Sintered neodymium-iron-boron permanent magnet with performance gradient and preparation method and application of sintered neodymium-iron-boron permanent magnet

Abstract

The invention discloses a sintered neodymium-iron-boron permanent magnet with performance gradient, the intrinsic coercive force of the sintered neodymium-iron-boron permanent magnet is continuously increased in a gradient manner from the core part to the outermost layer of the sintered neodymium-iron-boron permanent magnet along the non-orientation direction, and the difference value of the intrinsic coercive force from the core part to the outermost layer is 1.5-15KOe. The sintered neodymium-iron-boron permanent magnet meets the requirements of high coercive force in an easily demagnetized region and low heavy rare earth in a difficultly demagnetized region, and has better performance buffering. The invention further provides a preparation method and application of the sintered neodymium-iron-boron permanent magnet.

Description

Technical Field

[0001] This invention belongs to the field of permanent magnet technology for electric motors, specifically relating to a sintered NdFeB permanent magnet with performance gradient, its preparation method, and its application. Background Technology

[0002] In today's world, energy scarcity makes the development of a low-carbon economy imperative. In the global field of energy conservation and environmental protection, new energy vehicles are the most highly regarded emerging industry. The motor drive system is a core component for achieving energy conservation and emission reduction in new energy vehicles. Sintered NdFeB rare-earth permanent magnets, with their excellent magnetic properties such as high remanence, high energy product, and high coercivity, have become the mainstay of permanent magnet materials and are widely used in permanent magnet drive motors for new energy vehicles. Adding large amounts of precious rare-earth elements such as dysprosium and terbium during the preparation of magnets can improve the demagnetization resistance of motor magnets, but this method significantly increases the cost of the motor. Selective infiltration of heavy rare-earth elements into the magnets is currently a better option for improving the motor's demagnetization resistance. The demagnetization process of a magnet is related to the macroscopic non-uniform distribution of the internal demagnetization field. Micromagnetic simulation results show that the macroscopic demagnetization field is mainly distributed on the surfaces of the two magnetic poles of the oriented magnet. During reverse demagnetization, the grain boundary phase and grains near the corners of the magnetic pole surfaces preferentially undergo inversion. Theoretically, grain boundary diffusion should be used primarily to improve the coercivity of weak parts of the magnet. In response to the varying demagnetization requirements at different locations in the application of permanent magnet motors, some companies have proposed different degrees of heavy rare earth grain boundary diffusion in different parts of the magnet.

[0003] The invention patent application with publication number CN113035556A discloses a method for preparing an RTB magnet with a gradient distribution of magnet properties. Rare earth slurry is coated on different surfaces and positions of the magnet to generate different gradients of coercivity and remanence in different regions of the magnet, reducing the reduction of remanence caused by uniform coating on the penetration surface. Coating is also applied to local positions on non-oriented surfaces to improve the resistance to demagnetization at the corners. Although this can effectively improve the coercivity of easily demagnetized areas, it requires coating one position at a time before moving on to the next, resulting in a long process flow, extremely high tooling precision requirements, and high cost. It needs to be combined with specific market demands.

[0004] The invention patent application with publication number CN118366742A discloses a neodymium iron boron magnet and its preparation method, which uses magnets with different properties to bond together with glue to form a gradient magnet; however, it increases the bonding process, prolongs the process flow, and has a large performance gap between the easy-to-demagnetize and difficult-to-demagnetize areas, without performance buffer.

[0005] Statistical analysis revealed that over 80% of NdFeB magnets are designed with at least one dimension (length l) within the range of 10-35mm (width m is a non-orientation direction). Their manufacturing process is as follows: Figure 1As shown in (a), the sintered dense body L*W*H is ground on all four sides → cut with one cut or ground on two sides and cut with two cuts to achieve a finished product with approximate specifications l*w*h. Then, the thickness h direction is coated or sputtered and diffused, and the diffused magnet is further ground and electroplated. The process is lengthy, inefficient and has a long cycle. Summary of the Invention

[0006] This invention provides a sintered NdFeB permanent magnet with performance gradients. This sintered NdFeB permanent magnet satisfies the requirements of high coercivity in the easy demagnetization region and low heavy rare earth elements in the difficult demagnetization region, thus exhibiting good performance buffering.

[0007] The present invention provides a sintered NdFeB permanent magnet with a performance gradient, wherein the intrinsic coercivity increases continuously in a gradient from the core to the outermost layer along the non-oriented direction, and the difference in intrinsic coercivity from the core to the outermost layer is 1.5-15 kOe.

[0008] The core provided by this invention is a region with a radius of 1 mm centered on the center of the sintered NdFeB permanent magnet along the non-orientation direction, and the outermost layer is a region within 2 mm from the surface along the non-orientation direction. It can be understood that the orientation direction provided in this application refers to the direction of the easy magnetization axis of the magnetic grains of the magnet, and the non-orientation direction refers to the direction perpendicular to the easy magnetization direction (i.e., the orientation direction) of the magnet.

[0009] This invention controls the intrinsic coercivity of the permanent magnet from the core to the outermost layer along the non-orientation direction to gradually increase, while controlling the range of the difference in intrinsic coercivity. This results in the outermost layer, which is prone to demagnetization in the non-orientation direction, having higher coercivity, while the core, which is difficult to demagnetize, has lower coercivity. This directly enhances the anti-demagnetization performance of the outermost layer, which is at high risk of demagnetization. It maintains high magnetic flux in the central region, making it easier to achieve anti-demagnetization performance, while also saving heavy rare earth materials.

[0010] This invention improves waveforms and reduces harmonics and losses by increasing the intrinsic coercivity gradient in the non-orientation (in-plane / radial) direction.

[0011] Preferably, the difference in intrinsic coercivity between the core and the outermost layer is 1.68-14.4 kOe.

[0012] Preferably, along the non-oriented direction, the heavy rare earth content of the sintered NdFeB permanent magnet gradually decreases from the outermost layer to the core, and the difference in heavy rare earth content between the core and the outermost layer is 0.2-1.85 wt%. Along the orientation direction, the heavy rare earth content of the sintered NdFeB permanent magnet varies from 0 to 0.18 wt% from the outermost layer to the core.

[0013] More preferably, along the non-oriented direction, the heavy rare earth content of the sintered NdFeB permanent magnet gradually decreases from the outermost layer to the core, and the difference in heavy rare earth content between the core and the outermost layer is 0.2-1.5 wt%. Along the orientation direction, the heavy rare earth content of the sintered NdFeB permanent magnet varies from 0.03 to 0.18 wt% from the outermost layer to the core.

[0014] This invention achieves consistent heavy rare earth content in the core and outermost layers along the orientation direction through non-oriented diffusion, thus ensuring relatively consistent coercivity between the core and outermost layers in the orientation direction; while gradually decreasing the heavy rare earth content from the outermost layer to the core along the non-oriented direction, thus ensuring a gradual increase in coercivity from the core to the outermost layer in the non-oriented direction.

[0015] This invention ensures relatively consistent coercivity between the core and outermost layer along the orientation direction, resulting in almost no difference in the magnet's diamagnetic capability along the orientation direction. This eliminates the need to additionally increase the overall coercivity for locally weak layers (avoiding over-design) and also eliminates the risk of localized failure (avoiding under-design). While guaranteeing diamagnetic performance, it minimizes the use of heavy rare earth elements. Furthermore, it ensures uniform remanence and permeability across the entire thickness range, as well as a uniform magnetic flux density distribution, reducing harmonics and losses.

[0016] The permanent magnet provided by the present invention can have a length l or a width w along the non-orientation direction, while the orientation direction is along the orientation thickness h. The permanent magnet has a directional dimension range of 10mm-35mm in both the length l and width w directions in the non-orientation direction, which meets the general size requirements of neodymium iron boron magnets.

[0017] Preferably, the heavy rare earth element is Dy or Tb.

[0018] Preferably, in the preparation of the sintered NdFeB permanent magnet, the chemical formula of the green blank is RExFe. 100-x-y-z B y M z x is 28-34 wt%, y is 0.83-1.05 wt%, and z is 0.5-6 wt%; RE is one or more of rare earth elements Nd, Pr, Gd, Ho, Dy, Tb, Ho, La, and Y, and contains at least Nd; M contains one or more of Cu, Ga, Al, Co, Ti, Zr, Nb, and Si. The diffusion source is a heavy rare earth alloy, or an oxide, fluoride or hydride of heavy rare earth, and the content of the diffusion source in the sintered NdFeB permanent magnet is 0.2wt%-3wt%.

[0019] Preferably, the content of the diffusion source in the sintered NdFeB permanent magnet is 1wt%-3wt%.

[0020] The present invention also provides a method for preparing the sintered NdFeB permanent magnet with performance gradient, comprising: S1. Green body preparation: The green body preparation process is as follows: batching - rapid solidification melting - hydrogen crushing - air jet milling - isostatic pressing to obtain green body; The chemical formula of the green body is RExFe. 100-x-y-z B y M z x is 28-34 wt%, y is 0.83-1.05 wt%, and z is 0.5-6 wt%; RE is one or more of rare earth elements Nd, Pr, Gd, Ho, Dy, Tb, Ho, La, and Y, and contains at least Nd; M contains one or more of Cu, Ga, Al, Co, Ti, Zr, Nb, and Si elements in the formulation. S2. Place the diffusion source on the non-oriented parallel surface of the green billet to obtain the billet to be diffused. The diffusion source is a heavy rare earth alloy, or an oxide, fluoride or hydride of heavy rare earth. S3. The blank to be diffused obtained in step S2 is sintered and aged. The sintering process is a multi-stage sintering process performed in a vacuum environment. The multi-stage sintering process includes at least a first stage sintering and a second stage sintering performed sequentially. The first stage sintering is performed at a temperature of 200-350℃ for 0.5-3 hours, and the second stage sintering is performed at a temperature of 500-650℃ for 0.5-3 hours.

[0021] This invention, by controlling the process sequence, obtains the billet to be diffused and then directly performs sintering and aging treatment. Compared with the traditional process of first sintering and aging the green billet and then performing heavy rare earth grain boundary diffusion, this invention is equivalent to combining the sintering and diffusion heat treatment of the green billet in the prior art into the sintering process of this application. It eliminates the grain boundary diffusion, diffusion heat treatment and post-diffusion grinding steps in the traditional process, shortens the production process, improves production efficiency, and reduces energy consumption because the heat treatment process provided by this invention is uninterrupted. In order to simultaneously complete the decarburization of the diffusion source and the green billet, this invention extends the sintering time of the first and second stages, thereby meeting the requirement of combining the sintering and diffusion heat treatment of the green billet into a single sintering process.

[0022] Preferably, the density of the green body is 3.9-4.5 g / cm³. 3 A suitable green density allows for better pressing and molding, facilitating processing. At the same time, a suitable green density results in better porosity within the green, which is beneficial for the uniform and deep diffusion of the diffusion source.

[0023] Preferably, the sintering process includes the following steps: The billet to be diffused is sintered in a vacuum sintering furnace, and the vacuum is evacuated to ≤9.9×10.-1 Pa, start by heating to 200-350℃ and sintering for 0.5-3h, then heat to 500-650℃ and sinter for 0.5-3h, continue heating to 800-900℃ and sinter for 0.5-4h, then heat to 1010-1090℃ for high-temperature sintering for 8-15h, with a heating rate of <8℃ / min throughout the process; after high-temperature sintering, cool with the furnace to 700-900℃, then purge with argon and cool to room temperature.

[0024] Preferably, the aging process includes a two-stage heat treatment, followed by evacuation to 10°C. -1 Below Pa, the first-stage heat treatment includes: heating rate ≤6℃ / min, directly heating to 870-950℃, holding time 5-20h; after the first-stage heat treatment, argon is used to cool to ≤60℃ and then a second-stage heat treatment is performed, the second-stage heat treatment includes: directly heating to 450-550℃, holding time 4-8h, and then cooling to below 60℃ before being removed from the furnace.

[0025] Since the sintering process provided by this invention directly completes the decarburization of the green body and the diffusion source, the aging process is carried out directly after sintering with heating, without the need for a heat preservation process, which reduces energy consumption and improves preparation efficiency.

[0026] Preferably, the preparation method of the sintered NdFeB permanent magnet with performance gradient further includes machining and electroplating.

[0027] This invention grinds the non-oriented surface of the diffusion process, and grinds or slices the other two directions according to the finished product specifications. The machined permanent magnet sheet is then electroplated as required.

[0028] On the other hand, the present invention also provides the application of the sintered NdFeB permanent magnet with performance gradient in a permanent magnet drive motor.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention improves the magnet's resistance to demagnetization by matching the highest coercivity to the outermost layer in the non-orientation direction, i.e., the region with the highest demagnetization risk. In regions where the demagnetization risk gradually decreases, the coercivity decreases in a gradient along the core direction. This avoids the redundant design of excessively increasing the coercivity of the entire magnet, saves heavy rare earth elements, and makes efficient use of resources. Furthermore, because the coercivity in the core is lower, the decrease in remanence of the magnet is minimized, thus better preserving the magnetic flux output capability. Attached Figure Description

[0030] Figure 1 A process flow diagram is provided for the traditional process and specific embodiments of the present invention. Figure 1 (a) is a process flow diagram of a traditional technique. Figure 1 (b) is a process flow diagram of another traditional process. Figure 1 (c) A process flow diagram of the preparation method provided in this application; Figure 2 This is a sample image of the product tested in Embodiment 1 of the present invention; Figure 3 This is a microscopic morphology diagram of region jA of the sintered NdFeB permanent magnet obtained in Example 1 of the present invention. Detailed Implementation

[0031] To meet the requirements of new energy motors, this invention proposes a new performance gradient sintered NdFeB permanent magnet and its preparation method, taking into account the appearance and shape of the permanent magnet in new energy motors. The difference between the HCJ in the core and the HCJ on the surface of the permanent magnet is 1.5-15 KOe, which satisfies the requirements of high coercivity in the easy demagnetization area and low heavy rare earth in the difficult demagnetization area. This not only meets the requirements of motor use, but also shortens the process flow.

[0032] In a specific embodiment of the present invention, green body diffusion is employed, directly subjecting the formed green body to heavy rare earth diffusion. This method is simple to operate, significantly shortens the production process, and improves production efficiency. Figure 1 As shown, this is a comparison with the conventional grain boundary diffusion product manufacturing process. Figure 1 (a) The preparation process provided in a specific embodiment of the present invention Figure 1 (c) The steps of grain boundary diffusion, diffusion heat treatment and post-diffusion grinding are omitted; compared with the production process of selected grain boundary diffusion gradient magnet 1(b), which requires grain boundary diffusion in different regions, the preparation process provided by the specific embodiment of the present invention omits the steps of grain boundary diffusion (region 1), grain boundary diffusion (region 2), diffusion heat treatment and grinding. The production cycle of gradient magnets produced by the present invention is shortened by more than 50 hours, and production energy consumption is further reduced.

[0033] A specific embodiment of the present invention provides a sintered NdFeB magnet with a performance gradient, which forms a performance gradient from the core outward. The core, which is difficult to demagnetize, has low performance, saving the amount of heavy rare earth diffusion source used; the outer, easily demagnetized region has high coercivity, effectively resisting external demagnetizing fields, meeting the requirements of motor use, and reducing the material cost of sintered NdFeB permanent magnets.

[0034] The specific embodiment of the present invention uses non-oriented surface diffusion, which breaks through the limitation of the orientation thickness of grain boundary diffusion magnets.

[0035] In a specific embodiment of the present invention, diffusion is carried out in a green body while sintering. Compared with diffusion in a sintered body, the present invention has a lower diffusion driving force, higher diffusion efficiency, and a higher increase in magnet coercivity.

[0036] The specific embodiments of the present invention provide Examples 1-3 and Comparative Examples 1-2. The component contents of the green blanks of Examples 1-3 and Comparative Examples 1-2 are shown in Table 1. The diffusion direction, blank size and product size of the green blanks of Examples 1-3 and Comparative Examples 1-2 are shown in Table 2. The process parameters of Examples 1-3 and Comparative Examples 1-2 are shown in Table 3.

[0037] Example 1 This embodiment provides a method for preparing sintered NdFeB permanent magnets with performance gradients, including: S1. Green body preparation: The green body preparation process includes batching, rapid solidification melting, hydrogen crushing, air jet milling, and isostatic pressing to obtain a green body with a density of 3.98 g / cm³. 3 .

[0038] The chemical formula of the green body is RExFe. 100-x-y-z B y M z Prepare the ingredients.

[0039] The process parameters for this rapid solidification smelting are: maximum temperature 1485℃, casting temperature 1445℃, and copper roller linear speed 1.2m / s.

[0040] The process parameters for this hydrogenation process are: dehydrogenation temperature 550℃.

[0041] The process parameters for this airflow milling are: grinding pressure 0.5 MPa, rotation speed 5400 r / min.

[0042] S2. Place the diffusion source on the non-oriented parallel surface of the green blank to obtain the blank to be diffused.

[0043] S3. The blank to be diffused obtained in step S2 is sintered and aged.

[0044] The specific steps of the sintering include: sintering the billet to be diffused in a vacuum sintering furnace, wherein after evacuation, four stages of sintering are performed sequentially, and then argon gas is used to cool it to room temperature.

[0045] The aging process includes, after vacuuming, performing a first-stage heat treatment, a second-stage heat treatment, and then cooling.

[0046] S4: Machining: Grind the non-oriented surface of the diffusion process, and grind or slice the other two directions as required by the finished product specifications.

[0047] S5: Electroplating: Electroplating the machined permanent magnet sheets according to requirements.

[0048] Specifically, such as Figure 2 As shown, Figure 2As shown in (a), the dimensions of the blank after sintering and aging are 32*22*42, and its diffusion direction is along the direction with a dimension of 22mm, as shown in (a). Figure 2 As shown in (b), the four sides are ground to a size of 31*21*42. The 42mm section is then cut to a thickness of 2mm along the orientation direction, yielding 19 pieces, resulting in a finished 31*21*2 magnet. Figure 2 As shown in (c), 19 magnets were labeled from left to right as a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s. The magnetic properties of a, d, g, j, m, p, and s were tested, and the Tb content was tested by ICP. As shown in Table 5, there was no significant difference in their performance.

[0049] To verify its performance gradient, such as Figure 2 As shown in (d), the outermost layer a and the core j of the finished magnet were cut into 2mm thick sheets along the diffusion and non-orientation direction, yielding 9 sheets. These sheets were labeled A, B, C, D, E, F, G, H, and I from top to bottom. The coercivity of aA, aC, aE, aG, aI and jA, jC, jE, jG, jI were tested. Both a and j regions showed a trend of lower coercivity at the core and higher coercivity at the edges, corresponding to a lower Tb content at the core and higher coercivity at the edges, as shown in Figure 6. The microstructure of jA was also observed using a scanning electron microscope. Figure 3 The microstructure of region jA is shown in Table 7. Spot scans were performed at points A (core of the main phase grain), B (edge ​​of the main phase grain), and C (grain boundary phase). The test results are shown in Table 7. As can be seen from Table 7, there is no Tb inside the main phase grains, while the Tb content at the edge of the main phase grains is higher than that at the grain boundary phase. This indicates that Tb from the diffusion source effectively diffuses into the product and forms a Tb shell on the outer layer of the main phase grains.

[0050] like Figure 2 As shown in (e), (f), and (g), to further verify the consistency of its performance along the non-oriented, non-diffusion direction, slices were further prepared along the non-oriented, non-diffusion direction for aA and jE. 31 was sliced ​​to 2mm, yielding 14 slices, labeled 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 from front to back. The magnetic properties of aA-1, aA-5, aA-9, aA-14, and jE-1, jE-5, jE-9, and jE-14 were tested, and no significant differences were found, as shown in Table 8. The magnet of this invention exhibits a performance gradient along the non-oriented and diffusion direction, while its performance is relatively consistent in the other two directions.

[0051] Comparative Example 1 Unlike Example 1, as shown in Table 2, the diffusion direction is different. In Comparative Example 1, the diffusion of heavy rare earth elements is carried out along the orientation direction (the orientation direction with a size of 42 mm). As shown in Table 9, the resulting product has poor consistency in magnetic properties along the orientation direction, i.e., positions a, d, g, j, m, p, and s, and the difference in anti-demagnetization properties is large, which does not meet the application conditions.

[0052] Example 2 Compared with Example 1, the differences are that the heavy rare earth content is different, the size of the green body is different (see Table 2), and the parameters of the sintering process and aging process are different (see Table 3).

[0053] Comparative Example 2 Compared with Example 2, the sintering time for the first stage and the second stage are too short.

[0054] As shown in Table 2, the coercivity of both the core and surface layers of the comparative example is lower than that of Example 2. The difference in coercivity of the core is 0.98 kOe, and the difference in the surface layer is 3.30 kOe. The carbon content was tested by a carbon-sulfur analyzer. There was no significant difference in the carbon content of the core. However, the carbon content of the surface layer of the comparative example is 525 ppm higher than that of Example 2. This indicates that the sintering time of the first and second stages is too short, and there is more carbon residue in the green body and diffusion source, which affects the further diffusion of Tb from the surface layer to the core, thereby affecting the magnetic properties.

[0055] Example 3 Compared with Example 1, the content of heavy rare earth elements is different, as are the other elements and their contents, the preparation process parameters, the size of the blank (green blank), and the product.

[0056] The corresponding characterization data is also provided.

[0057] Table 1. Component content of green bodies in Examples 1-3 and Comparative Examples 1-2 Table 2. Diffusion direction, blank size, and product size of Examples 1-3 and Comparative Examples 1-2 Table 3. Process parameters for Examples 1-3 and Comparative Examples 1-2 Table 4 shows the magnetic properties of Examples 1-3 and Comparative Examples 1-2. Table 5 Magnetic properties of the finished product provided in Example 1 along the orientation direction Table 6 shows the magnetic properties of the finished magnets provided in Example 1 along the diffusion and non-orientation direction. Table 7: Point Scan Energy Dispersive Spectrum Table of Magnet jA Region Provided in Example 1 Table 8 shows the magnetic properties along the non-diffusion direction and non-orientation direction provided in Example 1. Table 9. Magnetic properties of the finished product provided in Comparative Example 1 along the orientation direction.

Claims

1. A sintered NdFeB permanent magnet with performance gradient, characterized in that, Along the non-oriented direction from the core to the outermost layer of the sintered NdFeB permanent magnet, the intrinsic coercivity increases continuously in a gradient, and the difference between the intrinsic coercivity of the core and the outermost layer is 1.5-15 Koe.

2. The sintered NdFeB permanent magnet with performance gradient according to claim 1, characterized in that, Along the non-oriented direction, the heavy rare earth content of the sintered NdFeB permanent magnet gradually decreases from the outermost layer to the core, with the difference in heavy rare earth content between the core and the outermost layer being 0.2-1.85 wt%. Along the orientation direction, the heavy rare earth content of the sintered NdFeB permanent magnet varies from 0 to 0.18 wt% from the outermost layer to the core.

3. The sintered NdFeB permanent magnet with performance gradient according to claim 2, characterized in that, The heavy rare earth element is Dy or Tb.

4. The sintered NdFeB permanent magnet with performance gradient according to claim 1, characterized in that, In the preparation of the sintered NdFeB permanent magnet, the chemical formula of the green blank is RE. x Fe 100-x-y-z B y M z x is 28-34 wt%, y is 0.83-1.05 wt%, and z is 0.5-6 wt%; RE is one or more of rare earth elements Nd, Pr, Gd, Ho, Dy, Tb, Ho, La, and Y, and contains at least Nd; M contains one or more of Cu, Ga, Al, Co, Ti, Zr, Nb, and Si. The diffusion source is a heavy rare earth alloy, or an oxide, fluoride or hydride of heavy rare earth, and the content of the diffusion source in the sintered NdFeB permanent magnet is 0.2wt%-3wt%.

5. The method for preparing a sintered NdFeB permanent magnet with performance gradient according to any one of claims 1-4, characterized in that, include: S1. Green body preparation: The green body preparation process is as follows: batching - rapid solidification melting - hydrogen crushing - air jet milling - isostatic pressing to obtain green body; The chemical formula of the green body is RExFe. 100-x-y-z B y M z x is 28-34 wt%, y is 0.83-1.05 wt%, and z is 0.5-6 wt%; RE is one or more of rare earth elements Nd, Pr, Gd, Ho, Dy, Tb, Ho, La, and Y, and contains at least Nd; M contains one or more of Cu, Ga, Al, Co, Ti, Zr, Nb, and Si elements in the formulation. S2. Place the diffusion source on the non-oriented parallel surface of the green billet to obtain the billet to be diffused. The diffusion source is a heavy rare earth alloy, or an oxide, fluoride or hydride of heavy rare earth. S3. The blank to be diffused obtained in step S2 is sintered and aged. The sintering process is a multi-stage sintering process performed in a vacuum environment. The multi-stage sintering process includes at least a first stage sintering and a second stage sintering performed sequentially. The first stage sintering is performed at a temperature of 200-350℃ for 0.5-3 hours, and the second stage sintering is performed at a temperature of 500-650℃ for 0.5-3 hours.

6. The method for preparing sintered NdFeB permanent magnets with performance gradients according to claim 5, characterized in that, The density of the green body is 3.9-4.5 g / cm³. 3 .

7. The method for preparing sintered NdFeB permanent magnets with performance gradients according to claim 5, characterized in that, The specific steps of the sintering include: The billet to be diffused is sintered in a vacuum sintering furnace, and the vacuum is evacuated to ≤9.9×10. -1 Pa, start by heating to 200-350℃ and sintering for 0.5-3h, then heat to 500-650℃ and sinter for 0.5-3h, continue heating to 800-900℃ and sinter for 0.5-4h, then heat to 1010-1090℃ for high-temperature sintering for 8-15h, with a heating rate of <8℃ / min throughout the process; after high-temperature sintering, cool with the furnace to 700-900℃, then purge with argon and cool to room temperature.

8. The method for preparing sintered NdFeB permanent magnets with performance gradients according to claim 5, characterized in that, The aging process includes a two-stage heat treatment, followed by evacuation to 10°C. -1 Below Pa, the first-stage heat treatment includes: heating rate ≤6℃ / min, directly heating to 870-950℃, holding time 5-20h; after the first-stage heat treatment, argon is used to cool to ≤60℃ and then a second-stage heat treatment is performed, the second-stage heat treatment includes: directly heating to 450-550℃, holding time 4-8h, and then cooling to below 60℃ before being removed from the furnace.

9. The method for preparing a sintered NdFeB permanent magnet with performance gradient according to claim 5, characterized in that, The method for preparing sintered NdFeB permanent magnets with performance gradients also includes machining and electroplating.

10. The application of the sintered NdFeB permanent magnet with performance gradient according to any one of claims 1-4 in a permanent magnet drive motor.

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