High-temperature-resistant rare-earth-free r-t-b permanent magnet and preparation method thereof

Abstract

The application discloses a high-temperature-resistant rare-earth-free R-T-B permanent magnet and a preparation method thereof. By optimizing the particle size and distribution of neodymium iron boron fine powder, X10 of the fine powder is improved, X90 is reduced, the size distribution range of the neodymium iron boron fine powder is narrowed, the average size and distribution of the grain of the rare-earth-free magnet after sintering are improved, the intrinsic coercive force of the rare-earth-free magnet at normal temperature is improved, the high-temperature resistance of the magnet at 150 DEG C is further optimized through magnet component formula design and process cooperation such as dehydrogenation temperature, the magnet provided by the application also has good bending strength, meets the requirement of the bending strength of the magnet for an automobile motor, and can be applied to the field of automobile motors, and has wide market application prospects.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnets, specifically relating to a high-temperature resistant, heavy rare earth-free RTB permanent magnet and its preparation method. Background Technology

[0002] Currently, the export of neodymium iron boron magnets containing heavy rare earth elements from China is restricted. Many domestic manufacturers have accelerated their research and development of magnets without heavy rare earth elements, especially those with grades of 48SH and above. However, the coercivity of these magnets decreases significantly at high temperatures (150℃ and above), and their coercivity temperature coefficient is large, limiting their application in high-temperature environments.

[0003] Conventional Dy-containing 42SH and 45SH magnets exhibit Br and coercivity temperature coefficients greater than -0.125% / ℃ and -0.53% / ℃ at 150℃, respectively. Adding rare-earth Pr can produce higher-grade heavy rare-earth-free magnets; however, due to excessive Pr addition, Pr magnets exhibit poorer high-temperature resistance compared to Dy-containing magnets of the same grade. At 150℃, the Br and Hcj temperature coefficients of Pr magnets are less than -0.125% / ℃ and -0.55% / ℃, respectively.

[0004] Therefore, it is necessary to develop a high-temperature resistant, heavy rare-earth-free magnet to improve the high-temperature resistance and magnetic properties at room temperature under a heavy rare-earth-free formulation. Summary of the Invention

[0005] To address the shortcomings of existing technologies where neodymium iron boron (NdFeB) magnets without heavy rare earth elements cannot simultaneously possess both high-temperature resistance and high magnetic properties, this invention provides a high-temperature resistant, heavy rare earth-free RTB permanent magnet and its preparation method. Without the addition of heavy rare earth elements, the magnet's performance at room temperature (20℃) is: Br≥13.7kGs, Hcj≥20kOe, SQ≥0.97; at 150℃, its α(Br)≥-0.120% / ℃ and β(Hcj)≥-0.532% / ℃ meet the requirements of the 48SH grade.

[0006] The technical solution adopted in this invention is as follows: A high-temperature resistant, heavy rare-earth-free RTB permanent magnet, wherein the elemental composition of the magnet is expressed as a mass percentage: R: 31.8~32.8 wt.%, R includes Pr and Nd, where the mass of Pr is less than 25% of the total mass of R and the mass of Nd is more than 75% of the total mass of R; Al: 0~0.1wt.%, 0 represents infinitely close to 0 but not 0; Cu: 0.25~0.35 wt.%; Ga: 0.35~0.45 wt.%; Co: 1~1.6wt.%; B: 0.88~0.91 wt.%; M: M is Zr or Zr and Ti. When M is pure Zr, the mass percentage of M is m1(Zr) = 0.25~0.35 wt.%; when M is Zr and Ti, the masses of Zr and Ti are m2(Zr) and m2(Ti) respectively, m2(Zr) is 0.05~0.25 wt.% and m2(Ti) is 0.05~0.15 wt.%, satisfying the mass fraction of M as m2(Zr) + m2(Ti) = 0.2~0.3 wt.%, m2(Ti) = (0.35 - m2(Zr)) × 0.5; T: Balance, where T represents Fe and other unavoidable impurities.

[0007] This invention does not contain any heavy rare earth elements, such as Dy, Tb, Gd, and Ho. Furthermore, unlike heavy rare earth-free magnets designed for high Pr content, in this invention's heavy rare earth-free magnet, the proportion of Pr to rare earth R(Pr+Nd) is m(Pr) / R ≤ 0.25, and the proportion of Nd to rare earth R(Pr+Nd) is m(Nd) / R ≥ 0.75. This compositional design is a prerequisite for preparing high-temperature resistant heavy rare earth-free magnets. Simultaneously, to obtain a magnet with high coercivity, a relatively high total rare earth content is also required.

[0008] Al has higher solubility in the main phase and can directly affect the intrinsic properties of the main phase. Al will preferentially replace Fe(8j2) crystal sites and improve the intrinsic magnetocrystalline anisotropy of the magnet. However, adding a large amount of Al will also deteriorate the magnetic energy product and high-temperature performance of the magnet. Therefore, the Al content in this magnet is designed to be no more than 0.1 wt.%. At the same time, in order to ensure the α(Br) of the magnet, the Co content of the magnet of this invention is ≥1 wt.%.

[0009] Adding Cu and Ga can increase the Re6Fe content in the magnet. 13 The amount of (Cu,Ga) phases increases, leading to the formation of wider and more continuous grain boundary phases, thereby reducing the magnetic coupling between grains and improving the intrinsic coercivity Hcj of the magnet; in addition, Cu can also improve the high-temperature resistance of the magnet.

[0010] The addition of high-melting-point elements Zr and Ti can form precipitates such as ZrB, ZrB2, and TiB2. These precipitates mainly exist in grain boundaries and can improve intrinsic coercivity (Hcj) and refine magnet grains. This invention limits the content of Zr or Zr and Ti. Too low a Zr or Zr and Ti content has limited effect on improving the magnet's coercivity, and at the same time, too little addition of high-melting-point elements can make the magnet sensitive to sintering temperature and prone to abnormal grain growth (AGG). Too high a Zr or Zr and Ti content will significantly reduce the proportion of the main phase grains in the magnet, thereby reducing the remanence (Br) of the magnet.

[0011] This invention also provides a method for preparing the high-temperature resistant, heavy rare-earth-free RTB permanent magnet, comprising the following steps: S1: Take the magnet raw materials according to the component ratio and prepare the alloy rapid solidification sheet by vacuum melting; S2: The alloy quick-solidification sheet obtained in step S1 above is subjected to hydrogen crushing treatment to obtain alloy coarse powder; S3: The alloy coarse powder from step S2 above is subjected to air jet milling. The air jet mill is continuously fed to obtain NdFeB fine powder. The surface area density (SMD) of NdFeB fine powder is 3.0 ± 0.1 μm, with X10 ranging from 1.60 to 1.80 μm and X90 ranging from 6.50 to 6.9 μm, and X90 / X10 = 3.8 to 4.25. S4: The NdFeB fine powder obtained in step S3 above is subjected to orientation molding, secondary sintering and two-stage aging treatment to obtain a high-temperature resistant, heavy rare earth-free RTB permanent magnet.

[0012] In step S2, during the hydrogen crushing process, the dehydrogenation temperature is 550~580℃. The dehydrogenation temperature of this invention is higher than that of conventional processes because the higher temperature used in the hydrogen crushing dehydrogenation heat treatment allows for the earlier generation of some Re6Fe. 13 The (Cu,Ga) phase is distributed in the coarse powder to prepare for subsequent Re6Fe production. 13 The (Cu,Ga) phase is uniformly distributed in the magnet, which prepares to improve the performance uniformity of different blanks.

[0013] In step S3, the adjustment of the process parameters of the air jet mill will significantly affect the particle size and particle size distribution of NdFeB fine powder.

[0014] The particle size and distribution of NdFeB fine powder are among the main factors influencing the average size and distribution of magnet grains. Therefore, improving the particle size and distribution of NdFeB fine powder can improve the grain size and distribution of magnets. Under the same SMD conditions, conventionally produced NdFeB fine powder has an X90 greater than 6.9 μm and an X90 / X10 ratio in the range of 4.25 to 4.6.

[0015] However, by controlling the air jet milling process, this invention increases X10 to 1.60~1.80μm and reduces X90 to 6.50~6.9μm, thereby reducing X90 / X10 to 3.8~4.25, thus giving the NdFeB fine powder a narrower particle size distribution range.

[0016] The traditional air jet mill production process involves continuous feeding, with the powder loading in the mill chamber accounting for approximately 80-100% of the designed powder weight.

[0017] The air jet mill process of the present invention is as follows: the air jet mill is continuously fed, and when the final remaining powder in the mill chamber drops to 50-85% (preferably 65-80%) of the designed weight of the powder in the mill chamber, the speed of the air jet mill sorting wheel is increased by 200-300 rpm and continues to run until the entire batch of powder is discharged.

[0018] The conventional value of the sorting wheel speed in an air jet mill is generally around 3000-3500 rpm. This invention requires that when the grinding reaches the later stage and the amount of powder remaining in the grinding chamber decreases to 50-85% of the designed weight of the powder in the grinding chamber, the speed of the sorting wheel in the air jet mill should be further increased by 200-300 rpm compared to the original speed.

[0019] During the operation of an air jet mill, the powder loading within the mill chamber is typically maintained within a stable range, and continuous operation is achieved by controlling the dynamic balance between the feed and discharge rates. However, as the grinding process progresses, when the same batch of material enters the final stage, the amount of remaining powder in the mill chamber gradually decreases, falling below the optimal loading amount designed for the mill chamber's weight. This leads to instability in the fluidized bed of the air jet mill, a decrease in grinding efficiency, and a larger particle size in this tailings. Therefore, when processing the final remaining tailings, this invention appropriately increases the speed of the sorting wheel to improve particle size consistency.

[0020] For example, the sorting wheel speed of an air jet mill is set to A rpm, where A ranges from 3000 to 3500 rpm. The air jet mill is continuously fed, and the sorting wheel speed is A rpm. When the air jet mill processes the material until the final residual powder in the mill chamber accounts for 50-85% of the designed powder weight, the sorting wheel speed is changed to A+B rpm, where B ranges from 200-300 rpm, and this continues until all the powder from that batch is discharged. More preferably, B = A * 8%.

[0021] Air classifier mills employ two structures for screening powder particles: a sorting wheel and a cyclone separator. The sorting wheel primarily controls the maximum particle size of NdFeB fine powder (usually denoted by X90; the screening process is mainly controlled by centrifugal force, similar to the cyclone separator). The cyclone separator primarily controls the minimum particle size of NdFeB fine powder (usually denoted by X10). This invention addresses this by reducing the proportion of coarse powder in the designed weight of the powder in the grinding chamber during the air classifier milling process. This controls the weight of coarse powder in the grinding chamber, thereby reducing the probability of coarse powder collisions. During coarse powder collisions, the process prepares the powder for subsequent cyclone separator screening, increasing the X10 of the fine powder and preventing it from becoming too small. Furthermore, to avoid particle coarsening due to the reduced proportion of powder in the grinding chamber, the speed of the sorting wheel is increased at the end of the air classifier milling process, further reducing the X90 of the fine powder. Through the combined improvement of these two processes, the size distribution range of NdFeB fine powder is narrowed, ultimately obtaining the NdFeB fine powder required by this invention.

[0022] In step S4, the magnetic field strength in the orientation forming process is 1.5~2.3T.

[0023] The secondary sintering process is as follows: hold at 1070~1100℃ for 4~8 hours, cool down to room temperature, and then heat up to 1000~1060℃ and hold for 4~6 hours.

[0024] The vacuum degree during secondary sintering is 1*10 -1 ~4.2*10 -3 Pa.

[0025] The preferred sintering process is as follows: hold at 1080℃ for 6 hours, then cool down to 1000~1060℃ and hold for 5 hours.

[0026] The two-stage aging process is as follows: the first-stage aging process is to keep the temperature at 880~910℃ for 3~4.5h, and the second-stage aging process is to keep the temperature at 455~465℃ for 3~4.5h.

[0027] During the second-stage aging process, the vacuum level is 1*10. -1 ~4.2*10 -3 Pa.

[0028] The high-temperature resistant, heavy rare-earth-free RTB permanent magnet provided by this invention has an average grain size ≤3.6μm, wherein the number of grains with a grain size >3.6μm accounts for 38~46%, and the number of grains with a grain size <3μm accounts for ≥39~45%. Furthermore, it is preferred that the number of grains with a grain size > 3.6 μm accounts for 40-45%, and the number of grains with a grain size < 3 μm accounts for 40-44%.

[0029] The high-temperature resistant, heavy rare-earth-free RTB permanent magnet provided by this invention has the following performance at room temperature (20℃): Br≥13.7kGs, Hcj≥20kOe, SQ≥0.97; and at 150℃, its α(Br)≥-0.120% / ℃ and β(Hcj)≥-0.532% / ℃.

[0030] The high-temperature resistant, heavy rare-earth-free RTB permanent magnet provided by this invention has a bending strength greater than 500 MPa. This bending strength meets the requirements for applications in the motor field.

[0031] This invention also provides the application of high-temperature resistant, heavy rare-earth-free RTB permanent magnets in the field of automotive motors.

[0032] The beneficial effects of this invention are as follows: This invention provides a high-temperature resistant, heavy rare earth-free RTB permanent magnet. Under heavy rare earth-free conditions, this invention optimizes the particle size and distribution of NdFeB fine powder, increases the X10 of the fine powder, and decreases the X90, thereby narrowing the size distribution range of the NdFeB fine powder. This improves the average grain size and distribution of the sintered heavy rare earth-free magnet and enhances the intrinsic coercivity of the heavy rare earth-free magnet at room temperature.

[0033] This invention redesigns and adjusts the magnet formulation, further optimizing the magnet's high-temperature resistance at 150℃. This invention increases the Cu content; Cu mainly exists in the magnet's grain boundaries and participates not only in Re6Fe... 13 The formation of the (Cu,Ga) phase further enhances the magnet's high-temperature resistance. This invention also limits the use of high-melting-point elements Zr or a combination of Zr and Ti, ensuring that the remanence (Br) of the magnet is not reduced while increasing coercivity.

[0034] This invention increases the dehydrogenation temperature; using a higher temperature for dehydrogenation can generate some Re6Fe earlier. 13 The (Cu,Ga) phase, distributed within the coarse powder, facilitates subsequent Re6Fe... 13 The (Cu,Ga) phase is uniformly distributed in the magnet, improving performance consistency.

[0035] Without the addition of heavy rare earth elements, the magnet of this invention exhibits the following performance at room temperature (20℃): Br≥13.7kGs, Hcj≥20kOe, SQ≥0.97; at 150℃, its α(Br)≥-0.120% / ℃ and β(Hcj)≥-0.532% / ℃, while also possessing good bending strength, which meets the bending strength requirements of automotive motors and has significant market application value. Attached Figure Description

[0036] Figure 1 The particle size distribution test diagram is for NdFeB fine powder A1 in Comparative Example 1.

[0037] Figure 2 This is a particle size distribution test diagram of NdFeB fine powder A2 from Example 1.

[0038] Figure 3 This is a particle size distribution test diagram of NdFeB fine powder A3 from Example 2.

[0039] Figure 4 Microstructure of magnet B1 prepared for Comparative Example 1.

[0040] Figure 5 The image shows the microstructure of magnet B3 prepared in Example 2.

[0041] Figure 6 The bending force F-displacement X curve is shown for magnet B3 prepared in Example 2. Detailed Implementation

[0042] The technical solution of the present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0043] A high-temperature resistant, heavy rare-earth-free RTB permanent magnet, with elemental composition by mass percentage: R: 31.8~32.8 wt.%, R includes Pr and Nd, but does not contain any heavy rare earth elements such as Dy, Tb, Gd and Ho. The masses of Pr and Nd are m(Pr) and m(Nd), respectively, and m(Pr) / R≤0.25 and m(Nd) / R≥0.75. Al: 0~0.1wt.%, where 0 represents infinitely close to 0 but not 0; Cu: 0.25~0.35 wt.%; Ga: 0.35~0.45 wt.%; Co: 1~1.6wt.%; B: 0.88~0.91 wt.%; M: M is Zr or Zr and Ti; when M is pure Zr, the mass percentage of M is m1(Zr) = 0.25~0.35 wt.%; when M is Zr and Ti, the masses of Zr and Ti are m2(Zr) and m2(Ti) respectively, m2(Zr) is 0.05~0.25 wt.% and m2(Ti) is 0.05~0.15 wt.%, satisfying the mass fraction of M as m2(Zr) + m2(Ti) = 0.2~0.3 wt.%, m2(Ti) = (0.35 - m2(Zr)) × 0.5; T: Balance, where T represents Fe and other unavoidable impurities.

[0044] This invention utilizes vacuum induction melting, hydrogen decomposition (HD), air jet milling (JM), orientation forming, and sintering aging to prepare high-temperature resistant, heavy rare-earth-free RTB permanent magnets. The process includes melting and hydrogen decomposition of the alloy rapidly solidifying sheet, as well as air jet milling, orientation forming, and sintering aging steps. S1 alloy rapid solidification sheet vacuum induction melting: S11: Ingredients. Weigh the raw materials according to the proportions of the ingredients in the formula, with Fe having a purity greater than 99.99%.

[0045] S12: Charging. Place the raw materials into the crucible in order of decreasing melting point, close the furnace door, and evacuate the furnace to a vacuum level below 6 Pa.

[0046] S13: Preheating + Vacuuming. Preheating time > 20 min, followed by vacuuming for 20 min.

[0047] S14: Argon-filled heating and melting. After preheating, argon gas is introduced into the furnace to achieve a furnace pressure of 30,000~50,000 Pa, preferably 27 kPa. After argon filling, the temperature is increased, and the alloy liquid is heated to 1480~1520℃ and then subjected to a heat-holding refining process for 1~3 minutes.

[0048] S15: Casting. After heat preservation and refining, the power is turned off and the alloy liquid is subjected to self-cooling treatment. Casting is performed after the alloy liquid temperature reaches 1410~1430℃ to obtain heavy rare earth-free alloy rapid-solidifying flakes. The thickness of the rapid-solidifying flakes is 0.3mm±0.05mm, and the proportion of rapid-solidifying flakes with a thickness in the range of 0.2~0.4mm is not less than 90%, and further, not less than 95%.

[0049] S2 alloy rapid solidification sheet hydrogen decomposition treatment: S21: Charging and Positive / Negative Inspection. After loading the heavy rare earth alloy rapid solidification sheets into the hydrogen crushing furnace, argon gas is introduced for a positive inspection lasting 5 minutes. During the positive inspection, the pressure change inside the furnace is less than 0.01 MPa / 5 minutes. After the positive inspection, the furnace is evacuated for at least 40 minutes. After the evacuation is completed, a negative inspection begins for 5 minutes. Before and after the negative inspection, the pressure rise inside the furnace is less than 30 Pa / 5 minutes.

[0050] S22: Hydrogen absorption. One hydrogen absorption cycle consists of a pressure of 0.068~0.098 MPa. When the furnace is full of hydrogen, the furnace pressure is 0.098 MPa. The criterion for hydrogen saturation is that the furnace pressure change is less than 0.01 MPa / 5 min.

[0051] S23: Dehydrogenation. After hydrogen absorption, heating and holding for dehydrogenation begins. The heating rate is ≥15℃ / min, the dehydrogenation temperature is 550~580℃, and the dehydrogenation time is 2~6h, yielding coarse alloy powder.

[0052] S3 airflow milling treatment: The above-mentioned coarse alloy powder was subjected to air jet milling to obtain NdFeB fine powder.

[0053] The grinding chamber of the air jet mill used in this invention is designed to hold 60 kg of powder, and the N2 gas pressure of the air jet mill is 0.65 MPa.

[0054] The air jet mill production process adopted in this invention is as follows: the air jet mill is continuously fed, and when the final remaining powder in the mill chamber accounts for about 50-85% of the designed weight of the powder in the chamber, preferably 60-80%, the rotation speed of the air jet mill sorting wheel is increased by 200-300 r / min.

[0055] The NdFeB fine powder of the present invention has a surface area density (SMD) of 3.0 ± 0.1 μm, an X10 of 1.60 ~ 1.80 μm, an X90 of 6.50 ~ 6.9 μm, and an X90 / X10 ratio of 3.8 ~ 4.25.

[0056] S4 orientation forming and sintering aging treatment: The above-mentioned NdFeB fine powder was oriented, sintered, and subjected to a two-stage aging process to obtain the final heavy rare earth NdFeB magnet.

[0057] The orientation forming process uses a magnetic field strength of 1.8~2.3T.

[0058] The sintering process is a two-stage sintering process, namely 1080℃*6h + 1000~1060℃*5h, with a vacuum degree of 1*10. -1 ~4.2*10 -3 Pa; the two-stage aging treatment is 880~910℃*3~4.5h + 455~465℃*3~4.5h, with a vacuum degree of 1*10 -1 ~4.2*10 -3 Pa.

[0059] The obtained magnet was subjected to the following tests: Performance testing: The test cylinder has a specification of φ10*10, and the test temperatures are 20℃ and 150℃. The testing equipment is NIM62000.

[0060] Metallographic testing: The tested cylinder was demagnetized, and then the plane perpendicular to the cylinder's orientation axis (plane C) was etched. After etching, the cylinder was placed under an optical microscope (model: Keyence VHX) for metallographic imaging. The obtained metallographic images were analyzed using nanomeasurer software to perform grain statistics, obtaining the average grain size and distribution. The demagnetization temperature was 400℃.

[0061] Bending strength test: A universal testing machine (model: ZTW-20D) was used to test the bending strength, and the bending force F-displacement X curve was obtained. The bending strength was then automatically calculated using a formula. The sample size was 25*6*5↑mm, with the 5mm direction being the orientation direction and the span being 14.5mm.

[0062] Comparative Example 1 The component used is (Pr) 0.25 Nd 0.75 ) 32.1 B 0.9 Al 0.1 Co1Cu 0.26 Ga 0.43 Zr 0.3 Fe Bal The alloy formula was used to smelt 800 kg of alloy quick-setting sheets. The alloy quick-setting sheets were divided into 3 parts. The smelting temperature was 1500℃, the casting temperature was 1425℃, the thickness of the quick-setting sheets was 0.302 mm, and the proportion of quick-setting sheets with a thickness in the range of 0.2~0.4 mm was 96%.

[0063] Take a sample of the alloy quick-setting sheet and perform hydrogen crushing at a temperature of 560℃ for 4.5 hours.

[0064] The coarse alloy powder was processed using a traditional air jet mill. The grinding chamber of the air jet mill was designed to hold 60 kg of powder, and the N2 gas pressure of the air jet mill was 0.65 MPa. Comparative Example 1 used a traditional air jet mill production process with continuous feeding. The air jet mill's sorting wheel speed was 3100 r / min. The final residual material in the grinding chamber accounted for approximately 95% of the designed powder weight. The air jet mill's sorting wheel speed was increased to 3348 r / min and continued until the grinding was completed, resulting in NdFeB fine powder Al. The fine powder size and distribution are shown in Table 1.

[0065] The fine NdFeB powder was oriented and sintered and aged. The sintering and aging process was 1080℃*6h+1050℃*5h+890℃*3h+460℃*3h to obtain the final magnet B1.

[0066] Example 1

[0067] A sample of the alloy quick-setting sheet from Comparative Example 1 was used to prepare a NdFeB magnet. Unlike Comparative Example 1, the air jet mill's grinding chamber was designed to hold 60 kg of powder, and the N2 gas pressure was 0.65 MPa. The air jet mill production process used in Example 1 involved continuous feeding, with the sorting wheel rotating at 3100 r / min. When the remaining material in the grinding chamber accounted for approximately 85% of the designed powder weight, the sorting wheel speed was increased to 3348 r / min and maintained until the grinding was complete, at which point the entire batch of powder was discharged. The NdFeB fine powder A2 from Example 1 had a narrower size distribution, as shown in Table 1. The resulting magnet was designated B2.

[0068] Example 2

[0069] A sample of the alloy quick-setting sheet from Comparative Example 1 was used to prepare a NdFeB magnet. Unlike Comparative Example 1, the air jet mill's grinding chamber was designed to hold 60 kg of powder, and the N2 gas pressure was 0.65 MPa. The air jet mill production process used in Example 1 involved continuous feeding, with the sorting wheel rotating at 3100 r / min. When the remaining material in the grinding chamber accounted for approximately 70% of the designed powder weight, the sorting wheel speed was increased to 3348 r / min and maintained until the grinding was complete. The NdFeB fine powder A3 from Example 1 had a narrower size distribution, as shown in Table 1. The resulting magnet was designated B3.

[0070] Example 3

[0071] The NdFeB fine powder from Example 2 was used to prepare a magnet. Unlike Example 2, the sintering and aging process in Example 3 was 1080℃*6h+890℃*3h+460℃*3h, resulting in the final magnet B4.

[0072] Example 4:

[0073] The same vacuum melting, hydrogen crushing, air jet milling, orientation forming, and sintering aging process as in Example 2 was used, except that the composition was (Pr 0.25 Nd 0.75 ) 32.1 B 0.9 Al 0.1 Co1Cu 0.20 Ga 0.43 Zr 0.3 Fe Bal The magnet prepared was B5.

[0074] Example 5:

[0075] The same vacuum melting, hydrogen crushing, air jet milling, orientation forming, and sintering aging processes as in Example 2 were used, except that the composition was (Pr 0.25 Nd 0.75 ) 32.1 B0.9 Al 0.1 Co1Cu 0.26 Ga 0.43 Zr 0.1 Fe Bal The magnet prepared was B6.

[0076] Example 6:

[0077] The same vacuum melting, hydrogen crushing, air jet milling, orientation forming, and sintering aging processes as in Example 2 were used, except that the composition was (Pr 0.25 Nd 0.75 ) 32.1 B 0.9 Al 0.1 Co1Cu 0.26 Ga 0.43 Zr 0.5 Fe Bal The magnet prepared was designated B7.

[0078] Example 7:

[0079] The same vacuum melting, hydrogen crushing, air jet milling, orientation forming, and sintering aging processes as in Example 2 were used, except that the composition was (Pr 0.25 Nd 0.75 ) 32.1 B 0.9 Al 0.1 Co1Cu 0.26 Ga 0.43 Zr 0.15 Ti 0.1 Fe Bal The magnet prepared was B8.

[0080] Example 8:

[0081] A neodymium iron boron magnet was prepared by taking a portion of the alloy quick-solidification sheet from Comparative Example 1. Unlike Example 2, in the hydrogen absorption process of Example 8, the hydrogen decomposition temperature was 500°C, resulting in the final magnet B9.

[0082] Example 9:

[0083] A neodymium iron boron magnet was prepared by taking a portion of the alloy quick-solidification sheet from Comparative Example 1. Unlike Example 2, in the hydrogen absorption process of Example 9, the hydrogen decomposition temperature was 600°C, and the final magnet B10 was obtained.

[0084] The average grain size and distribution of different magnets are shown in Table 2. The room temperature (20℃) performance of different magnets is shown in Table 3. The high temperature (150℃) performance of different magnets is shown in Table 4. The bending strength of magnet B3 is shown in Table 5.

[0085] Table 1: Size and distribution of different NdFeB fine powders

[0086] Table 2: Average grain size and distribution of different magnets

[0087] Table 3: Performance of different magnets at room temperature (20℃)

[0088] Table 4: High-Temperature (150℃) Performance of Different Magnets

[0089] Table 5: Bending strength of magnet B3

[0090] As shown in Table 1, without optimization of the air jet mill process, the NdFeB fine powder A1 (such as...) in Comparative Example 1... Figure 1 The X90 / X10 of (as shown) is compared to A2 (as shown) Figure 2 (as shown) and A3 (as shown) Figure 3 The particle size distribution of the fine powder is relatively large (as shown in Table 2). After sintering at 1080℃ for 6 hours and then at 1050℃ for 5 hours, the particle size distribution of magnet B1 is relatively large (as shown in Table 2). Figure 4 The average grain size of the NdFeB fine powder (as shown in Table 2) is relatively large, at 4.09 μm, with grains >3.6 μm accounting for 57.0% and grains <3 μm accounting for ≥29.5%. The particle size and distribution of NdFeB fine powder are among the main factors affecting the average grain size and distribution of magnets. Therefore, improving the particle size and distribution of NdFeB fine powder can improve the grain size and distribution of magnets. As shown in Table 2, after orientation molding and sintering, the improved NdFeB fine powder results in magnets B2 and B3 (as shown in Table 2). Figure 5The average grain size and distribution of magnet B2 (as shown) are superior to those of magnet B1. Magnet B2 has a smaller average grain size of 3.40 μm, with 40.5% of grains having a size > 3.6 μm and 43% having a size < 3 μm. Magnet B3 has a smaller average grain size of 3.43 μm, with 40.6% of grains having a size > 3.6 μm and 43.5% having a size < 3 μm. The sintering process of NdFeB magnets is mainly controlled by liquid-phase sintering. During liquid-phase sintering, after the temperature rises and a liquid phase forms, the particles slide, rotate, and rearrange under the action of the liquid phase. The liquid phase fills the gaps between particles, reducing the distance between particles, filling or eliminating a large number of pores, and the density of the blank increases sharply. This is the key stage of liquid-phase strengthened sintering, achieving preliminary densification through particle rearrangement and laying the foundation for subsequent sintering. In some cases, liquid phase infiltration along grain boundaries can lead to further particle subdivision and secondary rearrangement, but primary rearrangement contributes more to densification. Subsequently, small particles or protrusions and fine parts on the particle surface dissolve in the liquid phase, forming a supersaturated solution. When the concentration of the solid phase in the liquid phase reaches saturation, the solute redefines on the surface of large particles. This stage promotes material migration through a dissolution-precipitation mechanism, further increasing the compactness of the green body, while also affecting grain morphology and size distribution. The dissolution-precipitation process enhances diffusion and plays an important role in the densification and microstructure evolution of the green body.

[0091] Therefore, by improving the distribution of powder size, the probability of uniform precipitation of solute in the liquid phase on the surface of different large particles can be increased. After sintering, the compact exhibits a narrower grain size distribution, as shown in Table 2, for magnets B2 and B3. The narrower grain size distribution is beneficial to improving the intrinsic coercivity of the magnet. As shown in Table 3, the intrinsic coercivity of magnets B2 and B3 (20.11 kOe and 20.36 kOe, respectively) is higher than that of magnet B1 (19.56 kOe).

[0092] As shown in Table 2, magnet B4 was sintered using only a single sintering process. Although the average grain size of the magnet (3.3 μm) is smaller than that of magnet B3 (3.43 μm), the proportion of grains with a size <3 μm is as high as 49%, while the proportion of grains with a size >3.6 μm is only 33.5%. This indicates that the magnet did not undergo low-temperature reheating, and its grain size distribution is not uniform. The grain size distribution structure of the magnet affects the intrinsic coercivity of the magnet. As shown in Table 3, the coercivity of magnet B4 is only 19.31 kOe, which is 1.05 kOe lower than that of magnet B3 (20.36 kOe).

[0093] In magnet B6, the designed Zr content is 0.1%, which is less than the requirement of 0.25~0.35 wt.% for Zr content in the present invention. In magnet B7, the designed Zr content is 0.5%, which is greater than the Zr content limit. Magnet B8 contains Zr and Ti, and satisfies the requirement of the present invention that m2(Ti) = (0.35 - m2(Zr)) × 0.5.

[0094] Table 3 shows that magnet B6 exhibits significantly reduced coercivity and squareness, while magnet B7 shows significantly reduced remanence and coercivity. In B7, excessive Zr content affects sintering density, leading to abnormal Zr growth, which infiltrates the main phase, causing defects and hindering grain boundary optimization; therefore, the magnetic properties of B7 are significantly reduced. B8 shows significantly higher coercivity compared to B6 and B7, and also exhibits the highest squareness.

[0095] In Example 8, the hydrogen decomposition temperature was reduced to 500℃ to obtain magnet B9. Table 2 shows that while magnet B9 exhibited significantly improved coercivity, its squareness was significantly reduced. This may be because at lower dehydrogenation temperatures, not all hydrogen was removed from the powder. Hydrogen provides some protection to the rare-earth-rich phase, reducing oxidation, thus increasing the volume fraction of the grain boundary phase, leading to demagnetization and improved coercivity. However, during vacuum sintering, hydrogen atoms are released during the vacuum heating process, which can easily cause cracks in the magnet. Localized liquefaction of the neodymium-rich phase during sintering results in excessive localized demagnetization and poor consistency of microscopic coercivity, thus worsening the squareness.

[0096] In Example 9, the hydrogen decomposition temperature was 600℃, resulting in magnet B10. The results in Table 2 show that the coercivity of magnet B9 decreased significantly. This may be because the excessively high dehydrogenation temperature is similar to high-temperature aging, which can lead to the agglomeration and growth of rare earth phases, which is not conducive to the formation of continuous rare earth-rich phases during sintering and aging. The original hydrogen cracks will also re-weld, and the air jet mill is prone to producing polycrystalline powder with complex orientations, which is prone to local self-demagnetization, thus significantly reducing the magnetic properties.

[0097] As shown in Table 4, the Cu content of magnet B5 is designed to be 0.2 wt.%, which is lower than that of magnets B1 and B3 (0.26 wt.%). Therefore, it exhibits different high-temperature resistance at 150℃. The β(Hcj-150℃) of magnet B5 is -0.543% / ℃, while those of magnets B1 and B3 are -0.529% / ℃ and -0.531% / ℃, respectively. In the high-Ga, low-B system, although the magnet forms Re6Fe through aging... 13 The (Cu,Ga) phase exhibits high coercivity, but its high-temperature resistance is weaker compared to Dy-containing magnets of the same grade. This invention increases the Cu content in the magnet; Cu mainly exists in the grain boundaries of the magnet and participates not only in Re6Fe... 13 The formation of the (Cu,Ga) phase can further enhance the magnet's high-temperature resistance.

[0098] Magnet B3 was cut into three batches, and their bending strength was tested separately to obtain the bending force F-displacement X curve (as shown in the figure). Figure 6 As shown in Table 5, after conversion using the formula, their bending strength all reach over 500MPa, which meets the bending strength requirements (over 420MPa) of automotive motor magnets.

[0099] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A high-temperature resistant, heavy rare-earth-free RTB permanent magnet, characterized in that, The elemental composition of the magnet is expressed as a mass percentage: R: 31.8~32.8 wt.%, R includes Pr and Nd, where the mass of Pr is less than 25% of the total mass of R and the mass of Nd is more than 75% of the total mass of R; Al: 0~0.1wt.%; Cu: 0.25~0.35 wt.%; Ga: 0.35~0.45 wt.%; Co: 1~1.6wt.%; B: 0.88~0.91 wt.%; M: M is Zr or Zr and Ti. When M is pure Zr, the mass percentage of M is m1(Zr) = 0.25~0.35 wt.%; when M is Zr and Ti, the masses of Zr and Ti are m2(Zr) and m2(Ti) respectively, m2(Zr) is 0.05~0.25 wt.% and m2(Ti) is 0.05~0.15 wt.%, satisfying the mass fraction of M as m2(Zr) + m2(Ti) = 0.2~0.3 wt.%, m2(Ti) = (0.35 - m2(Zr)) × 0.5; T: Balance, where T represents Fe and other unavoidable impurities; The average grain size of the magnet is ≤3.6μm, wherein the number of grains with a grain size >3.6μm accounts for 38~46%, and the number of grains with a grain size <3μm accounts for ≥39~45%. In an environment of 150℃, its α(Br)≥-0.120% / ℃ and β(Hcj)≥-0.532% / ℃.

2. The high-temperature resistant, heavy rare-earth-free RTB permanent magnet as described in claim 1, characterized in that, The performance at room temperature (20℃) is: Br≥13.7kGs, Hcj≥20kOe, SQ≥0.

97.

3. The high-temperature resistant, heavy rare-earth-free RTB permanent magnet as described in claim 1, characterized in that, The flexural strength is greater than 500 MPa.

4. The method for preparing a high-temperature resistant, heavy rare-earth-free RTB permanent magnet as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1: Take the magnet raw materials according to the component ratio and prepare the alloy rapid solidification sheet by vacuum melting; S2: The alloy quick-solidification sheet obtained in step S1 above is subjected to hydrogen crushing treatment to obtain alloy coarse powder; S3: The alloy coarse powder from step S2 above is subjected to air jet milling. The air jet mill is continuously fed to obtain NdFeB fine powder. The surface area density (SMD) of NdFeB fine powder is 3.0 ± 0.1 μm, with X10 ranging from 1.60 to 1.80 μm and X90 ranging from 6.50 to 6.9 μm, and X90 / X10 = 3.8 to 4.

25. S4: The NdFeB fine powder obtained in step S3 above is subjected to orientation molding, secondary sintering and two-stage aging treatment to obtain a high-temperature resistant, heavy rare earth-free RTB permanent magnet.

5. The method as described in claim 4, characterized in that, In step S2, the dehydrogenation temperature during hydrogen crushing is 550~580℃.

6. The method as described in claim 4, characterized in that, In step S3, the air jet mill process is as follows: the air jet mill is continuously fed; when the final remaining powder in the mill chamber drops to 50-85% of the designed weight of the powder in the mill chamber, the speed of the air jet mill sorting wheel is increased by 200-300 rpm compared to the original speed, and continues to run until the entire batch of powder is discharged.

7. The method as described in claim 6, characterized in that, When the final remaining powder in the grinding chamber accounts for 65-80% of the designed weight of the powder in the grinding chamber, the speed of the air classifier wheel increases by 200-300 rpm compared to the original speed.

8. The method as described in claim 4, characterized in that, In step S4, the secondary sintering process is as follows: holding at 1070~1100℃ for 4~8 hours, then cooling down to 1000~1060℃ and holding for 4~6 hours.

9. The method as described in claim 4, characterized in that, In step S4, the two-stage aging process is as follows: the first-stage aging process is to keep the temperature at 880~910℃ for 3~4.5h, and the second-stage aging process is to keep the temperature at 455~465℃ for 3~4.5h.

10. The application of the high-temperature resistant, heavy rare earth-free RTB permanent magnet as described in any one of claims 1 to 3 in the field of automotive motors.

Images