A high-remanence near-stoichiometric neodymium-iron-boron magnet and a method of making the same

Abstract

This invention discloses a high-remanence near-positive NdFeB magnet and its preparation method. The preparation method includes: melting and preparing NdFeB alloy rapid solidification sheets according to the element ratio of the main phase alloy; subjecting the NdFeB alloy rapid solidification sheets to hydrogen crushing and secondary air jet milling to obtain near-positive NdFeB magnetic powder with low rare earth content; and subjecting the near-positive NdFeB magnetic powder to orientation molding, cyclic sintering, and tempering heat treatment to obtain a high-remanence near-positive NdFeB magnet. This invention, through synergistic optimization of composition design and process, avoids the generation of impurity phases and abnormal grain growth, achieving a high main phase ratio and sufficient densification of the magnet, significantly improving remanence performance. The process is simple and easy to implement, suitable for mass industrial production.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnet material production technology, specifically relating to a high remanence near-positive NdFeB magnet and its preparation method. Background Technology

[0002] The remanence of NdFeB permanent magnet materials directly reflects the strength of their external magnetic field and determines the magnetic circuit design configuration of permanent magnet devices. Higher remanence in NdFeB magnets allows for lighter, smaller devices with higher energy efficiency. Even a slight increase in remanence can advance permanent magnet applications, and the remanence level also reflects the material's manufacturing level and embodies the core competitiveness of related manufacturers. Therefore, improving the remanence of NdFeB magnets has become a focus in the rare-earth permanent magnet field. Research shows that NdFeB (Nd2Fe) magnets... 14 B) The theoretical remanence limit of permanent magnet materials is about 16.0 kGs. At present, the remanence of industrially produced sintered NdFeB magnets is still far from the theoretical value (usually less than ~15.0 kGs, indicating room for improvement).

[0003] Neodymium iron boron magnets are typically manufactured using powder metallurgy processes (including: rapid solidification casting, hydrogen crushing, air jet milling, orientation forming, sintering, and tempering). The empirical formula for their remanence (Br) is: (Where, A is the positive domain volume fraction, mainly affected by the proportion of the main phase and the technical magnetization process; β is the volume fraction of non-ferromagnetic dopants, that is, the closer the NdFeB alloy composition is to the positive phase ratio of 2:14:1, the larger the value of 1-β; d) 磁体 With d 理 These refer to the actual density of magnets prepared by powder metallurgy and the theoretical density upper limit of neodymium iron boron materials, respectively, indicating that the higher the actual densification level of the magnet, the closer the remanence is to the upper limit; The degree of magnet orientation is mainly affected by the orientation forming process, and further optimization is difficult; Js is the saturation magnetic polarization of the material, mainly affected by the intrinsic composition characteristics of the material. Therefore, according to the remanence formula, while ensuring sufficient magnet densification, the closer the composition of the NdFeB alloy is to the positive phase ratio of 2:14:1, and the higher the proportion of the main phase in the magnet, the higher the level of remanence that the NdFeB magnet can achieve.

[0004] In the actual production and preparation of magnets, the rare earth content in the NdFeB alloy composition is usually designed to be between 29.0 wt% and 32.0 wt%. This is because when the NdFeB alloy composition is close to the positive phase ratio of 2:14:1 (the rare earth content is ~26.68 wt%), a large number of impurity phases will be generated in the rapidly solidified casting during the magnet preparation process, resulting in a decrease in the proportion of the main phase. At the same time, the columnar crystal morphology of the rapidly solidified sheet deteriorates, and the rare earth-rich phase is missing or unevenly distributed, which hinders the hydrogen fragmentation process that relies on the hydrogen absorption and fracture or fragmentation of the rare earth-rich phase, which is not conducive to subsequent powder preparation. Furthermore, since the NdFeB powder metallurgy process requires a sintering densification process, conventional methods necessitate the presence of excess rare-earth-rich phases as sintering agents to participate in the liquid-phase sintering of NdFeB powder in order to achieve sufficient sintering densification of the magnet. However, reducing the rare-earth content of the alloy by using NdFeB close to the positive fraction will result in insufficient or absent sintering agents, making it difficult for the magnet to achieve a high level of densification. Moreover, due to the lack of low-melting-point rare-earth-rich melting channels, atomic migration is hindered, and abnormal grain growth such as grain engulfment is prone to occur. This makes it difficult to achieve both magnet densification and normal grain growth, which is not conducive to the final magnet simultaneously obtaining high remanence and high coercivity properties. This means that if we can solve the problems of impurity phases easily generated in the rapid solidification casting of near-positive NdFeB magnets with low rare earth content, the difficulty in powder preparation caused by rich rare earth distribution, and the contradiction between densification and grain growth, we will be able to prepare high remanence NdFeB magnets with low rare earth content. This will break through the NdFeB remanence bottleneck and is of great significance to the development of NdFeB permanent magnet materials and grain boundary diffusion technology. Summary of the Invention

[0005] The main objective of this invention is to provide a high remanence near-positive NdFeB magnet and its preparation method, so as to overcome the shortcomings of the prior art.

[0006] To achieve the aforementioned objectives, the technical solution adopted by this invention includes:

[0007] This invention provides a method for preparing a near-positive NdFeB magnet with high remanence, comprising:

[0008] Neodymium iron boron alloy rapid solidification sheets were prepared by melting according to the elemental ratio of the main phase alloy; wherein, the chemical formula of the main phase alloy is RE a B b M c Fe 100-a-b-c RE is selected from any one or more combinations of Pr, Nd, Dy, Tb, La, Ce, Y, Ho, and Gd; B is boron; M is selected from any one or more combinations of Co, Cu, Al, Ga, Zr, and Ti; and Fe is iron. In the chemical formula, a, b, c, and 100-abc represent the mass percentage of each element, satisfying 29.5≤a≤32, 0.94≤b≤1.0, and 0.5≤c≤4.

[0009] The NdFeB alloy rapid solidification sheet was subjected to hydrogen crushing and secondary air jet milling to obtain near-positive NdFeB magnetic powder with low rare earth content.

[0010] Furthermore, the near-positive NdFeB magnetic powder is oriented, sintered, and tempered to obtain a high remanence near-positive NdFeB magnet.

[0011] The present invention also provides a high remanence near-positive NdFeB magnet prepared by the aforementioned preparation method.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] (1) The technical solution of the present invention combines the rare earth content of the initial NdFeB alloy with the air jet milling process to ensure that the rapidly solidified sheets prepared by the rapid solidification casting process are basically free of segregated impurities, have excellent columnar crystal morphology, and have uniform distribution of rare earth-rich phases that are easy to break down with hydrogen. At the same time, through multiple crushing by air jet milling, the brittle rare earth-rich components are preferentially screened and separated from the magnetic powder, thereby obtaining near-positive NdFeB magnetic powder with low rare earth content without generating impurities or affecting powder making.

[0014] (2) The obtained magnetic powder has excellent particle size distribution. The relatively uniform particle size magnetic powder is close to single crystal and the size of each grain is similar. This not only facilitates orientation molding to obtain high orientation, but also, when combined with the cyclic sintering heat treatment of the present invention, it can avoid abnormal grain merging and growth caused by excessive differences in grain size. This allows low rare earth content NdFeB magnets to achieve full densification while ensuring appropriate grain size, and can realize the preparation of high remanence near-positive NdFeB magnets.

[0015] (3) The present invention is simple and easy to implement, and the operation method is simple, making it suitable for mass production and industrial production. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figures 1a-1b These are X-ray diffraction patterns of the magnets in Embodiment 1 and Comparative Example 1-1 of the present invention;

[0018] Figures 2a-2b These are microscopic morphology images of medium-speed solidified casting sheets from Embodiment 2 and Comparative Example 2-1 of the present invention;

[0019] Figures 3a-3bThese are SEM microstructure images of the magnets prepared in Example 1 and Comparative Example 3 of this invention;

[0020] Figure 4 This is a schematic diagram of the preparation process of a high remanence near-positive NdFeB magnet in a typical embodiment of the present invention. Detailed Implementation

[0021] In view of the deficiencies of the prior art, the inventors of this case, through long-term research and extensive practice, have proposed the technical solution of this invention. The technical solution of this invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0022] Specifically, as one aspect of the technical solution of this invention, a method for preparing a high remanence near-positive NdFeB magnet includes:

[0023] Neodymium iron boron alloy rapid solidification sheets were prepared by melting according to the elemental ratio of the main phase alloy; wherein, the chemical formula of the main phase alloy is RE a B b M c Fe 100-a-b-c RE is selected from any one or more combinations of Pr, Nd, Dy, Tb, La, Ce, Y, Ho, and Gd; B is boron; M is selected from any one or more combinations of Co, Cu, Al, Ga, Zr, and Ti; and Fe is iron. In the chemical formula, a, b, c, and 100-abc represent the mass percentage of each element, satisfying 29.5≤a≤32, 0.94≤b≤1.0, and 0.5≤c≤4.

[0024] The NdFeB alloy rapid solidification sheet was subjected to hydrogen crushing and secondary air jet milling to obtain near-positive NdFeB magnetic powder with low rare earth content.

[0025] Furthermore, the near-positive NdFeB magnetic powder is oriented, sintered, and tempered to obtain a high remanence near-positive NdFeB magnet.

[0026] In some preferred embodiments, the preparation method specifically includes:

[0027] (1) Prepare NdFeB alloy rapid solidification sheets by melting according to the element ratio of the main phase alloy; wherein the NdFeB alloy rapid solidification sheets have a columnar crystal structure and a uniform distribution of rare earth phase.

[0028] (2) The NdFeB alloy quick-setting sheet is subjected to hydrogen crushing treatment to obtain hydrogen crushed powder. Then, nitrogen or argon is used to grind and crush the obtained hydrogen crushed powder in sequence using a first air jet mill and a second air jet mill to obtain near-positive NdFeB magnetic powder with low rare earth content.

[0029] (3) The near-positive NdFeB magnetic powder is oriented and isostatically pressed in a magnetic field to obtain a blank with high orientation degree;

[0030] (4) The high-orientation blank is subjected to cyclic sintering and tempering heat treatment to obtain a high remanence near-positive NdFeB magnet.

[0031] Furthermore, the thickness of the neodymium iron boron alloy rapid solidification sheet is 200-350 μm.

[0032] Furthermore, the rotation speed used in the first air jet mill grinding and crushing process is 1000~3000 rpm.

[0033] Furthermore, the rotation speed used in the second air jet mill grinding and crushing process is 3000~5000 rpm.

[0034] Furthermore, the surface density (SMD) of the fine powder obtained by the first air jet mill grinding and crushing treatment is 10~20 μm.

[0035] Furthermore, the near-positive NdFeB magnetic powder has a surface area density (SMD) of 1.9~2.6 μm and a particle size distribution (D90 / D10) of <4.5.

[0036] Furthermore, the rare earth element (RE) content in the near-positive NdFeB magnetic powder is ≤28.8 wt.%.

[0037] Furthermore, the cyclic sintering process includes: subjecting the high-orientation green body to a first sintering treatment at 1000℃~1050℃ for 1~5 hours, then cooling it to room temperature, and then subjecting it to a second sintering treatment at 1000℃~1100℃ for 1~5 hours, with inert gas being introduced for air cooling after each sintering.

[0038] Furthermore, the preparation method further includes: subjecting the product obtained from the second sintering treatment to repeated sintering treatments until the density of the obtained product is greater than 7.52 g / cm³. 3 The temperature for each sintering process is 1050℃~1100℃, and the time is 1~5h.

[0039] Furthermore, the tempering heat treatment includes a first-stage tempering treatment and a second-stage tempering treatment. The temperature of the first-stage tempering treatment is 800-950℃ and the time is 2-4 hours. The temperature of the second-stage tempering treatment is 460-530℃ and the time is 2-4 hours.

[0040] Furthermore, the hydrogen crushing process includes: absorbing hydrogen into the neodymium iron boron alloy rapid solidification sheet for 3 hours under a hydrogen pressure of 0.2 MPa, followed by vacuum dehydrogenation at 450°C for 10 hours to obtain hydrogen-crushed powder.

[0041] Furthermore, the strength of the magnetic field is 1.8 T.

[0042] Furthermore, the pressure used for the orientation molding is 20~40MPa.

[0043] Furthermore, the isostatic pressing process uses a pressure of 180 MPa.

[0044] In some preferred embodiments, a schematic diagram of the fabrication process of the high remanence near-positive NdFeB magnet of the present invention is shown below. Figure 4 As shown.

[0045] This invention achieves a high main phase ratio and full densification of the magnet through synergistic optimization of composition design and process, avoiding the generation of impurity phases and abnormal grain growth, resulting in a significant improvement in remanent magnetic properties. The process is simple and easy to implement, making it suitable for mass industrial production.

[0046] Another aspect of the present invention provides a high remanence near-positive NdFeB magnet prepared by the aforementioned preparation method.

[0047] The technical solution of the present invention will be further described in detail below with reference to several preferred embodiments and accompanying drawings. This embodiment is implemented on the premise of the technical solution of the invention, and provides detailed implementation methods and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0048] Unless otherwise specified, the experimental materials used in the examples below can be purchased from conventional biochemical reagent companies.

[0049] Example 1

[0050] Rapidly solidified alloy castings are prepared by melting according to the proportions of each element, wherein the chemical formula of the alloy is Nd by mass percentage. 29.5 Co 0.5 Cu 0.2 Al 0.1 Ga 0.2 Zr 0.1 Fe 68.42 B 0.98Rapidly solidified sheets with an average thickness of 250 μm, free of segregated impurities, exhibiting well-grown columnar crystals and a uniform distribution of rare-earth-rich phases, were prepared through smelting. The rapidly solidified sheets were then subjected to hydrogen absorption at a hydrogen pressure of 0.2 MPa for 3 hours, followed by vacuum dehydrogenation at 450 °C for 10 hours to obtain hydrogen-crushed powder. This coarse powder was then crushed using an air jet mill. The first air jet milling process involved a sorting wheel speed of 2000 rpm to obtain pre-ground fine powder. This pre-ground fine powder was then subjected to a second air jet milling process at a sorting wheel speed of 4500 rpm to obtain finely ground powder. The final finely ground powder had the following particle size distribution: SMD = 2.46 μm, D90 / D10 = 4.42, and the measured rare-earth content was 28.8 wt%. The magnetic powder was then subjected to orientation molding at 30 MPa in a 1.8 T magnetic field, followed by isostatic pressing at 180 MPa to obtain a magnet green blank. Then, the magnet blanks were placed in a vacuum sintering furnace under inert gas protection in a glove box. The first sintering temperature was 1050℃. After holding at this temperature for 5 hours, nitrogen gas was introduced and the blanks were cooled to room temperature. The second sintering process was then carried out at a temperature of 1085℃. After holding at this temperature for 3 hours, nitrogen gas was introduced and the blanks were cooled to room temperature. The blanks were then heat-treated at 900℃ for 4 hours and finally at 480℃ for 2 hours to obtain near-positive NdFeB magnet blanks.

[0051] Comparative Example 1-1: (By comparing single-stage and double-stage airflow milling processes, it is shown that double-stage airflow milling plays an important role in improving the overall performance of magnets)

[0052] The preparation method described in Example 1 differs from that in that only one air jet milling process is performed, with the sorting wheel rotating at 4500 rpm, to obtain the magnetic powder. The final particle size distribution of the magnetic powder is: SMD = 2.58 μm, D90 / D10 = 4.65.

[0053] Comparative Examples 1-2:

[0054] The preparation method described in Example 1 differs from that in that only one air jet milling process is performed, with the sorting wheel rotating at 2000 rpm, to obtain magnetic powder. The final particle size distribution of the magnetic powder is: SMD = 12.3 μm, D90 / D10 = 8.9.

[0055] The remanence and coercivity of the magnet products prepared in Example 1, Comparative Examples 1-1 and 1-2 of this invention were tested using an NIM-6500C permanent magnet measuring instrument, and the density was measured using Archimedes' displacement method. The test results are as follows:

[0056] The magnet products of Example 1 and Comparative Examples 1-1 were crushed, and the powder was subjected to X-ray diffraction. The proportion of the main phase was then calculated using Rietveld precision calculations, and the results are as follows: Figures 1a-1b As shown.

[0057] The above analysis clearly shows that the lack of multiple air jet milling process resulted in insufficient separation of rare earth-rich components in the magnetic powder, leading to a high rare earth content and an excessive amount of rare earth-rich phase in the magnet. Meanwhile, the proportion of the main phase in the magnet did not reach a high level, affecting the magnetic properties. However, the technical solution proposed in this invention can obtain NdFeB permanent magnets with a high proportion of main phase and higher remanence.

[0058] Example 2:

[0059] Rapidly solidified alloy castings are prepared by melting according to the proportions of each element, wherein the chemical formula of the alloy is Nd by mass percentage. 29.5 Co 0.5 Cu 0.2 Al 0.1 Ga 0.2 Zr 0.1 Fe 68.42 B 0.98 Rapidly solidified sheets with an average thickness of 250 μm, free of segregated impurities, exhibiting well-grown columnar crystals and a uniform distribution of rare-earth-rich phases, were prepared through smelting. The rapidly solidified sheets were then subjected to hydrogen absorption at a hydrogen pressure of 0.2 MPa for 3 hours, followed by vacuum dehydrogenation at 450 °C for 10 hours to obtain hydrogen-crushed powder. This coarse powder was then crushed using an air jet mill. The first air jet milling process involved a sorting wheel speed of 2500 rpm to obtain pre-ground fine powder. This pre-ground fine powder was then subjected to a second air jet milling process at a sorting wheel speed of 5000 rpm to obtain finely ground powder. The final finely ground powder had the following particle size distribution: SMD = 2.25 μm, D90 / D10 = 4.36, and the measured rare-earth content was 28.5 wt%. The magnetic powder was then subjected to orientation molding at 30 MPa in a 1.8 T magnetic field, followed by isostatic pressing at 180 MPa to obtain a magnet green blank. Then, the magnet blanks were placed in a vacuum sintering furnace under inert gas protection in a glove box. The first sintering temperature was 1050℃, and after holding at that temperature for 5 hours, nitrogen gas was introduced for air cooling to room temperature. The second sintering process was then carried out at a temperature of 1080℃, and after holding at that temperature for 3 hours, nitrogen gas was introduced for air cooling to room temperature. Then, the blanks were heat-treated at 900℃ for 4 hours, and finally, they were heat-treated at 500℃ for 2 hours to obtain near-positive NdFeB magnet blanks.

[0060] Comparative Example 2-1: (By comparing the differences in rare earth content in the alloy composition, the key role of the preparation process of this invention in suppressing the formation of α-Fe impurity phase, optimizing the distribution of rare earth-rich phases, and improving comprehensive magnetic properties is verified.)

[0061] The preparation method is the same as in Example 2, except that the alloy chemical formula for the rapidly solidifying alloy casting is different (it is designed as a low rare earth content composition directly in the original rapid solidification alloy casting composition design), and the mass percentage is Nd. 28.5 Co0.5 Cu 0.2 Al 0.1 Ga 0.2 Zr 0.1 Fe 67.92 B 0.98 The final particle size distribution of the magnetic powder was: SMD=2.41μm, D90 / D10=4.43;

[0062] Comparative Example 2-2:

[0063] The method is the same as in Example 2, except that: the mass percentage of Nd is... 32.5 Co 0.5 Cu 0.2 Al 0.1 Ga 0.2 Zr 0.1 Fe 67.92 B 0.98 The final magnetic powder particle size distribution was obtained as follows: SMD = 2.88 μm, D90 / D10 = 4.67. The remanence and coercivity of the magnet products prepared in Example 2, Comparative Examples 2-1 and 2-2 were measured using an NIM-6500C permanent magnet measuring instrument, and the density was measured using Archimedes' displacement method. The results are as follows:

[0064] The microstructure morphology of the rapid-solidification alloy castings in Example 2 and Comparative Example 2-1 is as follows: Figures 2a-2b As shown, the rapidly solidified sheets prepared by melting low rare earth content in Comparative Example 2-1 contain a large amount of α-Fe impurity phase. Analysis of the above results clearly shows that because the rare earth content of the original alloy formula was directly reduced in the comparative example, Nd2Fe... 14 The B-phase is more likely to deviate from the phase formation condition window, leading to a deterioration of the columnar crystal morphology of the main phase. In addition, the segregated areas are more prone to the formation of impurity phases, resulting in a decrease in the proportion of the main phase in the rapidly solidified alloy, which will further affect the proportion of the main phase and the magnetic properties of the final magnet. The technical solution proposed in this invention has a high rare earth content in the rapid solidification process, and the rare earth component is sufficiently abundant, which can fully meet the compositional conditions for the formation of the main phase (if the rare earth is sufficient or abundant, the 2:14:1 phase is more likely to form). It can effectively avoid the formation of impurity phases in the rapidly solidified sheet. Subsequently, the excess rare earth-rich phase is separated from the magnet by secondary air jet milling, making the magnet composition closer to the positive composition. This reduces the overall rare earth content of the magnet and avoids the formation of impurity phases, resulting in a NdFeB permanent magnet with higher remanence.

[0065] Comparative Example 3: (By comparing the traditional one-stage sintering process with the two-stage cyclic sintering process, it is verified that the sintering process of the present invention can effectively suppress abnormal grain merging and growth and optimize the uniformity of microstructure.)

[0066] The preparation method in Example 1 differs from that in Example 1 in that: a traditional one-time sintering process is used, the sintering temperature is 1085℃, the temperature is held for 5 hours and then cooled to room temperature by nitrogen, then heat treatment is performed at 900℃ for 4 hours, and finally heat treatment is performed at 480℃ for 2 hours to obtain a near-positive NdFeB magnet blank.

[0067] The remanence and coercivity of the magnet products prepared in Example 1 and Comparative Example 3 of this invention were tested using an NIM-6500C permanent magnet measuring instrument, and the density was measured using Archimedes' displacement method. The test results are as follows:

[0068] The magnet products of Example 1 and Comparative Example 3 were subjected to SEM morphology analysis, and the results are as follows: Figure 3a , Figure 3b As shown.

[0069] The above analysis clearly shows that for low-rare-earth content magnets, the absence of the cyclic sintering heat treatment method described in this invention leads to uneven grain growth during sintering, with some grains abnormally merging and growing, resulting in an uneven internal structure and significantly reduced performance. Therefore, the cyclic sintering heat treatment method described in this invention can effectively avoid abnormal grain merging caused by excessive differences in grain size, enabling low-rare-earth content NdFeB magnets to achieve both sufficient densification and suitable grain size, thus achieving the fabrication of high-remanence near-positive NdFeB magnets.

[0070] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0071] It should be understood that the technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made to the technical solutions of the present invention without departing from the spirit and scope of the claims are within the scope of protection of the present invention.

Claims

1. A method for preparing a near-positive NdFeB magnet with high remanence, characterized in that, include: Neodymium iron boron alloy rapid solidification sheets were prepared by melting according to the elemental ratio of the main phase alloy; wherein, the chemical formula of the main phase alloy is RE a B b M c Fe 100-a-b-c RE is selected from any one or more combinations of Pr, Nd, Dy, Tb, La, Ce, Y, Ho, and Gd; B is boron; M is selected from any one or more combinations of Co, Cu, Al, Ga, Zr, and Ti; and Fe is iron. In the chemical formula, a, b, c, and 100-abc represent the mass percentage of each element, satisfying 29.5≤a≤32, 0.94≤b≤1.0, and 0.5≤c≤4. The NdFeB alloy rapid solidification sheet was subjected to hydrogen crushing and secondary air jet milling to obtain near-positive NdFeB magnetic powder with low rare earth content. Furthermore, the near-positive NdFeB magnetic powder is oriented, sintered, and tempered to obtain a high remanence near-positive NdFeB magnet.

2. The preparation method according to claim 1, characterized in that, Specifically, it includes: (1) Prepare NdFeB alloy rapid solidification sheets by melting according to the element ratio of the main phase alloy; wherein the NdFeB alloy rapid solidification sheets have a columnar crystal structure and a uniform distribution of rare earth phase. (2) The NdFeB alloy quick-setting sheet is subjected to hydrogen crushing treatment to obtain hydrogen crushed powder. Then, nitrogen or argon is used to grind and crush the obtained hydrogen crushed powder in sequence using a first air jet mill and a second air jet mill to obtain near-positive NdFeB magnetic powder with low rare earth content. (3) The near-positive NdFeB magnetic powder is oriented and formed in a magnetic field and isostatically pressed to obtain a blank with high orientation degree; (4) The high-orientation blank is subjected to cyclic sintering and tempering heat treatment to obtain a high remanence near-positive NdFeB magnet.

3. The preparation method according to claim 2, characterized in that: The thickness of the neodymium iron boron alloy rapid solidification sheet is 200-350 μm.

4. The preparation method according to claim 2, characterized in that: The first airflow mill grinding and crushing process uses a rotation speed of 1000~3000 rpm; And / or, the rotation speed used in the second air jet mill grinding and crushing process is 3000~5000 rpm; And / or, the surface density (SMD) of the fine powder obtained by the first air jet mill grinding and crushing treatment is 10~20 μm; And / or, the near-positive NdFeB magnetic powder has a surface area density (SMD) of 1.9~2.6 μm and a particle size distribution (D90 / D10) of <4.5; And / or, the content of rare earth elements in the near-positive NdFeB magnetic powder is ≤28.8wt.%.

5. The preparation method according to claim 2, characterized in that: The cyclic sintering process includes: subjecting the high-orientation green body to a first sintering treatment at 1000℃~1050℃ for 1~5 hours, then cooling it to room temperature, and then subjecting it to a second sintering treatment at 1000℃~1100℃ for 1~5 hours, and purging it with inert gas for air cooling after each sintering.

6. The preparation method according to claim 5, characterized in that, Also includes: The product obtained from the second sintering treatment was subjected to multiple sintering treatments until the density of the obtained product was greater than 7.52 g / cm³. The temperature of each sintering treatment was 1050℃~1100℃ and the time was 1~5h.

7. The preparation method according to claim 2, characterized in that: The tempering heat treatment includes a first-stage tempering treatment and a second-stage tempering treatment. The first-stage tempering treatment is performed at a temperature of 800-950℃ for 2-4 hours, and the second-stage tempering treatment is performed at a temperature of 460-530℃ for 2-4 hours.

8. The preparation method according to claim 2, characterized in that, The hydrogen crushing process includes: absorbing hydrogen into the neodymium iron boron alloy rapid solidification sheet for 3 hours under a hydrogen pressure of 0.2 MPa, followed by vacuum dehydrogenation at 450°C for 10 hours to obtain hydrogen-crushed powder.

9. The preparation method according to claim 2, characterized in that: The strength of the magnetic field is 1.8T; And / or, the pressure used for the orientation molding is 20~40MPa; And / or, the isostatic pressing process is performed at a pressure of 180 MPa.

10. A near-positive NdFeB magnet with high remanence obtained by the preparation method according to any one of claims 1-9.

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