Method for reducing anisotropy of neodymium-iron-boron permanent magnet in sintering shrinkage process

Abstract

The invention discloses a method for reducing anisotropy of a neodymium-iron-boron permanent magnet in the sintering shrinkage process, and belongs to the technical field of rare earth permanent magnet material preparation. The method comprises the following steps: smelting a main phase alloy which hardly contains a rare earth-rich phase; carrying out rapid hardening, hydrogen decrepitation and air jet pulverization to obtain main phase powder with D50 of 3.5-5.0 [mu] m; a rare earth-rich phase thin film layer with the thickness of 100-150 nm is formed on the surface of the powder through magnetron sputtering or steam deposition, and the thin film layer comprises at least one of the R element, Al, Cu, Ga, Zr and Nb; carrying out compression molding under a magnetic field of 1.5-2.0 T; and then carrying out multi-section vacuum sintering and tempering treatment. According to the method, rare earth-rich phases on the surfaces of main phase particles are uniformly distributed, neodymium-rich phase accumulation in the orientation direction is avoided, the shrinkage anisotropy is smaller than or equal to 5%, the yield of the radiation ring is larger than or equal to 96%, and the problem of cracking caused by uneven distribution of the rare earth-rich phases in a traditional process is effectively solved.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnet material preparation technology. Specifically, it relates to a method for reducing the anisotropy of the sintering shrinkage process of NdFeB permanent magnets. In particular, it is a preparation method that achieves uniform distribution of rare earth-rich phase through a coating process, thereby significantly reducing the shrinkage anisotropy of sintered NdFeB permanent magnets caused by uneven distribution of rare earth-rich phase during densification. Background Technology

[0002] Sintered NdFeB permanent magnets, as the most powerful rare-earth permanent magnet material currently available, have been widely used in high-end manufacturing fields such as drive motors for new energy vehicles, wind power generation equipment, and high-precision sensors. Their superior magnetic properties rely on a highly oriented microstructure, especially in irregularly shaped products, where magnetic field pressing is required to achieve the directional alignment of the main phase grains along the easy magnetization axis (c-axis). However, during the subsequent high-temperature sintering densification process, the material often exhibits significant shrinkage anisotropy; that is, the linear shrinkage rate along the orientation direction (30-40%) is much higher than that in the transverse direction (15%-20%), with the difference generally exceeding 20%. Therefore, for radiating rings and multipolar rings, this inevitably leads to cracking of the billet, resulting in a consistently low yield.

[0003] The key issue lies in the unbalanced spatial distribution of the rare-earth-rich phase on the surface of the main phase particles. In traditional manufacturing processes, the 200-micron sheet alloy has its C-axis perpendicular to the plane of the rapidly solidified sheet. The main phase consists of 5-10 micron thin sheets with a 300-600 nanometer rare-earth-rich phase distributed on the surface. The particles produced by hydrogen crushing and air jet milling are irregular polyhedra, with a small portion being exposed main phase particles and the majority having a rare-earth-rich phase layer on one face. For the production of block-shaped, axially cylindrical, and axially annular products, the shrinkage rate is highest (30-40%) in the orientation direction, lowest (12%-15%) in the pressing direction, and 15%-20% in the free direction, exhibiting anisotropic shrinkage, which has little impact on product production. It is sufficient to allow for the corresponding shrinkage rate in the mold design. However, for radial ring products, this presents serious problems, even leading to 100% sintering cracking, which is why radial ring products have not been mass-produced.

[0004] For a radiating ring with an outer diameter D and an inner diameter d, the circumference of the outer diameter shrinks to D*3.14*(1-20%). Inner diameter d*3.14* (1-20%) The wall thickness should be a = (Dd) * (1 - 20%) / 2 = 0.4 (Dd). However, from the perspective of orientation, the contraction is: b = (Dd) * (1 - 40%) / 2 = 0.3 (Dd). (ab) / a = 0.1 / 0.4 = 0.25 The same ring has two contradictory wall thicknesses, which is why the radiation ring cracked.

[0005] Therefore, there is an urgent need for a method that can fundamentally eliminate the anisotropy of sintering shrinkage in radiative rings and improve the yield of radiative ring NdFeB. Summary of the Invention

[0006] The purpose of this invention is to provide a method for reducing anisotropy during the sintering shrinkage process of NdFeB permanent magnets. By forming a uniform rare-earth-rich phase thin film layer on the surface of the main phase powder which contains almost no rare-earth-rich phase, the problem of shrinkage anisotropy caused by uneven distribution of rare-earth-rich phase is fundamentally solved, and the yield and dimensional accuracy of irregularly shaped magnets are significantly improved.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: This invention provides a method for reducing anisotropy during the sintering shrinkage process of NdFeB permanent magnets, comprising the following steps: (1) Melting the main phase alloy; the main phase alloy is a main phase alloy that contains almost no rare earth-rich phases, and the composition is close to the atomic ratio of R2Fe14B to form a single main phase structure, wherein the R element is selected from one or more of Nd, Pr, Dy, Tb, Ce, and La, and the total mass percentage of the R element is 26.0%-27.5%; (2) The main phase alloy after melting is rapidly solidified and melted to form sheet castings; (3) The main phase alloy after rapid solidification and cooling is subjected to hydrogen crushing treatment to obtain coarse powder with a particle size of less than 200 μm; (4) The coarse powder of the main phase alloy obtained in step (3) is subjected to air jet milling until the D50 is 3.5-5.0 μm; then a rare earth-rich phase thin film layer with a thickness of 100-150 nm is formed on the surface of the air jet milled main phase alloy powder by a coating process. The rare earth-rich phase thin film layer uniformly covers the surface of the main phase powder. The rare earth-rich phase alloy contains the R element and at least one of Al, Cu, Ga, Zr and Nb, wherein the total mass percentage of Al, Cu and Ga is 0.5%-1.5%, and the total mass percentage of Zr and Nb is 1%-6%. (5) Under an orientation magnetic field of 1.5-2.0 T, the coated main phase powder is pressed into a compact to obtain a pressed preform; (6) The pressed blank is sintered in multiple stages under vacuum conditions. First, the temperature is raised to 800-900℃ at a heating rate of 5-10℃ / min and held for 30-60 min. Then, the temperature is raised to 1020-1080℃ at a heating rate of 3-5℃ / min and held for 60-120 min to achieve densification sintering.

[0008] Furthermore, in step (1), the melting of the main phase alloy and the rare earth-rich phase alloy is carried out in a vacuum induction melting furnace with a vacuum degree of less than 10 Pa, the melting temperature is 1450-1550℃, and the holding time is 30-60 min.

[0009] Furthermore, in step (3), hydrogen gas with a purity of not less than 99.999% is introduced into the hydrogen crushing process at room temperature, with an initial pressure of 50-100 kPa; the coarse powder after hydrogen crushing is dehydrogenated at 500-600℃ for 2-4 h.

[0010] Furthermore, in step (4), the airflow pressure of the airflow pulverizer is 0.6-0.8 MPa.

[0011] Furthermore, in step (4), the coating process adopts magnetron sputtering or vapor deposition, and the substrate temperature is controlled at 80-150℃.

[0012] Furthermore, in step (5), the pressing pressure is 20-60 MPa and the holding time is 10-30 s.

[0013] Furthermore, step (6) also includes two tempering processes: first, high-temperature tempering at 850-950℃ for 0.5-2 h, and then low-temperature tempering at 450-550℃ for 1-3 h.

[0014] This invention also provides a radiation ring-type NdFeB permanent magnet prepared by the method, which exhibits shrinkage anisotropy. 5%, with a yield rate of ≥99.5%.

[0015] Compared with the prior art, the present invention has the following significant advantages: This invention fundamentally solves the cracking problem caused by anisotropic shrinkage due to uneven distribution of rare-earth-rich phases in traditional processes. By forming a 100-150 nm thick rare-earth-rich phase film layer on the powder surface after crushing a main phase alloy that contains almost no rare-earth-rich phase, this invention addresses the issue. During the orientation pressing process, the film layer aligns with the main phase particles, ensuring uniform distribution of the rare-earth-rich phase at all grain boundaries. This effectively avoids the accumulation of neodymium-rich phase in the orientation direction, preventing significant shrinkage after sintering. Consequently, the difference in linear shrinkage between the orientation direction and the transverse direction is stably controlled within 5%. This technical solution not only avoids the shrinkage imbalance problem caused by the random distribution of rare-earth-rich phases in traditional processes but also increases the yield of irregularly shaped magnets to ≥99.5%, significantly reducing the manufacturing cost of high-end magnets. Detailed Implementation

[0016] To enable those skilled in the art to better understand the technical solutions of this invention, the present application will be further described in detail below with reference to embodiments.

[0017] In both the embodiments and comparative examples of this invention, the same square mold was used to prepare sintered NdFeB permanent magnets. The design dimensions were: an outer diameter of 80 mm, an inner diameter of 60 mm, and a height of 20 mm for the radiating magnetic ring. Fifty samples were prepared in each batch, and all performance tests were performed according to the methods described below. The results were taken as statistical averages.

[0018] Example 1 (1) Melting the main phase alloy The main phase alloy is a single RFeB lattice structure without rare earth-rich phases, with the following composition: Nd+Pr 26.0 wt%, Fe 71.5 wt%, B 2.5 wt% (total mass percentage of R element is 26.0%).

[0019] The metal was melted in a vacuum induction melting furnace at a vacuum level of 8 Pa and a melting temperature of 1450°C for 30 minutes to ensure homogenization of the composition. Argon gas was then introduced at a pressure of 80,000 Pa.

[0020] (2) The molten alloy is then rapidly solidified and cooled to form a sheet casting with a thickness of 0.20 mm.

[0021] (3) Hydrogen crushing treatment The main phase ingot was placed in a hydrogen crushing furnace, and high-purity hydrogen gas with a purity of not less than 99.999% was introduced at room temperature with an initial hydrogen pressure of 190 kPa. The hydrogen absorption reaction continued until the hydrogen pressure in the furnace no longer decreased, and the ingot spontaneously fractured along the grain boundaries. The resulting coarse powder had a particle size of less than 200 μm, exhibited an irregular polyhedral morphology, and was rich in microcracks. Subsequently, it was dehydrogenated under vacuum at 580°C for 4 h, with a residual hydrogen content of 900 ppm.

[0022] (4) Powder refining and coating The coarse powder of the main phase was subjected to air jet milling under nitrogen protection at a pressure of 0.6 MPa until the D50 was 4.5 μm.

[0023] On the surface of the main phase powder after air jet milling, a rare earth-rich phase thin film was deposited by magnetron sputtering. The composition of the rare earth-rich phase alloy was 96.0 wt% Nd, 3.0 wt% Cu, and 1.0 wt% Zr (Al, Ga, and Nb contents were 0, of which the total content of Al, Cu, and Ga was 3 wt%, and the total content of Zr and Nb was 1 wt%). The deposition parameters were Ar gas pressure 0.5 Pa, power 300 W, substrate temperature 120℃, and deposition time 120 s, forming a rare earth-rich phase thin film with an average thickness of 120 nm.

[0024] (5) Orientation magnetic field pressing molding The coated main phase powder is filled into the cavity of a circular mold, and a uniaxial pressure of 30 MPa is applied under an orientation magnetic field of 1.5 T. The density is 4.2 g / cm3, and the pressure is held for 10 s to obtain a compact with a certain strength.

[0025] (6) Vacuum multi-segment sintering The compact was placed in a vacuum sintering furnace (with a vacuum level better than 10⁻² Pa) and heated to 800°C at a rate of 5°C / min, and held for 30 minutes. Then, it was heated to 1050°C at a rate of 3°C / min and held for 60 minutes to complete densification. Finally, a two-stage tempering process was performed: first, holding at 850°C for 3.5 hours, and then at 480°C for 3 hours. It was then rapidly cooled to room temperature.

[0026] Example 2 Except for the following parameters, the remaining processes in this embodiment are the same as in Embodiment 1: (1) Melting the main phase alloy The main phase alloy is a single RFeB lattice structure without rare earth-rich phases, with the following composition: Nd 26.3 wt%, Pr 0.2 wt%, Dy 0.5 wt%, Fe 70.5 wt%, B 2.5 wt% (total R element = 27.0 wt%).

[0027] Melting conditions: vacuum degree 5 Pa, 1500°C for 45 min.

[0028] (2) Hydrogen crushing treatment Initial hydrogen pressure 175 kPa, dehydrogenation process: dehydrogenation treatment at 550°C for 3 h under vacuum conditions; (3) Powder refining and coating Main phase powder: gas flow pressure 0.7 MPa, D50=4.2 μm.

[0029] A rare earth-rich phase thin film was formed on the surface of the main phase powder after air jet milling using a vapor deposition method. The rare earth-rich phase alloy had the following composition: Pr 40.0 wt%, Nd 44.0 wt%, Al 6.0 wt%, Cu 3.0 wt%, Ga 3.0 wt%, Zr 2.0 wt%, and Nb 2.0 wt% (total 100.0 wt%; of which the total content of Al, Cu, and Ga is 12 wt%, and the total content of Zr and Nb is 4 wt%). The deposition temperature was 100℃, resulting in a rare earth-rich phase thin film with an average thickness of 150 nm.

[0030] (4) Orientation magnetic field pressing molding Orientation magnetic field: 1.6 T, uniaxial pressure: 40 MPa, holding pressure for 20 s. Density: 4.3 g / cm³ (5) Vacuum multi-segment sintering The temperature was increased to 850°C at a rate of 7.5°C / min and held for 45 minutes. Then, the temperature was increased to 1065°C at a rate of 4°C / min and held for 90 minutes to complete densification. Finally, a two-stage tempering process was performed: first, the temperature was held at 900°C for 3.5 hours, and then at 500°C for 3 hours.

[0031] Example 3 Except for the following parameters, the remaining processes in this embodiment are the same as in Embodiment 1: (1) Melting the main phase alloy The main phase alloy is a single RFeB lattice structure without rare earth-rich phases, with the following composition: Nd 27.5 wt%, Fe 70.0 wt%, B 2.5 wt% (total R element = 27.5 wt%).

[0032] Melting conditions: vacuum degree 3 Pa, 1550°C for 60 min.

[0033] (2) Hydrogen crushing treatment Initial hydrogen pressure 190 kPa, dehydrogenation process: dehydrogenation treatment at 600°C for 4 h under vacuum conditions; residual hydrogen 800 ppm.

[0034] (3) Powder refining and coating Main phase powder: gas flow pressure 0.8 MPa, D50=5.0 μm.

[0035] A rare earth-rich phase thin film was formed on the surface of the main phase powder after air jet milling using magnetron sputtering. The rare earth-rich phase alloy had the following composition: La 10.0 wt%, Ce 10.0 wt%, Nd 59.0 wt%, Al 12.0 wt%, Cu 3.0 wt%, Zr 5.0 wt%, and Nb 1.0 wt% (total 100.0 wt%; of which the total content of Al, Cu, and Ga is 15 wt%, and the total content of Zr and Nb is 6 wt%). The deposition parameters were: Ar gas pressure 0.6 Pa, power 350 W, and substrate temperature 150℃, forming a rare earth-rich phase thin film with an average thickness of 100 nm.

[0036] (4) Powder modification treatment: Spray the coated main phase powder into 0.20 wt% tributyl borate and mix for 50 min.

[0037] (5) Orientation magnetic field pressing molding Orientation magnetic field: 2.0 T, uniaxial pressure: 50 MPa, holding pressure for 10 s. Density: 4.36 g / cm³ (6) Vacuum multi-segment sintering The temperature was increased to 900°C at a rate of 10°C / min and held for 60 minutes. Then, the temperature was increased to 1080°C at a rate of 5°C / min and held for 120 minutes to complete densification. Finally, a two-stage tempering process was performed: first, the temperature was held at 950°C for 2 hours, and then at 550°C for 3 hours.

[0038] Preferably, in Examples 1-3, the entire operating area must meet explosion-proof safety standards during hydrogen crushing. Specifically, the hydrogen crushing furnace and its surrounding electrical equipment should obtain a high level of explosion-proof electrical certification to ensure safety during operation in a high-purity hydrogen environment.

[0039] Comparative Example 1 The same alloy composition and all processes except for coating were used as in Example 2. The main phase coarse powder was air-jet milled (D50=3.2 μm) and then no powder coating treatment was performed.

[0040] Experimental Example To objectively evaluate the technical effects of this invention, the sintered NdFeB magnets (orientation 50 mm, free direction 60 mm, height 30 mm) prepared in Examples 1-3 and Comparative Example 1 were tested for the following 10 performance indicators. All tests were conducted under the same environmental conditions (temperature 23±2°C, humidity 50±5% RH), specifically as follows: the dimensions, weight, density, magnetic energy product, and coercivity were measured after magnetic field forming and sintering. The test data are summarized in Table 1.

[0041] Table 1. Properties of sintered NdFeB square magnets The shrinkage anisotropy test data in Table 1 show that the shrinkage anisotropy of Examples 1-3 is ≤5.9%, significantly better than Comparative Example 1 (17.7%). Comparative Example 1, using a traditional method, had rare earth-rich phases on the c-axis plane of the particles, causing the liquid phase to preferentially migrate from the orientation direction to other planes during the initial sintering stage, resulting in a sharp increase in shrinkage anisotropy. In contrast, this invention uses a coating process to uniformly cover the surface of the main phase powder with a rare earth-rich phase film layer. During orientation pressing, the film is synchronously oriented with the main phase particles, ensuring precise three-dimensional spatial distribution of the rare earth-rich phase at the grain boundaries. During sintering, the liquid phase uniformly wets the interfaces in all directions, effectively reducing excessive shrinkage due to liquid phase loss in the orientation direction and minimizing shrinkage anisotropy.

[0042] Examples 4, 5, 6, and Comparative Example 2 Press a radiating magnetic ring with a diameter of 50 mm, an inner diameter of 35 mm, and a height of 30 mm, and add 120 g of powder.

[0043] The magnetic property test data in Table 1 show that the remanence (Br) of Examples 1-3 is 13.9-14.2 kG, the coercivity (Hcj) is 16.1-16.8 kOe, and the maximum energy product ((BH)max) is 41.0-42.5 MGOe, all of which are superior to Comparative Example 1 (Br 13.9 kG, Hcj 16.5 kOe, (BH)max 41.2 MGOe). Comparative Example 1 has a larger shrinkage rate in the orientation direction due to the higher distribution of rare earth-rich phases perpendicular to the c-axis.

[0044] In summary, this invention achieves a neodymium-rich layer on the surface of the main phase particles by coating the main phase powder surface. During the sintering process, there is less liquid phase migration, which fundamentally eliminates sintering shrinkage anisotropy, reducing shrinkage anisotropy from 17.7% to 5.9% and achieving a yield of ≥96%. At the same time, it ensures dimensional accuracy of ±0.2 mm and high magnetic properties, effectively solving the problem of low yield in traditional processes.

[0045] For standard cubes, the pass rates were similar. However, for radiating ring products, the pass rate increased from 72% to 94%.

Claims

1. A method for reducing anisotropy of a Nd-Fe-B permanent magnet during sintering shrinkage, comprising the following steps: (1) smelting a main phase alloy; the main phase alloy is a main phase alloy containing almost no rare earth-rich phase, and the composition is close to the atomic ratio of R2Fe14B, forming a single main phase structure, wherein R elements are selected from one or more of Nd, Pr, Dy, Tb, Ce, La, and the total mass percentage of R elements is 26.0%-27.5%; (2) rapidly solidifying the smelted main phase alloy to form a flaky ingot; (3) hydrogen crushing the rapidly solidified main phase alloy to obtain coarse powder with a particle size of less than 200 μm; (4) air-jet milling the coarse powder of the main phase alloy obtained in step (3) to a D50 of 3.5-5.0 μm; then forming a rare earth-rich phase thin film layer on the surface of the air-jet milled main phase alloy powder by a plating process, with a film thickness of 100-150 nm, the rare earth-rich phase thin film layer uniformly covering the surface of the main phase powder, the rare earth-rich phase alloy containing the R elements and at least one of Al, Cu, Ga, Zr, and Nb, wherein the total mass percentage of Al, Cu, and Ga is 0.5%-1.5%, and the total mass percentage of Zr and Nb is 1%-6%; (5) pressing the plated main phase powder into a compact under an orienting magnetic field of 1.5-2.0 T to obtain a green compact; (6) multi-stage sintering the green compact under vacuum conditions, first increasing the temperature to 800-900 ℃ at a rate of 5-10 ℃ / min and maintaining for 30-60 min, and then increasing the temperature to 1020-1080 ℃ at a rate of 3-5 ℃ / min and maintaining for 60-120 min for densification sintering.

2. The method of claim 1, wherein, In step (1), the smelting of the main phase alloy and the rare earth-rich phase alloy is carried out in a vacuum induction melting furnace with a vacuum degree of less than 10 Pa, and the smelting temperature is 1450-1550 ℃, with a holding time of 30-60 min.

3. The method of claim 1, wherein, In step (3), the hydrogen crushing treatment is carried out at room temperature by introducing hydrogen gas with a purity of not less than 99.999%, with an initial pressure of 50-100 kPa; and the hydrogen-crushed coarse powder is dehydrogenated at 500-600 ℃ for 2-4 h.

4. The method of claim 1, wherein, In step (4), the air-jet milling pressure is 0.6-0.8 MPa.

5. The method of claim 1, wherein, In step (4), the plating process uses a magnetron sputtering or vapor deposition method, and the substrate temperature is controlled at 80-150 ℃.

6. The method of claim 1, wherein, In step (5), the pressing pressure is 20-60 MPa, and the holding time is 10-30 s.

7. The method of claim 1, wherein, The step (6) further comprises two-stage tempering treatment: first high-temperature tempering at 850-950 ℃ for 0.5-2 h, and then low-temperature tempering at 450-550 ℃ for 1-3 h.

8. Radiation ring class neodymium-iron-boron permanent magnet produced according to the method of any one of claims 1 to 7, characterized by The neodymium-iron-boron permanent magnet shrinkage anisotropy 5%, yield ≥ 99.5%.

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