Hot-deformed NdFeB Magnets Research Articles

Identifying Optimal Processing Variables and Investigating Mechanisms of Grain Alignment in Hot-Deformed NdFeB Magnets through Design of Experiments.

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.

Effects of Pressure Holding Time on Magnetic Properties and Corrosion Resistance of Hot-Deformed NdFeB Magnets

Effects of Pressure Holding Time on Magnetic Properties and Corrosion Resistance of Hot-Deformed NdFeB Magnets

Coercivity enhancement of hot-deformed NdFeB magnets by in situ two-end-diffusion with R70Cu30 alloy powders (R = NdPr and Ce)

We hereby present a simple process, called in situ two-end diffusion (TED), to reach coercivity enhancement with almost unchanged energy product for hot deformed (HD) NdFeB magnets. Coercivity of the HD magnets is increased from 15.1 to 18.7 kOe for the magnet TED with (Nd0.75Pr0.25)70Cu30 (NdPrCu), which is larger than 17.6 kOe for the magnet directly doped with 2 wt. % NdPrCu alloy. Besides, the magnet TED with NdPrCu exhibits good (00L) texture and therefore could support high (BH)max of 43.0 MGOe. In addition, the magnet TED with Ce70Cu30 (CeCu) exhibit superior magnetic properties of Br = 12.8 kG, iHc = 18.0 kOe and (BH)max = 40.7 MGOe to those of traditional doping with CeCu (Br = 11.7 kG, iHc = 16.4 kOe and (BH)max = 34.2 MGOe). For TED process, low-melting CeCu can infiltrate into the center of the magnet, and most Ce distribute at grain boundary. The distribution of Ce and Cu at grain boundary can reduce the magnetization of grain boundary phase, strengthen the decoupling effect between grains and contribute to the coercivity enhancement. This work demonstrates TED with low-melting HRE-free alloys is a simple method to effectively enhance coercivity and keep high Br and (BH)max for HD NdFeB magnets.

Magnetic Properties Enhancement of Hot Deformed Ndfeb Magnets by Eliminating Coarse Grains

Magnetic Properties Enhancement of Hot Deformed Ndfeb Magnets by Eliminating Coarse Grains

Electrochemical corrosion behavior of hot-deformed NdFeB magnet with different content of nano-TiC

Nano-TiC powders were doped to increase the corrosion resistance of hot-deformed NdFeB magnets. Effects of the dopant content on their electrochemical corrosion behavior in sulphuric acid solutions were studied by various measurement techniques including potentiodynamic polarization, electrochemical impedance spectroscopy, scanning electron microscopy and X-ray photoelectron spectrometer. Results showed that low additions of nano-TiC especially at the optimum content of 0.5 wt% can obviously increase the resistances and reduce the corrosion current density. It is ascribed to the dopant hindrance effects on the dissolution of Nd-rich phases and the coverage increase of intermediate products on NdFeB substrate induced by nano-TiC that acted as the favorable sites for adsorption reactions. Maximum increase in inductive resistance and the corresponding reduction in current density were found to be 60.6% and 36.1%, respectively. In addition, the effect mechanism of nano-TiC on corrosion behaviors was discussed based on experimental results. • Low additions of nano-TiC can increase corrosion resistance of NdFeB in sulphuric acid solutions. • Nano-TiC hinders Nd-rich phase dissolution and increases intermediates coverage on NdFeB substrate. • Effect mechanism of nano-TiC on corrosion behaviors of NdFeB was clarified.

Coercivity enhancement of hot-deformed NdFeB permanent magnets with AlCuZn eutectic alloy grain boundary diffusion

It is approved that grain boundary diffusion is an effective method to increase the coercivity of hot-deformed NdFeB magnet. In this paper, a new rare earth-free grain boundary diffusion source of hot-deformed magnet was studied. AlCuZn powders blended with commercial NdFeB powders were hot-compacted to obtain fully dense magnets, hot-deformed into anisotropic magnets and finally annealed to gain better homogeneity. Initially, the influences of annealing temperature and time on the magnetic properties of the specimens were studied and the optimal parameters of 600 °C and 60 min were achieved. Then, by changing the proportions of AlCuZn grain boundary diffusion, the coercivity, remanence and maximum energy product of the hot-deformed NdFeB magnets were examined. The result showed that with 1.0 wt% AlCuZn grain boundary diffusion and annealing at 600 °C for 60 min, the coercivity rose from 828 to 987 kA·m−1 without deteriorating the remanence. Microstructural analysis confirmed that AlCuZn diffused into the intergranular boundaries and the magnet diffused with AlCuZn possessed finer grains than that of without AlCuZn grain boundary diffusion.

Improvement of comprehensive magnetic properties for hot-deformed NdFeB magnets via intergranular additions of nano-TiC

Intergranular doping nano-TiC was used to enhance the comprehensive magnetic properties of hot-deformed NdFeB. Before 1.0 wt% more dopants increased the amount of strip-shaped Nd-rich phases at ribbon boundaries due to the ribbon fragmentation and the improved Nd2Fe14B orientation degree, raising the remanence. It coupled with better squareness enhanced the magnetic energy product (BH)max by 10.8% compared to the case of TiC-free. The increase of strip-shaped Nd-rich phases decreased the Nd-rich phases surrounded ultrafine matrix phase grains, reducing the magnetic isolation and harming the coercivity Hcj after 0.25 wt%. At the dopant range of 0.25–1.0 wt%, Hcj+(BH)max had almost the same values, higher than the corresponding value of TiC-free magnet, demonstrating that appropriate nano-TiC additions improved the magnetic performance.

Magnetic properties enhancement of hot-deformed NdFeB magnets by two different methods of CeNdCu diffusion

Two different ways were used to control the distribution of cerium for the enhancement of coercivity. One was by coating CeNdCu in the prepared magnet and annealing to make CeNdCu diffuse into the grain boundary to increase the coercivity, the other was by mixing CeNdCu with initial magnetic powders and then preparing the hot-deformed magnet. The SEM-EDS result indicates that cerium diffuses more easily into the main phase by the mixing way, while cerium is mainly distributed in the grain boundary via the coating way. The heat-treatment process may be one of the dominant influencing factors for the distribution of Ce. Multi-steps heat treatment in the mixing way, consisting of hot-pressing, hot-deforming and post heat process, easily introduces Ce into the main phase Nd2Fe14B, which forms the Ce2Fe14B shell resulting in the decrease of HA. But the coating way can ensure uniform dispersion of Ce in the grain boundary, which leads to the high coercivity.

Micromagnetic simulation of the effect of grain boundaries and secondary phases on the magnetic properties and recoil loops of hot-deformed NdFeB magnets

Abstract Hot-deformed NdFeB magnets have attracted considerable attention due to their high thermal stability and exceptional corrosion resistance. In this study, the dependence of magnetic properties and recoil loops on the presence of a grain boundary phase (GBP) and Nd-rich secondary phases in hot-deformed NdFeB magnets was explored using micromagnetic simulations. By adjusting the parameters of the simulation model, the influence of several factors (thickness and spontaneous magnetization (Ms) of the GBP, Ms, total volume fraction and spatial distribution of the secondary phase), on the main and recoil loops was investigated. A change in thickness and in Ms of the GBP affects the coercivity (Hc) and remanence of the main loop. In particular, an increase of Ms leads to a decrease in the magnet’s Hc. The influence of the secondary phase depends on its spatial distribution. Large volume grains result in lower Hc, but the role in decreasing Hc is weaker in contrast to a random distribution of smaller grains of the same overall volume content. Open recoil loops were interpreted by analyzing the distribution of calculated magnetic moments and energy terms. Loop opening is not directly related to Hc of the main loop. Rather it is mainly due to small irreversibilities in the dipolar energy of different magnetic configurations that appear along the recoil loop.

Coercivity enhancement of hot-deformed NdFeB magnets by doping R80Ga20 (R = Pr, Dy, Tb) alloys

Abstract Significant coercivity (iHc) enhancement of hot deformed NdFeB magnets made from commercial MQU-F powders doped with 2 wt% R80Ga20 eutectic alloy powders is demonstrated, where R represents Pr, Dy and Tb. The experimental result shows that the coercivity of the hot deformed magnet is improved from 15.0 kOe to 17.2–18.7 kOe by doping R80Ga20 alloys, but (BH)max is slightly decreased from 41.5 MGOe to 34.1–38.7 MGOe. The magnet doped with Tb80Ga20 exhibits higher iHc = 18.7 kOe with moderate (BH)max = 38.7 MGOe among the doped magnets post-annealed at 600 °C. In addition, hot deformed NdFeB magnets doped with Tb80Ga20 alloy powders annealed at various temperatures were also explored. Increasing the annealing temperature from 600 °C to 700 °C, iHc is significantly enhanced to 24.0 kOe with attractive (BH)max of 37.5 MGOe. Post-annealing treatment at higher temperature, i.e., 700 °C, could modify the microstructures and therefore improve the magnetic properties of the hot deformed magnets effectively. TMA analysis shows that the increase of TC with annealing temperature reveals the increased amount of Tb entered into 2:14:1 phase. TEM with EDX analysis shows that the preference of Ga at grain boundary may reduce nucleation sites of reverse domain and strengthen pinning effect, contributing to coercivity enhancement. This study provides a useful method to enhance the coercivity of hot deformed NdFeB magnets larger than 20 kOe, which is also suitable for making thicker NdFeB MQ 3 magnets for high temperature applications.

Recycling of NdFeB Magnets by Electrodischarge Sintering—Microstructure, Magnetic, and Mechanical Properties

In this work, we investigate the feasibility of recycling NdFeB magnets by means of electrodischarge sintering (EDS). We crushed, sintered, and hot-deformed NdFeB magnets in a jaw crusher, and the NdFeB fragments were further compacted to a round shape by EDS. The EDS technique is a fast and energy-saving compaction process for powders with sufficient electrical conductivity. The current is discharged from capacitors into a loose powder that has been precompacted by Cu punches into a ceramic die, thus resulting in fully dense magnets. In this study, we investigated the apparent density, particle size distribution, oxygen content, and morphology of the crushed powder. In addition, the microstructure, compressive strength, and the magnetic properties of the EDS-densified samples were examined. For all specimens, the energy product decreases with the increasing discharge energy during EDS processing and the increasing oxygen content of the initial powder. Furthermore, high apparent densities together with large particle sizes promote EDS densification of NdFeB magnets. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross section of the EDS-densified specimens. These volume fractions of the different microstructural constituents during the EDS process and the powder characteristics (oxygen content, morphology, etc.) determine the resulting mechanical and magnetic properties of the specimens.

Overview of the Ways for Enhancing the Coercivity of Hot-Deformed Nd2Fe14B-Type Magnets

Several ways for improving the coercivity of the hot-deformed NdFeB magnets, mainly made from commercial MQU-F powders, were explored. First, mixing another commercial Dy-containing MQU-G powders into MQU-F powders could enhance the coercivity of the hot-deformed magnets from 15.1 to 22.1 kOe. The increment of magnetocrystalline anisotropy field for R2Fe14B with Dy substitution leads to coercivity enhancement. In addition, doping 2 wt% R70Cu30 alloy powders [R = Dy, Dy + Nd, mischmetal (MM)] into the hot-deformed NdFeB magnets is also effective in enhancing the coercivity to 19–19.5 kOe, originated from different mechanisms. Dy70Cu30-doped magnets and (Nd0.5Dy0.5)70Cu30-doped magnets show an almost identical enhancement of coercivity of about 4.4 kOe. Importantly, the latter magnet shows a beneficial effect of reducing the usage of Dy from 1.6 to 0.8 wt%. Most interestingly, Dy-free MM70Cu30-doped magnet (MM = Pr71Nd27Ce2) can also exhibit a comparable coercivity increment of about 4 kOe and yet still persist high magnetic energy product. The microstructure analysis shows that the entrance of Pr into 2:14:1 phase, grain refinement, modified grain shape, and Ce-containing grain boundary layer play crucial roles in its coercivity enhancement.

Significant coercivity enhancement of hot deformed NdFeB magnets by doping Ce-containing (PrNdCe)70Cu30 alloys powders

Coercivity enhancement of hot-deformed NdFeB magnets made from commercially available NdFeB powders doped with Ce-containing alloys is demonstrated. By doping (Pr71Nd27Ce2)70Cu30 powders, where Pr71Nd27Ce2 is a commercial mischmetal, the hot-deformed magnets exhibit significant enhancement of coercivity from 15.0kOe to 19.0kOe, and it can reach 20.1kOe by doping higher Ce-content (Pr71Nd9Ce20)70Cu30 powders, yet sustains high energy product. The magnetic isolation effect with Ce-containing phase in the grain boundary and the microstructure refinement lead to coercivity enhancement. This study provides an economic way to enhance coercivity of hot deformed NdFeB magnets without using heavy rare earth.

Comparison on the Coercivity Enhancement of Hot-Deformed Nd2Fe14B-Type Magnets by Doping R70Cu30(R = Nd, Dy, and Tb) Alloy Powders

Coercivity enhancement effect of the anisotropic hot-deformed NdFeB magnets made from NdFeB MQU-F powders doped with 2 wt% low-melting R70Cu30 (R = Nd, Dy, and Tb) powders was explored. Post-annealing treatment at 600 °C and 700 °C was employed to modify the microstructures and therefore improve the magnetic properties of the hot-deformed magnets. Doping R70Cu30 alloy powders into the hot-deformed NdFeB magnets is effective in enhancing the coercivity from 15.1 kOe to 16.3–23.6 kOe, depending on the rare-earth elements adopted. The coercivity increment $\Delta _{i}\text{H}_{c}$ is 1.2 and 4.4 kOe for the magnets doped with Nd70Cu30 and Dy70Cu30 alloy powders, respectively. Most interestingly, Tb70Cu30-doped magnet followed with 700 °C post-annealing exhibits the highest coercivity i H c of 23.6 kOe and the highest coercivity increment of 8.5 kOe. TEM analysis shows that annealing treatment not only modifies the grain boundary of the platelet 2:14:1 phase but also promotes more Tb atoms entering into 2:14:1 crystal. As a result, the coercivity of the magnets is effectively enhanced.

Coercivity enhancement in hot deformed Nd2Fe14B-type magnets by doping low-melting RCu alloys (R = Nd, Dy, Nd + Dy)

Magnetic properties of the anisotropic NdFeB magnets prepared by hot pressing followed by die-upsetting NdFeB MQU-F powders doped with low-melting RCu alloy powders were explored, where RCu stands for Nd70Cu30, Dy70Cu30 and (Nd0.5Dy0.5)70Cu30, respectively. In addition, the post-annealing at 600°C was employed to modify the microstructures and the magnetic properties of the hot deformed magnets. It is found that doping RCu alloy powders is effective in enhancing the coercivity of the hot deformed NdFeB magnets from 15.1kOe to 16.3–19.5kOe. For Nd70Cu30-doped magnets, the increment of coercivity is only 1.2kOe. Meanwhile, Dy70Cu30-doped and (Nd0.5Dy0.5)70Cu30-doped magnets show an almost identical enhancement of coercivity of about 4.4kOe. Importantly, the latter magnet shows a beneficial effect of reducing the usage of Dy from 1.6wt% to 0.8wt%. TEM analysis shows that nonmagnetic Nd, Dy and Cu appear at grain boundary and isolate the magnetic grains, leading to an enhancement of coercivity. Doping lower melting point Dy-lean (Nd0.5Dy0.5)70Cu30 powders into commercial MQU-F powders for making high coercivity hot deformed NdFeB magnets might be a potential and economic way for mass production.

Effects of Fe fine powders doping on hot deformed NdFeB magnets

Abstract The composite NdFeB magnets with blending melt-spun flakes and Fe fine powders were prepared by the hot-pressed and hot-deformed route. Characterizations of the hot-deformed NdFeB magnets affected by the doped Fe powders were tested. The doped Fe powders decrease the hot-deformed pressure when the strain is between 15 and 50%. XRD patterns show that the doped Fe powders have little influence on the c-axis alignment of hot-deformed NdFeB magnets in the press direction. The Br and the (BH)max get improved when the doped Fe powders are less than 3 wt%. The doped Fe of hot-deformed NdFeB magnets exists in the elongated state and the spherical state surrounded by the Nd-rich phase. With the Fe fraction increasing, the potential of magnet moves to the positive direction and the diameter of the Nyquist arc becomes larger, which indicate that the corrosion resistance improved effectively. The bending strength was enhanced by the elongated α-Fe phase embedded in the matrix 2:14:1 phase.

Structural and magnetic properties of single-stage hot deformed NdFeB magnets prepared by different initial densities

The effects of initial density on the magnetic properties of NdFeB magnets prepared by single-stage hot deformation were investigated in this work. The results show that the values of maximum energy product (BH)m and coercivity H cj decrease with the increase of initial density. Under optimum condition, an anisotropic magnet with a maximum energy product of 264 kJ·m−3 was produced using the initial density of 4.4 g·cm−3. The influence of initial density on the magnetic properties was discussed on the basis of microstructure and strain energy. It is concluded that the thicker platelet grains are obtained along the Nd2Fe14B base plane with the initial density increasing. It is mainly because that grain rotation is restricted by high strain energy, which results from high initial density.

Effects of excessive grain growth on the magnetic and mechanical properties of hot-deformed NdFeB magnets

The magnetic and mechanical properties of rare-earth magnets hot-deformed at temperature range 750–950 °C have been investigated. The grains tended to grow excessively from dozens of nanometers to several microns at the temperatures above 850 °C. The alignment of grains was disrupted by the hot deformation at the high temperatures. The Nd-rich phase was extruded at the temperatures which are higher than 850 °C. The Nd-rich phase extrusion resulted in the reduction of density by 1% and the reduction of remanence from 1.42 to 0.72 T. The reduction of grain boundaries caused by flat platelet-shaped grains changing to spherical grains and the weak binding strength among large grains of Nd 2Fe 14B phase may be the main reasons for the low mechanical strength of hot-deformed magnets.

Effects of some additives on the magnetic properties of single stage hot deformed NdFeB magnets

The effects of the eight kinds of additives, such as Al, BN, C, MoS2, Si, Zn, oleic acid, and diethylene glycol on the magnetic properties of the single stage hot deformed NdFeB magnets had been investigated in this work. Among these additives, it was observed that MoS2 and Al were more effective in increasing remanence and maximum energy product, and Zn in increasing coercivity considerably.

Coercivity enhancement by Zn addition in hot deformed NdFeB magnets

Anisotropic NdFeB magnets were fabricated by the single-stage hot deformation method from commercial isotropic MQPA powder. The effect of Zn addition on the magnetic properties was investigated and the considerable increase of coercivity was observed. The increment of coercivity was about 57% in the case of 0.4 wt% Zn addition compared with that for additive-free magnets.