Grain boundary diffusion composite film, rare earth permanent magnet and preparation method

Abstract

The invention discloses a grain boundary diffusion composite film, a rare earth permanent magnet and a preparation method, and relates to the field of magnet preparation; the grain boundary diffusion composite film is attached to the surface of a sintered magnet, and the raw materials of the grain boundary diffusion composite film include any one or more combinations selected from rare earth borides, rare earth boron oxides, rare earth hydrides, and alloys composed of rare earth elements and other metal elements; the grain boundary diffusion composite film is attached to the surface of the sintered magnet through a screen printing or spraying process, and then a rare earth permanent magnet is prepared through diffusion heat treatment; the invention improves the binding energy of surface crystals and the crystal properties of grain boundaries, thereby inhibiting heavy rare earth elements from entering the main phase grains of the surface of the magnet, promoting the heavy rare earth to diffuse more efficiently into the depth of the magnet, reducing the concentration difference between the surface and the center of the magnet, and thus achieving the effects of reducing the amount of heavy rare earth, increasing the diffusion depth of the magnet, increasing the coercive force enhancement range, and reducing the residual magnetization reduction range.

Description

Technical Field

[0001] The invention relates to the field of magnet preparation, and in particular to a grain boundary diffusion composite film, a rare earth permanent magnet and a preparation method thereof. Background Art

[0002] As the third generation of rare earth permanent magnet materials, NdFeB has the characteristics of high remanence, high coercivity and maximum magnetic energy product. Sintered NdFeB magnets made of neodymium, iron and boron as the main raw materials are increasingly used for their excellent magnetic properties. They are widely used in the fields of nuclear magnetic resonance, computers, hybrid vehicles, various motors and wind turbines. Generally speaking, sintered NdFeB magnets made of rare earth praseodymium and neodymium have low coercivity, poor resistance to reverse magnetic field and high temperature, and are easy to lose magnetism. They can only be used in environments with low reverse magnetic field and not too high temperature.

[0003] In order to improve the coercivity of magnets and reduce the amount of heavy rare earth used in magnets, the method generally recognized in the industry is grain boundary diffusion technology. For example, the patent application with publication number CN106328367A mentions the use of heavy rare earth Tb for grain boundary diffusion, and the patent application with publication number CN108039259A uses an inner layer of low melting point metal (melting point ≤ 500°C) and an outer layer of heavy rare earth or heavy rare earth alloy layer for diffusion, which improves the utilization rate of heavy rare earth, solves the problem of high cost, and greatly improves the performance of magnets.

[0004] However, the existing grain boundary diffusion technology has obvious defects and pain points. In the process of heavy rare earth elements diffusing into the interior of the magnet along the grain boundaries, the heavy rare earth elements will inevitably enter the main phase grains on the surface of the magnet in large quantities, and cannot diffuse deeper into the magnet more efficiently, resulting in a large amount of enrichment of heavy rare earth elements on the surface of the magnet during the diffusion process, which greatly affects the diffusion speed and diffusion depth of the heavy rare earth elements, resulting in increased diffusion weight gain. In addition, excessive entry of heavy rare earth elements into the main phase will greatly increase the reduction in remanence. Summary of the invention

[0005] The invention aims to solve the technical problems that a large amount of heavy rare earth elements are enriched on the surface of magnets in the existing grain boundary diffusion technology, resulting in low heavy rare earth utilization rate, slow penetration speed, low penetration depth and large residual magnetization reduction. The invention aims to provide a grain boundary diffusion composite film, a rare earth permanent magnet and a preparation method, which achieves the effects of reducing the amount of heavy rare earth, increasing the diffusion depth of magnets, increasing the coercive force improvement range and reducing the residual magnetization reduction range.

[0006] The present invention is achieved through the following technical solutions:

[0007] The first object of the present invention is to provide a grain boundary diffusion composite film, which is attached to the surface of a sintered magnet. The raw materials of the grain boundary diffusion composite film include any one or more combinations selected from rare earth borides, rare earth boron oxides, rare earth hydrides, and alloys composed of rare earth elements and other metal elements.

[0008] As a further technical solution, the addition amount of the rare earth boride is less than 1.5% of the weight of the sintered magnet, and the addition amount of the rare earth boron oxide, the rare earth hydride and the alloy composed of the rare earth element and other metal elements is less than 5.0% of the weight of the sintered magnet.

[0009] As a further technical solution, the number of film layers of the grain boundary diffusion composite film is ≥1.

[0010] As a further technical solution, the rare earth elements include Dy, Tb, Pr, Ho, and Y, and the other metal elements include Ga, Cu, Al, Fe, Co, Ti, Zr, and Nb.

[0011] As a further technical solution, the rare earth element in the rare earth boride and rare earth boron oxide is any one of Dy and Tb or a combination of both.

[0012] The second object of the present invention is to provide a rare earth permanent magnet, comprising a sintered magnet and the aforementioned grain boundary diffusion composite film, wherein the grain boundary diffusion composite film is attached to the surface of the sintered magnet.

[0013] The third object of the present invention is to provide a method for preparing a rare earth permanent magnet, comprising the following steps:

[0014] Prepare RTBM sintered magnets, and perform surface activation treatment on the sintered magnets after machining;

[0015] Adhere the aforementioned grain boundary diffusion composite film on the surface of the sintered magnet by screen printing or spraying process;

[0016] The sintered magnet with the grain boundary diffusion composite film attached thereto is heat treated in a vacuum sintering furnace to diffuse the elements of the composite film layer into the interior of the sintered magnet.

[0017] As a further technical solution, in the RTBM sintered magnet, R is selected from one or more of La, Ce, Pr, Nd, Dy, Tb, Gd, Ho, and Y, with a total amount of 28.0wt%-32.5wt%; M is selected from one or more of Ga, Al, Cu, Co, Ti, Zr, and Nb, with a total amount of 0.2-5.0wt%; the total amount of B is 0.8wt%-1.2wt%; and the remaining elements are Fe.

[0018] As a further technical solution, the surface activation treatment includes degreasing, ultrasonic cleaning, pickling and sandblasting.

[0019] As a further technical solution, the heat treatment includes:

[0020] Primary heat treatment, temperature range is 850℃-950℃, heat treatment time is 5h-40h;

[0021] Secondary heat treatment, temperature range is 400℃-700℃, heat treatment time is 2.5h-6.5h;

[0022] The vacuum degree of the diffusion furnace is controlled at 10 -1 -10 -4 Pa, so that the elements of the composite film layer diffuse into the interior of the sintered magnet.

[0023] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0024] The present invention uses rare earth borides, rare earth boron oxides, rare earth hydrides, and alloys composed of rare earth elements and other metal elements as diffusion sources for grain boundary diffusion into the inner surface of the magnet, and improves the binding energy of surface crystals and the crystal properties of grain boundaries, thereby inhibiting heavy rare earth elements from entering the main phase grains on the surface of the magnet, promoting heavy rare earth elements to diffuse more efficiently into the depth of the magnet, reducing the concentration difference between the surface and the center of the magnet, and thereby achieving the effect of reducing the amount of heavy rare earth, increasing the diffusion depth of the magnet, increasing the coercive force enhancement range, and reducing the residual magnetization reduction range. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0026] Figure 1 This is a scanning electron microscope image of the surface layer of the magnet prepared in Example 2;

[0027] Figure 2 This is a scanning electron microscope image of the surface layer of the magnet prepared in Comparative Example 2. DETAILED DESCRIPTION

[0028] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the technical solutions in the embodiments of the present invention are clearly and completely described below in combination with the embodiments and drawings. Obviously, the schematic implementation modes of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0029] The following is a detailed description of the embodiments of a grain boundary diffusion composite film, a rare earth permanent magnet, and a preparation method of the present invention with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions and repeated descriptions of well-known matters are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art.

[0030] It should be noted that the experimental methods used in the examples are conventional methods unless otherwise specified. The materials, reagents, methods and instruments used are conventional materials, reagents, methods and instruments in the art unless otherwise specified, and can be obtained by those skilled in the art through commercial channels.

[0031] The technical solution of the present invention is to provide a grain boundary diffusion composite film, which is attached to the surface of a sintered magnet. The raw materials of the grain boundary diffusion composite film include any one or more combinations selected from rare earth borides, rare earth boron oxides, rare earth hydrides, and alloys composed of rare earth elements and other metal elements.

[0032] The raw material of the grain boundary diffusion composite film of the present invention can be one or more combinations of the above raw materials, and the number of film layers can be a combination of one or more layers. Rare earth borides, rare earth boron oxides, rare earth hydrides, and alloys composed of rare earth elements and other metal elements are used as diffusion sources to diffuse into the inner surface layer of the magnet, thereby improving the binding energy of the surface crystals, changing the crystal properties of the epitaxial layer of the main phase grains, reducing the enrichment of rare earth elements in the surface grains of the magnet, optimizing the concentration gradient of the rare earth elements in the diffusion direction, greatly improving the utilization rate of heavy rare earth elements, reducing the amount of rare earth used, reducing the reduction range of residual magnetism, and improving the performance of the magnet.

[0033] As a preferred embodiment, the addition amount of the rare earth boride is less than 1.5% of the weight of the sintered magnet, and the addition amount of the rare earth boron oxide, the rare earth hydride and the alloy composed of the rare earth element and other metal elements is less than 5.0% of the weight of the sintered magnet.

[0034] As a preferred embodiment, the number of film layers of the grain boundary diffusion composite film is ≥ 1. Preferably, the number of film layers of the grain boundary diffusion composite film is 2, 3 or more, and the influence of the film layers on the magnet performance is improved by combining different raw materials of the multiple film layers.

[0035] As a preferred embodiment, the rare earth elements include Dy, Tb, Pr, Ho, and Y, and the other metal elements include Ga, Cu, Al, Fe, Co, Ti, Zr, and Nb.

[0036] As a preferred embodiment, the rare earth element in the rare earth boride and rare earth boron oxide is any one or a combination of Dy and Tb. The rare earth boride is preferably DyB, TbB, and the rare earth boron oxide can be DyBO, TbBO, wherein DyB and the like can form a high melting point film layer on the surface of the surface main phase grains to inhibit Tb from diffusing into the surface main phase grains.

[0037] On this basis, the present invention provides a rare earth permanent magnet, comprising a sintered magnet and the aforementioned grain boundary diffusion composite film, wherein the grain boundary diffusion composite film is attached to the surface of the sintered magnet.

[0038] On this basis, the present invention also provides a method for preparing a rare earth permanent magnet, comprising the following steps:

[0039] Prepare RTBM sintered magnets, and perform surface activation treatment on the sintered magnets after machining;

[0040] Adhere the aforementioned grain boundary diffusion composite film on the surface of the sintered magnet by screen printing or spraying process;

[0041] The sintered magnet with the grain boundary diffusion composite film attached thereto is heat treated in a vacuum sintering furnace to diffuse the elements of the composite film layer into the interior of the sintered magnet.

[0042] In the preparation method of the present invention, the diffusion source can be prepared by a melt-spinning process, an arc melting and rapid quenching process, a gas atomization granulation process and the like; it can also be directly prepared in the step of preparing the RTBM sintered magnet, before air flow milling after being mixed evenly with hydrogen powder-breaking, or it can be directly mixed and stirred evenly with fine powder after air flow milling, and then formed and sintered to obtain a blank to act as a precursor, and then the blank is used for diffusion, which also has good experimental results.

[0043] As a preferred embodiment, in the RTBM sintered magnet, R is selected from one or more of La, Ce, Pr, Nd, Dy, Tb, Gd, Ho, and Y, with a total amount of 28.0wt%-32.5wt%; M is selected from one or more of Ga, Al, Cu, Co, Ti, Zr, and Nb, with a total amount of 0.2-5.0wt%; the total amount of B is 0.8wt%-1.2wt%; and the remaining elements are Fe.

[0044] As a preferred embodiment, the surface activation treatment includes degreasing, ultrasonic cleaning, acid cleaning and sand blasting.

[0045] As a preferred embodiment, the heat treatment comprises:

[0046] Primary heat treatment, temperature range is 850℃-950℃, preferably 905℃, heat treatment time is 5h-40h, preferably 12h;

[0047] Secondary heat treatment, the temperature range is 400℃-700℃, preferably 500℃, the heat treatment time is 2.5h-6.5h, preferably 4.5h;

[0048] The vacuum degree of the diffusion furnace is controlled at 10 -1 -10 -4 Pa, so that the elements of the composite film layer diffuse into the interior of the sintered magnet.

[0049] The technical solution of the present invention is further described in detail below in conjunction with embodiments.

[0050] Example 1

[0051] A method for preparing a rare earth permanent magnet comprises the following steps:

[0052] (1) A RTBM sintered magnet is prepared by a method known to those skilled in the art, wherein R is Pr, Nd, and the total amount thereof is 30.5wt%, M is Al, Cu, Ga, Co, Zr, and Ti, and the total amount thereof is 1.5wt%, the total amount of B is 0.95wt%, and the remaining elements are Fe, and the matrix properties are Br=14.6KGs and Hcj=14.00KOe. The preparation method of the RTBM sintered magnet in this embodiment is as follows:

[0053] 1) The magnet raw materials are vacuum melted and poured into 0.2-0.35mm thick sheets at a pouring temperature of about 1440℃;

[0054] 2) Hydrogen crushing for coarse crushing and dehydrogenation at 560°C;

[0055] 3) Jet milling to prepare powder with a particle size of 3.0 μm;

[0056] 4) Use a forming press to press a 60*40*36mm blank and perform cold isostatic pressing at 200MPa;

[0057] 5) Vacuum sintering is performed at 1070°C for 6 hours to obtain a sintered magnet.

[0058] (2) machining the sintered magnet (including multi-wire, wire cutting, slicing, double-sided grinding, etc.) into a product of the desired shape, with the specification of the sample to be infiltrated being 29.0×11.5×2.0 mm, and performing surface activation treatments such as degreasing, ultrasonic cleaning / pickling, and sandblasting;

[0059] (3) Use screen printing, spraying and other processes to attach Dy on the surface of the sintered magnet in sequence 85 B 15and Tb3H 1.5 , the number of attached film layers is 2.

[0060] (4) The sintered magnet with the film layer deposited is heat treated in a vacuum sintering furnace. The heat treatment includes a primary heat treatment temperature range of 850°C-950°C and a heat treatment time of 5h-40h, a secondary heat treatment temperature range of 400°C-700°C and a heat treatment time of 2.5h-6.5h, and the vacuum degree of the diffusion furnace is controlled at 10 -1 -10 -4 Pa, so that the elements of the composite film layer diffuse into the interior of the sintered magnet.

[0061] Example 2

[0062] The difference between this embodiment and embodiment 1 is that Dy 85 B 15 ,Tb3H 1.5 and Pr 94 Al6, the number of attached film layers is 3, Dy 85 B 15 Need to be attached to the innermost layer.

[0063] Example 3

[0064] The difference between this embodiment and embodiment 1 is that Dy 85 B 15 O and Pr 12 8T 80 , the number of attached film layers is 2, Dy 85 B 15 O needs to be attached to the innermost layer.

[0065] Example 4

[0066] The difference between this embodiment and embodiment 1 is that Dy 85 B 15 O、Tb3H 1.5 and Pr 94 Al6, the number of attached film layers is 3, Dy 85 B 15 O needs to be attached to the innermost layer.

[0067] Example 5

[0068] The difference between this embodiment and embodiment 1 is that Dy 18 B 3.5 P6A 2.5 Tb 70, the number of attached film layers is 1.

[0069] Comparative Example 1

[0070] The difference between this comparative example and Example 1 is that Tb3H 1.5 , the number of film layers is 1.

[0071] Comparative Example 2

[0072] The difference between this comparative example and Example 1 is that Pr 12 8T 80 , the number of film layers is 1.

[0073] The magnets prepared in Examples 1-5 and Comparative Examples 1-2 were subjected to performance tests and SEM analysis. The performance test results are shown in Table 1. The SEM analysis results are shown in Table 1. Figure 1 and Figure 2 shown.

[0074] Table 1 Performance test data of magnets prepared in Examples 1-5 and Comparative Examples 1-2

[0075]

[0076] Table 1 except Tb3H 1.5 Except for the atomic ratio, the rest are mass percentages. The mass percentage of Dy in DyB or DyBO is 75-95%, the mass percentage of B is 5%-25%, the content of O in DyBO is less than 10000ppm, and the content of O in this embodiment is 1400ppm. The mass percentage of Tb in PrAlTb is 25-90%, the mass percentage of Pr is 0-40%, the mass percentage of Al is 0-40%, the content of Al in PrAl is less than 9.5% of the content of Pr, and the content of Dy in DyBPrAlTb is less than 25%, the mass percentage of B is less than 5%, the mass percentage of Pr is less than 10%, the mass percentage of Al is less than 5%, and the content of Tb is greater than 45%;

[0077] The above Dy 85 B 15 or Dy 85 B 15 O addition ratio is 0.2Wt%; Pr 94 The Al6 addition ratio is 0.2Wt%.

[0078] The weight gain in Table 1 refers to the percentage of Tb in the weight of the magnet. The ICP center test method is to cut out the core of the magnet with a thickness of 0.5 mm, and then grind it into powder and then test the Tb result by ICP. Under the same weight gain, the greater the ICP of the center of the magnet, the less the enrichment of Tb / Dy on the surface of the magnet.

[0079] From the data in Table 1, it can be seen that the composite film layer magnet containing boride, boron oxide or boron element in the embodiment has significantly improved magnet performance, remanence and coercivity have been improved, more Tb is tested in the center of the magnet, and the enrichment of Tb on the surface of the magnet is less, thereby inhibiting the heavy rare earth elements from entering the main phase grains on the surface of the magnet, promoting the heavy rare earth elements to diffuse more efficiently into the depth of the magnet, reducing the concentration difference between the surface and the center of the magnet, and then reducing the amount of heavy rare earth and increasing the diffusion depth of the magnet. Among them, the remanence of Example 1 decreases very little, the coercivity is improved, and more Tb is tested in the center of the magnet. In Examples 2-5, the remanence and coercivity are improved, the ICP of the center of the magnet is significantly increased, and the enrichment of Tb on the surface of the magnet is less. The comparative example is a single film layer and does not contain boride, boron oxide or boron element, and the data in all aspects are significantly different from those in Examples 1-5.

[0080] Figure 1 and Figure 2 The scanning electron microscope images of the magnet surface prepared in Example 2 and Comparative Example 2 are shown in Figures 1 and 2. By comparison, the magnet surface prepared in the comparative example has more rare earth element enrichment ( Figure 2 The surface grains of the magnet prepared by the present invention are more intact, with complete grain outlines and more uniform distribution of rare earth elements.

[0081] The above indicates that the composite film layer magnet containing boride, boron oxide and boron element has a good improvement effect on the distribution of heavy rare earth, especially when multiple types of film layers are combined, it has a better effect of improving the magnet performance.

[0082] Example 6

[0083] The difference between this embodiment and embodiment 1 is that Y3H 1.5 ,Tb3H 1.5 , the number of film layers is 2, Y3H 1.5 Need to be attached to the innermost layer.

[0084] Example 7

[0085] The difference between this embodiment and embodiment 1 is that Y3H 1.5 、Dy 85 B 15 ,Tb3H 1.5 , the number of film layers is 3, Y3H 1.5 Need to be attached to the innermost layer.

[0086] Example 8

[0087] The difference between this embodiment and embodiment 1 is that Dy 85 B 15 ,Y3Tb 97 , the number of film layers is 2, Dy 85 B 15 Need to be attached to the innermost layer.

[0088] Example 9

[0089] The difference between this embodiment and embodiment 1 is that Dy 85 B 15 ,Y2Tb 90 Cu8, the number of film layers is 2, Dy 85 B 15 Need to be attached to the innermost layer.

[0090] Comparative Example 3

[0091] The difference between this embodiment and embodiment 1 is that TbH is attached to the surface of the sintered magnet by using a process such as silk screen printing or spraying, and the number of film layers is one.

[0092] Comparative Example 4

[0093] The difference between this embodiment and embodiment 1 is that TbCu is attached to the surface of the sintered magnet by using a process such as silk screen printing or spraying, and the number of film layers is one.

[0094] Table 2 Performance test data of magnets prepared in Examples 6-9 and Comparative Examples 3-4

[0095]

[0096]

[0097] In Table 2, except Tb3H 1.5 ,Y3H 1.5 Except for the atomic percentage, the rest are mass percentages. The mass percentage of Dy in DyB is 75-95%, the mass percentage of B is 5%-25%, the mass percentage of Tb in YTb is 90-99.9%, the amount of Y added to the film is 0.1-10%, the mass percentage of Tb in TbCu is 25-90%, the mass percentage of Cu is 10-75%, and the Y content in YTbCu is 0.1-10%, the mass percentage of Tb is 25-90%, and the mass percentage of Cu is 10-75%;

[0098] The above Dy 85 B 15 The addition ratio is 0.2Wt%; Y3H 1.5 The addition ratio was 0.43 wt%.

[0099] It can be seen from Table 2 that the magnet performance is obviously improved to a certain extent by using the magnet with Y element composite diffusion. The influence of Y on the magnet surface on the change of the crystal structure and the formation energy of the magnet has a certain effect of inhibiting the Tb element from entering the surface main phase grains.

[0100] Finally, it should be noted that the above specific embodiments are only used to explain in detail the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above description is only a specific implementation method of the present invention and is not used to limit the protection scope of the present invention. Although the present invention is described in detail with reference to the above specific embodiments, ordinary technicians in this field should understand that it is still possible to modify the technical solutions recorded in the above embodiments, or to replace or improve some or all of the technical features therein. These modifications, equivalent replacements and improvements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and description of the present invention.

Claims

1. A grain boundary diffusion composite film, characterized in that: The grain boundary diffusion composite film is attached to the surface of the NdFeB sintered magnet, and the raw materials of the grain boundary diffusion composite film include various combinations of rare earth borides, rare earth boron oxides, rare earth hydrides, and alloys composed of rare earth elements and other metal elements; The number of film layers of the grain boundary diffusion composite film is ≥ 2, one of which is a rare earth boride or a rare earth boron oxide, and the other layers are rare earth hydrides or alloys composed of rare earth elements and other metal elements; The rare earth elements include Dy, Tb, Ho, and Y, and the other metal elements include Ga, Cu, Al, Fe, Co, Ti, Zr, and Nb; The rare earth element in the rare earth boride and rare earth boron oxide is any one of Dy and Tb or a combination of both.

2. The grain boundary diffusion composite film according to claim 1, characterized in that: The addition amount of the rare earth boride is less than 1.5% of the weight of the sintered magnet, and the addition amount of the rare earth boron oxide, the rare earth hydride and the alloy composed of the rare earth element and other metal elements is less than 5.0% of the weight of the sintered magnet.

3. A rare earth permanent magnet, characterized in that: It comprises a NdFeB sintered magnet and a grain boundary diffusion composite film as claimed in any one of claims 1 to 2, wherein the grain boundary diffusion composite film is attached to the surface of the NdFeB sintered magnet.

4. A method for preparing a rare earth permanent magnet, characterized in that: The following steps are involved: Prepare RTBM sintered magnets, and perform surface activation treatment on the sintered magnets after machining; Adhere the grain boundary diffusion composite film as claimed in any one of claims 1 to 2 on the surface of the sintered magnet by screen printing or spraying; The sintered magnet with the grain boundary diffusion composite film attached thereto is heat treated in a vacuum sintering furnace to diffuse the elements of the composite film layer into the interior of the sintered magnet.

5. The method for preparing a rare earth permanent magnet according to claim 4, characterized in that: In the RTBM sintered magnet, R is selected from one or more of La, Ce, Pr, Nd, Dy, Tb, Gd, Ho, and Y, with a total amount of 28.0wt%-32.5wt%; M is selected from one or more of Ga, Al, Cu, Co, Ti, Zr, and Nb, with a total amount of 0.2wt%-5.0wt%; B has a total amount of 0.8wt%-1.2wt%; and the remaining elements are Fe.

6. The method for preparing a rare earth permanent magnet according to claim 4, characterized in that: The surface activation treatment includes degreasing, ultrasonic cleaning, pickling and sandblasting.

7. A method for preparing a rare earth permanent magnet according to any one of claims 4 to 6, characterized in that: The heat treatment comprises: Primary heat treatment, temperature range is 850℃-950℃, heat treatment time is 5h-40h; Secondary heat treatment, temperature range is 400℃-700℃, heat treatment time is 2.5h-6.5h; The vacuum degree of the diffusion furnace is controlled at 10 -1 -10 -4 Pa, so that the elements of the composite film layer diffuse into the interior of the sintered magnet.

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