United States

Abstract

The coercivity improvement in low-cost commercial heavy rare earth lean melt-spun Nd-Fe-B powders used for polymer-bonded magnets will be presented. We added low-temperature eutectic alloy Nd-Cu as diffusion source [1,2] to a powder with a composition that gives a very small proportion of the intergranular phase. With this addition and optimized procession parameters [3], we significantly increased the coercivity compared to the initial Nd-Fe-B powder. A commercial powder was mixed with a different amount (3 – 10 wt.%) of Nd70Cu30 powder. A mixture was annealed at different temperatures and times in a vacuum to optimize the annealing process and obtain maximum magnetic properties enhancement. After the optimized annealing treatment, the improved basic powder was mixed with polyphenylene sulphide polymer (PPS) in a weight ratio of 88:12, compressed in a press to get a green compact annealed at 300 °C for 40 min in vacuum to provide a polymer-bonded magnet [4].
For the polymer-bonded magnet, consisting of basic powder with the addition of only 3 wt.% of Nd70Cu30 powder, the coercivity increased from 745 kA/m to 855 kA/m. Meanwhile, the remanent magnetization decreased by only ≈ 6 % (from 0.51 T to 0.48 T). The microstructure characterization confirmed the diffusion and increasing proportion of Nd inside the Nd-Fe-B particles. In this way, we have shown that it is possible to improve commercial Nd-Fe-B magnetic powder&#39;s coercivity with a small addition of intergranular phase by diffusion of low-temperature eutectic alloy. The enhanced commercial magnetic powder&#39;s final result is a polymer-bonded magnet with enhanced coercivity, which is a significant step forward in the industrial production of low-cost polymer-bonded magnets [4].

[1] H. Sepehri-Amin et al., “High-coercivity ultrafine-grained anisotropic Nd-Fe-B magnets processed by hot deformation and the Nd-Cu grain boundary diffusion process,” Acta Mater., vol. 61, no. 17, pp. 6622–6634, 2013, doi: 10.1016/j.actamat.2013.07.049.
[2] T. G. Woodcock et al., “Atomic-scale features of phase boundaries in hot deformed Nd-Fe-Co-B-Ga magnets infiltrated with a Nd-Cu eutectic liquid,” Acta Mater., vol. 77, pp. 111–124, 2014, doi: 10.1016/j.actamat.2014.05.045. 
[3] M. Soderžnik, B. Ambrožič, K. Žagar Soderžnik, and M. Korent, “Limits of grain boundary engineering in nanocrystalline Nd-Fe-B melt-spun ribbons,” Mater. Lett., vol. 264, pp. 20–22, 2020, doi: 10.1016/j.matlet.2020.127338.
[4] M. Korent, et al. &quot;A process for improving the magnetic properties of commercial Nd-Fe-B magnetic powder with a small proportion of intergranular phase and a process for producing polymer-bonded magnets from these magnetic powders&quot;, Patent submitted 25. January, 2021


REPM 2021: The 26th International Workshop on Rare-Earth and Future Permanent Magnets and their Applications

The coercivity enhancement of melt-spun Nd-Fe-B magnetic powder by diffusion of low-temperature eutectic alloy for the purpose of manufacturing polymer-bonded magnets

remanence

diffusion process

melt-spun nd-fe-b powder

coercivity

It is our great pleasure to invite you to attend the 26th International Workshop on Rare Earth and Future Permanent Magnets and Their Applications (REPM2021) which will be held virtually on June 7-10, 2021. Please note, this is an updated schedule due to COVID-19 travel restrictions which forced us to postpone the conference from summer of 2020. The REPM workshop takes place every two years and is the premier conference on permanent magnets covering a wide range of topics including:

* Rare earth magnets
* Rare earth-free magnets
* Raw materials, mining and recycling
* Advanced synthesis, processing and characterization techniques
* Theoretical modeling of coercivity
* Fundamental properties of advanced magnetic materials
* Thin film magnets
* Applications of permanent magnets

***Workshop***
The workshop will be chaired by Professor George C. Hadjipanayis, University of Delaware, Newark, DE and Dr. Jinfang Liu, President & COO of Electron Energy Corporation, Landisville, PA.
The objectives of the workshop are to bring together engineers and scientists who work on permanent magnets and their applications to present their latest results and exchange ideas on a wide range of relevant topics including resources, raw materials, recycling, processing, fundamental and hard magnetic properties and applications of rare earth and future magnets.

To regsiter for this event, please follow the link below.

Abstract

The coercivity improvement in low-cost commercial heavy rare earth lean melt-spun Nd-Fe-B powders used for polymer-bonded magnets will be presented. We added low-temperature eutectic alloy Nd-Cu as diffusion source [1,2] to a powder with a composition that gives a very small proportion of the intergranular phase. With this addition and optimized procession parameters [3], we significantly increased the coercivity compared to the initial Nd-Fe-B powder. A commercial powder was mixed with a different amount (3 – 10 wt.%) of Nd70Cu30 powder. A mixture was annealed at different temperatures and times in a vacuum to optimize the annealing process and obtain maximum magnetic properties enhancement. After the optimized annealing treatment, the improved basic powder was mixed with polyphenylene sulphide polymer (PPS) in a weight ratio of 88:12, compressed in a press to get a green compact annealed at 300 °C for 40 min in vacuum to provide a polymer-bonded magnet [4].
For the polymer-bonded magnet, consisting of basic powder with the addition of only 3 wt.% of Nd70Cu30 powder, the coercivity increased from 745 kA/m to 855 kA/m. Meanwhile, the remanent magnetization decreased by only ≈ 6 % (from 0.51 T to 0.48 T). The microstructure characterization confirmed the diffusion and increasing proportion of Nd inside the Nd-Fe-B particles. In this way, we have shown that it is possible to improve commercial Nd-Fe-B magnetic powder's coercivity with a small addition of intergranular phase by diffusion of low-temperature eutectic alloy. The enhanced commercial magnetic powder's final result is a polymer-bonded magnet with enhanced coercivity, which is a significant step forward in the industrial production of low-cost polymer-bonded magnets [4].

[1] H. Sepehri-Amin et al., “High-coercivity ultrafine-grained anisotropic Nd-Fe-B magnets processed by hot deformation and the Nd-Cu grain boundary diffusion process,” Acta Mater., vol. 61, no. 17, pp. 6622–6634, 2013, doi: 10.1016/j.actamat.2013.07.049.
[2] T. G. Woodcock et al., “Atomic-scale features of phase boundaries in hot deformed Nd-Fe-Co-B-Ga magnets infiltrated with a Nd-Cu eutectic liquid,” Acta Mater., vol. 77, pp. 111–124, 2014, doi: 10.1016/j.actamat.2014.05.045. 
[3] M. Soderžnik, B. Ambrožič, K. Žagar Soderžnik, and M. Korent, “Limits of grain boundary engineering in nanocrystalline Nd-Fe-B melt-spun ribbons,” Mater. Lett., vol. 264, pp. 20–22, 2020, doi: 10.1016/j.matlet.2020.127338.
[4] M. Korent, et al. "A process for improving the magnetic properties of commercial Nd-Fe-B magnetic powder with a small proportion of intergranular phase and a process for producing polymer-bonded magnets from these magnetic powders", Patent submitted 25. January, 2021


poster

This paper investigates the viability of hydrogen decrepitation as a means for recovery and recycling of samarium cobalt (SmCo5) magnets. To hydrogen decrepitate SmCo5, a higher pressure is required compared to the atmospheric pressure used for NdFeB magnets, but it is essential to find the lowest energy route to make it economically viable. Samples of SmCo5 were characterised using scanning electron microscopy and X-ray diffraction (XRD), before using intelligent gravimetric analysis (IGA) to determine the hydrogen absorption plateau pressures at a range of temperatures. The samples were then subjected to a range of pressures from 5-20 bar, as well as a high temperature and pressure trial at 300 ⁰C and 20 bar, to find the conditions at which the magnets broke down into powders. IGA measurements showed that increasing the reaction temperature also increased the plateau pressure for absorption, and that the plateau pressure at room temperature was ~6 bar. Magnetised bulk samples of SmCo5, however did not react at this plateau pressure, instead requiring a minimum 10 pressure bar at room temperature over 24 hours, however they did not demagnetise. Recommendations were made to improve the viability of hydrogen decrepitation for recovery and recycling of SmCo5 magnets.


Recovery and Recycling of SmCo5 Using Hydrogen Decrepitation

This poster is on the use of the HPMS process (Hydorgen processing of magnet scrap) to liberate NdFeB alloy powders from water pumps, wind turbine magnets and loudspeakers. The separated powder has been subsequently processed into HDDR powders. The HDDR microstructures and magnetic properties are reported here.

HPMS Optimisation for End of Life NdFeB Containing Products

Usually, the production of rare-earth sintered magnets requires some care in their manufacturing process, as powder manipulation at controlled atmosphere to avoid oxidation. Oxygen-free environments are required to prevent oxidation of rare earth-based powders. In this work, sintered Nd-Fe-B permanent magnets were prepared using a commercial alloy, following the traditional powder metallurgy route. The alloy was hydrogen decrepitated (HD process), and the resultant coarse powder was mechanically milled. This step was performed employing several reduced graphene oxide (rGO) additions (0.02 to 0.10 wt %). It was observed that rGO acts as a lubricant and effective milling medium, preventing agglomeration and welding of the particles. Additionally, it was possible to handle the milled powders without the need for an inert atmosphere. Magnetic properties of sintered magnets decreased with increasing rGO addition, showing that carbon is deleterious to the magnetic properties at high concentrations (0.10 wt %). The highest values of remanence (Br) and intrinsic coercivity (iHc) values were obtained for the sintered magnet using 0.02 wt% rGO addition, Br = 1.1 T and iHc = 764.0 kAm-1.

Nd-Fe-B sintered magnets obtained by mechanical ball milling with rGO addition

NdFeB magnet waste flows in the light of recycling – A detailed assessment

We are investigating the novel possibilities and aspects of electrochemical recycling and reprocessing of Nd-Fe-(B)-based and Sm-Co magnets. First, we investigated the possibility of electrodeposition of Nd-Fe alloy at room T from ionic liquid based on 1-ethyl-3-methylimidizolium dicyanamide ([EMIM][DCA]) [1]. The electrodeposition of the rare earths (REs) at room T, is still a challenge because of their extremely low electrochemical potential. We observed that Nd(III) cannot be reduced independently, but it can be co-deposited with the addition of Fe(II). Further, we propose [2] a new concept of recycling the sintered (Dy,Nd)-Fe-B magnets selective electrochemical etching recovering the 2:14:1 matrix phase. Via an electrochemical etching method, we are able to recover pure individual 2:14:1 grains that can be re-used for new types of magnet production. Similar anodic etching was applied also for Sm-Co magnets, where it was shown, that it is possible to selectively etch the Sm, with simultaneous deposition of Co, Fe, and Cu on the cathode. With regards to the energy consumption of the new process, the proposed route is comparable to the established direct re-use methods like remelting and hydrogen processing of magnetic scrap. Further to tackle the issue of efficient and environmentally friendly rare earth recycling, we propose a novel electrochemical method for Nd–Fe–B PMs etching and REs recycling [3]. The proposed procedure enables complete electrochemical etching of the Nd–Fe–B magnet with further selective REs recovery and a simultaneous Fe metal deposition. The proposed procedure thus avoids the formation of solid Fe-based waste with the total re-use of the electrolyte resulting in no wastewater discharge.

Electrochemical Recycling and Reprocessing Aspects of Rare Earth-based Permanent Magnets

The emergence of contemporary processing techniques, like, for example, the additive manufacturing, makes a production of non-conventional-shape permanent magnets possible. The idea is to adjust the magnet geometry and magnetization in accordance with the prescribed resulting magnetic field as it follows from the design of the magnet-containing device. Although the relation between the magnetization and the field is described in terms of the classical electrodynamics, the solution of the problem is not straight forward due to the ill-conditioned respective equations. We will present the application of the adjoint method for eciently solving the inverse-magnetostatic problem by considering the abilities and limitations of the 3D-printing process.

Design of permanent magnets by solving the inverse magnetostatic problem

Effect of Mg layer on the crystal orientation and magnetic properties for FePt thin films

The recent trends in rare-earth usage and applications are discussed. Most of the motors of electric cars use Neodymium-iron-boron magnets, resulting in 1-2 kg of NdFeB magnets per vehicle. This can push significantly the demand for NdFeB magnets. Other application that may increase the demand of NdFeB magnets are giant off-shore wind turbines. The production of light rare-earths as neodymium and praseodymium can be easily increased. However, heavy rare-earths as terbium and dysprosium are scarce, and its price has increased significantly. Many Brazilian projects are stopped in present moment, waiting cheaper methods of rare-earth separation. However, Serra Verde project (state of Goias - Brazil) was scheduled to start in 2022. Recycling should be an economical activity. This explain why the recycling of rare-earths at the present time is very small. The recycling of rare-earths is estimated to be around 1% or less. The re-use of magnets is a possibility. This can be viable only since there is a standardization of manufactured magnets. 


Influence of grain size in the coercivity of sintered magnets

Influence of cooling conditions and alloy composition on the microstructure and mechanical properties of hot-deformed Nd-Fe-B produced by rotary swaging
Corinna Müller1, Franziska Staab1, Fansun Chi2, Peter Groche2, Enrico Bruder3, Karsten Durst3, Stefan Riegg1 und Oliver Gutfleisch1
1 Funktionale Materialien, FB Materialwissenschaft, TU Darmstadt 
2 Institut für Produktionstechnik und Umformmaschinen, FB Maschinenbau, TU Darmstadt
3 Physikalische Metallkunde, FB Materialwissenschaft, TU Darmstadt
The classical powder-metallurgical production route for Nd-Fe-B magnets includes jet milling and sintering to achieve homogeneous grain sizes of a few micrometers. New alternative production routes use metal forming techniques for magnet production. One advantage is the possibility to produce specific shapes in a continuous process, which reduces production and machine times and increase material utilization. In addition, the non-powder based route decreases the level of oxygen impurities and improves the process handling. The here introduced use of rotary swaging could be such an alternative and cost-efficient technique, allowing continuous processing [1]. During swaging in this top-down processing approach the grain size of a metal-cladded cast body is reduced by the deformation.
The challenges of metal forming techniques such as rolling or swaging are the creation of the required homogeneous microstructure. In rotary swaging a reduction of the grain size down to hundred nanometers is possible, but still some very big grains remain. In this work the relationship between alloy composition, resulting mechanical properties and microstructure is investigated. The specific microstructure of these magnets is discussed in relation to their magnetic performance.
The variation of doping elements such as Cu, Nb and Ga changes not only the mechanical properties but also the initial microstructure and phase composition [2,3,4]. Furthermore, the phases and microstructure can be influenced by the cooling geometry. With different diameters for casting molds, different cooling speeds can be reached which changes the grain size and affects the formation of alpha-iron for example. The aim is to develop a feasible starting microstructure and composition for subsequent rotary swaging.
Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 405553726 – TRR 270.
[1] Chi, F.; Wießner, L.; Gröb, T.; Bruder, E.; Sawatzki, S.; Löwe, K. et al. (2019): Towards manufacturing of Nd-Fe-B magnets by continuous rotary swaging of cast alloy. In Journal of Magnetism and Magnetic Materials 490, p. 165405. DOI: 10.1016/j.jmmm.2019.165405
[2] HU, Zhihua; LIU, Guojun; WANG, Huijie (2011): Effect of niobium on thermal stability and impact toughness of Nd-Fe-B magnets with ultra-high intrinsic coercivity. In Journal of Rare Earths 29 (3), pp. 243–246. DOI: 10.1016/S1002-0721(10)60439-5.
[3] Jiang, Jianhua; Zeng, Zhenpeng; Yu, Jian; Wu, Jiansheng; Tokunaga, M.: The effect of Co addition on the fracture strength of NdFeB sintered magnets. In Intermetallics 9 (4), pp. 269–272.
[4] Liu, J. F.; Vora, P.; Walmer, M. H.; Kottcamp, E.; Bauser, S. A.; Higgins, A.; Liu, S. (2005): Microstructure and magnetic properties of sintered NdFeB magnets with improved impact toughness. In Journal of Applied Physics 97 (10), 10H101. DOI: 10.1063/1.1847215.


Influence of cooling conditions and alloy composition on the microstructure and mechanical properties of hot-deformed Nd-Fe-B produced by rotary swaging

The Effects of Zr + Fe2B additions on NdFeB-type Material during the Hydrogen Ductilisation Process (HyDP)

F. Burkhardt1, O.P. Brooks1, A. Walton1, I.R. Harris1
1School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

It is well known that NdFeB-type alloys are extremely brittle at room temperature. Therefore, in order to shape the material into fully dense magnets, it can either be reduced to a powder and sintered at high temperatures between 1000-1200°C [1][2], or hot pressed at temperatures between 800-900°C [1]. Recent studies have shown that it is possible to overcome the brittle nature of NdFeB-type starting cast alloys by pre-processing the material in hydrogen prior to mechanical deformation, using a process termed the „Hydrogen Ductilisation Process (HyDP)“[3][4][5].
During the HyDP the alloy is processed in hydrogen to produce a solid disproportionated (s-HD) microstructure that can be deformed at room temperature. However, during pressing trials it has been shown that fracturing occurs in the minority NdFe4B4 phase [4], which will lower the density of the final magnet and possibly limits the range of plastic deformation techniques that can be applied.
One way to reduce the amount of NdFe4B4 phase in book mould cast NdFeB alloys, is to move towards a stoichiometric Nd2Fe14B alloy. However, this is associated with the formation of free α-Fe dendrites which in turn are detrimental to the magnetic properties. It has previously been shown that by adding zirconium and Fe2B to cast NdFeB alloys it is possible to avoid the formation of free α-Fe dendrites [6]. This study investigates the effect of Zr and Fe2B additions on the microstructure and deformation behaviour of NdFeB alloys during the Hydrogen Ductilisation Process (HyDP).

References
[1] Müller, K.-H. et al. (1998a) ‘Hot deformed NdFeB permanent magnets’, in Materials Letters, pp. 50–54. doi: 10.1016/S0167-577X(97)00141-9.
[2] Croat, J. J. (2018b) Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets. 1. Edition. Elsevier Ltd. ISBN: 978-0-08-102225-2.
[3] Harris, I. R., Walton, A. and Brooks, O. P. (2017c) ‘Patent: Magnet Production’. United Kingdom: World Intellectual Property Organization.
[4] Brooks, O. P. et al. (2017d) ‘The Hydrogen Ductilisation Process (HyDP) for shaping NdFeB magnets’, Journal of Alloys and Compounds. Elsevier B.V, 703, pp. 538–547. doi: 10.1016/j.jallcom.2016.12.177.
[5] Brooks, O. P. et al. (2018e) ‘Complete ductility in NdFeB-type alloys using the Hydrogen Ductilisation Process (HyDP)’, Acta Materialia. Acta Materialia Inc., 155, pp. 268–278. doi: 10.1016/j.actamat.2018.04.055.
[6] Kirby, K. et al. (2018f) The effects of AB2 (A=Zr,Hf,Ti) additions on the free-iron content in ingots with the initial composition of Nd2.2Fe14B1.1. Birmingham. (Awaiting publication in JMMM)



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Recovery and Recycling of SmCo5 Using Hydrogen Decrepitation

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