A grain boundary diffusion method for preparing high-performance shaped NdFeB magnets

Abstract

The present invention discloses a grain boundary diffusion method for preparing high-performance special-shaped NdFeB magnets. The method is as follows: on the surface of a special-shaped sintered NdFeB magnet without copper, a copper coating with a thickness of 0.1-1 μm is deposited on the magnet surface by electroplating. The diffusion source powder is mixed with a binder and an organic solvent to prepare a diffusion source slurry. The diffusion source slurry is uniformly covered on the copper coating on the magnet surface by impregnation or spraying; then it is dried in an oven; the magnet is subjected to grain boundary diffusion treatment and then secondary aging treatment to obtain the high-performance special-shaped NdFeB magnet. The diffusion source coating of the present invention has a uniform thickness, strong bonding force, and is not easy to fall off. Moreover, depositing a copper coating between the magnet and the diffusion source can prevent C atoms in the diffusion source from diffusing into the interior of the NdFeB magnet matrix, promote the grain boundary diffusion of heavy rare earth elements, thereby increasing the diffusion depth of heavy rare earth atoms, and further improving the grain boundary diffusion effect of the magnet and the coercivity of the magnet.

Description

Technical Field

[0001] The present invention relates to a grain boundary diffusion method for preparing high-performance special-shaped neodymium-iron-boron magnets, belonging to the field of rare earth magnets. Background Art

[0002] Neodymium-iron-boron rare earth permanent magnets are a kind of permanent magnetic materials with excellent performance. Compared with other permanent magnetic materials, they have the highest maximum magnetic energy product, so they are widely used in modern industry. The advantage of the high magnetic energy product of neodymium-iron-boron magnets is conducive to the miniaturization of equipment. In recent years, with the rapid development of electric vehicles, the consumption of neodymium-iron-boron magnets in automobile engines has increased year by year.

[0003] The Curie temperature of neodymium-iron-boron magnets is relatively low, and the magnetic properties of the magnets decrease with the increase of temperature. The working temperature of automobile engines is about 200°C. At present, the high-temperature magnetic properties of the magnets are mainly ensured by increasing the room-temperature coercivity of the magnets. By adding a certain amount of heavy rare earth elements such as Dy and Tb to the magnets, the room-temperature coercivity of the magnets can be significantly increased. However, since the heavy rare earth atoms such as Dy and Tb are antiferromagnetically coupled with iron atoms, the magnetization intensity of the magnets will be reduced. Although adding a large amount of heavy rare earth elements during the melting stage can increase the coercivity of the magnets, the remanence of the magnets will also be reduced accordingly.

[0004] Adopting grain boundary diffusion can obtain a large coercivity increment while slightly reducing the remanence. The grain boundary diffusion process first deposits heavy rare earth elements on the surface of the magnet by means of powder coating, PVD coating or impregnation, etc., and then heats to a specified temperature and holds for a period of time. At high temperature, the heavy rare earth element atoms will diffuse into the magnet along the molten grain boundary rich rare earth phase and form a shell layer with a high anisotropy field on the surface layer of the main phase grains. The higher anisotropy field can increase the nucleation field of the reverse magnetization domain, so as to achieve the purpose of increasing the coercivity of the magnet.

[0005] During the grain boundary diffusion process, the molten grain boundary rich rare earth phase is a fast channel for atomic diffusion. In addition, adjusting the distribution of the grain boundary rich rare earth phase by means of secondary aging treatment after grain boundary diffusion can further improve the coercivity of the magnet. However, due to the high chemical activity of the grain boundary rich rare earth phase, it is very easy to react with impurity elements such as oxygen and carbon to form high-melting-point compounds. The formation of high-melting-point compounds will reduce the number of molten grain boundary phases during the secondary aging process, and these compounds distributed between the main phase grains of the magnet will hinder the diffusion of heavy rare earth atoms and reduce the grain boundary diffusion rate. The accumulation of heavy rare earth elements on the surface layer of the magnet will promote the occurrence of volume diffusion, reduce the coercivity improvement amount of grain boundary diffusion, and increase the reduction amount of remanence. For the above reasons, when the magnet is subjected to grain boundary diffusion treatment at present, diffusion sources containing more organic solvents are not used as much as possible. Instead, the heavy rare earth element diffusion source is placed on the surface of the magnet in the form of PVD deposition or direct powder covering.

[0006] PVD coating is suitable as a diffusion source coating method for magnets with relatively regular shapes. However, when PVD is used to deposit diffusion sources on special-shaped NdFeB magnets (magnets with holes, magnets with half holes, tile-shaped magnets or arch-shaped magnets, etc.), there is a defect of uneven thickness of the diffusion source layer, or the diffusion source cannot be deposited in some areas of the special-shaped magnets, which will lead to insufficient increase in the coercive force of the magnet after grain boundary diffusion and low consistency of product performance. For powder-based diffusion sources, in order to avoid the influence of carbon elements on the performance of magnets, the amount of organic solvent added to the powder is usually reduced. This results in a weaker bonding force between the powder and the magnet, and the diffusion source powder is easy to fall off the product during product transportation, which is not suitable for mass production. If the organic solvent content in the diffusion source powder is increased and a powder slurry with greater viscosity is prepared, the bonding force between the diffusion source powder and the product can be significantly improved. Applying a high-viscosity powder slurry to the magnet surface by dipping and spraying can simplify the production process and effectively improve the problem of uniformity of diffusion source coating on special-shaped magnets. However, the increase in organic solvent content will inevitably lead to the influence of carbon elements on the performance of magnets, which becomes an irreconcilable contradiction.

[0007] How to solve the influence of organic solvents in high-viscosity diffusion source slurry on magnetic properties, optimize the diffusion source coating process by spraying or dipping methods, and improve the uniformity of diffusion source coating on special-shaped magnets are important issues that need to be urgently solved in the mass production of high-performance special-shaped NdFeB magnets. Summary of the invention

[0008] Aiming at the problems that the coating of the grain boundary diffusion diffusion source of the special-shaped NdFeB magnet is uneven and the carbon element in the organic solvent diffusion source reacts with the rare earth-rich phase at the surface grain boundary of the magnet to form carbides, resulting in a low increase in the coercive force and an increased decrease in the remanence after the grain boundary diffusion of the magnet, the present invention provides a grain boundary diffusion method for preparing high-performance special-shaped NdFeB magnets.

[0009] The technical solution adopted by the present invention is as follows:

[0010] A grain boundary diffusion method for preparing high-performance special-shaped NdFeB magnets, the method comprising the following steps:

[0011] (1) On the surface of a copper-free special-shaped sintered NdFeB magnet, a copper coating with a thickness of 0.1 to 1 μm is deposited on the surface of the magnet by electroplating, and the mass content of copper in the copper coating is ≥ 99.5%;

[0012] (2) mixing a diffusion source powder with a binder and an organic solvent to prepare a diffusion source slurry, wherein the diffusion source powder is a powder containing a heavy rare earth element, and the heavy rare earth element is at least one of Dy and Tb;

[0013] (3) Uniformly cover the diffusion source paste on the copper coating on the surface of the magnet, with the diffusion layer thickness being 5 μm to 1 mm (preferably 20 μm); the covering method is dipping or spraying, etc.

[0014] (4) Dry the magnet covered with the diffusion source paste; then perform magnet grain boundary diffusion treatment, with the diffusion temperature being 900 to 1000 °C and the holding time being 2 to 24 h; then perform secondary aging treatment, with the secondary aging temperature being 400 to 650 °C and the holding time being 2 to 6 h; thus obtaining the high-performance special-shaped neodymium iron boron magnet.

[0015] In the step (1), for the copper-free special-shaped sintered neodymium iron boron magnet, the special-shaped sintered neodymium iron boron magnet refers to an irregular magnet other than a regular magnet, including those with holes, cavities, curved surfaces, etc., such as a magnet with holes, a magnet with semi-holes, a tile-shaped magnet, or an arched magnet, etc.

[0016] The copper-free special-shaped sintered neodymium iron boron magnet is generally prepared by combining the preparation processes of melt-spinning SC flakes, hydrogen decrepitation, jet milling, orientation forming, isostatic pressing, vacuum sintering and machining. This is a well-known preparation technology for special-shaped sintered neodymium iron boron magnets in the art. The magnet raw material does not contain copper element.

[0017] In the step (1), the surface of the copper-free special-shaped sintered neodymium iron boron magnet is generally first subjected to surface treatment to expose a fresh surface of the magnet. The surface treatment method generally adopts sandblasting, pickling, grinding, etc. to remove oil stains, rust spots, etc. on the magnet surface, so as to expose a fresh surface of the magnet.

[0018] In the step (1), the thickness of the copper coating is preferably 0.1 to 0.9 μm, more preferably 0.3 to 0.5 μm.

[0019] In the step (2), the volume ratio of the binder to the organic solvent is 1:5 to 25 (preferably 1:20), and they are mixed to obtain a mixed organic solvent.

[0020] The volume ratio of the diffusion source powder to the mixed organic solvent is 1:1.2 to 3 (preferably 1:1.5).

[0021] Mix the diffusion source powder and the mixed organic solvent and stir evenly to prepare the diffusion source paste.

[0022] The organic solvent is an alcohol solvent of C1 to C5 or a benzene-based solvent. Further, the alcohol solvent of C1 to C5 is one or more of methanol, ethanol, propanol, butanol, ethylene glycol, and the benzene-based solvent is toluene or xylene.

[0023] The binder is one or more of vinyl resin binders, acrylic binders, and cellulose ether binders. Further, it is preferably a PVB binder (polyvinyl butyral), PMMA (polymethyl methacrylate), or PEMA (polyethyl methacrylate).

[0024] The diffusion source powder is one of heavy rare earth element fluoride powder, heavy rare earth element oxide powder, heavy rare earth element hydride powder, heavy rare earth element pure metal powder, heavy rare earth element alloy powder, or a mixture of at least two of the above powders

[0025] The particle size SMD of the diffusion source powder is 1 μm to 2 mm.

[0026] In step (4), drying is generally carried out in an oven, the baking temperature is generally 60 to 120 °C, and the baking time is 20 min to 3 h.

[0027] In step (4), when the thickness D of the copper plating layer satisfies 0.1 μm ≤ D ≤ 0.5 μm, the grain boundary diffusion time t should satisfy 2 h ≤ t ≤ 12 h; when the thickness D of the copper plating layer satisfies 0.5 μm < D ≤ 1.0 μm, the grain boundary diffusion time t should satisfy 12 h < t ≤ 24 h.

[0028] In step (4), the temperature of the secondary aging is 400 to 650 °C, and the holding time is 2 to 6 h.

[0029] To prevent the volatilization of rare earth elements, argon gas at 30 to 50 kPa is charged into the diffusion furnace when heating to the diffusion temperature, and it can be cooled with the furnace or air-cooled after the heat preservation is completed.

[0030] After the secondary aging treatment, it is cooled to below 80 °C at a speed of not less than 30 °C / min.

[0031] For the high-performance shaped NdFeB magnet provided by the present invention, the average carbon content A1 (mass ratio) of the magnet in the region within a distance < 30 μm from the magnet surface and the average carbon content A2 (mass ratio) of the magnet in the region within a distance > 50 μm from the magnet surface satisfy 0 ppm < A1 - A2 < 300 ppm.

[0032] Moreover, the copper content of the high-performance shaped NdFeB magnet in the region within a distance > 30 μm from the magnet surface is less than 5 ppm (mass ratio).

[0033] The present invention first prepares a shaped sintered neodymium iron boron magnet, and then deposits a copper coating on the surface of the magnet by electroplating. Electroplating can deposit a coating with relatively uniform thickness on the surface of the shaped magnet (including inner holes, curved surfaces, etc.). The diffusion source powder, main solvent and binder are mixed into a slurry in a certain proportion, and the diffusion source slurry is coated on the copper coating on the surface of the magnet by impregnation or spraying. Subsequently, it is dried in an oven and heated to the grain boundary diffusion temperature and held for a period of time.

[0034] The diffusion source slurry added with the binder has a relatively strong bonding force with the copper-plated magnet, and a diffusion source layer with relatively uniform thickness can be adhered to the surface of the shaped magnet by spraying and other methods. It can effectively solve the problems of weak bonding force between the traditional diffusion source powder and the substrate and easy shedding of the powder during product transportation, and is suitable for the preparation of large quantities of products.

[0035] In the traditional grain boundary diffusion process, when using a diffusion source with a high content of organic solvents, the carbon element in the diffusion source will react with the grain boundary rare earth-rich phase on the surface layer of the magnet to form high-melting-point compounds. The formation of high-melting-point compounds will reduce the number of molten grain boundary phases during the secondary aging process, and these compounds distributed between the main phase grains of the magnet will hinder the diffusion of heavy rare earth atoms and reduce the grain boundary diffusion rate. The accumulation of heavy rare earth elements on the surface layer of the magnet will promote the occurrence of volume diffusion, reduce the increase in coercivity of grain boundary diffusion, and increase the decrease in remanence. In the present invention, a copper coating is deposited by electroplating to isolate the surface of the magnet and the diffusion source layer containing the organic diffusion source. Since there is no phase change reaction between the two elements of Cu and C, and the solubility of C in Cu is very small at both room temperature and high temperature (the solubility of C in Cu is only 0.9±0.1 wt.ppm at 870°C), the Cu coating distributed between the magnet and the diffusion source can hinder the diffusion of C atoms in the diffusion source into the interior of the neodymium iron boron magnet substrate. The solubility of heavy rare earth elements in Cu is relatively large, and they can react with Cu to form low-melting-point alloys and diffuse into the magnet interior. Therefore, by means of the isolation effect of the Cu coating on C atoms, the reaction between C in the diffusion source and the grain boundary rare earth-rich phase on the surface layer of the magnet can be effectively inhibited, ensuring that there are sufficient amounts of molten grain boundary rare earth-rich phases on the surface of the magnet during high-temperature diffusion, and promoting the grain boundary diffusion of heavy rare earth elements.

[0036] In the present invention, during high-temperature grain boundary diffusion, heavy rare-earth elements react with Cu and diffuse along the grain boundaries into the interior of the magnet. The lower-melting Cu diffuses into the grain-boundary rare-earth-rich phase, which can reduce the melting point of the grain-boundary rare-earth-rich phase and increase the atomic diffusion ability, thereby increasing the diffusion depth of heavy rare-earth atoms. Appropriate addition of Cu element in the preparation of NdFeB magnets can reduce the melting point of the grain-boundary rare-earth-rich phase, improve the wettability between the grain-boundary phase and the main phase, and increase the coercivity of the magnet after secondary aging. However, excessive addition of Cu will significantly reduce the remanence of the magnet. In the present invention, since a copper coating with a certain thickness is deposited on the magnet surface, in order to avoid the deterioration of the magnetic properties of the magnet caused by excessive Cu element, Cu is not added to the diffusion matrix. In addition, the Cu element is mainly concentrated in the depth range of 30 μm from the magnet surface. When the distance from the magnet surface is > 30 μm, the Cu content in the magnet is < 5 ppm (mass ratio).

[0037] With the progress of grain boundary diffusion, the Cu coating will be continuously consumed. In the later stage of diffusion, as the copper coating is gradually consumed, the C-rich diffusion source contacts the magnet surface, and C atoms begin to diffuse into the interior of the magnet. However, at this time, the diffusion process of heavy rare-earth elements has been basically completed, and the carbides formed by the subsequent reaction of C with the grain-boundary rare-earth-rich phase will not hinder the grain boundary diffusion of heavy rare-earth atoms. Therefore, the coercivity of the grain boundary diffusion magnet can be effectively increased.

[0038] In addition, by controlling the diffusion process parameters, reducing the holding time in the later stage of grain boundary diffusion can effectively reduce the reaction between the C element in the diffusion source and the grain-boundary rare-earth-rich phase of the magnet in the later stage of diffusion. The grain boundary diffusion temperature adopted in the present invention is 900 - 1000 °C. When the thickness D of the copper coating satisfies 0.1 μm ≤ D ≤ 0.5 μm, the grain boundary diffusion time t should satisfy 2 h ≤ t ≤ 12 h; when the thickness D of the copper coating satisfies 0.5 μm < D ≤ 1.0 μm, the grain boundary diffusion time t should satisfy 12 h < t ≤ 24 h. By strictly controlling the diffusion time, the reaction between C and the grain-boundary rare-earth-rich phase can be inhibited, and the carbon content on the surface layer of the magnet can be effectively reduced. The average carbon content A1 (mass ratio) of the magnet in the region where the distance from the magnet surface is < 30 μm and the average carbon content A2 (mass ratio) of the magnet in the region where the distance from the magnet surface is > 50 μm satisfy 0 ppm < A1 - A2 < 300 ppm. By reducing the carbon content on the surface layer of the magnet and the number of carbides in the grain-boundary rare-earth-rich phase, the increase in coercivity after secondary aging can be promoted.

[0039] The beneficial effects of the present invention are reflected in that by preparing a diffusion source powder slurry with high viscosity and coating it by spraying or dipping, a diffusion source of heavy rare earth elements with uniform thickness can be deposited on the surface of the shaped NdFeB magnet. A Cu coating is deposited on the magnet surface by electroplating to isolate the diffusion source with a high content of organic solvents from direct contact with the magnet surface. By virtue of the characteristics that Cu and C do not undergo phase transformation and the solubility of C in Cu is low at high temperature, the C atoms in the diffusion source are hindered from diffusing into the interior of the NdFeB magnet matrix. This avoids the reaction between C and the grain boundary rich rare earth phase on the magnet surface layer to form carbides, which would hinder the grain boundary diffusion of heavy rare earth elements. In addition, the lower melting point Cu diffuses into the grain boundary rich rare earth phase, which can reduce the melting point of the grain boundary rich rare earth phase and increase the atomic diffusion ability, thereby increasing the diffusion depth of heavy rare earth atoms. By strictly controlling the diffusion process, the reaction between the C element in the diffusion source and the grain boundary rich rare earth phase of the magnet in the later stage of diffusion is reduced, and the increase in coercivity after secondary aging is increased, so as to prepare a high-performance shaped NdFeB magnet. Description of the Drawings

[0040] Figure 1 Magnet shape and size diagrams for Experiments No. 1 to No. 16.

[0041] Figure 2 (a) and (b) are SEM micrographs of the magnet within a certain depth range from the diffusion source coating surface for Experiment No. 1 and Experiment No. 4, respectively.

[0042] Figure 3 Variation curve of copper element content within a certain depth range of the magnet for Experiment No. 4 from the diffusion source coating surface.

[0043] Figure 4 Variation curve of carbon element content within a certain depth range of the magnet for Experiments No. 8 to No. 11 from the diffusion coating surface.

[0044] Figure 5 SEM image of the magnet for Experiment No. 11 within a certain depth range from the diffusion source coating surface.

[0045] Figure 6 Variation curve of carbon element content within a certain depth range of the magnet for Experiments No. 12 to No. 16 from the magnet diffusion source coating surface.

[0046] Figure 7 (a) is a schematic diagram of the Tb layer thickness at different positions of the magnet for Experiment No. 17 after PVD, and (b) and (c) are SEM images of the magnet at positions 1 and 2, respectively. Detailed Embodiments

[0047] The preparation process of melting and spinning SC flakes, hydrogen breaking, jet milling, orientation forming, isostatic pressing, and vacuum sintering is combined with machining to prepare non-Cu special-shaped sintered NdFeB magnets. The special-shaped magnets include magnet with holes, magnet with semi-holes, tile-shaped magnets, arch-shaped magnets, etc.

[0048] The surface of the magnet is treated by sandblasting, pickling, grinding, etc. to remove surface oil stains, rust spots, etc., so that the fresh surface of the magnet is exposed.

[0049] A copper coating layer with a thickness of 0.1 - 1 μm is deposited on the surface of the special-shaped magnet by electroplating, and the content of Cu element in the copper coating layer is ≥ 99.5% (mass ratio).

[0050] The binder and the main solvent are mixed and stirred at a volume ratio of 1:5 - 25 to prepare a mixed organic solvent. The diffusion source powder is mixed with the above-mentioned mixed organic solvent at a volume ratio of 1:1.2 - 3 and stirred evenly to prepare a diffusion source powder slurry. The main solvent can be selected from alcohols (methanol, ethanol, propanol, butanol, ethylene glycol) and benzenes (toluene, xylene). The binder can be selected from vinyl-based, acrylic-based, and cellulose ether-based. The heavy rare earth element diffusion source powder is one of the heavy rare earth element fluoride powder, heavy rare earth element oxide powder, heavy rare earth element hydride powder, heavy rare earth element pure metal powder, heavy rare earth element alloy powder, or a mixture of at least two of the above powders. The particle size SMD of the heavy rare earth element diffusion source powder is 1 μm - 2 mm.

[0051] The diffusion source slurry with a thickness of 5 μm - 1 mm is coated on the surface of the copper-plated special-shaped magnet by dipping or spraying, and then placed in an oven at 60 - 120 °C for baking for 20 min - 3 h.

[0052] The baked magnet is placed in a vacuum diffusion furnace and kept at 900 - 1000 °C for 2 - 24 h. Preferably, when the thickness D of the copper coating layer of the magnet satisfies 0.1 μm ≤ D ≤ 0.5 μm, the grain boundary diffusion time t satisfies 2 h ≤ t ≤ 12 h; when the copper coating layer thickness D satisfies 0.5 μm < D ≤ 1.0 μm, the grain boundary diffusion time t satisfies 12 h < t ≤ 24 h. To prevent the volatilization of rare earth elements, argon gas with a pressure of 30 - 50 kPa is filled into the diffusion furnace when heating to the holding temperature, and it can be cooled with the furnace or by air cooling after the holding is completed.

[0053] The magnet after grain boundary diffusion is heated to 400 - 650 °C for secondary aging treatment for 2 - 6 h, and after that, it is cooled to below 80 °C at a speed of not less than 30 °C / min.

[0054] The surface of the magnet after secondary aging is sandblasted to expose a fresh surface. The composition of the magnet is measured by ICP, and the microstructure and micro-area composition of the magnet are analyzed by SEM and EPMA. The magnetic flux of the shaped magnet is measured after magnetization saturation, and the magnetic loss is measured after holding at 120 °C for 2 h under open-circuit conditions.

[0055] Example 1:

[0056] A tile-shaped sintered NdFeB magnet without Cu is prepared by combining the preparation processes of melt-spinning SC flakes, hydrogen decrepitation, jet milling, orientation forming, isostatic pressing, and vacuum sintering with machining.

[0057] The surface of the magnet is treated by sandblasting, pickling, and polishing to remove surface oil stains, rust spots, etc., so as to expose a fresh surface of the magnet.

[0058] Copper coatings with different thicknesses are deposited on the surface of the shaped magnet by electroplating, and the content of Cu element in the copper coating is ≥99.5% (mass ratio). The thickness of the copper coating is shown in Table 1.

[0059] The binder and the organic solvent are mixed and stirred at a volume ratio of 1:20 to prepare a mixed organic solvent. The diffusion source powder is mixed with the prepared mixed organic solvent at a volume ratio of 1:1.5 and stirred evenly to prepare a diffusion source powder slurry. The organic solvent is selected as butanol, and the binder is selected as PVB. The heavy rare earth element diffusion source powder is selected as terbium fluoride powder with a particle size SMD of 3 μm.

[0060] The method of dipping is used to coat a 20-μm-thick slurry of terbium fluoride powder on the surface of the copper-plated shaped magnet, and then it is placed in an oven at 110 °C and baked for 1 h.

[0061] The baked magnet is placed in a vacuum diffusion furnace and held at 950 °C for 6 h. To prevent the volatilization of rare earth elements, argon gas at 30-50 kPa is charged into the diffusion furnace when heating to the holding temperature, and air cooling is used after the holding is completed.

[0062] The magnet after grain boundary diffusion is heated to 520 °C for 3 h of secondary aging treatment, and after completion, it is cooled to below 80 °C at a rate of not less than 30 °C / min.

[0063] The surface of the magnet after secondary aging is sandblasted to expose a fresh surface. The composition of the magnet is measured by ICP, and the microstructure and micro-area composition of the magnet are analyzed by SEM and EPMA. The magnetic flux of the shaped magnet is measured after magnetization saturation, and the magnetic loss is measured after holding at 120 °C for 2 h under open-circuit conditions.

[0064] The composition (mass percentage) of the magnet after grain boundary diffusion with different thickness copper coatings is shown in Table 1:

[0065] Table 1

[0066]

[0067] The magnetic flux of the magnet after secondary aging and the magnetic loss at 120℃ for 2h under open circuit conditions are shown in Table 2:

[0068] Table 2

[0069] Experiment No. Magnetic Flux (mWb) Magnetic Loss (%) 1 14.24 3.80 2 14.41 3.11 3 14.45 2.82 4 14.43 2.57 5 14.41 2.93 6 14.19 3.04 7 13.57 3.15

[0070] Comparing the magnetic properties of the secondary aged magnets, when the coating amount of the diffusion source is the same, when the thickness of the copper plating is less than 1μm, the magnetic loss of the magnet decreases with the increase of the thickness of the copper plating of the magnet, indicating that the coercive force of the magnet increases. When the thickness of the copper plating is above 1μm, the magnetic loss of the magnet increases with the increase of the thickness of the copper plating of the magnet. Moreover, when the thickness of the copper plating is greater than 1μm, the magnetic flux of the magnet decreases significantly with the increase of the thickness of the copper plating.

[0071] Figure 2 (a) and (b) are SEM microstructure photos of the magnets of Experiment No. 1 and Experiment No. 4 within a certain depth range from the diffusion source coating surface. There is no copper coating on the surface of the magnet in Experiment No. 1. After grain boundary diffusion, there are many granular carbides in the rare earth-rich phase of the grain boundary within a depth range of 5 to 25 μm from the magnet surface. In Experiment No. 4, there is a copper coating with a thickness of 0.5 μm between the magnet and the diffusion source layer. After diffusion, there are only a small amount of carbides within a depth range of 5 to 25 μm from the magnet surface.

[0072] EPMA point scanning analysis Figure 2 (a) The carbide composition is shown in Table 3 (mass percentage):

[0073] Table 3

[0074] Point. Fe Nd Pr C Al 1 55.71 18.42 5.34 20.30 0.23

[0075] EPMA surface scanning was used to analyze the average carbon content (ppm mass ratio) of the magnet in the area of 0 to 30 μm and 50 to 80 μm from the magnet surface. The results are shown in Table 4:

[0076] Table 4

[0077]

[0078] Figure 3 This is the variation of the copper content of the magnet No. 4 within a certain depth range from the magnet surface. It can be seen that the copper content of the magnet close to the diffusion source coating surface is higher, and the copper content gradually decreases with the increase of depth. When the distance from the magnet surface exceeds 30μm, the copper content of the magnet is less than 5ppm (mass ratio).

[0079] The chemical activity of the grain boundary phase of NdFeB magnets is relatively high. The carbon element in the diffusion source is very likely to react with the rare-earth-rich grain boundary phase on the surface layer of the magnet to form carbides distributed between the main-phase grains. The large amount of carbides will reduce the amount of molten rare-earth-rich grain boundary phase at high temperatures and hinder the diffusion of heavy rare-earth elements, thereby reducing the increase in the coercivity of the magnet after grain boundary diffusion. In addition, the large accumulation of heavy rare-earth elements on the magnet surface will promote the occurrence of volume diffusion, and the remanence of the magnet after grain boundary diffusion will be significantly reduced.

[0080] There is no phase change reaction between copper and carbon elements, and the solubility of carbon in copper is very small at high temperatures. By depositing a copper coating on the surface of the magnet, then coating a heavy rare-earth element diffusion source on the copper coating and performing grain boundary diffusion. The copper coating distributed between the magnet and the heavy rare-earth diffusion source can hinder the diffusion of carbon elements into the NdFeB matrix. The solubility of heavy rare-earth elements in copper is relatively large, and it can form a low-melting-point alloy with heavy rare-earth elements and diffuse into the magnet to improve the coercivity of the magnet. The present invention can effectively inhibit the reaction between carbon in the diffusion source and the rare-earth-rich grain boundary phase on the surface layer of the magnet through the isolation effect of the copper coating, ensuring that there is a sufficient amount of molten rare-earth-rich grain boundary phase on the magnet surface during high-temperature diffusion and promoting the grain boundary diffusion process of heavy rare-earth elements.

[0081] The experimental results show that the thicker the copper coating is not necessarily the better. When the thickness of the copper coating exceeds 1 μm, the magnetic flux of the magnet after grain boundary diffusion decreases significantly. Excessive Cu will significantly increase the number of grain boundary phases in the magnet surface layer and reduce the melting point of the grain boundary phase. During the grain boundary diffusion process, the main-phase grains are more likely to rotate, reducing the remanence and magnetic flux of the magnet. Therefore, in the present invention, it is preferably that the substrate does not contain Cu and the thickness of the copper coating does not exceed 1 μm.

[0082] Example Two:

[0083] Use the preparation processes of melt-spinning SC sheets, hydrogen decrepitation, jet milling, orientation forming, isostatic pressing, and vacuum sintering combined with machining to prepare a tile-shaped sintered NdFeB magnet without Cu.

[0084] Use methods such as sandblasting, pickling, and polishing to perform surface treatment on the magnet to remove surface oil stains, rust spots, etc., so that the fresh surface of the magnet is exposed.

[0085] Use electroplating to deposit a metal layer with a thickness of 0.5 μm and different copper contents on the surface of the special-shaped magnet. The element content of the metal coating is shown in Table 5.

[0086] The binder and the main solvent are mixed and stirred at a volume ratio of 1:20 to prepare a mixed organic solvent. The diffusion source powder is mixed with the prepared mixed organic solvent at a volume ratio of 1:1.5 and stirred evenly to prepare a diffusion source powder slurry. The main solvent is selected as butanol, and the binder is selected as PVB. The heavy rare-earth element diffusion source powder is selected as terbium fluoride powder with a particle size SMD of 3 μm.

[0087] The method of dipping is adopted to coat the surface of the shaped copper-plated magnet with terbium fluoride powder slurry with a thickness of 20 μm, and then it is placed in an oven at 110 °C and baked for 1 h.

[0088] The baked magnet mentioned above is placed in a vacuum diffusion furnace and kept at 950 °C for 6 h. To prevent the volatilization of rare earth elements, argon gas with a pressure of 30 - 50 kPa is filled into the diffusion furnace when heating to the holding temperature, and air cooling is adopted after the holding is completed.

[0089] The magnet after grain boundary diffusion is heated to 520 °C for a secondary aging treatment for 3 h, and after the end, it is cooled to below 80 °C at a speed of not less than 30 °C / min.

[0090] The surface of the magnet after the secondary aging treatment is subjected to sandblasting treatment to expose a fresh surface of the magnet. ICP is used to measure the composition of the magnet, and SEM and EPMA are used to analyze the microstructure and micro-area composition of the magnet. The magnetic flux is measured after the shaped magnet is magnetized to saturation, and the magnetic loss is measured after keeping at 120 °C for 2 h under open-circuit conditions.

[0091] In this embodiment, the matrix magnet compositions of each experimental group are the same as those of each experimental group in Example 1. Table 5 shows the coating compositions of different experimental groups and the main element contents (mass percentage) of the magnet after diffusion:

[0092] Table 5

[0093] Experiment No. Coating Composition Cu Content in Coating Tb Cu Al 8 Cu >99.5 0.175 0.035 0.30 9 Cu - Al 80.0 0.164 0.027 0.32 10 Cu - Al 50.0 0.159 0.019 0.38 11 Al 0 0.146 0 0.42

[0094] The magnetic flux of the magnet after the secondary aging treatment and the magnetic loss after keeping at 120 °C for 2 h under open-circuit conditions are shown in Table 6:

[0095] Table 6

[0096] Experiment No. Magnetic Flux (mWb) Magnetic Loss (%) 8 14.43 2.54 9 14.40 4.01 10 14.22 4.31 11 14.08 4.63

[0097] It can be seen from the results in Table 6 that when the copper content in the copper coating > 99.5%, the magnetic loss is the lowest, indicating that the coercivity of the magnet is the largest. When the copper content is below 99.5%, the magnetic loss gradually increases, the magnetic flux gradually decreases, and the magnetic properties deteriorate.

[0098] Figure 4 It is a curve graph of the carbon element content change within a certain depth range from the diffusion coating surface of the magnets of Experiment No. 8 - No. 11.

[0099] Figure 5 It is an SEM image of the magnet of Experiment No. 11 within a certain depth range from the diffusion source coating surface.

[0100] From Figure 4It can be seen that when the copper purity of the magnet copper layer is greater than 99.5 wt.%, the carbon content in the surface layer of the magnet after diffusion is relatively low. When copper-aluminum alloy is used instead of pure copper, the carbon content in the surface layer of the magnet after diffusion increases significantly. When aluminum is used to replace copper as the coating, the carbon content on the surface of the diffused magnet increases sharply. It can be seen from the SEM image of the magnet in Experiment No. 11 that some large carbide are formed in the grain boundary phase of the magnet surface layer.

[0101] There is no phase change reaction between copper element and carbon element, and the solubility of carbon in copper is very low. Therefore, coating a copper layer between the magnet and the diffusion source powder can inhibit the diffusion of carbon element into the magnet interior at the initial stage of diffusion, thereby reducing the number of carbide formed in the magnet surface layer, weakening the influence of carbide on the diffusion of heavy rare earth elements, and further increasing the coercivity increment of the magnet after diffusion. When a certain amount of copper element is replaced by other metal elements (especially low-melting-point metal elements), since these elements have a phase change reaction with carbon, they can participate in and promote the formation of carbide on the magnet surface.

[0102] In this embodiment, in Experiment No. 11, an aluminum coating is used to replace the copper coating. Since there is a phase change reaction between carbon and aluminum elements, and the solubility of carbon element in aluminum is relatively large. When heated to the diffusion temperature, the diffusion of aluminum element into the magnet interior will reduce the melting point of the grain boundary rare earth-rich phase in the magnet surface layer, thereby promoting the phase change reaction of carbon element with aluminum element and grain boundary rare earth elements to form large carbide. The carbide are mainly distributed between the main phase grains, reducing the number of molten grain boundary rare earth-rich phases and also hindering the diffusion of heavy rare earth elements, resulting in a decrease in the coercivity increment of the magnet after diffusion. In addition, the accumulation of a large amount of heavy rare earth elements on the magnet surface will promote the occurrence of volume diffusion, and the reduction amount of the remanence of the magnet after diffusion increases significantly. Therefore, in the present invention, a copper coating must be used, and the copper purity of the coating should be greater than 99.5 wt.%.

[0103] Example 3:

[0104] A tile-shaped sintered NdFeB magnet without Cu is prepared by combining the preparation processes of melt-spinning SC flakes, hydrogen decrepitation, jet milling, orientation forming, isostatic pressing, vacuum sintering and machining.

[0105] The magnet is surface-treated by means of sandblasting, pickling and polishing to remove surface oil stains, rust spots, etc., so as to expose a fresh surface of the magnet.

[0106] Copper coatings with different thicknesses are deposited on the surface of the shaped magnet by electroplating, and the content of Cu element in the copper coating is ≥99.5% (mass ratio). The thickness of the copper coating is shown in Table 7.

[0107] The binder and the main solvent are mixed and stirred at a volume ratio of 1:20 to prepare a mixed organic solvent. The diffusion source powder is mixed with the prepared mixed organic solvent at a volume ratio of 1:1.5 and stirred evenly to prepare a diffusion source powder slurry. The main solvent is selected as butanol, and the binder is selected as PVB. The heavy rare earth element diffusion source powder is selected as terbium fluoride powder with a particle size SMD of 3 μm.

[0108] The method of dipping is used to coat the surface of the copper-plated shaped magnet with a terbium fluoride powder slurry with a thickness of 20 μm, and then it is placed in an oven at 110 °C and baked for 1 h.

[0109] The baked magnet is placed in a vacuum diffusion furnace and kept at 950 °C for different times, as shown in Table 7. To prevent the volatilization of rare earth elements, argon gas with a pressure of 30 - 50 kPa is filled into the diffusion furnace when heating to the holding temperature, and air cooling is used after the holding is completed.

[0110] The magnet after grain boundary diffusion is heated to 520 °C for a secondary aging treatment for 3 h, and after the end, it is cooled to below 80 °C at a speed of not less than 30 °C / min.

[0111] The surface of the magnet after secondary aging is subjected to sandblasting treatment to expose a fresh surface of the magnet. ICP is used to measure the composition of the magnet, and SEM and EPMA are used to analyze the microstructure and micro-area composition of the magnet. The magnetic flux of the shaped magnet is measured after magnetization saturation, and the magnetic loss is measured after keeping warm at 120 °C for 2 h under open circuit conditions.

[0112] In this embodiment, the matrix magnet compositions of each experimental group are the same as those of each experimental group in Example 1. Table 7 shows the variables and main element contents (mass percentage) of different experimental groups:

[0113] Table 7

[0114]

[0115] The magnetic flux of the magnet after secondary aging and the magnetic loss after keeping warm at 120 °C for 2 h under open circuit conditions are shown in Table 8:

[0116] Table 8

[0117] Experiment No. Magnetic Flux (mWb) Magnetic Loss (%) 12 14.43 2.57 13 14.41 2.73 14 14.41 2.93 15 14.42 2.67 16 14.41 3.52

[0118] A copper plating layer with a certain thickness is used to isolate the magnet and the diffusion source powder, so that the two do not come into direct contact, which can effectively inhibit the reaction between carbon and the rare-earth-rich phase at the grain boundaries of the magnet in the initial stage of diffusion. However, as the diffusion progresses, the copper plating layer will be continuously consumed, and carbon elements will also continuously accumulate in the remaining diffusion source powder. With the complete consumption of the copper plating layer in the later stage of diffusion, the carbon-rich residual diffusion source powder will come into direct contact with the magnet surface, resulting in the reaction between the rare-earth-rich phase on the magnet surface and carbon elements to form carbides. Although the carbides formed in the later stage of diffusion do not hinder the diffusion of heavy rare-earth elements, they will reduce the amount of molten grain-boundary rare-earth-rich phase during the secondary aging process, affect the redistribution of the grain-boundary rare-earth-rich phase, and reduce the coercivity of the magnet after secondary aging. Therefore, it is necessary to reasonably control the grain-boundary diffusion time according to different copper plating layer thicknesses.

[0119] In the present invention, when the thickness D of the copper plating layer on the magnet surface satisfies 0.1 μm ≤ D ≤ 0.5 μm, the grain-boundary diffusion time t should satisfy 2 h ≤ t ≤ 12 h; when the thickness D of the copper plating layer on the magnet satisfies 0.5 μm < D ≤ 1.0 μm, the grain-boundary diffusion time t should satisfy 12 h < t ≤ 24 h. Prolonging the diffusion time will cause a phase change reaction between the carbon-rich diffusion source and the magnet surface, resulting in an increase in the carbon content of the magnet surface layer and a decrease in the coercivity increment after secondary aging. From Figure 6 It can be seen from the change in the carbon element content within a certain depth range from the magnet diffusion source coating surface of the magnets in Experiments No. 12 to No. 16 that after exceeding the specified diffusion time, the carbon element content of the magnet surface layer increases sharply, and the coercivity of the magnet decreases significantly. When the diffusion time is insufficient, the copper plating layer is not completely consumed, and the diffusion amount and diffusion depth of the heavy rare-earth elements are insufficient, which will also reduce the coercivity increment of the magnet grain-boundary diffusion.

[0120] Example 4:

[0121] A tile-shaped sintered NdFeB magnet without Cu is prepared by combining the preparation processes of melt-spinning SC flakes, hydrogen decrepitation, jet milling, orientation forming, isostatic pressing, vacuum sintering and machining.

[0122] The magnet is surface-treated by means of sandblasting, pickling and polishing to remove surface oil stains, rust spots, etc., so that the magnet exposes a fresh surface.

[0123] In Experiment No. 17, a 20-μm Tb layer is directly deposited on the magnet surface by the PVD method, and in Experiment No. 18, a 0.5-μm-thick copper plating layer is deposited on the surface of the special-shaped magnet by electroplating. The Cu element content in the copper plating layer is ≥ 99.5% (mass ratio), and then an organic slurry containing a diffusion source is coated on the copper plating layer of the magnet.

[0124] The binder and the main solvent are mixed and stirred at a volume ratio of 1:20 to prepare a mixed organic solvent. The diffusion source powder is mixed with the prepared mixed organic solvent at a volume ratio of 1:1.5 and stirred evenly to prepare a diffusion source powder slurry. The main solvent is selected as butanol, and the binder is selected as PVB. The heavy rare earth element diffusion source powder is selected as terbium fluoride powder with a particle size SMD of 3 μm.

[0125] The method of dipping is used to coat the surface of the copper-plated special-shaped magnet with a 20-μm-thick slurry of terbium fluoride powder, and then it is placed in an oven at 110 °C and baked for 1 h.

[0126] The above magnet is placed in a vacuum diffusion furnace and kept at 950 °C for 6 h. To prevent the volatilization of rare earth elements, argon gas at 30-50 kPa is filled into the diffusion furnace when heating to the holding temperature, and air cooling is used after the holding is completed.

[0127] The magnet after grain boundary diffusion is heated to 520 °C for a secondary aging treatment for 3 h, and after the end, it is cooled to below 80 °C at a rate of not less than 30 °C / min.

[0128] SEM is used to analyze the Tb layer thickness in different regions of the special-shaped magnet after PVD. The surface of the magnet after secondary aging is sandblasted to expose a fresh surface of the magnet. ICP is used to measure the magnet composition, the magnetic flux is measured after the special-shaped magnet is magnetized to saturation, and the magnetic loss is measured after keeping warm at 120 °C for 2 h under open-circuit conditions.

[0129] In this embodiment, the matrix magnet compositions of each experimental group are the same as those of each experimental group in Example 1. The main element contents of the magnets after diffusion in different experimental groups are shown in Table 9:

[0130] Table 9

[0131] Experiment No. Thickness of Copper Coating (μm) Cu (wt.%) b (wt.%) 17 0 0 0.251 18 0.5 0.035 0.175

[0132] SEM is used to analyze the thickness of the heavy rare earth diffusion source layer on the surface of the magnets in different experimental groups. The Tb layer thickness at different positions of the magnet in Experiment No. 17 after PVD is as Figure 7 shown. Figures (b) and (c) are SEM pictures of the magnet at positions 1 and 2 in Figure (a) respectively. It can be seen that the Tb layer at position 2 is thicker and has a higher bonding strength with the matrix. However, at position 1, due to the arc problem of the special-shaped magnet itself, the deposited Tb layer thickness is thinner during PVD, and the bonding force with the matrix is weaker. Since the heavy rare earth element diffusion layer is coated on the magnet in Experiment No. 18 by the dipping method, the diffusion layer thickness at different positions is basically the same.

[0133] The magnetic flux of the magnet after secondary aging and the magnetic loss after keeping warm at 120 °C for 2 h under open-circuit conditions are shown in Table 10:

[0134] Table 10

[0135] Experiment No. Magnetic Flux (mWb) Magnetic Loss (%) 17 14.42 3.85 18 14.43 2.56

[0136] When depositing the Tb layer on the surface of the magnet by the PVD method, the process is relatively complex and the product needs to be frequently turned over. In addition, the thickness of the Tb layer deposited in different regions of the special-shaped magnet is different, and the bonding force with the substrate is also different. Therefore, the increase in coercivity at different parts of the magnet after diffusion is also different. From the magnetic property data of the magnets in Table 10, it can be seen that the magnetic fluxes of the magnets after diffusion in Experiment No. 17 and No. 18 are basically the same, while the magnetic loss of Experiment No. 17 is much greater than that of Experiment No. 18. This is because the uneven thickness of the Tb layer leads to different increments of coercivity in different regions after magnetic diffusion, and finally the overall coercivity improvement effect is poor. Therefore, the method disclosed in this invention can not only obtain high-performance special-shaped NdFeB magnets, but also reduce the coating difficulty of the diffusion source layer of the special-shaped magnet.

Claims

1. A grain boundary diffusion method for preparing high-performance special-shaped Nd-Fe-B magnets, characterized in that The method includes the following steps: (1) On the surface of a non - copper - containing shaped sintered Nd - Fe - B magnet, a copper coating with a thickness of 0.1 - 1 μm is deposited on the magnet surface by electroplating, and the mass content of copper in the copper coating is ≥99.5%; the shaped sintered Nd - Fe - B magnet refers to an irregular magnet other than a regular magnet, containing holes, cavities, and curved surfaces; (2) Mix a diffusion source powder with a binder and an organic solvent to prepare a diffusion source slurry. The diffusion source powder is a powder containing heavy rare - earth elements, and the heavy rare - earth element is at least one of Dy or Tb; (3) Uniformly cover the diffusion source slurry on the copper coating on the magnet surface; the way of uniform covering is impregnation or spraying; (4) Dry the magnet covered with the diffusion source slurry; then perform magnet grain - boundary diffusion treatment, where the diffusion temperature is 900 - 1000 °C and the holding time is 2 - 24 h; then perform secondary aging treatment to obtain the high - performance shaped Nd - Fe - B magnet.

2. The method according to claim 1, characterized in that In step (2), the volume ratio of the binder to the organic solvent is 1:5 - 25, and the binder and the organic solvent are mixed to obtain a mixed organic solvent; the volume ratio of the diffusion source powder to the mixed organic solvent is 1:1.2 - 3.

3. The method according to claim 1, characterized in that In step (2), the organic solvent is an alcohol solvent with C1 - C5 or a benzene - based solvent; the binder is one or more of vinyl resin - type binders, acrylic - type binders, and cellulose ether - type binders.

4. The method according to claim 1, wherein In step (2), the diffusion source powder is one of heavy rare - earth element fluoride powder, heavy rare - earth element oxide powder, heavy rare - earth element hydride powder, heavy rare - earth element pure metal powder, heavy rare - earth element alloy powder, or a mixture of at least two of the above powders.

5. The method according to claim 1, characterized in that In step (2), the particle size SMD of the diffusion source powder is 1 μm - 2 mm.

6. The method according to claim 1, characterized in that In step (4), when the thickness D of the copper coating satisfies 0.1 μm ≤ D ≤ 0.5 μm, the grain - boundary diffusion time t should satisfy 2 h ≤ t ≤ 12 h; when the thickness D of the copper coating satisfies 0.5 μm < D ≤ 1.0 μm, the grain - boundary diffusion time t should satisfy 12 h < t ≤ 24 h.

7. The method according to claim 1, characterized in that In step (4), the temperature of the secondary aging is 400 - 650 °C and the holding time is 2 - 6 h.

8. The high-performance special-shaped NdFeB magnet prepared by the method according to any one of claims 1 to 7, characterized in that, In the magnet, the average carbon content A1 of the magnet in the region within a distance <30 μm from the magnet surface and the average carbon content A2 of the magnet in the region with a distance >50 μm from the magnet surface satisfy 0 ppm < A1 - A2 < 300 ppm.

9. The high-performance special-shaped neodymium iron boron magnet according to claim 8, characterized in that The copper content in the magnet in the region with a distance >30 μm from the magnet surface is less than 5 ppm.

Images