Anisotropic NdFeB Magnets Research Articles

Preparation of Anisotropic Nanocrystalline NdFeB Magnets from Amorphous Powders by One‐Step Hot Deformation

The preparation of nanocrystalline anisotropic NdFeB permanent magnets generally includes two steps, hot pressing (HP) and hot deformation (HD). Herein, one‐step HD process is proposed to fabricate NdFeB magnets using amorphous powders. The effects of deformation ratio on the microstructure, magnetic properties, and corrosion resistance are investigated. Compared to traditional HP and HD process, one‐step HD can significantly reduce the volume fraction of coarse‐grain zone in the magnet. The increase of deformation ratio can promote the formation of the textured platelet‐shaped grains and uniform grain boundary phase. The magnetic properties, temperature stability, and corrosion resistance of the magnets increase with increasing deformation ratio until 80%. The best magnetic properties with intrinsic coercivity Hcj = 930 kA m−1, remanence Jr = 1.17 T, and maximum energy product (BH)max = 263 kJ m−3 have been achieved for the magnet prepared by one‐step HD, which are comparable to those obtained by conventional HP/HD. In addition, different Co element segregations are observed in the magnets with different deformation ratios, which have important effects on the Curie temperature and thermal stability of the magnets. The results suggest that, by replacing HP/HD process and using amorphous powder, one‐step HD can simplify the process while maintaining good properties.

Comparisons of the Microstructure and Magnetic Properties of Anisotropic NdFeB Magnets Prepared by Hot Pressing and Spark Plasma Sintering

Comparisons of the Microstructure and Magnetic Properties of Anisotropic NdFeB Magnets Prepared by Hot Pressing and Spark Plasma Sintering

Effects of grain boundary configuration and characteristics on the demagnetization process and coercivity of anisotropic NdFeB magnets

A systematic investigation on the effects of grain boundary configuration and characteristics on the demagnetization process and magnetic properties of NdFeB magnets is carried out by micromagnetic simulation based on a periodic anisotropic model. The results indicate that, when the grain boundary phase is perpendicular to the easy axis, the demagnetization field is along the positive Z axis and the magnetic moments rotate difficultly due to the dipolar coupling. When the grain boundary phase is parallel to the easy axis, the demagnetization field is along the negative Z axis and the magnetic moments rotate easily. The best hard magnetic properties can be obtained by reducing the thickness and saturation magnetization of grain boundary phase distributed parallel to the easy axis and reducing the exchange stiffness of grain boundary phase perpendicular to the easy axis.

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.

Hydrogen Decrepitation Press-Less Process recycling of NdFeB sintered magnets

A Hydrogen Decrepitation Press-Less Process (HD-PLP) recycling method for recycling of anisotropic NdFeB magnets is demonstrated. The method combines hydrogen decrepitation (HD) disintegration of the initial magnet, powder sieving and the Press-Less Process (PLP), where hydride powder is sintered in a graphite mold. Coercivities up to 534kA/m were obtained in porous samples based on powder size d< 100µm. Adding a ball milling step resulted in full density isotropic magnets for d> 100µm. The coercivity reached Hci= 957kA/m being 86% of the original N48M material without addition of rare earth elements.

Hysteresis Modeling of NdFeB Magnets with High Nd

The Stoner-Wohlfarth hysteresis model can describe very well the hysteresis curves since the assumptions of the model are compatible with the studied samples. These conditions are (i) single domain particle size and (ii) inexistence of interactions between the particles. These conditions are obtained in Nd2Fe14B magnets with nanocrystalline grain size below 200 nm and with high Nd, which promotes a paramagnetic phase at the grain boundaries. In the present study, both isotropic and anisotropic NdFeB magnets are modeled.

Preparation of Isotropic and Anisotropic Nanocrystalline NdFeB Magnets by High-Velocity Compaction and Hot Deformation

NdFeB powders are consolidated into nanocrystalline bulk magnets by a near-net-shape process of high-velocity compaction (HVC) at room temperature with no binder employed. The magnets prepared under various conditions, including impact energy, filling weight, mold dimension, and plasticity and size of the starting powders, were investigated. The results showed that the density of as-compacted NdFeB magnets increased with the increasing impact energy and decreasing filling weight. The as-compacted magnets with relatively high density can inherit the coercivity and microstructure of the starting powders. The small flake powders with good plasticity and/or large mold diameter are beneficial to obtain high density. The relative green densities for the samples with low-Nd composition and high-Nd composition reach 92% and 87.5%, respectively. Using the HVCed magnet as the precursor, the anisotropic NdFeB magnets with enhanced magnetic properties have been prepared by hot deformation. This paper provides an alternative technique for preparing nanocrystalline NdFeB magnets.

High performance anisotropic NdFeB magnets prepared by dual-alloy die-upsetting

The anisotropic nanocrystalline magnets were produced by hot pressing and die-upsetting the mixed powders of MQU-F and MQU-G with a mass ratio of x:1−x (x = 0−1). With x increasing from 0 to 1, the intrinsic coercivity (Hci) decreased from 22.7 to 15.0 kOe, and the remanence (Br) and maximum energy product ((BH)max) ascended from 12.6 to 13.5 kG and from 38.0 to 44.1 MGOe respectively. The XRD patterns showed that (006), (105), and (008) peaks were strengthened, indicating that easy axes of the grains were aligned along the hot-deformation direction. Fine plate-like grains were observed by field emission scanning electron microscope. The components of the magnets prepared from the blends behaved in a cooperative way, which made the demagnetization curves a smooth profile. Both initial magnetization and demagnetization curve were measured. Additionally, the magnetization mechanism and magnetization behaviors were also analyzed.

Effect of pressure loading rate on the crystallographic texture of NdFeB nanocrystalline magnets

Plastic deformation of NdFeB nanocrystalline magnets has been performed at relatively low temperatures but high pressures compared to conventional hot deformation. The effect of deformation conditions on the crystallography texture of the anisotropic NdFeB magnets has been studied. It was observed that both deformation temperature and pressure loading rate significantly affect final crystallographic texture and thus magnetic properties. Energy product increases from 31 to 44 MGOe by simply reducing pressure loading rate from 300 MPa/min to 50 MPa/min. Microstructure observations confirm that better crystallographic texture has been obtained in the deformed magnets with a lower pressure loading rate.

Diffusion of Nd-rich phase in the spark plasma sintered and hot deformed nanocrystalline NdFeB magnets

The role of the Nd-rich phase in developing microstructure and properties of isotropic and anisotropic NdFeB magnets has been investigated. Melt spun Nd-rich Nd13.5Fe73.5Co6.7Ga0.5B5.6 and Fe-rich Nd7.7Pr2.6Fe84.1B5.5 alloy powders were mechanically mixed with different ratios. The mixed powders were consolidated into isotropic magnets and anisotropic magnets by spark plasma sintering (SPS) and SPS followed by hot deformation, respectively. The composition and microstructure of diffusion area between Nd-rich and Fe-rich compositions for isotropic and anisotropic magnets were investigated. The gradient distribution of Nd content from Nd-rich to Fe-rich area due to the diffusion of liquid Nd-rich phase in the SPS and hot deformation was observed, which leads to gradually changes in grain structure. The remanent polarization of 1.29 T, coercivity of 995 kA/m, and maximum energy product of 293 kJ/m3 are obtained for an anisotropic magnet. The role of the Nd-rich phase has to be carefully considered in order to achieve deformation anisotropy and fine grains in the NdFeB magnets.

High performance bonded neo magnets using high density compaction

This paper presents a manufacturing method called Combustion Driven Compaction (CDC) for the manufacture of isotropic bonded NdFeB magnets (bonded Neo). Magnets produced by the CDC method have density up to 6.5 g/cm3 which is 7–10% higher compared to commercially available bonded Neo magnets of the same shape. The performance of an actual seat motor with a representative CDC ring magnet is presented and compared with the seat motor performance with both commercial isotropic bonded Neo and anisotropic NdFeB rings of the same geometry. The comparisons are made at both room and elevated temperatures. The airgap flux for the magnet produced by the proposed method is 6% more compared to the commercial isotropic bonded Neo magnet. After exposure to high temperature due to the superior thermal aging stability of isotropic NdFeB powders the motor performance with this material is comparable to the motor performance with an anisotropic NdFeB magnet.

Microstructure and property evolution of isotropic and anisotropic NdFeB magnets fabricated from nanocrystalline ribbons by spark plasma sintering and hot deformation

Isotropic and anisotropic NdFeB magnets were synthesized by spark plasma sintering (SPS) and SPS+HD (hot deformation), respectively, using melt-spun ribbons as the starting materials. Spark plasma sintered magnets sintered at low temperatures (<700 °C) almost maintained the uniform fine grain structure inherited from rapid quenching. At higher temperatures, due to the local high-temperature field caused by the spark plasma discharge, the grain growth occurred at the initial particle surfaces and the coarse grain zones formed in the vicinity of the particle boundaries. Since the interior of the particles maintained the fine grain structure, a distinct two-zone structure was formed in the spark plasma sintered magnets. The SPS temperature and pressure have important effects on the widths of coarse and fine grain zones, as well as the grain sizes in two zones. The changes in grain structure led to variations in the magnetic properties. By employing low SPS temperature and high pressure, high-density magnets with negligible coarse grain zone and an excellent combination of magnetic properties can be obtained. An anisotropic magnet with a maximum energy product of ∼30 MG Oe was produced by the SPS+HD process. HD at 750 °C did not lead to obvious grain growth and the two-zone structure still existed in the hot deformed magnets. Intergranular exchange coupling was demonstrated in the spark plasma sintered magnets and was enhanced by the HD process, which reduced the coercivity. Good temperature stability was manifested by low temperature coefficients of remanence and coercivity. The results indicated that nanocrystalline NdFeB magnets without significant grain growth and with excellent properties could be obtained by SPS and HD processes.

The influence of the powder densification temperature on the microstructure and magnetic properties of anisotropic NdFeB magnets aligned by hot deformation

Abstract The relation between the temperature for the initial powder densification and the structure and magnetic properties of anisotropic magnets, oriented by means of die-upset forging is reported. It was found that, from the viewpoint of the efficiency of the orientation process, there is an optimum temperature for the densification. For the Magnequench MQP-A powder used in the study, this temperature is about 650 °C. At this temperature the isotropic densified precursor has the most suitable structure, characterized by small grains and a density close to the theoretical value. The use of precursors with a favourable structure resulted in anisotropic magnets with the following properties: remanence J r = 1.41 T, coercivity J H c = 780 kA/m and maximum energy product ( BH ) max = 390 kJ/m 3 .

Design and fabrication of anisotropic NdFeB magnets

Taguchi method for experimental design was used in the fabricating process of the anisotropic NdFeB magnet by the single stage hot deformation. The optimum conditions and the contribution of each experiment parameter were obtained by ANOVA calculation.

High-performance nanocrystalline NdFeB magnets by CAPA process

The anisotropic NdFeB magnets were prepared from the melt-spun isotropic powders by CAPA process. The precursor isotropic magnet shows the uniform magnetic properties according to the overall position in the magnet. In the case of the anisotropic magnet, the outer position shows higher remanence and energy product compared to the center position. The magnetic properties of the anisotropic magnet obtained from Nd 14Fe 80B 6 powders are B r = 15 kG , i H c = 4.1 kOe and B H max = 36 MG Oe . In the case of addition of Zn to Nd 14Fe 80B 6 powders, the energy product increased because of the improved coercivity. The magnetic properties of the Zn-added magnet are B r = 14.5 kG , i H c = 9.7 kOe and B H max = 52 MG Oe . The Zn addition is effective to depress Nd 2Fe 14B grain growth of the interparticle regions during plastic deformation.

NdFeB thin anisotropic magnets obtained by hot working process

Abstract The current-applied pressure-assisted process has been applied to obtain anisotropic thin NdFeB magnets. The magnetic properties and microstructure of Nd14Fe80B6 and Nd12Dy2Fe73.2Co6.6Ga0.6B5.6 magnets with thickness of millimeter and sub-millimeter scale were investigated. For the thin magnets, the remanence and coercivity are lower than those of the thick magnet in spite of higher thickness reduction ratio due to the poor c-axis texture and abnormal grain growth. The optimized magnetic properties of the thin Nd12Dy2Fe73.2Co6.6Ga0.6B5.6 magnet with thickness of 780 μm are Br=12.4 kG, iHc=164 kOe, and (BH)max=37.1 MGOe.

Near net shape production of radially oriented NdFeB ring magnets by backward extrusion

Anisotropic NdFeB magnets find application in electric motors where high flux densities combined with high coercivities are required. Such magnets can be produced by hot deformation of nanocrystalline NdFeB alloys. In particular, the backward extrusion process results in radially oriented ring magnets. The hot working process can be carried out in such a way that the ring magnets obtained after cutting off the top and the bottom are suited for direct use on the rotor in an electric motor. This is possible for a wide range of dimensions, which means that the ring magnets can be produced in a near net shape. Especially, smaller wall depths of the rings are realizable compared with sintered magnets. Major benefits are the simpler rotor construction, a greatly reduced rotor assembly time and simpler magnet retention on the rotor due to greater mechanical integrity of the magnetic structure. After a brief explanation of the backward extrusion process, the used dies and the corresponding rings are presented. Finally, the homogeneity of the magnetic properties and of the deformation process are discussed.

통전가압법으로 제조한 이방성 NdFeB 영구자석의 자기특성에 미치는 첨가제의 영향

Rapidly solidified NdFeB powders were mixed with fine powders of pure metal elements before CA-press employed to obtain a fully dense isotropic precursor. Subsequently, the precursor was deformed by CA-deformation to obtain an anisotropic magnet. The CA-deformed anisotropic NdFeB magnets with 0.3 wt.% Zn or Sn exhibited the coercivities about 80% higher (11.4. and 11.2 kOe, respectively) than that (6.4 kOe) of the additive-free magnet.

Design of powder aligning systems for the compression molding of radially anisotropic permanent magnet rings

The article describes the design of powder aligning fixtures for the compression molding of radially anisotropic NdFeB magnet rings. The implications of the high fields required are discussed, with particular reference to the limitations imposed on the aspect ratio of the magnets that can be satisfactorily aligned. Features for enhancing the magnitude of the aligning field in the mold cavity, e.g., a magnetic shunt to shield the inner core and tapering of the upper plate are discussed. These issues are illustrated by means of a design study on a powder aligning fixture for a NdFeB magnet ring with an outer diameter of 26 mm and a wall thickness of 3 mm. Extensive finite element analysis is employed to investigate the role of various features, and to optimize the geometry. The findings of the design study are validated by comparison with measurements on a prototype fixture.