A multi-pole neodymium-iron-boron magnet and a method for producing the same

Abstract

The application relates to the technical field of permanent magnet part machining, in particular to a multi-pole neodymium-iron-boron magnet and a preparation method thereof. The preparation method of the multi-pole neodymium-iron-boron magnet disclosed by the application can remove the bonding system in the green body through chemical and physical methods after the green body with a predetermined shape is obtained by changing the formula of the bonding system, so that the finally obtained magnet is basically composed of pure neodymium-iron-boron magnet, and the performance of the finished magnet is improved.

Description

Technical Field

[0001] This invention relates to the field of permanent magnet parts processing technology, and in particular to a multipole neodymium iron boron magnet and its preparation method. Background Technology

[0002] With the development of modern industry, the requirements for motor performance are becoming increasingly stringent. Multi-stage motors, due to their advantages such as high power density, high efficiency, and low torque ripple, have been widely used in new energy vehicles, industrial robots, aerospace, and other fields. As a key component of the motor, the stator's magnet performance directly affects the overall performance of the motor.

[0003] Neodymium iron boron (NdFeB) permanent magnets are the preferred material for stator magnets in multi-pole motors due to their superior magnetic properties, including high remanence, high coercivity, and high energy product. Currently, the main methods for preparing NdFeB magnets are sintering and bonding. While sintered NdFeB magnets offer high magnetic properties, they are difficult to manufacture, costly to produce, and have high raw material costs for multi-pole motor stator magnets with small differences in inner and outer diameters and complex shapes. This results in significant material waste, low yield rates, and a susceptibility to defects such as cracks. Bonded NdFeB magnets, while capable of forming complex shapes, have relatively lower magnetic properties, limiting further improvements in motor performance.

[0004] For example, application CN110931236B discloses a method and apparatus for injection molding anisotropic bonded neodymium iron boron magnets with radial orientation. Although this method can mold complex cylindrical magnets through injection molding, in this application, materials such as PPS used for injection molding were not removed, resulting in a smaller magnetic energy product of the magnet.

[0005] Therefore, existing technologies still need to be improved and developed. Summary of the Invention

[0006] This invention discloses a multipole neodymium iron boron magnet and its preparation method. In view of the defects of the prior art, it provides a solution that can remove the binder in the preparation process, so as to be particularly suitable for the production process of magnets with higher magnetic energy products.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a multipole neodymium iron boron magnet includes the following steps: a) Provide neodymium iron boron magnetic powder and a binder system, mix them and granulate to obtain granular material; b) The granular material is melted by an injection molding machine and injected into the cavity of a mold to obtain a green body of a predetermined shape; c) Remove the binding system from the green embryo to obtain a brown embryo; d) Sintering the brown embryo to form a uniformly densified coarse embryo; and e) Magnetize the blank to obtain the multipole NdFeB magnet; In step b), a multipolar magnetic field is applied to the periphery of the mold to obtain the preform oriented by the multipolar field; wherein the multipolar magnetic field is applied 2-3 seconds before the injection of the injection molding machine begins, the total application time of the magnetic field is 6-10 seconds, and the magnetic field is generated by a current of 200-400A.

[0008] Preferably, in the raw materials provided in step a), the average particle size of the neodymium iron boron magnetic powder is 6-10 μm; The bonding system comprises an adhesive, a coupling agent, and an antioxidant, wherein the adhesive is selected from one or more of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and paraffin wax.

[0009] Preferably, the mass fraction of each component in the bonding system is: Polyoxymethylene: 50-80%, or high-density polyethylene: 5-15%, or ethylene-vinyl acetate copolymer: 1-5%, or paraffin wax: 5-20%; Coupling agent: 0.5-1%, wherein the coupling agent is silane coupling agent KH550 and / or stearic acid; Antioxidant: 0.2-0.5%.

[0010] Preferably, the mixing and granulation in step a) are carried out under inert gas protection, and the binder is added in stages so that the melt index of the obtained granules is controlled at 100-200 g / 10min.

[0011] Preferably, in step b), during the injection molding process, the injection temperature is 150-280℃, the injection pressure is 80-120MPa, and the injection speed is 30-50 cm. 3 / s, mold temperature is 80-85℃; and Maintain pressure for 10-15 seconds after injection.

[0012] Preferably, the degreasing process in step c) is selected according to the type of adhesive: When the main component of the adhesive is a plastic-based system, a two-step method combining catalytic degreasing and thermal degreasing is used; or When the main component of the adhesive is a wax-based system, a two-step method combining extraction degreasing and thermal degreasing is adopted.

[0013] Preferably, the thermal degreasing stage is carried out in an atmosphere of hydrogen, nitrogen, argon or a combination thereof; and a segmented heating procedure is adopted: first heating at 200-250°C for 2-4 hours, and then heating at 400-550°C for 3-6 hours.

[0014] Preferably, the sintering in step d) is carried out in an inert atmosphere at 1000-1100℃ for 2-3 hours.

[0015] Preferably, the method further includes aging the blank after the sintering step.

[0016] The present invention also discloses a multipole neodymium iron boron magnet, which is prepared by the preparation method of a multipole neodymium iron boron magnet as described above.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention discloses a method for preparing multipole NdFeB magnets. By modifying the formulation of the bonding system, after obtaining a preform of a predetermined shape, the bonding system in the preform can be completely removed through chemical and physical methods, resulting in a final magnet composed primarily of pure NdFeB magnets, thereby improving the performance of the finished magnet. Furthermore, in this invention, a multipole magnetic field is arranged around the mold to orient the NdFeB magnetic powder during injection molding, thus obtaining a preform oriented by the multipole magnetic field. During the orientation of the NdFeB magnetic powder, the multipole magnetic field is applied 2-3 seconds before injection molding. Once injection begins, the multipole magnetic field stabilizes, and after a moderate current is applied continuously for 6-10 seconds, the NdFeB magnetic powder, before solidification, is highly oriented. This achieves a very high degree of orientation with a relatively small magnetic field, allowing the preform to obtain a large magnetic energy product after sintering and magnetization, significantly improving the energy efficiency ratio in the NdFeB magnet production process.

[0018] Furthermore, the present invention also discloses a multipole NdFeB magnet, which possesses all the advantages of the above-described preparation method. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the fabrication process of a multipole neodymium iron boron magnet according to an embodiment of the present invention. Detailed Implementation

[0020] 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.

[0021] In this invention, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "middle," "vertical," "horizontal," "lateral," and "longitudinal" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing the invention and its embodiments, and are not intended to limit the indicated devices, elements, or components to having a specific orientation, or to be constructed and operated in a specific orientation.

[0022] Furthermore, in addition to indicating direction or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in certain situations to indicate a dependency or connection. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0023] Furthermore, the terms "installation," "setup," "equipped with," "connection," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0024] Furthermore, the terms "first," "second," etc., are primarily used to distinguish different devices, elements, or components (which may be the same or different in specific type and construction), and are not intended to indicate or imply the relative importance or quantity of the indicated devices, elements, or components. Unless otherwise stated, "a plurality of" means two or more.

[0025] The technical solution of the present invention will be further described below with reference to the embodiments and accompanying drawings.

[0026] Example While some existing technical solutions can form relatively complex cylindrical multipole NdFeB magnets, they are plastic magnets. The PPS and other materials used in the injection molding process are still mixed with the magnetic powder, resulting in a small magnetic energy product and a narrow range of applicable scenarios.

[0027] Therefore, this invention provides a multipole NdFeB magnet and its preparation method, which uses metal powder injection molding (MIM) technology to form NdFeB magnets with complex shapes. In this invention, a plastic-based or wax-based binder is used to bind the magnet powder, and the preform is degreased through chemical and physical methods in subsequent processes, enabling the magnet to have a large magnetic energy product after sintering, thus improving the magnet's performance.

[0028] Furthermore, in this invention, a multipolar magnetic field is used to orient the magnetic powder during the injection molding process. The multipolar magnetic field is applied 2-3 seconds before injection into the injection molding machine. During the injection process, the magnetic field is already stable, and the application time of the magnetic field is controlled at 6-10 seconds. At this time, the magnetic powder injected into the mold is fully oriented. After the magnetic powder is fully oriented, the magnetic field is de-energized. Within a suitable energization time, the magnetic powder obtains a high degree of orientation. At the same time, the energy required to maintain the magnetic field is relatively small, resulting in high energy efficiency in the production of neodymium iron boron magnets.

[0029] Specifically, the paper cup method using neodymium iron boron magnets according to embodiments of the present invention includes the following steps: a) Provide neodymium iron boron magnetic powder and a binder system, mix them and granulate to obtain granular material.

[0030] The neodymium iron boron magnetic powder selected is a high-performance neodymium iron boron magnetic powder with Nd2Fe as its main phase. 14 B. The magnetic powder has a uniform particle size distribution with an average particle size of 6-10 μm. Furthermore, the intrinsic magnetic properties of the magnetic powder, such as intrinsic coercivity (Hcj) ≥1200kA / m and remanence (Br) ≥1.3T.

[0031] Specifically, the bonding system includes plastic-based bonding systems and wax-based bonding systems.

[0032] In the plastic-based adhesive system, the adhesive is one or more of polyoxymethylene, high-density polyethylene, and ethylene-vinyl acetate copolymer. In one embodiment of the present invention, a small amount of paraffin wax may also be added to the plastic-based adhesive, wherein the mass fraction of paraffin wax in the plastic-based adhesive is 3% to 10%.

[0033] In wax-based adhesive systems, the adhesive is primarily paraffin wax. In one embodiment of the present invention, a small amount of plasticizing components, such as one or more of polyoxymethylene, high-density polyethylene, and ethylene-vinyl acetate copolymer, may also be added to the wax-based adhesive. The plasticizing components constitute 20% to 35% of the wax-based adhesive as a skeleton agent by mass.

[0034] Furthermore, the bonding system also includes coupling agents and antioxidants. The coupling agent improves the interfacial bonding between the magnetic powder and the binder, thereby enhancing the overall performance of the magnet. The antioxidant prevents the magnetic powder from oxidizing during processing.

[0035] In one embodiment of the present invention, the mass fraction of each component in the plastic-based adhesive system is: polyoxymethylene: 50-80% by mass of magnetic powder, or high-density polyethylene: 5-15%, or ethylene-vinyl acetate copolymer: 1-5%, or paraffin wax: 3-5%.

[0036] In one embodiment of the present invention, the mass fraction of each component in the wax-based binder system is as follows: paraffin wax: 70-80% by mass of magnetic powder, or polyoxymethylene: 5-10%, or high-density polyethylene: 5-10%, or ethylene-vinyl acetate copolymer: 3-8%.

[0037] The coupling agent can be at least one of silane coupling agent KH550, stearic acid, etc., and the mass fraction of the coupling agent is 0.5%-1% of the mass of the magnetic powder.

[0038] The antioxidant can be selected from at least one of diethyl malonate, petroleum ether, polyethylene oxide monofatty acid, and triethylene glycol, and the mass fraction of the antioxidant is 0.1%-0.5% of the mass of the magnetic powder.

[0039] It should be noted that the mass fractions of the coupling agent and antioxidant should be adaptively adjusted depending on whether the system is plastic-based or wax-based. For example, when using a plastic-based bonding system, the mass fraction of the coupling agent is 0.5%-1% of the magnetic powder mass, and the mass fraction of the antioxidant is 0.1%-0.5% of the magnetic powder mass. When using a wax-based bonding system, the mass fraction of the coupling agent is 0.5%-1% of the magnetic powder mass, and the mass fraction of the antioxidant is 0.2%-0.5% of the magnetic powder mass.

[0040] After obtaining the magnetic powder and binder system, the binder, coupling agent, and antioxidant in the binder system are first mixed to obtain an additive, which is then divided into three portions. After measuring the mass of each raw material, the NdFeB magnetic powder and the first portion of the mixed additive are fed into a twin-screw internal mixer. The rotation of the screws in the mixer ensures thorough mixing of the magnetic powder and binder. After the first mixing time in the internal mixer, the second portion of the additive is added. After the second mixing time in the internal mixer, the third portion of the additive is added, and the mixing continues for a third time until the final melt flow index is controlled at 100-200 g / 10min.

[0041] In one embodiment of the present invention, the screw speed of the internal mixer is controlled at 80-120 r / min, and the mixing temperature is 160-200℃, thereby ensuring that the binder system is fully melted and thoroughly mixed with the NdFeB magnetic powder. The binder is evenly distributed in the system, and the mixing is carried out under inert gas protection. Simultaneously, the mixing time is controlled at 20-35 min to ensure uniform mixing of the materials, forming a NdFeB magnetic powder-binder composite with good processing properties.

[0042] After the mixing is completed, the mixed magnetic powder-binder composite is transferred to a twin-screw extruder and granulated through the twin-screw extruder. The transfer and granulation of the magnetic powder-binder composite are carried out under inert gas protection to obtain granules.

[0043] In one embodiment of the present invention, the extruder temperature is set to 230-260℃, and the screw speed is 120-150 r / min. After the material is melted and plasticized in the extruder, it is extruded into strips through a die, and then cut into uniform granules with a particle diameter of 2-3 mm by a pelletizer. The granulated material is convenient for subsequent injection molding processing.

[0044] b) The granular material is melted by an injection molding machine and injected into the cavity of a mold to obtain a green body of a predetermined shape.

[0045] In one embodiment of the present invention, the injection molding machine is an injection molding machine with an orientation device. Before injection, the orientation device is energized to apply a multipolar magnetic field to the periphery of the mold, so that the multipolar magnetic field is already in a stable state before injection. After the multipolar magnetic field is stable, injection is performed, and a better orientation effect can be achieved in a short time.

[0046] Specifically, the multipolar magnetic field is applied 2-3 seconds before the injection begins, the total application time of the magnetic field is 6-10 seconds, and the magnetic field is generated by a current of 200-400A.

[0047] The barrel temperature (injection temperature) can be set to 150-180℃ to ensure the material is fully melted. The mold temperature is controlled at 80-85℃ to reduce the temperature difference between the material and the mold while ensuring mold precision, thereby slowing down the solidification rate of the molten material and ensuring the dimensional accuracy and surface quality of the magnet during the molding process.

[0048] In one embodiment of the present invention, the injection pressure is 80-120 MPa and the injection speed is 30-50 cm. 3 The magnetic field allows the molten material to rapidly fill the mold cavity under its influence, resulting in more complete orientation of the magnetic powder within the material. The injection holding time is set to 10-15 seconds to ensure the density and dimensional stability of the magnet blank.

[0049] c) Remove the binding system from the green embryo to obtain a brown embryo.

[0050] Specifically, based on the changes in the bonding system of the bonded NdFeB magnetic powder, the preforms include plastic-based preforms and wax-based preforms. Depending on the different compositions of the preforms, the process of removing the bonding system in the preforms is also different.

[0051] In the case of green bodies formed from plastic-based binders, a preliminary degreasing treatment is first performed using catalytic degreasing. This catalytic degreasing employs nitric acid or oxalic acid vapor to catalytically remove polyoxymethylene (POM). Specifically, a strong acid gas phase is used to acid-catalyze the depolymerization of the acetal bonds in POM, converting the polymer into volatile formaldehyde and other small molecules that are then discharged. This selectively removes POM from the plastic-based binder, which forms the backbone, while simultaneously creating a porous network within the green body and maintaining its shape, facilitating subsequent thermal removal of the remaining binder.

[0052] The catalytically degreased green embryos are then placed in a hot degreasing furnace for hot degreasing to remove residual high-density polyethylene and / or ethylene-vinyl acetate copolymer and / or paraffin wax from the green embryos, thereby obtaining high-purity brown embryos.

[0053] Specifically, during catalytic degreasing, nitric acid reacts with the acetal structure, and the nitric acid vapor removes polyoxymethylene to form porous channels. These channels form a connected network, which in turn makes the thermal degreasing of high-density polyethylene and ethylene-vinyl acetate copolymer faster, with less stress on the preform and a lower risk of preform warping / cracks.

[0054] In one embodiment of the present invention, the catalytic degreasing temperature is 110-130℃ and the degreasing time is 5-8h.

[0055] In one embodiment of the present invention, the thermal degreasing temperature is 200-550℃, the heating rate is 0.5-5℃ / min, and the degreasing time is 2-8h.

[0056] It should be noted that when a small amount of paraffin is added to the plastic matrix system, during the catalytic degreasing process of nitric acid vapor at 110–130°C, the paraffin is mainly carried out through the formed channels by physical means of melt permeation and a small amount of volatilization.

[0057] When the binder system in the green body is mainly wax-based, the first step is to use extraction degreasing. The injection-molded green body is placed in an organic solvent for extraction degreasing. During the extraction process, the low molecular weight part of the binder is dissolved and extracted by the organic solvent, which reduces the binder content in the green body and increases the porosity, creating favorable conditions for subsequent thermal degreasing and sintering.

[0058] Similarly, the magnet blanks after catalytic degreasing are placed in a hot degreasing furnace for hot degreasing. This removes residual polyoxymethylene and / or high-density polyethylene and / or ethylene-vinyl acetate copolymer from the green blanks, thereby obtaining high-purity brown blanks.

[0059] Specifically, during extraction and defatting, it is only effective on paraffin. The organic solvent first wets and dissolves the paraffin, thereby forming pores on the embryo. The paraffin / solvent mixture diffuses outward along the concentration gradient and is carried away, forming open channels within the embryo skeleton.

[0060] In one embodiment of the present invention, the organic solvent is selected from one or more of n-heptane, aviation kerosene, and white oil.

[0061] In one embodiment of the present invention, the extraction temperature is 60-80℃ and the extraction time is 6-8h.

[0062] In one embodiment of the present invention, the thermal degreasing temperature is 200-550℃, the heating rate is 0.5-5℃ / min, and the degreasing time is 2-8h.

[0063] Furthermore, in this application, after catalytic degreasing, the plastic-based binder system forms a porous network structure within the green embryo. This allows high-density polyethylene and / or ethylene-vinyl acetate copolymer and / or paraffin to be more fully removed from the porous structure generated during the initial degreasing process while ensuring the stability of the embryo structure, resulting in a purer brown embryo.

[0064] Alternatively, after extraction and degreasing, the wax-based binder system forms a porous network structure within the green embryo. This allows polyoxymethylene and / or high-density polyethylene and / or ethylene-vinyl acetate copolymer to be more fully removed from the porous structure generated during the initial degreasing process, while ensuring the stability of the embryo structure, resulting in a purer brown embryo.

[0065] Furthermore, the thermal degreasing process is carried out in one or a mixture of hydrogen, nitrogen, or argon atmospheres. This avoids the bubbling or cracking of the green body caused by paraffin during thermal degreasing in plastic-based binder systems, as well as the exothermic oxidation and coking that can occur. It also avoids the cracking of the green body caused by the exothermic oxidation of formaldehyde during thermal degreasing in wax-based binder systems. Furthermore, in one embodiment of the present invention, the thermal degreasing process employs a segmented heating method.

[0066] Specifically, the temperature is first increased from room temperature to 200-250℃ at a rate of 1-2℃ / min, maintaining the shape of the green body and holding it at this temperature for 2-4 hours to remove residual organic solvents and some low-molecular-weight binders from the magnet blank. At this temperature, the paraffin in the plastic-based and wax-based binder systems completes volatilization and residual migration, preventing the paraffin from coking at high temperatures.

[0067] After completely removing the paraffin wax from the plastic-based or wax-based adhesive system, the temperature is increased to 300-450℃ at a rate of 0.5-3℃ / min and held for 2-4 hours. Finally, the temperature is increased to 500-650℃ at a rate of 0.5-1℃ / min and held for 2-4 hours to further remove the adhesive. When used in a high-temperature atmosphere and one or more of hydrogen, nitrogen, or argon, polyoxymethylene and / or high-density polyethylene and / or ethylene-vinyl acetate copolymer diffuse outwards after thermal decomposition / vaporization, while neodymium iron boron magnetic powder remains largely unoxidized under the mixed atmosphere, maintaining its superior performance.

[0068] In one embodiment of the present invention, during thermal degreasing, a sufficient amount of inert gas is required to purge the gas and prevent the accumulation of volatiles from affecting the degreasing effect. At the same time, the exhaust gas needs to be condensed and absorbed.

[0069] d) The brown embryo is sintered to form a uniformly dense coarse embryo.

[0070] Specifically, sintering refers to sintering the degreased brown embryos in a vacuum sintering furnace.

[0071] In one embodiment of the present invention, the sintering temperature is 1000-1100℃ and the holding time is 2-3h.

[0072] In one embodiment of the present invention, inert gas nitrogen or argon is introduced during sintering to prevent oxidation and gas absorption of the brown embryo; the magnetic powder particles in the brown embryo gradually become denser through atomic diffusion and rearrangement to form a rough embryo with good magnetic properties.

[0073] Furthermore, in one embodiment of the present invention, the magnet after sintering needs to undergo aging treatment.

[0074] In one embodiment of the present invention, the first-stage aging temperature of the sintered rough blank is 900-950℃, and the holding time is 2-5h; the second-stage aging temperature is 400-450℃, and the holding time is 1-2h. The rough blank is subjected to aging treatment to eliminate the stress inside the magnet, adjust the microstructure of the grain boundary phase of the magnet, and further improve the magnetic properties and stability of the magnet.

[0075] In one embodiment of the present invention, after the blank has been aged, it can be ground to make the blank size and appearance meet the requirements.

[0076] e) Magnetize the blank to obtain the multipole NdFeB magnet.

[0077] Specifically, the sintered and tempered magnets are placed in a magnetizer for magnetization. First, the magnets are weakly magnetized to confirm the magnetic pole direction. Then, they are placed on a multi-pole magnetizing coil for full magnetization. By controlling the direction and intensity of the magnetizing magnetic field, the magnets achieve the desired magnetic pole distribution and magnetic properties. For stator magnets of multi-pole motors, the magnetization parameters are precisely controlled according to their design requirements to achieve multi-pole magnetization.

[0078] Example 1 We provide 1000g of NdFeB magnetic powder with an average particle size of 6-10μm. The intrinsic coercivity (Hcj) of the NdFeB magnetic powder is ≥1250kA / m, and the remanence (Br) is ≥1.35T.

[0079] Provide a plastic-based adhesive system: Select 625g of a combination of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and paraffin wax, with the following addition ratios: polyoxymethylene 89%, high-density polyethylene 5%, ethylene-vinyl acetate copolymer 2%, and paraffin wax 2.5%. At the same time, add 1.0% stearic acid lubricant and 0.5% antioxidant. Mix the above adhesive, coupling agent, and antioxidant to obtain a mixed additive.

[0080] The above-mentioned mixed additive was divided into three portions. All the NdFeB magnetic powder and the first portion of the mixed additive were added to a twin-screw internal mixer and stirred. The screw speed was set to 100 r / min and the mixing temperature was 200℃. After mixing for 6 minutes, the second portion of the mixed additive was added, and mixing continued for another 6 minutes. Then, the third portion of the mixed additive was added. The mixing time was 20 minutes, and the final melt flow index was controlled at 130 g / 10min.

[0081] The intensively mixed magnetic powder-binder composite was transferred to a twin-screw extruder with an inert gas atmosphere for granulation under inert gas protection. The extruder temperature was 230℃ and the screw speed was 120r / min. After pelleting, particles with a diameter of about 2.5mm were obtained.

[0082] The granules were added to an injection molding machine equipped with an orientation device. The barrel temperature was set to 180℃, and the mold temperature to 85℃. Two seconds before injection, an orientation magnetic field of 0.6T and a magnetic field current of 200A were applied. Then, the molten granules were injected into the mold. The magnetization time was 10 seconds, the injection pressure was 100MPa, and the injection speed was 40cm / s. 3 / s, holding pressure for 12s, to obtain the embryo.

[0083] The green embryos were degreased using a two-step method: catalytic degreasing and thermal degreasing. Catalytic degreasing used nitric acid vapor at a temperature of 110℃ for 5 hours. Thermal degreasing was then carried out under a hydrogen atmosphere, consisting of two stages. The first stage degreasing was performed at 250℃ with a heating rate of 1℃ / min for 4 hours. The second stage degreasing was performed at 550℃ with a heating rate of 0.5℃ / min for 6 hours, resulting in brown embryos.

[0084] The degreased brown embryos were placed in a vacuum sintering furnace at a vacuum degree of 5×10⁻. 4 Under Pa conditions, the sintering temperature was 1050℃, and the holding time was 2.5h. Then, solution treatment was performed at 1010℃ for 1.5h. The aging temperature was 900℃, and the aging time was 5h.

[0085] The comprehensive performance and orientation energy consumption of the neodymium iron boron magnet obtained in this embodiment are shown in the table below. 1 11.2 11.8 30.5MGOe 94% 0.6T Approximately 118 kJ Example 2 We provide 1000g of NdFeB magnetic powder with an average particle size of 6-10μm. The intrinsic coercivity (Hcj) of the NdFeB magnetic powder is ≥1250kA / m, and the remanence (Br) is ≥1.35T.

[0086] Provide a wax-based binder system: Select a combination of paraffin wax, polyoxymethylene, high-density polyethylene, and ethylene-vinyl acetate copolymer totaling 625g, with the following addition ratios: paraffin wax 80%, high-density polyethylene 10%, and ethylene-vinyl acetate copolymer 8.5%. At the same time, add lubricant stearic acid 1.0% and antioxidant 0.5%. Mix the above binder, coupling agent, and antioxidant to obtain a mixed additive.

[0087] The above-mentioned mixed additive was divided into three portions. All the NdFeB magnetic powder and the first portion of the mixed additive were added to a twin-screw internal mixer and stirred. The screw speed was set to 100 r / min and the mixing temperature was 190℃. After mixing for 6 minutes, the second portion of the mixed additive was added, and mixing continued for another 6 minutes. Then, the third portion of the mixed additive was added. The mixing time was 20 minutes, and the final melt flow index was controlled at 130 g / 10min.

[0088] The intensively mixed magnetic powder-binder composite was transferred to a twin-screw extruder with an inert gas atmosphere for granulation under inert gas protection. The extruder temperature was 230℃ and the screw speed was 120r / min. After pelleting, particles with a diameter of about 2.5mm were obtained.

[0089] The granules were added to an injection molding machine equipped with an orientation device. The barrel temperature was set to 180℃, and the mold temperature to 85℃. Two seconds before injection, an orientation magnetic field of 0.8T and a magnetic field current of 240A were applied. The molten granules were then injected into the mold. The magnetization time was 8 seconds, the injection pressure was 100MPa, and the injection speed was 40cm. 3 / s, holding pressure for 12s, to obtain the embryo.

[0090] The green embryos were degreased using a two-step method: solvent extraction and thermal degreasing. Solvent extraction degreasing involved dissolving paraffin in n-heptane at 70°C for 7 hours. Subsequently, thermal degreasing was performed under a hydrogen atmosphere, comprising three stages: the first stage at 200°C with a heating rate of 2°C / min for 4 hours; the second stage at 500°C with a heating rate of 1°C / min for 6 hours; and the third stage at 650°C with a heating rate of 1°C / min for 6 hours, ultimately yielding brown embryos.

[0091] The degreased brown embryos were placed in a vacuum sintering furnace at a vacuum degree of 5×10⁻. 4 Under Pa conditions, the sintering temperature was 1070℃, and the holding time was 2.5h. Then, a solution treatment was performed at 1030℃ for 1.5h. The aging temperature was 900℃, and the aging time was 5h.

[0092] The comprehensive performance and orientation energy consumption of the neodymium iron boron magnet obtained in this embodiment are shown in the table below. 2 11.1 12.3 30.8MGOe 95% 0.8T Approximately 133 kJ Comparative Example 1 We provide 1000g of NdFeB magnetic powder with an average particle size of 6-10μm. The intrinsic coercivity (Hcj) of the NdFeB magnetic powder is ≥1250kA / m, and the remanence (Br) is ≥1.35T.

[0093] Provide a wax-based binder system: Select a combination of paraffin wax, polyoxymethylene, high-density polyethylene, and ethylene-vinyl acetate copolymer totaling 625g, with the following addition ratios: paraffin wax 80%, high-density polyethylene 10%, and ethylene-vinyl acetate copolymer 8.5%. At the same time, add lubricant stearic acid 1.0% and antioxidant 0.5%. Mix the above binder, coupling agent, and antioxidant to obtain a mixed additive.

[0094] The above-mentioned mixed additive was divided into three portions. All the NdFeB magnetic powder and the first portion of the mixed additive were added to a twin-screw internal mixer and stirred. The screw speed was set to 100 r / min and the mixing temperature was 190℃. After mixing for 6 minutes, the second portion of the mixed additive was added, and mixing continued for another 6 minutes. Then, the third portion of the mixed additive was added. The mixing time was 20 minutes, and the final melt flow index was controlled at 130 g / 10min.

[0095] The intensively mixed magnetic powder-binder composite was transferred to a twin-screw extruder with an inert gas atmosphere for granulation under inert gas protection. The extruder temperature was 230℃ and the screw speed was 120r / min. After pelleting, particles with a diameter of about 2.5mm were obtained.

[0096] The granules were added to an injection molding machine equipped with an orientation device. The barrel temperature was set to 180℃, and the mold temperature to 85℃. An orientation magnetic field of 1.5T and a magnetic field current of 350A were applied simultaneously with injection. Molten granules were then injected into the mold. The magnetization time was 14 seconds, the injection pressure was 100MPa, and the injection speed was 40cm. 3 / s, holding pressure for 12s, to obtain the embryo.

[0097] The green embryos were degreased using a two-step method: solvent extraction and thermal degreasing. Solvent extraction degreasing involved dissolving paraffin in n-heptane at 70°C for 7 hours. Subsequently, thermal degreasing was performed under a hydrogen atmosphere. This thermal degreasing consisted of two stages: the first stage was performed at 200°C with a heating rate of 2°C / min for 4 hours; the second stage was performed at 500°C with a heating rate of 1°C / min for 6 hours; and the third stage was performed at 650°C with a heating rate of 1°C / min for 6 hours, ultimately yielding brown embryos.

[0098] The degreased brown embryos were placed in a vacuum sintering furnace at a vacuum degree of 5×10⁻. 4 Under Pa conditions, the sintering temperature was 1070℃, and the holding time was 2.5h. Then, a solution treatment was performed at 1030℃ for 1.5h. The aging temperature was 900℃, and the aging time was 5h.

[0099] The comprehensive performance and orientation energy consumption of the neodymium iron boron magnet obtained in this embodiment are shown in the table below. 2 11.4 12.3 31.2MGOe 95% 1.5T Approximately 507 kJ By comparing Examples 1 and 2 with Comparative Example 1, it can be seen that turning on the orientation magnetic field before injection molding can achieve an orientation degree close to that of a stronger magnetic field that is turned on for a longer time during injection molding, with a shorter magnetic field duration. This results in a magnetic energy product close to that of the neodymium iron boron magnet prepared after orientation with a stronger magnetic field.

[0100] In some applications where the energy product requirement is not high, the fabrication process of NdFeB magnets uses a small current to maintain a weak magnetic field for a short time to orient the NdFeB magnet. Although the energy product is slightly smaller than that of magnets oriented by a stronger magnetic field, the NdFeB magnets obtained by the fabrication method in this application require less energy and are therefore more suitable for mass production to achieve cost savings.

[0101] At the same time, it should be clarified that applying a magnetic field in advance is not done aimlessly. Rather, it is necessary to take into account the technical limitations of maintaining a large current in the magnetic field and the energy consumption generated by maintaining a large current in the magnetic field. If the magnetic field is applied in advance before injection without constraining the application time, the duration of magnetic field maintenance will inevitably be longer, and the overall energy consumption will not be much different from that of a magnetic field applied simultaneously during injection.

[0102] It is understood that those skilled in the art can make equivalent substitutions or changes to the technical solution and inventive concept of the present invention, and all such changes or substitutions should fall within the protection scope of the present invention.

Claims

1. A method for preparing a multipole neodymium iron boron magnet, characterized in that, Includes the following steps: a) Provide neodymium iron boron magnetic powder and a binder system, mix them and granulate to obtain granular material; b) The granular material is melted by an injection molding machine and injected into the cavity of a mold to obtain a green body of a predetermined shape; c) Remove the binding system from the green embryo to obtain a brown embryo; d) The brown embryo is sintered to form a uniformly densified coarse embryo; as well as e) Magnetize the blank to obtain the multipole NdFeB magnet; In step b), a multipole magnetic field generated by an electromagnetic coil is applied to the periphery of the mold, and the temperature of the mold is controlled at 80-85°C. The process of applying the multipolar magnetic field includes: turning on the electromagnetic coil 2-3 seconds before the injection action of the injection molding machine, so that the magnetic field reaches a preset stable magnetic induction intensity before the molten granules enter the cavity, and the current is 200-400A; after the injection starts, the multipolar magnetic field is continuously applied, and the current is cut off after the magnetic powder is fully oriented, so that the total application time of the multipolar magnetic field is controlled within 6-10 seconds.

2. The method for preparing a multipole NdFeB magnet according to claim 1, characterized in that, In the raw materials provided in step a), the average particle size of the neodymium iron boron magnetic powder is 6-10 μm; The bonding system comprises an adhesive, a coupling agent, and an antioxidant, wherein the adhesive is selected from one or more of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and paraffin wax.

3. The method for preparing a multipole NdFeB magnet according to claim 2, characterized in that, The mass fraction of each component in the bonding system is a fraction of the mass of the magnetic powder: Polyoxymethylene: 50-80%, or high-density polyethylene: 5-15%, or ethylene-vinyl acetate copolymer: 1-5%, or paraffin wax: 5-20%; Coupling agent: 0.5-1%, wherein the coupling agent is silane coupling agent KH550 and / or stearic acid; Antioxidant: 0.2-0.5%.

4. A method for preparing a multipole NdFeB magnet according to claim 2 or 3, characterized in that, The mixing and granulation in step a) are carried out under inert gas protection, and the binder is added in stages so that the melt index of the obtained granules is controlled at 100-200 g / 10min.

5. The method for preparing a multipole NdFeB magnet according to claim 1, characterized in that, During the injection molding process in step b), the injection temperature is 150-280℃, the injection pressure is 80-120MPa, and the injection speed is 30-50 cm³ / s.

6. The method for preparing a multipole NdFeB magnet according to claim 3, characterized in that, The degreasing process in step c) is selected according to the type of adhesive: When the main component of the adhesive is a plastic-based system, a two-step method combining catalytic degreasing and thermal degreasing is used; or When the main component of the adhesive is a wax-based system, a two-step method combining extraction degreasing and thermal degreasing is adopted.

7. The method for preparing a multipole NdFeB magnet according to claim 6, characterized in that, The thermal degreasing stage is carried out in an atmosphere of hydrogen, nitrogen, argon or a combination thereof, and adopts a segmented heating program: first, heating at 200-250℃ for 2-4 hours, and then heating at 400-550℃ for 3-6 hours.

8. The method for preparing a multipole NdFeB magnet according to claim 1, characterized in that, The sintering in step d) is carried out in an inert atmosphere at 1000-1100℃ for 2-3 hours.

9. A method for preparing a multipole NdFeB magnet according to claim 8, characterized in that, The method further includes aging the blank after the sintering step.

10. A multipole neodymium iron boron magnet, characterized in that, It is prepared by any one of the preparation methods of a multipole neodymium iron boron magnet according to any one of claims 1-9.

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