Preparation method of high-performance grain boundary diffusion sintered neodymium-iron-boron material

Abstract

The invention provides a preparation method of a high-performance grain boundary diffusion sintered neodymium iron boron material, and belongs to the technical field of permanent magnet materials. The low-melting-point heavy-rare-earth-free alloy coating is coated and cured on the easy-to-diffuse surface of the neodymium-iron-boron base material magnet, and the heavy-rare-earth alloy coating is coated and cured on the low-melting-point heavy-rare-earth-free alloy coating; and the coated magnet is sequentially subjected to low-temperature pre-diffusion heat treatment, grain boundary diffusion heat treatment and aging treatment, and then the grain boundary diffusion neodymium-iron-boron magnet is obtained. According to the invention, the diffusion efficiency of heavy rare earth elements can be effectively improved, the coercive force of the magnet is improved, and relatively high residual magnetism can be achieved.

Description

A method for preparing high-performance grain boundary diffusion sintered NdFeB materials Technical Field

[0001] This invention belongs to the field of permanent magnet materials technology, specifically relating to a method for preparing high-performance grain boundary diffusion sintered NdFeB materials. Background Technology

[0002] Nd-Fe-B represents Nd2Fe, an intermetallic compound. 14 B's third-generation rare-earth permanent magnet material, Nd2Fe, has been known as the "King of Magnets" since its introduction in 1983. 14 The high saturation magnetization of the B phase (16 kGs) makes its (BH)max far exceed that of other permanent magnet materials, thus making it widely used in fields with stringent requirements for magnetic properties, such as smartphone components, new energy vehicles, medical diagnostics, and wind and solar power generation.

[0003] With the widespread application of neodymium iron boron (NdFeB), the performance requirements for NdFeB magnets are becoming increasingly stringent. However, the H of NdFeB magnets... cj If the temperature is not high, its magnetic properties will deteriorate rapidly at high temperatures. To meet the operating requirements of magnets at high temperatures, it is necessary to increase its H value. A In traditional production processes, heavy rare earth elements such as Dy / Tb are typically added directly during the smelting process. Because Dy2Fe... 14 B and Tb2Fe 14 B's H A Much higher than Nd2Fe 14 B, After adding Dy / Tb, Nd2Fe 14 When a portion of the Nd in the B main phase is replaced by Dy / Tb, a higher H will be formed. A (Nd,HRE)2Fe 14 The B phase increases the Hcj of the magnet, thus improving its high-temperature stability. However, the atomic magnetic moments of heavy rare earth Dy / Tb and Fe are antiferromagnetically coupled, and the addition of heavy rare earths will affect the B phase of the NdFeB magnet. r and (BH) max The cost has been significantly reduced. Furthermore, the scarcity and extremely high price of heavy rare earth resources have greatly increased the production cost of NdFeB magnets.

[0004] Grain boundary diffusion (GBD) is a method that can efficiently utilize rare earth elements and significantly improve the H of magnets. cj The technology is capable of using B r A slight decrease in cost significantly increases the H of the magnet. cjThis process aims to improve the utilization rate of rare earth resources and produce high-performance NdFeB magnets at a relatively low cost. However, the degree to which grain boundary diffusion enhances coercivity is closely related to the diffusion depth of heavy rare earth elements in the magnet. The diffusion efficiency of heavy rare earth elements is determined by two factors: the chemical composition of the substrate magnet, which affects the intrinsic properties and microstructure of the magnet, especially the composition and distribution of the grain boundary phase; and the chemical composition of the diffusion source, which affects the grain boundary structure during diffusion. Under conventional grain boundary diffusion processes, the diffusion depth of heavy rare earth elements is shallow, and they tend to accumulate in large quantities near the diffusion surface of the magnet, resulting in a small increase in coercivity and poor squareness. Furthermore, due to the low diffusion efficiency of heavy rare earth elements, the increase in coercivity is even more significantly reduced for magnets with greater thickness (above 5 mm). Therefore, improving the diffusion depth and efficiency of heavy rare earth elements in magnets is a major challenge affecting the widespread application of grain boundary diffusion technology. Patent CN118899161A discloses a grain boundary diffusion method for sintered NdFeB magnets. This method involves applying a heavy rare earth alloy infiltration source to a sintered NdFeB blank, followed by a light rare earth alloy infiltration source, and then performing grain boundary diffusion and tempering treatments to improve the magnet's coercivity. However, this method has a high remanence loss rate and weak demagnetization resistance, offering no inspiration for this invention. Furthermore, this invention effectively overcomes the problem of poor coercivity consistency by subjecting the coated magnet to a low-temperature pre-diffusion heat treatment. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing high-performance grain boundary diffusion sintered NdFeB materials. This invention solves the problems of shallow diffusion depth, poor squareness, and low coercivity of heavy rare earth elements by first coating a low-melting-point heavy rare earth-free alloy coating onto the easily diffused surface of the NdFeB substrate magnet, and then coating a heavy rare earth alloy coating on top of the low-melting-point heavy rare earth-free alloy coating. A further objective of this invention is to optimize the continuity of the grain boundary phase and lower its melting point through a low-temperature pre-diffusion heat treatment with a low-melting-point heavy rare earth-free alloy, thereby solving the problems of large remanence loss, poor coercivity consistency, uneven distribution of heavy rare earth elements, and / or aggregation near the diffusion surface.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The present invention provides a method for preparing high-performance grain boundary diffusion sintered NdFeB materials, comprising the following steps: S1: coating and curing a low-melting-point heavy rare earth alloy coating onto the easily diffused surface of the NdFeB substrate magnet, and coating and curing a heavy rare earth alloy coating onto the low-melting-point heavy rare earth alloy coating; S2: subjecting the coated magnet to a low-temperature pre-diffusion heat treatment, a grain boundary diffusion heat treatment, and an aging treatment in sequence to obtain a grain boundary diffusion NdFeB magnet.

[0007] This invention involves first coating a low-melting-point, heavy rare-earth-free alloy coating onto the easily diffused surface of a NdFeB substrate magnet. The easily diffused surface is a plane on which the diffusion source can be easily fabricated, not limited to an orientation plane. This invention opens grain boundary channels through low-melting-point, heavy rare-earth-free diffusion. Under a lower-temperature pre-diffusion heat treatment, the continuity of the grain boundary phase is enhanced, promoting the dispersion of heavy rare-earth-free elements and lowering the melting point of the grain boundary phase, thus widening the diffusion channels for heavy rare-earth elements. The grain boundary diffusion heat treatment also improves the diffusion efficiency of the heavy rare-earth alloy diffusion source, deepens the diffusion depth of heavy rare-earth elements, ensures uniform distribution of heavy rare-earth elements, reduces aggregation near the diffusion surface, effectively reduces remanence loss, and improves demagnetization resistance. Furthermore, by controlling the mass ratio of the low-melting-point, heavy rare-earth-free alloy coating to the heavy rare-earth alloy coating, the diffusion efficiency of heavy rare-earth elements is improved, resulting in a greater increase in coercivity while ensuring consistency in coercivity.

[0008] Preferably, in step S2, the low-temperature pre-diffusion treatment temperature is 500-700℃, the grain boundary diffusion treatment temperature is 850-950℃, and the aging temperature is 400-700℃. This invention adjusts the low-temperature pre-diffusion treatment temperature to be slightly higher than the aging temperature to facilitate the reconstruction of a continuous grain boundary phase. The coated magnet undergoes a first stage of low-temperature pre-diffusion heat treatment, where the low-melting-point, heavy rare-earth alloy melts and diffuses along the grain boundaries to form a continuous grain boundary phase. Then, a second stage of high-temperature grain boundary diffusion heat treatment is performed, where the relatively high-melting-point heavy rare-earth alloy coating melts, and the heavy rare-earth elements diffuse along the optimized grain boundaries to form a highly anisotropic core-shell structure. Finally, aging treatment is performed to obtain the diffused sintered NdFeB magnet.

[0009] Preferably, the mass ratio of the single-sided mass of the low-melting-point heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is (0.1-0.5):100; the mass ratio of the single-sided mass of the heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is (0.25-0.75):100. The heavy rare-earth-free coating of this invention mainly serves as a pretreatment to optimize grain boundary continuity and broaden diffusion channels. By controlling the mass ratio of the low-melting-point heavy rare-earth alloy coating and the heavy rare-earth alloy coating to the NdFeB substrate magnet, it avoids an excessively large proportion of the heavy rare-earth alloy coating, resulting in uneven diffusion or even occupying too many diffusion channels, which would affect the diffusion efficiency and dispersion effect of heavy rare-earth elements into the grain boundaries, making it difficult to improve coercivity and increasing remanence loss.

[0010] Preferably, by mass percentage, the low-melting-point heavy rare-earth alloy coating comprises 50-90% heavy rare-earth alloy, 10-30% solvent, and 1-6% binder; the heavy rare-earth alloy coating comprises 50-90% heavy rare-earth alloy, 10-30% solvent, and 1-6% binder. This invention involves mixing the heavy rare-earth alloy and heavy rare-earth alloy separately with solvent and binder to form a slurry, then coating and curing it in layers. The low-melting-point heavy rare-earth alloy coating is coated and cured on the easily diffused surface of the NdFeB substrate magnet, and the heavy rare-earth alloy coating is coated and cured on top of the low-melting-point heavy rare-earth alloy coating, forming a double-layer diffusion source coating with significant compositional differences.

[0011] Preferably, the average particle size of the heavy rare earth-free alloy is 2-4 μm; the average particle size of the heavy rare earth alloy is 2-4 μm. This invention, by controlling the average particle size of the heavy rare earth-free alloy, ensures good continuity of the grain boundary phase under pre-diffusion heat treatment at lower temperatures, opening grain boundary diffusion channels and widening diffusion channels for heavy rare earths; simultaneously, controlling the average particle size of the heavy rare earth alloy ensures its diffusion rate, avoids uneven distribution, and improves the consistency of coercivity.

[0012] Preferably, the heavy rare earth-free alloy has an LRE composition. x M 100-x By mass percentage, 70%≤x≤90%; the LRE is one or more of Pr, Nd, Ce, La, Sm, Eu, and Pm, and M is one or more of Al, Co, Cu, and Ga.

[0013] Preferably, the heavy rare earth alloy has an HRE composition. x LRE y M 100-x-y By mass percentage, 70%≤x≤90%, 0%≤y≤20%; the HRE is one or more of Dy, Tb, Gd, Ho, and Y, the LRE is one or more of Pr, Nd, Ce, La, Sm, Eu, and Pm, and the M is one or more of Al, Co, Cu, Ga, Zr, Ti, Nb, Zn, Fe, and Li.

[0014] Preferably, the composition of the neodymium iron boron substrate magnet is RE x Fe 100-x-y-z B y M z By mass percentage, 29%≤x≤33%, 0.9%≤y≤1.02%; RE is one or more of Pr, Nd, Dy, Tb, La, Ce, Y, Gd, and Ho, and M is one or more of Al, Co, Cu, Ga, Zr, Ti, Nb, Zn, and Li.

[0015] Preferably, in the low-melting-point heavy rare earth alloy coating and the heavy rare earth alloy coating, the solvent is one or more of anhydrous ethanol, diethyl ether, acetaldehyde, propanol, acetone, methanol, tetrahydrofuran, and terpineol, and the binder is one or more of polyvinyl butyral, polyvinyl alcohol, polyethylene glycol, and polyvinylpyrrolidone.

[0016] Preferably, the preparation of the neodymium iron boron substrate magnet includes the following steps: S1: preparing the raw material RE x Fe 100-x-y-z B y M z S1: Melting to obtain a rapidly solidified cast sheet; S2: After the rapidly solidified cast sheet is crushed by absorbing hydrogen to obtain coarse powder, it is then milled by air jet milling under oxygen-free conditions to obtain fine powder; S3: The fine powder is oriented and pressed into shape under conditions with an oxygen content ≤500ppm, and then isostatically pressed to obtain a NdFeB compact; S4: The NdFeB compact is subjected to vacuum sintering and two-stage aging treatment in sequence to obtain a NdFeB substrate.

[0017] Preferably, in the preparation of the NdFeB substrate magnet, the thickness of the rapidly solidified casting sheet is 0.1-0.4 mm; the average particle size of the fine powder is 2-4 μm; the vacuum sintering temperature is 1000-1100℃ and the time is 2-10 h; the two-stage aging treatment includes: the first-stage aging temperature is 850-950℃ and the time is 2-5 h, and the second-stage aging temperature is 400-700℃ and the time is 2-5 h.

[0018] Compared with existing technologies, this invention has the following advantages: By layering a low-melting-point heavy rare-earth alloy coating and a high-melting-point heavy rare-earth alloy coating on the magnet surface, and under a lower-temperature pre-diffusion heat treatment, the low-melting-point heavy rare-earth alloy effectively opens grain boundary diffusion channels, increases the diffusion depth of heavy rare-earth elements, and optimizes the uniform distribution of heavy rare-earth elements. Furthermore, by controlling the mass ratio and average particle size of the low-melting-point heavy rare-earth alloy coating and the heavy rare-earth alloy coating, this invention effectively improves the diffusion efficiency of heavy rare-earth elements and ensures the uniformity and stability of coercivity. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0020] The present invention proposes a method for preparing high-performance grain boundary diffusion sintered NdFeB materials, comprising the following steps: S1: coating and curing a low-melting-point heavy rare earth-free alloy coating onto the easily diffused surface of the NdFeB substrate magnet, and coating and curing a heavy rare earth alloy coating onto the low-melting-point heavy rare earth-free alloy coating; the mass ratio of the low-melting-point heavy rare earth-free alloy coating to the heavy rare earth alloy coating is 1: (2-4); S2: subjecting the coated magnet to a low-temperature pre-diffusion heat treatment, a grain boundary diffusion heat treatment, and an aging treatment in sequence to obtain a grain boundary diffusion NdFeB magnet.

[0021] In some preferred embodiments, the mass ratio of the single-sided mass of the low-melting-point heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is (0.1-0.5): 100; the mass ratio of the single-sided mass of the heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is (0.25-0.75): 100.

[0022] In some preferred embodiments, the average particle size of the heavy rare earth-free alloy is 2-4 μm; the average particle size of the heavy rare earth alloy is 2-4 μm.

[0023] In some preferred embodiments, the heavy rare earth-free alloy composition is LRE. x M 100-x By mass percentage, 70%≤x≤90%; the LRE is one or more of Pr, Nd, Ce, La, Sm, Eu, and Pm, and M is one or more of Al, Co, Cu, and Ga.

[0024] In some preferred embodiments, the heavy rare earth alloy has an HRE composition. x LRE y M 100-x-y By mass percentage, 70%≤x≤90%, 0%≤y≤20%; the HRE is one or more of Dy, Tb, Gd, Ho, and Y, the LRE is one or more of Pr, Nd, Ce, La, Sm, Eu, and Pm, and the M is one or more of Al, Co, Cu, Ga, Zr, Ti, Nb, Zn, Fe, and Li.

[0025] In some preferred embodiments, the neodymium iron boron substrate magnet has a composition of RE. x Fe 100-x-y-z B y M z By mass percentage, 29%≤x≤33%, 0.9%≤y≤1.02%; RE is one or more of Pr, Nd, Dy, Tb, La, Ce, Y, Gd, and Ho, and M is one or more of Al, Co, Cu, Ga, Zr, Ti, Nb, Zn, and Li.

[0026] In some preferred embodiments, in S2, the low-temperature pre-diffusion treatment temperature is 500-700℃, the grain boundary diffusion treatment temperature is 850-950℃, and the aging temperature is 400-700℃.

[0027] In some preferred embodiments, the low-melting-point heavy rare-earth alloy coating comprises, by mass percentage, 50-90% heavy rare-earth alloy, 10-30% solvent, and 1-6% binder; the heavy rare-earth alloy coating comprises 50-90% heavy rare-earth alloy, 10-30% solvent, and 1-6% binder. The heavy rare-earth alloy and heavy rare-earth alloy are respectively mixed with solvent and binder to form slurries, then coated in layers and cured to form a double-layer diffusion source coating with significantly different compositions.

[0028] In some preferred embodiments, the solvent in the low-melting-point heavy rare earth alloy coating and the heavy rare earth alloy coating is one or more of anhydrous ethanol, diethyl ether, acetaldehyde, propanol, acetone, methanol, tetrahydrofuran, and terpineol, and the binder is one or more of polyvinyl butyral, polyvinyl alcohol, polyethylene glycol, and polyvinylpyrrolidone.

[0029] In some preferred embodiments, the preparation of the neodymium iron boron substrate magnet includes the following steps: S1: preparing the raw material RE x Fe 100-x-y-z B y M z S1: Melting to obtain a rapidly solidified cast sheet; S2: After the rapidly solidified cast sheet is crushed by absorbing hydrogen to obtain coarse powder, it is then milled by air jet milling under oxygen-free conditions to obtain fine powder; S3: The fine powder is oriented and pressed into shape under conditions with an oxygen content ≤500ppm, and then isostatically pressed to obtain a NdFeB compact; S4: The NdFeB compact is subjected to vacuum sintering and two-stage aging treatment in sequence to obtain a NdFeB substrate.

[0030] In some preferred embodiments, during the preparation of the NdFeB substrate magnet, the thickness of the rapidly solidified cast sheet is 0.1-0.4 mm; the average particle size of the fine powder is 2-4 μm; the vacuum sintering temperature is 1000-1100℃ for 2-10 h; the two-stage aging treatment includes: a first-stage aging temperature of 850-950℃ for 2-5 h, and a second-stage aging temperature of 400-700℃ for 2-5 h. This invention requires selecting a suitable heat treatment temperature range based on different particle sizes and compositions.

[0031] In some preferred embodiments, in step S1 of the preparation of the NdFeB substrate magnet, the NdFeB raw material (RE) is smelted in a vacuum induction rapid solidification furnace. x Fe 100-x-y-z B y M zBy adjusting process parameters such as the rotation speed of the copper rod and the casting tilt angle, a rapid solidification casting sheet with a thickness of 0.1-0.4 mm can be obtained.

[0032] In some preferred embodiments, during S2 of the preparation of the NdFeB substrate magnet, the hydrogen absorption pressure is 0.08-0.15 MPa and the dehydrogenation temperature is 540-590℃ during the hydrogen annealing process; during the air jet milling process, the grinding gas pressure is 0.5-0.8 MPa, the classifying wheel speed is 3000-5000 rpm, and the average particle size of the powder after air jet milling is 2-4 μm, with D90 / D10 ≤ 5.0.

[0033] In some preferred embodiments, in step S3 of the preparation of the NdFeB substrate magnet, the magnetic field strength of the orientation pressing is 1.5-5T, and the isostatic pressing pressure is 150-300MPa.

[0034] In some preferred embodiments, during step S4 of the preparation of the NdFeB substrate magnet, the isostatically pressed NdFeB blank needs to be stripped and fed into the furnace in a stripping box with an oxygen content ≤500ppm to avoid oxidation of rare earth elements and ensure the magnetic properties of the magnet. After the NdFeB blank is placed in a vacuum sintering furnace and subjected to vacuum sintering and two-stage aging treatments, the NdFeB substrate is obtained. The NdFeB substrate is then cut into small pieces of appropriate size, and the surface is treated.

[0035] In some preferred embodiments, in step S4 of the preparation of the NdFeB substrate magnet, the sintered NdFeB substrate magnet is processed into small pieces by wire cutting or mechanical slicing, and the oxide layer on the easily diffused surface can be removed by methods such as sanding and acid etching. Finally, it is ultrasonically cleaned with anhydrous ethanol. The acid washing solution can be dilute nitric acid, citric acid, etc.

[0036] In some preferred embodiments, the preparation steps of the low-melting-point heavy rare-earth alloy and the heavy rare-earth alloy are the same as those of the neodymium iron boron substrate magnet, the difference being that the smelting raw materials are LRE. x M 100-x HRE x LRE y M 100-x-y .

[0037] Example 1: A method for preparing a high-performance grain boundary diffusion sintered NdFeB material, comprising the following steps: S1: Preparation of NdFeB substrate magnet, by mass percentage, sampling, PrNd 30.5%, Dy 0.5%, B 0.92%, Al 0.25%, Cu 0.2%, Co 0.8%, Ga 0.12%, Ti 0.18%, balance Fe to prepare raw materials (PrNd). 30.5% Dy 0.5% Fe 66.53%B 0.92% Al 0.25% Cu 0.2% Co 0.8% Ga 0.12% Ti 0.18% Rapidly solidified castings with an average thickness of 3±0.5 mm were prepared by vacuum rapid solidification furnace melting. Hydrogen crushing was then performed under a hydrogen absorption pressure of 0.12 MPa and a dehydrogenation temperature of 580 °C to obtain coarse hydrogen-crushed powder. This coarse powder was then subjected to an air jet mill at 3500 rpm to obtain fine powder with an SMD of 2.5 μm. The fine powder was then oriented and pressed into shape in a press with a magnetic field strength of 2.0 T under an oxygen content ≤500 ppm, and subjected to isostatic pressing at 250 MPa to obtain a compact. The compact was then sintered in a vacuum sintering furnace. The sintering temperature was 1070℃, the holding time was 5h, the first-stage aging temperature was 900h, the holding time was 2.5h, and the second-stage aging temperature was 500℃, the holding time was 5h, thus obtaining the sintered NdFeB substrate magnet; the substrate magnet was wire-cut into 20×20×4mm black sheets (4mm thickness direction is the orientation direction), the black sheets were sanded smooth and then ultrasonically cleaned with anhydrous ethanol; S2: low-melting-point heavy rare earth-free alloys with the composition Pr were prepared by rapid solidification casting, hydrogen crushing and air jet milling processes respectively. 70 Al 10 Cu 10 Ga 10 (By mass percentage, Pr 70%, Al 10%, Cu 10%, Ga 10%) and heavy rare earth alloy composition Tb 80 Pr 10 Cu5Co5 (by mass percentage, Tb 80%, Pr 10%, Cu 5%, Co 5%); Sampling was performed, and by mass percentage, a heavy rare earth alloy Pr was added. 70 Al 10 Cu 10 Ga 10 A heavy rare earth alloy-free coating slurry was prepared by combining 75% terpineol (solvent), 20% polyvinylpyrrolidone (binder), and 5% heavy rare earth alloy Tb. 80 Pr 10 A heavy rare earth alloy coating slurry was prepared by combining 75% Cu5Co5, 20% terpineol (solvent), and 5% polyvinylpyrrolidone (binder); S3: A low-melting-point heavy rare earth-free alloy (Pr 70 Al 10 Cu 10 Ga 10 The coating paste is applied and cured onto the orientation surface of the NdFeB substrate magnet, and the heavy rare earth alloy (Tb) is used. 80 Pr 10A Cu5Co5 coating slurry is applied and cured onto a low-melting-point heavy rare-earth alloy coating. The mass ratio of the low-melting-point heavy rare-earth alloy coating to the heavy rare-earth alloy coating is 1:2. The mass ratio of the single-sided mass of the low-melting-point heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is 0.2:100, and the mass ratio of the single-sided mass of the heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is 0.4:100. S4: The coated magnet is placed in a vacuum sintering furnace for heat treatment. The first stage of low-temperature pre-diffusion treatment is at a temperature of 600℃ and a holding time of 5h. The second stage of grain boundary diffusion treatment is at a temperature of 900℃ and a holding time of 10h. The aging temperature is at a temperature of 500℃ and a holding time of 5h, thus obtaining a grain boundary diffused NdFeB magnet.

[0038] Example 2: A method for preparing a high-performance grain boundary diffusion sintered NdFeB material, comprising the following steps: S1: Preparation of NdFeB substrate magnet, by mass percentage, sampling, PrNd 30.5%, Dy 0.5%, B 0.92%, Al 0.25%, Cu 0.2%, Co 0.8%, Ga 0.12%, Ti 0.18%, balance Fe to prepare raw materials (PrNd). 30.5% Dy 0.5% Fe 66.53% B 0.92% Al 0.25% Cu 0.2% Co 0.8% Ga 0.12% Ti 0.18% Rapidly solidified castings with an average thickness of 3±0.5 mm were prepared by vacuum rapid solidification furnace melting. Hydrogen crushing was then performed under a hydrogen absorption pressure of 0.12 MPa and a dehydrogenation temperature of 580 °C to obtain coarse hydrogen-crushed powder. This coarse powder was then subjected to an air jet mill at 3500 rpm to obtain fine powder with an SMD of 2.5 μm. The fine powder was then oriented and pressed into shape in a press with a magnetic field strength of 2.0 T under an oxygen content ≤500 ppm, and subjected to isostatic pressing at 250 MPa to obtain a compact. The compact was then sintered in a vacuum sintering furnace. The sintering temperature was 1070℃, the holding time was 5h, the first-stage aging temperature was 900h, the holding time was 2.5h, and the second-stage aging temperature was 500℃, the holding time was 5h, thus obtaining the sintered NdFeB substrate magnet; the substrate magnet was wire-cut into 20×20×4mm black sheets (4mm thickness direction is the orientation direction), the black sheets were sanded smooth and then ultrasonically cleaned with anhydrous ethanol; S2: low-melting-point heavy rare earth-free alloys with the composition Pr were prepared by rapid solidification casting, hydrogen crushing and air jet milling processes respectively. 70 Al 10 Cu 10 Ga 10 (By mass percentage, Pr 70%, Al 10%, Cu 10%, Ga 10%) and heavy rare earth alloy composition Tb80 Pr 10 Cu5Co5 (by mass percentage, Tb 80%, Pr 10%, Cu 5%, Co 5%); Sampling was performed, and by mass percentage, a heavy rare earth alloy Pr was added. 70 Al 10 Cu 10 Ga 10 A heavy rare earth alloy-free coating slurry was prepared by combining 75% terpineol (solvent), 20% polyvinylpyrrolidone (binder), and 5% heavy rare earth alloy Tb. 80 Pr 10 A heavy rare earth alloy coating slurry was prepared by combining 75% Cu5Co5, 20% terpineol (solvent), and 5% polyvinylpyrrolidone (binder); S3: A low-melting-point heavy rare earth-free alloy (Pr 70 Al 10 Cu 10 Ga 10 The coating paste is applied and cured onto the orientation surface of the NdFeB substrate magnet, and the heavy rare earth alloy (Tb) is used. 80 Pr 10 A Cu5Co5 coating slurry is applied and cured onto a low-melting-point heavy rare-earth alloy coating. The mass ratio of the low-melting-point heavy rare-earth alloy coating to the heavy rare-earth alloy coating is 1:3. The mass ratio of the single-sided mass of the low-melting-point heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is 0.1:100, and the mass ratio of the single-sided mass of the heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is 0.3:100. S4: The coated magnet is placed in a vacuum sintering furnace for heat treatment. The first stage of low-temperature pre-diffusion treatment is at a temperature of 600℃ and a holding time of 5h. The second stage of grain boundary diffusion treatment is at a temperature of 900℃ and a holding time of 10h. The aging temperature is at a temperature of 500℃ and a holding time of 5h, thus obtaining a grain boundary diffused NdFeB magnet.

[0039] Comparative Example 1: A method for preparing a grain boundary diffusion sintered NdFeB material, comprising the following steps: S1: Preparation of NdFeB substrate magnet, by mass percentage, sampling, PrNd 30.5%, Dy 0.5%, B 0.92%, Al 0.25%, Cu 0.2%, Co 0.8%, Ga 0.12%, Ti 0.18%, with the balance being Fe to prepare the raw material (PrNd). 30.5% Dy 0.5% Fe 66.53% B 0.92% Al 0.25% Cu 0.2% Co 0.8% Ga 0.12% Ti 0.18%Rapidly solidified castings with an average thickness of 3±0.5 mm were prepared by vacuum rapid solidification furnace melting. Hydrogen crushing was then performed under a hydrogen absorption pressure of 0.12 MPa and a dehydrogenation temperature of 580 °C to obtain coarse hydrogen-crushed powder. This coarse powder was then subjected to an air jet mill at 3500 rpm to obtain fine powder with an SMD of 2.5 μm. The fine powder was then oriented and pressed into shape in a press with a magnetic field strength of 2.0 T under an oxygen content ≤500 ppm, and subjected to isostatic pressing at 250 MPa to obtain a compact. The compact was then sintered in a vacuum sintering furnace. The sintering temperature was 1070℃, the holding time was 5h, the first-stage aging temperature was 900h, the holding time was 2.5h, and the second-stage aging temperature was 500℃, the holding time was 5h, thus obtaining the sintered NdFeB substrate magnet; the substrate magnet was wire-cut into 20×20×4mm black sheets (4mm thickness direction is the orientation direction), the black sheets were sanded smooth and then ultrasonically cleaned with anhydrous ethanol; S2: low-melting-point heavy rare earth-free alloys with the composition Pr were prepared by rapid solidification casting, hydrogen crushing and air jet milling processes respectively. 70 Al 10 Cu 10 Ga 10 (By mass percentage, Pr 70%, Al 10%, Cu 10%, Ga 10%) and heavy rare earth alloy composition Tb 80 Pr 10 Cu5Co5 (by mass percentage, Tb 80%, Pr 10%, Cu 5%, Co 5%); Sampling was performed, and by mass percentage, a heavy rare earth alloy Pr was added. 70 Al 10 Cu 10 Ga 10 A heavy rare earth alloy-free coating slurry was prepared by combining 75% terpineol (solvent), 20% polyvinylpyrrolidone (binder), and 5% heavy rare earth alloy Tb. 80 Pr 10 A heavy rare earth alloy coating slurry was prepared by combining 75% Cu5Co5, 20% terpineol (solvent), and 5% polyvinylpyrrolidone (binder); S3: A low-melting-point heavy rare earth-free alloy (Pr 70 Al 10 Cu 10 Ga 10 Coating paste and heavy rare earth alloy (Tb) 80 Pr 10The Cu5Co5) coating slurry is uniformly mixed, coated, and cured onto the orientation surface of the NdFeB substrate magnet. The mass ratio of the low-melting-point heavy rare-earth alloy coating to the heavy rare-earth alloy coating is 1:2, and the mass ratio of the single-sided mass of the mixed coating to the mass of the NdFeB substrate magnet is 0.6:100. Alternatively, the mass ratio of the single-sided mass of the low-melting-point heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is 0.2:100, and the mass ratio of the single-sided mass of the heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is 0.4:100. S4: The coated magnet is placed in a vacuum sintering furnace for heat treatment. The first stage of low-temperature pre-diffusion treatment is at 600℃ and held for 5 hours; the second stage of grain boundary diffusion treatment is at 900℃ and held for 10 hours; and the aging temperature is at 500℃ and held for 5 hours, thus obtaining a grain boundary diffused NdFeB magnet.

[0040] Comparative Example 2: A method for preparing a grain boundary diffusion sintered NdFeB material, comprising the following steps: S1: Preparation of NdFeB substrate magnet, by mass percentage, sampling, PrNd 30.5%, Dy 0.5%, B 0.92%, Al 0.25%, Cu 0.2%, Co 0.8%, Ga 0.12%, Ti 0.18%, with the balance being Fe to prepare the raw material (PrNd). 30.5% Dy 0.5% Fe 66.53% B 0.92% Al 0.25% Cu 0.2% Co 0.8% Ga 0.12% Ti 0.18% Rapidly solidified castings with an average thickness of 3±0.5 mm were prepared by vacuum rapid solidification furnace melting. Hydrogen crushing was then performed under a hydrogen absorption pressure of 0.12 MPa and a dehydrogenation temperature of 580 °C to obtain coarse hydrogen-crushed powder. This coarse powder was then subjected to an air jet mill at 3500 rpm to obtain fine powder with an SMD of 2.5 μm. The fine powder was then oriented and pressed into shape in a press with a magnetic field strength of 2.0 T under an oxygen content ≤500 ppm, and subjected to isostatic pressing at 250 MPa to obtain a compact. The compact was then sintered in a vacuum sintering furnace. The sintering temperature was 1070℃, the holding time was 5h, the first-stage aging temperature was 900h, the holding time was 2.5h, and the second-stage aging temperature was 500℃, the holding time was 5h, thus obtaining the sintered NdFeB substrate magnet; the substrate magnet was wire-cut into 20×20×4mm black sheets (4mm thickness direction is the orientation direction), the black sheets were sanded smooth and then ultrasonically cleaned with anhydrous ethanol; S2: low-melting-point heavy rare earth-free alloys with the composition Pr were prepared by rapid solidification casting, hydrogen crushing and air jet milling processes respectively. 70 Al 10 Cu 10 Ga 10(By mass percentage, Pr 70%, Al 10%, Cu 10%, Ga 10%) and heavy rare earth alloy composition Tb 80 Pr 10 Cu5Co5 (by mass percentage, Tb 80%, Pr 10%, Cu 5%, Co 5%); Sampling was performed, and by mass percentage, a heavy rare earth alloy Pr was added. 70 Al 10 Cu 10 Ga 10 A heavy rare earth alloy-free coating slurry was prepared by combining 75% terpineol (solvent), 20% polyvinylpyrrolidone (binder), and 5% heavy rare earth alloy Tb. 80 Pr 10 A heavy rare earth alloy coating slurry was prepared by combining 75% Cu5Co5, 20% terpineol (solvent), and 5% polyvinylpyrrolidone (binder); S3: A low-melting-point heavy rare earth-free alloy (Pr 70 Al 10 Cu 10 Ga 10 Coating paste and heavy rare earth alloy (Tb) 80 Pr 10 The Cu5Co5) coating slurry is uniformly mixed, coated, and cured onto the orientation surface of the NdFeB substrate magnet. The mass ratio of the low-melting-point heavy rare-earth alloy coating to the heavy rare-earth alloy coating is 1:2, and the mass ratio of the single-sided mass of the mixed coating to the mass of the NdFeB substrate magnet is 0.6:100. Alternatively, the mass ratio of the single-sided mass of the low-melting-point heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is 0.2:100, and the mass ratio of the single-sided mass of the heavy rare-earth alloy coating to the mass of the NdFeB substrate magnet is 0.4:100. S4: The coated magnet is placed in a vacuum sintering furnace for heat treatment. The grain boundary diffusion temperature is 900℃, the holding time is 10h, and the aging temperature is 500℃, the holding time is 5h, to obtain a grain boundary diffused NdFeB magnet.

[0041] The only difference between Comparative Example 3 and Example 1 is that in S4, the coated magnet was placed in a vacuum sintering furnace for heat treatment. The grain boundary diffusion temperature was 900°C and the holding time was 10h. The aging temperature was 500°C and the holding time was 5h, thus obtaining a grain boundary diffused NdFeB magnet.

[0042] Comparative Example 4: A method for preparing a grain boundary diffusion sintered NdFeB material, comprising the following steps: S1: Preparation of NdFeB substrate magnet, by mass percentage, sampling, PrNd 30.5%, Dy 0.5%, B 0.92%, Al 0.25%, Cu 0.2%, Co 0.8%, Ga 0.12%, Ti 0.18%, with the balance being Fe to prepare the raw material (PrNd). 30.5% Dy 0.5% Fe 66.53% B0.92% Al 0.25% Cu 0.2% Co 0.8% Ga 0.12% Ti 0.18% Rapidly solidified castings with an average thickness of 3±0.5 mm were prepared by vacuum rapid solidification furnace melting. Hydrogen crushing was then performed under a hydrogen absorption pressure of 0.12 MPa and a dehydrogenation temperature of 580 °C to obtain coarse hydrogen-crushed powder. This coarse powder was then subjected to an air jet mill at 3500 rpm to obtain fine powder with an SMD of 2.5 μm. The fine powder was then oriented and pressed into shape in a press with a magnetic field strength of 2.0 T under an oxygen content ≤500 ppm, and subjected to isostatic pressing at 250 MPa to obtain a compact. The compact was then sintered in a vacuum sintering furnace. The sintering temperature was 1070℃, the holding time was 5h, the first-stage aging temperature was 900h, the holding time was 2.5h, and the second-stage aging temperature was 500℃, the holding time was 5h, thus obtaining the sintered NdFeB substrate magnet; the substrate magnet was wire-cut into 20×20×4mm black sheets (4mm thickness direction is the orientation direction), the black sheets were sanded smooth and then ultrasonically cleaned with anhydrous ethanol; S2: low-melting-point heavy rare earth-free alloys with the composition Pr were prepared by rapid solidification casting, hydrogen crushing and air jet milling processes respectively. 70 Al 10 Cu 10 Ga 10 (By mass percentage, Pr 70%, Al 10%, Cu 10%, Ga 10%) and heavy rare earth alloy composition Tb 80 Pr 10 Cu5Co5 (by mass percentage, Tb 80%, Pr 10%, Cu 5%, Co 5%); Sampling was performed, and by mass percentage, a heavy rare earth alloy Pr was added. 70 Al 10 Cu 10 Ga 10 A heavy rare earth alloy-free coating slurry was prepared by combining 75% terpineol (solvent), 20% polyvinylpyrrolidone (binder), and 5% heavy rare earth alloy Tb. 80 Pr 10 A heavy rare earth alloy coating slurry was prepared by combining 75% Cu5Co5, 20% terpineol (solvent), and 5% polyvinylpyrrolidone (binder); S3: The heavy rare earth alloy (Tb) was then coated with the slurry. 80 Pr 10 Cu5Co5) coating paste is applied and cured onto the orientation surface of the NdFeB substrate magnet, and a low-melting-point heavy rare earth-free alloy (Pr) is formed. 70 Al 10 Cu 10 Ga 10The coating slurry is coated and cured on the heavy rare earth alloy coating; wherein, the mass ratio of the low melting point heavy rare earth alloy coating to the heavy rare earth alloy coating is 1:2, the mass ratio of the single side of the low melting point heavy rare earth alloy coating to the mass of the NdFeB substrate magnet is 0.2:100, and the mass ratio of the single side of the heavy rare earth alloy coating to the mass of the NdFeB substrate magnet is 0.4:100; S4: The coated magnet is placed in a vacuum sintering furnace for heat treatment. The first stage of low temperature pre-diffusion treatment is at a temperature of 600℃ and a holding time of 5h, the second stage of grain boundary diffusion treatment is at a temperature of 900℃ and a holding time of 10h, and the aging temperature is at a temperature of 500℃ and a holding time of 5h, thus obtaining a grain boundary diffused NdFeB magnet.

[0043] The only difference between Comparative Example 5 and Comparative Example 4 is that in Comparative Example 4, the coated magnet was placed in a vacuum sintering furnace for heat treatment. The grain boundary diffusion temperature was 900℃ and the holding time was 10h, and the aging temperature was 500℃ and the holding time was 5h, thus obtaining a grain boundary diffused NdFeB magnet.

[0044] The grain boundary diffusion sintered NdFeB magnets prepared in Examples 1-2 and Comparative Examples 1-5 were processed into small cylinders for magnetic performance testing. The test results are shown in Table 1 below. As shown in Table 1, this invention first coats a low-melting-point heavy rare-earth alloy coating onto the easily diffused surface of a NdFeB substrate magnet. By controlling the mass ratio of the low-melting-point heavy rare-earth alloy single-sided coating, the heavy rare-earth alloy single-sided coating, and the NdFeB substrate magnet, the low-melting-point heavy rare-earth alloy preferentially opens grain boundary channels. Under a pre-diffusion heat treatment at a lower temperature, the uniform diffusion of the heavy rare-earth alloy is promoted, remanence loss is reduced, squareness is improved, and while ensuring high coercivity, the stability of the coercivity is also improved.

[0045] Based on the test results in Table 1, and through Example 1 and Comparative Example 1, it can be seen that the overall magnetic performance of the double-layer coated diffusion magnet with a low-melting-point heavy rare-earth-free diffusion source and a heavy rare-earth alloy diffusion source is better than that of the uniformly mixed coated diffusion magnet with the same weight gain. Comparative Example 3, compared to Example 1, did not undergo low-temperature pre-diffusion treatment but directly entered high-temperature grain boundary diffusion treatment. As shown in the test results, its coercivity and squareness were significantly reduced. Direct high-temperature sintering caused the low-melting-point heavy rare-earth-free alloy to fail to diffuse uniformly along the grain boundaries to form a continuous grain boundary phase, resulting in a decrease in heavy rare-earth diffusion efficiency. Furthermore, through Comparative Examples 1-2, it can be seen that the performance of the diffusion magnet after lower pre-diffusion temperature treatment is also significantly better than that of the magnet treated at conventional diffusion temperatures.

[0046] Comparative Example 4, by first coating a heavy rare earth alloy coating onto the easily diffused surface of a NdFeB substrate magnet, and then coating a low-melting-point heavy rare earth-free alloy coating on top of the heavy rare earth alloy coating, showed a significantly larger remanence loss, weaker demagnetization resistance, and less significant improvement in coercivity compared to Example 1. This invention, by diffusing heavy rare earth elements along optimized grain boundaries to form a highly anisotropic core-shell structure, increases the diffusion depth of heavy rare earth elements, effectively widening the uniform diffusion channels, effectively reducing remanence loss, and improving demagnetization resistance. Compared to Comparative Example 4, which did not undergo low-temperature pre-diffusion treatment, the performance degradation of this example was more significant, further demonstrating that after pre-diffusion treatment with a low-melting-point heavy rare earth-free diffusion source, the grain boundary phase of the magnet was optimized, the melting point of the grain boundary phase decreased, and its continuity was enhanced. Therefore, the diffusion efficiency of heavy rare earth elements was higher, the diffusion depth was deeper, and the magnet achieved a higher level of coercivity improvement. Furthermore, the overall uniformity of heavy rare earth elements in the diffused magnet was improved, and the loss of remanence after diffusion was also less.

Claims

1. A method for preparing high-performance grain boundary diffusion sintered NdFeB materials, characterized in that, Includes the following steps: S1: A low-melting-point heavy rare-earth alloy coating is applied and cured onto the easily diffused surface of the NdFeB substrate magnet, and a heavy rare-earth alloy coating is applied and cured onto the low-melting-point heavy rare-earth alloy coating; S2: The coated magnet is subjected to a low-temperature pre-diffusion heat treatment, a grain boundary diffusion heat treatment, and an aging treatment in sequence to obtain a grain boundary diffused NdFeB magnet.

2. The method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 1, characterized in that, In S2, the low-temperature pre-diffusion treatment temperature is 500-700℃, the grain boundary diffusion treatment temperature is 850-950℃, and the aging temperature is 400-700℃.

3. The method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 1, characterized in that, The mass ratio of the single-sided mass of the low-melting-point non-heavy rare earth alloy coating to the mass of the NdFeB substrate magnet is (0.1-0.5):100; the mass ratio of the single-sided mass of the heavy rare earth alloy coating to the mass of the NdFeB substrate magnet is (0.25-0.75):

100.

4. A method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 1 or 2, characterized in that, By mass percentage, the low-melting-point heavy rare-earth-free alloy coating comprises 50-90% heavy rare-earth-free alloy, 10-30% solvent, and 1-6% binder; the heavy rare-earth alloy coating comprises 50-90% heavy rare-earth alloy, 10-30% solvent, and 1-6% binder.

5. The method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 3, characterized in that, The heavy rare earth-free alloy has an LRE composition. x M 100-x By mass percentage, 70%≤x≤90%; the LRE is one or more of Pr, Nd, Ce, La, Sm, Eu, and Pm, and M is one or more of Al, Co, Cu, and Ga.

6. The method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 3, characterized in that, The heavy rare earth alloy has the composition HRE. x LRE y M 100-x-y By mass percentage, 70%≤x≤90%, 0%≤y≤20%; the HRE is one or more of Dy, Tb, Gd, Ho, and Y, the LRE is one or more of Pr, Nd, Ce, La, Sm, Eu, and Pm, and the M is one or more of Al, Co, Cu, Ga, Zr, Ti, Nb, Zn, Fe, and Li.

7. A method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 1 or 2, characterized in that, The neodymium iron boron substrate magnet has the following composition: RE x Fe 100-x-y-z B y M z By mass percentage, 29%≤x≤33%, 0.9%≤y≤1.02%; RE is one or more of Pr, Nd, Dy, Tb, La, Ce, Y, Gd, and Ho, and M is one or more of Al, Co, Cu, Ga, Zr, Ti, Nb, Zn, and Li.

8. The method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 3, characterized in that, In the low-melting-point heavy rare earth alloy coating and the heavy rare earth alloy coating, the solvent is one or more of anhydrous ethanol, diethyl ether, acetaldehyde, propanol, acetone, methanol, tetrahydrofuran, and terpineol, and the binder is one or more of polyvinyl butyral, polyvinyl alcohol, polyethylene glycol, and polyvinylpyrrolidone.

9. The method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 7, characterized in that, The preparation of the neodymium iron boron substrate magnet includes the following steps: S1: preparing the raw material RE x Fe 100-x-y-z B y M z S1: Melting to obtain a rapidly solidified cast sheet; S2: After the rapidly solidified cast sheet is crushed by absorbing hydrogen to obtain coarse powder, it is then milled by air jet milling under oxygen-free conditions to obtain fine powder; S3: The fine powder is oriented and pressed into shape under conditions with an oxygen content ≤500ppm, and then isostatically pressed to obtain a NdFeB compact; S4: The NdFeB compact is subjected to vacuum sintering and two-stage aging treatment in sequence to obtain a NdFeB substrate.

10. The method for preparing a high-performance grain boundary diffusion sintered NdFeB material according to claim 9, characterized in that, In the preparation of the NdFeB substrate magnet, the thickness of the rapidly solidified cast sheet is 0.1-0.4 mm; the average particle size of the fine powder is 2-4 μm. Vacuum sintering temperature is 1000-1100℃, time is 2-10h; two-stage aging treatment includes: first-stage aging temperature is 850-950℃, time is 2-5h, and second-stage aging temperature is 400-700℃, time is 2-5h.

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