A GaCu co-doped sintered permanent magnet and its manufacturing process

Abstract

This invention discloses a GaCu co-doped sintered permanent magnet and its manufacturing process. The magnet comprises, by weight percentage, 25–35 wt% Ce, 15–25 wt% Nd+Pr, 0.9–1.1 wt% B, 0.2–0.3 wt% Ga, 0.1–0.2 wt% Cu, with the balance being Fe. The manufacturing process includes the following steps: S1 Raw material melting; S2 Powder preparation under a hydrogen atmosphere at a pressure of 0.1–0.2 wt%. The material is treated at 300℃ for 2.5-3.5 hours under MPa and then air-jet milled under inert gas protection to obtain anisotropic powder with an average particle size of 3-5 μm. Step S3 involves pressing and shaping. Step S4 involves vacuum sintering: the pressed compact is pre-sintered under vacuum at 600-700℃ for 0.8-1.2 hours, then heated to 1040-1080℃ for 2-4 hours, followed by cooling to 500-600℃ for 1-2 hours. Cooling yields the GaCu co-doped sintered permanent magnet. By doping with GaCu and achieving spontaneous passivation of grain boundaries during vacuum sintering and low-temperature annealing, the corrosion resistance and structural stability of the magnet are significantly improved without plating.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet material preparation technology, specifically to a production process for GaCu co-doped sintered permanent magnets. Background Technology

[0002] Sintered NdFeB permanent magnets possess excellent magnetic properties, but the Nd-rich phase at the grain boundaries is highly reactive and easily oxidized or corroded, especially in high humidity or salt spray environments, leading to rapid decay of magnetic properties. Currently, industrial applications often employ chemical plating of Ni, Zn, or epoxy coatings for protection. Literature from *Surface Technology* (Vol. 49, No. 8, 2020) and *Rare Earths* (Vol. 40, No. 6, 2019) points out that while such external coatings can improve corrosion resistance, the large difference in thermal expansion coefficients between the coating and the substrate easily leads to cracks during thermal cycling, causing protective failure. Furthermore, these processes are complex, costly, and environmentally unfriendly.

[0003] The existing technology has the following technical problems: 1. High-cerium magnets are prone to corrosion at grain boundaries and lack coating-free protection solutions; 2. The outer coating is complex and has a large environmental burden; 3. Single doping has limited improvement. Summary of the Invention

[0004] In order to overcome the deficiencies of the above-mentioned related technologies, this application provides a GaCu co-doped sintered permanent magnet and its production process. In the Ce–Fe–B magnet system, GaCu is doped and spontaneous passivation of grain boundaries is achieved during vacuum sintering and low-temperature annealing, thereby significantly improving the corrosion resistance and structural stability of the magnet under coating-free conditions.

[0005] This application provides a GaCu co-doped sintered permanent magnet comprising, by weight percentage, 25–35 wt% Ce, 15–25 wt% Nd+Pr, 0.9–1.1 wt% B, 0.2–0.3 wt% Ga, 0.1–0.2 wt% Cu, with the balance being Fe.

[0006] Preferably, the composition comprises, by weight percentage, 30 wt% Ce, 20 wt% Nd+Pr, 1 wt% B, 0.3 wt% Ga, 0.1 wt% Cu and 48.4 wt% Fe.

[0007] A manufacturing process for GaCu co-doped sintered permanent magnets includes the following steps:

[0008] S1 Raw Material Smelting:

[0009] The raw materials weighed according to the formula are heated to 1450-1500 ℃ under inert gas protection until completely melted, and then cooled to form a master alloy ingot;

[0010] S2 powder preparation:

[0011] After the master alloy ingot is melted, it is rapidly solidified into alloy sheets by a strip spinning machine. The alloy sheets are then treated in a hydrogen atmosphere at a pressure of 0.1-0.2 MPa and a temperature of 300 ℃ for 2.5-3.5 hours. Finally, they are air-jet milled under inert gas protection to obtain anisotropic powder with an average particle size of 3-5 μm.

[0012] S3 compression molding:

[0013] Anisotropic powder is oriented and cold-pressed into a compact under the action of a magnetic field along the main axis.

[0014] S4 Vacuum Sintering:

[0015] The compact is pre-sintered under vacuum conditions at a temperature of 600-700 ℃ for 0.8-1.2 hours. The temperature is then increased to 1040-1080 ℃ and held for 2-4 hours. The temperature is then decreased to 500-600 ℃ and held for 1-2 hours. The GaCu co-doped sintered permanent magnet is obtained after cooling.

[0016] Preferably, step S1 specifically involves using a medium-frequency vacuum induction melting furnace to heat the material to 1450-1500 ℃ under the protection of high-purity argon gas until it is completely melted, and repeating the melting process 3-4 times.

[0017] Preferably, the speed of the strip spinning machine in step S2 is 25–30 m / s, and the average thickness of the alloy sheet is 30–50 μm.

[0018] Preferably, step S3 pressing and forming specifically involves using an in-mold magnetic field orientation press with a magnetic field strength of 1.5–2.0 T. Under the action of the magnetic field, the orientation is cold-pressed along the main axis with a pressure of 80–120 MPa to obtain an orientation compact. The compact is then pre-sintered under vacuum at a temperature of 600–700 ℃ for 1 hour.

[0019] Compared with related technologies, the present invention has the following advantages:

[0020] This invention significantly improves the corrosion resistance and structural stability of a high-cerium Ce–Fe–B magnet by introducing trace amounts of Ga and Cu synergistic doping into the system, achieving spontaneous passivation of grain boundaries during vacuum sintering and low-temperature annealing. The magnet prepared by this invention exhibits a salt spray test lifetime exceeding 120 hours in the uncoated state, compared to approximately 72 hours for conventional Ce–Fe–B magnets. Weight loss is reduced by about 80%, and no obvious pitting or erosion is observed on the surface. Furthermore, the magnet's remanence (Br≈1.09 T), intrinsic coercivity (Hcj≈890 kA / m), and maximum energy product (BH)max≈285 kJ / m are also improved. 3All three phases maintained excellent performance, showing improvements of approximately 14%, 12%, and 10% respectively compared to undoped magnets. Microstructural analysis revealed that Ga and Cu formed continuous Nd–M (M=Ga, Cu) intermetallic compound phases at the grain boundaries, uniformly coating the main phase particles, significantly enhancing the grain boundary electrochemical potential and suppressing corrosion galvanic reactions.

[0021] This invention can be implemented on a conventional pressing-sintering production line without the need for chemical plating or electroplating processes. The production process is simplified and the cost is reduced by about 20%. At the same time, it avoids the use of harmful plating materials such as Ni and Cr, and has significant environmental and economic advantages.

[0022] This invention achieves an innovative transformation of high-cerium magnets from "external coating protection" to "internal grain boundary passivation," which unifies high corrosion resistance and green manufacturing while ensuring magnetic properties, demonstrating outstanding technological progress and industrialization value. Attached Figure Description

[0023] Figure 1 This is a SEM microstructure image of the sintered permanent magnet in Example 1;

[0024] Figure 2 The image shows the SEM microstructure of the sintered permanent magnet in Comparative Example 1. Detailed Implementation

[0025] First, those skilled in the art should understand that these embodiments are only used to explain the technical principles of the embodiments of this application and are not intended to limit the scope of protection of the embodiments of this application. Those skilled in the art can make adjustments as needed to adapt to specific application scenarios.

[0026] Example 1

[0027] A GaCu co-doped sintered permanent magnet, with the following mass percentages: Ce 30.0%, Nd+Pr 20.0%, Fe 48.7%, B 1.0%, Ga 0.25%, Cu 0.15%, etc. This composition system balances high cerium content, magnetic properties and grain boundary electrochemical stability, providing a chemical basis for the formation of a stable passivation layer.

[0028] A manufacturing process for GaCu co-doped sintered permanent magnets includes the following steps:

[0029] Raw material design and weighing:

[0030] High-purity metals Ce (≥99.5%), Nd (≥99.5%), Pr (≥99.5%), Fe (≥99.9%), Ga (≥99.9%), Cu (≥99.9%), and Fe–B master alloy (B content approximately 18 wt%) were selected as raw materials.

[0031] S1, Vacuum Melting and Homogenization

[0032] Equipment: Medium-frequency vacuum induction melting furnace (vacuum degree ≤ 5×10) -3 Pa).

[0033] Steps: Weigh the raw materials according to the proportion and add them in batches to the melting furnace in a graphite crucible. Heat to 1450–1500 ℃ under the protection of high-purity argon gas to completely melt them into a melt. Repeat the melting process 3 times to ensure the uniformity of chemical composition.

[0034] Pouring and cooling: The molten material is poured into a copper mold and quickly solidifies to form a master alloy ingot.

[0035] S2, Rapid Solidification and Powder Preparation

[0036] Strip casting: After the master alloy ingot is melted, it is rapidly solidified by a strip casting machine at a wheel speed of about 30 m / s to obtain a metal strip with an average thickness of about 40 μm.

[0037] Hydrogenation and air jet milling: The strip was treated in a hydrogen atmosphere (0.15 MPa, 300 ℃) for 3 hours, and then hydrogenated and air jet milled under inert gas protection to obtain anisotropic powder with an average particle size of 3–5 μm.

[0038] Powder protection: The entire powder preparation process is carried out under an argon or nitrogen atmosphere to prevent oxidation.

[0039] S3, Orientation Press Molding

[0040] Orientation equipment: an in-mold magnetic field orientation press with a magnetic field strength of 1.8 T is used.

[0041] Pressing parameters: Under the action of a magnetic field, the orientation is cold-pressed along the main axis, and the pressure is 100 MPa to obtain an orientation compact.

[0042] Demolding and pre-sintering: The compact is pressed under vacuum conditions (approximately 10 minutes). -3 Pre-sintering (Pa) at a temperature of 600–700 ℃ for 1 hour to remove adsorbed gases and increase the density of the green body.

[0043] S4, Vacuum sintering and grain boundary passivation formation

[0044] Sintering stage: The temperature is increased to 1070℃ at 10℃ / min in a vacuum sintering furnace and held for 3 hours to allow the powder to fully diffuse and bond together to form a dense structure.

[0045] Passivation layer formation mechanism: At this stage, Ga and Cu have high diffusion activity and preferentially segregate towards the grain boundaries to form Nd-Ga and Nd-Cu intermetallic compounds with Nd. They then continuously coat the main phase grains at the grain boundaries to construct a high-potential passivation layer.

[0046] Annealing stage: The temperature is then lowered to 550℃ and held for 1.5 hours, followed by furnace cooling to room temperature to promote grain boundary phase stabilization and micro-stress release.

[0047] Post-processing and performance testing

[0048] Machining: After sintering, the sample is machined into a Φ10 mm×3 mm disc without the need for chemical plating or coating treatment.

[0049] Magnetic properties were tested: Br, Hcj and (BH)max were measured using a Riken BHS-40 magnetic property analyzer (Japan); grain boundary structure was observed using SEM / EDS.

[0050] Corrosion performance test: Neutral salt spray test (5% NaCl solution, 35℃, relative humidity ≥95%) was conducted according to national standard GB / T 10125-2021 for 120 hours.

[0051] Test results:

[0052] Br = 1.09 T, Hcj = 890 kA / m, (BH)max = 285 kJ / m 3 After 120 h of salt spray, the weight loss was 0.58 mg / cm², and no obvious corrosion pits were observed on the surface. A continuous Nd–Ga / Cu phase coating layer with a thickness of 20–50 nm was observed at the grain boundaries.

[0053] Technical effects:

[0054] This embodiment achieves corrosion resistance comparable to nickel-plated magnets without plating, while maintaining excellent magnetic properties, thus achieving a balance between grain boundary self-passivation and process simplification.

[0055] Example 2:

[0056] A GaCu co-doped sintered permanent magnet, comprising, by mass percentage: Ce 30.0%, Nd+Pr 20.0%, Fe 48.5%, B 1.0%, Ga 0.20%, and Cu 0.20%.

[0057] A manufacturing process for GaCu co-doped sintered permanent magnets:

[0058] In step S3, the orientation pressing is performed with a magnetic field strength of 1.8 T, a pressing pressure of 100 MPa, a pre-sintering temperature of 1050℃, and a holding time of 2 h.

[0059] In step S4, during vacuum sintering and grain boundary passivation, the annealing stage involves cooling to 550°C, holding for 1 hour, and then furnace cooling to room temperature.

[0060] The rest is the same as in Example 1.

[0061] Test results:

[0062] Br=1.07T, Hcj=860kA / m, (BH)max=275kJ / m 3 The weight loss rate during the salt spray test (120h) was approximately 0.46 mg / cm³. 2 SEM-EDS results showed significant Cu segregation at grain boundaries, forming a continuous Fe–Cu–Nd phase distribution.

[0063] Effect Analysis:

[0064] A higher Cu ratio promotes rapid passivation layer formation, improves initial protection efficiency, and has the lowest weight loss rate, making it suitable for applications with higher corrosion resistance requirements, but with slightly lower coercivity.

[0065] Example 3:

[0066] A GaCu co-doped sintered permanent magnet, comprising, by mass percentage: Ce 30.0%, Nd+Pr 20.0%, Fe 48.4%, B 1.0%, Ga 0.30%, and Cu 0.10%.

[0067] A manufacturing process for GaCu co-doped sintered permanent magnets:

[0068] In step S3, during orientation pressing, the magnetic field strength is 2.0 T.

[0069] During step S4, vacuum sintering and grain boundary passivation are being performed.

[0070] Sintering stage: The temperature is increased to 1080℃ at 10℃ / min in a vacuum sintering furnace and held for 3 hours. The sintering temperature is 1080℃ and held for 3 hours.

[0071] Annealing stage: Then cool to 600℃, hold for 2 hours, and then furnace cool to room temperature;

[0072] The rest is the same as in Example 1.

[0073] Test results:

[0074] Br = 1.10 T, Hcj = 905 kA / m, (BH)max = 287 kJ / m 3 The weight loss rate after 120 hours of salt spray testing was approximately 0.60 mg / cm³. 2 The surface showed only slight discoloration and no localized corrosion. Microscopic analysis revealed a higher proportion of Ga phase at the grain boundaries, further enhanced grain boundary potential, and better interface continuity than the control sample.

[0075] Effect Analysis:

[0076] A high Ga ratio improves grain boundary potential and oxidation resistance, exhibiting better high-temperature stability and making it suitable for humid or high-temperature service conditions.

[0077] Comparative Example 1

[0078] The only difference from the example is that it does not contain Ga and Cu; otherwise, it is the same as Example 1.

[0079] Test results:

[0080] The weight loss rate in the salt spray test reached 3.2 mg / cm³. 2 The presence of obvious pitting and erosion phenomena indicates severe grain boundary corrosion.

[0081] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A manufacturing process for GaCu co-doped sintered permanent magnets, characterized in that, Includes the following steps: S1 Raw Material Smelting: The raw materials weighed according to the formula include, by weight percentage, 25-35 wt% Ce, 15-25 wt% Nd+Pr, 0.9-1.1 wt% B, 0.2-0.3 wt% Ga, 0.1-0.2 wt% Cu, and the balance being Fe. They are heated to 1450-1500℃ under inert gas protection and completely melted, then cooled to form a master alloy ingot. S2 powder preparation: The master alloy ingot is melted and rapidly solidified into alloy sheets. The alloy sheets are then treated in a hydrogen atmosphere at a pressure of 0.1-0.2 MPa and a temperature of 300℃ for 2.5-3.5 hours. Finally, they are air-jet milled under inert gas protection to obtain anisotropic powder with an average particle size of 3-5 μm. S3 compression molding: Anisotropic powder is oriented and cold-pressed into a compact under the action of a magnetic field along the main axis. S4 Vacuum Sintering: The compact is pre-sintered under vacuum conditions at a temperature of 600-700℃ for 0.8-1.2 hours. The temperature is then increased to 1040-1080℃ and held for 2-4 hours. The temperature is then decreased to 500-600℃ and held for 1-2 hours. The GaCu co-doped sintered permanent magnet is obtained after cooling.

2. The manufacturing process of a GaCu co-doped sintered permanent magnet according to claim 1, characterized in that, It comprises, by weight percentage, 30 wt% Ce, 20 wt% Nd+Pr, 1 wt% B, 0.3 wt% Ga, 0.1 wt% Cu and 48.4 wt% Fe.

3. The manufacturing process of a GaCu co-doped sintered permanent magnet according to claim 1, characterized in that, Step S1 specifically involves using a medium-frequency vacuum induction melting furnace to heat the material to 1450-1500℃ under the protection of high-purity argon gas until it is completely melted, and repeating the melting process 3-4 times.

4. The production process of a GaCu co-doped sintered permanent magnet according to claim 1, characterized in that, In step S2, the master alloy ingot is melted and then rapidly solidified into alloy sheets by a strip spinning machine. The speed of the strip spinning machine is 25-30 m / s, and the average thickness of the alloy sheets is 30-50 μm.

5. The production process of a GaCu co-doped sintered permanent magnet according to claim 1, characterized in that, Step S3 pressing and forming specifically involves using an in-mold magnetic field orientation press with a magnetic field strength of 1.5-2.0T. Under the action of the magnetic field, the orientation is cold-pressed along the main axis with a pressure of 80-120MPa to obtain an orientation compact. The pressed blank is pre-sintered under vacuum at a temperature of 600-700℃ for 1 hour.

6. A GaCu co-doped sintered permanent magnet, characterized in that, It is prepared by the production process of a GaCu co-doped sintered permanent magnet as described in any one of claims 1-5.

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