A method for producing a fine-grain sintered neodymium-iron-boron magnet

Abstract

The application discloses a preparation method of fine-grain sintered Nd-Fe-B magnets, wherein a first stage is set to have a sintering temperature lower than that of a second stage, and a plurality of temperature points and corresponding holding time are set for each stage according to a preset gradient, so that the sintering curve of each sintering stage is in a ladder shape; the holding time is determined based on the temperature points and average grain size expectation values corresponding to the temperature points; the correlation among the holding time, the average grain size expectation values and the temperature points is established, so that the temperature points and the holding time are matched with the average grain size expectation values, and thus the actual average grain size in the sintering process is kept near the corresponding average grain size expectation value; and the experience selection of the sintering temperature and the holding time is changed into scientific calculation, so that the densification is realized, the average grain size of the final magnet can be accurately controlled to be below 3 µm, and the contradiction between the densification and the grain growth is successfully solved.

Description

Technical Field

[0001] This invention relates to sintered NdFeB magnets, and more particularly to a method for preparing fine-grained sintered NdFeB magnets. Background Technology

[0002] Sintered neodymium iron boron (Nd-Fe-B) permanent magnets play an irreplaceable role in modern industry due to their extremely high magnetic energy product, and are widely used in new energy vehicles, wind power generation, precision instruments, and other fields. The key performance indicators of sintered Nd-Fe-B magnets, especially the intrinsic coercivity (Hcj), are closely related to their microstructure. Based on the Stoner-Wohlfarth single-domain theory, refining the main phase Nd2Fe in sintered Nd-Fe-B magnets... 14 B-grains are one of the most effective ways to improve their coercivity.

[0003] Currently, the average grain size of commercially available sintered NdFeB magnets is typically around 5µm. To achieve higher coercivity, the industry has been working to reduce the grain size to below 3µm. However, this goal faces a fundamental technical contradiction: the conflict between densification and grain growth.

[0004] In traditional sintering processes, achieving high density (>99% of theoretical density) requires high sintering temperatures (typically 1040-1080℃) and long holding times. However, this leads to rapid grain growth, even coarsening the originally fine precursor powder to over 5µm, making it impossible to achieve a fine-grained structure. If low-temperature sintering is used to suppress grain growth, the diffusion rate is insufficient, making it difficult to complete the densification process. This results in a large number of pores in the sintered NdFeB magnets, which in turn leads to a significant decrease in remanence and energy product.

[0005] To address this contradiction, researchers have explored various methods. For example, a two-step sintering method has been employed, involving initial densification at a lower temperature followed by final sintering at a slightly higher temperature. However, the temperature selection and time control in existing two-step sintering processes largely rely on experience, making it difficult to stably achieve a balance between complete densification and grain size control within a wide process window. Furthermore, ultrafine powders (D50 < 3µm) exhibit extremely high sintering activity, and their grain growth kinetics differ significantly from those of conventional powders. Traditional process parameters are difficult to apply directly, easily leading to abnormal grain growth or insufficient densification. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for preparing fine-grained sintered NdFeB magnets that can achieve densification while keeping the grain size stably controlled below 3µm.

[0007] The technical solution adopted by this invention to solve the above-mentioned technical problem is as follows: a method for preparing fine-grained sintered NdFeB magnets, comprising first preparing NdFeB powder, then pressing the NdFeB powder into a green blank, and then sintering the green blank. The specific method for sintering the green blank is as follows: under an inert gas protective atmosphere, two stages of sintering process are performed sequentially, followed by aging treatment, to obtain the fine-grained sintered NdFeB magnet; wherein, the sintering temperature range of the first stage is 850℃-900℃, and M temperature points are selected from this temperature range according to a preset gradient, where M is an integer greater than or equal to 3, and a corresponding average grain size period is set for each temperature point. The expected average grain size is determined by the following steps: First, the temperature is increased sequentially to each temperature point in ascending order, and then held for a set holding time to implement the first stage sintering process. Second, the sintering temperature range is 970℃-1000℃. F temperature points are selected from this range based on a preset gradient, where F is an integer greater than or equal to 3. A corresponding expected average grain size is set for each temperature point. Then, the temperature is increased sequentially to each temperature point in ascending order, and held for a set holding time to implement the second stage sintering process. The holding time for each temperature point in both stages of the sintering process is calculated using formula (1): (1) In equation (1), T represents the temperature point, and the unit is K; K 0 indicates the pre-exponential factor (μm) n / s), which was obtained in advance through experimental fitting, is approximately 1.77906E+17. t The holding time at temperature point T is expressed in seconds (s). d 0 This represents the initial average grain size at temperature T, in μm. d 1 represents the expected average grain size at temperature T, in μm; n This represents the grain growth index, with a value ranging from 25 to 30, and a preferred value of 28. e Represents the base of the natural logarithm; Q represents the grain boundary diffusion activation energy, ranging from 100 to 300, with units of kJ / mol; R represents the molar gas constant, with a value of 8.314 J / (mol·K); in the first stage, the expected average grain size at the previous temperature point is the initial average grain size at the next temperature point, and the initial average grain size at the first temperature point is the average grain size of the green body; in the second stage, the expected average grain size at the previous temperature point is the initial average grain size at the next temperature point, and the initial average grain size at the first temperature point is the expected average grain size at the last temperature point of the first stage, with the expected average grain size at the last temperature point being 3 μm.

[0008] Compared with the prior art, the advantages of the present invention are that when the sintering process is carried out by the two-step sintering method, that is, the two-stage sintering process, the sintering temperature of the first stage is set to be lower than that of the second stage, and multiple temperature points and corresponding holding times are set for each stage according to the preset gradient, so that the sintering curve of each sintering stage is stepped. The holding time is determined based on the temperature point value and the expected value of the average grain size corresponding to the temperature point. The expected value of the average grain size is the test value obtained by experimental testing or the average of multiple test values. Formula (1) is constructed based on the grain growth kinetic model, and the relationship between the holding time, the expected value of the average grain size and the temperature point is established, so that the temperature point and the holding time are matched with the expected value of the average grain size, thereby keeping the actual average grain size near the corresponding expected value of the average grain size during the sintering process. Thus, the present invention transforms the empirical selection of sintering temperature and holding time into scientific calculation, thereby achieving densification while accurately controlling the average grain size of the final magnet below 3µm, successfully solving the contradiction between densification and grain growth.

[0009] Furthermore, M=6, F=4; the six temperature points in the first stage are 850℃, 860℃, 870℃, 880℃, 890℃, and 900℃, with corresponding expected average grain sizes of 2.2μm, 2.25μm, 2.3μm, 2.35μm, 2.4μm, and 2.45μm, respectively; the four temperature points in the second stage are 970℃, 980℃, 990℃, and 1000℃, with corresponding expected average grain sizes of 2.7μm, 2.8μm, 2.9μm, and 3μm, respectively.

[0010] Furthermore, the specific process for preparing NdFeB powder is as follows: Step 1: Preparation of NdFeB alloy rapidly solidified sheets: Molten NdFeB raw materials are poured onto the surface of a high-speed rotating copper roller with internal double-spiral groove cooling channels for rapid cooling, resulting in NdFeB alloy rapidly solidified sheets with a thickness of 0.15mm-0.25mm. The microstructure consists of Nd2Fe alloy particles with a size ≤3μm. 14 B consists of primary grains and uniformly distributed grain boundary phases; Step 2: The neodymium iron boron alloy rapid solidification sheet is subjected to hydrogen crushing and air jet milling under inert gas protection to prepare an ultrafine narrow distribution powder with a median particle size D50 of 1.8μm-2.5μm and a particle size distribution value [(D90-D10) / D50]≤3.0; Step 3: Under an argon or nitrogen protective atmosphere, the ultrafine narrow distribution powder is uniformly mixed with nano-alumina powder, which serves as a grain boundary growth inhibitor, to obtain NdFeB powder; the proportion of nano-alumina powder in the NdFeB powder is 0.03wt%-0.08wt%.

[0011] Furthermore, the double spiral groove cooling water channel consists of two parallel spiral grooves with opposite directions of rotation. The cross-sectional area of ​​the spiral groove is 0.8%-1.5% of the working area of ​​the copper roller, the spiral angle is 25°-35°, the cooling water inlet pressure is ≥0.6MPa, the flow rate is ≥8m / s, and the temperature fluctuation of the copper roller surface is controlled within ±3℃.

[0012] Furthermore, the ratio of the base circle diameter D to the height H of the sorting wheel in the air jet mill is 1.8 to 2.2, and the rotational speed of the sorting wheel is 4200 rpm to 4800 rpm.

[0013] Furthermore, the particle size of the nano-alumina powder is 30 nm to 50 nm.

[0014] Furthermore, the specific method for pressing NdFeB powder into green blanks is as follows: under the conditions of pulse magnetic field strength ≥2T and pressing pressure of 60MPa-100MPa, NdFeB powder is pressed into green blanks.

[0015] Furthermore, the timeliness processing is a two-level or multi-level timeliness processing. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to the embodiments.

[0017] Example 1: A method for preparing a fine-grained sintered NdFeB magnet. First, NdFeB powder is prepared, then the NdFeB powder is pressed into a green compact, and then the green compact is sintered. The specific sintering method for the green compact is as follows: under an inert gas protective atmosphere, two stages of sintering are performed sequentially, followed by aging treatment to obtain a fine-grained sintered NdFeB magnet. The first stage sintering temperature range is 850℃-900℃. M temperature points are selected from this temperature range according to a preset gradient, where M is an integer greater than or equal to 3. A corresponding expected average grain size is set for each temperature point, and then the grains are arranged in ascending order of size. The first stage sintering process is carried out by sequentially heating to each temperature point and holding it for a set time. The second stage sintering temperature range is 970℃-1000℃. F temperature points are selected from this temperature range according to the preset gradient, where F is an integer greater than or equal to 3. The expected average grain size is set for each temperature point. Then, the temperature is sequentially heated to each temperature point in ascending order and held for a set time to carry out the second stage sintering process. The holding time for each temperature point in the two stages of sintering process is calculated using formula (1): (1) In equation (1), T represents the temperature point, and the unit is K; K 0 indicates the pre-exponential factor (μm)n / s), which can be obtained through experimental fitting, is approximately 1.77906E+17, t The holding time at temperature point T is expressed in seconds (s). d 0 This represents the initial average grain size at temperature T, in μm. d 1 represents the expected average grain size at temperature T, in μm; n This represents the grain growth index, with a value ranging from 25 to 30, and a preferred value of 28. e Represents the base of the natural logarithm; Q represents the grain boundary diffusion activation energy, ranging from 100 to 300, with units of kJ / mol; R represents the molar gas constant, with a value of 8.314 J / (mol·K); in the first stage, the expected average grain size at the previous temperature point is the initial average grain size at the next temperature point, and the initial average grain size at the first temperature point is the average grain size of the green body (the average grain size of the green body is characterized by the median particle size D50 of the NdFeB powder); in the second stage, the expected average grain size at the previous temperature point is the initial average grain size at the next temperature point, and the initial average grain size at the first temperature point is the expected average grain size at the last temperature point of the first stage, with the expected average grain size at the last temperature point being 3 μm.

[0018] In this embodiment, when implementing the sintering process using a two-step sintering method (i.e., a two-stage sintering process), the sintering temperature of the first stage is set to be lower than that of the second stage. Multiple temperature points and corresponding holding times are set for each stage, so that the sintering curve of each sintering stage is stepped. The holding time is determined based on the temperature point value and the expected average grain size corresponding to the temperature point. The expected average grain size is the test value obtained through experimental testing or the average of multiple test values. A correlation is established between the holding time, the expected average grain size, and the temperature point, so that the temperature point and the holding time match the expected average grain size. This ensures that the actual average grain size during the sintering process remains near the corresponding expected average grain size. Thus, the empirical selection of sintering temperature and holding time is transformed into scientific calculation, thereby achieving densification while accurately controlling the average grain size of the final magnet to below 3µm, successfully resolving the contradiction between densification and grain growth.

[0019] Example 2: This example is basically the same as Example 1, except that: in this example, M=6, F=4; the six temperature points in the first stage are 850℃, 860℃, 870℃, 880℃, 890℃, and 900℃, and the corresponding expected average grain size values ​​are 2.2μm, 2.25μm, 2.3μm, 2.35μm, 2.4μm, and 2.45μm, respectively; the four temperature points in the second stage are 970℃, 980℃, 990℃, and 1000℃, and the corresponding expected average grain size values ​​are 2.7μm, 2.8μm, 2.9μm, and 3μm, respectively.

[0020] Example 3: This example is basically the same as Example 1, except that the specific process for preparing NdFeB powder in this example is as follows: Step 1: Preparation of NdFeB alloy rapidly solidified sheets: Molten NdFeB raw materials are poured onto the surface of a high-speed rotating copper roller with internal double-spiral groove cooling channels for rapid cooling, resulting in NdFeB alloy rapidly solidified sheets with a thickness of 0.15mm-0.25mm. The microstructure consists of Nd2Fe alloy particles with a size ≤3μm. 14 B consists of primary grains and uniformly distributed grain boundary phases; Step 2: Under inert gas protection, the rapidly solidified NdFeB alloy flakes are subjected to hydrogen crushing and air jet milling to prepare an ultrafine narrow-distribution powder with a median particle size D50 of 1.8μm-2.5μm and a particle size distribution value [(D90-D10) / D50]≤3.0. Step 3: Under an argon or nitrogen protective atmosphere, the ultrafine narrow-distribution powder is uniformly mixed with nano-alumina powder, which serves as a grain boundary growth inhibitor, to obtain NdFeB powder; the particle size of the nano-alumina powder is 30 nm. The proportion of nano-alumina powder in the NdFeB powder is 0.03wt%-0.08wt%.

[0021] In this embodiment, a dual mechanism of "raw material optimization" and "process inhibition" is introduced. Ultrafine, narrowly distributed powder is the material basis for achieving a fine-grained structure; nano-alumina powder, as a grain boundary growth inhibitor, actively suppresses grain boundary migration chemically during sintering, further inhibiting abnormal grain growth. This dual inhibition mechanism, combined with the step-like two-stage low-temperature sintering control, resolves the contradiction between "densification and grain growth." Example 4: This example is basically the same as Example 3, except that the particle size of the nano-alumina powder is 50 nm.

[0022] Example 5: This example is basically the same as Example 3, except that: in this example, the double spiral groove cooling water channel consists of two parallel spiral grooves with opposite directions of rotation. The cross-sectional area of ​​the spiral groove is 0.8%-1.5% of the working area of ​​the copper roller, the spiral angle is 25°-35°, the cooling water inlet pressure is ≥0.6MPa, the flow velocity is ≥8m / s, and the temperature fluctuation of the copper roller surface is controlled within ±3℃. The ratio of the base circle diameter D to the height H of the sorting wheel in the air jet mill is 1.8 to 2.2, and the sorting wheel speed is 4200 rpm to 4800 rpm.

[0023] Example 6: This example is basically the same as Example 3, except that: in this example, the specific method of pressing NdFeB powder into a green blank is as follows: under the conditions of pulse magnetic field strength ≥2T and pressing pressure of 60MPa-100MPa, NdFeB powder is pressed into a green blank.

[0024] Example 7: This example is basically the same as Example 3, except that the time-sensitivity processing in this example is two-level or multi-level time-sensitivity processing.

[0025] Before actual implementation, the method for preparing fine-grained sintered NdFeB magnets of the present invention selects a required number of temperature points within its sintering temperature range according to its preset gradient at each stage. These temperature points can be evenly spaced or non-uniformly distributed. Then, through actual testing or experimental fitting, the average grain size or the average value of the average grain size at these temperature points is obtained as the expected value of the average grain size (i.e., the target value of the average grain size). The more temperature points selected, the closer the actual average grain size at each temperature point will be to its expected value of average grain size during the actual preparation process, and the higher the accuracy of the average grain size.

[0026] In summary, this invention transforms the empirical selection of sintering temperature and holding time into scientific calculation, thereby achieving densification while precisely controlling the average grain size of the final magnet to below 3µm, successfully resolving the contradiction between densification and grain growth.

Claims

1. A method for preparing a fine-grained sintered NdFeB magnet, comprising first preparing NdFeB powder, then pressing the NdFeB powder into a green blank, and then sintering the green blank, characterized in that, The specific method for sintering the green blank is as follows: under an inert gas protective atmosphere, two stages of sintering process are performed sequentially, followed by aging treatment, to obtain the fine-grained sintered NdFeB magnet; wherein, the first stage sintering temperature range is 850℃-900℃, M temperature points are selected from this temperature range according to a preset gradient, M is an integer greater than or equal to 3, and a corresponding expected value of average grain size is set for each temperature point, and then the temperature is raised to each temperature point in ascending order and held for a period of time according to the set holding time to implement the first stage sintering process; the second stage sintering temperature range is 970℃-1000℃, F temperature points are selected from this temperature range according to a preset gradient, F is an integer greater than or equal to 3, and a corresponding expected value of average grain size is set for each temperature point, and then the temperature is raised to each temperature point in ascending order and held for a period of time according to the set holding time to implement the second stage sintering process; in the two stages of sintering process, the holding time of each temperature point is calculated by formula (1): （1） In equation (1), T represents the temperature point, and the unit is K; K 0 indicates the pre-exponential factor, with units of μm. n / s; t The holding time at temperature point T is expressed in seconds (s). d 0 This represents the initial average grain size at temperature T, in μm. d 1 represents the expected average grain size at temperature T, in μm; n This represents the grain growth index, with a value range of 25 to 30. e Represents the base of the natural logarithm; Q represents the grain boundary diffusion activation energy, ranging from 100 to 300, with units of kJ / mol; R represents the molar gas constant, with a value of 8.314 J / (mol·K); in the first stage, the expected average grain size at the previous temperature point is the initial average grain size at the next temperature point, and the initial average grain size at the first temperature point is the average grain size of the green body; in the second stage, the expected average grain size at the previous temperature point is the initial average grain size at the next temperature point, and the initial average grain size at the first temperature point is the expected average grain size at the last temperature point of the first stage, with the expected average grain size at the last temperature point being 3 μm.

2. The method for preparing fine-grained sintered NdFeB magnets according to claim 1, characterized in that, M=6, F=4; the six temperature points in the first stage are 850℃, 860℃, 870℃, 880℃, 890℃, and 900℃, with corresponding expected average grain sizes of 2.2μm, 2.25μm, 2.3μm, 2.35μm, 2.4μm, and 2.45μm, respectively; the four temperature points in the second stage are 970℃, 980℃, 990℃, and 1000℃, with corresponding expected average grain sizes of 2.7μm, 2.8μm, 2.9μm, and 3μm, respectively.

3. The method for preparing fine-grained sintered NdFeB magnets according to claim 1, characterized in that, The specific process for preparing NdFeB powder is as follows: Step 1: Preparation of NdFeB alloy rapidly solidified sheets: Molten NdFeB raw materials are poured onto the surface of a high-speed rotating copper roller with internal double-spiral groove cooling channels for rapid cooling, resulting in NdFeB alloy rapidly solidified sheets with a thickness of 0.15mm-0.25mm. The microstructure consists of Nd2Fe alloy particles with a size ≤3μm. 14 B consists of primary grains and uniformly distributed grain boundary phases; Step 2: The neodymium iron boron alloy rapid solidification sheet is subjected to hydrogen crushing and air jet milling under inert gas protection to prepare an ultrafine narrow distribution powder with a median particle size D50 of 1.8μm-2.5μm and a particle size distribution value [(D90-D10) / D50]≤3.0; Step 3: Under an argon or nitrogen protective atmosphere, the ultrafine narrow distribution powder is uniformly mixed with nano-alumina powder, which serves as a grain boundary growth inhibitor, to obtain NdFeB powder; the proportion of nano-alumina powder in the NdFeB powder is 0.03wt%-0.08wt%.

4. The method for preparing the fine-grained sintered NdFeB magnet according to claim 3, characterized in that, The double spiral groove cooling water channel consists of two parallel spiral grooves with opposite directions of rotation. The cross-sectional area of ​​the spiral groove is 0.8%-1.5% of the working area of ​​the copper roller, the spiral angle is 25°-35°, the cooling water inlet pressure is ≥0.6MPa, the flow rate is ≥8m / s, and the temperature fluctuation of the copper roller surface is controlled within ±3℃.

5. The method for preparing the fine-grained sintered NdFeB magnet according to claim 3, characterized in that, The ratio of the base circle diameter D to the height H of the sorting wheel in the air jet mill is 1.8 to 2.2, and the rotational speed of the sorting wheel is 4200 rpm to 4800 rpm.

6. The method for preparing the fine-grained sintered NdFeB magnet according to claim 3, characterized in that, The particle size of the nano-alumina powder is 30 nm to 50 nm.

7. The method for preparing the fine-grained sintered NdFeB magnet according to claim 3, characterized in that, The specific method for pressing NdFeB powder into green blanks is as follows: under the conditions of pulse magnetic field strength ≥2T and pressing pressure of 60MPa-100MPa, NdFeB powder is pressed into green blanks.

8. The method for preparing the fine-grained sintered NdFeB magnet according to claim 3, characterized in that, The timeliness processing is a two-level or multi-level timeliness processing.

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