Die-upsetting Process Research Articles

Effects of Length-to-Width Ratio on Magnetic and Microstructural Properties of Die-Upset Nd–Fe–B Magnets

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.

High frequency properties of 2D Sm2Fe14B nanoflakes with bianisotropy

In this work, the high frequency and microwave absorption properties of two-dimensional (2D) Sm2Fe14B nanoflakes were investigated. Both planar magnetocrystalline and shape anisotropy was obtained for the 2D Sm2Fe14B nanoflakes prepared via die-upset process, which can effectively increase the effective out-of-plane anisotropy field and improve the high frequency intrinsic magnetization (the product of initial permeability and resonance frequency). The nanoflakes have a diameter of approximately 1–2 μm and a typical thickness of approximately 200 nm. Compared with irregular Sm2Fe14B powder, a higher complex permeability and a lower complex permittivity were obtained for the 2D Sm2Fe14B nanoflakes because of the suppressing eddy effect and the oxidized grain boundary phase (GBP) layer, which facilitates the microwave absorption performance. A minimum RL value reaches −16.9 dB at the perfect matching frequency of 12.1 GHz.

Comparison of Hot-deformation Behavior and Magnetic Properties between Nd-Fe-B HDDR and MQU-F Powder

Hot-deformation behavior of Nd-Fe-B HDDR powder was investigated in order to understand the difference in texturing mechanism of HDDR and MQU-F powder during hot-deformation. The composition of the HDDR powder was the same as that of MQU-F powder. However, the grain size of HDDR powder (~300 nm) was eight times larger than that of MQU-F powder (~40 nm). After being subjected to hot-pressing at 700 °C under 200 MPa in a vacuum, the grains of the magnets made from HDDR powder and MQU-F powder have globular and platelet shapes, respectively. The remanence was 12.2 kG after die-upsetting process at the strain of 0.5. And it increased up to 13 kG at the strain of 1.4 although the grains still maintained globular-like shapes. The remanence of 13 kG was almost the same as that obtained the magnet from MQU-F powder at the same deformation condition. These results indicate that the hot-deformation behavior is quite different between HDDR and MQU-F powder.

Prospect of developing Nd–Fe–B-type magnet with high electrical resistivity

Nd–Fe–B-type magnet is exclusively used as a rotor magnet in the traction motor of hybrid electric vehicle (HEV) and electric vehicle (EV), but its overly high operating temperature is a lingering problem attached to the magnet. The major cause of the high operating temperature is eddy current, which is readily generated in the highly conductive metallic magnet under alternating magnetic field from stator ripple. In this article, temperature rise in the Nd–Fe–B-type magnet with varying electrical resistivity under alternating magnetic field is discussed with the intention of highlighting the importance of enhancing the electrical resistivity for reducing the operating temperature of the Nd–Fe–B-type rotor magnet. Temperature rise in the Nd–Fe–B-type magnet (dielectric salt-added die-upset magnet) with high electrical resistivity is noticeably lower compared to the magnet (commercial sintered rotor magnet) with lower electrical resistivity, substantiating the theory that enhancing the electrical resistivity in the rotor magnet is fairly effective for suppressing the over-rise of its operating temperature during operation. Die-upset process is revealed to be particularly pertinent for the fabrication of highly dense salt-added magnet with high electrical resistivity.

Magnetic properties improvement of hot-deformed Nd–Fe–B permanent magnets by Pr–Cu eutectic pre-diffusion process

Pre-diffusion grain boundary strategy was proposed to fabricate die-upset Nd-Fe-B magnets with excellent magnetic properties. Microstructure modification of die-upset magnets played a key role in determining the magnetic properties. The effective coercivity enhancement of die-upset magnets through pre-diffusion process was enhanced from 0.076 T/wt%Pr to 0.114 T/wt%Pr with a slight remanence loss. Driven by heat, the Pr–Cu eutectic alloys were diffused into melt-spun ribbons with weakened exchange couple of matrix phase, which led to enhanced coercivity. The uniform distribution of intergranular phase was obtained in die-upset magnet with pre-diffusion process and the remanence reduction was limited with a high squareness factor. Microstructure analysis confirmed that the pre-diffusion process suppressed the longitudinal/lateral ratio of platelet-shaped grains and grain growth in die-upset process. Simultaneously, the pre-diffusion process facilitated the formation of continuous and uniform grain boundaries (GBs). The grain size was distributed over a narrow range of value with similar local critical nucleation field, which resulted in the improved squareness. The thick and continuous intergranular phase strengthened the magnetic isolation between neighboring Nd2Fe14B phases and offered more “pinning” sites for domain wall shift. The modified structure hindered the nucleation and spread of reverse domains in a low magnetic field in favor of the coercivity enhancement. The view-direct time-dependent behavior of reverse magnetic domains indicated that the coercivity mechanism for die-upset magnet was a combination of “pinning” effect and nucleation model.

High performance (La, Ce, Pr, Nd)-Fe-B die-upset magnets based on misch-metal

High performance (La, Ce, Pr, Nd)-Fe-B anisotropic nanocrystalline magnets were fabricated through die-upsetting process by incorporating misch-metal in Nd-Fe-B magnets via dual alloy method. Although coercivity decrease with the increase of misch-metal monotonously, there are no significant decrease in remanence. Magnet with remanence of 13.46 kGs and maximum energy product of 43.5 MGOe was obtained with 30 wt% misch-metal substitution, which is quite promising in practical application. High remanence was resulted from good texture and microstructure analysis revealed that Ce tended to diffuse into 2:14:1 phase uniformly, while the excessive La tended to assemble into the interface of melt-spun ribbons.

Effects of Strain and Stain Rate on Microstructure and Magnetic Properties of Nd–Fe–B Magnets During Die-Upsetting Process

The effect of strain and stain rate on microstructure and the magnetic properties of Nd–Fe–B magnets during hot-deformation process have been studied with commercial melt-spun flakes with a composition of Nd13.6Fe73.6Co6.6Ga0.6B5.6. The compacts produced by hot pressing at 700 °C under 100 MPa in vacuum were subjected to die upsetting at 700 °C with different deformation conditions of strain rate 0.1–0.001 $\text{s}^{\mathrm {-1}}$ and height reduction 40%–75%. The remanence and coercivity tended to increase and decrease with increasing strain, respectively. At below the strain of 0.5, the remanence increased more quickly with decreasing the strain rate, but the tendency was gradually reversed with increasing the strain. This was because the prolonged deformation induced the squeeze-out of Nd-rich phase from grain boundaries and the formation of coarse Nd-rich phase. In addition, the sample subjected to the prolonged deformation had quite thin and discontinuous grain boundary phase, which resulted in the decrease of the coercivity.

Impact of Nd–Cu diffusion on microstructure and coercivity in hot-pressed and die-upset nanocomposite magnets

The microstructure evolution in hot-pressed and die-upset magnets was studied to clarify the impact of Nd-Cu diffusion on microstructure and coercivity. It was found that Nd-Cu diffusion mainly occurs at the interface of the raw powders during the hot-pressing process. In the die-upsetting process, grain boundary diffusion is main reason for the homogeneous distribution of Nd-Cu. Moreover, Nd-Cu grain boundary diffusion decoupling the hard phases, grain morphology and the presence of Nd2O3 contribute to the higher coercivity in die-upset magnets with low height reduction. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Simulation of the die-upsetting process of hot-deformed magnets

A plastic deformation simulation was performed using a three-dimensional finite-element method based deformation modeling software in order to clearly understand the die-upsetting process in detail. The simulation indicates that effective strains are different for different regions in the die-upsetting process because of a limitation of the dies. Effective strains of Nd-Fe-B die-upset magnets have a parabolic reduction from the middles to the ends of the magnetic specimens along the c-axis. Experimentally, anisotropic Nd-Fe-B magnets were prepared from commercial melt-spun powders (MQU-F) by hot pressing and subsequent die upsetting. The magnetic properties, the Xray diffraction patterns and the microstructures of die-upset magnet specimens of different heights were studied. The magnetic properties increased with the effective strains of the die-upset magnets. Experimental research confirmed that the microstructural characterics of die-upset magnets is due to the inhomogeneity of the die-upsetting process.

Microstructure and Properties of Die-Upset Nd-Fe-B/Dy$_{2}$O$_{3}$ Composite Magnets

Fabrication of rare-earth-based permanent magnets with both high electrical resistivity and excellent magnetic properties for the traction motor applications is highly challenging. In this paper, Nd-Fe-B/Dy2O3 composite magnets have been fabricated by hot-pressing and die-upset. The influence of Dy2O3 filler on the microstructures, electrical resistivity and magnetic properties of the Nd-Fe-B magnets have been investigated by scanning electron microscope, four-probe resistivity meter and hysteresis graph. The results showed that the electrical resistivities of the Nd-Fe-B/Dy2O3 composite magnets increased with the increase of Dy2O3 amount and reached 1270 cm when the Dy2O3 amount increased to 20 wt.%. Whereas the electrical resistivity of the nondoped Nd-Fe-B magnets is 230 cm. The cross-section SEM images of the composite magnets showed that the Dy O phase formed laminated structure and some agglomerate phenomenon. The agglomerate areas became larger with the increase of Dy2O3 amount. The residual magnetic flux density and maximum energy product of the Nd-Fe-B/Dy2O3 composite magnets decreased due to the nonmagnetic Dy2O3 filler. It is interesting to find that coercivity of the Nd-Fe-B/Dy2O3 composite magnets increased from 7.8 kOe to 9.64 kOe when the Dy O amount increased from 10 wt.% to 20 wt.%. The reason may be that some of the dysprosium diffused into the Nd-Fe-B matrix and formed (Nd,Dy)2Fe14B compounds. In the mean time, some of neodymium-rich phase diffused to Dy2O3 phase, replaced dysprosium and formed Nd O . This probably happened during the die-upset process at 850°C.

Enhanced mechanical properties in die-upset Nd-Fe-B magnets via die-upsetting process

The mechanical properties of die-upset Nd-Fe-B magnets produced at different die-upset processes were investigated. The results showed that the optimum comprehensive mechanical properties of die-upset Nd-Fe-B magnets were obtained at the deformation temperature of 680 °C. The anisotropy of Vickers hardness was more obvious at the die-upset level of 55%, and the Vickers hardness measured parallel to the c-axis was significantly lower than that perpendicular to the c-axis. The fracture toughness measured parallel to the c-axis first increased, and then decreased with increase in die-upset level. The maximum fracture toughness of Nd-Fe-B magnets was obtained at the die-upset level of 60%. The microstructure showed that the width of defect layers and the average size of large grains increased, and the layered structure of die-upset Nd-Fe-B magnets was obviously different with increase in the die-upset level.

Effect of Die-upset Process on Magnetic Properties and Deformation Behavior of Nanostructured Nd-Fe-B Magnets

Nd-Fe-B high performance magnets were prepared by die-upset forging. The effects of the deformation parameters on magnetic properties and flow stress were studied. Deformation temperatures in the range of 600~900 ℃ enable to achieve an effective anisotropy and temperature 800 ℃ proves to be suitable for deformation of Nd-Fe-B magnets. The amount of c-axis alignment along the press direction seems to depend on the amount of deformation and a saturation behavior is shown at deformation ratio of 75%. Magnetic properties are also related to strain rate, and maximum energy product is attained at an optimum strain rate of Φ= 1 × 10?² s?¹. By analyzing the relationship of stress and strain at different deformation temperature during die-upset forging process, deformation behavior of Nd-Fe-B magnets was studied and parameters for describing plastic deformation were obtained. Nd-rich boundary liquid phase, which is additionally decreasing the flow stress during deformation, is supposed to play the role of diffusion path and enhance the diffusion rate.

Enhanced magnetic properties in Nd-Fe-B magnets prepared by spark plasma sintering via die-upsetting process

The magnetic properties and microstructure of Nd-Fe-B magnets prepared by spark plasma sintering with different die-upsetting processes were investigated. The results showed that the optimum magnetic properties of die-upset Nd-Fe-B magnets were obtained at 680 °C when the die-upset level was 60%, and the degree of magnetic alignment was 0.84. The microstructures showed that the coarse grains occurred predominantly within certain areas, and abnormal grain growth was not observed within the major areas of well-aligned grains. There existed many small spherical grains, which stacked together and were not aligned during die upsetting when the deformation temperature was 650 °C. These small spherical grains grew up, and were aligned when the deformation temperature increased from 650 to 680 °C, which could improve the crystallographic alignment of die-upset Nd-Fe-B magnets.

Magnetic Properties of Anisotropic Didymium-Fe-Co-Nb-V-Y-B Magnets by using Hot-Forming Process

The experiment was aiming for making anisotropic magnet by using hot-forming process. And this experiment was carried out to investigate the effects of Cu addition, and hot-forming temperature on magnetic properties of Didymium-Fe-Co-Nb-Y-B and Didymium-Fe-Co-Nb-V-Y-B alloys. The results are summarized as follows:1) In the X-ray diffraction patterns which were taken in the direction of applied compressive stress in the hot-forming process, large peaks corresponding to (004), (105) and (006) planes were observed. These planes are facing the direction of the c-axis of the Nd2Fe14B-type crystal. These observations suggest that crystallites are oriented toward the c-axis by the hot-forming process.2) The optimized preparatory conditions for the anisotropic magnet Di12.5Fe68.3Co10Nb1V1Y0.7Cu0.5B6 are as follows: roller velocity in the process of preparing the melt-spun ribbons : 25 m/s, and temperature of the hot-forming process: 840°C. Typical magnetic properties of the anisotropic magnets obtained: Jr=1.041T, HcJ=812.5 kA/m, HcB=561.7 kA/m, (BH)max=176.8 kJ/m3, (Jr - Jr')/Jr × 100 =54.5 %. The α-Fe type phase is contained several percent in the sample. Good results are obtained in this study, which can be related to the industrial “die-upset process”.

Magnetic properties and microstructures of mischmetal-FeB-(Al, Ti and Al-Co) permanent magnets

The magnetic characteristics of anisotropic MM-FeB- (Al, Ti and Al-Co) permanent magnets have been investigated by using hot-pressing and die-upsetting process. The best magnetic properties obtained in these studies were HC = 5.1 kOe, Br = 5.4 kG with (BH)max = 5.1 MGOe for hot-pressed MM-FeB-Al-Co magnets and HC = 3.6 kOe, Br = 6.7 kG, (BH)max = 6.8 MGOe for die-upset MM-FeB-Al-Co magnets. Higher squareness of demagnetization curve was obtained in anisotropic die-upset MM-FeB- (Al, Al-Co) magnets. X-ray diffraction and STEM investigations revealed that the higher magnetic properties in die-upset magnets were resulted from alignment of the c-axis along the die-upsetting direction. The magnetic anisotropy of the die-upset magnets and the densification of the hot-pressed magnets were increased by partial substitution of Al and Al-Co for Fe.

Plastic deformation modeling of die-upset process for magnequench NdFeB magnets

The die upset of hot pressed NdFeB magnets modifies the equiaxed grains to platelets, develops the c-axis texture parallel to press direction, and improves magnetic properties. The mechanism of c-axis alignment has been suggested to be a combination of grain-boundary sliding and anisotropic grain growth in a direction normal to the applied stress. To clearly understand the role of deformation process in grain-boundary sliding or anisotropic grain growth simulations of the die-upset process were performed using ANATARES, a three-dimensional finite element method based deformation modeling software. The stresses and strains in the different regions of a cylindrical Nd–Fe–B magnet at different stages of the die-upset process were determined. The average value of the maximum principal stress (parallel to the upset direction) and total effective strain increased as the upset increased from 50% to 70%. The maximum principal stress and total effective strain show a maximum at the center and decrease in both the thickness and the radial direction due to friction at the die-wall/magnet interface. The stress and effective strain uniformity improves with increase in upset. The data agree well with the variations of texture in the magnet observed using pole figure measurements in this study and texture studies using a synchrotron source.

Preparation of anisotropic NdFeB magnets with different Nd contents by hot deformation (die-upsetting) using hot-pressed HDDR powders

In this work HDDR powders were used for the preparation of highly densified anisotropic NdFeB magnets by the die-upsetting process. HDDR powders with different Nd contents (14 at %, 16 at % and 18 at %) and different grain sizes were used in order to determine the most favourable composition and microstructure for a production of crack-free magnets with maximum texture and optimum magnetic properties. The powders were hot compacted in vacuum at 725°C and 150 MPa. Then the samples were hot deformed in an argon atmosphere at 750°C with a strain rate of 10 −3 s −1. Using HDDR powders with grain sizes around 0.3 μm, corresponding to a coercivity μ 0 j H c of 1 T, a Nd content of 14 at % and a deformation degree of 0.9, a remanence of 1.2 T and a coercivity of 0.7 T were obtained. However a large amount of cracks were present in the die-upset sample with 14 at % Nd. By increasing the Nd content suppression of the cracks in the die-upset samples was achieved and an increase of the coercivity was observed, but the degree of texture and the remanence of the sample are reduced. For a Nd content of 16 at %, crack-free magnets with a coercivity of 0.95 T and a remanence of 1 T could be produced.