High-consistency neodymium-iron-boron magnet and method for manufacturing the same

Abstract

The application discloses a kind of high consistency neodymium-iron-boron magnets and preparation method thereof, method is carried out sand blasting treatment to the orientation surface of magnet to form pit, then its surface roughness Ra is reduced to 0.05-0.6 μm by double-side grinding;After cleaning and drying, with heavy rare earth diffusion source and auxiliary ball in proportion in rotary furnace at 600-750 ℃ for 6-30 h to carry out low-temperature rotary film coating;Subsequently, high-temperature diffusion is carried out at 800-950 ℃ for 4-10 h, and then aging is carried out at 450-600 ℃ for 3-5 h.The application combines surface morphology control and rotary dynamic coating technology, and under the premise of not changing the process parameters of rotary dynamic coating, the contradiction between diffusion uniformity and high heavy rare earth weight gain is alleviated, and finally a magnet with gradient structure is obtained, which improves the magnetic properties and consistency of the magnet.

Description

Technical Field

[0001] This invention relates to the field of sintered NdFeB magnet preparation technology, and particularly to a highly consistent NdFeB magnet and its preparation method. Background Technology

[0002] As a third-generation rare-earth permanent magnet material, sintered NdFeB magnets are known as the "King of Magnets" due to their extremely high magnetic properties. They are high-performance magnetic materials composed of rare-earth elements (RE, Nd, Pr, etc.), transition metals (TM, Fe, Co, etc.), and boron in specific proportions, smelted together, then pressed using powder metallurgy and sintered. The applications of sintered NdFeB materials are becoming increasingly widespread, especially in the consumer electronics industry, where products are required to be small in size, high in performance, and have good consistency.

[0003] Traditional grain boundary diffusion methods mainly include coating, electrodeposition, and magnetron sputtering. These methods require arranging the magnets on a disk before coating, which is not only complex but also requires sophisticated equipment, making them unsuitable for the industrialization of small-sized magnets.

[0004] Rotary diffusion, developed to address the aforementioned issues, effectively solves the grain boundary diffusion problems faced by small and irregularly shaped magnets. The process involves mixing the magnet with heavy rare earth metals or alloys, then placing it in a rotary furnace for heat treatment, high-temperature diffusion, and aging treatment to complete the grain boundary diffusion process. The rotation of the furnace ensures more thorough and uniform contact between the magnet and the heavy rare earth diffusion source.

[0005] However, rotational diffusion presents the following challenges: ① Due to the limitations of the rotational diffusion process, it is difficult to simultaneously achieve both diffusion efficiency and coating consistency. During production, to ensure coating consistency, the diffusion temperature needs to be lowered and the holding time extended, which significantly reduces coating efficiency; ② To ensure coating effectiveness, several times or even ten times the mass of the magnet needs to be added as a heavy rare earth metal diffusion source, resulting in a waste of resources; ③ Prolonged contact between the magnet and the diffusion source leads to excessive consumption of heavy rare earth elements, resulting in over-diffusion and a significant decrease in the magnet's remanence. Summary of the Invention

[0006] To address the aforementioned shortcomings, this invention proposes a highly consistent NdFeB magnet and its preparation method. By combining surface morphology control with rotational dynamic low-temperature coating technology, supplemented by segmented precision temperature-controlled diffusion, the contradiction between the uniformity of the heavy rare earth diffusion layer and the weight gain of high heavy rare earth is mitigated without changing the rotational dynamic coating process parameters. Ultimately, the industrial-scale preparation of NdFeB magnets with high intrinsic coercivity, high remanence, and high performance consistency is achieved.

[0007] This invention provides the following technical solution: a method for preparing a highly uniform NdFeB magnet, comprising the following steps: S1. Sandblasting treatment: The orientation surface of the NdFeB magnet is sandblasted to form pits with a depth of 3 to 10 μm on its surface, increasing the surface roughness of the orientation surface. Then, the NdFeB magnet is cleaned with anhydrous ethanol and dried. S2. Double-sided grinding treatment: The NdFeB magnets that have been sandblasted are subjected to double-sided grinding treatment to reduce the surface roughness Ra of the orientation surface to 0.05-0.6μm and the depth of the pits to 0.3-4μm; S3. Cleaning and drying: Clean the NdFeB magnets again with anhydrous ethanol and then dry them. S4. Rotary coating: The neodymium iron boron magnets treated in step S3, the heavy rare earth diffusion source and the stirring ball are mixed in a mass ratio of 1:(1~10):(0.1~2) and placed in a rotary heat treatment furnace. After vacuuming, the mixture is rotated and kept at 600~750℃ for 6~30h to form a coating on the surface of the magnets by the heavy rare earth diffusion source. S5. High-temperature diffusion and aging: After cooling to room temperature, the coated magnet is separated from the heavy rare earth diffusion source and stirring ball. The coated magnet is then subjected to a vacuum degree better than 1.0 × 10⁻⁶. -2 High-temperature diffusion treatment was carried out at Pa and temperature of 800–950℃ for 4–10 hours, followed by a vacuum degree better than 5.0 × 10⁻⁶. -2 Aging treatment was carried out for 3 to 5 hours under conditions of Pa and temperature of 450 to 600℃.

[0008] As an improvement, in step S4, the rotational speed of the rotary heat treatment furnace is 0.1–10 r / min, and the vacuum level is evacuated to a value better than 5.0 × 10⁻⁶. -3 Pa.

[0009] As an improvement, in step S4, the heavy rare earth diffusion source is at least one of metal Dy, metal Tb, Dy alloy or Tb alloy, and its form is blocky or granular with a size of less than 15 mm.

[0010] As an improvement, in step S4, the mixing balls are at least one of alumina balls, zirconium oxide balls, silicon oxide balls, or silicon nitride balls, and their particle size is 2 to 10 mm.

[0011] As an improvement, in step S5, during the high-temperature diffusion treatment, the heating rate is 8-12℃ / min before the temperature reaches 600℃, and the heating rate is reduced to 3-7℃ / min after the temperature reaches 600℃.

[0012] As an improvement, the drying temperature in step S3 is 70–120°C, and the drying time is 10–60 min.

[0013] A highly uniform NdFeB magnet is prepared using any of the methods described above. The intermediate body of the magnet has a gradient structure in which the intrinsic coercivity gradually decreases from the surface to the center. The concentration of heavy rare earth elements (RH) on the surface is 2 to 10 times that of the center. The total amount of heavy rare earth elements in the 100 μm range of the surface layer of the intermediate body is 1.2 to 3 times that of the center.

[0014] As an improvement, the intermediate is subjected to high-temperature diffusion and aging treatment, which makes the heavy rare earth element RH concentration of the magnet uniformly distributed from a depth of 100μm from the surface to the center.

[0015] Compared with the prior art, the advantages of the present invention are as follows: First, a creative combination of sandblasting and double-sided grinding was employed. The former creates uniformly deep and distributed micro-pits on the magnet orientation surface, significantly increasing the specific surface area and providing more adhesion points for subsequent heavy rare earth diffusion. The latter precisely controls the surface roughness to an extremely low and uniform level, with Ra ranging from 0.05 to 0.6 μm. Simultaneously, the pit depth on the magnet orientation surface decreases and the curvature becomes gentler, retaining some of the optimized pit structure. This combined process achieves a smooth surface macroscopically while increasing the area of ​​the heavy rare earth adhesion surface, creating a favorable morphology for diffusion. Without altering the rotational dynamic coating process parameters, it mitigates the conflict between diffusion layer uniformity and the weight gain of high heavy rare earth elements. Furthermore, by adjusting the pit depth, the rotational diffusion coating effect is effectively controlled, preventing excessive enrichment of heavy rare earth elements in the pits that could lead to waste during diffusion. This improves the utilization rate of the heavy rare earth diffusion source and lays a crucial structural foundation for subsequent processes.

[0016] Building upon this foundation, the rotary coating process introduces stirring balls that are mixed with the magnet and diffusion source at a specific mass ratio, followed by dynamic heat treatment at a relatively low temperature of 600–750°C. During this dynamic heat treatment, the rotary furnace continuously and uniformly rotates clockwise or counterclockwise, causing the stirring balls, magnet, and diffusion source to rotate together. This keeps them in a state of alternating contact and relative motion. The presence of the stirring balls effectively isolates the magnet and diffusion source, preventing adhesion and sintering at low temperatures. At 600–750°C, the heavy rare earth elements on the diffusion source surface are activated and released in gaseous form, adhering to the magnet surface. Simultaneously, continuous tumbling and stirring ensures extreme uniformity in heat and diffusion material transfer, allowing the heavy rare earth elements to be uniformly deposited in gaseous form onto the pretreated magnet surface, forming a pre-diffusion layer with uniform composition and controllable thickness. This facilitates the formation of a gradient structure with gradually decreasing intrinsic coercivity from the surface to the center during subsequent high-temperature diffusion. The uniform pre-diffusion layer lays a crucial compositional and structural foundation for the subsequent deep diffusion of elements at high temperatures, and its uniformity directly determines the consistency of the final product.

[0017] The subsequent high-temperature diffusion and aging treatment, by separating the coated magnet from the heavy rare earth diffusion source and stirring balls before high-temperature diffusion and aging treatment, eliminated the risk of melting and adhesion between the magnet and the diffusion source due to direct contact during subsequent higher-temperature heat treatment, ensuring a high yield of the final product. Furthermore, the furnace cavity was kept under negative pressure throughout the high-temperature diffusion and aging treatment process, eliminating oxidation contamination and improving performance consistency within and between batches. From the diffusion mechanism perspective, a segmented heating method was adopted: a relatively rapid heating rate was used before 600℃, and the heating rate was actively reduced when the temperature approached the range where the activity of heavy rare earth elements significantly increased. This made the diffusion process more gradual and controllable, promoting the full release of heavy rare earth elements from the pre-diffusion layer and uniform diffusion inward along the grain boundaries, thereby forming a layer inside the magnet... Figure 4 The gradient structure shown in figure a exhibits a gradual decrease in the concentration of heavy rare earth elements (RH) from the surface to the center, while the intrinsic coercivity of the magnet gradually weakens.

[0018] Ultimately, the synergistic effect of all the above process steps is reflected in the final product: the NdFeB magnet not only possesses high intrinsic coercivity, remanence, and maximum energy product, but more importantly, it exhibits high performance consistency. The concentration of heavy rare earth elements between the magnet surface and the core is controlled within a small and stable range, meaning that the magnetic performance dispersion of the entire batch of magnets is extremely low. Simultaneously, due to the effective isolation and protection during the relatively low-temperature coating stage, the proportion of missing corners in the magnet is significantly reduced, resulting in a significant improvement in product yield and reliability. In summary, this solution, through a series of interconnected process designs, successfully integrates surface modification, low-temperature uniform coating, and high-temperature precise diffusion, synergistically solving the key technical challenges in the industrial-scale preparation of highly consistent NdFeB magnets. Attached Figure Description

[0019] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments: Figure 1 This is a schematic diagram of the surface morphology of a neodymium iron boron magnet, in which... Figure 1 Figure 'a' is a schematic diagram of the untreated surface morphology of a neodymium iron boron magnet. Figure 1 b represents the surface morphology of a neodymium iron boron magnet that has only undergone sandblasting. Figure 1 c is a schematic diagram of the surface morphology of the neodymium iron boron magnet after sandblasting and double-sided grinding; Figure 2 These are actual images taken using a scanning electron microscope (SEM) of neodymium iron boron magnets. Figure 2 Image 'a' is an electron microscope image of the untreated surface morphology of a neodymium iron boron magnet. Figure 2 b represents the electron microscope surface morphology of a neodymium iron boron magnet after sandblasting only. Figure 2c is an actual electron microscope image of the surface morphology of the neodymium iron boron magnet after sandblasting and double-sided grinding; Figure 3 This is a schematic diagram of the rotary coating device. Figure 4 This is a schematic diagram of the microstructure of a neodymium iron boron magnet, in which... Figure 4 Figure 'a' is a schematic diagram of the microstructure of the magnet after rotational coating. Figure 4 b is a schematic diagram of the microstructure of the magnet after diffusion; Figure 5 This is a scanned distribution map of the heavy rare earth element Tb in neodymium iron boron magnets, where... Figure 5 'a' represents the scanned distribution map of heavy rare earth elements in a traditional coating. Figure 5 b is the Tb scan distribution map of heavy rare earth elements in this invention. Detailed Implementation

[0020] Example 1 like Figures 1 to 4 As shown, a method for preparing a highly uniform NdFeB magnet includes the following steps: S1. Sandblasting: Pretreatment of NdFeB magnets. Cut NdFeB magnets into several square samples of grade N52, each 10×10×2mm with the 2mm direction as the orientation direction. After removing oil, sandblast the orientation surface of the NdFeB magnets to form pits with a depth of 3μm on the surface, thereby increasing the surface roughness of the orientation surface. Among them, neodymium iron boron magnets are those with R2Fe 14 R-Fe-B rare earth sintered magnets with B-type compound grains as the main phase; S2. Double-sided grinding treatment: The NdFeB magnets that have been sandblasted are subjected to double-sided grinding treatment to reduce the surface roughness Ra of the orientation surface to 0.15±0.02μm and the depth of the pits to 1±0.2μm; S3. Cleaning and drying: Clean the NdFeB magnet with anhydrous ethanol and dry it at 120℃ for 60 minutes. Mark it as sample A0. S4. Rotary Coating: The neodymium iron boron magnets treated in step S3, the heavy rare earth diffusion source, and the stirring balls are mixed in a mass ratio of 1:3:1 and placed in a rotary heat treatment furnace. Preferably, the stirring balls are zirconium oxide with a particle size of 3 mm, and the heavy rare earth diffusion source is a Dy cube with a side length of 8 mm, labeled as Dy diffusion source. The furnace is evacuated to a vacuum level better than 5.0 × 10⁻⁶. -3 After Pa, the magnet is rotated and held at 720℃ for 6 hours. The rotation speed of the rotary heat treatment furnace is 2.5 r / min, so that the heavy rare earth diffusion source forms a coating on the magnet surface. S5. High-temperature diffusion and aging: After cooling to room temperature, the coated magnet is separated from the heavy rare earth diffusion source and the stirring ball. The coated magnet is then subjected to a vacuum degree better than 8×10⁻⁶. -3 High-temperature diffusion treatment was carried out at 850℃ and Pa for 10 hours. The heating rate was 8℃ / min before reaching 600℃, and the heating rate was reduced to 4℃ / min after reaching 600℃. Subsequently, the treatment was carried out under a vacuum degree better than 3×10⁻⁶. -2 The aging treatment was carried out at 450℃ for 5 hours.

[0021] The magnets in Example 1 underwent visual inspection, and the magnetic properties of the blank sample A0 and the magnets in Example 1 were tested using a permanent magnet material measurement system according to the requirements of GB / T3217-2013 "Permanent Magnet (Hard Magnet) Materials - Magnetic Test Methods". The test results are shown in Table 1. The intrinsic coercivity of the original sample A0 magnet is 13.8 kOe, the remanence of the magnet is 14.35 kGs, and the maximum energy product of the magnet is 51.12 MGOe; In Example 1, the intrinsic coercivity of the magnet is 20.03 kOe, the remanence of the magnet is 14.23 kGs, the maximum magnetic energy product of the magnet is 50.71 MGOe, and the magnet has a corner missing ratio of 1.3%. During the coating stage of the diffusion treatment, i.e., after relatively low temperature heat treatment at 600-750℃, no substantial connection due to sintering, melting or chemical reaction occurred between magnets or between the magnet and the diffusion source or stirring ball.

[0022] Among them, the percentage of magnets with missing corners % = number of magnets with missing corners / total number of magnets.

[0023] The magnet of Example 1 underwent a high-temperature irreversible magnetic flux loss test. The heating temperature was 120℃, and the holding time was 2 hours, under semi-open circuit conditions. The test results are shown in Table 2. In Example 1, the maximum value of the high-temperature magnetic flux irreversible loss test result was 3.9%, the minimum value was 3.17%, the average value was 3.50%, and the standard deviation was 0.21.

[0024] Comparative Example 1 The pretreatment of the NdFeB magnet in step S1 only removes oil stains from the surface of the NdFeB magnet and does not perform roughening treatment. Other parameters and steps are the same as in Example 1, so they will not be repeated here.

[0025] The magnet in Comparative Example 1 underwent a visual inspection, and its magnetic properties were tested using a permanent magnet material measurement system according to the requirements of GB / T3217-2013 "Permanent Magnet (Hard Magnet) Materials - Magnetic Test Methods". The test results are shown in Table 1. Comparative Example 1 magnet has an intrinsic coercivity of 18.98 kOe, a remanence of 14.24 kGs, a maximum magnetic energy product of 50.63 MGOe, and a corner-missing ratio of 1.4%. During the coating stage of the diffusion treatment, i.e., after relatively low-temperature heat treatment at 600-750℃, no substantial connection due to sintering, melting, or chemical reaction occurred between magnets or between magnets and diffusion sources or stirring balls.

[0026] High-temperature irreversible flux loss tests were conducted on the magnet of Comparative Example 1. The heating temperature was 120℃, and the holding time was 2 hours, under semi-open circuit conditions. The test results are shown in Table 2. The maximum value of the irreversible magnetic flux loss test results for the comparative example 1 at high temperature was 4.65%, the minimum value was 3.42%, the average value was 3.86%, and the standard deviation was 0.40.

[0027] Example 2 S4. Rotary Coating: The NdFeB magnets treated in step S3, the heavy rare earth diffusion source, and the mixing balls are mixed in a mass ratio of 2:3:1 and placed in a rotary heat treatment furnace. Preferably, the mixing balls are alumina with a particle size of 4 mm, and the heavy rare earth diffusion source is a 10 mm diameter Tb ball, labeled as Tb diffusion source. The furnace is evacuated to a vacuum level better than 4.0 × 10⁻⁶. -3 After Pa, the magnet is rotated and held at 700℃ for 8 hours. The rotation speed of the rotary heat treatment furnace is 2 r / min, so that the heavy rare earth diffusion source forms a coating on the magnet surface. S5. High-temperature diffusion and aging: After cooling to room temperature, the coated magnet is separated from the heavy rare earth diffusion source and the stirring ball. The coated magnet is then subjected to a vacuum degree better than 7×10⁻⁶. -3 High-temperature diffusion treatment was carried out for 12 hours at 900℃ and Pa. The heating rate was 9℃ / min before reaching 600℃, and the heating rate was reduced to 5℃ / min after reaching 600℃. Subsequently, the treatment was carried out under a vacuum degree better than 2×10⁻⁶. -2 The aging process was carried out at 500℃ for 4 hours.

[0028] Other parameters and steps are the same as in Example 1, so they will not be repeated here.

[0029] The magnets in Example 2 underwent visual inspection, and their magnetic properties were tested using a permanent magnet material measurement system according to the requirements of GB / T3217-2013 "Permanent Magnet (Hard Magnet) Materials - Magnetic Test Methods". The test results are shown in Table 1. In Example 2, the intrinsic coercivity of the magnet is 23.7 kOe, the remanence of the magnet is 14.27 kGs, the maximum magnetic energy product of the magnet is 50.89 MGOe, and the magnet has a corner loss ratio of 1.2%. During the coating stage of the diffusion treatment, i.e., after relatively low temperature heat treatment at 600-750℃, no substantial connection due to sintering, melting or chemical reaction occurred between magnets or between the magnet and the diffusion source or stirring ball.

[0030] The magnet of Example 2 underwent a high-temperature irreversible magnetic flux loss test. The heating temperature was 120°C, and the holding time was 2 hours, under semi-open circuit conditions. The test results are shown in Table 2. In Example 2, the maximum value of the high-temperature magnetic flux irreversible loss test result was 1.05%, the minimum value was 0.22%, the average value was 0.53%, and the standard deviation was 0.26.

[0031] Comparative Example 2 In step S5, the coated magnet is not separated from the heavy rare earth diffusion source and the stirring ball; instead, the coated magnet is directly subjected to a vacuum degree better than 7×10⁻⁶. -3 High-temperature diffusion treatment was carried out for 12 hours at 900℃ and Pa. The heating rate was 9℃ / min before reaching 600℃, and the heating rate was reduced to 5℃ / min after reaching 600℃. Subsequently, the treatment was carried out under a vacuum degree better than 2×10⁻⁶. -2 The aging process was carried out at 500℃ for 4 hours.

[0032] The other parameters and steps are the same as in Example 2, so they will not be repeated here.

[0033] The magnets in Comparative Example 2 underwent visual inspection, and their magnetic properties were tested using a permanent magnet material measurement system according to the requirements of GB / T3217-2013 "Permanent Magnet (Hard Magnet) Materials - Magnetic Test Methods". The test results are shown in Table 1. Comparative Example 2 magnet has an intrinsic coercivity of 23.51 kOe, a remanence of 14.26 kGs, a maximum energy product of 50.73 MGOe, and a corner-missing ratio of 1.3%. During the coating stage of the diffusion treatment, i.e. after high-temperature heat treatment at 900℃, severe adhesion occurred between magnets and between magnets and diffusion sources or stirring balls due to sintering, melting, or chemical reactions.

[0034] High-temperature irreversible magnetic flux loss tests were conducted on the magnet of Comparative Example 2. The heating temperature was 120℃, and the holding time was 2 hours, under semi-open circuit conditions. The test results are shown in Table 2. The maximum value of the high-temperature magnetic flux irreversible loss test results in Comparative Example 2 was 1.07%, the minimum value was 0.18%, the average value was 0.59%, and the standard deviation was 0.25.

[0035] Table 1. Performance Comparison of N52 Neodymium Iron Boron Permanent Magnets After Treatment Under Different Conditions Table 2 Comparison of irreversible high-temperature magnetic flux loss results of N52 neodymium iron boron permanent magnet materials after treatment under different conditions In Example 1, the orientation surface of the NdFeB magnet was roughened by sandblasting to form pits with a depth of 3 μm. Subsequent double-sided grinding reduced the surface roughness Ra to 0.15 ± 0.02 μm. This combined process significantly improved the uniformity and activity of the magnet surface, providing a more effective carrier for subsequent heavy rare earth diffusion. Test results showed that Example 1 had a heavy rare earth content of 0.35% and an intrinsic coercivity of 20.03 kOe, a significant improvement over the original sample A0's 13.8 kOe. Furthermore, the average high-temperature irreversible flux loss was only 3.50%, with a standard deviation of 0.21, demonstrating highly consistent performance. In contrast, Comparative Example 1, which only removed oil, had a heavy rare earth content of only 0.29%, resulting in a decrease in intrinsic coercivity to 18.98 kOe, an increase in the average high-temperature irreversible flux loss to 3.86%, and a larger standard deviation of 0.4. The data above indicate that insufficient surface treatment not only reduces the adhesion of heavy rare earth elements but also leads to uneven diffusion, thereby reducing the overall performance and consistency of the magnet. This comparison highlights the crucial role of sandblasting roughening treatment: it not only enhances the adhesion of the diffusion layer but also optimizes the penetration path of rare earth elements, thus improving intrinsic coercivity and thermal stability.

[0036] In Example 2, the parameters of the spin coating step were adjusted: the mass ratio of the magnet, heavy rare earth diffusion source, and mixing balls was changed to 2:3:1; alumina mixing balls with a particle size of 4 mm and Tb ball diffusion sources with a diameter of 10 mm were used; and the vacuum degree was adjusted to 4.0 × 10⁻⁶. -3The magnet was heated to 700℃ for 8 hours at a rotation speed of 2 r / min. Subsequent high-temperature diffusion and aging conditions were changed to 900℃×12h and 500℃×4h. These changes resulted in significant performance improvements: the heavy rare earth element RH content was 0.38%, the intrinsic coercivity further increased to 23.7 kOe, and the average irreversible high-temperature flux loss was significantly reduced to 0.53% with a standard deviation of 0.26. This indicates that the Tb diffusion source has a higher intrinsic coercivity improvement than the Dy diffusion source, and the optimized temperature and time parameters promote a more uniform rare earth element distribution, thereby greatly enhancing the thermal stability and consistency of the magnet. Comparative Example 2 omitted the separation step compared to Example 2, directly performing high-temperature diffusion without removing the diffusion source and stirring ball. This resulted in severe adhesion between the magnet and the diffusion source or stirring ball. Although the intrinsic coercivity slightly decreased to 23.51 kOe and the average irreversible loss of high-temperature magnetic flux slightly increased to 0.59%, the adhesion problem highlighted the necessity of the separation step: it avoided the adhesion problem caused by direct contact between the heavy rare earth diffusion source and the magnet at high temperatures, ensuring the yield and performance consistency of the magnet. This comparison emphasizes the importance of the separation step in the process; it is not only an operational improvement but, more importantly, prevents adverse reactions, thereby maintaining the high performance and high consistency of the magnet.

[0037] Example 3 A method for preparing a highly uniform NdFeB magnet includes the following steps: S1. Sandblasting: Pretreatment of neodymium iron boron magnets. Commercial neodymium iron boron magnets are cut into strips of 10×3×2mm, with the 2mm direction as the orientation direction. They are unmagnetized and have the grade 45M. After removing oil, the orientation surface of the neodymium iron boron magnets is sandblasted to form pits with a depth of 4μm on the surface, increasing the surface roughness of the orientation surface. Among them, neodymium iron boron magnets are R-Fe-B type rare earth sintered magnets with R2Fe14B type compound grains as the main phase; S2. Double-sided grinding treatment: The NdFeB magnets that have been sandblasted are subjected to double-sided grinding treatment to reduce the surface roughness Ra of the orientation surface to 0.2±0.02μm and the depth of the pits to 1.5±0.2μm; S3. Cleaning and drying: Clean the NdFeB magnet with anhydrous ethanol and dry it at 90℃ for 30 minutes. Mark it as sample B0. S4. Rotary Coating: The neodymium iron boron magnet treated in step S3, the heavy rare earth diffusion source, and the stirring ball are mixed in a mass ratio of 3:3:1 and placed in a rotary heat treatment furnace. Preferably, the stirring ball is silicon oxide with a diameter of 5 mm, and the heavy rare earth diffusion source is a Dy cylinder with a diameter of 10 mm and a height of 7 mm, labeled as Dy diffusion source. After evacuating to 3.0 × 10-3 Pa, the furnace is rotated and held at 680 °C for 10 h. The rotation speed of the rotary heat treatment furnace is 1.5 r / min, so that the heavy rare earth diffusion source forms a coating on the surface of the magnet. S5. High-temperature diffusion and aging: After cooling to room temperature, the coated magnet is separated from the heavy rare earth diffusion source and the stirring ball. The coated magnet is then subjected to a vacuum degree better than 6×10⁻⁶. -3 High-temperature diffusion treatment was carried out for 14 hours under conditions of Pa and temperature of 900℃. The heating rate was 10℃ / min before reaching 600℃, and the heating rate was reduced to 6℃ / min after reaching 600℃. Subsequently, the treatment was carried out under a vacuum degree better than 3×10 -2 The aging treatment was carried out at 480°C for 5 hours.

[0038] The magnets in Example 3 underwent visual inspection, and the magnetic properties of the blank sample A0 and the magnets in Example 1 were tested using a permanent magnet material measurement system according to the requirements of GB / T3217-2013 "Permanent Magnet (Hard Magnet) Materials - Magnetic Test Methods". The test results are shown in Table 3. The intrinsic coercivity of the original sample B0 magnet is 15.03 kOe, the remanence of the magnet is 13.62 kGs, and the maximum energy product of the magnet is 45.88 MGOe; In Example 3, the intrinsic coercivity of the magnet is 22.65 kOe, the remanence of the magnet is 13.52 kGs, the maximum magnetic energy product of the magnet is 45.53 MGOe, and the magnet has a corner missing ratio of 1.3%. During the coating stage of the diffusion treatment, i.e., after relatively low temperature heat treatment at 600-750℃, no substantial connection due to sintering, melting or chemical reaction occurred between magnets or between the magnet and the diffusion source or stirring ball.

[0039] Among them, the percentage of magnets with missing corners % = number of magnets with missing corners / total number of magnets.

[0040] Comparative Example 3 S5. High-temperature diffusion and aging: After cooling to room temperature, the coated magnet is separated from the heavy rare earth diffusion source and the stirring ball. The coated magnet is then subjected to a vacuum degree better than 6×10⁻⁶. -3 High-temperature diffusion treatment was carried out for 14 hours under the conditions of Pa, 900℃, and a rotation speed of 1.5 r / min. The heating rate was 10℃ / min before reaching 600℃, and the heating rate was reduced to 6℃ / min after reaching 600℃. Subsequently, the treatment was carried out under a vacuum degree better than 3×10 -2The aging treatment was carried out at 480°C for 5 hours.

[0041] The other parameters and steps are the same as in Example 3, so they will not be repeated here.

[0042] The magnets in Comparative Example 3 underwent visual inspection, and the magnetic properties of blank sample B0 and the magnets in Example 1 were tested using a permanent magnet material measurement system according to the requirements of GB / T3217-2013 "Permanent Magnet (Hard Magnet) Materials - Magnetic Test Methods". The test results are shown in Table 3. The intrinsic coercivity of the three magnets in the comparative example is 22.72 kOe, the remanence of the magnet is 13.50 kGs, the maximum magnetic energy product of the magnet is 45.48 MGOe, and the proportion of missing corners of the magnet is 8.1%. During the coating stage of the diffusion treatment, that is, after the relatively low temperature heat treatment at 600-750℃, no substantial connection due to sintering, melting or chemical reaction occurred between the magnets or between the magnet and the diffusion source or stirring ball.

[0043] Example 4 In step S1, a pit with a depth of 5 μm is formed on its surface; In step S2, the surface roughness Ra of the orientation surface is reduced to 0.4±0.02μm, and the depth of the pit is reduced to 3±0.2μm; In step S4, the neodymium iron boron magnets treated in step S3, the heavy rare earth diffusion source, and the stirring balls are mixed in a mass ratio of 3:5:1 and placed in a rotary heat treatment furnace. Preferably, the stirring balls are silicon nitride with a diameter of 6 mm, and the heavy rare earth diffusion source is Tb shavings with a diameter of 12 mm and a height of 6 mm, labeled as Tb diffusion source. The furnace is evacuated to a vacuum level better than 2.0 × 10⁻⁶. -3 After Pa, the magnet is rotated and held at 660℃ for 12 hours. The rotation speed of the rotary heat treatment furnace is 1 r / min, so that the heavy rare earth diffusion source forms a coating on the magnet surface. After cooling to room temperature in step S5, the coated magnet is separated from the heavy rare earth diffusion source and the stirring ball. The coated magnet is then subjected to a vacuum level better than 5 × 10⁻⁶. -3 High-temperature diffusion treatment was carried out at 850℃ and Pa for 16 hours. The heating rate was 11℃ / min before reaching 600℃, and the heating rate decreased to 7℃ / min after reaching 600℃. Subsequently, the treatment was carried out under a vacuum of 2×10⁻⁶ Pa. -2 The aging treatment was carried out at 460℃ for 5 hours.

[0044] The magnet in Example 4 underwent a visual inspection, and the magnetic properties of the magnet in Example 1 were tested using a permanent magnet material measurement system according to the requirements of GB / T3217-2013 "Permanent Magnet (Hard Magnet) Materials - Magnetic Test Methods". The test results are shown in Table 3. In Example 4, the intrinsic coercivity of the magnet is 24.54 kOe, the remanence of the magnet is 13.52 kGs, the maximum magnetic energy product of the magnet is 45.64 MGOe, and the magnet has a corner missing ratio of 1.5%. During the coating stage of the diffusion treatment, i.e., after relatively low temperature heat treatment at 600-750℃, no substantial connection due to sintering, melting or chemical reaction occurred between magnets or between the magnet and the diffusion source or stirring ball.

[0045] The other parameters and steps are the same as in Example 3, so they will not be repeated here.

[0046] Comparative Example 4 In step S5, high-temperature diffusion and aging: After cooling to room temperature, the coated magnet is separated from the heavy rare earth diffusion source and the stirring ball. The coated magnet is then subjected to a vacuum degree better than 5×10⁻⁶. -3 High-temperature diffusion treatment was carried out for 16 hours at 660℃ and Pa. The heating rate was 11℃ / min before reaching 600℃, and the heating rate decreased to 7℃ / min after reaching 600℃. Subsequently, the diffusion was carried out under a vacuum degree better than 2×10⁻⁶. -2 The aging treatment was carried out at 460℃ for 5 hours.

[0047] The other parameters and steps are the same as in Example 4, so they will not be repeated here.

[0048] The magnets in Comparative Example 4 underwent visual inspection, and the magnetic properties of the magnets in Example 1 were tested using a permanent magnet material measurement system according to the requirements of GB / T3217-2013 "Permanent Magnet (Hard Magnet) Materials - Magnetic Test Methods". The test results are shown in Table 3. The intrinsic coercivity of the four magnets was 20.83 kOe, the remanence was 13.47 kGs, the maximum energy product was 45.36 MGOe, and the corner loss rate was 1.4%. During the coating stage of the diffusion treatment, i.e., after relatively low temperature heat treatment at 600-750℃, no substantial connection was formed between the magnets or between the magnets and the diffusion source or stirring ball due to sintering, melting or chemical reaction.

[0049] Table 3. Performance Comparison of 45M NdFeB Permanent Magnet Materials After Treatment Under Different Conditions Example 3 used a 10×3×2mm strip-shaped sample of grade 45M magnet. The initial magnetic properties of the original sample B0 were lower than those of the N52 magnet in Example 1. The surface roughness Ra was reduced to 0.2±0.02μm by sandblasting to create pits combined with double-sided grinding, providing an optimized surface morphology for subsequent diffusion. In the spin coating stage, a higher magnet / diffusion source / stirring ball mass ratio of 3:3:1 and a better vacuum level of 3.0×10⁻⁶ were used. -3The conditions of Pa, a relatively low holding temperature of 680℃, and a slow rotation speed of 1.5 r / min facilitated the formation of a uniform and well-adhered heavy rare earth element (Dy) coating on the magnet surface, while preventing adhesion between magnets, resulting in a corner-missing ratio of only 1.3%. Subsequent high-temperature diffusion at 900℃ for 14 hours provided sufficient energy and time for the heavy rare earth elements to fully penetrate into the magnet, ultimately significantly increasing the intrinsic coercivity from 15.03 kOe to 22.65 kOe, demonstrating excellent diffusion performance.

[0050] Comparative Example 3 introduced an additional rotation speed of 1.5 r / min during the high-temperature diffusion stage (S5). This single change caused the magnet chipping rate to deteriorate sharply from the excellent 1.3% in Example 3 to 8.1%, while the magnetic properties remained essentially unchanged. The experimental results of Comparative Example 3 demonstrate that rotation during the 900°C high-temperature diffusion stage is a harmful and non-obvious operation. The decreased mechanical strength of the magnet at high temperatures, coupled with continuous rotational collisions with the stirring balls, directly leads to physical damage, which is the direct cause of the significant increase in the chipping rate. This comparison highlights the importance of stopping rotation during the diffusion and aging processes. It can be seen that strictly limiting rotation to the low-temperature coating stage (S4) to achieve uniform adhesion of the heavy rare earth source, while avoiding this operation during the high-temperature diffusion stage (S5) to prevent mechanical damage, thus minimizing mechanical damage while ensuring excellent magnetic properties, i.e., high intrinsic coercivity, and further improving the yield.

[0051] Example 4 further optimized the diffusion process by using Tb shavings as the diffusion source, whose larger specific surface area may have improved diffusion efficiency. The spin coating stage employed a higher diffusion source ratio (mass ratio of 3:5:1) and a superior vacuum level of 2.0 × 10⁻⁶. -3 Pa and a lower processing temperature of 660°C and a rotation speed of 1 r / min. This combination of conditions was designed to achieve a finer and more controllable coating process. High-temperature diffusion was then carried out at 850°C for 16 hours. This temperature is lower than the 900°C in Example 3, but the longer processing time may have compensated for the lack of driving force, ultimately achieving a higher intrinsic coercivity of 24.54 kOe, while the corner loss ratio of 1.5% was also kept at a low level.

[0052] In Comparative Example 4, the high-temperature diffusion temperature in Example 4 was reduced from 850°C to 660°C, while other parameters remained unchanged. The intrinsic coercivity decreased significantly to 20.83 kOe. This demonstrates that a temperature of 660°C is insufficient to provide enough energy to drive the effective diffusion of the heavy rare earth element Tb into the depths of the magnet grains, only forming a shallow concentration gradient and failing to significantly improve the overall intrinsic coercivity. This comparison strongly demonstrates the necessity of a specific temperature range, such as 850°C, in the high-temperature diffusion step; this parameter is a decisive factor in forming effective grain boundary diffusion and thus obtaining high intrinsic coercivity.

[0053] Depend on Figure 5 It can be seen that, compared with traditional coated magnets, spin-coated magnets exhibit a heavy rare earth element (RH) enrichment zone between the pre-diffusion layer and the magnet core. This is because, during the spin-coating process, heavy rare earth elements adhere to the magnet surface in gaseous form, forming a pre-diffusion layer; simultaneously, the heavy rare earth elements already attached to the magnet surface penetrate inward, widening the diffusion channels and forming the RH enrichment zone. After high-temperature diffusion and aging, the heavy rare earth elements in traditional coated magnets exhibit a "high-low" distribution with a large concentration range; while the heavy rare earth elements in spin-coated magnets exhibit a "slightly high-medium-slightly low" gradient distribution with a more gradual concentration range and higher performance consistency.

[0054] The above description only illustrates the preferred embodiments of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All modifications made within the scope of the independent claims of this invention are also within the scope of protection of this invention.

Claims

1. A method for preparing a highly consistent NdFeB magnet, characterized in that, Includes the following steps: S1. Sandblasting treatment: The orientation surface of the NdFeB magnet is sandblasted to form pits with a depth of 3 to 10 μm on its surface, thereby increasing the surface roughness of the orientation surface. Then, the NdFeB magnet is cleaned with anhydrous ethanol and dried. S2. Double-sided grinding treatment: The NdFeB magnet that has been sandblasted is subjected to double-sided grinding treatment to reduce the surface roughness Ra of the orientation surface to 0.05-0.6μm and the depth of the pit to 0.3-4μm; S3. Cleaning and drying: Clean the NdFeB magnet again with anhydrous ethanol and then dry it. S4. Rotary coating: The neodymium iron boron magnets treated in step S3, the heavy rare earth diffusion source, and the stirring ball are mixed in a mass ratio of 1:(1~10):(0.1~2) and placed in a rotary heat treatment furnace. After vacuuming, the mixture is rotated and kept at 600~750℃ for 6~30h to form a coating on the surface of the magnets by the heavy rare earth diffusion source. S5. High-temperature diffusion and aging: After cooling to room temperature, the coated magnet is separated from the heavy rare earth diffusion source and stirring ball. The coated magnet is then subjected to a vacuum degree better than 1.0 × 10⁻⁶. -2 High-temperature diffusion treatment was carried out at Pa and temperature of 800–950℃ for 4–10 hours, followed by a vacuum degree better than 5.0 × 10⁻⁶. -2 Aging treatment was carried out for 3 to 5 hours under conditions of Pa and temperature of 450 to 600℃.

2. The method for preparing a highly uniform NdFeB magnet according to claim 1, characterized in that, In step S4, the rotational speed of the rotary heat treatment furnace is 0.1–10 r / min, and the vacuum level is better than 5.0 × 10⁻⁶. -3 Pa.

3. The method for preparing a highly uniform NdFeB magnet according to claim 1, characterized in that, In step S4, the heavy rare earth diffusion source is at least one of metal Dy, metal Tb, Dy alloy or Tb alloy, and its form is blocky or granular with a size of less than 15 mm.

4. The method for preparing a highly uniform NdFeB magnet according to claim 1, characterized in that, In step S4, the mixing balls are at least one of alumina balls, zirconium oxide balls, silicon oxide balls, or silicon nitride balls, and their particle size is 2 to 10 mm.

5. The method for preparing a highly uniform NdFeB magnet according to claim 1, characterized in that, During the high-temperature diffusion treatment in step S5, the heating rate is 8-12℃ / min before the temperature reaches 600℃, and the heating rate drops to 3-7℃ / min after the temperature reaches 600℃.

6. The method for preparing a highly uniform NdFeB magnet according to claim 1, characterized in that, In step S3, the drying temperature is 70–120°C and the drying time is 10–60 min.

7. A highly uniform NdFeB magnet, comprising the method for preparing a highly uniform NdFeB magnet as described in any one of claims 1-5, characterized in that, The intermediate body of the magnet has a gradient structure in which the intrinsic coercivity gradually decreases from the surface to the center, and the RH concentration of heavy rare earth elements on the surface is 2 to 10 times that of the RH concentration in the center. The total amount of heavy rare earth elements in the 100μm range of the surface of the intermediate body is 1.2 to 3 times that of the total amount of heavy rare earth elements in the center of the magnet.

8. A highly uniform NdFeB magnet according to claim 7, characterized in that, After the intermediate is subjected to high-temperature diffusion and aging treatment, the concentration of heavy rare earth element RH in the magnet is uniformly distributed from a depth of 100μm from the surface to the center.

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