A medium-high performance neodymium iron boron magnet and preparation method thereof

Abstract

The present invention discloses a medium-high performance neodymium iron boron and a preparation method thereof. Through the master-slave phase double alloy process, the master phase alloy is cerium iron boron with a low total rare earth content, and the slave phase is a neodymium iron boron alloy with a high total rare earth content, avoiding the entry of light rare earths in the master phase into the grain boundaries, while the surplus rare earths in the slave phase enter the grain boundaries and are recombined into rare earth-rich phases. The prepared magnet is processed into a thin magnetic sheet, and a low-melting-point HReM alloy is magnetron sputtered, realizing low-temperature grain boundary diffusion. This alloy improves the grain boundary wettability and also improves the fluidity of the rare earth-rich phase, enabling the cerium-containing neodymium iron boron master and slave phase grains to be quickly isolated during grain boundary diffusion. The low-temperature diffusion avoids the replacement of the diffused heavy rare earths by praseodymium, neodymium, and cerium elements, inhibits the exchange coupling between cerium iron boron grains, and makes the heavy rare earths free in the grain boundaries or interact with the rare earth-rich phase to form a core-shell structure on the surface layer of the master phase grains, enabling the rare earth-rich phase to be continuously and uniformly distributed, effectively enhancing the effectiveness of the grain boundary diffusion of heavy rare earth elements in improving the intrinsic coercivity of the cerium-containing magnet.

Description

Technical Field

[0001] The invention belongs to the technical field of sintered NdFeB rare earth permanent magnet materials, and more specifically, it adopts a main-auxiliary phase dual alloy process to prepare a Cerium Iron Boron magnet, and then performs grain boundary diffusion on it to prepare a low-cost, medium-performance sintered NdFeB magnet. Background Art

[0002] With the expansion of rare earth permanent magnet application and the surge in demand, key rare earth elements (Pr, Nd, Dy, Tb) are overused, while high-abundance rare earth elements (La, Ce, Y) are constantly accumulating. High-abundance rare earth permanent magnet materials can not only reduce costs, but also achieve comprehensive and balanced utilization of rare earth resources. Considering national strategic security and raw material costs, the research and development of cost-effective high-abundance rare earth permanent magnet materials is imperative.

[0003] In order to reduce costs, in recent years, many manufacturers have adopted light rare earth Ce and La to replace part of Nd in applications with low requirements for magnetic energy product, and great progress has been achieved. However, for some medium- and high-performance magnets with certain coercivity requirements, after adding Ce and La to replace part of Nd, studies have found that Ce in the grain boundary phase will become a "sponge" that absorbs heavy rare earths, reducing the efficiency of heavy rare earths in improving the coercivity of magnets, and restricting the application of light rare earth Ce and La in grain boundary diffusion magnets.

[0004] Chinese patent CN110634669A, diffusion of heavy rare earth fluoride RF 3 Elemental fluorine will preferentially capture oxygen in the magnet material to form a more stable CeOF. In the process of forming CeOF, the diffusion elements will be freed at the grain boundary, thereby improving the efficiency of grain boundary diffusion and effectively improving the coercivity of the rare earth magnet material. This invention effectively solves the replacement of Dy and Tb by Ce in the main phase, but does not solve the replacement of Dy and Tb free at the grain boundary with the main phase Pr and Nd elements, so it is impossible to reduce the amount of heavy rare earth Dy and Tb. In addition, the use of paraffin wax coated on the magnet surface in the preparation process also puts greater pressure on the vacuum system of the diffusion equipment during the subsequent diffusion process.

[0005] Chinese patent CN201910434469.9, by increasing the grain boundary diffusion temperature to the melting point of the REFe phase for diffusion, the REFe term is diffused into the phase by grain boundary diffusion. Since the diffusion temperature is already higher than the melting point of the rare earth-rich phase, the rare earth-rich phase has completely become a liquid phase when diffusing at the grain boundary, and the diffusion efficiency of rare earth elements at the grain boundary is improved. This invention effectively solves the grain boundary diffusion efficiency of cerium-containing magnets, but fails to solve the replacement of heavy rare earth elements Dy and Tb diffused into the magnet with the main phase Pr, Nd, and Ce elements. Moreover, due to the high diffusion temperature, the replacement and adsorption of Dy and Tb elements by the main phase Ce are aggravated, which also affects the effectiveness of heavy rare earth elements in improving the intrinsic coercivity of cerium-containing magnets, so the amount of heavy rare earth Dy and Tb cannot be reduced.

[0006] Chinese patent CN202011355320.0, through two-stage diffusion, first one-stage diffusion PrNd 0.86 Cu 0.1 Ga 0.04 The primary diffusion displaces part of the cerium in the rare earth-rich phase, in order to reduce the Ce and secondary diffusion Tb 0.88 PcqI 0.04 Fe 0.04 Al 0.02 Ga 0.02 The replacement and adsorption of heavy rare earth Tb, the amount and depth of cerium replaced by the primary diffusion, have an impact on the secondary diffusion, and the replaced Ce still exists in the grain boundary phase, which still affects the efficiency of heavy rare earth Tb entering by the secondary diffusion on the intrinsic coercivity of the magnet. Secondary diffusion is not convenient for industrial mass production, because a thin oxide layer will form on the surface after the primary diffusion, and grinding must be performed before the secondary diffusion can be carried out, otherwise it will affect the secondary diffusion effect, which will lead to an extension of the production cycle and an increase in production costs. Summary of the invention

[0007] In view of the problems existing in the prior art, the present invention is designed to provide a medium-to-high performance NdFeB magnet and a preparation method thereof.

[0008] This is achieved specifically through the following technical solutions:

[0009] The medium-high performance NdFeB magnet comprises a main phase alloy and an auxiliary phase alloy, wherein the main phase alloy accounts for 80-100% by weight and the auxiliary phase alloy accounts for 0-20% by weight:

[0010] The main phase alloy characteristics are as follows:

[0011] The mass percentage of the main phase alloy is (Ce x Re 1-x ) a M b Fe 100-a-b-c-d-f Bc Co d ;

[0012] Ce in the main phase alloy is cerium, and the rare earth element Re is two or more of Pr, Nd, Gd, and Y; where 0 < x < 1; preferably, 0.5 < x < 1;

[0013] M in the main phase alloy is two or more of Al, Cu, Zr, and Nb;

[0014] Among them, a = 28 - 30 wt%, b = 0 - 2.5 wt%, c = 0.85 - 0.95 wt%, d = 0.1 - 4 wt%, and the balance is pure iron;

[0015] Its secondary phase alloy has the following characteristics:

[0016] The component mass percentage of the secondary phase alloy is Re a M b Fe 100-a-b-c-d-f B c Co d ;

[0017] The rare earth element Re in the secondary phase alloy is two or more of Pr, Nd, Gd, and Y;

[0018] M in the secondary phase alloy is two or more of Al, Cu, Ga, Zr, Nb, and Ti;

[0019] Among them, a = 30 - 33 wt%, b = 0 - 3 wt%, c = 0.85 - 1 wt%, d = 0.1 - 4 wt%, and the balance is pure iron;

[0020] Preferably, the total rare earth content a of the secondary phase alloy is 30 - 33 wt%, and a is related to the magnetic properties of the target magnet to be prepared and the addition ratio of the secondary phase. The lower the addition ratio of the secondary phase, the higher the total rare earth content a of the secondary phase.

[0021] For the preparation method of the medium and high performance NdFeB described above, the steps for preparing the main phase alloy powder are as follows:

[0022] S1. Melting: Weigh and proportion the industrial pure metal raw materials according to the NdFeB alloy composition by mass percentage, and use a vacuum rapid solidification melting furnace for the target raw materials, and then rapidly quench them into alloy ingots;

[0023] S2. Hydrogen crushing: The main-phase NdFeB alloy sheet is hydrogen-crushed in a hydrogen-crushing furnace for 2 - 4 hours, then dehydrogenated for 4 - 8 hours at a dehydrogenation temperature of 450 - 550 °C. After dehydrogenation, the alloy cast sheet is cooled to 35 °C, then the cooling is stopped and it is left standing for 60 - 120 minutes. After waiting for the cooling temperature to stop rising, 100 - 300 ppm of pure oxygen is introduced into the hydrogen-crushing reaction device. At this time, the furnace has a certain temperature and the reaction kettle of the hydrogen-crushing furnace does not stop rotating. When supplementing oxygen, the coarse powder is continuously stirred to make the coarse powder absorb oxygen evenly and controllably. When the equipment shows that the oxygen inside is reduced to below 10 ppm, the addition of pure oxygen is stopped. After the oxygen content is stable without fluctuation, the cooling continues to obtain the main-phase NdFeB alloy coarse powder;

[0024] S3. Jet milling for powder making: 0.1 - 0.5% of the lubricant based on the mass of the coarse powder is added during the mixing of the main-phase NdFeB alloy coarse powder; after the main-phase alloy coarse powder is stirred and mixed, the coarse powder is further milled with a fluidized bed jet mill without oxygen supplementation to obtain the main-phase NdFeB alloy fine powder; an antioxidant is added to the obtained main-phase NdFeB alloy fine powder and mixed evenly; the SMD of the alloy powder is 2.8 - 3.2 μm, and D90 / D10 < 5.3, D10 > 1.6, and the distribution value of 3 - 10 μm > 72%.

[0025] The preparation steps of the auxiliary-phase alloy powder are as follows:

[0026] A1. Melting: The industrial pure metal raw materials are weighed and proportioned according to the NdFeB alloy composition by mass percentage, and the target raw materials are melted in a vacuum rapid solidification melting furnace and then rapidly quenched into auxiliary-phase alloy cast sheets;

[0027] A2. Hydrogen crushing: The NdFeB auxiliary-phase alloy sheet is hydrogen-crushed in a hydrogen-crushing furnace for 2 - 4 hours, then dehydrogenated for 4 - 8 hours at a dehydrogenation temperature of 450 - 550 °C. After dehydrogenation, the alloy cast sheet is cooled to 35 °C, then the cooling is stopped and it is left standing for 60 - 120 minutes. After waiting for the cooling temperature to stop rising, 100 - 300 ppm of pure oxygen is introduced into the hydrogen-crushing reaction device. At this time, the furnace has a certain temperature and the reaction kettle of the hydrogen-crushing furnace does not stop rotating. When supplementing oxygen, the coarse powder is continuously stirred to make the coarse powder absorb oxygen evenly and controllably. When the equipment shows that the oxygen inside is reduced to below 10 ppm, the addition of pure oxygen is stopped. After the oxygen content is stable without fluctuation, the cooling continues to obtain the NdFeB auxiliary-phase alloy coarse powder;

[0028] A3. Jet milling for powder making: 0.1 - 0.5% of the lubricant based on the mass of the coarse powder is added during the mixing of the NdFeB auxiliary-phase alloy coarse powder; after the auxiliary-phase alloy coarse powder is stirred and mixed, the coarse powder is further milled with a fluidized bed jet mill without oxygen supplementation to obtain the NdFeB auxiliary-phase alloy fine powder; an antioxidant is added to the obtained NdFeB auxiliary-phase alloy fine powder and mixed evenly; the SMD of the alloy powder is 2.6 - 3.0 μm, and D90 / D10 < 4.8, D10 > 1.8, and the distribution value of 3 - 10 μm > 75%.

[0029] A preparation method of medium and high performance NdFeB magnets. The steps for preparing the magnets are as follows:

[0030] S4. Mixing: Add the NdFeB matrix alloy powder and the auxiliary phase alloy powder into a stainless steel bucket in a certain proportion, and mix them using a three-dimensional mixer.

[0031] S5. Orientation and forming: Orient and form the alloy powder under the protection of nitrogen to obtain a green compact.

[0032] S6. Sintering and tempering: After the green compact is subjected to vacuum sintering and tempering treatments, it is cooled to obtain the NdFeB magnet. Sintering means sintering at 1000 - 1100 °C for 2 - 5 h, followed by tempering at 800 - 900 °C for 1 - 2 h and tempering at 450 - 550 °C for 2 - 5 h.

[0033] S7. Machining and cutting: Machine and cut the magnet into thin magnetic sheets with a thickness of 1 - 10 mm required by the customer. The magnetic sheets can be square sheets, tile-shaped sheets, or circular sheets. Debond and degrease the magnetic sheets and clean them to ensure a clean surface.

[0034] S8. Magnetron sputtering of HReM alloy: Magnetron sputter the HReM alloy on the clean surface of the magnetic sheet. Put the magnetic sheet with both sides coated into a molybdenum sintering box in a nitrogen protection box. Preferably, magnetron sputtering coating is used, and the film thickness is more uniform.

[0035] S9. Grain boundary diffusion: Put the magnetic sheet loaded into a graphite box into a sintering furnace, evacuate it to a high vacuum, heat it to 600 - 750 °C and hold for 8 - 12 h for grain boundary diffusion, and then temper at 450 - 550 °C for 2 - 5 h to obtain the medium and high performance NdFeB magnet.

[0036] Furthermore, for magnetron sputtering or evaporation of HReM alloy, its component mass percentage is HRe a Al b Cu c Ga 100-a-b-c ; HRe is Dy or Tb, a = 50 - 70 wt%, b = 5 - 30 wt%, c = 5 - 20%, and the balance is Ga. The melting point of this HReM alloy is 600 - 900 °C.

[0037] Furthermore, the low melting point of this alloy is mainly related to the relatively high mass percentage of Ga. The purity of various raw materials used for preparing the HReM alloy target is not less than 99.95%. After the target blank is manufactured by sand casting, it needs to be subjected to forging and densification treatment, and then processed into the required size, which can improve the life of the target and avoid thermal deformation of the magnetron sputtering target.

[0038] The present invention designs a main phase alloy with low total rare earth content and high abundance light rare earth content, and adds Zr element to the main phase, which effectively inhibits abnormal grain growth during high-temperature sintering caused by low melting point of light rare earth. Since the main phase with low total rare earth content and high abundance light rare earth content is adopted, the high abundance light rare earth elements are effectively prevented from entering the grain boundary. A small amount of Ce precipitates into the grain boundary and combines with the 100-300ppm high oxygen added during hydrogen crushing of the main phase alloy to form a stable Ce. 2 O 3 Phase. The present invention designs an auxiliary phase alloy with a high total rare earth content, so that the surplus rare earth elements in the auxiliary phase alloy enter the grain boundary and are reconstructed into a rare earth-rich phase. The present invention designs an HReM alloy, and magnetron sputters or evaporates the processed thin magnetic sheet to form an HReM alloy. The melting point of the alloy is relatively low, only 700-900°C, which is much lower than the melting point of pure heavy rare earths Dy and Tb. In this way, effective grain boundary diffusion can be achieved at a lower temperature, which is also beneficial to the improvement of the grain boundary diffusion rate. In addition, the Al, Cu, and Ga in the HReM alloy improve the wettability of the main phase and the grain boundary phase, and also greatly improve the fluidity of the rare earth-rich phase during grain boundary diffusion, which promotes the displacement of the magnetic domain wall at the grain boundary interface of the main phase, so that the low total rare earth content and high abundance of light rare earth The main phase grains and auxiliary phase grains with a relatively high proportion are quickly and completely wrapped and isolated by Al, Cu and Ga during the grain boundary diffusion process, which prevents the heavy rare earth Dy and Tb elements diffused into the grain boundaries from being replaced and adsorbed by the main phase Ce elements, and also avoids being replaced by Pr and Nd elements in the main phase or auxiliary phase, inhibiting the abnormal grain annealing and exchange coupling that are prone to occur in Cerium Iron Boron magnets; moreover, the entry of Al, Cu and Ga elements into the grain boundaries increases the fluidity of the rare earth-rich phase, effectively repairs the micropores and defects of the neodymium-rich phase at the grain boundaries, makes the neodymium-rich phase continuously and evenly distributed, and is also beneficial to the improvement of the intrinsic coercivity. The HReM alloy designed by the present invention is subjected to low-temperature diffusion, and the diffusion temperature is much lower than the Ce-Nd-Fe-B eutectic temperature and the melting point of Ce, which effectively avoids the substitution of heavy rare earth elements Dy, Tb, etc. when the cerium of the main phase diffuses with the grain boundary, and avoids Ce from becoming a "sponge" for absorbing heavy rare earth, which effectively improves the efficiency of heavy rare earth elements in improving the coercive force of cerium-containing magnets; and in the HReM alloy diffused from the grain boundary to the grain boundary phase, Dy and Tb elements are effectively distributed around the grain boundary, exist in the grain boundary in a free state, or interact with the rare earth-rich phase on the surface of the grain to form (PrNdDy) 2 Fe 14 B or (PrNdTb) 2 Fe 14In the B modification region, the HRe elements form a core-shell structure at the edges of the main-phase grains, preventing the heavy rare earths from replacing Pr, Nd, and Ce elements inside the main-phase or secondary-phase grains. This effectively reduces the usage of Dy and Tb elements and avoids a decrease in the remanence. At the same time, the intrinsic coercivity is significantly increased. Moreover, the use of the HReM alloy instead of pure dysprosium or pure terbium also reduces the heavy rare earth usage by 30 - 50%, achieving better results than the diffusion of pure heavy rare earth elements. Additionally, the low-temperature and short-time diffusion also reduces energy consumption. Brief Description of the Drawings

[0039] Figure 1 This is the process flow chart of the present invention;

[0040] Figure 2 This is a schematic diagram of the grain boundary situation after grain boundary diffusion. Detailed Embodiments

[0041] The following further describes the present invention in detail with specific embodiments to better understand the technical solution.

[0042] Embodiment 1

[0043] The component mass percentages of the main-phase alloy are (Ce x Re 1-x ) a M b Fe 100-a-b-c-d-f B c Co d ; Ce in the main-phase alloy is cerium, and the rare earth elements Re are Pr and Nd; where a = 30 wt%, x = 0.5, Pr = 3 wt%, Nd = 12 wt%, Ce = 15 wt%; M in the main-phase alloy is Al, Cu, Zr; b = 0.8 wt%; among them, Al = 0.4 wt%, Cu = 0.1 wt%, Zr = 0.3 wt%; B in the main-phase alloy, c = 0.92 wt%; Co in the main-phase alloy, d = 0.5 wt%; the balance is pure iron;

[0044] The steps for preparing the main-phase alloy powder are as follows:

[0045] S1. Melting

[0046] Weigh and proportion the industrial pure metal raw materials according to the component mass percentages of the NdFeB alloy, and then use a vacuum rapid solidification melting furnace for the target raw materials and quickly quench them into alloy cast sheets;

[0047] S2. Hydrogen decrepitation

[0048] The main-phase neodymium-iron-boron alloy sheets are hydrogenated using a hydrogenation crusher for 4 hours of hydrogenation crushing, and then dehydrogenated for 8 hours at a dehydrogenation temperature of 550 °C. After dehydrogenation, the alloy cast sheets are cooled to 35 °C, then the cooling is stopped and they are left standing for 120 minutes. After waiting for the cooling temperature to stop rising, 200 ppm of pure oxygen is introduced into the hydrogenation reaction device. At this time, the furnace has a certain temperature and the reaction kettle of the hydrogenation crusher does not stop rotating. During oxygen supplementation, the coarse powder is continuously stirred to make the coarse powder absorb oxygen evenly and controllably. When the equipment shows that the oxygen inside is reduced to below 10 ppm, the addition of pure oxygen is stopped. After the oxygen content is stable without fluctuations, the cooling is continued to obtain the main-phase neodymium-iron-boron alloy coarse powder;

[0049] S3. Grinding into powder by jet mill

[0050] During the mixing process of the main-phase neodymium-iron-boron alloy coarse powder, a lubricant accounting for 0.1% of the mass of the coarse powder is added; after the main-phase alloy coarse powder is stirred and mixed, the coarse powder is further ground using a fluidized bed jet mill without oxygen supplementation during jet milling to obtain the main-phase neodymium-iron-boron alloy fine powder; an antioxidant is added and mixed evenly into the obtained main-phase neodymium-iron-boron alloy fine powder; the SMD of the alloy powder is 3.1 μm, and D90 / D10 = 5.25, D10 = 1.65, and the distribution value of 3 - 10 μm = 73%;

[0051] The component mass percentages of the secondary-phase alloy are Re a M b Fe 100-a-b-c-d-f B c Co d ; the Re rare earth elements of the secondary-phase alloy are Pr, Nd, Gd; a = 32 wt%, where Gd = 2 wt%, Pr = 6 wt%, Nd = 24 wt%; the M of the secondary-phase alloy is Al, Cu, Ga, Zr; b = 0.8 wt%, where Al = 0.4 wt%, Cu = 0.1 wt%, Ga = 0.1 wt%, Zr = 0.2 wt%; the B of the secondary-phase alloy, c = 0.94 wt%; the Co of the secondary-phase alloy, d = 0.5 wt%; the balance is pure iron;

[0052] The preparation steps of the secondary-phase alloy powder are as follows:

[0053] A1. Melting

[0054] The industrial pure metal raw materials are weighed and proportioned according to the NdFeB alloy composition by mass percentage, and the target raw materials are melted using a vacuum rapid solidification melting furnace and then rapidly quenched into secondary-phase alloy cast sheets;

[0055] A2. Hydrogenation

[0056] The neodymium-iron-boron secondary-phase alloy flakes are hydrogenated using a hydrogenation crusher for 4 hours of hydrogen crushing, and then dehydrogenated for 6 hours at a dehydrogenation temperature of 450°C. After dehydrogenation, the alloy ingots are cooled to 35°C, then cooling is stopped and they are left standing for 120 minutes. After waiting for the cooling temperature to stop rising, 100 ppm of pure oxygen is introduced into the hydrogenation reaction device. At this time, there is a certain temperature in the furnace, and the reaction kettle of the hydrogenation crusher does not stop rotating. During oxygen supplementation, the coarse powder is continuously stirred to make the coarse powder absorb oxygen evenly and controllably. When the equipment shows that the oxygen inside is reduced to below 10 ppm, the addition of pure oxygen is stopped. After the oxygen content stabilizes without fluctuation, cooling is continued to obtain the neodymium-iron-boron secondary-phase alloy coarse powder.

[0057] A3. Grinding into powder by jet mill

[0058] During the mixing process of the neodymium-iron-boron secondary-phase alloy coarse powder, a lubricant accounting for 0.1% of the mass of the coarse powder is added; after the secondary-phase alloy coarse powder is stirred and mixed, the coarse powder is further ground into fine powder using a fluidized bed jet mill without oxygen supplementation during jet milling; an antioxidant is added to and mixed evenly with the obtained neodymium-iron-boron secondary-phase alloy fine powder; the SMD of the alloy powder is 2.9 μm, and D90 / D10 = 4.75, D10 = 1.85, and the distribution value of 3 - 10 μm = 76.5%;

[0059] The steps for magnet preparation are as follows:

[0060] S4. Mixing

[0061] The neodymium-iron-boron main-phase alloy powder and the secondary-phase alloy powder are added to a stainless-steel bucket in a ratio of 90%:10% and mixed using a three-dimensional mixer;

[0062] S5. Orientation and forming

[0063] The alloy powder is magnetically oriented and formed under nitrogen protection to obtain a green compact;

[0064] S6. Sintering and tempering

[0065] After the green compact is subjected to vacuum sintering and tempering treatments, it is cooled to obtain a neodymium-iron-boron magnet. Sintering means sintering at 1050°C for 4 h, followed by tempering at 900°C for 2 h and tempering at 500°C for 4 h;

[0066] S7. Machining and cutting

[0067] The magnet is machined and cut into thin magnetic sheets with a thickness of 50 * 40 * 5 mm, and the magnetic sheets are degummed, degreased, and cleaned to ensure a clean surface;

[0068] S8. Magnetron sputtering of HReM alloy

[0069] The composition of the HReM alloy is by mass percentage HRe a Al b Cu c Ga100-a-b-c ; HRe is Tb, a = 50 wt%, b = 15 wt%, c = 15 wt%, and the balance is Ga.

[0070] Perform magnetron sputtering of HReM alloy on the surface-cleaned magnetic disk. For the magnetic disk with both sides coated, the measured weight gain of the magnet is 0.5012%, and place it in a molybdenum sintering box in a nitrogen protection box;

[0071] S9. Grain boundary diffusion

[0072] Place the magnetic disk in the graphite box into the sintering furnace, evacuate to high vacuum, heat up to 650 °C and hold for 8 h for grain boundary diffusion, and then temper at 500 °C for 4 h; Sample 1 is prepared. The process flow chart of the present invention is as Figure 1 shown, and the schematic diagram of the grain boundary situation after grain boundary diffusion is shown in Figure 2 .

[0073] Comparative Example 1

[0074] S1. Magnetron sputtering of pure terbium

[0075] Perform magnetron sputtering of pure terbium on the 50*40*5 mm prepared in step S7 of Example 1. The measured weight gain of the magnet is 0.5015%. Place the magnetic disk with both sides coated in a molybdenum sintering box in a nitrogen protection box;

[0076] S2. Grain boundary diffusion

[0077] Place the magnetic disk in the graphite box into the sintering furnace, evacuate to high vacuum, heat up to 900 °C and hold for 10 h for grain boundary diffusion, and then temper at 500 °C for 4 h; Sample 2 is prepared.

[0078] Comparative Example 2

[0079] By calculating the magnet alloy composition before grain boundary diffusion in Example 1, use exactly the same alloy composition. The mass percentage of the alloy composition is Re a M b Fe 100-a-b-c-d-f B c Co d ; The rare earth element Re is Pr, Nd, Gd, Ce; a = 30.2 wt%, where Pr = 3.3 wt%, Nd = 13.2 wt%, Gd = 0.2 wt%, Ce = 13.5 wt%; M of the alloy is Al, Cu, Zr, Ga; b = 0.8 wt%; among them, Al = 0.4 wt%, Cu = 0.1 wt%, Zr = 0.29 wt%, Ga = 0.01 wt%; B of the alloy, c = 0.922 wt%; Co of the alloy, d = 0.5 wt%; the balance is pure iron;

[0080] The steps for preparing the magnet are as follows:

[0081] S1, Melting

[0082] Weigh and proportion the industrial pure metal raw materials according to the composition of the NdFeB alloy by mass percentage. Then, use a vacuum rapid solidification melting furnace for the target raw materials and quickly quench them into alloy ingots;

[0083] S2, Hydrogen Crushing

[0084] Use a hydrogen crushing furnace to crush the NdFeB alloy ingots. Conduct hydrogen crushing for 4 hours, then dehydrogenate for 8 hours at a dehydrogenation temperature of 550°C. After dehydrogenation, cool the alloy ingots to 35°C, stop cooling, and let them stand for 120 minutes. After waiting for the cooling temperature to stop rising, introduce 200 ppm of pure oxygen into the hydrogen crushing reaction device. At this time, the furnace has a certain temperature, and the reaction kettle of the hydrogen crushing furnace rotates without stopping. During oxygen supplementation, the coarse powder is continuously stirred to make the coarse powder evenly and controllably absorb oxygen. When the equipment shows that the oxygen inside is below 10 ppm, stop adding pure oxygen. After the oxygen content is stable without fluctuation, continue cooling to obtain NdFeB alloy coarse powder;

[0085] S3, Jet Milling for Powder Preparation

[0086] Add a lubricant accounting for 0.1% of the mass of the coarse powder during the mixing process of the NdFeB alloy coarse powder; after stirring and mixing the alloy coarse powder, further mill the coarse powder with a fluidized bed jet mill without oxygen supplementation during jet milling to obtain NdFeB alloy fine powder; add an antioxidant to the obtained NdFeB alloy fine powder and mix evenly; the SMD of the alloy powder is 3.0 μm, and D90 / D10 = 4.95, D10 = 1.75, and the distribution value of 3 - 10 μm = 74.5%;

[0087] S4, Orientation Forming

[0088] Orient and form the alloy powder under the protection of nitrogen to obtain a green compact;

[0089] S5, Sintering and Tempering

[0090] After vacuum sintering and tempering treatment of the green compact, cool to obtain NdFeB magnets. Sintering means sintering at 1050°C for 4 hours, followed by tempering at 900°C for 2 hours and 500°C for 4 hours;

[0091] S6, Machining and Cutting

[0092] Machine and cut the magnet into thin magnetic sheets with a thickness of 50*40*5 mm, and degum, degrease, and clean the magnetic sheets to ensure a clean surface;

[0093] S7, Magnetron Sputtering of Pure Terbium

[0094] Conduct magnetron sputtering of pure terbium on the thin magnetic sheets. For the magnetic sheets with both sides coated, the measured weight gain of the magnet is 0.502%, and place them in a molybdenum sintering box in a nitrogen protection box;

[0095] S8, Grain boundary diffusion

[0096] The magnetic discs placed in the graphite box were put into a sintering furnace, evacuated to a high vacuum, heated to 900 °C and held for 10 h for grain boundary diffusion, and then tempered at 500 °C for 4 h; Sample 3 was obtained.

[0097] In Comparative Example 1, the thin magnetic discs prepared in step S7 of Example 1 were directly used, and then pure terbium was directly magnetron sputtered. In Example 1, the HReM alloy of the present invention was magnetron sputtered. Therefore, the magnet compositions before grain boundary diffusion in Example 1 and Comparative Example 1 were exactly the same; Comparative Example 2 also used the same magnet composition as Example 1, except that the magnet was prepared by a single alloy process, rather than the master-slave phase double alloy preparation used in Example 1. The magnetic properties were detected using a NIM-2000 demagnetization curve tester. The irreversible loss of magnetic moment was detected using an ESPEC PHH201 high-temperature box, and the open-circuit detection method was used in both cases. A HIRAYAMA PC-304R8 accelerated aging test chamber was used to detect the weight loss corrosion resistance of the magnets. A universal testing machine was used to detect the compressive strength.

[0098] The sample preparation methods and sample sizes were the same, and will not be repeated here. The magnetic properties and process parameters of Example 1, Comparative Example 1, and Comparative Example 2 are compared in Table 1.

[0099] Table 1

[0100]

[0101] From the comparison of the magnetic properties and process parameters of Example 1 and Comparative Example 1 in Table 1, the following conclusions can be drawn. The preparation process of the magnetic body before grain boundary diffusion is exactly the same. However, since a higher temperature was used for grain boundary diffusion in Comparative Example 2, although pure terbium was magnetron sputtered, the effect of improving the intrinsic coercivity of the magnetic body was not good. The analysis shows that mainly because cerium in the cerium magnet replaces and adsorbs the heavy rare earth terbium diffused through the grain boundary, it seriously affects the efficiency of the heavy rare earth in enhancing the intrinsic coercivity. From the comparison of the magnetic properties and process parameters of Comparative Example 1 and Comparative Example 2, the following conclusions can be drawn. For magnets with the same composition before grain boundary diffusion, in Comparative Example 1, a high-abundance light rare earth sintered NdFeB magnet prepared by the main and auxiliary phase double alloy process of the present invention was used, while in Comparative Example 2, a high-abundance light rare earth sintered NdFeB magnet prepared by the traditional single alloy process was used. Both were magnetron sputtered with 0.5% pure terbium, and the magnetic properties of the finally prepared magnets were very different. The analysis shows that in Comparative Example 2, due to the use of the single alloy process, more cerium enters the rare earth-rich phase, which has a very large impact on the subsequent grain boundary diffusion, seriously restricting the enhancement of the intrinsic coercivity by the heavy rare earth terbium diffused into the grain boundary. Moreover, too much cerium enters the grain boundary rare earth-rich phase, causing exchange coupling of the main phase grains and also reducing the remanence of the magnet. From the compressive strength and weight loss results of Example 1, Comparative Example 1 and Comparative Example 2, it can also be seen that using the main and auxiliary phase double alloy process can significantly improve the mechanical properties and corrosion resistance of the cerium-containing magnet.

Claims

1. A medium and high performance neodymium iron boron magnet, characterized in that, the neodymium iron boron magnet comprises a main phase alloy and an auxiliary phase alloy, the mass ratio of the main phase alloy is 80-90%, and the mass ratio of the auxiliary phase alloy is 10-20%; The component mass percentage of the main phase alloy is (Ce x Re 1-x ) a M b Fe 100-a-b-c-d B c Co d ; Ce in the main phase alloy is cerium; the Re rare earth elements are two or more of Pr, Nd, Gd, and Y, 0.5 < x < 1, and Ce replaces more than 50% of the Pr / Nd elements; M in the main phase alloy is two or more of Al, Cu, Zr, and Nb, and contains Zr. When the main phase alloy is hydrogenated and crushed, 100-300 ppm of pure oxygen is added to form a stable Ce2O3 phase, wherein, a = 28-30 wt%, b = 0-2.5 wt%, c = 0.85-0.95 wt%, d = 0.1-4 wt%, and the rest is pure iron; The component mass percentage of the auxiliary phase alloy is Re a M b Fe 100-a-b-c-d B c Co d ; the Re rare earth elements in the auxiliary phase alloy are two or more of Pr, Nd, Gd, and Y; M in the auxiliary phase alloy is two or more of Al, Cu, Ga, Zr, Nb, and Ti; wherein, a = 32-33 wt%, b = 0-3 wt%, c = 0.85-1 wt%, d = 0.1-4 wt%, and the rest is pure iron; the medium and high performance neodymium iron boron magnet is prepared by the following steps: 1) Preparation of the main phase alloy powder; 2) Preparation of the auxiliary phase alloy powder; 3) Mixing: Add the neodymium iron boron main phase alloy powder and the auxiliary phase alloy powder into a stainless steel bucket according to the formula ratio, and mix them with a three-dimensional mixer; 4) Orientation molding: Orient and mold the alloy powder under nitrogen protection to obtain a green compact; 5) Sintering and tempering: After the green compact is subjected to vacuum sintering and tempering treatment, it is cooled to obtain a neodymium iron boron magnet; 6) Machining and cutting: Machine and cut the magnet into thin magnetic sheets with a thickness of 1-10 mm required by the customer. The magnetic sheets are square sheets, tile-shaped sheets or circular sheets. Debond and degrease the magnetic sheets and clean them to ensure the surface is clean; 7) Magnetron sputtering or evaporation of HReM alloy: Magnetron sputter or evaporate the HReM alloy on the clean surface of the magnetic sheet. Put the magnetic sheet with both sides coated into a molybdenum sintering box in a nitrogen protection box, and magnetron sputter or evaporate the HReM alloy. The mass percentage of its components is HReaAlbCucGa100-a-b-c; HRe is Dy or Tb, a = 50-70 wt%, b = 5-30 wt%, c = 5-20%, and the remainder is Ga. The melting point of the HReM alloy is 600-900 °C; 8) Grain boundary diffusion: Put the magnetic sheet loaded into a graphite box into a sintering furnace, evacuate to high vacuum, heat up to 600-750 °C and hold for 8-12 h for grain boundary diffusion, and then temper at 450-550 °C for 2-5 h to obtain a medium and high performance neodymium iron boron magnet.

2. A medium and high performance neodymium iron boron magnet according to claim 1, characterized in that in step 1), the preparation of the main phase alloy powder is carried out by the following steps: a) Melting: Weigh and mix industrial pure metal raw materials according to the NdFeB alloy composition by mass percentage. Use a vacuum rapid solidification melting furnace for the target raw materials, and then quickly quench them into alloy castings. b) Hydrogen crushing: The main-phase NdFeB alloy cast sheet is subjected to hydrogen crushing in a hydrogen crushing furnace for 2 - 4 hours, then dehydrogenated for 4 - 8 hours at a dehydrogenation temperature of 450 - 550 °C. After dehydrogenation, the alloy cast sheet is cooled to 35 °C, then the cooling is stopped and it is left standing for 60 - 120 minutes. After the cooling temperature no longer rises, 100 - 300 ppm of pure oxygen is introduced into the hydrogen crushing reaction device. When the equipment shows that the oxygen inside is reduced to below 10 ppm, the addition of pure oxygen is stopped. After the oxygen content is stable without fluctuation, the cooling is continued to obtain the NdFeB main-phase alloy coarse powder; c) Jet milling for powder making: 0.1 - 0.5% of the lubricant by mass of the coarse powder is added during the mixing process of the NdFeB main-phase alloy coarse powder; after the main-phase alloy coarse powder is stirred and mixed, the coarse powder is further milled with a fluidized bed jet mill without oxygen supplementation during jet milling to obtain the NdFeB main-phase alloy fine powder; an antioxidant is added and mixed evenly into the obtained NdFeB main-phase alloy fine powder; the SMD of the alloy powder is 2.8 - 3.2 μm, and D90 / D10 < 5.3, D10 > 1.6, and the distribution value of 3 - 10 μm > 72%.

3. The medium and high performance NdFeB magnet according to claim 1, characterized in that For the preparation of the auxiliary-phase alloy powder in step 2), the following steps are adopted: a) Melting: The industrial pure metal raw materials are weighed and proportioned according to the NdFeB alloy composition by mass percentage, and the target raw materials are melted by a vacuum rapid solidification melting furnace and then rapidly quenched into auxiliary-phase alloy cast sheets; b) Hydrogen crushing: The NdFeB auxiliary-phase alloy cast sheet is subjected to hydrogen crushing in a hydrogen crushing furnace for 2 - 4 hours, then dehydrogenated for 4 - 8 hours at a dehydrogenation temperature of 450 - 550 °C. After dehydrogenation, the alloy cast sheet is cooled to 35 °C, then the cooling is stopped and it is left standing for 60 - 120 minutes. After waiting for the cooling temperature no longer to rise, 100 - 300 ppm of pure oxygen is introduced into the hydrogen crushing reaction device. When the equipment shows that the oxygen inside is reduced to below 10 ppm, the addition of pure oxygen is stopped. After the oxygen content is stable without fluctuation, the cooling is continued to obtain the NdFeB auxiliary-phase alloy coarse powder; c) Jet milling for powder making: 0.1 - 0.5% of the lubricant by mass of the coarse powder is added during the mixing process of the NdFeB auxiliary-phase alloy coarse powder; after the auxiliary-phase alloy coarse powder is stirred and mixed, the coarse powder is further milled with a fluidized bed jet mill without oxygen supplementation during jet milling to obtain the NdFeB auxiliary-phase alloy fine powder; an antioxidant is added and mixed evenly into the obtained NdFeB auxiliary-phase alloy fine powder; the SMD of the alloy powder is 2.6 - 3.0 μm, and D90 / D10 < 4.8, D10 > 1.8, and the distribution value of 3 - 10 μm > 75%.

4. The medium and high performance NdFeB magnet according to claim 1, characterized in that The sintering in step 2) means sintering at 1000 - 1100 °C for 2 - 5 h, then tempering at 800 - 900 °C for 1 - 2 h and tempering at 450 - 550 °C for 2 - 5 h.

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