A heat treatment process for cerium-rich neodymium-iron-boron magnets

Abstract

The application discloses a heat treatment process of cerium-rich neodymium-iron-boron magnet, and relates to the technical field of rare earth permanent magnet materials; the application comprises the following steps: firstly, performing first heat treatment on a sintered cerium-rich neodymium-iron-boron magnet base body in a high-purity Ar atmosphere and at 450-550 DEG C, and adopting inert gas protection gas to air cool to room temperature; secondly, coating the magnet surface with an MM heavy rare earth diffusion source, performing second heat treatment in a high-purity Ar+5%H2 atmosphere and at 600-700 DEG C, and adopting inert gas protection gas to air cool to room temperature; thirdly, performing third heat treatment in a high-purity Ar atmosphere and at 800-900 DEG C, and adopting Ar quenching to rapidly cool to room temperature; and finally, performing fourth heat treatment in a high-purity Ar atmosphere and at 450-650 DEG C, and adopting inert gas protection gas to air cool to room temperature; the application has the advantages of simple process, no need of new raw materials and new equipment, and only change of heat treatment time, temperature, atmosphere, cooling mode and speed, etc., so that the comprehensive magnetic performance of the cerium-rich neodymium-iron-boron magnet is improved under the condition of less heavy rare earth diffusion source.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet materials technology, specifically a heat treatment process for cerium-rich neodymium iron boron magnets. Background Technology

[0002] Rare earth permanent magnet materials are not only the fastest-growing, largest-scale, and most complete development direction in the entire rare earth field, but also an irreplaceable and indispensable key raw material for national strategic emerging industries. Sintered NdFeB permanent magnets are currently the most widely used, fastest-growing, and best-performing permanent magnet materials in the world, and are widely used in new energy vehicles, industrial robots, energy-saving home appliances, wind power generation, and other fields.

[0003] With the rapid development of the new energy industry, heavy rare earth grain boundary diffusion technology has gradually become the most common method to improve the overall magnetic properties of sintered NdFeB permanent magnets. Grain boundary diffusion technology utilizes elemental, compound, or alloyed heavy rare earth elements such as Dy and Tb as diffusing agents. Through diffusion heat treatment, heavy rare earth elements are diffused from the magnet surface along the grain boundaries into the magnet's interior, distributing themselves on the grain boundaries and grain surfaces to enhance the magnet's coercivity and high-temperature stability. However, with the widespread application of grain boundary diffusion technology in sintered NdFeB permanent magnets, the demand for rare earth elements has also experienced rapid and unbalanced growth. On the one hand, heavy rare earth elements are an expensive strategic resource, with strict restrictions on their use and export. Producing large quantities of high-coercivity sintered NdFeB permanent magnets faces increased costs and raw material supply difficulties. On the other hand, the high abundance of cerium in the Earth's crust has not been rationally utilized, resulting in a large stockpile. The price of cerium metal is only about 1 / 20th that of praseodymium-neodymium metal and 1 / 100th that of heavy rare earth elements. To address the issue of balanced utilization of rare earth resources, neodymium iron boron permanent magnets have emerged by replacing Pr / Nd with high-abundance Ce, becoming an important research direction in the field of rare earth permanent magnet materials.

[0004] The saturation magnetization and anisotropic field of the cerium-iron-boron (CFeB) phase are significantly lower than those of the neodymium-iron-boron (NdFeB) phase. Adding large amounts of cerium to NdFeB magnets deteriorates their overall magnetic properties (coercivity, remanence, thermal stability, etc.), especially remanence and coercivity. To reduce the magnetic dilution effect and improve the coercivity of cerium-rich NdFeB magnets, heavy rare earth grain boundary diffusion technology has been introduced into Ce-based NdFeB magnets. However, this approach has not significantly improved the coercivity of Ce-based NdFeB magnets. Current research on Ce-based NdFeB magnets mainly focuses on matrix composition design and diffusion source selection, achieving some positive progress. Despite increasing research on grain boundary diffusion technology for Ce-based NdFeB magnets, some problems remain. To achieve the same increase in coercivity, Ce-based NdFeB magnets require the diffusion of more heavy rare earth elements, while the increase in coercivity is relatively small for the same amount of heavy rare earth elements.

[0005] Grain boundary diffusion heat treatment is a key technology for improving the coercivity of magnets, but its interface problems (such as inhomogeneous diffusion, phase reaction, and grain boundary structure evolution) directly affect the overall magnetic properties of the magnets. Existing grain boundary diffusion heat treatment processes for Ce-based NdFeB magnets typically employ high-temperature diffusion at 850–950℃ and two-stage tempering. Furthermore, when the cerium content exceeds 5% (by weight), it heavily relies on heavy rare earth elements to ensure the overall magnetic properties of the magnets.

[0006] The invention patent with announcement number CN114914046B, entitled "A Medium-High Performance NdFeB Magnet and its Preparation Method," discloses a method that uses an HReM alloy with a melting point of 600-900℃ to change the diffusion temperature to 600-750℃. This method reduces the amount of Dy and Tb elements used, avoids a decrease in remanence, and improves intrinsic coercivity. However, this method requires the development of a new low-melting-point alloy diffuser, necessitating the use of a special HReM alloy, which is not readily available on the market. This increases production costs and process difficulty, adds control points to the alloy, and is not conducive to industrial application. In summary, this invention aims to further expand the development prospects of cerium-rich NdFeB magnets by studying other aspects to improve the comprehensive magnetic properties of cerium-rich NdFeB magnets. Summary of the Invention

[0007] The purpose of this invention is to provide a heat treatment process for cerium-rich NdFeB magnets, so as to improve the overall magnetic properties of cerium-rich NdFeB magnets by controlling the heat treatment process alone.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a heat treatment process for cerium-rich NdFeB magnets, comprising the following steps:

[0009] S1 Low-Temperature Annealing Pretreatment: The sintered cerium-rich NdFeB magnet matrix undergoes a first heat treatment at 450–550°C in a high-purity Ar atmosphere to reduce Ce / Nd grain boundary segregation and suppress CeFe2 phase formation. After heat treatment, it is air-cooled to room temperature under an inert gas protective atmosphere. The general chemical formula of the cerium-rich NdFeB magnet matrix is ​​(PrNd). a Ce b M c Fe 100-a-b-c-d B d In the formula, by mass percentage, 15≤a≤30, 5≤b≤20, 0≤c≤2, 0.95≤d≤1.05; M is selected from at least one of Cu, Ga, Al, Co, Zr, Nb, and Ti;

[0010] S2 medium-temperature diffusion: The surface of the magnet after low-temperature annealing pretreatment is coated with MM heavy rare earth diffusion source, and a second heat treatment is carried out in a high-purity Ar+5% H2 atmosphere at 600~700℃ to form a low-melting-point liquid phase channel, so that the heavy rare earth MM diffuses into the interior of the magnet along the grain boundary. After heat treatment, it is air-cooled to room temperature with inert gas protective gas.

[0011] S3 High-Temperature Diffusion: The magnets after medium-temperature diffusion undergo a third heat treatment in a high-purity Ar atmosphere at 800–900°C. After heat treatment, they are rapidly cooled to room temperature by Ar quenching to form (MM,Nd)₂Fe. 14 B. Hard magnetic shell;

[0012] S4 tempering treatment: The magnet after high-temperature diffusion is subjected to a fourth heat treatment in a high-purity Ar atmosphere at 450-650℃ to repair and improve the high-temperature diffusion grain boundary interface. After heat treatment, it is air-cooled to room temperature using an inert gas protective gas.

[0013] Preferably, in step S1 above, the heating rate of the first heat treatment is 3-5℃ / min, and the holding time after the temperature is reached is 0.5-2 hours; the cooling rate of the air cooling is 3-5℃ / s.

[0014] Preferably, in step S2 above, the heating rate of the second heat treatment is 3-5℃ / min, and the holding time after the temperature is reached is 3-10 hours; the cooling rate of air cooling is 3-5℃ / s.

[0015] Preferably, in step S3 above, the heating rate of the third heat treatment is 3-5℃ / min, the holding time after the temperature is reached is 0.5-2 hours, and the cooling rate is 15-30℃ / s.

[0016] Preferably, in step S4 above, the heating rate of the fourth heat treatment is 3-5℃ / min, and the holding time after the temperature is reached is 1-5 hours; the cooling rate of air cooling is 1-3℃ / s.

[0017] Preferably, the grade of the above-mentioned cerium-rich NdFeB magnet matrix is ​​selected from N38, N40, N42, N45, 38H, 40H, 42H, 38SH, and 40SH.

[0018] Preferably, the preparation process of the above-mentioned cerium-rich NdFeB magnet matrix includes the steps of melting and rapid solidification, hydrogen breaking, air jet milling, magnetic field orientation, isostatic pressing and sintering.

[0019] In a preferred embodiment, the MM heavy rare earth diffusion source includes at least one of the following: hydrides, fluorides, oxides, and alloys of Dy or Tb, and the amount added does not exceed 1.5% of the mass of the cerium-rich NdFeB magnet matrix.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. The heat treatment process of this cerium-rich NdFeB magnet, without changing the original process and raw materials as much as possible, significantly improves the coercivity of Ce-based NdFeB magnets by synergistic regulation of low-temperature annealing pretreatment, secondary diffusion, and tempering, and by controlling the heat treatment time, temperature, atmosphere, cooling method and rate, while maintaining high remanence. It solves the problems of diffusion inhomogeneity, interface reaction and phase transformation, and grain boundary structure degradation that exist in the conventional Ce-based NdFeB magnet grain boundary diffusion process. The upper limit of cerium addition can reach 20% (mass percentage).

[0022] 2. The heat treatment process of this cerium-rich NdFeB magnet is simple and easy to operate, and the cost is controllable. Compared with existing methods, it reduces the amount of heavy rare earth elements used while maintaining high remanence performance, achieving the same coercivity performance.

[0023] 3. The heat treatment process of this cerium-rich NdFeB magnet incorporates a low-temperature annealing pretreatment before grain boundary diffusion. This reduces Ce / Nd grain boundary segregation, suppresses CeFe2 phase formation, and improves subsequent diffusion uniformity. The invention also employs a two-stage diffusion heat treatment process. First, a medium-temperature diffusion at 600–700°C forms a low-melting-point liquid phase channel, allowing heavy rare earth elements to diffuse along the grain boundaries into the magnet's interior. Then, a high-temperature diffusion at 800–900°C forms (Dy / Tb, Nd)2Fe. 14 The B-hard magnetic shell process avoids problems such as abnormal grain growth, unnecessary excessive phase precipitation, and uneven diffusion of the magnet, significantly improving the coercivity of cerium-rich NdFeB magnets, while also helping to maintain high remanence and thermal stability. Detailed Implementation

[0024] Current conventional grain boundary diffusion processes for Ce-based NdFeB magnets mainly suffer from problems such as diffusion inhomogeneity, interfacial reactions and phase transitions, and grain boundary structure degradation. One issue is diffusion inhomogeneity, due to the Nd2Fe... 14 B and Ce2Fe 14 The difference in grain boundary energy of boron (B) and the tendency of Ce to combine with oxygen to form CeO2, blocking diffusion channels, easily cause diffusing elements such as Dy and Tb to preferentially penetrate along the grain boundaries of the neodymium-rich phase, hindering diffusion in the cerium-rich phase region. Secondly, interfacial reactions and phase transitions are significant. Because Ce is highly reactive, it easily undergoes unintended reactions with diffusing elements, such as the formation of low-melting-point phases TbCeFe2 and Ce2Fe with diffusing element Tb. 14Bo has a lower decomposition temperature, making it prone to abnormal growth of the main phase grains during high-temperature diffusion. Thirdly, the grain boundary structure degrades because local supersaturation of components during diffusion leads to phase separation, and thermal stress concentration during cooling can cause pores or cracks to form at the interface after diffusion, resulting in discontinuous grain boundary phases and reducing the demagnetizing coupling effect.

[0025] To address the existing problems in the grain boundary diffusion process of Ce-based NdFeB magnets, and to improve the performance of cerium-rich NdFeB magnets while preserving the original preparation process framework and raw material model as much as possible, and to avoid designing solutions that are difficult to apply, this invention has made a great deal of conceptualization and design, and after numerous experiments and improvements, has finally achieved the following important progress in improving the heat treatment process.

[0026] A heat treatment process for cerium-rich NdFeB magnets includes the following steps:

[0027] S1 Low-temperature annealing pretreatment: The sintered cerium-rich NdFeB magnet matrix is ​​subjected to a first heat treatment in a high-purity Ar atmosphere at 450-550℃. The holding time after reaching the temperature is preferably 0.5-2 hours. After the heat treatment, it is cooled to room temperature by inert gas protection gas. The cooling rate should be controlled at 3-5℃ / s.

[0028] The purpose of the first heat treatment is to homogenize the chemical composition of the grain boundaries, reduce Ce / Nd grain boundary segregation, inhibit the formation of CeFe2 phase, promote the continuous distribution of Nd-rich phase, and provide channels for subsequent diffusion.

[0029] S2 Medium-Temperature Diffusion: The surface of the magnet after low-temperature annealing pretreatment is coated with MM heavy rare earth diffusion source (MM heavy rare earth diffusion source includes at least one of Dy or Tb hydride, fluoride, oxide, alloy, the amount added does not exceed 1.5% of the mass of the cerium-rich NdFeB magnet matrix, the specific amount can be determined according to the magnet grade), and a second heat treatment is carried out in a high-purity Ar+5% H2 atmosphere and at 600-700℃. The holding time after the temperature is reached is preferably 3-10 hours. After heat treatment, the magnet is air-cooled to room temperature with inert gas protective gas. The cooling rate should be controlled at 3-5℃ / s.

[0030] This temperature is higher than the phase formation temperature of CeFe2 and lower than the diffusion temperature of the main phase grains, thus forming a low-melting-point liquid phase channel, allowing heavy rare earth elements to diffuse into the interior of the magnet along the grain boundaries as much as possible, while almost no substitution reaction between heavy rare earth elements and grains occurs, thereby minimizing abnormal grain growth and uneven diffusion of the magnet.

[0031] S3 High-Temperature Diffusion: The magnet after medium-temperature diffusion is subjected to a third heat treatment in a high-purity Ar atmosphere at 800-900℃. The holding time after reaching the temperature is preferably 0.5-2 hours. After heat treatment, Ar gas quenching is used to rapidly cool to room temperature. The cooling rate should be controlled at 15-30℃ / s.

[0032] This step achieves the substitution reaction between heavy rare earth elements and light rare earth elements within the grains, with heavy rare earth elements (MM) deeply substituting for the main phase grains to form (MM, Nd)2Fe. 14 B hard magnetic shell, compared with the traditional diffusion process, has a first-stage low-temperature annealing pretreatment process and a medium-temperature diffusion process. High-temperature diffusion can greatly reduce the diffusion time and avoid abnormal grain growth, unnecessary excessive phase precipitation and uneven diffusion of magnets.

[0033] S4 tempering treatment: The magnet after high temperature diffusion is subjected to a fourth heat treatment in a high-purity Ar atmosphere at 450-650℃. The holding time after reaching the temperature is preferably 1-5 hours. After the heat treatment, the magnet is air-cooled to room temperature using an inert gas protective gas. The cooling rate should be controlled at 1-3℃ / s.

[0034] The fourth heat treatment uses a traditional tempering process, which can repair and improve the high-temperature diffusion grain boundary interface, further demagnetize and couple the grains, and enhance coercivity.

[0035] The heating rate for all four heat treatments can be 3–5 °C / min.

[0036] The general chemical formula of the aforementioned cerium-rich NdFeB magnet matrix is ​​(PrNd). a Ce b M c Fe 100-a-b-c-d B d In the formula, by mass percentage, 15≤a≤30, 5≤b≤20, 0≤c≤2, 0.95≤d≤1.05; M is selected from at least one of Cu, Ga, Al, Co, Zr, Nb, and Ti, specifically using the following grades: N38, N40, N42, N45, 38H, 40H, 42H, 38SH, and 40SH. The preparation process includes steps such as melting and rapid solidification, hydrogen breaking, air jet milling, magnetic field orientation, isostatic pressing, and sintering. The above formula is a matrix formula adapted to the heat treatment method of the present invention, but the above matrix preparation process is a known technology, and this application has not made any improvements, so it will not be described again.

[0037] The present invention will be further illustrated below through several embodiments and comparative examples. However, the following embodiments are not all embodiments of the present invention and should not be regarded as an absolute limitation of the present invention.

[0038] Example 1

[0039] The cerium-rich NdFeB magnet matrix in this embodiment takes the N38 matrix formulation as an example. It is prepared by a series of process steps, including melting and rapid solidification, hydrogen breaking, air jet milling, magnetic field orientation, isostatic pressing, and sintering, according to the composition ratio of Pr-Nd 15.5%, Ce 20%, B 0.97%, Cu 0.2wt%, Al 0.2wt%, Zr 0.15wt%, Ti 0.05wt%, and Fe balance.

[0040] First, the sintered cerium-rich NdFeB magnet substrate was pretreated by low-temperature annealing in a high-purity Ar atmosphere. The heating rate was 5℃ / min, reaching 500℃, and held for 1 hour. Then, it was air-cooled to room temperature using an inert gas shield at a cooling rate of 3℃ / s. The sample was then removed. Next, dysprosium hydride was coated onto the surface of the pretreated magnet for medium-temperature diffusion. This was done in a high-purity Ar + 5% H2 atmosphere, with a heating rate of 5℃ / min, reaching 650℃, and held for 5 hours. The sample was then air-cooled to room temperature using an inert gas shield at a cooling rate of 3℃ / s. The amount of dysprosium added was 0.6% of the mass of the cerium-rich NdFeB magnet substrate (Dy mass fraction). Next, the magnets that had undergone intermediate-temperature diffusion were subjected to high-temperature diffusion in a high-purity Ar atmosphere at a heating rate of 5℃ / min, reaching 900℃ and holding for 1 hour. They were then rapidly cooled to room temperature using Ar quenching at a cooling rate of 20℃ / s, and the samples were removed. Finally, the magnets that had undergone high-temperature diffusion were tempered in a high-purity Ar atmosphere at a heating rate of 5℃ / min, reaching 500℃ and holding for 3 hours. They were then air-cooled to room temperature using an inert gas protective gas at a cooling rate of 2℃ / s, and the samples were removed.

[0041] The prepared grain boundary diffused cerium-rich NdFeB magnet was placed in a PFM12cn ultra-high coercivity permanent magnet tester, and the magnetic properties were measured and recorded in Table 1 below.

[0042] Example 2

[0043] The cerium-rich NdFeB magnet matrix in this embodiment takes the N38 matrix formulation as an example. It is prepared by a series of process steps including melting and rapid solidification, hydrogen breaking, air jet milling, magnetic field orientation, isostatic pressing, and sintering, according to the composition ratio of Pr-Nd 20%, Ce 15%, B 0.97%, Cu 0.1wt%, Al 0.1wt%, Ti 0.15wt%, and Fe balance.

[0044] First, the sintered cerium-rich NdFeB magnet substrate was pretreated by low-temperature annealing in a high-purity Ar atmosphere. The heating rate was 5℃ / min, reaching 550℃, and held for 2 hours. Then, it was air-cooled to room temperature using an inert gas shield at a cooling rate of 4℃ / s. The sample was then removed. Next, dysprosium hydride was coated onto the pretreated magnet surface for medium-temperature diffusion. The substrate was then heated to 650℃ in a high-purity Ar + 5% H2 atmosphere at a heating rate of 5℃ / min, held for 4 hours, and air-cooled to room temperature using an inert gas shield at a cooling rate of 4℃ / s. The dysprosium content (Dy mass fraction) was 0.4% of the mass of the cerium-rich NdFeB magnet substrate. Next, the magnets that had undergone intermediate-temperature diffusion were subjected to high-temperature diffusion in a high-purity Ar atmosphere at a heating rate of 5℃ / min, reaching 850℃ and holding for 2 hours. They were then rapidly cooled to room temperature using Ar quenching at a cooling rate of 25℃ / s, and the samples were removed. Finally, the magnets that had undergone high-temperature diffusion were tempered in a high-purity Ar atmosphere at a heating rate of 5℃ / min, reaching 600℃ and holding for 5 hours. They were then air-cooled to room temperature using an inert gas protective gas at a cooling rate of 1℃ / s, and the samples were removed.

[0045] The prepared grain boundary diffused cerium-rich NdFeB magnet was placed in a PFM12cn ultra-high coercivity permanent magnet tester, and the magnetic properties were measured and recorded in Table 1 below.

[0046] Comparative Example 1

[0047] Comparative Example 1 is the same as Example 1, except that it does not use low-temperature annealing pretreatment and medium-temperature diffusion treatment processes.

[0048] The prepared grain boundary diffused cerium-rich NdFeB magnet was placed in a PFM12cn ultra-high coercivity permanent magnet tester, and the magnetic properties were measured and recorded in Table 1 below.

[0049] Comparative Example 2

[0050] Comparative Example 2 is the same as Example 2, except that it does not use low-temperature annealing pretreatment and medium-temperature diffusion treatment. In addition, in order to obtain similar coercivity performance, the amount of dysprosium added is increased to 0.7% of the mass of the cerium-rich NdFeB magnet matrix by mass fraction of Dy.

[0051] The prepared grain boundary diffused cerium-rich NdFeB magnet was placed in a PFM12cn ultra-high coercivity permanent magnet tester, and the magnetic properties were measured and recorded in Table 1 below.

[0052] Example 3

[0053] The cerium-rich NdFeB magnet matrix in this embodiment takes the 38SH matrix formulation as an example. It is prepared by a series of process steps including melting and rapid solidification, hydrogen breaking, air jet milling, magnetic field orientation, isostatic pressing, and sintering, according to the composition ratio of Pr-Nd 25%, Ce 10%, B 0.97%, Cu 0.3wt%, Al 0.2wt%, Co 0.5wt%, Ga 0.1wt%, Zr 0.05wt%, and Fe balance.

[0054] First, the sintered cerium-rich NdFeB magnet substrate was pretreated by low-temperature annealing in a high-purity Ar atmosphere. The heating rate was 3℃ / min, reaching 550℃, and held for 2 hours. Then, it was air-cooled to room temperature using an inert gas shield at a cooling rate of 5℃ / s. The sample was then removed. Next, terbium fluoride was coated onto the pretreated magnet surface for medium-temperature diffusion. The substrate was then heated to 650℃ in a high-purity Ar + 5% H2 atmosphere at a heating rate of 3℃ / min, held for 6 hours, and air-cooled to room temperature using an inert gas shield at a cooling rate of 5℃ / s. The terbium addition amount, calculated as Tb mass fraction, was 0.25% of the mass of the cerium-rich NdFeB magnet substrate. Next, the magnet, after medium-temperature diffusion, underwent high-temperature diffusion in a high-purity Ar atmosphere at a heating rate of 3℃ / min, reaching 900℃ and holding for 1.5 hours. It was then rapidly cooled to room temperature using Ar quenching at a cooling rate of 30℃ / s, and the sample was removed. Finally, the magnet, after high-temperature diffusion, underwent tempering in a high-purity Ar atmosphere at a heating rate of 3℃ / min, reaching 650℃ and holding for 3 hours. It was then air-cooled to room temperature using an inert gas protective gas at a cooling rate of 1℃ / s, and the sample was removed.

[0055] The prepared grain boundary diffused cerium-rich NdFeB magnet was placed in a PFM12cn ultra-high coercivity permanent magnet tester, and the magnetic properties were measured and recorded in Table 1 below.

[0056] Example 4

[0057] The cerium-rich NdFeB magnet matrix in this embodiment takes the 38SH matrix formulation as an example. It is prepared by a series of process steps including melting and rapid solidification, hydrogen breaking, air jet milling, magnetic field orientation, isostatic pressing, and sintering, according to the composition ratio of Pr-Nd 30%, Ce 5%, B 0.97%, Cu 0.2wt%, Al 0.2wt%, Co 0.3wt%, Ga 0.15wt%, Nb 0.05wt%, and Fe balance.

[0058] First, the sintered cerium-rich NdFeB magnet substrate was pretreated by low-temperature annealing in a high-purity Ar atmosphere. The heating rate was 3℃ / min, reaching 500℃, and held for 1 hour. Then, it was air-cooled to room temperature using an inert gas shield at a cooling rate of 4℃ / s. The sample was then removed. Next, terbium fluoride was coated onto the pretreated magnet surface for medium-temperature diffusion. This was done in a high-purity Ar + 5% H2 atmosphere, with a heating rate of 3℃ / min, reaching 600℃, and held for 8 hours. The sample was then air-cooled to room temperature using an inert gas shield at a cooling rate of 4℃ / s. The amount of terbium added was 0.18% of the mass of the cerium-rich NdFeB magnet substrate (Tb mass fraction). Next, the magnets that had undergone intermediate-temperature diffusion were subjected to high-temperature diffusion in a high-purity Ar atmosphere at a heating rate of 3℃ / min, reaching 850℃ and holding for 3 hours. They were then rapidly cooled to room temperature using Ar quenching at a cooling rate of 25℃ / s, and the samples were removed. Finally, the magnets that had undergone high-temperature diffusion were tempered in a high-purity Ar atmosphere at a heating rate of 3℃ / min, reaching 500℃ and holding for 5 hours. They were then air-cooled to room temperature using an inert gas protective gas at a cooling rate of 2℃ / s, and the samples were removed.

[0059] The prepared grain boundary diffused cerium-rich NdFeB magnet was placed in a PFM12cn ultra-high coercivity permanent magnet tester, and the magnetic properties were measured and recorded in Table 1 below.

[0060] Comparative Example 3

[0061] Comparative Example 3 is the same as Example 3, except that it does not use low-temperature annealing pretreatment and medium-temperature diffusion treatment.

[0062] The prepared grain boundary diffused cerium-rich NdFeB magnet was placed in a PFM12cn ultra-high coercivity permanent magnet tester, and the magnetic properties were measured and recorded in Table 1 below.

[0063] Comparative Example 4

[0064] Comparative Example 4 is the same as Example 4, except that it does not use low-temperature annealing pretreatment and medium-temperature diffusion treatment. In addition, in order to obtain similar coercivity performance, the amount of terbium added is 0.4% of the mass of the cerium-rich NdFeB magnet matrix, expressed as Tb mass fraction.

[0065] The prepared grain boundary diffused cerium-rich NdFeB magnet was placed in a PFM12cn ultra-high coercivity permanent magnet tester, and the magnetic properties were measured and recorded in Table 1 below.

[0066] Table 1. Magnetic performance test results of the examples and comparative examples

[0067]

[0068] From the comparison of Example 1 and Comparative Example 1, and Example 3 and Comparative Example 3 in the table above, it is easy to see that the present invention can significantly improve the coercivity performance of cerium-rich NdFeB magnets without changing other processes, under the same amount of heavy rare earth diffusion source added. However, through the comparison of Example 2 and Comparative Example 2, and Example 4 and Comparative Example 4, it is easy to find that in order to make the coercivity of the product prepared by the existing method comparable to that prepared by the method of the present invention, it is necessary to add more heavy rare earth diffusion sources. After increasing the amount of diffusion source, the remanence and coercivity of Comparative Example 2 and Comparative Example 4 are not as good as those of the process of the present invention. The method of the present invention only changes the timing, number of times, temperature and time of heat treatment, etc., and achieves the improvement of the overall magnetic properties of cerium-rich NdFeB magnets with less heavy rare earth diffusion source.

[0069] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

[0070] Any aspects of this invention not described in detail are well-known to those skilled in the art.

Claims

1. A heat treatment process for cerium-rich NdFeB magnets, characterized in that, Includes the following steps: S1 Low-Temperature Annealing Pretreatment: The sintered cerium-rich NdFeB magnet matrix undergoes a first heat treatment at 450–550°C in a high-purity Ar atmosphere. The heating rate is 3–5°C / min, and the holding time after reaching the temperature is 0.5–2 hours. This first heat treatment is used to reduce Ce / Nd grain boundary segregation and suppress CeFe2 phase formation. After the heat treatment, the matrix is ​​air-cooled to room temperature using an inert gas protective gas at a cooling rate of 3–5°C / s. The general chemical formula of the cerium-rich NdFeB magnet matrix is ​​(PrNd). a Ce b M c Fe 100-a-b-c-d B d In the formula, by mass percentage, 15≤a≤30, 5≤b≤20, 0≤c≤2, 0.95≤d≤1.05; M is selected from at least one of Cu, Ga, Al, Co, Zr, Nb, and Ti; S2 medium-temperature diffusion: The surface of the magnet after low-temperature annealing pretreatment is coated with MM heavy rare earth diffusion source, and a second heat treatment is carried out in a high-purity Ar + 5% H2 atmosphere at 600-700℃. The heating rate is 3-5℃ / min, and the holding time after reaching the temperature is 3-10 hours. The second heat treatment is used to form a low-melting-point liquid phase channel, so that the heavy rare earth MM diffuses into the interior of the magnet along the grain boundary. After the heat treatment, the magnet is air-cooled to room temperature with an inert gas protective gas at a cooling rate of 3-5℃ / s. S3 High-Temperature Diffusion: The magnets after medium-temperature diffusion are subjected to a third heat treatment in a high-purity Ar atmosphere at 800–900°C, with a heating rate of 3–5°C / min and a holding time of 0.5–2 hours after reaching the desired temperature. After the third heat treatment, the magnets are rapidly cooled to room temperature by Ar quenching at a cooling rate of 15–30°C / s to form (MM,Nd)₂Fe. 14 B. Hard magnetic shell; S4 tempering treatment: The magnet after high-temperature diffusion is subjected to a fourth heat treatment in a high-purity Ar atmosphere at 450-650℃, with a heating rate of 3-5℃ / min and a holding time of 1-5 hours after reaching the temperature. The fourth heat treatment is used to repair and improve the high-temperature diffusion grain boundary interface. After the heat treatment, it is air-cooled to room temperature with an inert gas protective gas at a cooling rate of 1-3℃ / s.

2. The heat treatment process for a cerium-rich NdFeB magnet according to claim 1, characterized in that: The grade of the cerium-rich NdFeB magnet matrix is ​​selected from N38, N40, N42, N45, 38H, 40H, 42H, 38SH, and 40SH.

3. The heat treatment process for a cerium-rich NdFeB magnet according to claim 1, characterized in that: The preparation process of the cerium-rich NdFeB magnet matrix includes melting and rapid solidification, hydrogen breaking, air jet milling, magnetic field orientation, isostatic pressing, and sintering steps.

4. The heat treatment process for a cerium-rich NdFeB magnet according to claim 1, characterized in that: The MM heavy rare earth diffusion source includes at least one of the hydrides, fluorides, oxides, and alloys of Dy or Tb, and the amount added does not exceed 1.5% of the mass of the cerium-rich NdFeB magnet matrix.

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