Cerium-containing high-abundance rare earth sintered neodymium iron boron permanent magnet and preparation method thereof

Abstract

The invention discloses a cerium-containing high-abundance rare earth sintered neodymium iron boron permanent magnet which does not contain heavy rare earth and does not need a grain boundary diffusion process and a preparation method thereof. The magnet is formed by crushing and mixing three alloy casting pieces, the three alloy casting pieces are an Nd / Pr-rich phase, a Ce-La-rich phase and a Y-Nd-rich phase respectively, and Ce accounts for more than or equal to 30wt% of the total amount of all rare earth elements; the trace elements (Al, Cu, Ga, Nb, Zr and the like) are uniformly introduced in the smelting stage of each main phase, and grain boundary reconstruction is induced through in-situ hydrotreating to form a thin and continuous rare earth-rich grain boundary phase; and after orientation compression molding, vacuum sintering and multi-stage heat treatment, a sintered magnet with excellent magnetic performance is obtained. According to the product, the intrinsic coercive force Hcj is larger than or equal to 15 kOe, the residual magnetism Br is larger than or equal to 13.7 kG, and the maximum magnetic energy product (BH) max is larger than or equal to 39 MGOe. The problems that a high-Ce magnet is low in coercive force, discontinuous in grain boundary phase and the like are solved, the method does not need to depend on Dy / Tb heavy rare earth and a grain boundary diffusion technology thereof, the cost is reduced by 30% or above, and the method is suitable for the fields of new energy automobiles, wind power generation and the like and has a good industrialization prospect.

Description

Technical Field

[0001] This invention belongs to the field of functional materials and magnetic materials technology, specifically relating to a cerium (Ce)-rich rare earth sintered Nd-Ce-Fe-B permanent magnet and its preparation method. In particular, it describes a technique that achieves a synergistic improvement in high coercivity, high remanence, and high energy product through independent melting of three main phases, pre-doping with trace elements, and in-situ hydrogenation-induced grain boundary reconstruction. This technique is suitable for applications such as high-performance permanent magnet motors, energy-saving home appliances, and intelligent manufacturing. Background Technology

[0002] Sintered NdFeB permanent magnets are among the best permanent magnet materials in terms of overall magnetic properties and are widely used in new energy vehicle drive motors, wind turbines, and consumer electronics. However, their large-scale application is limited by the scarcity of light rare earth Nd / Pr resources and the high cost of adding heavy rare earth Dy / Tb to improve coercivity.

[0003] Ce, one of the most abundant rare earth elements in the Earth's crust (approximately 3–5 times that of Nd), is only 1 / 5–1 / 10 the price of Nd, making it extremely economical. However, directly introducing high Ce content can lead to the following problems: (Nd,Ce)₂Fe 14 The saturation magnetization Js and magnetocrystalline anisotropy field HA of B decrease; it readily precipitates paramagnetic CeFe2 phase, becoming a source of antimagnetization nuclei; Ce segregates at grain boundaries, disrupting grain boundary continuity and weakening magnetohardening ability. Existing modification methods include the dual-principal-phase method, grain boundary diffusion (GBD), and trace element addition. While GBD can significantly improve Hcj, it relies on expensive Dy / Tb resources and only improves surface properties; while the traditional dual-principal-phase method is difficult to precisely control element distribution.

[0004] Recent studies have shown that in-situ generated rare earth hydrides (such as NdHx) can promote liquid phase migration and grain boundary reconstruction during sintering, which is beneficial for the formation of continuous grain boundary phases; multi-stage annealing helps to control the precipitation behavior of grain boundary phases; La co-doping can stabilize the Ce³⁺ valence state and reduce lattice distortion caused by Ce²⁺; and elements such as Nb / Zr have a strong inhibitory effect on grain growth. However, no technology has yet been developed that organically combines "independent melting of multiple principal phases + pre-uniform distribution of trace elements + in-situ hydrogenation-induced grain boundary reconstruction" to achieve high comprehensive magnetic properties without relying on heavy rare earth grain boundary diffusion.

[0005] Therefore, there is an urgent need for a new preparation method that can make full use of Ce resources to reduce costs, and achieve synergistic optimization of high coercivity, high remanence and high magnetic energy product through internal structural regulation. Summary of the Invention

[0006] The primary technical objective of this invention is to provide a high-abundance Ce-based sintered Nd-Ce-Fe-B permanent magnet that does not contain heavy rare earth elements and requires no subsequent grain boundary diffusion treatment. The second technical objective of this invention is to provide a method for preparing a high-abundance Ce-based sintered Nd-Ce-Fe-B permanent magnet that does not contain heavy rare earth elements and does not require subsequent grain boundary diffusion treatment.

[0007] The first technical objective of this invention is achieved through the following technical solution: A cerium-rich rare earth sintered NdFeB permanent magnet is made by crushing and mixing three alloy castings: a Nd / Pr-rich alloy, a Ce-La-rich alloy, and a Y-Nd-rich alloy, wherein Ce accounts for ≥30 wt.% of the total rare earth elements. The permanent magnet also includes 0.1–2.0 wt.% of trace element M, selected from one or more combinations of trace elements from Al, Cu, Ga, Nb, and Zr. The trace element is uniformly introduced during the melting stages of each main phase to form a thin and continuous rare earth-rich grain boundary phase.

[0008] This invention solves the following problems existing in the prior art: high Ce content leads to a serious decrease in coercivity; discontinuous grain boundary phases and precipitation of harmful phases (such as CeFe2); uneven distribution of trace elements leads to local microstructure deterioration; and there is a lack of systematic composition-structure-performance synergistic regulation strategy.

[0009] This invention achieves a unified high Hcj, high Br and high (BH)max without using heavy rare earth elements such as Dy / Tb through a four-in-one strategy of "independent melting of three main phases + pre-doping of trace elements + in-situ hydrogenation-induced grain boundary reconstruction + sintering temperature control".

[0010] As a transition phase, its lattice constant lies between that of the Nd / Pr-rich and Ce-rich phases, which helps alleviate lattice mismatch, optimize the distribution gradient of Ce elements within the grains (gradient design), and reduce the magnetic dilution effect of Ce on the main phase grains, thereby contributing to maintaining high remanence (Br) and maximum magnetic energy product ((BH)max). While Y itself has weak magnetism, its introduction is crucial for improving the microstructure and elemental distribution.

[0011] Ce Total Quantity Control: Ce accounts for ≥ 30 wt.% of the total rare earth (RE) content to achieve significant cost reduction. Key Trace Element Addition (M): 0.1 ~ 2.0 wt.% of trace element M is added simultaneously during the mixing of the matrix magnetic powder, where M is selected from one or more of Al, Cu, Ga, Nb, and Zr. The synergistic effect of these elements is crucial to the final magnetic properties. Al (aluminum): Its main function is to improve the wettability and fluidity of the grain boundary phase, promoting its uniform distribution and continuity. In subsequent hydrogenation and sintering / heat treatment processes, it helps to form a thinner, more continuous grain boundary isolation layer (especially with REH). x (Combined), significantly improving coercivity (Hcj). At the same time, Al can inhibit grain growth, refine grains, indirectly improve coercivity, and help suppress the formation of REFe2 phase.

[0012] Cu (Copper): A key element in the formation of low-melting-point grain boundary phases. It significantly lowers the melting point of the grain boundary phase, promoting sintering densification and uniform distribution of the grain boundary phase. During sintering and heat treatment, the presence of Cu promotes good distribution and continuity of the grain boundary phase. Continuous, thin, and uniform rare-earth-rich grain boundary phases are crucial for hindering the propagation of antimagnetic domains and enhancing coercivity (Hcj). Cu also improves the corrosion resistance of the grain boundary phase.

[0013] Gallium (Ga): Similar to Cu, it forms a low-melting-point eutectic phase, improving grain boundary fluidity. The addition of Ga can optimize the composition of the grain boundary phase, significantly suppress the formation of the harmful REFe2 phase, and improve coercivity (Hcj). Ga has a relatively small negative impact on maintaining high remanence (Br).

[0014] Nb (niobium) / Zr (zirconium): Their main function is to refine the main phase grains. During sintering, they inhibit grain growth, resulting in a finer, more uniform main phase grain structure. Finer grains are beneficial for achieving higher coercivity (Hcj) (because smaller grains make the formation and growth of antimagnetic nuclei more difficult, i.e., the small size effect). Simultaneously, grain refinement has a relatively small negative impact on remanence (Br).

[0015] When Al and Cu are added together, they have a good synergistic effect on the wettability and melting point of the grain boundary phase; after Nb / Zr refines the grains, it can synergistically hinder demagnetization with the grain boundary phase optimized by Al and Cu.

[0016] Preferably, a cerium-rich rare-earth sintered NdFeB permanent magnet is prepared by crushing three alloy castings of different compositions and mixing them in a specific mass ratio. These alloys are Nd / Pr-rich phase alloys: NdFeB (… 0.6–0.8 Pr 0.2–0.4 ) x Fe y B z Of which x = 30.0–32.0 wt.%, y = 65.0–67.5 wt.%, z = 0.9–1.1 wt.%, accounting for 60–65% of the total alloy mass; Ce-La phase-rich alloys: chemical formula Ce a La b Fe cB d Of which a = 28.0–30.0 wt.%, b = 0.5–1.5 wt.%, c = 67.0–69.0 wt.%, d = 0.9–1.1 wt.%, accounting for 20–25% of the total alloy mass; Y-Nd phase-rich alloys: chemical formula Y e Nd f Fe g B h Of which e = 26.0–28.5 wt.%, f = 2.0–4.0 wt.%, g = 67.0–69.5 wt.%, h = 0.9–1.1 wt.%, accounting for 10–15% of the total alloy mass; Of these, Ce accounts for no less than 30 wt.% of the total mass fraction of all rare earth elements.

[0017] The La content of this invention is 0.5–1.5 wt.%, used to stabilize the Ce³⁺ valence state. The Y content of this invention is 26.0–28.5 wt.%, used to adjust grain boundary fluidity and oxidation resistance.

[0018] Preferably, the permanent magnet further includes 0.1–2.0 wt.% of trace element M, selected from one or more combinations of Al, Cu, Ga, Nb, and Zr.

[0019] Preferably: Al: 0.2–0.5 wt.%, improves grain boundary wettability; Cu: 0.2–0.6 wt.%, promotes liquid phase migration; Ga: 0.1–0.3 wt.%, inhibiting CeFe2 phase precipitation; Nb or Zr: 0.3–0.6 wt.%, to refine grain size.

[0020] In this invention, all trace elements M are added during their respective alloy smelting stages to ensure uniform distribution in each main phase.

[0021] The second technical objective of this invention is achieved through the following technical solution: A method for preparing a cerium-rich rare-earth sintered NdFeB permanent magnet includes the following steps: (1) Independent smelting and casting preparation The three alloys were weighed separately and melted independently in a vacuum induction furnace. During the melting process, trace element M was added in the corresponding proportion. Thin strip alloy sheets with a thickness of 0.2–0.5 mm were produced by strip casting.

[0022] (2) Crushing and powder preparation The three types of castings were mixed in a set ratio and then subjected to hydrogen crushing (HDDR pretreatment), followed by grinding with an air jet mill to an average particle size of 3.0–5.0 μm to obtain composite magnetic powder.

[0023] (3) In-situ hydrogenation treatment The composite magnetic powder is placed in an Ar atmosphere, H2 is introduced to a pressure of 0.05–0.10 MPa, the temperature is raised to 450–520°C, and the temperature is held for 2–4 hours to generate a nanoscale REHx layer on the powder surface.

[0024] (4) Orientation pressing molding Orientation pressing is performed under magnetic field strength ≥ 2.0 T to obtain green blanks.

[0025] (5) Vacuum sintering The material is held at 1040–1060°C for 3.0–4.0 hours, then cooled in the furnace to below 800°C and rapidly cooled to obtain a dense sintered body.

[0026] (6) Multi-stage heat treatment.

[0027] During sintering, the addition of elements M (Al, Cu, Ga) promotes the formation and uniform distribution of low-melting-point grain boundary phases, while Nb / Zr inhibits grain growth. REH x It decomposes and participates in the formation of optimized grain boundary phases.

[0028] The in-situ hydrogenation treatment described in this invention generates a REHx layer on the powder surface, which promotes grain boundary reconstruction and liquid phase redistribution during sintering. The multi-stage heat treatment of this invention is used to optimize grain boundary phase precipitation behavior and improve interfacial bonding and magnetic hardening ability.

[0029] During cooling after sintering or subsequent heat treatment, REHx decomposes (dehydrogenates) or reacts with surrounding materials (including added trace element M) to form a continuous, thin, and magnetically isolated novel grain boundary phase in situ. This phase effectively hinders the nucleation and propagation of antimagnetic domains and is one of the core mechanisms that enables a breakthrough improvement in coercivity (Hcj) (≥15 kOe).

[0030] Synergistic effect with trace element M: The presence of trace elements M (especially Al, Cu, Ga) can optimize the composition, melting point, and distribution of the grain boundary phases formed after the hydrogenation reaction process and the decomposition / reaction of REHx, further enhancing their magnetic isolation effect and continuity, and jointly contributing to high Hcj. The grain-refining effect of Nb / Zr increases the total grain boundary area, which, combined with REH... x The grain boundary layer further enhances the coercivity.

[0031] Preferably, step (6) multi-stage heat treatment includes first performing a first stage at 900–950°C for 2.0–3.0 h; then rapidly cooling to the second stage at 500–550°C for 2.0–3.0 h; and then performing a third stage at 400–450°C for 1.5–2.5 h.

[0032] After sintering, necessary heat treatment (multi-stage tempering) is performed to further optimize the grain boundary phase structure and magnetic properties. The presence of trace element M helps to form a grain boundary phase morphology that is more conducive to high coercivity during the tempering process.

[0033] Preferably, the magnetic powder obtained by air jet milling has an average particle size of 3.0–5.0 μm.

[0034] The beneficial effects of this invention are as follows: This invention achieves high coercivity (Hcj ≥ 15kOe) without the need for heavy rare earth grain boundary diffusion, thanks to grain boundary reconstruction and fine grain strengthening; Ce utilization is high, accounting for ≥30 wt.% of the total rare earth content, and the material cost is reduced by more than 30% compared with traditional N52 magnets; the grain boundary phase is continuous and uniform in thickness, effectively blocking the propagation of antimagnetic domains; the content of harmful phases is extremely low, the main phase grains are fine (<5 μm), and the structure is uniform; it has excellent corrosion resistance, and the salt spray test life is increased by more than 2 times; it has strong process compatibility, is suitable for existing powder metallurgy production lines, and is easy to scale up production. Attached Figure Description

[0035] Figure 1 This is a scanning electron microscope (SEM) image of the grain boundary microstructure of a cerium-rich rare earth sintered NdFeB permanent magnet according to Example 1 of the present invention. Detailed Implementation

[0036] Examples and Comparative Examples

[0037] The raw materials used in the following examples and comparative examples were all industrially pure metals (purity ≥ 99.5%), and the equipment was conventional powder metallurgy equipment. All tests were performed in accordance with GB / T 13560-2017.

[0038] Example

[0039] Nd / Pr rich phase alloys: (Nd0.6–0.8Pr0.2–0.4) x Fe y B z x = 30.0–32.0 wt.%, y = 65.0–67.5 wt.%, z = 0.9–1.1 wt.%, accounting for 60–65%; Ce-La phase-rich alloys: Ce a La b Fe c Bd , a=28.0–30.0 wt.%, b=0.5–1.5 wt.%, c=67.0–69.0 wt.%, d=0.9–1.1 wt.%, accounting for 20–25%; Y-Nd phase-rich alloys: Y e Nd f Fe g B h e=26.0–28.5 wt.%, f=2.0–4.0 wt.%, g=67.0–69.5 wt.%, h=0.9–1.1 wt.%, accounting for 10–15%; The element contains Ce, which accounts for ≥30 wt.% of the total rare earth elements, and also contains 0.1–2.0 wt.% of one or more of Al, Cu, Ga, Nb, and Zr.

[0040] Preparation process of Examples 1-8: Airflow mill particle size: 3.5±0.5 μm Hydrogenation conditions: 480°C × 3 h, H2 pressure 0.08 MPa Sintering: 1050°C × 3.5 h Multi-stage heat treatment: 930°C×2.5h→530°C×2.5h→430°C×2h Comparative example C4 preparation process: Using the formula in Example 1, Airflow mill particle size: 3.5±0.5 μm Hydrogenation conditions: 480°C × 3 h, H2 pressure 0.08 MPa Sintering: 1080°C × 3.5 h Multi-stage heat treatment: 930°C×2.5h→530°C×2.5h→430°C×2h.

[0041] Comparative Examples

[0042] The remaining process parameters for comparative examples C1–C3 are the same as those in Example 1.

[0043] Summary table of performance test results for examples and comparative examples

[0044] Results Analysis Comparative example C1 (single main phase + integral melting) showed the worst performance: Hcj was only 10.3 kOe, and CeFe2 phase was as high as 3.8%, indicating that the lack of composition gradient design and microstructure control will seriously degrade the magnetic properties; The comparative example C2 (post-doped) Hcj was 12.6 kOe, which was lower than that of the example, indicating that if trace elements are not uniformly distributed during the melting stage, it is difficult to achieve effective grain boundary modification. The comparative example C3 (without hydrogenation) Hcj was only 12.1 kOe, verifying the key role of "in-situ hydrogenation" in inducing grain boundary reconstruction; The comparative example C4 (with increased sintering temperature) has an Hcj of only 12.6 kOe, which verifies the key role of "sintering temperature" in the sintering process of Ce-containing magnets. In contrast, Examples 2 and 7 exhibited the best overall performance (Hcj > 15.5 kOe, (BH)max > 41 MGOe), demonstrating the significant inventiveness of the four-in-one strategy of "independent melting + pre-doping + hydrogenation reconstruction + controlled sintering temperature" proposed in this invention.

Claims

1. A cerium-rich rare-earth sintered NdFeB permanent magnet, characterized in that, The magnet is made by crushing and mixing three alloy castings: a Nd / Pr-rich alloy, a Ce-La-rich alloy, and a Y-Nd-rich alloy, wherein Ce accounts for ≥30 wt.% of the total rare earth elements. The permanent magnet also includes 0.1–2.0 wt.% of trace element M, selected from one or more combinations of trace elements from Al, Cu, Ga, Nb, and Zr. The trace element is uniformly introduced during the melting stages of each main phase to form a thin and continuous rare earth-rich grain boundary phase.

2. The cerium-rich rare-earth sintered NdFeB permanent magnet according to claim 1, characterized in that, It is made by crushing and mixing three alloy castings, the three alloys being: Nd / Pr rich phase alloys: (Nd 0.6-0.8, Pr 0.2-0.4) x Fe y B z , x=30.0-32.0 wt.%, y=65.0-67.5 wt.%, z=0.9-1.1 wt.%, accounting for 60-65%; Ce-La phase-rich alloys: Ce a La b Fe c B d , a=28.0–30.0 wt.%, b=0.5–1.5 wt.%, c=67.0–69.0 wt.%, d=0.9–1.1 wt.%, accounting for 20–25%; Y-Nd phase-rich alloys: Y e Nd f Fe g B h , e=26.0–28.5 wt.%, f=2.0–4.0 wt.%, g=67.0–69.5 wt.%, h=0.9–1.1 wt.%, accounting for 10–15%; The element contains Ce, which accounts for ≥30 wt.% of the total rare earth elements, and also contains 0.1–2.0 wt.% of one or more of Al, Cu, Ga, Nb, and Zr.

3. The cerium-rich rare-earth sintered NdFeB permanent magnet as described in claim 1, characterized in that, The average size of the main phase grains is less than 5 μm, the grain boundary phase is continuous and uniform in thickness, and the CeFe2 phase area ratio is less than 2.0%.

4. The cerium-rich rare-earth sintered NdFeB permanent magnet as described in claim 1, characterized in that, Its magnetic properties are Hcj ≥ 15 kOe, Br ≥ 13.7 kG, and (BH)max ≥ 39 MGOe.

5. A cerium-rich rare-earth sintered NdFeB permanent magnet as described in claim 1, characterized in that, Al: 0.2–0.5 wt.%; Cu: 0.2–0.6 wt.%; Ga: 0.1–0.3 wt.%; Nb or Zr: 0.3–0.6 wt.%.

6. A method for preparing a cerium-rich rare-earth sintered NdFeB permanent magnet as described in any one of claims 1–5, characterized in that, Including steps (1) The three alloys, namely, the Nd / Pr-rich alloy, the Ce-La-rich alloy, and the Y-Nd-rich alloy, were melted and cast separately; (2) After mixing, the mixture is crushed by hydrogen and then milled by air jet milling; (3) Perform in-situ hydrogenation treatment at 450–520°C for 2–4 h; (4) Magnetic field orientation pressing molding; (5) Vacuum sintering, held at 1040–1060°C for 3.0–4.0 h; (6) Multi-stage heat treatment.

7. The method for preparing cerium-rich rare-earth sintered NdFeB permanent magnets according to claim 6, characterized in that, Specifically, it includes the steps. (1) Independent melting and casting preparation: The above three alloys are weighed separately and melted independently in a vacuum induction furnace. During the melting process, the corresponding proportion of trace element M is added simultaneously. Thin strip alloy sheets with a thickness of 0.2–0.5 mm are prepared by rapid cooling casting process. (2) Crushing and powder preparation: The three types of castings are mixed in a set ratio and then subjected to hydrogen crushing, and then ground by air jet mill to an average particle size of 3.0–5.0 μm to obtain composite magnetic powder; (3) In-situ hydrogenation treatment: The composite magnetic powder is placed under an Ar atmosphere, H2 is introduced to a pressure of 0.05–0.10 MPa, the temperature is raised to 450–520°C, and the temperature is maintained for 2–4 hours to generate a nanoscale REHx layer on the powder surface; (4) Orientation pressing molding: Orientation pressing molding is performed under magnetic field strength ≥ 2.0 T to obtain green blanks; (5) Vacuum sintering: The green body obtained after orientation pressing is held at 1040–1060°C for 3.0–4.0 hours, cooled in the furnace to below 800°C and then rapidly cooled to obtain a dense sintered body; (6) Multi-stage heat treatment.

8. The method for preparing cerium-rich rare-earth sintered NdFeB permanent magnets according to claim 7, characterized in that, Step (6) Multi-stage heat treatment includes first stage 900–950°C, heat treatment for 2.0–3.0 h; then rapid cooling to the second stage: 500–550°C, heat treatment for 2.0–3.0 h; then the third stage: 400–450°C, heat treatment for 1.5–2.5 h.

9. The method for preparing a cerium-rich rare-earth sintered NdFeB permanent magnet as described in claim 7, characterized in that, The in-situ hydrogenation treatment generates REH on the powder surface. x Layers are used to promote grain boundary reconstruction and liquid phase redistribution during sintering.

10. The method for preparing a cerium-rich rare-earth sintered NdFeB permanent magnet as described in claim 7, characterized in that, The average particle size of the magnetic powder obtained by air jet milling in step (2) is 3.0–5.0 μm.

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