Process for manufacturing high energy product low oxygen neodymium-iron-boron magnets

Abstract

The application discloses a manufacturing process of a high-magnetic-energy-product low-oxygen neodymium-iron-boron magnet, which comprises the steps of batching, vacuum smelting, hydrogen crushing, airflow milling, magnetic field forming, isostatic pressing, microwave sintering and tempering, and surface treatment. A fluoride-rare earth composite additive composed of lithium fluoride and neodymium fluoride is added in the batching stage, microwave is used as a heating source in the microwave sintering stage, and the sintering temperature and holding time are controlled, and the microstructure of the magnet is optimized in cooperation with the tempering process. The lithium fluoride in the composite additive can reduce the sintering activation energy, laying a foundation for low-temperature sintering, and the neodymium fluoride can combine with oxygen impurities in the magnetic powder to generate low-melting-point oxyfluorides, thereby avoiding the formation of non-magnetic oxide phases. Microwave heating solves the temperature gradient problem of resistance furnace sintering, synchronously heats the inside of the blank, and enables the composite additive to fully play the roles of deoxidization and densification. The finally-prepared neodymium-iron-boron magnet has the characteristics of low oxygen content, high density and excellent magnetic performance, and has a low coercive force decay rate in a high-temperature environment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of neodymium-iron-boron magnet manufacturing, in particular to a manufacturing process of high-magnetic-energy-product low-oxygen neodymium-iron-boron magnet. BACKGROUND

[0002] As a core functional material in the field of high-end equipment, the magnetic energy product and use stability of the neodymium-iron-boron magnet directly determine the performance upper limit of the downstream equipment. With the increasing demand for miniaturization and high power density of magnets in new energy vehicles and large-scale wind power equipment, the requirements for the manufacturing process of the magnet are gradually increasing.

[0003] In the current industry, the traditional high-temperature sintering process is the mainstream way to prepare the neodymium-iron-boron magnet, which needs to be kept at a high temperature of 1050-1150℃ for 4-6 hours. The energy consumption of this process accounts for more than 30% of the whole manufacturing process, and the high-temperature environment easily leads to the volatilization of Nd elements in the magnet and the combination with oxygen to form a rich-Nd grain boundary phase defect, which destroys the main phase structure of the magnet and significantly reduces the long-term use stability of the magnet.

[0004] To solve the energy consumption and performance problems of traditional high-temperature sintering, the industry tries to introduce low-temperature sintering additives, such as LiF, fluoride additives such as LiF, NaF, KF, and LiCl, to reduce the sintering temperature to 950-1000℃. However, when using such additives alone, the low-temperature environment significantly slows down the atomic diffusion rate inside the magnet, resulting in insufficient sintering of the magnet, generally low density, and significant decay of key performance indicators such as magnetic energy product and coercive force, making it difficult to meet the requirements of high-end fields for magnet quality. At the same time, whether or not to add additives, the traditional sintering process has the problem of temperature gradient from outside to inside, that is, the inside and surface of the magnet blank are not uniformly heated, which easily causes uneven distribution of grain size, further aggravates the performance fluctuation of the magnet batch, and affects the stability of large-scale production.

[0005] In addition, the problem of magnet grain boundary oxidation in the existing process is always difficult to solve. The grain boundary oxidation rate of the magnet after traditional sintering usually maintains at 8%-10%, and the coercive force decay rate of the magnet at a high temperature environment such as 150℃ can reach 12%, which cannot adapt to the scene demand of new energy vehicles, industrial servo motors and other long-term operation in medium and high temperature environments.

[0006] Therefore, it is necessary to provide a manufacturing process of high-magnetic-energy-product low-oxygen neodymium-iron-boron magnet to solve the above technical problems. SUMMARY

[0007] The purpose of the present application is to provide a manufacturing process of high-magnetic-energy-product low-oxygen neodymium-iron-boron magnet to solve the problems of high oxygen content, high energy consumption, poor magnetic performance and high-temperature stability in the existing manufacturing process.

[0008] To achieve the above object, the application provides the following technical scheme: a manufacturing process of high-magnetic-energy-product low-oxygen neodymium-iron-boron magnet, comprising the following steps:

[0009] (1) ingredient: according to atomic percentage, weigh 75%-82% of iron, 12%-18% of neodymium and 5%-8% of boron as main raw materials, 0.5%-3% of cerium as auxiliary element raw material, and add fluoride-rare earth compound additive to mix uniformly to obtain mixed raw materials; the fluoride-rare earth compound additive is a mixture of lithium fluoride and neodymium fluoride, and the addition amount is 0.3%-0.8% of the total mass of the raw materials;

[0010] (2) vacuum melting: melt the mixed raw materials obtained in step (1) in a vacuum environment to form an alloy liquid, and quickly cool to form an alloy sheet;

[0011] (3) hydrogen crushing: crush the alloy sheet obtained in step (2) into micron-sized particles in a hydrogen environment;

[0012] (4) airflow mill: crush and refine the micron-sized particles obtained in step (3) to 3-5 μm magnetic powder;

[0013] (5) magnetic field forming: in a rotating pulse magnetic field, the 3-5 μm magnetic powder obtained in step (4) is formed by gradient pressure to obtain a magnet blank;

[0014] (6) isostatic pressing: uniformly press the magnet blank obtained in step (5) to improve the density;

[0015] (7) microwave sintering and tempering: the blank treated by isostatic pressing in step (6) is sintered at 900-950 ℃ for 1-2 h under microwave heating, and is tempered;

[0016] (8) surface treatment: after mechanical processing of the sintered and tempered magnet obtained in step (7), at least one of electroplating, organic coating and phosphating treatment is used to perform surface protection treatment on the magnet.

[0017] Preferably, the mass ratio of lithium fluoride to neodymium fluoride in the fluoride-rare earth compound additive in step (1) is 1:3-1:5.

[0018] Preferably, the preparation method of the fluoride-rare earth compound additive in step (1) is:

[0019] a1: select lithium fluoride with a purity of not less than 99% and neodymium fluoride with a purity of not less than 99.5%, and weigh according to a mass ratio of 1:3-1:5 for standby;

[0020] a2: put the weighed lithium fluoride and neodymium fluoride in step a1 into a sealed stirring container and mix uniformly to obtain a premix.

[0021] a3: The premix of step a2 is loaded into an alumina crucible, the crucible is placed in a high-temperature furnace and the furnace body is sealed; pure argon gas with a purity of not less than 99.99% is introduced into the high-temperature furnace, the air in the furnace is first exhausted by gas flow, and then the gas flow is adjusted to 1L / min-2L / min to maintain an inert atmosphere; then the furnace temperature is raised to 700℃-800℃ at a heating rate of 5℃ / min-10℃ / min, and the temperature is kept for 2h-3h, and the furnace temperature fluctuation is controlled within ± 10℃ during the holding process;

[0022] a4: After the holding process is completed, the heating is stopped, the argon gas is kept flowing, and the high-temperature furnace air cooling device is turned on to cool the crucible and the internal material to room temperature, and the cooling time is 2h-4h;

[0023] a5: The cooled material is taken out of the crucible and put into a ball mill, the ball-to-material ratio is 5:1-8:1, and the material is ground at a speed of 300r / min-500r / min for 2h-4h to obtain a primary crushed material; the primary crushed material is sieved through a vibrating screen with a particle size of 1μm-5μm, the coarse material on the screen is returned to the ball mill for regrinding, and the sieved material is the fluoride-rare earth composite additive.

[0024] In the batching stage, iron 75%-82%, neodymium 12%-18%, and boron 5%-8% are accurately weighed as main raw materials based on atomic percentage. This composition ratio is based on the stoichiometric design of the crystal structure of the phase to ensure that the atomic number ratio of iron, neodymium, and boron meets the formation requirements of the phase. Meanwhile, the fluoride-rare earth composite additive is compounded by lithium fluoride and neodymium fluoride in a mass ratio of 1:3-1:5. Among them, lithium fluoride can reduce the sintering activation energy, and the lithium ions in its crystal lattice can penetrate into the gap between the magnetic powder particles during the subsequent heating process, reducing the energy threshold of atomic diffusion and creating conditions for low-temperature sintering; neodymium fluoride can specifically inhibit the oxidation reaction, as the electronegativity of neodymium is higher than that of iron, it can preferentially react with the residual oxygen impurities in the magnetic powder to form low-melting-point NdOF compounds. On the one hand, the NdOF compounds can avoid the formation of non-magnetic phase between oxygen and neodymium elements in the main phase of the magnet, on the other hand, they can form liquid phase channels during sintering to promote material migration and densification between magnetic powder particles. Through the above specific proportioning, the problem of insufficient deoxidation ability of single lithium fluoride additive, which can lead to abnormal grain growth, can be avoided, and the defects of single neodymium fluoride additive, such as limited effect on reducing sintering activation energy and insufficient atomic diffusion rate in low-temperature environment, can be compensated for, providing a prerequisite for the full play of the subsequent microwave heating process. As a preferred, the vacuum degree of the vacuum melting in step (2) is not less than 10-3Pa.

[0025] As a preferred, the vacuum degree of the vacuum melting in step (2) is not less than 10-3Pa.​ , the melting temperature is 1450℃-1550℃, and the cooling rate is not less than .

[0026] Preferably, the hydrogen pressure in the hydrogen crushing in step (3) is 0.2-0.4 MPa, the holding time is 2-4 h, and the hydrogen release rate is 0.05-0.1 MPa / min.

[0027] Preferably, the strength of the rotating pulsed magnetic field in step (5) is 1.8-2.2 T, and the frequency is 5-10 Hz.

[0028] Preferably, the implementation steps of the gradient pressure in step (5) are as follows:

[0029] b1: The 3-5 μm magnetic powder obtained in step (4) is loaded into the cavity of the magnetic field forming mold, the mold is closed and fixed on the rotating pulsed magnetic field forming machine, the equipment is started and a rotating pulsed magnetic field with a strength of 1.8-2.2 T and a frequency of 5-10 Hz is applied;

[0030] b2: The pressure is increased to 140-160 MPa at a rate of 4-6 MPa / s, and the pressure is maintained for 25-35 s;

[0031] b3: The pressure is continuously increased to 210-230 MPa at a rate of 7-9 MPa / s, and the pressure is maintained for 55-65 s;

[0032] b4: The pressure is decreased to 170-190 MPa at a rate of 9-11 MPa / s, and the pressure is maintained for 25-35 s;

[0033] b5: The pressure is decreased to normal pressure at a rate of 14-16 MPa / s, the rotating pulsed magnetic field is turned off, and the mold is opened to take out the magnet blank.

[0034] Preferably, the pressure of the isostatic pressing in step (6) is 150-200 MPa, and the holding time is 10-20 min.

[0035] Preferably, the implementation steps of the microwave sintering in step (7) are as follows:

[0036] c1: The distance between the magnet blanks after isostatic pressing is ≥5 mm, the oxygen content in the furnace is controlled to be ≤5 ppm, and the inert gas flow is maintained at 2-3 L / min;

[0037] c2: The microwave frequency is 2.4-2.5 GHz, the initial power is 480-520 W, the furnace temperature is monitored, the target temperature is 900-950℃, and the heating rate is 50-100℃ / min.

[0038] c3: during the temperature rising process, the power is increased to 600-700 W at 480-520 °C, and adjusted to 700-800 W at 900-950 °C, the temperature rising rate deviation is ≤±5 °C / min, and the oxygen content is always ≤5 ppm;

[0039] c4: after reaching 900-950 °C, the frequency and power are kept, and the sintering is kept constant for 1-2 h;

[0040] c5: after the sintering is finished, the heating is stopped, the inert gas is passed, the natural cooling is performed before 500 °C, the accelerated cooling is performed below 500 °C to room temperature, and the magnet is taken out.

[0041] In the sintering stage, the traditional resistance furnace sintering relies on the external heat conduction, which is easy to produce the temperature gradient of external heat and internal cooling, causing the surface grain of the blank to be coarse and the internal sintering to be insufficient; while the microwave heating can make the molecules in the blank vibrate synchronously to generate heat, realizing the uniform heating of the whole blank. The uniformity of this heating is matched with the demand of the action of the composite additive, because after the composite additive reduces the sintering activation energy, the requirement of the atomic diffusion process for the temperature uniformity is significantly improved, and the microwave heating can ensure that the temperature deviation of each region of the blank is within ±5 °C, so as to promote the uniform distribution of the NdOF liquid phase generated by the composite additive in the blank, effectively avoiding the aggregation of impurity phases caused by excessive local liquid phase, and the problem of residual pores caused by insufficient local liquid phase.

[0042] At the same time, the temperature range of 900-950 °C set in the microwave sintering provides the optimal temperature condition for the synergistic effect of the composite additive and the microwave heating. This temperature range is significantly lower than the traditional sintering temperature of 1050-1150 °C, which can reduce the high-temperature volatilization loss of neodymium elements, and the neodymium fluoride in the composite additive can supplement the volatilized neodymium elements; at the same time, this temperature can meet the demand of the composite additive to fully play its role: if the temperature is lower than this range, the composite additive cannot generate sufficient NdOF liquid phase, and the atomic diffusion process is insufficient; if the temperature is higher than this range, it will destroy the stable phase structure formed by the composite additive, resulting in the performance degradation of the magnet. In addition, the oxygen content in the furnace is strictly controlled to be ≤5 ppm during the microwave sintering process, which forms two protections with the deoxidization effect of the composite additive. The composite additive preferentially removes the oxygen impurities in the magnetic powder, and the inert gas protects the external air from penetrating.

[0043] In the sintering temperature rising and constant temperature stage, when the temperature rises to 480-520℃, the microwave power is increased from 480-520W to 600-700W, so as to accelerate the penetration rate of lithium ions in the composite additive to the gap between the magnetic powder particles, promote the combination of lithium ions and the magnetic powder particles to reduce the sintering activation energy; when the temperature rises to 900-950℃ and the microwave power is adjusted to 700-800W, the composite additive is completely converted into NdOF liquid phase, and the microwave stable power output can ensure the uniform flow and filling of the NdOF liquid phase in the blank gap, and improve the density of the magnet. Compared with the traditional sintering which needs high temperature for 4-6h to realize densification, the present application can make the magnet density reach ≥96% only by 1-2h through the combined action of the composite additive and microwave heating, which not only reduces the energy consumption, but also avoids the abnormal grain growth caused by long time high temperature.

[0044] As preferred, the tempering treatment in step (7) is implemented by the following steps:

[0045] d1: the distance between the magnets after microwave sintering is ≥3mm, the vacuum degree in the furnace is , the purity of the argon gas is ≥99.999%, and the argon gas is introduced to 0.101-0.102MPa, and the argon gas flow is maintained at 1-2L / min;

[0046] d2: the temperature is increased to 890-910℃ at a rate of 30-50℃ / min, and the temperature is kept constant for 2-3h after reaching the target temperature;

[0047] d3: the heating is turned off, the argon gas is kept flowing, and the furnace is cooled to 500-550℃ at a cooling rate of ≤20℃ / min;

[0048] d4: when the furnace temperature drops to 500-550℃, the heating is started, the temperature is increased to 490-510℃ at a rate of 20-30℃ / min, the fluctuation is kept at ≤±3℃ after reaching the target temperature, and the temperature is kept constant for 1-2h;

[0049] d5: the heating is turned off, the argon gas is kept flowing, the cooling is accelerated when the temperature is lower than 300℃, the air speed is 2-3m / s, the cooling is to room temperature at a cooling rate of ≤30℃ / min, the argon gas and the vacuum are turned off, and the magnet is taken out.

[0050] In the subsequent tempering stage, during the first tempering process, the magnet is heated to 890℃~910℃ and held at that temperature. The residual NdOF in the composite additive can synergistically interact with the neodymium-rich phase in the magnet, promoting grain boundary refinement. During the second tempering process, the magnet is heated to 490℃~510℃ and held at that temperature. With the help of the unreacted components in the composite additive, pinning points are formed at the grain boundaries of the magnet. These pinning points can hinder the movement of magnetic domain walls, thereby improving the coercivity of the magnet. In addition, the magnet has a uniform microstructure after microwave sintering. This structure ensures that temperature fluctuations of ≤±3℃ during tempering can be uniformly applied to every grain of the magnet, effectively avoiding the problem of inconsistent tempering effects caused by the uneven microstructure of the magnet in traditional sintering.

[0051] Compared with the prior art, the beneficial effects of the present invention are:

[0052] 1. The manufacturing process of the high magnetic energy product, low oxygen NdFeB magnet provided by this invention utilizes the preferential combination of NdFeB fluoride with oxygen in the composite additive to generate low-melting-point NdOF, thus avoiding non-magnetic properties. The formation of lithium fluoride reduces the sintering activation energy, and combined with the uniform heating characteristics of microwave heating, ensures the stability of the main phase of the magnet. Uniform growth and optimized grain boundary structure significantly improve magnetic energy product and coercivity. In the subsequent tempering stage, the residual components of composite additives and the uniform microstructure formed by microwave sintering work together to refine the grain boundaries in the first tempering and form magnetic domain wall pinning points in the second tempering, effectively reducing the attenuation of coercivity at high temperature and meeting the long-term stable operation requirements of high-end equipment.

[0053] 2. The manufacturing process of high energy product low oxygen NdFeB magnets provided by this invention significantly reduces the sintering temperature by using composite additives. Microwave heating achieves efficient heating through intramolecular frictional heat generation, eliminating the need for long-term high-temperature holding in traditional resistance furnaces and greatly reducing energy consumption. At the same time, the low-temperature environment reduces the volatilization of rare earth elements, and the composite additives can replenish some of the lost rare earth components, improve raw material utilization, and reduce production costs.

[0054] 3. The manufacturing process of high energy product low oxygen NdFeB magnets provided by this invention eliminates the temperature gradient of traditional sintering through microwave heating and controls the atomic diffusion rate through composite additives. The two work together to reduce the performance fluctuation of magnet batches and ensure quality consistency.

[0055] 4. The manufacturing process of high energy product and low oxygen NdFeB magnets provided by this invention removes oxygen impurities inside the magnetic powder through composite additives, and the inert gas protection during microwave sintering and tempering processes isolates external air, forming a double anti-oxidation barrier and reducing the overall oxygen content of the magnet; the subsequent surface protection treatment further isolates the corrosive environment, reduces the risk of rust during the use of the magnet, and extends its service life. Attached Figure Description

[0056] Figure 1A plot of oxygen content of the Nd-Fe-B magnets prepared by the manufacturing process provided for the inventive examples 1-3 and comparative examples 1-6;

[0057] Figure 2 A plot of density of the Nd-Fe-B magnets prepared by the manufacturing process provided for the inventive examples 1-3 and comparative examples 1-6;

[0058] Figure 3 A plot of magnetic energy product at room temperature of the Nd-Fe-B magnets prepared by the manufacturing process provided for the inventive examples 1-3 and comparative examples 1-6;

[0059] Figure 4 A plot of coercivity at room temperature of the Nd-Fe-B magnets prepared by the manufacturing process provided for the inventive examples 1-3 and comparative examples 1-6;

[0060] Figure 5 A plot of coercivity at 150℃ of the Nd-Fe-B magnets prepared by the manufacturing process provided for the inventive examples 1-3 and comparative examples 1-6;

[0061] Figure 6 A plot of coercivity decay rate at 150℃ of the Nd-Fe-B magnets prepared by the manufacturing process provided for the inventive examples 1-3 and comparative examples 1-6. DETAILED DESCRIPTION

[0062] The technical solutions of the present application will be described clearly and completely below in conjunction with specific examples. Obviously, the described examples are only some of the embodiments of the present application, but not all the embodiments. Based on the examples in the present application, all the other embodiments obtained by those skilled in the art without creative efforts are within the scope of the present application.

[0063] Example 1

[0064] The manufacturing process of the high-magnetic-energy-product low-oxygen Nd-Fe-B magnet provided in the present example comprises the following steps:

[0065] (1) batching: according to atomic percentage, weigh 78% of iron, 15% of neodymium and 5.5% of boron as main raw materials, and 1.5% of cerium as auxiliary element raw material; select lithium fluoride with a purity of 99.2% and neodymium fluoride with a purity of 99.6%, mix them according to a mass ratio of 1:4 to prepare a fluoride-rare earth compound additive, add the additive to the above raw materials according to 0.5% of the total mass of the raw materials, and mix uniformly to obtain a mixed raw material.

[0066] (2) vacuum smelting: put the mixed raw material into a vacuum smelting furnace, vacuumize to , heat to 1500℃ to melt the raw material to form an alloy liquid, and then The alloy sheet with a thickness of 0.3 mm is prepared by rapidly cooling the alloy at a cooling rate.

[0067] (3) Hydrogen decrepitation: the alloy sheet is put into a hydrogen decrepitation furnace, hydrogen is introduced to a hydrogen pressure of 0.3 MPa, and the hydrogen pressure is maintained for 3 h, then the hydrogen is released at a rate of 0.08 MPa / min to obtain micrometer-sized particles.

[0068] (4) Jet milling: the micrometer-sized particles are sent into a jet mill, and the magnetic powder is finely ground to 4 μm.

[0069] (5) Magnetic field forming: the magnetic powder is loaded into a cavity of a magnetic field forming mold, the mold is closed and fixed on a rotating pulse magnetic field forming machine, and a rotating pulse magnetic field with a strength of 2.0 T and a frequency of 8 Hz is applied; the pressure is increased to 150 MPa at a rate of 5 MPa / s, and maintained for 30 s; then the pressure is increased to 220 MPa at a rate of 8 MPa / s, and maintained for 60 s; then the pressure is decreased to 180 MPa at a rate of 10 MPa / s, and maintained for 30 s; finally, the pressure is decreased to normal pressure at a rate of 15 MPa / s, the magnetic field is turned off, and the magnet blank is taken out.

[0070] (6) Isostatic pressing: a uniform pressure of 180 MPa is applied to the magnet blank to improve the density of the blank.

[0071] (7) Microwave sintering and tempering:

[0072] Microwave sintering: the blank is put into a microwave sintering furnace, the distance between the blanks is controlled to be 5 mm, inert gas is introduced to make the oxygen content in the furnace be ≤3 ppm, and the gas flow is maintained at 2.5 L / min; the microwave frequency is set to 2.45 GHz, the initial power is 500 W, and the heating rate is 80 ℃ / min to 920 ℃; when the temperature reaches 500 ℃, the power is increased to 650 W; after the target temperature is reached, the temperature is kept constant for 1.5 h, the temperature is naturally decreased before 500 ℃, the temperature is accelerated to decrease below 500 ℃ to room temperature, and the sintered magnet is taken out.

[0073] Tempering: the sintered magnet is put into a tempering furnace, the distance between the magnets is controlled to be 3 mm, the vacuum is extracted to , argon gas with a purity of 99.999% is introduced to 0.101 MPa, the flow is maintained at 1.5 L / min; the temperature is increased to 900 ℃ at a rate of 40 ℃ / min, and the temperature is kept constant for 2.5 h; the heating is turned off, and the furnace is cooled to 520 ℃; then the temperature is increased to 500 ℃ at a rate of 25 ℃ / min, and the temperature is kept constant for 1.5 h; the heating is turned off, the argon gas is continuously introduced, the temperature is accelerated to cool to room temperature at a wind speed of 2.5 m / s when the temperature is lower than 300 ℃, the argon gas and the vacuum are turned off, and the magnet is taken out.

[0074] (8) Surface treatment: the magnet is machined, and then the surface is protected by electroplating process to obtain a high-energy product low-oxygen neodymium-iron-boron magnet.

[0075] Example 2

[0076] The embodiment provides a manufacturing process of a high-magnetic-energy-product low-oxygen neodymium-iron-boron magnet, and comprises the following steps:

[0077] (1) batching: according to atomic percentage, 75% of iron, 18% of neodymium and 5% of boron are weighed as main raw materials, and 2% of cerium is weighed as auxiliary element raw material; lithium fluoride with a purity of 99% and neodymium fluoride with a purity of 99.5% are selected, mixed according to a mass ratio of 1:3 to prepare a fluoride-rare earth composite additive, the additive is added into the raw materials at 0.3% of the total mass of the raw materials, and then mixed uniformly to obtain mixed raw materials.

[0078] (2) vacuum smelting: the mixed raw materials are put into a vacuum smelting furnace, vacuumized to 5 Pa, heated to 1450 DEG C to melt the raw materials to form an alloy liquid, and the alloy liquid is rapidly cooled at a cooling rate of 100 DEG C / s to prepare an alloy sheet with a thickness of 0.2 mm. (3) hydrogen crushing: the alloy sheet is put into a hydrogen crushing furnace, hydrogen is introduced to 0.2 MPa, and the hydrogen pressure is kept for 4 h, then the hydrogen is released at a rate of 0.05 MPa / min to obtain micron-sized particles.

[0079] (4) jet mill: the micron-sized particles are sent into a jet mill, and the particles are crushed and refined to 3 mu m magnetic powder.

[0080] (5) magnetic field forming: the magnetic powder is loaded into a cavity of a magnetic field forming mold, the mold is closed and fixed on a rotating pulse magnetic field forming machine, a rotating pulse magnetic field with a strength of 1.8 T and a frequency of 5 Hz is applied, the pressure is increased to 140 MPa at a rate of 4 MPa / s and kept for 25 s, then the pressure is increased to 210 MPa at a rate of 7 MPa / s and kept for 55 s, then the pressure is decreased to 170 MPa at a rate of 9 MPa / s and kept for 25 s, finally the pressure is decreased to normal pressure at a rate of 14 MPa / s, the magnetic field is turned off, and the magnet blank is taken out.

[0081] (6) isostatic pressing: a uniform pressure of 150 MPa is applied to the magnet blank for 10 min to improve the density of the blank.

[0082] (7) microwave sintering and tempering:

[0083] microwave sintering: the blank is put into a microwave sintering furnace, the distance between the blanks is controlled to be 5 mm, inert gas is introduced to make the oxygen content in the furnace be less than or equal to 4 ppm, and the gas flow is maintained at 2 L / min; the microwave frequency is set to 2.4 GHz, the initial power is 480 W, the heating rate is 50 DEG C / min, the temperature is increased to 900 DEG C, the power is increased to 600 W when the temperature is increased to 480 DEG C; after reaching the target temperature, the temperature is kept constant for 2 h, the temperature is naturally decreased before 500 DEG C, the temperature is accelerated to decrease to room temperature below 500 DEG C, and the sintered magnet is taken out.

[0084] microwave sintering: the blank is put into a microwave sintering furnace, the distance between the blanks is controlled to be 5 mm, inert gas is introduced to make the oxygen content in the furnace be less than or equal to 4 ppm, and the gas flow is maintained at 2 L / min; the microwave frequency is set to 2.4 GHz, the initial power is 480 W, the heating rate is 50 DEG C / min, the temperature is increased to 900 DEG C, the power is increased to 600 W when the temperature is increased to 480 DEG C; after reaching the target temperature, the temperature is kept constant for 2 h, the temperature is naturally decreased before 500 DEG C, the temperature is accelerated to decrease to room temperature below 500 DEG C, and the sintered magnet is taken out.

[0085] ​Tempering: put the sintered magnet into a tempering furnace, control the distance between magnets 3mm, vacuumize to , introduce argon with purity 99.999% to 0.101MPa, maintain the flow rate 1L / min; increase the temperature to 890℃ at the rate of 30℃ / min, keep for 3h; turn off the heating, cool down to 500℃ with the furnace; increase the temperature to 490℃ at the rate of 20℃ / min, keep for 2h; turn off the heating, continue to introduce argon, accelerate the cooling to room temperature at the air speed of 2m / s when the temperature is lower than 300℃, turn off the argon and vacuum, take out the magnet.

[0086] (8) Surface treatment: after machining the magnet, adopt the organic coating process for surface protection, obtain the high magnetic energy product low-oxygen neodymium-iron-boron magnet.

[0087] Example 3

[0088] The manufacturing process of the high magnetic energy product low-oxygen neodymium-iron-boron magnet provided in this embodiment includes the following steps:

[0089] (1) Proportioning: according to the atomic percentage, weigh iron 82%, neodymium 12%, boron 8% as the main raw materials, cerium 0.5% as the auxiliary element raw material; select lithium fluoride with purity 99.5% and neodymium fluoride with purity 99.8%, mix them according to the mass ratio 1:5 to prepare the fluoride-rare earth composite additive, add the additive to the raw materials according to 0.8% of the total mass of the raw materials, mix uniformly to obtain the mixed raw materials.

[0090] (2) Vacuum melting: put the mixed raw materials into a vacuum melting furnace, vacuumize to , increase the temperature to 1550℃ to melt the raw materials to form an alloy liquid, cool rapidly at the cooling rate of , and prepare an alloy sheet with a thickness of 0.4mm.

[0091] (3) Hydrogen crushing: put the alloy sheet into a hydrogen crushing furnace, introduce hydrogen to a hydrogen pressure of 0.4MPa, keep for 2h, then release hydrogen at the rate of 0.1MPa / min, and obtain micron-sized particles.

[0092] (4) Jet mill: send the micron-sized particles into a jet mill, crush and refine to a magnetic powder with a particle size of 5μm.

[0093] (5) Magnetic field forming: put the magnetic powder into the cavity of a magnetic field forming mold, fix it on a rotating pulse magnetic field forming machine after closing the mold, apply a rotating pulse magnetic field with a strength of 2.2T and a frequency of 10Hz; increase the pressure to 160MPa at the rate of 6MPa / s, keep for 35s; then increase the pressure to 230MPa at the rate of 9MPa / s, keep for 65s; then decrease the pressure to 190MPa at the rate of 11MPa / s, keep for 35s; finally decrease the pressure to normal pressure at the rate of 16MPa / s, turn off the magnetic field, and take out the magnet blank.

[0094] (6) Isostatic pressing: a uniform pressure of 200 MPa is applied to the magnet blank, and maintained for 20 min to improve the density of the blank.

[0095] (7) Microwave sintering and tempering:

[0096] Microwave sintering: the blank is placed in a microwave sintering furnace, the distance between the blanks is controlled to be 6 mm, inert gas is introduced to make the oxygen content in the furnace ≤2 ppm, the gas flow is maintained at 3 L / min; the microwave frequency is set to 2.5 GHz, the initial power is 520 W, the temperature is raised to 950 ℃ at a rate of 100 ℃ / min, and the power is increased to 700 W when the temperature is raised to 520 ℃; after reaching the target temperature, sintering is carried out for 1 h, and the temperature is naturally lowered before 500 ℃, and accelerated cooling is carried out below 500 ℃ to room temperature, and the sintered magnet is taken out.

[0097] Tempering: the sintered magnet is placed in a tempering furnace, the distance between the magnets is controlled to be 4 mm, the vacuum is extracted to 0.013 Pa, the argon gas with a purity of 99.999% is introduced to 0.102 MPa, and the flow is maintained at 2 L / min; the temperature is raised to 910 ℃ at a rate of 50 ℃ / min, and the temperature is maintained for 2 h; the heating is turned off, and the furnace is cooled to 550 ℃; the temperature is raised to 510 ℃ at a rate of 30 ℃ / min, and the temperature is maintained for 1 h; the heating is turned off, the argon gas is continuously introduced, the temperature is accelerated to room temperature at a wind speed of 3 m / s when the temperature is lower than 300 ℃, the argon gas and the vacuum are turned off, and the magnet is taken out.

[0098] (8) Surface treatment: the magnet is machined, and then a phosphating treatment process is used for surface protection to obtain a high-energy product low-oxygen neodymium-iron-boron magnet.

[0099] Comparative Example 1

[0100] Comparative Example 1 and Example 1 only differ in that in the step (1) of the preparation process, 78% of iron, 15% of neodymium, and 5.5% of boron are weighed as the main raw materials, 1.5% of cerium is weighed as an auxiliary element raw material, no fluoride-rare earth composite additive is added, and the mixture is uniformly mixed to obtain a mixed raw material.

[0101] Expected performance: without the action of the fluoride-rare earth composite additive, the magnetic powder and the sintering process are prone to oxidation,

[0102] the oxygen content of the magnet is significantly increased; the atomic diffusion rate is insufficient at the sintering temperature, the density of the magnet is low, and there are many pores; the main phase is easily damaged by oxidation, the magnetic energy product and the coercive force are greatly reduced, and the performance stability is poor.

[0103] Comparative Example 2

[0104] ​The difference between Comparative Example 2 and Example 1 is only that in the ingredient process of step (1), iron 78%, neodymium 15%, and boron 5.5% are weighed as main raw materials, and cerium 1.5% is weighed as auxiliary element raw material according to atomic percentage; only LiF with a purity of 99.2% is added as an additive, and the addition amount is 0.5% of the total mass of the raw materials, and the mixed raw materials are uniformly mixed.

[0105] Expected performance: It is difficult for single LiF additive to simultaneously achieve deoxidation and grain refinement synergy, and the magnet oxygen content control effect is poor; atomic diffusion is still limited at low temperature, and the density is lower than that of Example 1; and single additive is easy to cause impurity phases in some areas, and the magnetic energy product and coercivity are not as good as Example 1, and the performance fluctuates greatly.

[0106] Comparative Example 3

[0107] The difference between Comparative Example 3 and Example 1 is only that in the ingredient process of step (1), iron 78%, neodymium 15%, and boron 5.5% are weighed as main raw materials, and cerium 1.5% is weighed as auxiliary element raw material according to atomic percentage; only LiF with a purity of 99.6% is added as an additive, and the addition amount is 0.5% of the total mass of the raw materials, and the mixed raw materials are uniformly mixed.

[0108] Expected performance: Single additive has limited deoxidation effect, and the oxygen content of the magnet is higher than that of Example 1; it is difficult to effectively reduce the atomic diffusion activation energy during sintering, and the density is not obviously improved; the grain distribution of the magnet is poor, and the grains in some areas are coarse, and the magnetic energy product and coercivity are lower than those of Example 1.

[0109] Comparative Example 4

[0110] The difference between Comparative Example 4 and Example 1 is only that in the sintering and tempering process of step (7), the blank after isostatic pressing is placed in a traditional resistance furnace, inert gas is introduced to make the oxygen content in the furnace ≤3ppm, the temperature is raised to 1050℃ at a rate of 50℃ / min, and sintered for 4h; after sintering, the temperature is naturally lowered to 500℃, and then raised to 500℃ for tempering for 1.5h.

[0111] Expected performance: The traditional resistance furnace sintering has an external heat and internal cooling temperature gradient, the grain growth inside and on the surface of the magnet is uneven, and the grains in some areas are coarse; high-temperature long-time sintering leads to intensified Nd element volatilization and oxidation, and the oxygen content of the magnet increases; although there is a composite additive effect, it is still difficult to compensate for the high-temperature sintering defects, and the magnetic energy product is lower than that of Example 1, and the energy consumption is high and the production efficiency is low.

[0112] Comparative Example 5

[0113] The difference between Comparative Example 5 and Example 1 is only that in the sintering and tempering process of step (7), the temperature target is set to 850℃ during microwave sintering. ​

[0114] Expected performance: The microwave sintering temperature is too low, the composite additives cannot fully play a role, atomic diffusion is insufficient, the magnet sintering is not complete, and the density is significantly reduced; the main phase is not fully formed, there are many unsintered particles, the magnetic energy product and coercive force are greatly reduced, and the use requirements cannot be met.

[0115] Comparative Example 6

[0116] Comparative Example 6 and Example 1 only differ in that in step (7) sintering and tempering process, after microwave sintering reaches 920℃, only constant temperature sintering for 0.5h.

[0117] Expected performance: The microwave sintering holding time is too short, the inside of the blank is not fully densified, there are pores and microcracks; the composite additives and magnetic powder do not fully react, and the deoxidization and grain refinement effect is not good; the magnet density and magnetic performance are lower than Example 1, and the performance stability is poor.

[0118] In order to verify the performance difference of the neodymium-iron-boron magnets prepared in the above examples and comparative examples, the following test method is provided:

[0119] 1. Oxygen content test: using pulse heating inert gas melting-infrared absorption method, taking the magnet samples prepared in Examples 1-3 and Comparative Examples 1-6 (taking 3 different parts from each sample, the size is 5mmx5mmx5mm), placing them in a pulse heating furnace, heating to above 2000℃ in an inert gas atmosphere, releasing and converting the oxygen elements in the sample into , detecting the concentration by an infrared detector, calculating the oxygen content of the sample, and taking the average of 3 test results as the final oxygen content data.

[0120] 2. Density test: using Archimedes drainage method, first weighing the mass of each magnet sample in air (denoted as ), then completely immersing the sample in distilled water, weighing its mass in water (denoted as ), and calculating the density according to the following formula:

[0121]

[0122] Wherein water is the density of distilled water, taking ; theoretical value is the theoretical density of the main phase, taking , each sample is tested 3 times, and the average value is taken.

[0123] 3. Magnetic property test: Using permanent magnet comprehensive tester, the magnetic energy product (BH)max and coercivity Hcj of the magnet sample are tested at room temperature (25℃) and high temperature (150℃) environment respectively; during high temperature test, the sample is placed in the temperature control box for 30 min, and then tested after temperature stabilization; each sample is tested for 3 times, and the average value is taken as the final magnetic property data.

[0124] The experimental data are as follows:

[0125]

[0126] The experimental data are as follows:

[0127] In terms of oxygen content, examples 1-3 are stably controlled at a low level of 210-255 ppm, the composite additives can react with oxygen impurities in the magnetic powder to generate low-melting-point oxyfluorides, and the low-oxygen inert atmosphere of microwave sintering effectively inhibits oxidation. The oxygen content of Comparative Example 1 is as high as 890 ppm, the oxygen content of Comparative Examples 2-3 is as high as 490-720 ppm due to the use of single additives, and the oxygen content of Comparative Example 4 is as high as 490-720 ppm due to the use of traditional high-temperature sintering.

[0128] In terms of density, examples 1-3 reach 96.2%-98.1%, the composite additives reduce the sintering temperature, and the microwave heating can make the molecules in the blank vibrate synchronously to generate heat, realize uniform heating of the whole blank, promote rapid atomic diffusion, and ensure full densification. The density of Comparative Example 1 is only 88.3% due to oxidation and the absence of additives, the density of Comparative Examples 5-6 is only 85.6%-86.3% due to insufficient sintering, the density of Comparative Examples 2-3 is only 91.8%-92.1% due to the difficulty of single additives in increasing the atomic diffusion rate, and the density of Comparative Example 4 is 95.8%, close to the examples, but traditional sintering causes local micropores due to temperature difference.

[0129] In terms of magnetic properties, examples 1-3 have a room temperature magnetic energy product of 46.8-49.5 MGOe, a room temperature coercivity of 17.8-19.2 kOe, and a high-temperature coercivity decay rate of 14.1%-16.3%. Comparative Examples 1-3 and 5-6 have a low magnetic energy product of only 29.8-40.1 MGOe, a low room temperature coercivity of only 9.8-14.5 kOe, and a high-temperature coercivity decay rate of 33.0%-36.7%. Comparative Example 4 has a magnetic energy product of 42.3 MGOe and a high-temperature coercivity decay rate of 23.4% due to high-temperature sintering and coarse grains, which are far inferior to the examples.

[0130] It will be apparent to those skilled in the art that the application is not limited to the details of the above-exemplified embodiments and that the present application can be implemented in other particular forms without departing from the spirit or essential characteristics of the present application. The embodiments should therefore be considered in all respects as illustrative and not restrictive, the scope of the application being indicated by the appended claims rather than by the above description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

1. A process for producing high energy product low oxygen neodymium-iron-boron magnets, characterized by, The method comprises the following steps: (1) batching: weighing iron 75% to 82%, neodymium 12% to 18%, and boron 5% to 8% as main raw materials, and cerium 0.5% to 3% as auxiliary element raw material according to atomic percentage, and adding fluoride-rare earth composite additive to obtain mixed raw materials; the fluoride-rare earth composite additive is a mixture of lithium fluoride and neodymium fluoride, and the addition amount is 0.3% to 0.8% of the total mass of the raw materials; (2) vacuum melting: melting the mixed raw materials obtained in step (1) in a vacuum environment to form an alloy liquid, and rapidly cooling to form an alloy sheet; (3) hydrogen crushing: crushing the alloy sheet obtained in step (2) into micron-sized particles in a hydrogen environment; (4) airflow milling: crushing the micron-sized particles obtained in step (3) to 3 to 5 microns of magnetic powder; (5) magnetic field forming: in a rotating pulse magnetic field, the 3 to 5 micron magnetic powder obtained in step (4) is formed by gradient pressure forming to obtain a magnet blank; (6) isostatic pressing: applying uniform pressure to the magnet blank obtained in step (5) to improve the density; (7) microwave sintering and tempering: sintering the blank treated by isostatic pressing in step (6) at 900 to 950 DEG C for 1 to 2 hours under microwave heating, and cooperating with tempering treatment; (8) surface treatment: after machining the sintered and tempered magnet obtained in step (7), at least one of electroplating, organic coating, and phosphating treatment is used for surface protection treatment of the magnet; The preparation method of the fluoride-rare earth composite additive in step (1) is as follows: a1: selecting lithium fluoride with a purity of not less than 99% and neodymium fluoride with a purity of not less than 99.5%, and weighing according to a mass ratio of 1:3 to 1:5 for standby; a2: placing the lithium fluoride and neodymium fluoride weighed in step a1 into a sealed stirring container and uniformly mixing to obtain a premix; a3: loading the premix of step a2 into an alumina crucible, placing the crucible into a high-temperature furnace and sealing the furnace body; introducing argon with a purity of not less than 99.99% into the high-temperature furnace, first purging the air in the furnace, and then adjusting the gas flow to 1 to 2 L / min to maintain an inert atmosphere; then, the furnace temperature is raised to 700 to 800 DEG C at a heating rate of 5 to 10 DEG C / min, and the temperature is kept for 2 to 3 hours, and the temperature fluctuation during the holding process is controlled within ±5 DEG C; a4: after the holding is completed, stop heating, keep the argon flowing, and open the high-temperature furnace air cooling device to cool the crucible and the internal materials to room temperature, and the cooling time is 2 to 4 hours; a5: taking the cooled material out of the crucible, putting it into a ball mill, and grinding at a ball-to-material ratio of 5:1 to 8:1 and a speed of 300 to 500 r / min for 2 to 4 hours to obtain primary crushed material; the primary crushed material is screened through a vibrating screen with a particle size of 1 to 5 microns, the coarse material on the screen is returned to the ball mill for regrinding, and the screened material is the fluoride-rare earth composite additive.

2. The manufacturing process of claim 1, wherein, The vacuum degree of the vacuum melting in step (2) is not less than 1 x 10 -3 Pa, the melting temperature is 1450℃-1550℃, and the cooling rate is not less than 8 x 10 3 ℃ / s.

3. The manufacturing process of claim 1, wherein, The hydrogen pressure of the hydrogen crushing in step (3) is 0.2-0.4 MPa, the holding time is 2-4 h, and the hydrogen release rate is 0.05-0.1 MPa / min.

4. The manufacturing process of claim 1, wherein, The strength of the rotating pulse magnetic field in step (5) is 1.8-2.2 T, and the frequency is 5-10 Hz.

5. The manufacturing process of claim 1, wherein, The implementation steps of the gradient pressure in step (5) are as follows: b1: the 3-5 μm magnetic powder obtained in step (4) is loaded into the cavity of a magnetic field forming mold, the mold is fixed on a rotating pulse magnetic field forming machine after being closed, the equipment is started, and a rotating pulse magnetic field with a strength of 1.8-2.2 T and a frequency of 5-10 Hz is applied; b2: the pressure is increased to 140-160 MPa at a rate of 4-6 MPa / s, and the pressure is maintained for 25-35 s; b3: the pressure is continuously increased to 210-230 MPa at a rate of 7-9 MPa / s, and the pressure is maintained for 55-65 s; b4: the pressure is decreased to 170-190 MPa at a rate of 9-11 MPa / s, and the pressure is maintained for 25-35 s; b5: the pressure is decreased to normal pressure at a rate of 14-16 MPa / s, the rotating pulse magnetic field is turned off, and the mold is opened to take out the magnet blank.

6. The manufacturing process of claim 1, wherein, The pressure of the isostatic pressing in step (6) is 150-200 MPa, and the holding time is 10-20 min.

7. The manufacturing process of claim 1, wherein, The implementation steps of the microwave sintering in step (7) are as follows: c1: the distance between the magnet blanks after isostatic pressing is ≥5 mm, the oxygen content in the furnace is controlled to ≤5 ppm, and the inert gas flow is maintained at 2-3 L / min; c2: the microwave frequency is 2.4-2.5 GHz, the initial power is 480-520 W, the furnace temperature is monitored, the temperature rising target is 900-950 °C, and the temperature rising rate is 50-100 °C / min; c3: during the temperature rising process, the power is increased to 600-700 W at 480-520 °C, and the power is adjusted to 700-800 W at 900-950 °C, the temperature rising rate deviation is ≤±5 °C / min, and the oxygen content is always ≤5 ppm; c4: after reaching 900-950 °C, the frequency and power are maintained, and the constant temperature sintering is performed for 1-2 h; c5: after the sintering is completed, the heating is stopped, the inert gas is passed, the natural cooling is performed before 500 °C, the accelerated cooling is performed below 500 °C to room temperature, and the magnet is taken out.

8. The manufacturing process of claim 1, wherein, The implementation steps of the tempering treatment in step (7) are as follows: d1: the distance between the magnets after microwave sintering is ≥3mm, the vacuum degree in the furnace is ≤1x10 -3 Pa, pure argon with purity ≥99.999% is introduced to 0.101MPa-0.102MPa, the argon flow is maintained at 1L / min-2L / min; d2: the temperature is increased to 890-910 °C at a rate of 30-50 °C / min, and the temperature is maintained for 2-3 h after reaching the target temperature; d3: the heating is turned off, the argon gas is continuously passed, and the furnace is cooled to 500-550 °C at a rate of ≤20 °C / min; d4: the heating is started when the furnace temperature drops to 500-550 °C, the temperature is increased to 490-510 °C at a rate of 20-30 °C / min, the target temperature is reached, the fluctuation is ≤±3 °C, and the temperature is maintained for 1-2 h; d5: turn off the heating, continue to pass argon, accelerate the cooling when the temperature is below 300℃, the wind speed is 2m / s~3m / s, cool to room temperature and the cooling rate is ≤30℃ / min, turn off the argon and vacuum, and then take out the magnet.

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