Manufacturing of More Sustainable Nd-Fe-B Permanent Magnets using the 2-Powder Method

Permanent magnets based on rare earth elements (REEs), as the Nd-Fe-B system, are a key material in overcoming climate change, due to their usage in traction motors of electric vehicles or generators in wind turbines. However, if Nd-Fe-B magnets are used at elevated temperatures, the critical heavy rare earth elements (HREs) such as Dy or Tb have been added to increase temperature stability. To reduce the usage of HREs to a minimum, an additional time- and energy-intensive diffusion process, the so-called grain boundary diffusion process (GBDP), is conventionally used. The GBDP leads to a decreased criticality but to an enhanced energy demand during magnet manufacturing. In order to maintain the advantage of the GBDP of reducing criticality but without the disadvantages of requiring additional energy-intensive thermal treatments, the 2-powder method for manufacturing of Nd-Fe-B sintered magnets was developed. The 2-powder method (2PM) for the production of more sustainable and less critical high-performance Nd2Fe14B-based sintered magnets is investigated by blending coarse HRE-free main phase powders (MPs) with finer HRE-containing anisotropy powders (APs). In the first part of this work, a proof-of-concept is demonstrated with all processing steps up to a 20 kg range by using the manufacturing methods of strip casting, hydrogen decrepitation, and jet milling on an industrially relevant pilot line for magnet production at Fraunhofer IWKS, Hanau. Blending a HRE-free MP (D50 = 5.3 μm) and a finer HRE-containing AP (D50 = 2.6 μm) with a Dy content of 10 wt.%, the development of a HRE-enriched core-shell structure is observed, and the magnetic properties and the microstructure are analyzed in detail. As a result, the coercivity was enhanced from an initial value of 1342 kA/m without Dy to 1457 kA/m, 1601 kA/m, and finally 1735 kA/m, having 1, 2, and 3 wt.% Dy respectively without a significant decrease in remanence. Further microstructural investigations show the formation of a core-shell development, in which the HREs are only located in the outer regions of the grains, the shell regions. For a better understanding of the core-shell development, the diffusion behavior of the 2PM is investigated, and the diffusion coefficient for Dy is determined to be D = (1.83 ± 0.54) 10-11 cm2/s. In the second part of this work, the enabling of the production of big HRE lean magnets with a homogeneous microstructure is shown, and thus the utilization of the 2PM for industrial magnet manufacturing is demonstrated. At first, the particle size effect is investigated by engineering a finer and a larger MP (D50 = 3.7 and 5.4 μm) and four different APs with average particle sizes of D50 = 2.5, 3.1, 3.6, and 4.0 μm. As a result, the coercivity gain is determined to be between 350 - 450 kA/m, and roughly 100 kA/m is attributed to the particle size effect, meaning that finer grain sizes lead to higher coercivity enhancements. The possibility of producing magnets with a core-shell structure independent of their size by using the 2PM is demonstrated on a 340g big cylindrical magnet with dimensions of approximately 45 mm in height and 40 mm in diameter, containing 2 wt.% Dy. The magnetic properties are determined to be 1.32 T in remanence and 1408 kA/m in coercivity, leading to an energy product of 336 kJ/m3. Microstructural investigations of the core-shell development show that the grain size and core thickness after sintering increase with increasing size of the initial powder mixtures. On the other hand, the shell thickness is always in the same range of approximately 2 μm, independent of the particle size of the MP and AP, showing that precipitation of the Dy atoms of the AP has a crucial role in core-shell development. Next, the approach of removing the finest powder particle fraction of less than 1 μm is investigated, leading to better magnetic properties for magnets produced out of the MP only. However, the coercivity gain by using the 2PM is less significant and is attributed to an increased nitrogen pickup during jet milling, which could be avoided if, for example, argon is chosen as milling gas. Finally, a life-cycle-assessment (LCA) compares the global warming potentials of conventional magnet manufacturing, the industrial used GBDP, and the 2PM. The LCA shows that about 8 % more CO2-eq./kg is consumed in the additional GBDP steps. On the other hand, the 2PM consumes only 2 % more CO2-eq./kg. Therefore, the potential of the 2PM as an alternative manufacturing method for the production of more sustainable and less critical Nd-Fe-B magnets for climate-friendly applications is proven and demonstrated on an industrially relevant scale.