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Fabrication, Characterization, and Development of Hot-Deformed Magnets
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Fabrication, Characterization, and Development of Hot-Deformed Magnets

A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Materials Physics".

Deadline for manuscript submissions: closed (20 April 2025) | Viewed by 13189

Special Issue Editors

Dr. Tae-Hoon Kim

E-Mail Website
Guest Editor
Korea Institute of Materials Science, Changwon 51508, Korea
Interests: magnetic materials; permanent magnets; rare-earth magnets; rare-earth free magnets; sintering; hot deformation; microstructure characterization
Dr. Xin Tang

E-Mail Website
Guest Editor
Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba, Japan
Interests: Nd-Fe-B; hot-deformed magnets

Special Issue Information

Dear Colleagues,

1) High-performance permanent magnets are a core material in the production of motors of next-generation mobility driven by environmentally friendly electric energy. In particular, among various magnet manufacturing processes, the hot-deformation process is a promising next-generation industrial process for the production of high-coercivity permanent magnets comprising nano-sized grains. Therefore, for sustainable R&D in the permanent magnet field, significant research results regarding the fabrication, characterization, and development of hot-deformed magnets should be accumulated and shared.

2) The aim of this Special Issue, entitled “Fabrication, Characterization, and Development of Hot-Deformed Magnets“, is to share the state-of-the-art development in high-performance hot-deformed permanent magnets. We cordially invite you to contribute to the permanent magnet field both academically and industrially by submitting your meaningful research results regarding hot-deformed magnets. We are sure that your contribution will be of great help to the growth and expansion of the worldwide permanent magnet field. Manuscripts in the form of full research papers, communications, and review articles are encouraged.

Dr. Tae-Hoon Kim
Dr. Xin Tang
Guest Editors

Manuscript Submission Information

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Keywords

  • permanent magnets
  • rare-earth based magnets
  • rare-earth free magnets
  • hot deformation
  • microstructure
  • magnetic properties

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Published Papers (7 papers)

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Research

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12 pages, 2789 KiB  
Article
Effects of Length-to-Width Ratio on Magnetic and Microstructural Properties of Die-Upset Nd–Fe–B Magnets
by Sang-Hyub Lee, Min-Suk Kang, Yang-Do Kim and Dong-Hwan Kim
Materials 2024, 17(21), 5236; https://doi.org/10.3390/ma17215236 - 28 Oct 2024
Viewed by 714
Abstract
In this study, the effects of the length-to-width ratio on the magnetic and microstructural properties of die-upset Nd–Fe–B magnets were examined. A die-upset magnet with a uniform shape and no significant cracking was successfully developed. During the die-upset process, the applied pressure was [...] Read more.
In this study, the effects of the length-to-width ratio on the magnetic and microstructural properties of die-upset Nd–Fe–B magnets were examined. A die-upset magnet with a uniform shape and no significant cracking was successfully developed. During the die-upset process, the applied pressure was not uniform across the magnet and varied depending on its shape and position. In the case of the magnet with a 4:1 ratio, which had the largest length-to-width ratio, there was a higher concentration of stress at the edges compared to the center, resulting in easier grain deformation and growth at the edges, which led to the lowest remanence and (BH)max. As the length-to-width ratio approached 1:1, the grain size increased slightly, reducing the coercivity; however, the magnets maintained a uniform grain-size distribution. The change from a rectangular to a square pressing surface resulted in a more uniform stress distribution, particularly at the center and corners, which improved the consistency of the magnetic properties across different regions of the magnet. Consequently, an 11.9% enhancement in (BH)max, reaching 285.6 kJ/m3, was achieved in the 1:1 ratio die-upset magnet, which is attributed to the uniform grain size and improved grain alignment. Full article
(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)
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11 pages, 3983 KiB  
Article
Effect of Pressure on Ce-Substituted Nd-Fe-B Hot-Deformed Magnets in the Hot-Pressing Process
by Ye Ryeong Jang, Wonjin Kim, Sumin Kim and Wooyoung Lee
Materials 2024, 17(15), 3769; https://doi.org/10.3390/ma17153769 - 31 Jul 2024
Cited by 2 | Viewed by 1083
Abstract
With the increasing demand for Nd-Fe-B magnets across various applications, the cost-effective substitution of Ce has garnered significant interest. Many studies have been conducted to achieve the high magnetic properties of Nd-Ce-Fe-B hot deformation magnets in which Nd is replaced with Ce. We [...] Read more.
With the increasing demand for Nd-Fe-B magnets across various applications, the cost-effective substitution of Ce has garnered significant interest. Many studies have been conducted to achieve the high magnetic properties of Nd-Ce-Fe-B hot deformation magnets in which Nd is replaced with Ce. We propose a method to improve magnetic properties of the Ce-substituted Nd-Ce-Fe-B hot-deformed magnets by optimizing the hot-pressing process. This study investigates the microstructure and properties following hot deformation of Ce-substituted Nd-Ce-Fe-B magnets fabricated at a constant temperature and different pressures (100–300 MPa) during the hot-pressing process. The results highlight the influence of pressure from previous hot-pressing processes on grain alignment and microstructure during hot deformation. Magnets subjected to hot pressing at 200 MPa followed by hot deformation achieved superior magnetic properties, with Hci = 8.9 kOe, Br = 12.2 kG, and (BH)max = 31 MGOe with 40% of Nd replaced with Ce. Conversely, precursors prepared at 100 MPa exhibited low density due to high porosity, resulting in poor microstructure and magnetic properties after hot deformation. In magnets using precursors prepared at 300 MPa, coarsened grains and a condensed h-RE2O3 phase were observed. Incorporating Ce into the magnets led to insufficient formation of RE-rich phases due to the emergence of REFe2 secondary phases, disrupting grain alignment and hindering the homogeneous distribution of the RE-rich phase essential for texture formation. Precursors prepared under suitable pressure exhibited uniform distribution of the RE-rich phase, enhancing grain alignment along the c-axis and improving magnetic properties, particularly remanence. In conclusion, our findings present a strategy for achieving the ideal microstructure and magnetic properties of hot-deformed magnets with high Ce contents. Full article
(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)
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15 pages, 10448 KiB  
Article
Identifying Optimal Processing Variables and Investigating Mechanisms of Grain Alignment in Hot-Deformed NdFeB Magnets through Design of Experiments
by Jongbin Ahn, Jung-Goo Lee and Wooyoung Lee
Materials 2024, 17(13), 3371; https://doi.org/10.3390/ma17133371 - 8 Jul 2024
Viewed by 1139
Abstract
This study introduces a novel approach for investigating hot-deformed NdFeB magnets by combining the minimal stress deformation process (MSDP) with the design of experiment (DoE) methodology. This study focused on enhancing the crystallographic alignment, particularly the c-axis alignment of the Nd2Fe [...] Read more.
This study introduces a novel approach for investigating hot-deformed NdFeB magnets by combining the minimal stress deformation process (MSDP) with the design of experiment (DoE) methodology. This study focused on enhancing the crystallographic alignment, particularly the c-axis alignment of the Nd2Fe14B grains, to optimize the magnetic properties. By utilizing the Box-Behnken design matrix and response surface regression, critical processes and variables were identified, determining that a hot-pressing temperature of 700 °C is crucial for achieving optimal grain alignment. Changing the strain rate to 0.019 mm/s under a stress of 110 MPa led to significant enhancements in the alignment, yielding magnets with a remanence of approximately 13.4 kG and a coercivity of 21 kOe. These findings highlight the effectiveness of combining the MSDP and DoE for predicting and achieving improved magnetic properties. Despite the challenges associated with understanding the complexity of crystal alignment mechanisms, this integrated approach successfully improved magnetic characteristics. The methodology represents a significant advancement in the fabrication of high-performance hot-deformed NdFeB magnets, marking a notable contribution to the field. Full article
(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)
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11 pages, 3282 KiB  
Article
Design of High-Remanence Nd-Fe-B Hot-Pressed Magnets by Manipulating Coercivity of Hydrogenation-Disproportionation-Desorption-Recombination Treated Anisotropic Precursors
by Jae-Gyeong Yoo, Tae-Hoon Kim, Hee-Ryoung Cha, Yang-Do Kim and Jung-Goo Lee
Materials 2023, 16(24), 7599; https://doi.org/10.3390/ma16247599 - 11 Dec 2023
Cited by 3 | Viewed by 1723
Abstract
We propose a method of manipulating the coercivity of anisotropic hydrogenation-disproportionation-desorption-recombination (HDDR) powders to fabricate high-remanence and fine-grained Nd-Fe-B magnets using only hot-pressing without a subsequent hot-deformation process. By reducing the Nd content of anisotropic HDDR precursors such that their coercivity (Hcj [...] Read more.
We propose a method of manipulating the coercivity of anisotropic hydrogenation-disproportionation-desorption-recombination (HDDR) powders to fabricate high-remanence and fine-grained Nd-Fe-B magnets using only hot-pressing without a subsequent hot-deformation process. By reducing the Nd content of anisotropic HDDR precursors such that their coercivity (Hcj) is lowered, the c-axis of each HDDR particle is well-aligned parallel to the direction of the applied magnetic field during the magnetic alignment step. This is because the magnetic repulsive force between adjacent particles, determined by their remanent magnetization, decreases as a result of the low coercivity of each particle. Therefore, after hot-pressing the low-Hcj HDDR powders, a significantly higher remanence (11.2 kG) is achieved in the bulk than that achieved by hot-pressing the high-Hcj HDDR powders (8.2 kG). It is clearly confirmed by the large-scale electron backscatter diffraction (EBSD) analysis that the alignment of the c-axis of each anisotropic HDDR particle in the bulk is improved when low-Hcj HDDR powders are used to fabricate hot-pressed magnets. This coercivity manipulation of HDDR powders can be a helpful method to expand the use of HDDR powders in fabricating anisotropic Nd-Fe-B bulk magnets. Full article
(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)
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12 pages, 2967 KiB  
Article
Inhomogeneity of the Backward Extruded NdFeB Ring Magnet Prepared from Amorphous Powders
by Weida Huang, Ke Xu, Xuefeng Liao, Bang Zhou, Hongya Yu, Xichun Zhong and Zhongwu Liu
Materials 2023, 16(14), 5117; https://doi.org/10.3390/ma16145117 - 20 Jul 2023
Cited by 1 | Viewed by 1496
Abstract
Back extrusion is an important process to prepare radially oriented NdFeB ring magnets. In this work, we fabricate the ring magnets using amorphous magnetic powders as the raw material. The microstructure, magnetic properties, corrosion resistance, and mechanical properties of the backward extruded magnet [...] Read more.
Back extrusion is an important process to prepare radially oriented NdFeB ring magnets. In this work, we fabricate the ring magnets using amorphous magnetic powders as the raw material. The microstructure, magnetic properties, corrosion resistance, and mechanical properties of the backward extruded magnet at different positions along the axial direction have been investigated, and the inhomogeneity of the magnet is clarified. The results showed that the grains in the middle region of the ring magnet exhibit a strong c-axis orientation, whereas the grains at the bottom and top regions are disordered with random orientation. The microstructure variation is related to the distribution of the grain boundary phase and the degree of grain deformation. Due to the microstructure difference, the magnetic properties, temperature stability, corrosion resistance, and mechanical properties in the middle region of the magnet are higher than those in the top and bottom regions. The exchange coupling between grains also varies in different regions, which is related to the grain size and grain boundary thickness. In addition, different Co element segregations were observed in different regions, which has a crucial effect on the Curie temperature and thermal stability of the magnet. The microstructure difference also leads to the variation of corrosion resistance and mechanical properties for the samples from different regions of the magnet. This work suggests that the amorphous powder can be used to directly prepare radially oriented ring magnets, and the inhomogeneity of the magnet should be fully understood. Full article
(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)
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Review

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17 pages, 7478 KiB  
Review
Development of High-Performance Hot-Deformed Neodymium–Iron–Boron Magnets without Heavy Rare-Earth Elements
by Keiko Hioki
Materials 2023, 16(19), 6581; https://doi.org/10.3390/ma16196581 - 6 Oct 2023
Cited by 3 | Viewed by 2191
Abstract
Neodymium–iron–boron magnet is an essential material for the traction motors of green vehicles because it exhibits the highest maximum energy product, (BH)max, out of all permanent-magnet materials. However, heavy rare-earth elements such as dysprosium and terbium, which are scarce [...] Read more.
Neodymium–iron–boron magnet is an essential material for the traction motors of green vehicles because it exhibits the highest maximum energy product, (BH)max, out of all permanent-magnet materials. However, heavy rare-earth elements such as dysprosium and terbium, which are scarce resources, are added to these magnets to improve their heat resistance. To address this resource problem, considerable efforts have been made to reduce the composition of heavy rare-earth elements in these magnets without causing a significant reduction in coercivity. Hot-deformed Nd-Fe-B magnets are a category of Nd-Fe-B magnets where precious materials such as heavy rare-earth elements can be eliminated or reduced to maintain high coercivity owing to their fine microstructure. Although they are not often used for the fabrication of high-performance magnets due to their complicated production process and the difficulty in controlling their fine microstructure, after the rare-earth crisis in 2020, these magnets have begun to attract attention as a material that could increase coercivity when controlling their microstructures. This paper provides an overview of hot-deformed magnets and the efforts made to improve their properties by controlling their microstructures. Full article
(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)
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25 pages, 23142 KiB  
Review
Significant Progress for Hot-Deformed Nd-Fe-B Magnets: A Review
by Renjie Chen, Xianshuang Xia, Xu Tang and Aru Yan
Materials 2023, 16(13), 4789; https://doi.org/10.3390/ma16134789 - 3 Jul 2023
Cited by 11 | Viewed by 3793
Abstract
High-performance Nd-Fe-B-based rare-earth permanent magnets play a crucial role in the application of traction motors equipped in new energy automobiles. In particular, the anisotropic hot-deformed (HD) Nd-Fe-B magnets prepared by the hot-press and hot-deformation process show great potential in achieving high coercivity due [...] Read more.
High-performance Nd-Fe-B-based rare-earth permanent magnets play a crucial role in the application of traction motors equipped in new energy automobiles. In particular, the anisotropic hot-deformed (HD) Nd-Fe-B magnets prepared by the hot-press and hot-deformation process show great potential in achieving high coercivity due to their fine grain sizes of 200–400 nm, which are smaller by more than an order of magnitude compared to the traditional sintered Nd-Fe-B magnets. However, the current available coercivity of HD magnets is not as high as expected according to an empirical correlation between coercivity and grain size, only occupying about 25% of its full potential of the anisotropy field of the Nd2Fe14B phase. For the sake of achieving high-coercivity HD magnets, two major routes have been developed, namely the grain boundary diffusion process (GBDP) and the dual alloy diffusion process (DADP). In this review, the fundamentals and development of the HD Nd-Fe-B magnets are comprehensively summarized and discussed based on worldwide scientific research. The advances in the GBDP and DADP are investigated and summarized based on the latest progress and results. Additionally, the mechanisms of coercivity enhancement are discussed based on the numerous results of micromagnetic simulations to understand the structure–property relationships of the HD Nd-Fe-B magnets. Lastly, the magnetization reversal behaviors, based on the observation of magneto-optic Kerr effect microscopy, are analyzed to pinpoint the weak regions in the microstructure of the HD Nd-Fe-B magnets. Full article
(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)
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