Grain boundary diffusion method and application thereof in preparation of sintered neodymium-iron-boron magnet

Abstract

The invention discloses a grain boundary diffusion method and application thereof in preparation of a sintered neodymium-iron-boron magnet. The grain boundary diffusion method comprises the following steps: providing a rare earth permanent magnet base material; after a heavy rare earth metal layer and a metal protection layer are sequentially formed on the surface of the rare earth permanent magnet base material, permeation and diffusion treatment is carried out, and grain boundary diffusion is completed; wherein the heavy rare earth metal layer is composed of heavy rare earth metal dysprosium, and the metal protection layer is composed of metal copper. According to the grain boundary diffusion method, a metal protection layer copper is formed on the surface of a heavy rare earth metal layer dysprosium, evaporation of dysprosium elements under the vacuum high-temperature condition is avoided, and it is guaranteed that the heavy rare earth metal dysprosium is diffused into a magnet grain boundary phase to the maximum extent; meanwhile, the metal protective layer copper reacts with the heavy rare earth diffusion source to form a binary alloy diffusion source, the binary alloy diffusion source is diffused into a grain boundary, uniform distribution of a rare earth-rich phase is improved, the diffusion effect of heavy rare earth metal elements is improved, the coercivity of the magnet is obviously improved, and efficient utilization of heavy rare earth resources is achieved.

Description

Technical Field

[0001] The present application belongs to the technical field of permanent magnetic materials, and in particular relates to a grain boundary diffusion method, and also relates to the application of the grain boundary diffusion method in the preparation of sintered NdFeB magnets. Background Art

[0002] In recent years, new energy vehicles have developed rapidly. Since their automotive magnets require high temperature resistance, they need to use high remanence and high coercive force permanent magnets. The demand for sintered rare earth permanent magnet materials in the field of new energy vehicles has increased sharply year by year.

[0003] Due to the high temperature resistance of permanent magnets for new energy vehicles, sintered NdFeB permanent magnets must have high coercivity to meet the requirements of use. Considering the use requirements and manufacturing costs of permanent magnets, the commonly used method is to use the grain boundary diffusion infiltration process. Specifically, first, a sintered NdFeB substrate blank permanent magnet with high remanence and low coercivity is manufactured (low formulation cost). The substrate is then processed into a semi-finished magnet. The surface of the magnet is then plated with heavy rare earth metals dysprosium Dy and / or terbium Tb, or a heavy rare earth dysprosium Dy and / or terbium Tb alloy using a coating or coating process. Grain boundary diffusion treatment is then performed under high temperature and vacuum, allowing the heavy rare earth elements dysprosium Dy and / or terbium Tb to enter the grain boundary phase and the surface layer of the main phase grains, ultimately increasing the coercivity of the grain surface layer, thereby obtaining a sintered NdFeB magnet with high coercivity. In addition, since the heavy rare earth element terbium Tb is more expensive than the heavy rare earth metal dysprosium Dy, considering the cost factor, it is usually more competitive to diffuse the heavy rare earth metal dysprosium Dy at the grain boundary.

[0004] However, the current magnet grain boundary diffusion process, heavy rare earth metal Dy is usually in vacuum (vacuum degree less than 10 -2 Pa) and high temperature (between 800℃ and 920℃) to diffuse and penetrate in the magnet grain boundary. However, the heavy rare earth metal dysprosium Dy has a high saturated vapor pressure (about 1200×(10 -2 –10 -1 )Pa), resulting in a large loss of heavy rare earth technology dysprosium Dy due to high-temperature volatilization, resulting in a reduction of heavy rare earth metal dysprosium Dy elements diffusing into the magnet grain boundaries, which not only reduces the improvement of the magnet's coercive force, but also wastes heavy rare earth metal dysprosium Dy resources.

[0005] Existing solutions add a layer of high-melting-point metal to the surface of the heavy rare earth elements. This layer prevents oxidation and thermal volatilization of the heavy rare earth layer, taking advantage of the fact that the high-melting-point metal remains insoluble during heat treatment. Simultaneously, the molten heavy rare earth layer, influenced by the gravity of the high-melting-point metal layer, continuously penetrates into the substrate, enhancing penetration efficiency. Although high-melting-point metals do not melt at high temperatures, some will diffuse into the magnet's surface. Furthermore, some high-melting-point metals, such as tungsten and molybdenum, do not form a eutectic reaction with the rare-earth-rich phase on the magnet's surface, thus reducing the magnet's magnetic properties. Summary of the Invention

[0006] In view of this, it is necessary for the present application to provide a grain boundary diffusion method to form a metal protective layer on the surface of the heavy rare earth metal layer, thereby avoiding the evaporation of heavy rare earth metal elements under vacuum and high temperature conditions, and ensuring that the maximum amount of heavy rare earth metal elements diffuse into the grain boundary phase of the magnet. At the same time, the high-temperature metal protective film layer in the present application reacts with the heavy rare earth diffusion source to form a new binary alloy diffusion source, which diffuses into the grain boundary, improves the uniform distribution of the rare earth-rich phase, improves the diffusion effect of the heavy rare earth metal elements, significantly improves the coercive force of the magnet, and also realizes the efficient utilization of heavy rare earth resources.

[0007] In order to achieve the above objectives, this application adopts the following technical solutions:

[0008] One aspect of the present application provides a grain boundary diffusion method, comprising the following steps:

[0009] Provide rare earth permanent magnet substrate;

[0010] After forming a heavy rare earth metal layer and a metal protective layer in sequence on the surface of the rare earth permanent magnet substrate, performing an infiltration diffusion treatment to complete grain boundary diffusion;

[0011] Wherein, the heavy rare earth metal layer is composed of heavy rare earth metal dysprosium, and the metal protective layer is composed of metal copper.

[0012] Another aspect of the present application provides the use of the grain boundary diffusion method described above in preparing sintered NdFeB magnets.

[0013] Another aspect of the present application provides a method for preparing a sintered NdFeB magnet, comprising the following steps:

[0014] The heavy rare earth metal is diffused using the grain boundary diffusion method described above;

[0015] The diffused magnet is subjected to aging treatment to obtain a sintered NdFeB magnet.

[0016] Compared with the prior art, this application has the following beneficial effects:

[0017] This application forms a heavy rare earth metal dysprosium Dy as a heavy rare earth diffusion layer on the surface of a permanent magnet, and then forms a layer of metallic copper (Cu) on the surface. On the one hand, the metallic copper (Cu) layer protects the metallic Dy from evaporation, ensuring that the coated heavy rare earth metal dysprosium Dy diffuses into the magnet grain boundary phase to the maximum extent. On the other hand, under vacuum and high temperature, the metallic copper (Cu) reacts with the dysprosium Dy to form a low-melting-point Dy-Cu binary alloy. This binary alloy serves as a new diffusion source, diffusing into the grain boundaries, improving the uniform distribution of the grain boundary phase, increasing coercivity, and achieving efficient utilization of heavy rare earth resources. At the same time, the metal protective film layer protects the heavy rare earth dysprosium Dy from oxidation, so the diffusion of dysprosium Dy into the magnet grain boundaries does not increase the oxygen content of the magnet grain boundaries and reduce the diffusion effect (the higher the oxygen content, the worse the diffusion effect of the heavy rare earth element), thereby reducing the magnetic properties (coercivity). DETAILED DESCRIPTION

[0018] The embodiments of the present application are described in detail below. The embodiments described below are exemplary and are only used to explain the present application, and should not be understood as limiting the present application.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0020] In a first aspect, the present application provides a grain boundary diffusion method for sintering NdFeB magnets, which mainly comprises the following steps:

[0021] S100, providing rare earth permanent magnet substrate

[0022] Specifically, the rare earth permanent magnet substrate can be obtained by conventional methods in the art, that is, by machining and surface cleaning. According to the embodiments of the present application, the rare earth permanent magnet blank is machined (such as slicing or multi-wire cutting slicing) into small-sized magnets, and then the processed magnets are surface treated, that is, cleaned. It can be understood that the cleaning here adopts the conventional methods in the art, generally including degreasing, pickling and ultrasonic cleaning, etc., but is not limited to this. Since it is a conventional means, it is not further elaborated here. In addition, the rare earth permanent magnet blank used in this article is not particularly limited. In the embodiments of the present application, the composition of the rare earth permanent magnet blank is: the total rare earth RE mass content is ≤31.5%, wherein the total rare earth RE is the sum of light rare earth elements and heavy rare earth elements, including one or more of praseodymium Pr, neodymium Nd, dysprosium Dy, terbium Tb, holmium Ho, and gadolinium Gd.

[0023] S200, forming a heavy rare earth metal layer and a metal protective layer

[0024] In step S100, a heavy rare earth metal layer composed of a heavy rare earth metal dysprosium Dy film is formed on the surface of the rare earth permanent magnet substrate. Furthermore, the heavy rare earth metal layer can be formed by conventional coating methods in the art, and specific examples include magnetron sputtering, vacuum coating or coating. Preferably, in the embodiment of the present application, PVD vacuum coating is used, and the coating vacuum is controlled at 1×10 –1 Below Pa.

[0025] Subsequently, a metal protective layer composed of copper (Cu) is formed on the surface of the heavy rare earth metal layer. It is understood that the metal protective layer is also formed by conventional coating methods in the art. In the embodiment of the present application, the first thin film layer is formed by PVD vacuum coating, and the coating vacuum is controlled at 1×10 –1 Below Pa.

[0026] The heavy rare earth metal layer is protected by a metal protective layer to avoid the evaporation and waste of the heavy rare earth element Dy in the heavy rare earth metal layer during the vacuum high-temperature diffusion process, thereby allowing the heavy rare earth element Dy to diffuse into the magnet in the largest amount; at the same time, under vacuum and high temperature, metallic copper Cu reacts with dysprosium Dy to form a low-melting-point Dy-Cu binary alloy, which is used as a new diffusion source to diffuse into the grain boundary, thereby improving the uniform distribution of the grain boundary phase, increasing the coercive force, and realizing the efficient utilization of heavy rare earth resources. It should be noted that in this application, the diffusion degree and diffusion amount of the heavy rare earth element can be controlled by adjusting the thickness of the heavy rare earth metal layer. As an example, the thickness of the formed heavy rare earth metal layer is between 10-50 μm, for example, it can be any thickness of 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm and 50 μm, or a range value between any two thicknesses. The thickness of the metal protection layer is between 1 μm and 6 μm, for example, it can be any thickness of 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or 6 μm, or a range between any two thicknesses.

[0027] S300, high temperature diffusion treatment

[0028] The magnet obtained in step S200 is subjected to an infiltration diffusion treatment under high temperature and vacuum conditions. The specific infiltration diffusion treatment process can adopt the conventional infiltration diffusion treatment in the art. In the infiltration diffusion treatment process, the vacuum degree is lower than 8.0×10 –3 Pa, under the temperature condition of 850℃-910℃, keep warm for 6-20 hours, and the heavy rare earth elements will quickly diffuse into the deep grain boundaries along the liquid phase grain boundaries at high temperature. Among them, the penetration and diffusion time at 850℃-910℃ can be adjusted according to the penetration thickness of the magnet. Generally speaking, the time for magnet penetration is 3-4 hours for every 1mm of magnet thickness. The thicker the magnet, the longer the penetration and diffusion time.

[0029] Through the above-mentioned grain boundary diffusion method, metallic copper (Cu) reacts with dysprosium (Dy) under vacuum and high temperature to form a low-melting-point Dy-Cu binary alloy. The binary alloy serves as a new diffusion source and diffuses into the grain boundaries, thereby improving the uniform distribution of the grain boundary phase, increasing the coercive force, and realizing efficient utilization of heavy rare earth resources. At the same time, the metal protective film layer protects the heavy rare earth dysprosium (Dy) from evaporation and oxidation, allowing the maximum amount of Dy to diffuse into the interior of the magnet. Moreover, the diffusion of dysprosium (Dy) into the magnet grain boundaries does not lead to an increase in the oxygen content of the magnet grain boundaries and reduce the diffusion effect (the higher the oxygen content, the worse the diffusion effect of the heavy rare earth element), thereby reducing the magnetic properties (coercive force).

[0030] The second aspect of the present application provides the application of the grain boundary diffusion method described in the first aspect of the present application in preparing sintered NdFeB magnets. The specific preparation method of the sintered NdFeB magnets is not particularly limited and will not be elaborated here.

[0031] In some specific embodiments of the present application, a method for preparing a sintered NdFeB magnet is provided, specifically, including grain boundary diffusion and aging treatment of the magnet after grain boundary diffusion, wherein the grain boundary diffusion is performed using the grain boundary diffusion method described in the first aspect of the present application. It is understood that aging heat treatment is a conventional magnet processing process in the art. According to the embodiments of the present application, aging heat treatment is performed under high temperature and vacuum conditions to improve the performance of the sintered NdFeB magnet. In one or more embodiments of the present application, the vacuum degree of the aging heat treatment is 9×10 –1 Pa below, the aging temperature is 480-510°C, and the holding time is 4-6 hours. The sintered NdFeB magnet prepared in the present application has high coercive force and excellent magnetic properties.

[0032] The present application is described below through specific examples. It should be noted that the following specific examples are only for illustrative purposes and do not limit the scope of the present application in any way. In addition, unless otherwise specified, methods without specific conditions or steps are conventional methods, and the reagents and materials used can be obtained from commercial channels.

[0033] Example 1

[0034] This embodiment discloses a method for preparing sintered NdFeB permanent magnets using grain boundary diffusion, and the specific steps are as follows:

[0035] S1. Select sintered NdFeB permanent magnet blank with a total rare earth mass percentage of 31, the specific composition is: (PrNd) 30.4 Tb 0.6 B 0.97 Al 0.2 Co 01.5 Cu 0.15 Ga 0.2Zr 0.2 Fe 65.78 The multi-wire slices were processed into 20mm×20mm×5mm(M) magnets. After degreasing, pickling and ultrasonic cleaning, the magnets were coated with conventional fully automatic PVD equipment in a vacuum of 9.5×10 –2 Automatically deposit the film under Pa conditions to form a heavy rare earth metal dysprosium Dy film with a thickness of 50μm on the surface of the permanent magnet;

[0036] S2, the permanent magnet obtained in step S1 is subjected to PVD process at a vacuum degree of 8.0×10 –2 Under Pa conditions, a metal copper Cu film layer with a thickness of 6 μm is formed on the surface of the permanent magnet;

[0037] S3, placing the permanent magnet obtained in step S2 in a heat treatment furnace at a vacuum degree of 5.0×10 –3 Under Pa conditions, heat to 900℃ and keep warm for 15 hours to carry out infiltration and diffusion treatment; after the infiltration and diffusion time is over, fill with argon Ar and cool to below 85℃ with an air cooler;

[0038] S4, placing the permanent magnet after heat treatment in step S3 in a heat treatment furnace, and –1 The magnets were heated to 500°C under Pa conditions and kept at this temperature for 5 hours for aging heat treatment. After the aging time, argon gas was filled and the magnets were cooled to below 55°C with a blower, and then taken out of the furnace to obtain high coercive force sintered NdFeB magnets.

[0039] Comparative Example 1-1

[0040] This comparative example adopts the same implementation method as Example 1, except that: in this comparative example, only a heavy rare earth metal dysprosium Dy film with a thickness of 50 μm is plated on the surface of the permanent magnet, and a metal copper Cu film layer is not formed. The other process steps and parameter conditions are the same as those in Example 1.

[0041] Comparative Example 1-2

[0042] This comparative example uses the same method as Example 1, except that a 50 μm thick heavy rare earth metal dysprosium (Dy) film is plated on the surface of the permanent magnet and a 6 μm thick tungsten layer is formed. Other process steps and parameter conditions are the same as Example 1.

[0043] The sintered NdFeB magnets obtained in Example 1 and Comparative Examples 1-1 and 1-2 were respectively subjected to room temperature (25°C) magnetic property tests (see the "Guide to Permanent Magnet (Hard Magnet) Pulse Measurement Methods" GB / T 29628-2013 for the test standards). The test results are shown in Table 1.

[0044] Table 1 Test results of room temperature magnetic properties of sintered NdFeB magnets in Example 1 and Comparative Examples 1-1 and 1-2

[0045]

[0046] Example 2

[0047] This embodiment discloses a method for preparing sintered NdFeB permanent magnets using grain boundary diffusion, and the specific steps are as follows:

[0048] S1. Select sintered NdFeB permanent magnet blank with a total rare earth mass percentage of 30.8, the specific composition is: (PrNd) 29.8 Dy 1.0 B 0.98 Al 0.25 Co 1.0 Cu 0.15 Ga 0.2 Nb 0.15 Fe 66.47 The magnets were sliced ​​and processed into 20mm×20mm×4mm(M). After degreasing, pickling and ultrasonic cleaning, the magnets were coated with conventional fully automatic PVD equipment at a vacuum degree of 7.0×10 –2 Automatically deposit the film under Pa conditions to form a 30μm thick heavy rare earth metal dysprosium Dy film on the surface of the permanent magnet;

[0049] S2, the permanent magnet obtained in step S1 is subjected to PVD process at a vacuum degree of 6.0×10 –2 Under Pa conditions, a metal copper Cu film layer with a thickness of 5 μm is formed on the surface of the permanent magnet;

[0050] S3, placing the permanent magnet obtained in step S2 in a heat treatment furnace at a vacuum degree of 5.5×10 –3 Under Pa conditions, the mixture was heated to 900°C and kept at this temperature for 12 hours to perform the infiltration and diffusion treatment. After the infiltration and diffusion time was over, the mixture was filled with argon gas and cooled to below 80°C using an air cooler.

[0051] S4, placing the permanent magnet obtained in step S3 in a heat treatment furnace at a vacuum degree of 7.0×10 –1 Under Pa conditions, the magnets were heated to 480°C and kept at that temperature for 5 hours for secondary aging treatment. After the aging time, argon gas was filled and the magnets were cooled to below 50°C with a blower, and then taken out of the furnace to obtain high coercive force sintered NdFeB magnets.

[0052] Comparative Example 2-1

[0053] This comparative example adopts the same implementation method as Example 2, with the only difference being that in this comparative example, only the heavy rare earth metal terbium Dy with a thickness of 30 μm is plated on the surface of the permanent magnet, without forming a metal copper Cu film layer. The remaining process steps and parameter conditions are the same as Example 2.

[0054] The permanent magnets obtained in Example 2 and Comparative Example 2-1 were respectively subjected to room temperature (25°C) magnetic property tests (for the test standard, see the "Guide to Permanent Magnet (Hard Magnet) Pulse Measurement Method" GB / T 29628-2013). The test results are shown in Table 2.

[0055] Table 2 Test results of room temperature magnetic properties of sintered NdFeB magnets in Example 2 and Comparative Example 2-1

[0056]

[0057] Example 3

[0058] This embodiment discloses a method for preparing sintered NdFeB permanent magnets using grain boundary diffusion, and the specific steps are as follows:

[0059] S1. Select sintered NdFeB permanent magnet blank with a total rare earth mass percentage of 31.5, the specific composition is: (PrNd) 30.4 Dy 0.6 Tb 0.5 B 0.98 Co 1.5 Cu 0.2 Ga 0.2 Zr 0.2 Fe 64.92 The magnets were sliced ​​and processed into 20mm×20mm×3mm(M). After degreasing, pickling and ultrasonic cleaning, the magnets were coated with conventional fully automatic PVD equipment in a vacuum of 7.5×10 –2 Automatically depositing film under Pa conditions, a heavy rare earth metal dysprosium Dy film with a thickness of 25μm is formed on the surface of the permanent magnet;

[0060] S2, the permanent magnet obtained in step S1 is subjected to PVD process at a vacuum degree of 8.0×10 –2 Under Pa conditions, a copper Cu film with a thickness of 4 μm is formed on the surface of the permanent magnet;

[0061] S3, placing the permanent magnet obtained in step S2 in a heat treatment furnace at a vacuum degree of 7.5×10 –3 Under Pa conditions, the mixture was heated to 910°C and kept for 9 hours for infiltration and diffusion treatment. After the infiltration and diffusion time was over, the mixture was filled with argon gas and cooled to below 90°C using an air cooler.

[0062] S4, placing the permanent magnet obtained in step S3 in a heat treatment furnace at a vacuum degree of 4.0×10–1 The magnets were heated to 510°C under Pa conditions and kept at that temperature for 4 hours for secondary aging treatment. After the aging time, argon gas was filled and the magnets were cooled to below 55°C with a blower, and then taken out of the furnace to obtain high coercive force sintered NdFeB magnets.

[0063] Comparative Example 3-1

[0064] This comparative example adopts the same implementation method as Example 3, with the only difference being that in this comparative example, only the heavy rare earth metal dysprosium Dy with a thickness of 25 μm is plated on the surface of the permanent magnet, without forming a metal copper Cu film layer. The remaining process steps and parameter conditions are the same as those in Example 3.

[0065] The sintered NdFeB magnets obtained in Example 3 and Comparative Example 3-1 were respectively subjected to room temperature (25°C) magnetic property tests (for the test standard, see the "Guide to Permanent Magnet (Hard Magnet) Pulse Measurement Methods" GB / T 29628-2013). The test results are shown in Table 3.

[0066] Table 3 Room temperature magnetic properties test results of sintered NdFeB magnets in Example 3 and Comparative Example 3-1

[0067]

[0068] Example 4

[0069] This embodiment discloses a method for preparing sintered NdFeB permanent magnets using grain boundary diffusion, and the specific steps are as follows:

[0070] S1. Select sintered NdFeB permanent magnet blank with a total rare earth mass percentage of 30.5, the specific composition is: (PrNd) 27 Dy 1.0 Ho 1.0 Gd 1.5 B 0.96 Co 1.0 Cu 0.2 Ga 0.2 Nb 0.1 Zr 0.15 Fe 66.89 The magnets were sliced ​​and processed into 20mm×20mm×2mm(M). After degreasing, pickling and ultrasonic cleaning, the magnets were coated with conventional fully automatic PVD equipment at a vacuum degree of 5.5×10 –2 Automatically depositing film under Pa conditions, a heavy rare earth metal dysprosium Dy film with a thickness of 15μm is formed on the surface of the permanent magnet;

[0071] S2, the permanent magnet in step S1 is made by PVD process at a vacuum degree of 7.0×10 –2 Under Pa conditions, a metal copper Cu film layer with a thickness of 3 μm is formed on the surface of the permanent magnet;

[0072] S3, placing the permanent magnet obtained in step S2 in a heat treatment furnace at a vacuum degree of 7.5×10 –3 Under Pa conditions, the mixture was heated to 890°C and kept at this temperature for 6 hours for infiltration and diffusion treatment. After the infiltration and diffusion time was over, the mixture was filled with argon gas and cooled to below 85°C using an air cooler.

[0073] S4, placing the permanent magnet obtained in step S3 in a heat treatment furnace at a vacuum degree of 7.0×10 –2 The magnets were heated to 480°C under Pa conditions and kept at this temperature for 4 hours for secondary aging treatment. After the aging time, argon gas was filled and the magnets were cooled to below 55°C with a blower, and then taken out of the furnace to obtain high coercive force sintered NdFeB magnets.

[0074] Comparative Example 4-1

[0075] This comparative example adopts the same implementation method as Example 4, except that: this comparative example only plates a heavy rare earth metal dysprosium Dy film with a thickness of 15 μm on the surface of the permanent magnet, and does not form a metal copper Cu film layer. The remaining process steps and parameter conditions are the same as Example 4.

[0076] The permanent magnets obtained in Example 4 and Comparative Example 4-1 were respectively subjected to room temperature (25°C) magnetic property tests (for the test standard, see the "Guide to Permanent Magnet (Hard Magnet) Pulse Measurement Method" GB / T 29628-2013), and the test results are shown in Table 4.

[0077] Table 4 Room temperature magnetic properties test results of sintered NdFeB magnets in Example 4 and Comparative Example 4-1

[0078]

[0079] Example 5

[0080] This embodiment discloses a method for preparing sintered NdFeB permanent magnets using grain boundary diffusion, and the specific steps are as follows:

[0081] S1. Select sintered NdFeB permanent magnet blank with a total rare earth mass percentage of 30.3, the specific composition is: (PrNd) 28.1 Dy 1.2 Gd 1.0 B 0.98 Al 0.35 Co 1.5 Cu 0.2 Ga 0.2 Nb 0.2 Fe 66.27The magnets were sliced ​​and processed into 20mm×23mm×1.5mm(M). The magnets were degreased, pickled, and ultrasonically cleaned. Conventional fully automatic PVD coating equipment was used with a vacuum degree of 4.5×10 –2 Automatically depositing under Pa conditions, a heavy rare earth metal dysprosium Dy film with a thickness of 10 μm is formed on the surface of the permanent magnet;

[0082] S2, the permanent magnet obtained in step S1 is subjected to PVD process at a vacuum degree of 7.0×10 –2 Under the conditions of Pa, a metal copper Cu layer with a thickness of 1 μm is formed on the surface of the permanent magnet;

[0083] S3, placing the permanent magnet obtained in step S2 in a heat treatment furnace at a vacuum degree of 3.5×10 –3 Under Pa conditions, the mixture was heated to 900°C and kept at this temperature for 6 hours to perform the infiltration and diffusion treatment. After the infiltration and diffusion time was over, the mixture was filled with argon gas and cooled to below 85°C using an air cooler.

[0084] S4, the permanent magnet obtained in step S3 is placed in a heat treatment furnace with a vacuum degree of 9.0×10 –2 The magnets were heated to 500°C under Pa conditions and kept at this temperature for 5 hours for secondary aging treatment. After the aging time, argon gas was filled and the magnets were cooled to below 50°C with a blower, and then taken out of the furnace to obtain high coercive force sintered NdFeB magnets.

[0085] Comparative Example 5-1

[0086] This comparative example adopts the same implementation method as Example 5, except that: this comparative example only plates a heavy rare earth metal dysprosium Dy film layer with a thickness of 10 μm on the surface of the permanent magnet, and does not form a metal copper Cu film layer. The remaining process conditions and step parameters are the same as Example 5.

[0087] The sintered NdFeB magnets obtained in Example 5 and Comparative Example 5-1 were respectively subjected to room temperature (25°C) magnetic property tests (for the test standard, see the "Guide to Permanent Magnet (Hard Magnet) Pulse Measurement Methods" GB / T 29628-2013), and the test results are shown in Table 5.

[0088] Table 5 Room temperature magnetic properties test results of sintered NdFeB magnets in Example 5 and Comparative Example 5-1

[0089]

[0090] From the test results of the NdFeB magnets in Examples 1-5 above, it can be seen that the preparation method in the present application adds a layer of metal copper Cu film as a protective layer after the magnet is coated with heavy rare earth metal dysprosium Dy, which prevents the evaporation of metal dysprosium Dy and ensures that the coated heavy rare earth metal dysprosium Dy diffuses into the magnet grain boundary phase in the largest amount; and under vacuum and high temperature, metal copper Cu reacts with dysprosium Dy to form a low-melting-point Dy-Cu binary alloy, which serves as a new diffusion source and diffuses into the grain boundary, thereby improving the uniform distribution of the grain boundary phase, increasing the coercive force, and realizing efficient utilization of heavy rare earth resources.

[0091] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0092] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. A grain boundary diffusion method, characterized in that: The following steps are involved: Provide rare earth permanent magnet substrate; After forming a heavy rare earth metal layer and a metal protective layer in sequence on the surface of the rare earth permanent magnet substrate, performing an infiltration diffusion treatment to complete grain boundary diffusion; Wherein, the heavy rare earth metal layer is composed of heavy rare earth metal dysprosium, and the metal protective layer is composed of metal copper.

2. The grain boundary diffusion method according to claim 1, wherein: The rare earth permanent magnet substrate is obtained by mechanically processing a sintered rare earth permanent magnet blank and then cleaning the surface to obtain the rare earth permanent magnet substrate.

3. The grain boundary diffusion method according to claim 2, wherein: The mechanical processing method is slicing or multi-wire cutting slicing.

4. The grain boundary diffusion method according to claim 1, wherein: The thickness of the heavy rare earth metal layer is between 10 and 50 μm.

5. The grain boundary diffusion method according to claim 1, wherein: The thickness of the metal protective layer is between 1 and 6 μm.

6. The grain boundary diffusion method according to claim 1, wherein: The method of forming the heavy rare earth metal layer and the metal protective layer is magnetron sputtering or coating; Preferably, the coating is spraying or printing.

7. The grain boundary diffusion method according to claim 1, wherein: The infiltration diffusion treatment process is specifically as follows: keeping the temperature at 850° C. to 910° C. for 6 to 20 hours under vacuum conditions.

8. Use of the grain boundary diffusion method according to any one of claims 1 to 7 in preparing sintered NdFeB magnets.

9. A method for preparing a sintered NdFeB magnet, characterized in that: The following steps are involved: Diffusion of heavy rare earth metals by the grain boundary diffusion method according to any one of claims 1 to 7; The diffused magnet is subjected to aging treatment to obtain a sintered NdFeB magnet.

10. The method for preparing a sintered NdFeB magnet according to claim 9, wherein: The aging treatment process is specifically as follows: keeping the temperature at 480° C. to 510° C. for 4 to 6 hours under vacuum conditions.

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