Long-life neodymium-iron-boron magnet, preparation method and life evaluation method thereof

Abstract

The invention provides a long-service-life neodymium-iron-boron magnet, a preparation method and a service life evaluation method thereof, belongs to the technical field of rare earth permanent magnet material preparation, and solves the technical problem that the service life of a neodymium-iron-boron magnet is short. The long-life neodymium-iron-boron magnet comprises the following components in percentage by mass: (NdxRE1-x) aCobCucAldZreMfFebalBg, wherein x is more than 0 and less than or equal to 1, a is more than or equal to 27 and less than or equal to 33, b is more than 0 and less than or equal to 25, c is more than 0 and less than or equal to 1, d is more than 0 and less than or equal to 1, b / (c + d) is 10-15, e is more than or equal to 0.1 and less than or equal to 0.5, f is more than or equal to 0 and less than or equal to 0.4, g is more than or equal to 0.85 and less than or equal to 1.1, and the balance is bal. RE is one or a combination of more of Pr, Ce, La, Dy, Tb, Ho and Gd; m is one or a combination of more of Nb, Ti, V and Mo; the volume of the magnet surface layer area accounts for 10-30% of the total volume of the magnet The mass content of Co in the surface layer area of the magnet is 1.5-5% more than that in the core area, the mass content of Cu in the surface layer area of the magnet is 0.05-0.25% more than that in the core area, and the mass content of Al in the surface layer area of the magnet is 0.05-0.25% more than that in the core area; the average size of crystal grains in the surface layer area is 2.8-4.5 microns, and the average size of crystal grains in the core area is 4.6-10 microns. Compared with magnets of the same brand, the service life of the neodymium-iron-boron magnet is prolonged by at least 10 years or above.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnet material preparation technology, and relates to a long-life neodymium iron boron magnet, its preparation method and its life assessment method. Background Technology

[0002] Sintered neodymium iron boron (NdFeB) permanent magnets, as an important class of rare-earth functional materials, are widely used in new energy vehicles, intelligent robots, wind power generation, and other fields. In permanent magnet devices, NdFeB magnets primarily provide a stable magnetic field environment, and the decay of the magnet's magnetic properties to a certain level is typically considered a device failure condition. Currently, the main indicator characterizing magnetic property decay is irreversible flux loss, which is the percentage of magnetic flux loss relative to the initial flux after changes in the external environment. Generally, irreversible flux loss below 5% is considered acceptable. Related research indicates that the main factors causing magnetic decay in NdFeB magnets are temperature and a reverse magnetic field, with the magnet surface being most susceptible to demagnetization.

[0003] Therefore, to reduce magnetic attenuation and improve the lifespan of NdFeB magnets, two approaches are needed: first, improving the temperature resistance of NdFeB magnets; and second, improving the magnet's orientation, i.e., reducing magnetic declination. Improving the temperature resistance of NdFeB magnets typically involves increasing the magnet's Hcj (highest temperature coefficient), but this often requires adding expensive heavy rare earth elements like Dy or Tb, significantly increasing the magnet's cost. Another method is to increase the Curie temperature of the magnet by adding metallic elements. Chinese invention patent CN107134338B raised the Curie temperature of NdFeB to 370–390°C by adding Zn and Gd elements. However, this method has the problem that Gd is expensive, and when Gd replaces Nd in the main phase, it reduces the magnet's remanence and magnetic energy product. On the other hand, improving the magnet's orientation usually requires increasing the orientation magnetic field. However, due to limitations in the performance of the magnetic materials in the press's magnetic circuit, the magnetic field is generally difficult to exceed 2T; alternatively, multiple magnetization and orientation processes can be performed during pressing, but this significantly reduces production efficiency and is ineffective.

[0004] Furthermore, current assessments of permanent magnet degradation during service rely solely on intrinsic magnetic properties such as remanence (Br), coercivity (Hcb), intrinsic coercivity (Hcj), and maximum energy product (BH)max, as well as the "thermal demagnetization" index under open-circuit conditions. However, in actual service, magnets are not used in an open-circuit state; the reverse demagnetization field they experience is directly related to the workload. Therefore, short-term thermal demagnetization tests are insufficient to accurately reflect the magnet's durability and lifespan.

[0005] Therefore, how to improve the service life of NdFeB magnets in a low-cost and efficient manner, and how to establish a reliable method for assessing the service life of magnets, have become urgent technical problems to be solved. Summary of the Invention

[0006] Based on the above analysis, this invention proposes a long-life NdFeB magnet, its preparation method, and its life assessment method, which solves the technical problems of short lifespan of NdFeB magnets in the prior art, or the high cost, low efficiency, and poor effect of improving the lifespan of NdFeB magnets, as well as the lack of reliable magnet service life assessment methods.

[0007] On one hand, the present invention provides a long-life neodymium iron boron magnet, wherein the composition of the magnet by mass percentage is: (Nd x RE 1-x ) a Co b Cu c Al d Zr e M f Fe bal B g , 0<x≤1, 27≤a≤33, 0<b≤25, 0<c≤1, 0<d≤1, b / (c+d)=10~15, 0.1≤e≤0.5, 0≤f≤0.4, 0.85≤g≤1.1, bal is the balance; RE is one or more of Pr, Ce, La, Dy, Tb, Ho, Gd; M is one or more of Nb, Ti, V, Mo;

[0008] The surface region of the magnet accounts for 10-30% of the total volume of the magnet in the pressing direction, and the upper and lower surface regions have equal volumes. The Co mass content in the surface region is 1.5-5% higher than that in the core region, the Cu mass content is 0.05-0.25% higher, and the Al mass content is 0.05-0.25% higher. The average grain size in the surface region is 2.8-4.5 μm, and the average grain size in the core region is 4.6-10 μm.

[0009] Furthermore, the Co mass content in the surface region of the magnet is 10 to 15 times the sum of the Cu and Al mass contents.

[0010] Furthermore, the composition of the magnet by mass percentage is: (Nd x RE 1-x ) a Co b Cu c Al d Zr e M f Fe bal B g, 0.1≤x≤0.8, 29≤a≤31, 2≤b≤20, 0.1≤c≤0.8, 0.1≤d≤0.8, b / (c+d)=10~15, 0.2≤e≤0.4, 0.1≤f≤0.3, 0.90≤g≤1.0, bal is the balance; RE is one or more of Pr, Ce, La, Dy, Tb, Ho, Gd; M is one or more of Nb, Ti, V, Mo.

[0011] Furthermore, the remanence Br of the magnet is 10.5–15.5 kGs, the intrinsic coercivity Hcj is 8–40 kOe, the Curie temperature of the surface region of the magnet is 325–585 °C, and the Curie temperature of the core region is 310–570 °C.

[0012] Furthermore, when the magnet operates at high temperature and in a reverse magnetic field, 0 < magnetic declination ≤ 1.5°.

[0013] On the other hand, the present invention also provides a method for preparing the above-mentioned magnet, comprising the following steps:

[0014] Step 1: Preparation of NdFeB alloy powder for the surface region: Prepare the alloy powder for the surface region according to the process steps of batching - rapid solidification and spinning - hydrogen crushing - air jet milling - powder mixing. The thickness of the rapid solidification and spinning sheet is 0.15-0.25 mm; the grinding gas pressure of the air jet mill is 0.7-0.9 MPa, the classifier speed is 4000-4500 rpm, and no oxygen is added during the air jet milling; add additives to the fine powder of the air jet mill, and the mass of the additives is 0.025-0.05% of the powder mass;

[0015] Step 2: Preparation of NdFeB alloy powder in the core region: The alloy powder in the core region is prepared according to the following steps: batching, rapid solidification and slaking, hydrogen breaking, air jet milling, and powder mixing. Compared with the surface region, the alloy composition has reduced Co by 1.5-5%, Cu by 0.05-0.25%, and Al by 0.05-0.25%. The thickness of the rapid solidification slaking sheet is 0.25-0.45 mm. The grinding gas pressure of the air jet mill is 0.5-0.8 MPa, the classifier wheel speed is 3200-3800 rpm, and oxygen is supplemented during air jet milling. Additives are added to the fine powder of the air jet mill, and the mass of the additives is 0.01-0.02% of the powder mass.

[0016] Step 3: Fabrication of NdFeB green blanks: The two types of powders from Step 1 and Step 2 are added to a forming mold with an orientation magnetic field in the following order: NdFeB alloy powder in the surface region - NdFeB alloy powder in the core region - NdFeB alloy powder in the surface region. The powders are then oriented and formed, with localized magnetic field enhancement applied to the orientation magnetic field. The upper and lower surface regions of the blank have equal volumes, and the sum of the volumes of the upper and lower surface regions accounts for 10-30% of the total volume of the blank. The oriented blank is then placed in a cold isostatic press for further densification.

[0017] Step 4, Vibration Sintering and Secondary Tempering: The green blank is pushed into a vacuum sintering furnace for sintering. After pre-vacuuming, the temperature is raised and held between 400 and 600°C for degreasing, then held between 750 and 850°C for dehydrogenation. The temperature is then raised to the sintering temperature, which is between 1030 and 1100°C. After holding at this temperature for 1 to 3 hours, the temperature is cooled to below 850°C. The temperature is then raised again to the sintering temperature, and this process is repeated 2 to 5 times. Finally, the blower is turned on in the furnace, and argon gas is introduced to rapidly cool the temperature to below 70°C. The temperature is then raised to 850 to 950°C and held for 2 to 6 hours for primary tempering. The blower is then turned on and argon gas is introduced to rapidly cool the temperature to below 70°C. Finally, the temperature is raised to 450 to 650°C and held for 2 to 6 hours for secondary tempering. The blower is then turned on and argon gas is introduced to rapidly cool the temperature to below 70°C to obtain the final magnet.

[0018] Furthermore, in step 3, the orientation magnetic field is 1.5 to 2 T, and the local magnetic field enhancement is 0.1 to 0.5 T higher than the orientation magnetic field. The material used for the local magnetic field enhancement is a high-permeability soft magnetic alloy reinforcement block.

[0019] Furthermore, the reinforcing blocks are symmetrically arranged on both sides of the upper and lower surface areas of the blank along the orientation magnetic field direction. The shape of the reinforcing blocks is a trapezoidal platform magnetic focusing design. The area ratio of the lower and upper surfaces of the trapezoidal platform is 10:9 to 10:4, and the ratio of the height of the upper surface of the trapezoidal platform to the height of the surface area is 10:5 to 10:1.

[0020] Finally, the present invention also provides a method for evaluating the lifetime of a magnet prepared by the above-described magnet or by the above-described method, comprising the following steps:

[0021] S1: Determine the temperature T and the reverse magnetic field H experienced by the neodymium iron boron magnet during service. d ;

[0022] S2: Determine the irreversible flux loss h of the neodymium iron boron magnet. irr Criterion;

[0023] S3: Select the sample with the smallest knee coercivity Hk and the smallest orientation direction dimension from the batch of magnets as the test and evaluation sample;

[0024] S4: Magnetize the sample to be evaluated. The magnetizing magnetic field is 2 to 3 times or more than the intrinsic coercivity Hcj of the sample. Measure the initial magnetic flux value of the sample at room temperature and record it as φ(t0).

[0025] S5: Place the magnet in the reverse magnetic field device, and then place the whole thing in a high-temperature oven, simultaneously applying temperature T and a reverse magnetic field H. dAfter being kept at a certain temperature for a period of time, the sample was cooled to room temperature and removed. The magnetic flux value φ(t) after the experiment was tested. The experiment was conducted at least three times at different holding times to obtain three different magnetic flux values ​​φ(t).

[0026] S6: Substitute the at least three different magnetic flux values ​​φ(t) obtained in step S5 into the formula φ(t)=φ(t0)-Aln(t / t0) to calculate the coefficient A and plot h. irr The fitted plot of (t)=[φ(t)-φ(t0)] / φ(t0)=[-Aln(t / t0)] / φ(t0);

[0027] S7: In the fitting plot of step S6, based on h in step S2 irr The value is used as a control line, which is parallel to h. irr The t value corresponding to the intersection point of (t) is the lifetime of the magnet, which represents the minimum lifetime value of a batch of magnets.

[0028] Furthermore, in step S3, sampling is conducted according to the national standard GB / T 2828.1-2012 "Sampling Procedures for Inspection by Attributes - Part 1: Sampling Schemes for Lot-by-Lot Inspection Retrieved by Acceptable Quality Limit (AQL)" to obtain evaluation samples by sampling different batch quantities and quality requirement levels.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] 1. This invention achieves a balance in the demagnetization effect between the surface and core regions of a neodymium iron boron magnet by designing differences in composition and grain size between the two regions, thereby extending the service life of the neodymium iron boron magnet.

[0031] 2. The neodymium iron boron magnet of the present invention specifies Co, Al and Cu as essential elements. While adding Co to increase the Curie temperature, appropriate amounts of Cu and Al are added to optimize the microstructure and offset the adverse effects of Co.

[0032] 3. This invention achieves the control of the magnetic declination of NdFeB magnets within 1.5° by combining component stratification control, powder particle size zoning control, additive zoning control (e.g., the additive mass in step 1 is 0.025-0.05% of the powder mass, and the additive mass in step 2 is 0.01-0.02% of the powder mass) and magnetic field orientation compensation technology. At the same time, the surface region of the magnet that is prone to demagnetization has a higher Curie temperature and intrinsic coercivity, which improves the magnet's resistance to demagnetization under high temperature and reverse magnetic field, resulting in a long-life NdFeB magnet.

[0033] 4. This invention improves the wetting effect of rare earth phases on the main phase grains by oscillating sintering at a melting point of 850-900℃ for the rare earth-rich grain boundary phase. On the one hand, this can improve the density of the magnet, reduce the porosity in the magnet, and thus achieve high corrosion resistance of the magnet. On the other hand, it enhances the demagnetizing coupling effect of the rare earth phase, significantly improves the intrinsic coercivity of the magnet, improves the magnet's resistance to demagnetization, and extends the service life of NdFeB magnets.

[0034] 5. Compared with magnets of the same grade, the neodymium iron boron magnets of the present invention have a lifespan that is increased by at least 10 years under the same service conditions and failure criteria; the difference in the coefficient of thermal expansion between the parallel magnetization direction and the perpendicular magnetization direction is 5-50% lower than that of magnets of the same grade; and the Curie temperature is 15-200°C higher than that of magnets of the same grade.

[0035] 6. This invention simulates the working conditions of magnets under actual use, and combines the counting sampling inspection method to select the conditions with the highest probability of demagnetization for testing. By fitting the irreversible magnetic flux loss versus time curve, the minimum service life of permanent magnets can be obtained. This method can reduce quality loss and quality risks, providing strong support for users to select and use materials, while also avoiding material waste caused by the inability of permanent magnet materials to fully perform. Attached Figure Description

[0036] Figure 1 This is a three-dimensional orientation diagram of the neodymium iron boron magnet of the present invention;

[0037] Figure 2 This is a schematic diagram of the partitioning of the neodymium iron boron magnet of the present invention;

[0038] Figure 3 This is a schematic diagram of the orientation and molding process of the neodymium iron boron magnet of the present invention;

[0039] Figure 4 This is a three-dimensional schematic diagram of the reinforcing block during the orientation forming of the NdFeB blank of the present invention;

[0040] Figure 5 This is a process diagram of the oscillation sintering and secondary tempering of the neodymium iron boron magnet of the present invention;

[0041] Figure 6 These are grain morphology diagrams of the surface region (a) and core region (b) of the neodymium iron boron magnet of the present invention;

[0042] Figure 7 This is a schematic diagram of the reverse magnetic field device used in the lifetime assessment of this invention.

[0043] Figure 8 The graph shows the predicted relationship between irreversible magnetic loss of magnets and service time for Example 1, Comparative Example 1, and Comparative Example 2.

[0044] In the diagram: 1-NdFeB magnet; X, Y, Z-Spatial rectangular coordinate axes; N, S-Magnetic field poles; 11-Surface area; 1101-Upper surface area; 1102-Lower surface area; 12-Core area; 2-Lower pressure head; 3-Mold side plate; 4-Reinforcement block; 41-Lower bottom surface of reinforcement block; 42-Upper bottom surface of reinforcement block; L-Length of upper bottom surface of reinforcement block; H-Height of upper bottom surface of reinforcement block; 5-NdFeB magnet blank; 51-Surface area of ​​blank; 5101-Upper surface area of ​​blank; 5102-Lower surface area of ​​blank; 52-Core area of ​​blank; 6-Upper pressure head; 7-Orientation magnetic field; 8-Magnet under test; 9-Magnetic pole shoe; 10-Electromagnetic coil; 11-Magnetic frame; 12-Direction of reverse magnetic field; 13-Direction of magnetic field of the magnet under test. Detailed Implementation

[0045] The following detailed description, in conjunction with specific embodiments, provides a long-life neodymium iron boron magnet, its preparation method, and its life assessment method. These embodiments are for illustrative purposes only, and the present invention is not limited to these embodiments.

[0046] On one hand, the present invention provides a long-life neodymium iron boron magnet, wherein the composition of the magnet by mass percentage is: (Nd x RE 1-x ) a Co b Cu c Al d Zr e M f Fe bal B g , 0<x≤1, 27≤a≤33, 0<b≤25, 0<c≤1, 0<d≤1, b / (c+d)=10~15, 0.1≤e≤0.5, 0≤f≤0.4, 0.85≤g≤1.1, bal is the balance; RE is one or more of Pr, Ce, La, Y, Dy, Tb, Ho, Gd; M is one or more of Nb, Ti, V, Mo;

[0047] For example (Nd) x Pr 1-x ) 30 Co b Cu c Al d Zr e Fe bal B, where 0 < x ≤ 1, 27 ≤ a ≤ 33, 0 < b ≤ 25, 0 < c ≤ 1, 0 < d ≤ 1, b / (c+d) = 10~15, 0.1 ≤ e ≤ 0.5, 0.85 ≤ g ≤ 1.1, and bal is the remainder. For example, such as (Nd 0.75 Pr 0.25 ) 30 Co2.5 Cu 0.12 Al 0.12 Zr 0.17 Fe bal B 0.9 .

[0048] For example (Nd) x Pr x 'Dy 1-x-x ') a Co b Cu c Al d Zr e Nb f Fe bal B g Where 0 < x ≤ 1, 0 < x' + x ≤ 1, 27 ≤ a ≤ 33, 0 < b ≤ 25, 0 < c ≤ 1, 0 < d ≤ 1, b / (c + d) = 10 ~ 15, 0.1 ≤ e ≤ 0.5, 0 ≤ f ≤ 0.4, 0.85 ≤ g ≤ 1.1, and bal is the remainder. For example, (Nd) 0.70 Pr 0.10 Dy 0.20 ) 31 Co 26.35 Cu 0.875 Al 0.875 Zr 0.4 Nb 0.4 Fe bal B 1.1 ;

[0049] such as Nd a Co b Cu c Al d Zr e Fe bal B g Where 27≤a≤33, 0<b≤25, 0<c≤1, 0<d≤1, b / (c+d)=10~15, 0.1≤e≤0.5, 0.85≤g≤1.1, and bal is the remainder; for example, such as Nd 27 Co 13 Cu 0.5 Al 0.5 Zr 0.3 Fe bal B 1.0 wait.

[0050] The remanence Br of neodymium iron boron magnets is 10.5–15.5 kGs, and the intrinsic coercivity Hcj is 8–40 kOe.

[0051] Figure 1This is a three-dimensional orientation diagram of the neodymium iron boron magnet of the present invention. The direction of the Z-axis in the figure is the pressing direction for preparing the magnet, and the direction of the Y-axis is the direction of the magnetic field. The size range of the neodymium iron boron magnet in the Z-axis direction is 10-100mm; the size range in the Y-axis direction is 10-90mm; and the size range in the X-axis direction is 10-150mm.

[0052] Figure 2 This is a schematic diagram of the partitioning of the neodymium iron boron magnet of the present invention. The composition and grain size of the neodymium iron boron magnet differ between the surface region 11 and the core region 12 in the pressing direction. The Co mass content in the surface region 11 is 1.5–5% higher than that in the core region 12, e.g., 3%; the Cu mass content is 0.05–0.25% higher than that in the core region 12, e.g., 0.15%; the Al mass content is 0.05–0.25% higher than that in the core region 12, e.g., 0.15%; the average grain size in the surface region 11 is 2.8–4.5 μm, e.g., 3.5 μm; the average grain size in the core region 12 is 4.6–10 μm, e.g., 7.5 μm.

[0053] The Curie temperature of the surface region 11 of the magnet is 325–585℃, and that of the core region 12 is 310–570℃.

[0054] The volume of the surface region 11 of the magnet accounts for 10-30% of the total volume of the magnet. The upper surface region 1101 and the lower surface region 1102 have equal volumes. The density of the magnet is 7.4-7.8 g / cm³. 3 .

[0055] When neodymium iron boron magnets operate at high temperatures and in a reverse magnetic field, 0 < magnetic declination ≤ 1.5°.

[0056] It should be noted that, compared with the conventional NdFeB magnet composition, this invention limits Co, Al and Cu to three essential elements. Co is added to increase the Curie temperature, but this will lead to the deterioration of the microstructure. Therefore, appropriate amounts of Cu and Al are added at the same time to optimize the microstructure and offset the adverse effects of Co. For example, the mass of Co is 10 to 15 times the total mass of Cu and Al.

[0057] Furthermore, the Co mass content in the surface region of the magnet is 10 to 15 times the sum of the Cu and Al mass contents.

[0058] The surface region of a magnet is typically a "weak zone" prone to demagnetization failure, primarily because its self-demagnetization effect is higher than that of the core region. To balance the demagnetization effects between the surface and core regions, the intrinsic coercivity (Hcj) and Curie temperature of the surface region must be higher than those of the core region. Increasing Hcj involves grain refinement, and the most effective way to increase the Curie temperature is by adding Co. Therefore, in this invention, the average grain size of the surface region is smaller than that of the core region, and the Co content is higher. Since Co degrades the microstructure, appropriate amounts of Al and Cu must be added simultaneously to optimize the microstructure and offset the adverse effects of Co. Because the Co content is higher in the surface region than in the core region, naturally, the Al and Cu content in the surface region must also be higher than in the core region. In addition, in order to maintain the same demagnetization effect between the surface region and the core region of the magnet, the Co, Al, and Cu content in the surface region should be appropriately higher than that in the core region, and the average grain size in the surface region should also be appropriately smaller than that in the core region. At the same time, the volume of the surface region should be limited to 10% to 30% of the total volume of the magnet, such as 15%, 20%, 25%, etc.

[0059] Furthermore, the composition of the long-life NdFeB magnet was optimized, and the mass percentage was: (Nd x RE 1-x ) a Co b Cu c Al d Zr e M f Fe bal B g , 0.1≤x≤0.8, 29≤a≤31, 2≤b≤20, 0.1≤c≤0.8, 0.1≤d≤0.8, b / (c+d)=10~15, 0.2≤e≤0.4, 0.1≤f≤0.3, 0.90≤g≤1.0, bal is the balance; RE is one or more of Pr, Ce, La, Dy, Tb, Ho, Gd; M is one or more of Nb, Ti, V, Mo.

[0060] On the other hand, the present invention also provides a method for preparing the above-mentioned long-life neodymium iron boron magnet, comprising the following steps:

[0061] Step 1: Preparation of NdFeB alloy powder for the surface region: Prepare the alloy powder for the surface region according to the process steps of batching - rapid solidification and spinning - hydrogen crushing - air jet milling - powder mixing. The thickness of the rapid solidification and spinning sheet is 0.15-0.25 mm; the grinding gas pressure of the air jet mill is 0.7-0.9 MPa, the classifier speed is 4000-4500 rpm, and no oxygen is added during the air jet milling; add additives to the fine powder of the air jet mill, and the mass of the additives is 0.025-0.05% of the powder mass;

[0062] Step 2: Preparation of NdFeB alloy powder in the core region: The alloy powder in the core region is prepared according to the following steps: batching, rapid solidification and slaking, hydrogen breaking, air jet milling, and powder mixing. Compared with the surface region, the alloy composition has reduced Co by 1.5-5%, Cu by 0.05-0.25%, and Al by 0.05-0.25%. The thickness of the rapid solidification slaking sheet is 0.25-0.45 mm. The grinding gas pressure of the air jet mill is 0.5-0.8 MPa, the classifier wheel speed is 3200-3800 rpm, and oxygen is supplemented during air jet milling. Additives are added to the fine powder of the air jet mill, and the mass of the additives is 0.01-0.02% of the powder mass.

[0063] Step 3: Fabrication of NdFeB green blanks: The two types of powders from Step 1 and Step 2 are added to a forming mold with an orientation magnetic field in the following order: NdFeB alloy powder in the surface region - NdFeB alloy powder in the core region - NdFeB alloy powder in the surface region. The powders are then oriented and formed, with localized magnetic field enhancement applied to the orientation magnetic field. The upper and lower surface regions of the blank have equal volumes, and the sum of the volumes of the upper and lower surface regions accounts for 10-30% of the total volume of the blank. The oriented blank is then placed in a cold isostatic press for further densification.

[0064] Step 4: Vibration Sintering and Secondary Tempering: The green blank is pushed into a vacuum sintering furnace for sintering. After pre-vacuuming, the temperature is raised and held between 400 and 600°C for degreasing, then held between 750 and 850°C for dehydrogenation. The temperature is then raised to the sintering temperature of 1030–1100°C and held for 1–3 hours. The temperature is then cooled to below 850°C, and the temperature is raised again to the sintering temperature. This process is repeated 2–5 times to achieve the vibration sintering effect. Finally, the furnace fan is turned on, and argon gas is introduced for rapid cooling to below 70°C. The temperature is then raised to 850–950°C and held for 2–6 hours for primary tempering. The temperature is then rapidly cooled to below 70°C with the fan turned on and argon gas introduced. Finally, the temperature is raised to 450–650°C and held for 2–6 hours for secondary tempering. The temperature is then rapidly cooled to below 70°C with the fan turned on and argon gas introduced, yielding the final magnet.

[0065] Figure 5 This is a process diagram of the oscillation sintering and secondary tempering of the neodymium iron boron magnet of the present invention. Figure 6 The figures show the grain morphology of the surface region (a) and core region (b) of the NdFeB magnet of the present invention. As can be seen from the figures, the average grain size of the surface region of the magnet is 4.5 μm, and the average grain size of the core region of the magnet is 7 μm.

[0066] Specifically, in step 1, the grinding gas used in the air jet mill is any one of nitrogen, argon, or helium, with nitrogen or argon being commonly used. The oxygen content of the powder obtained by the air jet mill is 20–100 ppm, and the median particle size, or D50, of the powder is 2.5–3.5 μm.

[0067] In step 2, the grinding gas used in the air jet mill is any one of nitrogen, argon, or helium, with nitrogen or argon being commonly used. The oxygen content of the powder obtained by the air jet mill is 150–200 ppm, and the median particle size, or D50, of the powder is 3.5–5.5 μm.

[0068] In steps 1 and 2, additives are added to the air-jet mill powder. The additives play a role in lubrication and anti-oxidation. The main components of the additives are organic liquids such as esters, alkanes, alcohols, and acids, or mixtures of several, such as glyceryl monostearate, tributyl borate, oleic acid, isopropanol, etc.

[0069] It should be noted that the difference in process parameters between steps 1 and 2 is to ensure that the particle size of the NdFeB alloy powder in the surface region is finer than that in the core region. This results in a smaller grain size in the surface region after sintering of the NdFeB green compact compared to the core region. Ultimately, this grain refinement improves the coercivity Hcj of the magnet's surface region, thereby enhancing its demagnetization resistance. Specifically, to achieve a finer powder particle size, compared to step 2, step 1 requires reducing the thickness of the rapid-setting sheet while increasing the grinding pressure and classifying wheel speed of the air jet mill. After refining the particle size of the surface NdFeB alloy powder, the proportion of additives in the surface region powder needs to be increased to improve powder flowability and prevent oxidation. The purpose of oxygen supplementation during air jet milling is to reduce the activity of the powder and prevent local oxidation. In step 1, oxygen supplementation is not performed on the surface NdFeB alloy powder during air jet milling because the powder particle size is relatively fine and the flowability is poor, making it difficult to achieve uniform oxygen supplementation. In step 2, the NdFeB alloy powder in the core area has a larger particle size and better flowability, so oxygen supplementation can be performed appropriately to reduce the powder activity, and at the same time, the proportion of additives can be reduced.

[0070] In step 3, during orientation molding, the orientation magnetic field is 1.5 to 2T, the pressure is 20 to 40 MPa, and the local magnetic field enhancement is 0.1 to 0.5T higher than the orientation magnetic field. The material used for local magnetic field enhancement is the reinforcing block of a high-permeability soft magnetic alloy, such as 1J22 / 1J50 / DT4. Figure 3 This is a schematic diagram of the orientation molding process of the neodymium iron boron magnet of the present invention. In the diagram, the Z-axis direction is the pressing direction, the Y-axis direction is the direction of the orientation magnetic field, and four reinforcing blocks 4 are symmetrically arranged on both sides of the upper surface region 5101 and the lower surface region 5102 of the blank along the orientation magnetic field direction. Figure 4 This is a three-dimensional schematic diagram of the reinforcing block during the orientation molding of the NdFeB blank of the present invention. The reinforcing block 4 has a trapezoidal platform-like magnetic design. The area ratio of the lower base 41 to the upper base 42 of the trapezoidal platform ranges from 10:9 to 10:4. The ratio of the height H of the upper base of the trapezoidal platform to the height of the surface area is 10:5 to 10:1. The length L of the upper base of the trapezoidal platform is approximately equal to the length of the surface area. The density of the blank after orientation molding is 3.6–3.9 g / cm³.3 The oriented blank is then placed in a cold isostatic press for further densification at a pressure of 180–250 MPa, resulting in a green blank density of 3.9–4.3 g / cm³. 3 In steps 1 to 3, this invention achieves magnetic declination of NdFeB magnets within 1.5° by combining component stratification control, powder particle size zoning control, additive zoning control, and magnetic field orientation compensation technology. At the same time, the surface region of the magnet, which is prone to demagnetization, has a higher Curie temperature and intrinsic coercivity, which improves the magnet's resistance to demagnetization under high temperature and reverse magnetic field, resulting in long-life NdFeB magnets.

[0071] In step 4, the green billet is pushed into a vacuum sintering furnace for sintering. Before heating, a pre-vacuum is drawn, and the temperature is raised after the vacuum level is below 0.1 Pa. The billet is held at 400-600℃ for 2-4 hours for degreasing, and then held at 750-850℃ for 2-4 hours for dehydrogenation. The temperature is then raised to the sintering temperature at a rate not exceeding 5℃ / min, and the sintering temperature is between 1030-1100℃. After holding at this temperature for 1-3 hours, the temperature is cooled to below 850℃ at a rate of 5-20℃ / min. The temperature is then raised to the sintering temperature again at a rate not exceeding 5℃ / min. This process is repeated 2-5 times to achieve an oscillating sintering effect. During sintering, the vacuum level is kept below 0.1 Pa. Finally, the blower is turned on in the furnace, and argon gas is introduced to rapidly cool the furnace to below 70℃. Then, the temperature is increased to 850–950℃ at a rate not exceeding 10℃ / min and held for 2–6 hours for primary tempering. Then, the temperature is rapidly cooled to below 70℃ by turning on the fan and introducing argon gas. Finally, the temperature is increased to 450–650℃ at a rate not exceeding 10℃ / min and held for 2–6 hours for secondary tempering. Then, the temperature is rapidly cooled to below 70℃ by turning on the fan and introducing argon gas to obtain the final magnet, with a density of 7.4–7.8 g / cm³. 3 .

[0072] This invention proposes an oscillating sintering process. By oscillating sintering at around 850–900°C (the melting point of the rare earth-rich grain boundary phase), such as between 850 and 1100°C (the upper limit of the sintering temperature exceeds the melting point due to the need for sintering overheating), the wetting effect of the rare earth-rich phase on the main phase grains is improved. On the one hand, this can increase the density of the magnet, reduce the porosity in the magnet, and thus achieve high corrosion resistance of the magnet. On the other hand, it enhances the demagnetizing coupling effect of the rare earth-rich phase, which can significantly improve the intrinsic coercivity of the magnet, improve the magnet's resistance to demagnetization, and extend the service life of NdFeB magnets.

[0073] Compared with magnets of the same grade, the NdFeB magnets of this invention have a lifespan that is increased by at least 10 years under the same service conditions and failure criteria. "Same grade magnets" refers to similar magnets whose remanence (Br) differs from that of the NdFeB magnets of this invention by less than 0.3 kGs and whose intrinsic coercivity (Hcj) differs by less than 2 kOe. The difference in the coefficient of thermal expansion between the parallel magnetization direction and the perpendicular magnetization direction of the NdFeB magnets of this invention is 5 × 10⁻⁶. -6 ~9×10 -6 K -1 That is, the thermal expansion anisotropy is 5-50% lower than that of the same grade of magnet. The Curie temperature of the neodymium iron boron magnet of the present invention is 15-200°C higher than that of the same grade of magnet.

[0074] This invention reduces the overall magnetic declination of the magnet by controlling particle size, lubrication characteristics, and enhancing the orientation of the magnetic field. At the same time, it improves the Curie temperature, coercivity, and density of the magnet by controlling the composition in different zones and using an oscillating sintering process. Under the durability test conditions of high temperature reverse magnetic field, it significantly reduces the rate at which the magnetic flux decays over time and improves the lifespan of the magnet.

[0075] Finally, in order to evaluate the lifetime of the long-life NdFeB magnet prepared by the invention, the present invention also proposes a method for evaluating the lifetime of NdFeB magnets, including the following steps:

[0076] S1: Determine the temperature T and the reverse magnetic field H experienced by the neodymium iron boron magnet during service. d ;

[0077] S2: Determine the irreversible flux loss h of the neodymium iron boron magnet. irr Criterion;

[0078] S3: Select the sample with the smallest knee coercivity Hk and the smallest orientation direction dimension from the batch of magnets as the test and evaluation sample;

[0079] S4: Magnetize the sample to be evaluated. The magnetizing magnetic field is 2 to 3 times or more than the intrinsic coercivity Hcj of the sample. Measure the initial magnetic flux value of the sample at room temperature and record it as φ(t0).

[0080] S5: Place the magnet in the reverse magnetic field device, and then place the whole thing in a high-temperature oven, simultaneously applying temperature T and a reverse magnetic field H. d After holding the sample at room temperature for a period of time, the sample is cooled to room temperature and the magnetic flux value φ(t) is measured. The test is conducted at least three times at different holding times to obtain three different magnetic flux values ​​φ(t). The holding time is in hours. For example, after holding the sample at room temperature for t1, t2, and t3 hours, the sample is cooled to room temperature and the magnetic flux values ​​φ(t1), φ(t2), and φ(t3) are obtained for different holding times.

[0081] S6: Substitute the at least three different magnetic flux values ​​φ(t) obtained in step S5 into the formula φ(t)=φ(t0)-Aln(t / t0) to calculate the coefficient A and plot h. irr The fitted plot of (t)=[φ(t)-φ(t0)] / φ(t0)=[-Aln(t / t0)] / φ(t0);

[0082] S7: In the fitting plot of step S6, based on h in step S2 irr The value is used as a control line, which is parallel to h. irr The t value corresponding to the intersection point of (t) is the lifetime of the magnet, which represents the minimum lifetime value of a batch of magnets.

[0083] It should be noted that in step S2, different products have different effects on irreversible magnetic flux loss h. irr The criteria vary, comparing the room temperature magnetic flux value of the magnet after service with the initial room temperature magnetic flux value before service. Generally, if the room temperature magnetic flux value of the magnet after service is 95-98% of the initial room temperature magnetic flux value before service, then the irreversible magnetic flux loss h is considered acceptable. irr A value of -5% to -2% indicates that the magnet is considered to be in failure.

[0084] Specifically, in step S3, the minimum value of Hk and the minimum value of the sample orientation direction are obtained by sampling different batches and quality requirement levels according to the national standard GB / T2828.1-2012 "Sampling Procedures for Inspection by Attributes Part 1: Sampling Schemes Retrieved by Acceptable Quality Limit (AQL)". The orientation direction refers to the magnetization direction of the sample magnet.

[0085] In step S4, the sample size tolerance is ±0.01mm, and the volume of chipped or broken edges accounts for less than 0.1% of the total sample volume.

[0086] In steps S4 and S5, the accuracy of the magnetic flux testing equipment for the sample is ±0.1%, and the test is performed 30 minutes after powering on.

[0087] In step S5, the accuracy of the sample loading temperature device is ±0.5℃, and the magnetic field deviation of the reverse magnetic field loading device is ≤0.01T. Figure 7 This is a schematic diagram of the reverse magnetic field device used in the lifetime assessment of this invention.

[0088] The error between the predicted irreversible flux loss value and the measured value by the method of this invention is ≤15%.

[0089] This invention simulates the actual operating conditions of magnets in use, combining a counting sampling inspection method. It selects samples based on the minimum values ​​of the magnet's knee coercivity (Hk) and the orientation direction of the permanent magnet, comprehensively considering both material magnetic properties and processing dimensions. The test is conducted under conditions with the highest probability of demagnetization, and the minimum service life of the permanent magnet is obtained by fitting a curve of irreversible magnetic flux loss versus time. It should be noted that selecting the conditions with the highest probability of demagnetization to obtain the minimum service life of the permanent magnet is crucial for product design, safety assessment, and performance optimization. It represents a more scientific, rigorous, and engineering-oriented approach, transcending the traditional one-sided approach that only focuses on the "optimal performance" of materials, and instead pursuing a comprehensive and accurate understanding of material properties. Therefore, the life assessment method of this invention can reduce quality loss and quality risks, providing strong support for users in material selection and use, while also avoiding material waste caused by the underutilization of permanent magnet material performance.

[0090] Example 1

[0091] A long-life neodymium iron boron magnet, measuring 80mm × 80mm × 80mm, with the following composition: (Nd... 0.75 Pr 0.25 ) 30 Co 2.5 Cu 0.12 Al 0.12 Zr 0.17 Fe bal B 0.9 The surface region of the magnet contains 5% more Co, 0.25% more Cu, and 0.25% more Al than the core region. The surface region accounts for 30% of the total volume of the magnet, and the upper and lower surface regions have equal volumes.

[0092] The above-mentioned method for preparing long-life NdFeB magnets includes the following steps:

[0093] Step 1: Preparation of NdFeB alloy powder in the surface region: The surface region alloy powder is prepared according to the process steps of batching, rapid solidification and belt spinning, hydrogen crushing, air jet milling, and powder mixing. The batching ratio is: (NdFeB...) 0.75 Pr 0.25 ) 30 Co6Cu 0.295 Al 0.295 Zr 0.17 Fe bal B 0.9The thickness of the quick-setting flakes is 0.20 mm; the grinding gas in the air jet mill is nitrogen, the pressure is 0.9 MPa, the classifier speed is 4500 rpm, no oxygen is added during air jet milling, the oxygen content of the powder obtained by air jet milling is 85 ppm, and the median particle size, i.e., D50, of the powder is 2.8 μm; an additive is added to the fine powder of air jet milling, the additive plays the role of lubrication and anti-oxidation, the mass of the additive is 0.05% of the mass of the powder, the additive is glyceryl monostearate, tributyl borate, and oleic acid, the ratio of the three is 50:30:20;

[0094] Step 2: Preparation of NdFeB alloy powder in the core region: The core region alloy powder is prepared according to the following steps: batching - rapid solidification and belt spinning - hydrogen crushing - air jet milling - powder mixing. Compared with the surface region, the alloy composition has 5% less Co, 0.25% less Cu, and 0.25% less Al. The batching ratio is: (NdFeB) 0.75 Pr 0.25 ) 30 Co1Cu 0.045 Al 0.045 Zr 0.17 Fe bal B 0.9 The thickness of the rapid-setting flakes is 0.45 mm; the grinding gas in the air jet mill is nitrogen, the pressure is 0.5 MPa, the classifier speed is 3200 rpm, and oxygen is supplemented during the air jet milling process. The oxygen content of the powder obtained by the air jet milling is 200 ppm, and the median particle size, i.e., D50, of the powder is 4.5 μm; an additive is added to the fine powder of the air jet mill. The additive plays a role in lubrication and anti-oxidation. The mass of the additive is 0.01% of the mass of the powder. The additive is glyceryl monostearate, tributyl borate, and oleic acid in a ratio of 50:30:20.

[0095] Step 3: Fabrication of NdFeB green blanks: The powders from Steps 1 and 2 are added to a forming mold with an orientation magnetic field in the following order: NdFeB alloy powder in the surface region - NdFeB alloy powder in the core region - NdFeB alloy powder in the surface region. The orientation magnetic field is 2T, and the pressure is 20MPa. Local magnetic field enhancement is applied to the orientation magnetic field; the local magnetic field enhancement is 0.3T higher than the orientation magnetic field. The material used for local magnetic field enhancement is 1J22 reinforcement blocks. Four reinforcement blocks 4 are symmetrically arranged on both sides of the upper and lower surface regions of the blank along the orientation magnetic field direction. The reinforcement blocks 4 have a trapezoidal truncated pyramidal magnetic design, with the area ratio of the lower to upper surface of the trapezoidal pyramidal pyramid being 10:4, and the height ratio of the upper surface of the trapezoidal pyramidal pyramid to the height of the surface region being 10:5. The upper and lower surface layers of the blank have equal volumes, and the sum of their volumes accounts for 30% of the total volume of the blank; the density of the blank after orientation molding is 3.6 g / cm³. 3 The oriented blank is then placed in a cold isostatic press for further densification at a pressure of 180 MPa, resulting in a green blank density of 3.9 g / cm³.3 ;

[0096] Step 4, Vibration Sintering and Secondary Tempering: The green billet is pushed into a vacuum sintering furnace for sintering. Before heating, a pre-vacuum is drawn. After the vacuum level is below 0.1 Pa, the temperature is raised. The billet is held at 400℃ for 3 hours for degreasing, and then held at 750℃ for 4 hours for dehydrogenation. The temperature is then raised to the sintering temperature at a rate of 5℃ / min. The sintering temperature is 1050℃. After holding at 10℃ / min for 2 hours, the temperature is cooled to below 850℃. The temperature is then raised to the sintering temperature at a rate of 5℃ / min. This process is repeated 3 times to achieve the vibration sintering effect. During sintering, the vacuum level is kept below 0.1 Pa. Finally, the blower is turned on in the furnace, and argon gas is introduced to rapidly cool the temperature to below 70℃. Then, the temperature was increased to 910℃ at a rate of 9℃ / min and held for 4 hours for primary tempering. Afterwards, the temperature was rapidly cooled to below 70℃ by turning on the fan and introducing argon gas. Finally, the temperature was increased to 480℃ at a rate of 10℃ / min and held for 6 hours for secondary tempering. Afterwards, the temperature was rapidly cooled to below 70℃ by turning on the fan and introducing argon gas to obtain the final magnet with a density of 7.59 g / cm³. 3 .

[0097] The magnet from Example 1 was processed and tested. A cylindrical sample with a diameter of 10mm × 10mm was processed, and its magnetic properties were tested using a NIM-2000 permanent magnet material testing system. A cylindrical sample with a diameter of 6mm × 20mm was processed, and its coefficient of thermal expansion was tested using a Netzsch thermal expansion tester. A cubic sample with a diameter of 1mm × 1mm × 1mm was processed, and its TG curve was tested using a differential thermal analyzer. The inflection point of the TG curve was determined as the Curie temperature of the sample. The density ρ of the magnet was tested using the water displacement method. Using a scanning electron microscope (SEM) at 2000x magnification, the average diameter of the grains at the fracture surface of the magnet was observed, which approximates the average grain size d of the sample. The magnet was processed using a high-precision grinding machine, with 2mm of grinding on one side to achieve full light exposure. The magnetic declination of the sample was read using a three-dimensional Helmholtz coil. The results of the above tests are shown in Table 1.

[0098] The lifetime assessment of the long-life NdFeB magnet prepared in this embodiment includes the following steps:

[0099] S1: Based on the application of magnets in wind turbines, determine the temperature T and the reverse magnetic field H experienced by the neodymium iron boron magnet during service. d The reverse magnetic field of the magnet is 0.75T, and the maximum operating temperature is 105℃.

[0100] S2: Determine the irreversible magnetic flux loss h of the neodymium iron boron magnet according to industry standards and wind turbine reliability requirements. irr The criterion is -5%;

[0101] S3: Select the sample with the smallest knee coercivity Hk and the smallest orientation direction dimension from the batch of magnets as the test and evaluation sample;

[0102] The magnets are processed into sheets measuring 50mm × 20mm × 6mm, with an orientation direction of 6mm and a tolerance of ±0.05mm. Sampling is conducted according to GB / T 2828.1-2012, at inspection level II, grade 2.5. Specifically, for wind turbine units requiring 30,000 sheets each time, 125 sheets are sampled according to the sampling inspection standard, equivalent to 13 magnets. The demagnetization curves of all 13 magnets are inspected, and the minimum tested Hk value is 8.53kOe.

[0103] S4: Magnetize the sample to be evaluated with a magnetic field of 5T, and test the initial magnetic flux value of the sample at room temperature φ(t0) = 8.272uWb;

[0104] S5: Place the magnet in the reverse magnetic field device, and then place the whole thing in a high-temperature oven, simultaneously applying temperature T and a reverse magnetic field H. d After being kept at temperatures of t1 = 1 h, t2 = 10 h, and t3 = 100 h respectively, the samples were cooled to room temperature and removed. Three different magnetic flux values ​​were obtained: φ(t1) = 8.255 uWb, φ(t2) = 8.239 uWb, and φ(t3) = 8.222 uWb.

[0105] S6: Substitute the φ(t1), φ(t2), and φ(t3) obtained in step S5 into the formula φ(t)=φ(t0)-Aln(t / t0) to calculate the coefficient A=0.002, and plot h. irr The fitted plot of (t)=[φ(t)-φ(t0)] / φ(t0)=[-0.002ln(t / t0)] / φ(t0), Figure 8 This includes a predicted graph showing the relationship between irreversible magnetic loss of the magnet and service time in this embodiment;

[0106] S7: In the fitting plot of step S6, based on h in step S2 irr = -5% is used as a control line, and this line is parallel to h. irr The value of t corresponding to the intersection point of (t) is 8.2 × 10. 6 The term "years" refers to the lifespan of the magnet, representing the minimum lifespan of a batch of magnets.

[0107] It should be noted that the t-value obtained by the above-described lifespan assessment method greatly exceeds the conventional time limit. This indicates that the magnet lifespan in this embodiment is indeed relatively long; secondly, it can be predicted that after the magnet's service life far exceeds the conventional limit, it may no longer follow the logarithmic decay law; and thirdly, it indicates the irreversible flux loss h of the neodymium iron boron magnet in this embodiment. irrThe criterion was set too low, so further, using the aforementioned lifetime assessment method, the irreversible flux loss h of the S2 NdFeB magnet was determined. irr The criterion was reduced to -3%, and the final prediction was that the lifespan of the magnet in this embodiment was 373 years.

[0108] Example 2

[0109] A long-life neodymium iron boron magnet, measuring 80mm × 80mm × 80mm, with the following composition: (Nd... 0.70 Pr 0.10 Dy 0.20 ) 31 Co 25 Cu 0.83 Al 0.83 Zr 0.4 Nb 0.4 Fe bal B 1.1 The surface region of the magnet contains 1.5% more Co, 0.05% more Cu, and 0.05% more Al than the core region. The surface region accounts for 10% of the total volume of the magnet, while the upper and lower surface regions have equal volumes.

[0110] The above-mentioned method for preparing long-life NdFeB magnets includes the following steps:

[0111] Step 1: Preparation of NdFeB alloy powder in the surface region: The surface region alloy powder is prepared according to the process steps of batching, rapid solidification and belt spinning, hydrogen crushing, air jet milling, and powder mixing. The batching ratio is: (NdFeB...) 0.70 Pr 0.10 Dy 0.20 ) 31 Co 26.35 Cu 0.875 Al 0.875 Zr 0.4 Nb 0. 4Fe bal B 1.1 The thickness of the rapid-setting flakes is 0.15 mm; the grinding gas of the air jet mill is argon, the pressure is 0.8 MPa, the classifier speed is 4400 rpm, no oxygen is added during air jet milling, the oxygen content of the powder obtained by air jet milling is 93 ppm, and the median particle size, i.e., D50 of the powder is 2.5 μm; an additive is added to the fine powder of air jet milling, the additive plays the role of lubrication and anti-oxidation, the mass of the additive is 0.04% of the powder mass, and the additive is isopropanol;

[0112] Step 2: Preparation of NdFeB alloy powder in the core region: The core region alloy powder was prepared according to the steps of batching, rapid solidification and belt spinning, hydrogen crushing, air jet milling, and powder mixing. Compared with the surface region, the alloy composition showed a 1.5% reduction in Co, a 0.05% reduction in Cu, and a 0.05% reduction in Al. The batching ratio was: (NdFeB...)0.70 Pr 0.10 Dy 0.20 ) 31 Co 24.85 Cu 0.825 Al 0.825 Zr 0.4 Nb 0.4 Fe bal B 1.1 The thickness of the rapid-setting flakes is 0.25 mm; the grinding gas of the air jet mill is argon, the pressure is 0.8 MPa, the classifier speed is 3800 rpm, oxygen is supplemented during air jet milling, the oxygen content of the powder obtained by air jet milling is 150 ppm, and the median particle size of the powder, i.e., D50, is 3.5 μm; an additive is added to the fine powder of air jet milling, the additive plays the role of lubrication and anti-oxidation, the mass of the additive is 0.02% of the powder mass, and the additive is isopropanol;

[0113] Step 3: Fabrication of NdFeB green blanks: The powders from Steps 1 and 2 are added to a forming mold with an orientation magnetic field in the following order: NdFeB alloy powder in the surface region - NdFeB alloy powder in the core region - NdFeB alloy powder in the surface region. The orientation magnetic field is 1.5T, and the pressure is 40MPa. Local magnetic field reinforcement is applied to the orientation magnetic field; the local magnetic field reinforcement is 0.5T higher than the orientation magnetic field. The material used for local magnetic field reinforcement is DT4C reinforcement blocks. Four reinforcement blocks 4 are symmetrically arranged on both sides of the upper and lower surface regions of the blank along the orientation magnetic field direction. The reinforcement blocks 4 have a trapezoidal truncated pyramidal magnetic design, with the area ratio of the lower to upper surface of the trapezoidal pyramidal pyramid being 10:9, and the height ratio of the upper surface of the trapezoidal pyramidal pyramid to the height of the surface region being 10:1. The upper and lower surface layers of the blank have equal volumes, and the sum of their volumes accounts for 10% of the total volume of the blank; the density of the blank after orientation molding is 3.9 g / cm³. 3 The oriented blank is then placed in a cold isostatic press for further densification at a pressure of 250 MPa, resulting in a green blank density of 4.3 g / cm³. 3 ;

[0114] Step 4, Vibration Sintering and Secondary Tempering: The green billet is pushed into a vacuum sintering furnace for sintering. Before heating, a pre-vacuum is drawn. After the vacuum level is below 0.1 Pa, the temperature is raised. The billet is held at 600℃ for 2 hours for degreasing, and then held at 850℃ for 2 hours for dehydrogenation. The temperature is then raised to the sintering temperature of 1030℃ at a rate of 3℃ / min. After holding at 1030℃ for 3 hours, the temperature is cooled to below 850℃ at a rate of 5℃ / min. The temperature is then raised to the sintering temperature again at a rate of 3℃ / min. This process is repeated twice to achieve the vibration sintering effect. During sintering, the vacuum level is kept below 0.1 Pa. Finally, the blower is turned on in the furnace, and argon gas is introduced to rapidly cool the furnace to below 70℃. Then, the temperature was increased to 850℃ at a rate of 5℃ / min and held for 2 hours for primary tempering. Afterwards, the temperature was rapidly cooled to below 70℃ by turning on the fan and introducing argon gas. Finally, the temperature was increased to 650℃ at a rate of 5℃ / min and held for 2 hours for secondary tempering. Afterwards, the temperature was rapidly cooled to below 70℃ by turning on the fan and introducing argon gas, yielding the final magnet with a density of 7.79 g / cm³. 3 .

[0115] The magnet of this embodiment was processed and tested according to the method of Example 1. The results of the relevant tests are shown in Table 1.

[0116] The magnet of this embodiment was subjected to lifetime assessment according to the method of Embodiment 1, and the irreversible magnetic flux loss h was measured. irr The criterion is -5%, but because the intrinsic coercivity Hcj of the magnet in this embodiment is low, it is not suitable for the working conditions of a reverse magnetic field of 0.75T and a maximum operating temperature of 105℃. Therefore, the maximum operating temperature of the magnet in this embodiment during service is adjusted to 90℃ and the reverse magnetic field to 0.3T. The life assessment results are shown in Table 1.

[0117] Example 3

[0118] A long-life neodymium iron boron magnet, measuring 80mm × 80mm × 80mm, with the following composition: Nd 27 Co 13 Cu 0.5 Al 0.5 Zr 0.3 Fe bal B 1.0 The surface region of the magnet contains 3.5% more Co, 0.13% more Cu, and 0.13% more Al than the core region. The surface region accounts for 20% of the total volume of the magnet, while the upper and lower surface regions have equal volumes.

[0119] The above-mentioned method for preparing long-life NdFeB magnets includes the following steps:

[0120] Step 1: Preparation of NdFeB alloy powder in the surface region: The surface region alloy powder is prepared according to the following process steps: batching - rapid solidification and belt spinning - hydrogen crushing - air jet milling - powder mixing. The batching ratio is: Nd... 27 Co 15.8 Cu 0.604 Al 0.604 Zr 0.3 Fe bal B 1.0 The thickness of the quick-setting flakes is 0.25 mm; the grinding gas in the air jet mill is nitrogen, the pressure is 0.7 MPa, the classifier speed is 4000 rpm, no oxygen is added during air jet milling, the oxygen content of the powder obtained by air jet milling is 20 ppm, and the median particle size, i.e., D50, of the powder is 3.5 μm; an additive is added to the fine powder of air jet milling, the additive plays the role of lubrication and anti-oxidation, the mass of the additive is 0.025% of the powder mass, the additive is glyceryl monostearate, tributyl borate, and oleic acid, the ratio of the three is 50:30:20;

[0121] Step 2: Preparation of NdFeB alloy powder in the core region: The core region alloy powder was prepared according to the following steps: batching, rapid solidification and belt spinning, hydrogen crushing, air jet milling, and powder mixing. Compared with the surface region, the alloy composition showed a 3.5% reduction in Co, a 0.13% reduction in Cu, and a 0.13% reduction in Al. The batching ratio was: Nd... 27 Co 12.3 Cu 0.474 Al 0.474 Zr 0.3 Fe bal B 1.0 The thickness of the rapid-setting flakes is 0.35 mm; the grinding gas in the air jet mill is nitrogen, the pressure is 0.65 MPa, the classifier speed is 3500 rpm, and oxygen is supplemented during the air jet milling process. The oxygen content of the powder obtained by the air jet milling is 180 ppm, and the median particle size, i.e., D50, of the powder is 5.5 μm; an additive is added to the fine powder of the air jet mill. The additive plays a role in lubrication and anti-oxidation. The mass of the additive is 0.015% of the mass of the powder. The additive is glyceryl monostearate, tributyl borate, and oleic acid in a ratio of 50:30:20.

[0122] Step 3: Fabrication of NdFeB green blanks: The powders from Steps 1 and 2 are added to a forming mold with an orientation magnetic field in the following order: NdFeB alloy powder in the surface region - NdFeB alloy powder in the core region - NdFeB alloy powder in the surface region. The orientation magnetic field is 1.8T, and the pressure is 30MPa. Local magnetic field reinforcement is applied to the orientation magnetic field; the local magnetic field reinforcement is 0.1T higher than the orientation magnetic field. The material used for local magnetic field reinforcement is 1J50 reinforcement blocks. Four reinforcement blocks 4 are symmetrically arranged on both sides of the upper and lower surface regions of the blank along the orientation magnetic field direction. The reinforcement blocks 4 have a trapezoidal truncated pyramidal magnetic design, with the area ratio of the lower to upper surface of the trapezoidal pyramidal pyramid being 10:7, and the height ratio of the upper surface of the trapezoidal pyramidal pyramid to the height of the surface region being 10:3. The upper and lower surface layers of the blank have equal volumes, and the sum of their volumes accounts for 20% of the total volume of the blank; the density of the blank after orientation molding is 3.8 g / cm³. 3 The oriented blank is then placed in a cold isostatic press for further densification at a pressure of 320 MPa, resulting in a green blank density of 4.1 g / cm³. 3 ;

[0123] Step 4, Vibration Sintering and Secondary Tempering: The green billet is pushed into a vacuum sintering furnace for sintering. Before heating, a pre-vacuum is drawn. After the vacuum level is below 0.1 Pa, the temperature is raised. The billet is held at 500℃ for 4 hours for degreasing, and then held at 800℃ for 3 hours for dehydrogenation. The temperature is then raised to the sintering temperature at a rate of 1℃ / min. The sintering temperature is 1100℃. After holding at this temperature for 1 hour, the temperature is cooled to below 850℃ at a rate of 20℃ / min. The temperature is then raised to the sintering temperature at a rate of 1℃ / min. This process is repeated 5 times to achieve the vibration sintering effect. During sintering, the vacuum level is kept below 0.1 Pa. Finally, the blower is turned on in the furnace, and argon gas is introduced to rapidly cool the temperature to below 70℃. Then, the temperature was increased to 950℃ at a rate of 1℃ / min and held for 6 hours for primary tempering. Afterwards, the temperature was rapidly cooled to below 70℃ by turning on the fan and introducing argon gas. Finally, the temperature was increased to 550℃ at a rate of 1℃ / min and held for 4 hours for secondary tempering. Afterwards, the temperature was rapidly cooled to below 70℃ by turning on the fan and introducing argon gas, yielding the final magnet with a density of 7.65 g / cm³. 3 .

[0124] The magnet of this embodiment was processed and tested according to the method of Example 1. The results of the relevant tests are shown in Table 1.

[0125] The magnet of this embodiment was subjected to lifetime assessment according to the method of Embodiment 1, and the irreversible magnetic flux loss h was measured. irrThe criterion is -5%, but because the intrinsic coercivity Hcj of the magnet in this embodiment is low, it is not suitable for the working conditions of a reverse magnetic field of 0.75T and a maximum operating temperature of 105℃. Therefore, the maximum operating temperature of the magnet in this embodiment during service is adjusted to 80℃ and the reverse magnetic field to 0.2T. The life assessment results are shown in Table 1.

[0126] Comparative Example 1

[0127] A sintered NdFeB magnet with the same composition as in Example 1 was prepared, the magnet composition being (NdFeB) 0.75 Pr 0.25 ) 30 Co 2.5 Cu 0.1 2Al 0.12 Zr 0.17 Fe bal B 0.9 The process was carried out according to conventional procedures, with key process parameters including an average thickness of 0.35 mm for the rapid-setting sheets; NdFeB powder was prepared by air jet milling, with a median particle size (D50) of 3.8 μm. No oxygen was added during the air jet milling process, resulting in an oxygen content of approximately 50 ppm. Additives were added to the air-jet milled powder, serving as lubricants and antioxidants. The additives accounted for 0.03% of the powder mass and consisted of glyceryl monostearate, tributyl borate, and oleic acid in a 50:30:20 ratio. The air-jet milled powder was then added to a molding die without magnetic field compensation, with an orientation magnetic field of 2 T. The oriented blank was further densified in a cold isostatic press at a pressure of 180 MPa, resulting in a green blank density of 3.81 g / cm³. 3 The green blank was pushed into a vacuum sintering furnace for sintering. Before heating, a pre-vacuum was applied for 2 hours. Heating began at a vacuum level of 0.09 Pa. The blank was held at 400℃ for 3 hours for degreasing, and then held at 750℃ for 4 hours for dehydrogenation. The temperature was then increased to the sintering temperature of 1050℃ at a rate of 5℃ / min, and held for 6 hours. A second tempering heat treatment was then performed using the same process as in Example 1 to obtain the final magnet.

[0128] The magnet of Comparative Example 1 was processed and tested in accordance with the method of Example 1. The results of the relevant tests are shown in Table 1.

[0129] The magnet of Comparative Example 1 was subjected to a lifespan assessment entirely according to the method of Example 1:

[0130] S3: The minimum Hk value tested is 7.77kOe;

[0131] S4: The initial magnetic flux value of the sample at room temperature was measured to be φ(t0) = 8.254 uWb;

[0132] S5: Three different magnetic flux values ​​were obtained from the test: φ(t1) = 8.223 uWb, φ(t2) = 8.191 uWb, and φ(t3) = 8.160 uWb;

[0133] S6: Calculate the coefficient A = 0.0038, and plot h. irr The fitted plot of (t)=[φ(t)-φ(t0)] / φ(t0)=[-0.0038ln(t / t0)] / φ(t0), Figure 8 The figure includes a prediction of the relationship between irreversible magnetic loss and service time for the comparative magnet.

[0134] S7: The t value is 59 years, which is the lifespan of the magnet. This time represents the minimum lifespan of a batch of magnets.

[0135] Using the above-mentioned lifetime assessment method, the irreversible magnetic flux loss h of the NdFeB magnet in S2 is... irr The criterion was lowered to -3%, and the final predicted lifespan of this comparative magnet was 0.3 years.

[0136] Comparative Example 2

[0137] Based on the magnetic performance test results of Example 1, 50M commercial magnets of the same grade range were selected from the market for comparative testing and analysis. ICP composition analysis revealed that the magnet's composition percentage was (Nd... 0.75 Pr 0.25 ) 29.5 Dy 0.5 Co 0.5 Cu 0.1 Al 0.1 Ga 0.2 Zr 0.1 Nb 0.1 Fe bal B 0.98 Compared to Example 1, this magnet uses expensive heavy rare earth element Dy, which costs 2000 yuan / kg at the current market price, while PrNd costs 600 yuan / kg. Adding an additional 0.5% Dy increases the formulation cost by approximately 4% per kilogram of magnet.

[0138] The magnet of Comparative Example 2 was processed and tested in exactly the same way as in Example 1. The results of the relevant tests are shown in Table 1.

[0139] The lifespan of the magnet in Comparative Example 2 was assessed exactly according to the method in Example 1:

[0140] S3: The minimum Hk value tested is 7.02 kOe;

[0141] S4: The initial magnetic flux value of the sample at room temperature was measured to be φ(t0) = 8.225 uWb;

[0142] S5: Three different magnetic flux values ​​were obtained from the test: φ(t1) = 8.142 uWb, φ(t2) = 8.059 uWb, and φ(t3) = 7.976 uWb;

[0143] S6: Calculate the coefficient A = 0.0101, and plot h. irr The fitted plot of (t)=[φ(t)-φ(t0)] / φ(t0)=[-0.0101ln(t / t0)] / φ(t0), Figure 8 The figure includes a prediction of the relationship between irreversible magnetic loss and service time for the comparative magnet.

[0144] S7: The t value is 141 hours, which is the lifespan of the magnet. This time represents the minimum lifespan of a batch of magnets, which is 0.016 years.

[0145] To further verify the accuracy of the NdFeB magnet lifetime assessment method of this invention, the sample from Comparative Example 2, which had undergone a 100-hour test, was kept at a constant temperature for another 41 hours, then cooled to room temperature. Its magnetic flux value was then measured, and its irreversible magnetic loss was calculated. The calculations showed that the measured value under these conditions was -4.88%, very close to the -5% obtained through fitting, with an error of less than 3%.

[0146] Table 1. Magnet performance of the embodiments and comparative examples

[0147]

[0148] As can be seen from Table 1, the magnet density prepared in the embodiments of the present invention is 7.59 g / cm³. 3 The following conditions must be met: magnetic declination below 0.78°, surface Curie temperature above 376°C, and difference in thermal expansion coefficient below 6.1×10⁻⁶. -6 K -1 Compared with the comparative examples, it has advantages such as high density, small magnetic declination, high Curie temperature, and low difference in thermal expansion coefficient. Compared with neodymium iron boron magnets of the same grade, the Br, Hk, and Hcj of Example 1 are all higher than those of Comparative Examples 1 and 2. Further high-temperature reverse magnetic field durability tests show that under the same working conditions, the neodymium iron boron magnet of this invention has a minimum lifespan of 92 years, which is a significant improvement compared with Comparative Examples 1 and 2.

[0149] The above description of the present invention represents only some embodiments, but the present invention is not limited to the specific implementations described above. The specific implementations described above are illustrative and not restrictive. All specific extensions using the materials and methods of the present invention, without departing from the spirit and scope of the claims, are within the protection scope of the present invention.

Claims

1. A long-life neodymium iron boron magnet, characterized in that, The composition of the magnet by mass percentage is: (Nd) x RE 1-x ) a Co b Cu c Al d Zr e M f Fe bal B g , 0<x≤1, 27≤a≤33, 0<b≤25, 0<c≤1, 0<d≤1, b / (c+d)=10~15, 0.1≤e≤0.5, 0≤f≤0.4, 0.85≤g≤1.1, bal is the balance; RE is one or more of Pr, Ce, La, Dy, Tb, Ho, Gd; M is one or more of Nb, Ti, V, Mo; The surface region of the magnet accounts for 10-30% of the total volume of the magnet in the pressing direction, and the upper and lower surface regions have equal volumes. The Co mass content in the surface region is 1.5-5% higher than that in the core region, the Cu mass content is 0.05-0.25% higher, and the Al mass content is 0.05-0.25% higher. The average grain size in the surface region is 2.8-4.5 μm, and the average grain size in the core region is 4.6-10 μm.

2. The magnet according to claim 1, characterized in that, The Co mass content in the surface region of the magnet is 10 to 15 times the sum of the Cu and Al mass contents.

3. The magnet according to claim 1, characterized in that, The composition of the magnet by mass percentage is: (Nd) x RE 1-x ) a Co b Cu c Al d Zr e M f Fe bal B g , 0.1≤x≤0.8, 29≤a≤31, 2≤b≤20, 0.1≤c≤0.8, 0.1≤d≤0.8, b / (c+d)=10~15, 0.2≤e≤0.4, 0.1≤f≤0.3, 0.90≤g≤1.0, bal is the balance; RE is one or more of Pr, Ce, La, Dy, Tb, Ho, Gd; M is one or more of Nb, Ti, V, Mo.

4. The magnet according to any one of claims 1 to 3, characterized in that, The remanence Br of the magnet is 10.5–15.5 kGs, the intrinsic coercivity Hcj is 8–40 kOe, the Curie temperature of the surface region of the magnet is 325–585 °C, and the Curie temperature of the core region is 310–570 °C.

5. The magnet according to any one of claims 1 to 3, characterized in that, When the magnet operates under high temperature and reverse magnetic field, 0 < magnetic declination ≤ 1.5°.

6. A method for preparing a magnet as described in any one of claims 1 to 3, characterized in that, The method includes the following steps: Step 1: Preparation of NdFeB alloy powder for the surface region: Prepare the alloy powder for the surface region according to the process steps of batching - rapid solidification and spinning - hydrogen crushing - air jet milling - powder mixing. The thickness of the rapid solidification and spinning sheet is 0.15-0.25 mm; the grinding gas pressure of the air jet mill is 0.7-0.9 MPa, the classifier speed is 4000-4500 rpm, and no oxygen is added during the air jet milling; add additives to the fine powder of the air jet mill, and the mass of the additives is 0.025-0.05% of the powder mass; Step 2: Preparation of NdFeB alloy powder in the core region: The alloy powder in the core region is prepared according to the following steps: batching, rapid solidification and slaking, hydrogen breaking, air jet milling, and powder mixing. Compared with the surface region, the alloy composition has reduced Co by 1.5-5%, Cu by 0.05-0.25%, and Al by 0.05-0.25%. The thickness of the rapid solidification slaking sheet is 0.25-0.45 mm. The grinding gas pressure of the air jet mill is 0.5-0.8 MPa, the classifier wheel speed is 3200-3800 rpm, and oxygen is supplemented during air jet milling. Additives are added to the fine powder of the air jet mill, and the mass of the additives is 0.01-0.02% of the powder mass. Step 3: Fabrication of NdFeB green blanks: The two types of powders from Step 1 and Step 2 are added to a forming mold with an orientation magnetic field in the following order: NdFeB alloy powder in the surface region - NdFeB alloy powder in the core region - NdFeB alloy powder in the surface region. The powders are then oriented and formed, with localized magnetic field enhancement applied to the orientation magnetic field. The upper and lower surface regions of the blank have equal volumes, and the sum of the volumes of the upper and lower surface regions accounts for 10-30% of the total volume of the blank. The oriented blank is then placed in a cold isostatic press for further densification. Step 4, Vibration Sintering and Secondary Tempering: The green blank is pushed into a vacuum sintering furnace for sintering. After pre-vacuuming, the temperature is raised and held between 400 and 600°C for degreasing, then held between 750 and 850°C for dehydrogenation. The temperature is then raised to the sintering temperature, which is between 1030 and 1100°C. After holding at this temperature for 1 to 3 hours, the temperature is cooled to below 850°C. The temperature is then raised again to the sintering temperature, and this process is repeated 2 to 5 times. Finally, the blower is turned on in the furnace, and argon gas is introduced to rapidly cool the temperature to below 70°C. The temperature is then raised to 850 to 950°C and held for 2 to 6 hours for primary tempering. The blower is then turned on and argon gas is introduced to rapidly cool the temperature to below 70°C. Finally, the temperature is raised to 450 to 650°C and held for 2 to 6 hours for secondary tempering. The blower is then turned on and argon gas is introduced to rapidly cool the temperature to below 70°C to obtain the final magnet.

7. The method according to claim 6, characterized in that, In step 3, the orientation magnetic field is 1.5 to 2T, the local magnetic field enhancement is 0.1 to 0.5T higher than the orientation magnetic field, and the material used for the local magnetic field enhancement is a high-permeability soft magnetic alloy reinforcement block.

8. The method according to claim 7, characterized in that, The reinforcing blocks are symmetrically arranged on both sides of the upper and lower surface areas of the blank along the orientation magnetic field direction. The shape of the reinforcing blocks is a trapezoidal platform magnetic focusing design. The area ratio of the lower and upper surfaces of the trapezoidal platform is in the range of 10:9 to 10:4, and the ratio of the height of the upper surface of the trapezoidal platform to the height of the surface area is 10:5 to 10:

1.

9. A method for evaluating the lifetime of a magnet as described in any one of claims 1 to 3 or a magnet prepared by the method as described in any one of claims 6 to 8, characterized in that, The method includes the following steps: S1: Determine the temperature T and the reverse magnetic field H experienced by the neodymium iron boron magnet during service. d ; S2: Determine the irreversible flux loss h of the neodymium iron boron magnet. irr Criterion; S3: Select the sample with the smallest knee coercivity Hk and the smallest orientation direction dimension from the batch of magnets as the test and evaluation sample; S4: Magnetize the sample to be evaluated. The magnetizing magnetic field is 2 to 3 times or more than the intrinsic coercivity Hcj of the sample. Measure the initial magnetic flux value of the sample at room temperature and record it as φ(t0). S5: Place the magnet in the reverse magnetic field device, and then place the whole thing in a high-temperature oven, simultaneously applying temperature T and a reverse magnetic field H. d After being kept at a certain temperature for a period of time, the sample was cooled to room temperature and removed. The magnetic flux value φ(t) after the experiment was tested. The experiment was conducted at least three times at different holding times to obtain three different magnetic flux values ​​φ(t). S6: Substitute the at least three different magnetic flux values ​​φ(t) obtained in step S5 into the formula φ(t)=φ(t0)-Aln(t / t0) to calculate the coefficient A and plot h. irr The fitted plot of (t)=[φ(t)-φ(t0)] / φ(t0)=[-Aln(t / t0)] / φ(t0); S7: In the fitting plot of step S6, based on h in step S2 irr The value is used as a control line, which is parallel to h. irr The t value corresponding to the intersection point of (t) is the lifetime of the magnet, which represents the minimum lifetime value of a batch of magnets.

10. The method according to claim 9, characterized in that, Step S3, in accordance with the national standard GB / T 2828.1-2012 "Sampling Procedures for Inspection by Attributes - Part 1: Sampling Schemes for Lot-by-Lot Inspection Retrieved by Acceptable Quality Limit (AQL)," involves sampling for different batch quantities and quality requirement levels to obtain the evaluation samples.

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