NdFeB magnet with high magnetic energy product and preparation method thereof

Abstract

The invention belongs to the technical field of rare earth permanent magnet materials, and particularly relates to a high-magnetic-energy-product neodymium iron boron magnet and a preparation method thereof. Through cooperation of core-shell coating, microwave sintering and two-stage magnetic field heat treatment processes, the coercive force and residual magnetism of the magnet are remarkably improved, energy-saving and efficient production is achieved, the product consistency is good, and the method has outstanding industrialization value.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnet materials technology, specifically relating to a high magnetic energy product neodymium iron boron magnet and its preparation method. Background Technology

[0002] Since its inception, neodymium iron boron (Nd-Fe-B) permanent magnets have been hailed as the "King of Magnets" due to their highest magnetic energy product ((BH)max) to date, and have become a core material in high-tech fields such as new energy vehicle drive motors, wind turbine generators, precision instruments, consumer electronics, and aerospace. Downstream applications are placing higher demands on motor efficiency, power density, and equipment miniaturization, which directly translates into more stringent requirements for the comprehensive performance of Nd-Fe-B magnets, especially the simultaneous possession of high remanence (Br) and high intrinsic coercivity (Hcj). High remanence means the magnet can provide stronger magnetic flux, while high coercivity ensures the stability of the magnet's magnetic properties under harsh working environments such as high temperatures and reverse magnetic fields, preventing demagnetization. Therefore, developing magnets with both high remanence and high coercivity is a major direction for current industrial technology research.

[0003] However, the remanence and coercivity of NdFeB magnets are mutually restrictive to some extent. The most effective traditional method to improve coercivity is to add heavy rare earth elements (such as dysprosium (Dy) and terbium (Tb) to the alloy. These elements tend to accumulate at grain boundaries, forming a (Nd,Dy)₂Fe₁₄B shell with a high magnetocrystalline anisotropy (HA) field, thus significantly enhancing the pinning ability of grain boundaries to antimagnetic domains. However, the addition of heavy rare earth elements dilutes the volume fraction of the main phase (Nd₂Fe₁₄B) and may form a non-magnetic phase, leading to a significant decrease in remanence and energy product. More seriously, heavy rare earth resources are unevenly distributed globally, scarce in reserves, expensive, and highly volatile in price. Over-reliance on heavy rare earths not only significantly increases magnet costs but also poses challenges to supply chain security. Therefore, how to achieve a fundamental improvement in magnet coercivity while using as little or no heavy rare earth as possible is a core technical challenge that the industry urgently needs to solve.

[0004] To address this challenge, the industry has developed technologies such as grain boundary diffusion and dual alloying. Dual alloying, which involves mixing rare-earth-rich auxiliary alloys with the main phase alloy to optimize grain boundary composition and structure, is currently the mainstream approach to improving coercivity. However, traditional dual alloying processes typically use mechanical mixing to combine the two alloy powders. This method struggles to achieve a uniform and continuous distribution of auxiliary alloying elements in the grain boundary region, easily leading to component segregation and localized enrichment. On one hand, this limits the full utilization of heavy rare-earth elements, resulting in limited improvement in coercivity; on the other hand, the uneven distribution exacerbates the damage to the main phase magnetic phase, causing unnecessary loss of remanence. Simultaneously, traditional vacuum sintering processes require prolonged (usually exceeding 200 minutes) maintenance at high temperatures (typically above 1040°C), resulting in high energy consumption, long production cycles, and the potential for abnormal grain growth and excessive element diffusion during prolonged high-temperature processes. This hinders the acquisition of fine, uniform microstructures, thus affecting the uniformity and stability of the magnet's final performance. In addition, conventional heat treatment processes often lack the means to actively control the microscopic magnetic structure of magnets, and fail to further optimize the grain boundary phase and magnetic domain structure formed after sintering, thus limiting the full release of the magnet's potential.

[0005] Therefore, developing an innovative preparation method can fundamentally improve the distribution of rare earth-rich phases in magnets, achieve energy-saving and efficient processes, and precisely control the final microstructure and magnetic structure. This will synergistically enhance the remanence and coercivity of NdFeB magnets with low heavy rare earth addition, which has significant scientific and industrial value for promoting the low-cost and high-performance development of high-end NdFeB magnets. Summary of the Invention

[0006] The purpose of this invention is to provide a high-energy-product NdFeB magnet and its preparation method. This invention significantly improves the coercivity and remanence of the magnet through the synergistic process of core-shell coating, microwave sintering and two-stage magnetic field heat treatment, and achieves energy-saving and efficient production with good product consistency.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A method for preparing a high-energy-product neodymium iron boron magnet includes the following steps: (1) Ingredients: The main phase alloy 1 and rare earth-rich auxiliary alloy 2 are prepared. The main phase alloy 1 has the following mass percentage composition: Nd 27.5~29.8%, Fe 68.45~70.85%, B 0.95%, Ag 0.10~0.35%, Mo 0.10~0.30%, Zn 0.15~0.40%; the rare earth-rich auxiliary alloy 2 contains Nd, Ho, Dy, Ga, Cu, Zr, Al, Hf, Fe and B; (2) Alloy preparation: The main phase alloy 1 raw material prepared in step (1) is smelted and rapidly solidified into a strip to obtain an alloy strip with a thickness of 0.2-0.35 mm; The rare earth-rich auxiliary alloy 2 raw material prepared in step (1) is smelted and cast into alloy ingots, and then the alloy ingots are subjected to homogenization heat treatment and surface processing to obtain target alloy 2. (3) Hydrogen crushing: The alloy strip obtained in step (2) is subjected to hydrogen absorption-dehydrogenation treatment to obtain hydrogen crushed alloy 1 micro powder; (4) Air jet milling: The hydrogen-crushed alloy 1 micro powder obtained in step (3) is subjected to air jet milling under a protective atmosphere to obtain alloy 1 fine powder with an average particle size SMD of 1.45~2.51μm; (5) Core-shell coated powder preparation: The target alloy 2 obtained in step (2) is sputtered and coated on the surface of the fine powder of alloy 1 obtained in step (4) by physical vapor deposition to form a core-shell structure powder. The weight of the rare earth-rich auxiliary alloy 2 is controlled to be 0.5% to 3.0% of the total weight. (6) Magnetic field forming: Under a protective atmosphere with an oxygen content of less than 10 ppm, the core-shell structure powder obtained in step (5) is magnetically oriented and pressed to form a compact. (7) Sintering: The compact obtained in step (6) is microwave sintered under vacuum or protective atmosphere; (8) Heat treatment: The magnet sintered in step (7) is subjected to two-stage magnetic field heat treatment. The first stage heat treatment temperature is 870~920℃, held for 1~4h, and a magnetic field of 1.5~2.5T is applied. After air cooling, the second stage heat treatment is performed at a temperature of 490~520℃, held for 2~6h, and a magnetic field of 1.5~2.0T is applied to finally obtain the high magnetic energy product NdFeB magnet.

[0008] Furthermore, in step (2), the temperature of the rapid solidification belt spinning is 1470~1480℃; the temperature of the homogenization heat treatment is 600~1000℃, and the time is 6~12h.

[0009] Furthermore, in step (3), the hydrogen absorption process is carried out at ≤100℃, using hydrogen gas with a purity of 99.99%; the temperature of the dehydrogenation process is 560~580℃, and the dehydrogenation ends when the vacuum degree reaches below 20Pa.

[0010] Furthermore, in step (4), the protective atmosphere is argon, and the oxygen content is controlled below 5 ppm during the air jet milling process.

[0011] Furthermore, in step (5), the physical vapor deposition method is sputtering performed by a small powder PVD coating system.

[0012] Further, in step (6), the orientation magnetic field strength for magnetic field forming is 1.90~2.0T, and the density of the pressed blank is 4.3±0.1 g / cm³. 3 .

[0013] Furthermore, in step (7), the vacuum degree of microwave sintering is 1.0E-2 Pa, the sintering temperature is 900~1000℃, the holding time is 30~50min, and the microwave frequency is 1.50~2.5 Kw.

[0014] Further, in step (1), the mass percentage composition of the rare earth-rich auxiliary alloy 2 is as follows: Nd 20~39%, Ho 8~15%, Dy 5~15%, Ga 2~8%, Cu 5~15%, Zr 3~10%, Al 3.5~12%, Hf 2~6%, Fe 11.8~16.5%, with the balance being B.

[0015] A high-energy-product neodymium iron boron magnet is prepared by the above-described preparation method.

[0016] Compared with the prior art, the present invention has the following advantages: This invention achieves significant comprehensive benefits in the preparation of NdFeB magnets through an innovative process combination of "core-shell coated powder design - microwave rapid sintering - two-stage magnetic field heat treatment".

[0017] (1) Magnetic properties are significantly improved A core-shell structure was constructed using PVD sputtering, achieving a nanoscale uniform distribution of rare-earth-rich elements at the grain boundaries, which greatly enhanced the grain boundary pinning effect. With the same amount of auxiliary alloying, the intrinsic coercivity (Hcj) of the magnet was generally 2-5 kOe higher than that of traditional processes. For example, in Example 1 (3.0% addition), the coercivity reached 22.21 kOe, a 20.7% improvement over traditional processes, while retaining better remanence (Br), achieving a synergistic improvement in both high coercivity and high remanence.

[0018] (2) High efficiency and energy saving of process By replacing traditional vacuum sintering with microwave sintering, the sintering temperature is reduced from 1040–1070℃ to 900–1000℃, and the time is shortened from 240 minutes to 30–50 minutes. This process significantly reduces energy consumption, shortens the production cycle, and improves equipment utilization and production economy while suppressing grain growth.

[0019] (3) Enhanced product consistency and ingredient adaptability Core-shell coating ensures compositional uniformity from the outset, and combined with two-stage magnetic field heat treatment for directional control of grain boundary and magnetic domain structure, it significantly improves batch stability of magnet performance. This process system is applicable to various main phases and auxiliary alloy compositions, providing a reliable platform for flexible design and optimization of material properties.

[0020] In summary, this invention improves the overall performance of magnets while achieving energy saving, high efficiency and controllability of the process, and has outstanding industrialization value. Attached Figure Description

[0021] Figure 1 Particle size distribution diagram of the main phase alloy airflow mill powder in Example 1; Figure 2 SEM image of the main phase alloy air-jet milled powder in Example 1 at 2000x magnification; Figure 3 SEM image of the sintered magnet after PVD coating with 3.0% rare earth-rich auxiliary alloy in Example 1; Figure 4 Particle size distribution diagram of the main phase alloy airflow mill powder in Example 2; Figure 5 SEM image of the main phase alloy air-jet milled powder in Example 2 at 1200x magnification; Figure 6 SEM image of the sintered magnet coated with 2.4% auxiliary alloy in Example 2; Figure 7 Particle size distribution diagram of the main phase alloy airflow mill powder in Example 3; Figure 8 SEM image of the main phase alloy air-jet milled powder in Example 3 at 2000x magnification; Figure 9 SEM image of the sintered magnet coated with 2.1% auxiliary alloy in Example 3. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 A high-energy-product neodymium iron boron magnet and its preparation method, the preparation method comprising the following steps: (1) Batching: Weigh the main phase alloy 1 (composition by mass percentage: Nd 27.5%, Fe 70.85%, Ag 0.35%, Mo 0.20%, Zn 0.15%, B 0.95%) and rare earth-rich auxiliary alloy 2 (composition by mass percentage: Nd 39%, Ho 12%, Dy 10%, Ga 4.2%, Cu 5.2%, Zr 3%, Al 9.8%, Hf 4%, Fe 11.8%, B balance) according to the design ratio; (2) Alloy preparation: The main phase alloy 1 raw material was melted in a fully sealed vacuum rapid solidification strip casting furnace and cast at 1470–1480℃ to obtain alloy strip sheets with a thickness of 0.2–0.35 mm and a content of 96–97%. The rare earth-rich auxiliary alloy 2 raw material was melted uniformly in a vacuum melting furnace under argon protection, cast into ingots, and then placed in a vacuum heat treatment furnace for homogenization at 600℃ for 12 h, followed by air cooling; then the alloy billet was surface polished and shaped to obtain target alloy 2.

[0024] (3) Hydrogen crushing: The strip is loaded into the fully sealed hydrogen explosion furnace reactor, and after vacuuming, 99.99% high-purity hydrogen is introduced. Saturated hydrogen absorption is carried out below 100℃. After hydrogen absorption is completed, the temperature is raised to 570℃ to dehydrogenate to a vacuum degree ≤20 Pa, and then water-cooled to obtain hydrogen crushed alloy 1 micro powder. (4) Gas flow milling: The hydrogen-rich powder is placed in an argon gas flow mill for powdering. The oxygen content is ≤5ppm throughout the process. The operation is carried out under argon protection to obtain alloy 1 fine powder with an average particle size SMD=1.45 μm. (5) Preparation of core-shell coated powder: A small-scale powder PVD coating system was used to sputter and coat the rare earth-rich auxiliary alloy target onto the surface of alloy 1 fine powder to form a core-shell structure. The amount of auxiliary alloy added was controlled to be 1%, 1.5%, 2.0%, 2.5%, and 3.0% of the total mass, respectively. (6) Magnetic field forming: The coated powder is placed in a fully sealed magnetic field forming press with an oxygen content of <10 ppm and oriented under a magnetic field of 1.92T. The density of the pressed blank is 4.3±0.1 g / cm³. 3 ; (7) Sintering: The green blank is placed in a high-temperature microwave sintering furnace under nitrogen protection and the vacuum is drawn to 1.0×10 -2 Pa, heat to 900℃ and hold for 40 min, microwave frequency 2.2 kW; after holding, air cool to below 50℃ under argon protection and then remove from the furnace; (8) Heat treatment: The sintered magnet is heated to 870℃ under a magnetic field of 1.5 T and held for 1 h, then air-cooled to below 70℃; then heated to 490℃ under a magnetic field of 2.0 T and held for 2 h, and then air-cooled to below 60℃ under argon protection to obtain sintered NdFeB magnet.

[0025] Figure 1 This is a particle size distribution diagram of the main phase alloy air-jet milled powder in Example 1, showing that the average particle size SMD of the powder is 1.45 μm, and the particle size distribution is concentrated, which is beneficial for subsequent coating and molding.

[0026] Figure 2 The image shows the SEM image of the main phase alloy air-jet milled powder in Example 1 at a magnification of 2000, indicating that the powder has a regular morphology and a clean surface, making it suitable for PVD coating.

[0027] Figure 3 The image shows the SEM image of the sintered magnet after PVD coating with 3.0% rare earth-rich auxiliary alloy in Example 1. It shows that the core-shell structure is intact and the auxiliary alloy is uniformly coated on the surface of the main phase particles.

[0028] Example 2 A high-energy-product neodymium iron boron magnet and its preparation method, the preparation method comprising the following steps: (1) Ingredients: Main phase alloy 1 composition (mass percentage): Nd 28.7%, Fe 69.75%, Ag 0.20%, Mo 0.10%, Zn 0.30%, B 0.95%; Rare earth rich auxiliary alloy 2 composition (mass percentage): Nd 30%, Ho 8%, Dy 15%, Ga 8%, Cu 10%, Zr 6%, Al 3.5%, Hf 2%, Fe 16.5%, B balance; (2) Alloy preparation: The main phase alloy 1 was rapidly solidified in a vacuum and spun into a strip to obtain a spun strip with a thickness of 0.2–0.35 mm and a content of 96–97%. Auxiliary alloy 2 is melted and cast into ingots, homogenized at 800℃ for 8 hours and then air-cooled, and then surface-processed to obtain target alloy 2; (3) Hydrogen crushing: After the strip absorbs hydrogen at a temperature below 100°C, it is dehydrogenated at 560°C to a vacuum degree ≤20 Pa, and then cooled with water to obtain hydrogen crushed alloy 1 micro powder; (4) Air jet milling: Powdering is carried out under argon protection, with an oxygen content ≤5 ppm, to obtain alloy 1 fine powder with an average particle size SMD=2.05 μm; (5) Preparation of core-shell coated powder: The auxiliary alloy target was coated onto the surface of alloy 1 fine powder by PVD sputtering, and the amount of auxiliary alloy added was controlled to be 0.8%, 1.2%, 1.6%, 2.0%, and 2.4% of the total mass; (6) Magnetic field forming: Orientation forming is performed under an oxygen content of <10 ppm and a magnetic field strength of 1.90 T, with a compact density of 4.3±0.1 g / cm³. 3 .

[0029] (7) Sintering: In a high-temperature microwave sintering furnace, a vacuum of 1.0 × 10⁻⁶ is drawn. -2 Pa, heat at 950℃ for 50 min, microwave frequency 2.5 kW, then air-cooled under argon protection to below 50℃; (8) Heat treatment: First, keep at 890℃ for 2 h under a 2.0 T magnetic field and air cool to below 70℃; then keep at 500℃ for 3 h under a 1.5 T magnetic field and air cool to below 60℃ under argon protection to obtain sintered NdFeB magnets.

[0030] Figure 4 This is a particle size distribution diagram of the main phase alloy airflow milled powder in Example 2, showing that the average particle size SMD is 2.05 μm, indicating good particle size control.

[0031] Figure 5 The image shows the SEM image of the main phase alloy airflow milled powder in Example 2 at a magnification of 1200, which shows that the particles have a uniform shape and no agglomeration.

[0032] Figure 6 The image shows a SEM image of the sintered magnet coated with 2.4% auxiliary alloy in Example 2, which shows that the auxiliary alloy layer is continuous and tightly bonded to the main body.

[0033] Example 3 A high-energy-product neodymium iron boron magnet and its preparation method, the preparation method comprising the following steps: (1) Ingredients: Main phase alloy 1 composition (mass percentage): Nd 29.8%, Fe 68.45%, Ag 0.10%, Mo 0.30%, Zn 0.40%, B 0.95%; Rare earth rich auxiliary alloy 2 composition (mass percentage): Nd 20%, Ho 15%, Dy 5%, Ga 2%, Cu 15%, Zr 10%, Al 12%, Hf 6%, Fe 14%, B balance; (2) Alloy preparation: The main phase alloy 1 was spun to obtain spun sheets with a thickness of 0.2–0.35 mm and a content of 96–97%. Auxiliary alloy 2 is melted into ingots, homogenized at 1000℃ for 6 hours, and then air-cooled to process into target alloy 2; (3) Hydrogen crushing: After the strip absorbs hydrogen, it is dehydrogenated at 580℃ to a vacuum degree ≤20 Pa, and then cooled with water to obtain hydrogen crushed alloy 1 micro powder; (4) Air jet milling: Powdering is carried out under argon protection, with an oxygen content ≤5 ppm, to obtain alloy 1 fine powder with an average particle size SMD=2.51 μm; (5) Preparation of core-shell coated powder: The auxiliary alloy target was coated onto the surface of alloy 1 fine powder by PVD sputtering, and the amount of auxiliary alloy added was controlled to be 0.5%, 0.9%, 1.3%, 1.7%, and 2.1% of the total mass; (6) Magnetic field forming: Orientation forming is performed under an oxygen content of <10 ppm and a magnetic field strength of 2.0 T, with a compact density of 4.3±0.1 g / cm³. 3 .

[0034] (7) Sintering: In a high-temperature microwave sintering furnace, a vacuum of 1.0 × 10⁻⁶ is drawn. -2 Pa, heat at 1000℃ for 30 min, microwave frequency 1.50 kW, then argon-protected air cooling to below 50℃; (8) Heat treatment: First, keep at 920℃ for 4 h under a 2.5 T magnetic field and air cool to below 70℃; then keep at 520℃ for 6 h under a 2.0 T magnetic field and air cool to below 60℃ under argon protection to obtain sintered NdFeB magnets.

[0035] Figure 7 This is a particle size distribution diagram of the main phase alloy airflow milled powder in Example 3, showing that the average particle size SMD is 2.51 μm, which meets the process requirements.

[0036] Figure 8 This is a SEM image of the main phase alloy air-jet milled powder in Example 3 at 2000x magnification, showing that the powder surface is flat and suitable for sputter coating.

[0037] Figure 9 The image shows the SEM image of the sintered magnet coated with 2.1% auxiliary alloy in Example 3. It shows that the core-shell structure is clear, the auxiliary alloy is evenly distributed, and there is no obvious agglomeration.

[0038] To illustrate the technical effects of this invention, the applicant has set up comparative examples and conducted a performance comparison analysis as follows: Comparative Example 1 This comparative example uses conventional processes, and differs from Example 1 in the method of adding auxiliary alloys, sintering, and heat treatment processes: (1) The ingredients are the same as in Example 1; (2) Alloy preparation: The main phase alloy 1 and rare earth rich auxiliary alloy 2 were melted in a vacuum rapid solidification strip casting furnace and then cast at 1470–1480℃ to obtain two alloy strips with a thickness of 0.2–0.35 mm and a content of 96–97%. (3) Hydrogen crushing: The two types of strips were subjected to hydrogen crushing, the process being the same as in Example 1, to obtain hydrogen crushed alloy 1 micro powder and auxiliary alloy 2 powder; (4) Air jet milling: The two types of hydrogen-rich powders were subjected to air jet milling respectively, and the oxygen content was controlled to be ≤5 ppm to obtain fine powder of alloy 1 (particle size distribution: X 10 =0.71 μm, X 50 =2.08 μm, X 90 =3.96 μm, SMD=1.45 μm) and auxiliary alloy 2 fine powder (particle size distribution: X 10 =0.70 μm, X 50 =2.02 μm, X 90 =3.90 μm, SMD=1.42 μm); (5) Preparation of dual alloy powder: The fine powder of auxiliary alloy 2 is mixed with the fine powder of alloy 1 in a mixer at a ratio of 1%, 1.5%, 2.0%, 2.5%, and 3.0%; (6) Magnetic field forming: Same as in Example 1, magnetic field strength 1.92 T, compact density 4.3±0.1 g / cm³ 3 ; (7) Sintering: The green blanks are placed in a vacuum sintering furnace under nitrogen protection and the vacuum is evacuated to 1.0 × 10⁻⁶. -2 Pa, heated to 1040℃ and held for 240 min, then cooled to below 50℃ under argon protection and removed from the furnace; (8) Heat treatment: The blank is kept at 910℃ for 2 h and then air-cooled to below 70℃; then kept at 510℃ for 4 h and then air-cooled to below 60℃ under argon protection to obtain sintered NdFeB magnets.

[0039] Comparative Example 2 This comparative example uses a conventional process, and differs from Example 2 in the method of adding auxiliary alloys, sintering, and heat treatment processes: (1) The ingredients are the same as in Example 2; (2) Alloy preparation: The main phase alloy 1 and the auxiliary alloy 2 are respectively spun into strips to obtain corresponding spun strips; (3) Hydrogen crushing: Hydrogen crushing was carried out separately, with the same process as in Example 2, to obtain two kinds of hydrogen crushed powder; (4) Air jet milling: powdering was performed separately to obtain fine powder of alloy 1 (X). 10 =1.16 μm, X 50 =2.90 μm, X 90 =5.14μm, SMD=2.05 μm) and auxiliary alloy 2 fine powder (X 10 =1.22 μm, X 50 =3.02 μm, X 90 =5.19 μm, SMD=2.17 μm); (5) Preparation of dual alloy powder: The fine powder of auxiliary alloy 2 is mixed with the fine powder of alloy 1 in proportions of 0.8%, 1.2%, 1.6%, 2.0%, and 2.4%; (6) Magnetic field forming: Same as in Example 2, magnetic field strength 1.92 T, compact density 4.3±0.1 g / cm³ 3 ; (7) Sintering: Hold at 1060℃ for 240 min in a vacuum sintering furnace, then cool to below 50℃ under argon protection; (8) Heat treatment: Hold at 890℃ for 2 h, air cool to below 70℃; then hold at 500℃ for 3 h, air cool to below 60℃ under argon protection to obtain sintered NdFeB magnets.

[0040] Comparative Example 3 This comparative example uses a conventional process, and differs from Example 3 in the method of adding auxiliary alloys, sintering, and heat treatment processes: (1) The ingredients are the same as in Example 3; (2) Alloy preparation: The main phase alloy 1 and the auxiliary alloy 2 are respectively spun into strips to obtain corresponding spun strips; (3) Hydrogen crushing: Hydrogen crushing was carried out separately, with the same process as in Example 3, to obtain two kinds of hydrogen crushed powder; (4) Air jet milling: powdering was performed separately to obtain fine powder of alloy 1 (X). 10 =1.35 μm, X 50 =3.56 μm, X 90 =6.44μm, SMD=2.51 μm) and auxiliary alloy 2 fine powder (X 10 =1.38 μm, X 50 =3.60 μm, X 90 =6.41 μm, SMD=2.55 μm); (5) Preparation of dual alloy powder: fine powder of auxiliary alloy 2 is mixed with fine powder of alloy 1 in proportions of 0.5%, 0.9%, 1.3%, 1.7% and 2.1%; (6) Magnetic field forming: Same as in Example 3, magnetic field strength 2.0 T, compact density 4.3±0.1 g / cm³ 3 ; (7) Sintering: Hold at 1070℃ for 240 min in a vacuum sintering furnace, then cool to below 50℃ under argon protection; (8) Heat treatment: Hold at 910℃ for 3 h, air cool to below 70℃; then hold at 520℃ for 6 h, air cool to below 60℃ under argon protection to obtain sintered NdFeB magnets.

[0041] The performance of the magnets in each embodiment and comparative example was tested, and the results are shown in Tables 1-3.

[0042] Table 1: The performance of the products from Example 1 and Comparative Example 1 is as follows:

[0043] Table 2: The product performance of Example 2 and Comparative Example 2 is as follows:

[0044] Table 3: The product performance of Example 3 and Comparative Example 3 is as follows:

[0045] Performance comparison analysis: The difference between the embodiments and the comparative examples lies in the following: in embodiments 1-3, the auxiliary alloy is introduced by PVD sputtering coating, the sintering process is microwave sintering, and the heat treatment process uses two-stage magnetic field heat treatment; while in comparative examples 1-3, the auxiliary alloy is introduced by mechanical mixing, the sintering process is traditional vacuum sintering, and the heat treatment uses conventional (no magnetic field) heat treatment. By comparing the magnetic property data of the embodiments of the present invention with those of the conventional process comparative examples (Tables 1-3), it can be seen that the present invention has significant advantages in both remanence (Br) and intrinsic coercivity (Hcj), especially in the improvement of coercivity. 1. Core-shell coating structure enables precise elemental distribution. This invention employs physical vapor deposition (PVD) sputtering coating technology to uniformly coat rare-earth-rich auxiliary alloys onto the surface of the main phase powder in the form of a nanoscale thin film, forming a "core-shell" structure. This structure ensures the uniform distribution of rare-earth elements at grain boundaries, effectively enhancing the grain boundary pinning effect and thus significantly improving coercivity. In contrast, the comparative method using traditional mechanical powder mixing results in uneven distribution of the auxiliary alloy, easily forming local agglomerations, leading to limited improvement in coercivity and a significant decrease in remanence.

[0046] 2. Microwave sintering promotes densification and grain boundary optimization. This invention employs microwave sintering technology to achieve densification within 30–50 minutes at 900–1000℃. Microwave sintering features rapid and uniform heating, which can inhibit grain growth, promote grain boundary cleaning and homogenization, and improve the overall magnetic properties of the magnet. In contrast, the comparative example uses traditional vacuum sintering, which requires holding at 1040–1070℃ for 240 minutes. Prolonged high-temperature treatment can easily lead to grain coarsening and uneven element diffusion, affecting performance.

[0047] 3. Two-stage magnetic field heat treatment optimizes the magnetic structure. This invention employs a two-stage magnetic field heat treatment: the first stage applies a 1.5–2.5 T magnetic field at 870–920℃ to promote the orientation of the main phase grains and the adjustment of the grain boundary structure; the second stage applies a 1.5–2.0 T magnetic field at 490–520℃ to further stabilize the magnetic domain structure and improve coercivity and thermal stability. This process achieves precise control over the microstructure of the magnet, significantly outperforming comparative single-stage or non-magnetic field heat treatments.

[0048] 4. Overall performance is significantly better than traditional processes. (1) Significant improvement in coercivity: Under the same auxiliary alloy addition ratio, the Hcj of the examples is generally 2–5 kOe higher than that of the comparative examples. For example, the Hcj of Example 1 reached 22.21 kOe when 3.0% auxiliary alloy was added, while that of Comparative Example 1 was only 18.40 kOe.

[0049] (2) Excellent retention of remanence: While improving coercivity, the remanence of the embodiment decreased significantly less than that of the comparative example, indicating that the core-shell coating structure effectively reduced the dilution effect on the magnetism of the main phase.

[0050] (3) Strong process controllability: The combination of PVD coating + microwave sintering + two-stage magnetic field heat treatment realizes the systematic control of magnet composition, structure and performance, and has good repeatability and scalability.

[0051] In summary, this invention achieves a synergistic improvement in high coercivity, high remanence, and high energy product of NdFeB magnets through innovative processes such as core-shell coated powder design, microwave rapid sintering, and two-stage magnetic field heat treatment. This is significantly superior to traditional dual-alloy hybrid processes and has outstanding industrial application value.

[0052] The foregoing description illustrates and describes preferred embodiments of the present invention. As previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A method for preparing a high-energy-product neodymium iron boron magnet, characterized in that, Includes the following steps: (1) Ingredients: The main phase alloy 1 and rare earth-rich auxiliary alloy 2 are prepared. The main phase alloy 1 has the following mass percentage composition: Nd 27.5~29.8%, Fe 68.45~70.85%, B 0.95%, Ag 0.10~0.35%, Mo 0.10~0.30%, Zn 0.15~0.40%; the rare earth-rich auxiliary alloy 2 contains Nd, Ho, Dy, Ga, Cu, Zr, Al, Hf, Fe and B; (2) Alloy preparation: The main phase alloy 1 raw material prepared in step (1) is smelted and rapidly solidified into a strip to obtain an alloy strip with a thickness of 0.2-0.35 mm; The rare earth-rich auxiliary alloy 2 raw material prepared in step (1) is smelted and cast into alloy ingots, and then the alloy ingots are subjected to homogenization heat treatment and surface processing to obtain target alloy 2. (3) Hydrogen crushing: The alloy strip obtained in step (2) is subjected to hydrogen absorption-dehydrogenation treatment to obtain hydrogen crushed alloy 1 micro powder; (4) Air jet milling: The hydrogen-crushed alloy 1 micro powder obtained in step (3) is subjected to air jet milling under a protective atmosphere to obtain alloy 1 fine powder with an average particle size SMD of 1.45~2.51μm; (5) Core-shell coated powder preparation: The target alloy 2 obtained in step (2) is sputtered and coated on the surface of the fine powder of alloy 1 obtained in step (4) by physical vapor deposition to form a core-shell structure powder. The weight of the rare earth-rich auxiliary alloy 2 is controlled to be 0.5% to 3.0% of the total weight. (6) Magnetic field forming: Under a protective atmosphere with an oxygen content of less than 10 ppm, the core-shell structure powder obtained in step (5) is magnetically oriented and pressed to form a compact. (7) Sintering: The compact obtained in step (6) is microwave sintered under vacuum or protective atmosphere; (8) Heat treatment: The magnet sintered in step (7) is subjected to two-stage magnetic field heat treatment. The first stage heat treatment temperature is 870~920℃, held for 1~4h, and a magnetic field of 1.5~2.5T is applied. After air cooling, the second stage heat treatment is performed at a temperature of 490~520℃, held for 2~6h, and a magnetic field of 1.5~2.0T is applied to finally obtain the high magnetic energy product NdFeB magnet.

2. The preparation method according to claim 1, characterized in that, In step (2), the temperature of the rapid solidification belt is 1470~1480℃; the temperature of the homogenization heat treatment is 600~1000℃ and the time is 6~12h.

3. The preparation method according to claim 1, characterized in that, In step (3), the hydrogen absorption process is carried out at ≤100℃, using hydrogen gas with a purity of 99.99%; the temperature of the dehydrogenation process is 560~580℃, and the dehydrogenation ends when the vacuum degree reaches below 20Pa.

4. The preparation method according to claim 1, characterized in that, In step (4), the protective atmosphere is argon, and the oxygen content is controlled below 5 ppm during the gas flow milling process.

5. The preparation method according to claim 1, characterized in that, In step (5), the physical vapor deposition method is sputtering performed by a small powder PVD coating system.

6. The preparation method according to claim 1, characterized in that, In step (6), the orientation magnetic field strength for magnetic field forming is 1.90~2.0T, and the density of the pressed blank is 4.3±0.1 g / cm³. 3 .

7. The preparation method according to claim 1, characterized in that, In step (7), the vacuum degree of microwave sintering is 1.0E-2 Pa, the sintering temperature is 900~1000℃, the holding time is 30~50min, and the microwave frequency is 1.50~2.5 Kw.

8. The preparation method according to claim 1, characterized in that, In step (1), the mass percentage composition of the rare earth-rich auxiliary alloy 2 is as follows: Nd 20~39%, Ho 8~15%, Dy 5~15%, Ga 2~8%, Cu 5~15%, Zr 3~10%, Al 3.5~12%, Hf 2~6%, Fe 11.8~16.5%, with the balance being B.

9. A high-energy-product neodymium iron boron magnet, characterized in that, It is prepared by any one of claims 1 to 8.

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