spark-plasma sintering approach is potentially suitable for the net-shape manufacture of anisotropic, fine-grained Nd-Fe-B magnets based on jet-milled powders. However, the electrical effects specific to the rapid, non-equilibrium SPS govern the microstructure's formation, with the nonhomogeneous temperature distribution during a typical SPS run being the most critical factor [5]. Hot spots generated at particle junctions due to localized Joule heating facilitate the decomposition of the RE2Fe14B matrix phase and the formation of the soft-magnetic α-Fe. Several approaches to refine the microstructure of bulk SPS magnets prepared from monocrystalline powders were considered. By lowering the heating rate or tailoring the pressing tools' electrical properties, it is possible to lower the probability of local overheating. Alternatively, reducing the contact resistance by preheating the powder in a vacuum furnace before SPS also prevents the formation of α-Fe. REFERENCES: [1] O. Guillon et al., "Field‐assisted sintering technology/spark plasma sintering: mechanisms, materials, and technology developments," Advanced Engineering Materials, vol. 16, no. 7, pp. 830-849, 2014. [2] K. Žagar, A. Kocjan, and S. Kobe, "Magnetic and microstructural investigation of high-coercivity net-shape Nd–Fe–B-type magnets produced from spark-plasma-sintered melt-spun ribbons blended with DyF3," Journal of Magnetism and Magnetic Materials, vol. 403, pp. 90-96, 2016/04/01/ 2016. [3] T. Tomše et al., "Properties of SPS-processed permanent magnets prepared from gas-atomized Nd-Fe-B powders," Journal of Alloys and Compounds, vol. 744, pp. 132-140, 2018/05/05/ 2018. [4] T. Tomše et al., "Multicomponent permanent magnets for enhanced electrical device efficiency," Journal of Magnetism and Magnetic Materials, vol. 494, p. 165750, 2020/01/15/ 2020. [5] T. Tomše et al., "A spark-plasma-sintering approach to the manufacture of anisotropic Nd-Fe-B permanent magnets," vol. 502, p. 166504, 2020.