Sintered neodymium-iron-boron permanent magnet material without heavy rare earth and method for producing same

Abstract

The application discloses a sintered Nd-Fe-B permanent magnet material without heavy rare earth and a preparation method thereof, and belongs to the technical field of rare earth permanent magnet materials, wherein S1, ingredients of a sintered Nd-Fe-B main alloy are prepared, and the prepared ingredients are sequentially subjected to vacuum smelting and rapid solidification casting to obtain a main alloy rapid solidification piece; and a first light rare earth grain boundary alloy and a second light rare earth diffusion alloy adopt different compositions. The first light rare earth grain boundary alloy is relatively rich in Cu, has a relatively low melting point and a relatively good liquid phase wetting capacity, and a continuous light rare earth-rich grain boundary phase is preferentially formed in a sintering process; the second light rare earth diffusion alloy is relatively rich in Pr and can provide Pr with a relatively high diffusion driving force to the inside of the magnet. The two kinds of alloys respectively undertake the functions of constructing a grain boundary channel and strengthening an outer edge of a main phase.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnet materials technology, specifically relating to a sintered NdFeB permanent magnet material without heavy rare earth elements and its preparation method. Background Technology

[0002] Sintered NdFeB permanent magnet materials possess high remanence, coercivity, and maximum energy product, and are widely used in new energy vehicle drive motors, wind power generation equipment, industrial servo motors, robots, compressors, and consumer electronics. As permanent magnet motors develop towards higher power density, miniaturization, and higher operating temperatures, higher requirements are placed on the intrinsic coercivity, thermal stability, and magnetic property consistency of sintered NdFeB permanent magnet materials.

[0003] Sintered NdFeB permanent magnet materials are mainly composed of R2Fe 14 The sintered NdFeB magnet consists of a main phase (B), rare-earth-rich grain boundary phases, and a small amount of boron-rich phases, where R typically includes Nd and Pr. The intrinsic coercivity of sintered NdFeB magnets is not only affected by the grain size and orientation of the main phase, but also closely related to the degree of magnetic isolation between the main phase grains, the continuity of the grain boundary phase, and the local magnetocrystalline anisotropy of the main phase grain surface.

[0004] In existing technologies, to improve the intrinsic coercivity of sintered NdFeB magnets, heavy rare earth elements such as Dy and Tb are typically added during the alloy smelting stage, or diffusion sources containing Dy and Tb are placed on the surface of the sintered magnet. Through grain boundary diffusion, the heavy rare earth elements form a shell structure with a high anisotropic field at the outer edge of the main phase grains. This method can significantly improve coercivity, but resources such as Dy and Tb are relatively scarce, raw material prices are high, and the incorporation of heavy rare earth elements into R2Fe... 14 The B main phase is prone to unfavorable magnetic coupling with Fe, which may cause a decrease in remanence and maximum magnetic energy product. Summary of the Invention

[0005] The purpose of this invention is to provide a sintered NdFeB permanent magnet material without heavy rare earth elements and its preparation method, so as to solve the problems of discontinuous grain boundary channels, insufficient diffusion depth, low utilization rate of diffusion source, and difficulty in simultaneously improving coercivity and maintaining remanence during the existing light rare earth grain boundary diffusion process.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing sintered NdFeB permanent magnet material without heavy rare earth elements, comprising the following steps.

[0007] S1. Prepare the raw materials according to the composition of the sintered NdFeB main alloy, and then perform vacuum melting and rapid solidification casting of the raw materials in sequence to obtain the rapid solidification sheet of the main alloy.

[0008] The main alloy comprises, by mass percentage: R 28.5%–31.5%, B 0.90%–1.05%, Co 0.20%–1.00%, Cu 0.03%–0.20%, Al 0.05%–0.30%, Ga 0.03%–0.20%, Zr 0.02%–0.12%, with the balance being Fe and unavoidable impurities; R is composed of Nd and Pr, with Pr accounting for 5% to 30% of the total mass of R.

[0009] The average thickness of the main alloy quick-setting sheet is 0.20–0.50 mm.

[0010] In a cross-section perpendicular to the surface of the main alloy rapid solidification sheet, two-dimensional cross-sectional statistics are performed using backscattered electron imaging. The area of ​​columnar crystal regions accounts for 60% to 90% of the total area of ​​the cross-section in the thickness direction.

[0011] The rare earth-rich phase in the main alloy rapid solidification sheet is distributed in a strip, continuous network, or semi-continuous network along the columnar grain boundaries and lamellar interfaces, and at least 60% of the total area of ​​the rare earth-rich phase is located at the columnar grain boundaries or lamellar interfaces.

[0012] S2. Prepare the first light rare earth grain boundary alloy powder and the second light rare earth diffusion alloy powder respectively.

[0013] The first light rare earth grain boundary alloy powder is a low-melting-point alloy powder rich in Cu compared to the second light rare earth diffusion alloy powder. It is mainly used to form a light rare earth-rich liquid phase with good fluidity and wettability during sintering and to form a continuous grain boundary phase between the main phase grains.

[0014] The second light rare earth diffusion alloy powder is a low melting point alloy powder rich in Pr compared to the first light rare earth grain boundary alloy powder. It is mainly used to provide Pr in the grain boundary diffusion process and to allow Pr to enter the interior of the sintered magnet substrate along the continuous grain boundary phase.

[0015] The first light rare earth grain boundary alloy comprises, by mass percentage: The first light rare earth element is R160%–70%, Cu20%–28%, Al5%–9%, ​​Ga2%–5%, with the balance being Fe and unavoidable impurities.

[0016] The second light rare earth diffusion alloy comprises, by mass percentage: The second light rare earth element composition is 72%–82% R, 12%–20% Cu, 3%–7% Al, and 1%–4% Ga, with the balance being Fe and unavoidable impurities.

[0017] The first light rare earth element R1 is Pr, Nd or Pr-Nd alloy, and Pr accounts for 50% to 100% of the total mass of R1; the second light rare earth element R2 is Pr or Pr-Nd alloy, and Pr accounts for 80% to 100% of the total mass of R2.

[0018] The mass percentage of the second light rare earth element R2 in the second light rare earth diffusion alloy is 8 to 18 percentage points higher than the mass percentage of the first light rare earth element R1 in the first light rare earth grain boundary alloy.

[0019] The mass percentage of Cu in the first light rare earth grain boundary alloy is 5 to 12 percentage points higher than that in the second light rare earth diffusion alloy.

[0020] S3. The main alloy rapid solidification sheet is subjected to hydrogen crushing, dehydrogenation and air jet milling in sequence to obtain main alloy powder; the first light rare earth grain boundary alloy powder is mixed with the main alloy powder, and the first light rare earth grain boundary alloy powder accounts for 1.5% to 3.5% of the total mass of the composite magnetic powder after mixing.

[0021] The main alloy quick-condensing sheet was subjected to hydrogen absorption at 20–200℃ and hydrogen pressure of 0.08–0.18 MPa for 1–4 hours, and then dehydrogenated at 500–580℃ for 2–5 hours.

[0022] Nitrogen or argon gas with a purity of not less than 99.999% is used as the grinding gas in the air jet mill. Grinding is carried out in a closed-loop air jet mill system. During the grinding process, the oxygen content of the circulating gas is not higher than 50 ppm and the dew point is not higher than -40℃.

[0023] The median particle size D50 of the prepared main alloy powder is 2.8–4.0 μm, and the D90 is not greater than 7.0 μm; the median particle size D50 of the first light rare earth grain boundary alloy powder is 0.5–2.0 μm; the oxygen content in the composite magnetic powder after mixing is not higher than 1200 ppm.

[0024] S4. The composite magnetic powder is subjected to magnetic field orientation pressing, cold isostatic pressing, and vacuum sintering in sequence, so that the first light rare earth grain boundary alloy melts and wets R2Fe during the sintering process. 14 B main phase grains, in adjacent R2Fe 14 A continuous light rare earth-rich grain boundary phase is formed between the B main phase grains to obtain a sintered magnet substrate, wherein R is Nd and Pr.

[0025] The composite magnetic powder is placed in an orientation magnetic field with a magnetic field strength of 1.5 to 2.2 T and oriented under a pressure of 60 to 120 MPa. Then, it is cold isostatically pressed under a pressure of 150 to 250 MPa to obtain a compact.

[0026] Vacuum degree not higher than 5×10 -Under the condition of ³Pa, the compact is heated to 1020~1060℃ and held for 2~4h for vacuum sintering, then cooled to 880~930℃ and held for 1~3h, and after cooling, the sintered magnet substrate is obtained.

[0027] R2Fe in sintered magnet substrate 14 The average grain size of the B main phase is 2.5–5.0 μm.

[0028] S5. A second light rare earth diffusion alloy powder is placed on at least two opposite surfaces of the sintered magnet substrate, and then subjected to grain boundary diffusion treatment and low-temperature aging treatment in sequence.

[0029] During the grain boundary diffusion process, Pr in the second light rare earth diffusion alloy uses the light rare earth-rich continuous grain boundary phase as a diffusion channel to diffuse from the surface of the sintered magnet substrate to the interior along the light rare earth-rich continuous grain boundary phase, and forms the outer edge region of the main phase grain with Pr concentration gradient in the surface region 0.3 to 1.5 mm away from the diffusion surface.

[0030] The sintered magnet substrate is processed to a thickness of 1.5–6.0 mm along the diffusion direction. The diffusion surface of the sintered magnet substrate is then subjected to degreasing, pickling, deionized water cleaning, and anhydrous ethanol cleaning in sequence.

[0031] The second light rare earth diffusion alloy powder with a median particle size D50 of 0.5 to 3.0 μm was dispersed in anhydrous ethanol containing 0.2 to 0.8 wt% polyvinyl butyral to obtain a diffusion slurry.

[0032] The diffusion slurry was coated on two opposing diffusion surfaces of the sintered magnet substrate, with the loading of the second light rare earth diffusion alloy powder on each diffusion surface being 0.5–2.5 mg / cm², and dried under vacuum conditions at 60–100 °C.

[0033] Vacuum degree not higher than 5×10 - Grain boundary diffusion treatment is carried out under ³Pa or inert gas protection conditions and kept at 830–900℃ for 4–10 hours.

[0034] Those skilled in the art will understand that the grain boundary diffusion temperature and diffusion time are not independent of each other, but should be matched according to the diffusion rate of Pr along the continuous light rare earth-rich grain boundary phase, the thickness of the sintered magnet substrate, and the diffusion source loading.

[0035] At lower diffusion temperatures, the diffusion rate of Pr atoms is relatively low, requiring a longer diffusion time to ensure sufficient diffusion depth and Pr enrichment factor. For example, when the diffusion temperature is 830–850 °C, the preferred diffusion time is 8–10 h.

[0036] When the diffusion temperature is in the intermediate temperature range of greater than 850℃ and not higher than 880℃, the diffusion time is preferably 6 to 8 hours.

[0037] At higher diffusion temperatures, the diffusion rate of Pr along grain boundaries increases, which is necessary to avoid R2Fe. 14 If the B-phase grains grow abnormally, the light rare earth-rich grain boundary phase thickens excessively, or the diffusion source dissolves excessively into the interior of the main phase grains, the diffusion time should be shortened accordingly. For example, when the diffusion temperature is greater than 880℃ but not higher than 900℃, the preferred diffusion time is 4 to 6 hours.

[0038] The aforementioned matching relationship between diffusion temperature and diffusion time is within the scope of this invention. As long as the appropriate temperature-time combination is used to form the continuous light rare earth-rich grain boundary phase, Pr concentration gradient, and Pr-rich region as defined in the claims and this specification, the technical effects of this invention can be achieved.

[0039] After completing the grain boundary diffusion treatment, the temperature is lowered to 480-540℃ at a rate of 1-5℃ / min, and held at that temperature for 2-4 hours for low-temperature aging treatment, followed by cooling to room temperature.

[0040] The thickness of the outer edge region of the main phase grain is 10–80 nm, and the atomic ratio of Pr to the total amount of Pr and Nd in the outer edge region of the main phase grain is the same as that of R2Fe. 14 The atomic ratio of Pr to the total amount of Pr and Nd in the central region of the B main phase grain is 1.15 to 1.80 times, and the outer edge region of the main phase grain constitutes a Pr enrichment region.

[0041] No heavy rare earth elements are actively added during the preparation process, and the total mass content of heavy rare earth elements in the sintered NdFeB permanent magnet material is no higher than 0.05%.

[0042] The heavy rare earth elements mentioned in this invention are one or more of Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y.

[0043] The "continuous light rare earth-rich grain boundary phase" described in this invention is determined according to the following method.

[0044] Two-dimensional cross-sectional specimens were cut along a direction perpendicular to the diffusion surface. Transmission electron microscope specimens were prepared in an area 0.3–1.5 mm from the diffusion surface and observed using bright-field images of transmission electron microscopes or bright-field images of scanning transmission electron microscopes.

[0045] For each sample, at least 10 fields of view were randomly selected, and at least 200 pairs of adjacent R2Fe were statistically analyzed. 14 B-phase grains.

[0046] For any pair of adjacent R2Fe 14For B main phase grains, measure the continuous coverage length L1 of the light rare earth-rich grain boundary phase along the common interface between the two grains and the total length L0 of the common interface. When L1 / L0 is not less than 80%, it is determined that a continuous light rare earth-rich grain boundary phase is formed between the pair of adjacent main phase grains.

[0047] The proportion of continuous grain boundary phases is defined as the ratio of the number of grain pairs forming continuous light rare earth-rich grain boundary phases to the total number of adjacent main phase grain pairs.

[0048] Within a region extending 0.3–1.5 mm from the diffusion surface into the material interior, statistically at least 60% of adjacent R₂Fe₂... 14 A continuous light rare earth-rich grain boundary phase is formed between the B main phase grains.

[0049] In a preferred embodiment of the present invention, the proportion of continuous grain boundary phase can further reach 70% or more. When the composition, amount added, oxygen content of composite magnetic powder, and sintering and diffusion regimes of the first light rare earth grain boundary alloy are matched, the proportion of continuous grain boundary phase can reach 79% or more, and in some embodiments it can reach 80% or more.

[0050] The “Pr-rich region” described in this invention is determined according to the following method.

[0051] Scanning transmission electron microscopy combined with energy dispersive spectroscopy was used to perform line or surface scanning from the outer edge of the main phase grain to the grain center. The atomic ratio of Pr to the total amount of Pr and Nd in the outer region of the main phase grain was denoted as K1, and the corresponding atomic ratio of Pr to the total amount of Pr and Nd in the central region of the main phase grain was denoted as K0.

[0052] When K1 / K0 is 1.15 to 1.80, the outer edge region of the main phase grain is defined as the Pr enrichment region.

[0053] For each sample, at least 30 complete main phase grains are measured, and their arithmetic mean is taken as the Pr enrichment factor of the sample.

[0054] The heavy rare earth-free sintered NdFeB permanent magnet material prepared by the above method comprises, by mass percentage: R 29.0%–32.5%, B 0.88%–1.06%, Co 0.20%–1.20%, Cu 0.08%–0.45%, Al 0.08%–0.40%, Ga 0.05%–0.30%, Zr 0.02%–0.12%, with the balance being Fe and unavoidable impurities.

[0055] R is composed of Nd and Pr, with Pr accounting for 8% to 40% of the total mass of R, and the total mass content of heavy rare earth elements not exceeding 0.05%.

[0056] The sintered NdFeB permanent magnet material comprises R2Fe with an average particle size of 2.5–5.0 μm. 14 B main phase grains, and adjacent R2Fe 14 Light rare earth-rich grain boundary phases between B main phase grains.

[0057] Within a region extending 0.3–1.5 mm from the diffusion surface into the material interior, according to the aforementioned two-dimensional cross-sectional statistical method, at least 60% of adjacent R2Fe are statistically significant. 14 A continuous light rare earth-rich grain boundary phase is formed between the B main phase grains.

[0058] In the preferred embodiment, the proportion of continuous grain boundary phase reaches 70% or more; in a further preferred embodiment, the proportion of continuous grain boundary phase reaches 79% or more.

[0059] The sintered NdFeB permanent magnet material has a Pr concentration gradient that gradually decreases from the diffusion surface to the center. In the surface region 0.3–1.5 mm from the diffusion surface, R2Fe 14 The outer edge of the B main phase grains has a Pr enrichment region with a thickness of 10–80 nm.

[0060] The atomic ratio of Pr to the total amount of Pr and Nd in the Pr-enriched region is 1.15 to 1.80 times that of the atomic ratio of Pr to the total amount of Pr and Nd in the central region of the corresponding main phase grain.

[0061] Under preferred conditions, the density of the sintered NdFeB permanent magnet material is 7.45–7.60 g / cm³; at 20±2℃, the remanence Br is 1.28–1.32 T, the intrinsic coercivity Hcj is 1550–1680 kA / m, and the maximum energy product (BH)max is 305–318 kJ / m³.

[0062] Compared with existing technologies, this invention provides a heavy rare earth-free sintered NdFeB permanent magnet material and its preparation method, which has the following beneficial effects: the first light rare earth grain boundary alloy and the second light rare earth diffusion alloy have different compositions. The first light rare earth grain boundary alloy is relatively rich in Cu, has a lower melting point and better liquid phase wetting ability, and preferentially forms a continuous light rare earth-rich grain boundary phase during sintering; the second light rare earth diffusion alloy is relatively rich in Pr, and can provide Pr with a high diffusion driving force into the magnet interior. The two alloys respectively undertake the functions of grain boundary channel construction and main phase outer edge strengthening.

[0063] The continuous grain boundary phase formed during the sintering stage of the first light rare earth grain boundary alloy not only improves the magnetic isolation between adjacent main phase grains but also provides a continuous channel for subsequent Pr diffusion. In the second light rare earth diffusion alloy, Pr can diffuse from the magnet surface to the interior using the continuous grain boundary phase, thereby improving the Pr diffusion depth and diffusion uniformity.

[0064] This invention causes Pr to be mainly enriched at the outer edge of the main phase grains, rather than uniformly entering all the main phase grains. This can suppress the nucleation and expansion of antimagnetic domains, while reducing the adverse effects of excessive nonmagnetic grain boundary phases or excessive Pr on remanence.

[0065] By controlling the addition amount of the first light rare earth grain boundary alloy to 1.5%–3.5% and the loading amount of the second light rare earth diffusion alloy to 0.5–2.5 mg / cm², a balance can be achieved between grain boundary continuity, Pr diffusion degree and main phase volume fraction.

[0066] By appropriately matching the diffusion time according to the diffusion temperature, extending the diffusion time under low-temperature conditions and shortening the diffusion time under high-temperature conditions, the risk of abnormal growth of main phase grains and excessive thickening of grain boundary phase can be reduced while ensuring the diffusion depth and Pr enrichment.

[0067] By controlling the oxygen content and dew point in the grinding gas of the air jet mill, the degree of oxidation of the composite magnetic powder can be reduced, which is beneficial to maintaining the fluidity and wettability of the light rare earth-rich grain boundary phase and reducing the obstruction of Pr diffusion by grain boundary oxides.

[0068] This invention does not actively add heavy rare earth elements such as Dy and Tb, and includes Y, which may weaken the surface strengthening effect of Pr, in the restricted element range. Under the condition that the total mass content of heavy rare earth elements is not higher than 0.05%, it can still obtain high intrinsic coercivity and maintain high remanence and maximum magnetic energy product. Attached Figure Description

[0069] Figure 1 In Example 4 of this invention, R2Fe was located 0.8 mm from the diffusion surface. 14 STEM bright-field image of continuous light rare earth-rich grain boundary phases between B main phase grains; Figure 2 Comparative Example 3 of this invention: R2Fe at a distance of 0.8 mm from the diffusion surface 14 STEM bright-field image of discontinuous light rare earth-rich grain boundary phases between B main phase grains; Figure 3 R2Fe in Embodiment 4 of the present invention 14 STEM-EDS line scans of Pr, Nd, and Cu elements from the outer edge of the B main phase grain to the grain center; Figure 4 R2Fe in Comparative Example 4 of this invention 14 STEM-EDS line scans of Pr, Nd, and Cu elements from the outer edge of the B main phase grain to the grain center. Detailed Implementation

[0070] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0071] Please see Figures 1-4 This invention provides a technical solution for sintered NdFeB permanent magnet materials without heavy rare earth elements and a method for preparing the same: Unless otherwise specified, the raw materials used in the examples and comparative examples are all industrially pure or analytically pure raw materials; all percentages are mass percentages; all vacuum degrees are absolute pressures; and all particle sizes D50 and D90 are volume distribution particle sizes.

[0072] Table 1 Main Alloy Composition

[0073] Table 2 Composition of the first light rare earth grain boundary alloy

[0074] Table 3 Composition of the Second Light Rare Earth Diffusion Alloy

[0075] Table 4. Parameters for rapid setting, powdering, and molding / sintering

[0076] Table 5 Diffusion and Low Temperature Aging Parameters

[0077] Example 1 The main alloy composition of Example 1 in Table 1 was prepared, and the ingredients were placed in a vacuum induction melting furnace. After evacuation, high-purity argon gas was introduced and the furnace was melted at 1480°C. The melt was poured onto the surface of a water-cooled copper roller through an intermediate ladle to obtain a main alloy rapid solidification sheet with an average thickness of 0.20 mm.

[0078] Backscattered electron microscopy was performed on the cross section along the thickness of the rapidly solidified sheet. The columnar crystal region accounted for 60.5% of the area, and the rare earth-rich phase located at the grain boundaries of columnar crystals and the lamellar interfaces accounted for 60.8% of the total rare earth-rich phase area.

[0079] The main alloy rapidly solidified flakes were subjected to hydrogen absorption at 20℃ and 0.08MPa hydrogen pressure for 1 hour, followed by dehydrogenation at 500℃ for 2 hours. The mixture was then subjected to air jet milling with nitrogen gas of not less than 99.999% purity to obtain main alloy powder with a D50 of 2.8μm and a D90 of 5.8μm.

[0080] The first light rare earth grain boundary alloy powder was prepared according to the composition of Example 1 in Table 2, with a powder D50 of 0.5 μm. The first light rare earth grain boundary alloy powder was mixed with the main alloy powder so that the first light rare earth grain boundary alloy powder accounted for 1.5% of the total mass of the composite magnetic powder.

[0081] The composite magnetic powder was placed in a 1.5T magnetic field and oriented under a pressure of 60MPa, followed by cold isostatic pressing under a pressure of 150MPa. The vacuum degree was not higher than 5×10⁻⁶. - Under the condition of ³Pa, the substrate was sintered at 1020℃ for 2 hours, then cooled to 880℃ and held for 1 hour to obtain the sintered magnet substrate.

[0082] The sintered magnet substrate was processed into a sample with a thickness of 1.5 mm. The second light rare earth diffusion alloy powder was prepared according to the composition of Example 1 in Table 3, and it was made into a diffusion slurry and coated on the two opposite surfaces of the magnet with a single-sided loading of 0.5 mg / cm².

[0083] Grain boundary diffusion treatment was performed at 830℃ for 10 hours, then the temperature was reduced to 480℃ at a rate of 1℃ / min and held for 2 hours, and then cooled to room temperature to obtain the heavy rare earth-free sintered NdFeB permanent magnet material of Example 1.

[0084] Example 2 Example 2 was prepared using essentially the same method as Example 1, except that the parameters listed in Tables 1 to 5 for Example 2 were used.

[0085] The grain boundary diffusion temperature is 865℃ and the diffusion time is 7h, which is a matching combination of intermediate diffusion temperature and intermediate diffusion time.

[0086] Example 3 Example 3 was prepared using essentially the same method as Example 1, except that the parameters listed in Tables 1 to 5 for Example 3 were used.

[0087] In Example 3, the grain boundary diffusion temperature was 900°C and the diffusion time was 4 hours. A higher diffusion temperature can increase the diffusion rate of Pr along the grain boundaries, and by shortening the diffusion time to 4 hours, abnormal growth of the main phase grains and excessive thickening of the grain boundary phase can be reduced.

[0088] Examples 1, 2, and 3 correspond to the low, middle, and high values ​​of the main components and process parameters, respectively.

[0089] Example 4 The main alloy composition in Example 4 of Table 1 was prepared by batching, the batching was subjected to vacuum induction melting, and the main alloy quick-solidified sheet with an average thickness of 0.35 mm was produced by quick-solidification casting process.

[0090] The rapidly solidified flakes were subjected to hydrogen absorption at 25℃ and 0.12MPa hydrogen pressure for 2 hours, followed by dehydrogenation at 550℃ for 4 hours. A main alloy powder with a D50 of 3.2μm and a D90 of 6.0μm was obtained by air jet milling with high-purity nitrogen.

[0091] A first light rare earth grain boundary alloy powder comprising 65% Pr, 24% Cu, 7% Al, 3% Ga and 1% Fe was added to the main alloy powder, and the first light rare earth grain boundary alloy powder accounted for 2.5% of the total mass of the composite magnetic powder.

[0092] The composite magnetic powder was oriented and pressed under a magnetic field of 1.8T and a pressure of 90MPa, then cold isostatically pressed under a pressure of 200MPa, sintered at 1040℃ for 3h, and then held at 900℃ for 2h to obtain the sintered magnet substrate.

[0093] The sintered magnet substrate was processed to a thickness of 3.0 mm along the diffusion direction. A second light rare earth diffusion alloy powder, consisting of 78% Pr, 15% Cu, 4% Al, 2% Ga, and 1% Fe, was coated on the two opposite surfaces of the magnet, with a single-sided loading of 1.5 mg / cm².

[0094] Grain boundary diffusion treatment was performed by holding the temperature at 860℃ for 8 hours, and then the temperature was lowered to 510℃ at a rate of 3℃ / min and held for 3 hours for low-temperature aging treatment. After cooling, the heavy rare earth-free sintered NdFeB permanent magnet material of Example 4 was obtained.

[0095] Example 5 Example 5 was carried out according to the same basic process as Example 1, using the parameters of Example 5 in Tables 1 to 5.

[0096] In Example 5, the diffusion temperature was 850°C and the diffusion time was 9 hours, verifying the effectiveness of using a longer diffusion time in low-temperature regions.

[0097] Example 6 Example 6 was carried out according to the same basic process as Example 1, using the parameters of Example 6 in Tables 1 to 5.

[0098] In Example 6, the diffusion temperature was 885°C and the diffusion time was 5 hours, verifying the effectiveness of using a shorter diffusion time in a higher temperature region.

[0099] Comparative Example 1 differs only in that the first light rare earth grain boundary alloy powder is not added; all other conditions are the same as in Example 4.

[0100] Comparative Example 2 differs only in that the second light rare earth diffusion alloy powder is not placed on the surface of the sintered magnet substrate, and the grain boundary diffusion treatment at 860℃ is not performed. Instead, the sintered magnet substrate is directly subjected to a low-temperature aging treatment at 510℃ for 3 hours.

[0101] Comparative Example 3 differs only in that the first light rare earth grain boundary alloy is replaced with the same composition as the second light rare earth diffusion alloy, while the other conditions are the same as in Example 4.

[0102] Comparative Example 4 differs only in that the second light rare earth diffusion alloy is replaced with the same composition as the first light rare earth grain boundary alloy, while the other conditions are the same as in Example 4.

[0103] Comparative Example 5 differs only in that the oxygen content of the circulating gas during the air jet milling process is adjusted to 500 ppm and the dew point is adjusted to -10°C; the other conditions are the same as in Example 4.

[0104] Comparative Example 6 differs only in that it is directly cooled to room temperature after the grain boundary diffusion treatment is completed, without undergoing a low-temperature aging treatment at 510°C for 3 hours. All other conditions are the same as in Example 4.

[0105] Performance testing The sintered NdFeB permanent magnet materials prepared in Examples 1-6 and Comparative Examples 1-6 were tested as follows.

[0106] 1. Chemical composition testing Elements such as B, Co, Cu, Al, and Ga were tested using inductively coupled plasma atomic emission spectrometry (ICP-AES). Nd, Pr, and heavy rare earth elements were determined using ICP-MS or ICP-AES, with each sample tested in triplicate.

[0107] 2. Oxygen content test The oxygen content in the composite magnetic powder and the final magnet was determined using pulsed heating infrared absorption method.

[0108] 3. Density test The magnet density was tested according to GB / T 3850—2015 using the draining method, with 5 samples taken from each group.

[0109] 4. Magnetic property test Remanence Br, intrinsic coercivity Hcj, and maximum energy product (BH)max were tested in accordance with GB / T 44474—2024.

[0110] The test temperature was 20±2℃. The sample was saturated magnetized along the orientation direction. Five samples were taken in each group, and the test results were taken as the arithmetic mean.

[0111] 5. Tissue testing of quick-setting tablets Samples were prepared and microstructures were observed in accordance with GB / T 13298—2015. The area ratio of columnar crystal regions and the proportion of rare earth-rich phases at grain boundaries were statistically analyzed using scanning electron microscopy in backscatter mode.

[0112] 6. Main phase grain size test For each sample, at least 10 fields of view are randomly selected, and at least 500 main phase grains are counted cumulatively. The average grain size of the main phase grains is calculated using the equivalent circle diameter method.

[0113] 7. Continuous grain boundary phase ratio test TEM samples were prepared in the region 0.3–1.5 mm from the diffusion surface, and each group had a total of no less than 200 pairs of adjacent main phase grains.

[0114] When the continuous coverage length of the grain boundary phase accounts for more than 80% of the total length of the common interface, it is determined that a continuous grain boundary phase has been formed.

[0115] 8. Pr enrichment region test STEM-EDS was used to perform line or surface scanning of the main phase grains, and the K1 / K0 values ​​of the outer edge region and the grain center region were calculated. Each group of measurements included no less than 30 main phase grains.

[0116] 9. Effective diffusion depth test Composition scans are performed every 0.10 mm from the diffusion surface toward the center of the magnet. The maximum distance at which the Pr ratio increases by at least 10% relative to the reference value at the center of the magnet is defined as the effective diffusion depth.

[0117] Test Results Table 6. Tissue structure test results of the embodiments

[0118] Table 7 Magnetic performance test results of the embodiments

[0119] Table 8 Comparative Test Results

[0120] Test Result Analysis As can be seen from Examples 1 to 6, within the scope of the present invention, by reasonably matching the diffusion temperature and diffusion time, continuous light rare earth-rich grain boundary phases and Pr-rich regions can be formed.

[0121] Example 1 employed a low diffusion temperature of 830°C and extended the diffusion time to 10 hours. Despite the low diffusion source loading, the proportion of continuous grain boundary phase still reached 61.5%, and the effective diffusion depth reached 0.31 mm, demonstrating the feasibility of the low-temperature, long-duration combination.

[0122] Example 3 used a high diffusion temperature of 900℃ and shortened the diffusion time to 4 hours. The effective diffusion depth reached 1.49 mm, while the average particle size of the main phase was controlled at 5.0 μm, indicating that the combination of high temperature and short time can balance diffusion efficiency and grain size control.

[0123] The proportion of continuous grain boundary phase in Example 4 was 79.2%, demonstrating that under the preferred parameter combination, the proportion of continuous grain boundary phase can reach more than 79%. The proportions of continuous grain boundary phase in Examples 3 and 6 reached 82.0% and 80.5% respectively, further demonstrating that some implementation schemes can reach more than 80%.

[0124] As can be seen from Example 4 and Comparative Example 1, when the first light rare earth grain boundary alloy is not added, the proportion of continuous grain boundary phase decreases from 79.2% to 38.6%, the effective diffusion depth decreases from 1.10 mm to 0.58 mm, and Hcj decreases from 1658 kA / m to 1310 kA / m, indicating that the pre-formed continuous grain boundary phase has a significant diffusion channel effect.

[0125] As can be seen from Example 4 and Comparative Example 2, when the second light rare earth diffusion alloy is not provided, although the proportion of continuous grain boundary phase still reaches 77.8%, a Pr enrichment region that conforms to the definition of this invention is not formed, and Hcj is only 1385kA / m, indicating that both continuous grain boundary phase and Pr outer edge enrichment are indispensable.

[0126] As can be seen from Example 4 and Comparative Example 3, after replacing the first light rare earth grain boundary alloy with a Pr-rich and Cu-low composition, the proportion of continuous grain boundary phase decreased to 54.2%, indicating that the Cu-rich characteristic of the first light rare earth grain boundary alloy is conducive to the formation of low-melting-point liquid phase and continuous grain boundary channels.

[0127] As can be seen from Example 4 and Comparative Example 4, after replacing the second light rare earth diffusion alloy with a low Pr and high Cu composition, the proportion of continuous grain boundary phases changed little, but the Pr enrichment factor decreased from 1.55 to 1.16, proving that the Pr-rich characteristic of the second light rare earth diffusion alloy mainly acts on the strengthening of the outer edge of the main phase grains.

[0128] As can be seen from Example 4 and Comparative Example 5, increasing the oxygen content and dew point in the powder-making atmosphere will significantly reduce the proportion of continuous grain boundary phase and the effective diffusion depth, indicating that low-oxygen powder-making control plays an important role in maintaining the fluidity of grain boundary phase and the effectiveness of diffusion channels.

[0129] As can be seen from Example 4 and Comparative Example 6, even after omitting the low-temperature aging treatment, the continuous grain boundary phase and Pr enrichment region still exist, but Hcj decreases from 1658 kA / m to 1518 kA / m, indicating that low-temperature aging can further optimize the distribution of grain boundary phase and the magnetic isolation state between the main phase grains.

[0130] Figure 4In the linear scan, the scanning position starts from the grain boundary phase, perpendicularly passes through the outer edge of the main phase grain, and extends to the grain center; the scan step size is 0.5 nm. The results show that: in Comparative Example 4, the Pr enrichment degree at the outer edge of the main phase grain is relatively low, the Pr enrichment region is relatively thin, K1 / K0 = 1.16, which is significantly lower than that in Example 4 (K1 / K0 = 1.55).

[0131] In summary, the technical effects of this invention stem from the synergy of the following technical features: the first light rare earth grain boundary alloy is relatively Cu-rich, forming a continuous light rare earth-rich grain boundary phase during the sintering stage; the second light rare earth diffusion alloy is relatively Pr-rich, providing the Pr required to form the Pr-rich region at the outer edge of the main phase grains; the Pr in the second light rare earth diffusion alloy utilizes the continuous light rare earth-rich grain boundary phase as a diffusion channel, diffusing from the magnet surface to the interior; the diffusion temperature and diffusion time are matched to each other, ensuring the diffusion depth while suppressing abnormal grain growth; low-oxygen powder preparation and low-temperature aging further maintain and optimize the grain boundary channels and Pr-rich structure.

Claims

1. A method for preparing a sintered NdFeB permanent magnet material without heavy rare earth elements, characterized in that, The steps include the following: S1. Prepare the raw materials according to the composition of the sintered NdFeB main alloy, and then perform vacuum melting and rapid solidification casting of the raw materials in sequence to obtain the rapid solidification sheet of the main alloy. S2. Prepare a first light rare earth grain boundary alloy powder and a second light rare earth diffusion alloy powder respectively, wherein the first light rare earth grain boundary alloy powder is a low melting point alloy powder rich in Cu relative to the second light rare earth diffusion alloy powder, and the second light rare earth diffusion alloy powder is a low melting point alloy powder rich in Pr relative to the first light rare earth grain boundary alloy powder. S3. The main alloy rapid solidification sheet is subjected to hydrogen crushing, dehydrogenation and air jet milling in sequence to obtain main alloy powder; the first light rare earth grain boundary alloy powder is mixed with the main alloy powder, wherein the first light rare earth grain boundary alloy powder accounts for 1.5% to 3.5% of the total mass of the mixed composite magnetic powder; S4. The composite magnetic powder is subjected to magnetic field orientation pressing, cold isostatic pressing, and vacuum sintering in sequence, so that the first light rare earth grain boundary alloy melts and wets R2Fe during the sintering process. 14 B main phase grains, in adjacent R2Fe 14 A continuous light rare earth-rich grain boundary phase is formed between the B main phase grains to obtain a sintered magnet substrate, wherein R is Nd and Pr; S5. The second light rare earth diffusion alloy powder is disposed on at least two opposite surfaces of the sintered magnet substrate, and grain boundary diffusion treatment and low temperature aging treatment are performed in sequence, so that Pr in the second light rare earth diffusion alloy uses the light rare earth-rich continuous grain boundary phase as a diffusion channel to diffuse from the surface of the sintered magnet substrate to the interior along the light rare earth-rich continuous grain boundary phase, and forms a main phase grain outer edge region with Pr concentration gradient in the surface region 0.3 to 1.5 mm away from the diffusion surface, thereby obtaining a heavy rare earth-free sintered NdFeB permanent magnet material; The thickness of the outer edge region of the main phase grain is 10–80 nm, and the atomic ratio of Pr to the total amount of Pr and Nd in the outer edge region of the main phase grain is the same as that of R2Fe. 14 The atomic ratio of Pr to the total amount of Pr and Nd in the central region of the B main phase grain is 1.15 to 1.80 times, and the outer edge region of the main phase grain constitutes a Pr enrichment region; No heavy rare earth elements are actively added during the preparation process, and the total mass content of heavy rare earth elements in the sintered NdFeB permanent magnet material is not higher than 0.05%; the heavy rare earth elements are one or more of Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y.

2. The method for preparing heavy rare earth-free sintered NdFeB permanent magnet material according to claim 1, characterized in that, The main alloy comprises, by mass percentage: R 28.5%–31.5%, B 0.90%–1.05%, Co 0.20%–1.00%, Cu 0.03%–0.20%, Al 0.05%–0.30%, Ga 0.03%–0.20%, Zr 0.02%–0.12%, with the balance being Fe and unavoidable impurities; R is composed of Nd and Pr, with Pr accounting for 5% to 30% of the total mass of R.

3. The method for preparing heavy rare earth-free sintered NdFeB permanent magnet material according to claim 1, characterized in that: The average thickness of the main alloy quick-setting sheet is 0.20–0.50 mm; In a two-dimensional cross-section perpendicular to the thickness direction of the main alloy rapid solidification sheet, backscattered electron imaging shows that the area of ​​columnar crystal regions accounts for 60% to 90% of the total area of ​​the thickness direction cross-section. The rare earth-rich phase in the main alloy rapid solidification sheet is distributed in a strip, continuous network, or semi-continuous network along the columnar grain boundaries and lamellar interfaces, and at least 60% of the total area of ​​the rare earth-rich phase is located at the columnar grain boundaries or lamellar interfaces.

4. The method for preparing heavy rare earth-free sintered NdFeB permanent magnet material according to claim 1, characterized in that: In step S3, the main alloy quick-condensing sheet is subjected to hydrogen absorption at 20-200°C and hydrogen pressure of 0.08-0.18 MPa for 1-4 hours, and then subjected to dehydrogenation treatment at 500-580°C for 2-5 hours. Nitrogen or argon with a purity of not less than 99.999% is used as the grinding gas in the air jet mill. Grinding is carried out in a closed-loop air jet mill system. During the grinding process, the oxygen content of the circulating gas is not higher than 50 ppm and the dew point is not higher than -40℃. The median particle size D50 of the prepared main alloy powder is 2.8-4.0 μm and D90 is no greater than 7.0 μm. The median particle size D50 of the first light rare earth grain boundary alloy powder is 0.5-2.0 μm. The oxygen content in the composite magnetic powder after mixing is no higher than 1200 ppm.

5. The method for preparing heavy rare earth-free sintered NdFeB permanent magnet material according to claim 1, characterized in that: The first light rare earth grain boundary alloy comprises, by mass percentage: 60%–70% of the first light rare earth element R1, 20%–28% of Cu, 5%–9% of Al, 2%–5% of Ga, with the balance being Fe and unavoidable impurities; The second light rare earth diffusion alloy comprises, by mass percentage: 72%–82% of the second light rare earth element R2, 12%–20% of Cu, 3%–7% of Al, 1%–4% of Ga, with the balance being Fe and unavoidable impurities; The first light rare earth element R1 is Pr, Nd, or a Pr-Nd alloy, and Pr accounts for 50% to 100% of the total mass of R1; the second light rare earth element R2 is Pr or a Pr-Nd alloy, and Pr accounts for 80% to 100% of the total mass of R2. The mass percentage of the second light rare earth element R2 in the second light rare earth diffusion alloy is 8 to 18 percentage points higher than the mass percentage of the first light rare earth element R1 in the first light rare earth grain boundary alloy. The mass percentage of Cu in the first light rare earth grain boundary alloy is 5 to 12 percentage points higher than that in the second light rare earth diffusion alloy.

6. The method for preparing heavy rare earth-free sintered NdFeB permanent magnet material according to claim 1, characterized in that, Step S4 includes: The composite magnetic powder is placed in an orientation magnetic field with a magnetic field strength of 1.5 to 2.2 T and oriented under a pressure of 60 to 120 MPa. Then, it is cold isostatically pressed under a pressure of 150 to 250 MPa to obtain a compact. Vacuum degree not higher than 5×10 - Under the condition of ³Pa, the pressed blank is heated to 1020-1060℃ and held for 2-4 hours for vacuum sintering, then cooled to 880-930℃ and held for 1-3 hours, and the sintered magnet substrate is obtained after cooling. R2Fe in sintered magnet substrate 14 The average grain size of the B main phase is 2.5–5.0 μm.

7. The method for preparing heavy rare earth-free sintered NdFeB permanent magnet material according to claim 1, characterized in that, Step S5 includes: The sintered magnet substrate is processed to a thickness of 1.5 to 6.0 mm along the diffusion direction, and the diffusion surface of the sintered magnet substrate is sequentially degreased, pickled, cleaned with deionized water and cleaned with anhydrous ethanol. The second light rare earth diffusion alloy powder with a median particle size D50 of 0.5 to 3.0 μm was dispersed in anhydrous ethanol containing 0.2 to 0.8 wt% polyvinyl butyral to obtain a diffusion slurry. The diffusion slurry is coated on two opposing diffusion surfaces of a sintered magnet substrate, with the loading of the second light rare earth diffusion alloy powder on each diffusion surface being 0.5 to 2.5 mg / cm², and then dried under vacuum conditions at 60 to 100°C. Vacuum degree not higher than 5×10 - Grain boundary diffusion treatment is carried out under ³Pa or inert gas protection conditions at 830–900℃ for 4–10 hours. After the grain boundary diffusion treatment is completed, the temperature is lowered to 480-540°C at a cooling rate of 1-5°C / min, and held at that temperature for 2-4 hours for low-temperature aging treatment, and then cooled to room temperature.

8. The method for preparing heavy rare earth-free sintered NdFeB permanent magnet material according to claim 1, characterized in that: The main alloy comprises, by mass percentage, 23.8% Nd, 6.7% Pr, 0.98% B, 0.60% Co, 0.10% Cu, 0.18% Al, 0.08% Ga, and 0.05% Zr, with the balance being Fe and unavoidable impurities; The first light rare earth grain boundary alloy comprises, by mass percentage, 65% Pr, 24% Cu, 7% Al, 3% Ga, and 1% Fe; The second light rare earth diffusion alloy comprises, by mass percentage, 78% Pr, 15% Cu, 4% Al, 2% Ga, and 1% Fe; The first light rare earth grain boundary alloy powder accounts for 2.5% of the total mass of the composite magnetic powder; The vacuum sintering temperature was 1040℃ and the holding time was 3h, followed by holding at 900℃ for 2h; The thickness of the sintered magnet substrate along the diffusion direction is 3.0 mm, and the loading of the second light rare earth diffusion alloy powder on each diffusion surface is 1.5 mg / cm². The temperature for the grain boundary diffusion treatment is 860℃ and the holding time is 8h; the temperature for the low-temperature aging treatment is 510℃ and the holding time is 3h. On a two-dimensional cross-section perpendicular to the diffusion surface, using a bright-field image from a transmission electron microscope or a bright-field image from a scanning transmission electron microscope, at least 10 fields of view are randomly selected, and at least 200 pairs of adjacent R2Fe are statistically analyzed. 14 For B main phase grains, the criteria for determining the formation of continuous light rare earth-rich grain boundary phases are that the continuous coverage length of the light rare earth-rich grain boundary phase along the common interface of adjacent main phase grains accounts for more than 80% of the total length of the common interface. According to the above statistical method, in a region extending 0.3–1.5 mm from the diffusion surface into the material interior, statistically at least 70% of adjacent R2Fe 14 A continuous light rare earth-rich grain boundary phase is formed between the B main phase grains, and the atomic ratio of Pr to the total amount of Pr and Nd in the Pr enrichment region is 1.25 to 1.60 times that of the atomic ratio of Pr to the total amount of Pr and Nd in the corresponding main phase grain center region.

9. A sintered NdFeB permanent magnet material free of heavy rare earth elements, characterized in that, Including by mass percentage: R 29.0%–32.5%, B 0.88%–1.06%, Co 0.20%–1.20%, Cu 0.08%–0.45%, Al 0.08%–0.40%, Ga 0.05%–0.30%, Zr 0.02%–0.12%, with the balance being Fe and unavoidable impurities; R is composed of Nd and Pr, with Pr accounting for 8% to 40% of the total mass of R, and the total mass content of heavy rare earth elements not exceeding 0.05%. The sintered NdFeB permanent magnet material comprises R2Fe with an average particle size of 2.5–5.0 μm. 14 B main phase grains, and adjacent R2Fe 14 Light rare earth-rich grain boundary phases between B main phase grains.

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