Preparation method and application of organic suspension for improving coercive force of sintered neodymium-iron-boron magnet

Abstract

The invention discloses a preparation method and application of an organic suspension for improving the coercive force of a sintered neodymium-iron-boron magnet, and relates to the technical field of rare earth permanent magnets, the preparation method comprises the following specific steps: S1, raw material pretreatment and preparation; s2, preparing heavy rare earth powder; and S3, preparing an organic suspension. According to the preparation method and application of the organic suspension for improving the coercive force of the sintered neodymium-iron-boron magnet, the used heavy rare earth powder is smaller in particle size and can enter the magnet more easily in the grain boundary diffusion process, the coercive force of the magnet is effectively improved, more medium rare earth is saved, and the cost benefit is more remarkable; the polymer resin used in the organic suspension is low in melting point and low in C content, and can be converted into volatile components to be effectively removed during sintering, so that the influence of impurity elements on the performance of the magnet is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of rare earth permanent magnets, in particular to a preparation method and application of an organic suspension for improving the coercivity of sintered neodymium-iron-boron magnets. BACKGROUND

[0002] Sintered neodymium-iron-boron is widely used in the fields of new energy vehicles, industrial motors, motors, consumer electronics, medical devices, etc. due to its excellent magnetic properties. In recent years, the rise of new energy vehicles has made the application market of sintered neodymium-iron-boron magnets more high-end, which means that the performance of sintered neodymium-iron-boron magnets also needs to rise to a new level. However, the actual coercivity of sintered neodymium-iron is much lower than its theoretical value, and neodymium-iron-boron without special treatment cannot meet some application scenarios.

[0003] It is common in the art to introduce heavy rare earth elements during the preparation of magnets to significantly improve their coercivity in the form of grain boundary diffusion. Generally, magnetron sputtering, coating, electrophoretic deposition, etc. are used for heavy rare earth diffusion, but the above methods have the disadvantages of low utilization rate of heavy rare earth and high production cost. There are also coating suspensions in the prior art that are specifically used for grain boundary diffusion of sintered neodymium-iron-boron magnets, but due to improper solvent matching, powder particle size and operation process, there are generally problems such as poor dispersion effect of the suspension and poor coating consistency. Therefore, the present application relates to a new type of coating diffusion source suspension, which has a higher utilization rate of heavy rare earth, lower cost and more excellent magnet performance compared to the above methods. SUMMARY

[0004] In view of the deficiencies of the prior art, the present application provides a preparation method and application of an organic suspension for improving the coercivity of sintered neodymium-iron-boron magnets, which solves the problems raised in the background art.

[0005] To achieve the above purpose, the present application realizes the following technical scheme: a preparation method and application of an organic suspension for improving the coercivity of sintered neodymium-iron-boron magnets, comprising the following specific steps: S1, raw material pretreatment and preparation; S2, heavy rare earth powder preparation, specifically as follows: S201, alloying by melting, adding the pretreated dysprosium or terbium element, praseodymium or neodymium element and metal raw materials into a vacuum induction melting furnace, vacuumizing to 1x10 -3 Pa, introducing argon protection, heating to 1350-1500℃, keeping for 2-3 hours to make the raw materials completely melt, uniformly stirring and then pouring by copper mold rapid cooling to obtain a thickness of 0.05-0.2mm of the rapid solidification casting piece; S202, hydrogen crushing treatment; S203, superfine grinding; S204, mixed modification: In an argon-protected powder mixer, first add heavy rare earth fine powder, then add 0.1-0.5% lubricant and 0.01-0.1% antioxidant in proportion to improve powder dispersibility and antioxidant properties; S3. Preparation of organic suspension, as detailed below: S301. Mix in an inert environment. In a glove box protected by nitrogen or argon, weigh the raw materials according to the mass percentage and add them to the mixing container in sequence. S302, graded stirring and dispersion; S303, Viscosity and Stability Control: After stirring, check the viscosity of the suspension. If it deviates from 150,000-200,000... Adjust by adding dispersant or resin; let the suspension stand for 18-30 hours, and check that the amount of clear liquid in the upper layer is ≤5% to ensure dispersion stability.

[0006] Optionally, step S1 specifically includes: S101, Selected heavy rare earth raw materials, including elemental dysprosium or terbium, praseodymium or neodymium with a purity ≥99.9%, and one or more metals selected from copper, aluminum, gallium, and cobalt, according to the chemical formula RE1 x RE2 y M 100-x-y Weigh the raw materials according to the proportions where 10≤x≤70 and 10≤y≤30, and remove the oxide layer from the surface of the raw materials. S102, Pretreatment of organic auxiliary materials.

[0007] Optionally, step S102 specifically includes: A. Polymer resin: Select polyvinyl acetate or polyvinyl butyral, pulverize to a particle size ≤50μm, and dry in a vacuum drying oven; B. Dispersant: Mix ethanol and acetone at a mass ratio of 2-4:1-1.5 and pre-stir for 5-12 minutes until homogeneous; C. Defoamer: Mix silicone defoamer with a small amount of dispersant and disperse and activate it using ultrasound.

[0008] Optionally, the raw material mass percentage in step 301 is: The ingredients include 40-70% heavy rare earth powder, 2-10% polymer resin, 10-50% dispersant, and 1-3% defoamer.

[0009] Optionally, step S202 specifically includes: Place the rapidly solidified casting into the hydrogen breaker furnace and evacuate it to a vacuum level of 5×10. -2 After Pa, high-purity hydrogen gas is introduced, and the hydrogen pressure is controlled at 0.2-0.3 MPa. The mixture is kept at room temperature for 2-4 hours to allow the alloy to absorb hydrogen and become embrittled. Then, a vacuum is applied to 1×10⁻⁶ MPa. -2Pa, heated to 450-600℃ for dehydrogenation, to obtain coarse powder with a particle size of 80-100μm.

[0010] Optionally, step S203 specifically includes: The coarse powder was fed into an air jet mill, where nitrogen was used as the grinding medium to grind it into fine powder with a particle size of 1.0-3.0 μm; the particle size distribution was detected by a laser particle size analyzer.

[0011] Optionally, the graded mixing and dispersion in step S302 includes coarse mixing and fine mixing, specifically including: The coarse mixing process specifically involves: The mixed raw materials are transferred to a multi-functional vacuum mixing vessel, vacuumed, and stirred to allow the resin and dispersant to initially blend. Fine stirring specifically involves: The coarsely mixed slurry is transferred to a horizontal sand mill, and the particle size of the agglomerates is monitored by a dynamic light scattering instrument.

[0012] An application of an organic suspension for improving the coercivity of sintered NdFeB magnets, the specific application process is as follows: (1) Select sintered NdFeB blanks, use ultrasonic cleaning to remove surface oil stains, and then use plasma treatment to improve surface roughness and enhance coating adhesion; (2) A uniform base coating is formed on the surface of the blank by air spraying, and then the coating is scraped and repaired with a precision scraper to control the coating thickness to 50-100μm; for complex shaped magnets, dip coating combined with infrared thermometry is used. (3) Place the coated magnet into a hot air circulating oven and keep it at 120-180℃ for 20-40 minutes to allow the polymer resin to initially cure and prevent the coating from peeling off during subsequent heat treatment; after baking, cool to room temperature and use a thickness gauge to check the coating thickness deviation ≤5μm. (4) Place the magnet into a vacuum sintering furnace and evacuate it to a vacuum level of 5×10⁻⁶. -4 Pa, heating in stages: room temperature → 500-600℃ → 910-950℃; after holding, cooling with the furnace to below 200℃; (5) After diffusion, the magnet is transferred to a tempering furnace, protected by argon gas, heated to 450-550℃, and held for 3-6 hours to eliminate internal stress and stabilize the microstructure of the magnet; secondary tempering is used to further optimize the coercivity stability. (6) Post-processing and performance testing.

[0013] Optionally, the post-processing and performance testing are as follows: After heat treatment, the magnet was ultrasonically cleaned with anhydrous ethanol to remove residual carbides on the surface. After drying, the surface roughness Ra was measured to be ≤0.2μm.

[0014] This invention provides a method for preparing an organic suspension for improving the coercivity of sintered NdFeB magnets and its application, which has the following beneficial effects: The organic suspension uses heavy rare earth powder with smaller particle size, which makes it easier to enter the magnet during grain boundary diffusion, effectively improving the coercivity of the magnet and saving more medium rare earth content, resulting in more significant cost benefits. The polymer resin used in the organic suspension has a low melting point and low carbon content, which can be converted into volatiles during sintering and effectively removed, reducing the impact of impurity elements on the magnet performance. Furthermore, the equipment used in this invention is all used in the process of preparing sintered NdFeB, eliminating the need to purchase other equipment and simplifying the preparation process, thus enabling mass production. Attached Figure Description

[0015] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0016] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0017] Example 1 (1) Weigh one of dysprosium and terbium or an alloy mixture and add it to the melting furnace. Set the melting temperature to 1450℃. The thickness of the rapidly solidified casting after melting and casting is 0.2 mm. The particle size of the coarse powder after hydrogen crushing is 92 μm. The average particle size of the fine powder after air jet milling is 2.8 μm. (2) Weigh 40% heavy rare earth powder and alloy, 10% polyvinyl acetate, 49% acetone, and 1% organosilicon in sequence according to weight percentage; mix and stir 10% polyvinyl acetate, 49% acetone, and 1% organosilicon and transfer them into a vacuum glove box; add 40% heavy rare earth powder and alloy to the mixed organic solvent; and stir in a multi-functional vacuum stirring kettle and a horizontal sand mill to obtain an organic suspension. (3) The organic suspension was coated on the surface of the sintered NdFeB blank and heated and baked at 150°C for 30 minutes to obtain a heavy rare earth coating. (4) The coated sintered NdFeB blank is placed in a vacuum sintering furnace for heat treatment at a temperature of 950°C for 5 hours. Then it is tempered at a temperature of 550°C for 3 hours to obtain diffused sintered NdFeB.

[0018] Example 2 (1) Weigh one of dysprosium and terbium or an alloy mixture and add it to the melting furnace. Set the melting temperature to 1450℃. The thickness of the rapidly solidified casting after melting and casting is 0.2 mm. The particle size of the coarse powder after hydrogen crushing is 92 μm. The average particle size of the fine powder after air jet milling is 2.8 μm. (2) Weigh 50% of heavy rare earth powder and alloy, 8% of polyvinyl acetate, 41% of acetone, and 1% of organosilicon in sequence according to weight percentage; mix and stir 8% of polyvinyl acetate, 41% of acetone, and 1% of organosilicon and transfer them into a vacuum glove box; add 50% of heavy rare earth powder and alloy to the mixed organic solvent; and stir in a multi-functional vacuum stirring kettle and a horizontal sand mill to obtain an organic suspension. (3) The organic suspension was coated on the surface of the sintered NdFeB blank and heated and baked at 120°C for 30 minutes to obtain a heavy rare earth coating. (4) The coated sintered NdFeB blank is placed in a vacuum sintering furnace for heat treatment at a temperature of 940°C for 5 hours. Then it is tempered at a temperature of 550°C for 3 hours to obtain diffused sintered NdFeB.

[0019] Example 3 (1) Weigh one of dysprosium and terbium or an alloy mixture and add it to the melting furnace. Set the melting temperature to 1450℃. The thickness of the rapidly solidified casting after melting and casting is 0.2 mm. The particle size of the coarse powder after hydrogen crushing should be 92 μm. The average particle size of the fine powder after air jet milling is 2.8 μm. (2) Weigh 60% of heavy rare earth powder and alloy, 6% of polyvinyl acetate, 33% of acetone, and 1% of organosilicon in sequence according to weight percentage; mix and stir 6% of polyvinyl acetate, 33% of acetone, and 1% of organosilicon and transfer them into a vacuum glove box; add 60% of heavy rare earth powder and alloy to the mixed organic solvent; and stir in a multi-functional vacuum stirring kettle and a horizontal sand mill to obtain an organic suspension. (3) The organic suspension was coated on the surface of the sintered NdFeB blank and heated and baked at 120°C for 30 minutes to obtain a heavy rare earth coating. (4) The coated sintered NdFeB blank is placed in a vacuum sintering furnace for heat treatment. The sintering temperature is 930℃ and the holding time is 5 hours. Then, it is tempered at 550℃ for 3 hours to obtain diffused sintered NdFeB.

[0020] Example 4 (1) Weigh one of dysprosium and terbium or an alloy mixture and add it to the melting furnace. Set the melting temperature to 1450℃. The thickness of the rapidly solidified casting after melting and casting is 0.2 mm. The particle size of the coarse powder after hydrogen crushing should be 92 μm. The average particle size of the fine powder after air jet milling is 2.8 μm. (2) Weigh 70% of heavy rare earth powder and alloy, 4% of polyvinyl acetate, 25% of acetone, and 1% of organosilicon in sequence according to weight percentage; mix and stir 4% of polyvinyl acetate, 25% of acetone, and 1% of organosilicon and transfer them into a vacuum glove box; add 70% of heavy rare earth powder and alloy to the mixed organic solvent; and stir in a multi-functional vacuum stirring kettle and a horizontal sand mill to obtain an organic suspension. (3) The organic suspension was coated on the surface of the sintered NdFeB blank and heated and baked at 120°C for 30 minutes to obtain a heavy rare earth coating. (4) The coated sintered NdFeB blank is placed in a vacuum sintering furnace for heat treatment at a temperature of 920°C for 5 hours. Then it is tempered at a temperature of 550°C for 3 hours to obtain diffused sintered NdFeB.

[0021] Comparative Example 1 (1) Weigh one of dysprosium and terbium or an alloy mixture and add it to the melting furnace. Set the melting temperature to 1450℃. The thickness of the quick-solidified casting after melting and casting is 0.18 mm. The particle size of the coarse powder after hydrogen crushing is 84 μm. The average particle size of the fine powder after air jet milling is 2.5 μm. (2) Weigh 40% heavy rare earth powder and alloy, 10% polyvinyl alcohol, 49% ethanol, and 1% organosilicon in sequence according to weight percentage; mix 10% polyvinyl alcohol, 49% ethanol, and 1% organosilicon and transfer them into a vacuum glove box; add 40% heavy rare earth powder and alloy to the mixed organic solvent; and stir in a multi-functional vacuum stirring kettle and a horizontal sand mill to obtain an organic suspension. (3) The organic suspension was coated on the surface of the sintered NdFeB blank and heated and baked at 150°C for 30 minutes to obtain a heavy rare earth coating. (4) The coated sintered NdFeB blank is placed in a vacuum sintering furnace for heat treatment at a temperature of 950°C for 5 hours. Then it is tempered at a temperature of 550°C for 3 hours to obtain diffused sintered NdFeB.

[0022] Comparative Example 2 (1) Weigh one of dysprosium and terbium or an alloy mixture and add it to the melting furnace. Set the melting temperature to 1450℃. The thickness of the quick-solidified casting after melting and casting is 0.18 mm. The particle size of the coarse powder after hydrogen crushing is 84 μm. The average particle size of the fine powder after air jet milling is 2.5 μm. (2) Weigh 50% of heavy rare earth powder and alloy, 8% of polyvinyl alcohol, 41% of ethanol and 1% of organosilicon in sequence according to weight percentage; mix and stir 8% of polyvinyl alcohol, 41% of ethanol and 1% of organosilicon and transfer them into a vacuum glove box; add 40% of heavy rare earth powder and alloy to the mixed organic solvent; and stir in a multi-functional vacuum stirring kettle and a horizontal sand mill to obtain an organic suspension. (3) The organic suspension was coated on the surface of the sintered NdFeB blank and heated and baked at 150°C for 30 minutes to obtain a heavy rare earth coating. (4) The coated sintered NdFeB blank is placed in a vacuum sintering furnace for heat treatment at a temperature of 940°C for 6 hours. Then it is tempered at a temperature of 550°C for 4 hours to obtain diffused sintered NdFeB.

[0023] Comparative Example 3 (1) Weigh one of dysprosium and terbium or an alloy mixture and add it to the melting furnace. Set the melting temperature to 1450℃. The thickness of the quick-solidified casting after melting and casting is 0.18 mm. The particle size of the coarse powder after hydrogen crushing is 84 μm. The average particle size of the fine powder after air jet milling is 2.5 μm. (2) Weigh 60% of heavy rare earth powder and alloy, 6% of polyvinyl alcohol, 33% of ethanol, and 1% of organosilicon in sequence according to weight percentage; mix and stir 6% of polyvinyl alcohol, 33% of ethanol, and 1% of organosilicon and transfer them into a vacuum glove box; add 40% of heavy rare earth powder and alloy to the mixed organic solvent; and stir in a multi-functional vacuum stirring kettle and a horizontal sand mill to obtain an organic suspension. (3) The organic suspension was coated on the surface of the sintered NdFeB blank and heated and baked at 150°C for 30 minutes to obtain a heavy rare earth coating. (4) The coated sintered NdFeB blank is placed in a vacuum sintering furnace for heat treatment. The sintering temperature is 930℃ and the holding time is 7 hours. Then, it is tempered at 550℃ for 5 hours to obtain diffused sintered NdFeB.

[0024] Comparative Example 4 (1) Weigh one of dysprosium and terbium or an alloy mixture and add it to the melting furnace. Set the melting temperature to 1450℃. The thickness of the quick-solidified casting after melting and casting is 0.18 mm. The particle size of the coarse powder after hydrogen crushing is 84 μm. The average particle size of the fine powder after air jet milling is 2.5 μm. (2) Weigh 70% of heavy rare earth powder and alloy, 4% of polyvinyl alcohol, 25% of ethanol and 1% of organosilicon in sequence according to weight percentage; mix 4% of polyvinyl alcohol, 25% of ethanol and 1% of organosilicon and transfer them into a vacuum glove box; add 40% of heavy rare earth powder and alloy to the mixed organic solvent; and stir in a multi-functional vacuum stirring kettle and a horizontal sand mill to obtain an organic suspension. (3) The organic suspension was coated on the surface of the sintered NdFeB blank and heated and baked at 150°C for 30 minutes to obtain a heavy rare earth coating. (4) The coated sintered NdFeB blank was placed in a vacuum sintering furnace for heat treatment. The sintering temperature was 920℃ and the holding time was 8 hours. Then, it was tempered at 550℃ for 6 hours to obtain diffused sintered NdFeB. The magnetic properties of the diffused magnet were measured using a Belgian Metis pulsed field magnetometer. The results are shown in Table 1. The average value was taken from five tests. Table 1 As can be seen from Table 1, the magnetic properties of the diffusion-sintered NdFeB magnets prepared using suspension were significantly improved, and the remanence was reduced by a small amount. This invention prepares a coated organic suspension for improving the coercivity of sintered NdFeB magnets. By selecting a suitable organic carrier and utilizing the strong mechanical stirring action of a multifunctional vacuum stirring vessel and a horizontal sand mill, the agglomerated powder particles are dispersed through high-speed shearing and grinding, thereby achieving high dispersibility and suspension stability of the organic suspension.

[0025] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A process for the preparation of an organic suspension for increasing the coercivity of sintered neodymium-iron-boron magnets, characterized in that The method comprises the following specific steps: S1, raw material pretreatment and preparation; S2, heavy rare earth powder preparation, specifically as follows: S201、Melt alloying, the pre-treatment of dysprosium or terbium single element, praseodymium or neodymium single element, metal raw materials into vacuum induction melting furnace, vacuum to 1 × 10 -3 Pa, argon protection, heating to 1350-1500 DEG C, 2-3 hours to make the raw materials completely melted, stirring uniform after using copper mold quick cooling casting, obtain the thickness of 0.05-0.2 mm of quick solidification casting piece; S202, hydrogen crushing treatment; S203, superfine crushing; S204, mixing modification, in an argon-protected powder mixer, first add heavy rare earth fine powder, then add 0.1-0.5% lubricant and 0.01-0.1% antioxidant according to the proportion, to improve the powder dispersibility and oxidation resistance; S3, organic suspension preparation, specifically as follows: S301, mixing in an inert environment, in a nitrogen or argon-protected glove box, raw materials are weighed according to the mass percentage, and then added into a stirring container; S302, hierarchical stirring and dispersion; S303、Viscosity and stability control, after stirring is completed, the viscosity of the suspension is detected, if it deviates from 150000-200000 , adjust by adding dispersant or resin; the suspension is left for 18-30 hours, the amount of supernatant is detected ≤5%, to ensure the dispersion stability.

2. The production method according to claim 1, characterized by, Step S1 specifically includes: S101, heavy rare earth raw material selection, select purity ≥ 99.9% dysprosium or terbium single element, praseodymium or neodymium single element, and one or more metals of copper, aluminum, gallium, cobalt, weighing according to the chemical formula RE1 x RE2 y M 100-x-y , wherein 10≤x≤70, 10≤y≤30, remove the surface oxide layer of the raw material; S102, organic auxiliary material pretreatment.

3. The preparation method according to claim 2, characterized in that, Step S102 specifically includes: A, high molecular resin: polyvinyl acetate or polyvinyl butyral is selected, crushed to a particle size of ≤50μm, and dried in a vacuum drying box; B, dispersant: mix ethanol and acetone according to the mass ratio of 2-4:1-1.5, pre-stir for 5-12 minutes until uniform; C, defoaming agent: mix silicone defoaming agent with a small amount of dispersant, and disperse and activate by ultrasonic.

4. The method of claim 1, wherein, The mass percentage of raw materials in step 301 is: Heavy rare earth powder 40-70%, high molecular resin 2-10%, dispersant 10-50%, and defoaming agent 1-3%.

5. The preparation method according to claim 1, characterized in that, Step S202 specifically includes: The rapidly solidified ingot was put into a hydrogen decrepitation furnace, vacuumed to 5x10 -2 Pa, high purity hydrogen was introduced, hydrogen pressure was controlled at 0.2-0.3 MPa, and the alloy was kept at room temperature for 2-4 hours to make the alloy hydrogen embrittlement; then vacuumed to 1x10 -2 Pa, and dehydrogenated by heating to 450-600°C to obtain coarse powder with a particle size of 80-100 μm.

6. The method of claim 1, wherein, Step S203 specifically includes: The hydrogen coarse powder is sent into an air flow mill, nitrogen is used as the grinding medium, and fine powder with a particle size of 1.0-3.0μm is obtained by grinding; the particle size distribution is detected by a laser particle size analyzer.

7. The preparation method according to claim 1, characterized in that, The hierarchical stirring and dispersion in step S302 includes coarse stirring and fine stirring, specifically including: Coarse stirring specifically includes: Transfer the mixed raw materials to a multifunctional vacuum stirring kettle, stir after vacuumizing, and make the resin and dispersant preliminarily fuse; Fine stirring specifically includes: Transfer the slurry after coarse stirring into a horizontal sand mill, and monitor the particle size of particle agglomerates by a dynamic light scattering instrument.

8. Use of an organic suspension for increasing the coercivity of sintered neodymium-iron-boron magnets, produced using a method for producing an organic suspension for increasing the coercivity of sintered neodymium-iron-boron magnets according to any one of claims 1 to 7, characterized in that The specific application process is as follows: (1) Select sintered neodymium iron boron blanks, use ultrasonic cleaning to remove surface oil stains, and then use plasma treatment to improve surface roughness and enhance coating adhesion; (2) Form a uniform base coating on the surface of the blank by air spraying, and then use a precision doctor blade to scrape and coat for finishing, control the coating thickness to be 50-100μm; for complex-shaped magnets, use immersion coating combined with infrared temperature measurement; (3) Put the coated magnet into a hot air circulation oven, heat at 120-180℃ for 20-40 minutes to preliminarily cure the high molecular resin, so as to avoid coating falling off during subsequent heat treatment; after baking, cool to room temperature, and use a thickness gauge to detect the coating thickness deviation ≤5μm; (4) Put the magnet into the vacuum sintering furnace, vacuumize to 5 x 10 -4 Pa, temperature rise in stages: room temperature→500-600°C→910-950°C; after the end of the holding, cool to below 200°C with the furnace; (5) After diffusion, the magnet is transferred into a tempering furnace, argon is introduced for protection, heated to 450-550℃, and kept for 3-6 hours to eliminate internal stress and stabilize the microstructure of the magnet; use secondary tempering to further optimize the coercive force stability; (6) Post-treatment and performance detection.

9. Use according to claim 8, characterized in that, The post-treatment and performance detection are specifically as follows: after heat treatment, the magnet is ultrasonically cleaned with anhydrous ethanol to remove surface residual carbide, dried, and then the surface roughness Ra≤0.2μm is detected.

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