Ultrahigh-coercivity neodymium-iron-boron magnet and preparation method

Abstract

The invention discloses an ultrahigh-coercivity neodymium-iron-boron magnet and a preparation method, the ultrahigh-coercivity neodymium-iron-boron magnet comprises a main phase alloy, a first diffusion alloy and a second diffusion alloy, the main phase alloy comprises 10.0-16.0 wt% of Nd, 0-3.0 wt% of Pr, 0.8-1.5 wt% of B, and the balance Fe; the first diffusion alloy comprises 20-70 wt% of Dy and / or Ho, 10-40 wt% of Nd and / or Pr and the balance of Fe; the second diffusion alloy comprises the following components in percentage by weight: 5-30% of Ga, 1-10% of Al, 0-5% of Cu and the balance of Nd or Fe; the preparation method comprises the following steps: S1, smelting the main phase alloy, the first diffusion alloy and the second diffusion alloy, and respectively preparing alloy powder or organic solvent type slurry; s2, the surface of the blank body is sequentially coated with a first diffusion alloy and a second diffusion alloy; s3, sintering and high-temperature short-time diffusion; and S4, the blank with the thickness treated in the step S3 is heated to 450-650 DEG C under the controlled oxygen partial pressure, heat preservation is conducted for 0.5-12 h, and therefore the ultrahigh coercive force and the grain boundary self-passivation corrosion resistance of the neodymium-iron-boron magnet are improved at the same time.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet material preparation technology, specifically to a method for preparing an ultra-high coercivity neodymium iron boron magnet. Background Technology

[0002] Neodymium iron boron (NdFeB) rare earth permanent magnets are widely used in high-end fields such as new energy vehicle drive motors, wind power generation, and servo motors due to their excellent magnetic properties. As the application environment becomes increasingly demanding, the market is placing higher requirements on magnets: they not only need to have extremely high coercivity (Hcj) to resist high-temperature demagnetization, but also excellent corrosion resistance to ensure long-term operation.

[0003] To improve the coercivity and stability of NdFeB magnets, existing technologies mainly employ the following mainstream process routes, but all have significant shortcomings. The traditional single-alloy smelting process with added heavy rare earth elements involves directly adding large amounts of dysprosium (Dy) and terbium (Tb) during the NdFeB main phase raw material smelting stage. This ensures the elements are uniformly distributed within the main phase grains, thereby enhancing the coercivity by increasing the magnetocrystalline anisotropy field (Ha) of the main phase.

[0004] Existing defects and shortcomings: Extremely low utilization rate of heavy rare earth elements: Heavy rare earth elements entering the core of the main phase grain are considered "ineffective waste." In reality, only when distributed at the grain edges (shell) can the nucleation of antimagnetic domains be most effectively suppressed. Magnetic properties deteriorate; the antiferromagnetic coupling between heavy rare earth atoms and iron atoms leads to a significant decrease in the remanent magnetization (Br) and maximum energy product ((BH)max) of the magnet. High cost; Dy / Tb is expensive, and the fully alloyed addition method results in persistently high raw material costs.

[0005] Traditional grain boundary diffusion (GBD) technology involves fabricating a NdFeB substrate and then attaching heavy rare earth sources to the magnet surface through coating, sputtering, or evaporation. High-temperature heat treatment then allows the heavy rare earth elements to penetrate along the grain boundaries from the outside in, forming a "hard magnetic shell" structure on the grain surface. However, this technology has several drawbacks: size limitation (skin effect), typically limiting diffusion depth (usually effective depth <5mm), resulting in only a slight increase in coercivity in the central region for thick magnets, and uneven overall magnet performance (deteriorated squareness). Furthermore, it suffers from low production efficiency, often requiring prolonged high-temperature processing, leading to high energy consumption and limited production capacity.

[0006] Conventional bi-alloy (or multi-alloy) blending processes employ the mixing and sintering of two alloy powders with different compositions (main phase alloy + auxiliary phase alloy), attempting to optimize grain boundary structure through the auxiliary phase alloy. Nd-rich or Dy-rich alloys are used as auxiliary phases to improve sintering density. However, elemental distribution control is difficult; existing bi-alloy processes struggle to precisely control the "hierarchical diffusion" behavior of different elements during sintering. This often leads to the simultaneous diffusion or mixing of heavy rare earth elements (Dy / Ho) with auxiliary elements (Ga / Al), failing to form an ideal, clear core-shell structure. Furthermore, grain boundary corrosion resistance is lacking; the grain boundary phases formed by traditional processes are mostly Nd-rich phases with extremely low electrochemical potentials and highly reactive chemical properties, making them highly susceptible to intergranular corrosion. Current technologies lack a mechanism for "in-situ" generation of dense oxide passivation films at grain boundaries, causing high-coercivity magnets to easily pulverize and fail under high-temperature and high-humidity environments. Summary of the Invention

[0007] To overcome the shortcomings of the above-mentioned related technologies, this application provides an ultra-high coercivity NdFeB magnet and its preparation method, thereby simultaneously improving the ultra-high coercivity and grain boundary self-passivation corrosion resistance of the NdFeB magnet.

[0008] The technical solution adopted by this invention to solve the technical problem is: an ultra-high coercivity neodymium iron boron magnet, comprising a main phase alloy, a first diffusion alloy, and a second diffusion alloy. The main phase alloy consists of 10.0-16.0 wt% Nd, 0-3.0 wt% Pr, 0.8-1.5 wt% B, with the balance being Fe; The first diffusion alloy consists of 20-70 wt% Dy and / or Ho, 10-40 wt% Nd and / or Pr, with the balance being Fe; The second diffusion alloy consists of 5-30 wt% Ga, 1-10 wt% Al, 0-5 wt% Cu, with the balance being Nd or Fe. The total mass of the first diffusion alloy and the second diffusion alloy is 0.1-5 wt% of the mass of the main phase alloy, and the mass ratio of the first diffusion alloy to the second diffusion alloy is 2:1.

[0009] Preferably, the main phase alloy consists of 12.0-14.0 wt% Nd, 0-3.0 wt% Pr, 0.8-1.5 wt% B, 0-3 wt% Co, 0-1 wt% Cu, and the balance being Fe; The first diffusion alloy is 30-60 wt% Dy and / or Ho, 10-40 wt% Nd and / or Pr, with the balance being Fe; The second diffusion alloy consists of 8-20 wt% Ga, 2-6 wt% Al, 0-5 wt% Cu, with the balance being Nd or Fe.

[0010] A method for preparing an ultra-high coercivity neodymium iron boron magnet includes the following steps: S1 is smelted into a main phase alloy, crushed into powder, and pressed into a blank. The first diffusion alloy and the second diffusion alloy are smelted separately and made into alloy particles or organic solvent slurry respectively. S2 is coated with a first diffusion alloy and a second diffusion alloy in sequence on the surface of the blank; S3 The embryo after step S2 is heated to 1040-1080℃ in an inert atmosphere and held for 15-120 min, then held at 900-1050℃ for 0.5-6 h, then cooled at a medium speed to 500-600℃, and then slowly cooled to room temperature. S4 involves heating the preform, which has undergone the thickness treatment in step S3, to 450-650℃ under controlled oxygen partial pressure or an inert atmosphere containing ppm-level O2, holding it at that temperature for 0.5-12 hours, and then slowly cooling it to room temperature, wherein the oxygen partial pressure is 10. -6 -10 -3 Pa.

[0011] Preferably, in step S2, the thickness of the first diffusion alloy is 20 μm and the thickness of the second diffusion alloy is 10 μm.

[0012] Preferably, in step S2, the holding time for heating to 1040-1080℃ is 30-60 min, and at 900-1050℃, the holding time is 0.5-3 h, the medium-speed cooling is 10-100℃ / min, and the slow cooling in steps S3 and S4 is <50℃ / h.

[0013] Preferably, the step S1 of melting the main phase alloy and then pulverizing it into powder specifically involves: melting the main phase alloy in an induction furnace or vacuum arc furnace, melting and alloying high-purity metals according to a ratio, preferably at a melting temperature of 1200-1400℃, holding for 10-60 minutes, then clarifying and casting into alloy ingots, and mechanically pulverizing or ball milling under nitrogen / argon protection until the powder particle size d50 is 3-10μm.

[0014] Preferably, the pressing process in step S1 specifically involves isotropic pressing or magnetic field-oriented pressing under inert atmosphere or dry conditions, wherein the pressure of isotropic pressing is 100-400 MPa, and the pressure of magnetic field-oriented pressing is 50-200 MPa.

[0015] Compared with related technologies, the present invention has the following advantages: The first diffusion alloy, rich in Dy / Ho (or Dy+Ho), is used to form a hard magnetic shell. The second diffusion alloy, rich in Ga / Al (and may contain small amounts of Cu and Pr), is used to generate a dense Nd(Ga,Al)Ox passivation film in situ at low temperatures and optimize grain boundary wettability. Through the coordinated control of particle size, pressing density, sintering curve, and staged heat treatment process parameters, preferential surface enrichment of heavy rare earth elements and directional reconstruction at grain boundaries are achieved. Staged heat treatment under inert atmosphere or vacuum protection (sintering at 1040-1080℃, short-term high-temperature diffusion at 900-1050℃ for 0.5-6h) avoids excessive oxidation of the surface / grain boundaries. Simultaneously, at low temperatures (450-650℃), controlled oxidation or low oxygen partial pressure treatment promotes the formation of a dense passivation layer. Attached Figure Description

[0016] Figure 1 This is the SEM image of Example 1. Detailed Implementation

[0017] First, those skilled in the art should understand that these embodiments are merely 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.

[0018] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0019] Example 1 Raw material composition Main phase alloy formulation (rare earth-iron-boron type): Nd 12.5wt%, Pr 1.0wt%, B 1.0wt%, Fe balance (Fe~84.5wt%), Co 1.0wt% (optional) to adjust thermal stability and wettability.

[0020] The first diffusion alloy formulation consists of 45wt% Dy, 30wt% Nd, and 25wt% Fe, and is used to preferentially enrich the surface layer of grains in a high-temperature, short-time process to form a hard magnetic shell.

[0021] The second diffusion alloy formulation consists of 12 wt% Ga, 4 wt% Al, 40 wt% Nd, and 44 wt% Fe, used to form Nd(Ga,Al)O at low temperatures. x / Al2O3-rich oxide passivation layer improves grain boundary electrochemical stability.

[0022] The total addition amount of the first and second diffusion alloys (after coating / printing) is 0.8 wt% relative to the mass ratio of the main alloy powder.

[0023] Preparation process Step A: Preparation of the matrix alloy Melting and alloying: High-purity (≥99.5%) metal is melted and alloyed according to a specified ratio in an induction furnace or vacuum arc furnace. Inclined casting or vacuum induction is used. The preferred melting temperature is 1200-1400℃, held for 10-60 minutes (depending on the furnace type and alloy quantity). The metal is then clarified and cast into ingots. Equipment: Induction melting furnace, vacuum arc melting furnace.

[0024] Rapid solidification / ingot crushing: To refine the grains and facilitate subsequent crushing, gas-water atomization or rapid solidification (single roller or belt quenching) can be used to prepare strip / powder blanks; (or directly cast ingots followed by heat treatment to improve phase composition).

[0025] Step B: Preparation of main phase powder Crushing method: Mechanical crushing or ball milling (ball-to-material ratio 5:1-15:1) of alloy ingots or strips under nitrogen / argon protection, with a target powder particle size d50≈6μm. The grinding time and energy should avoid introducing excessive oxides (can be carried out under an inert atmosphere / oil mist lubrication).

[0026] Powder degreasing and sieving: After ball milling, the powder is sieved (the mesh size depends on the target particle size) and then degassed under low temperature vacuum in an inert atmosphere to remove surface-adsorbed gases.

[0027] Step C: Preparation of auxiliary phase diffusion alloy (or diffusion paste) The first and second diffusion alloys were respectively used to prepare organic solvent-based slurries (for screen printing). Preparation equipment included: a vacuum melting furnace, a low-melting-point alloy casting mold, a ball mill (for preparing micron-sized particles), and stirring and dispersing equipment (for preparing the slurry).

[0028] Step D: Pressing and Shaping The main powder and trace amount of lubricant are mixed evenly, vacuum dried and isotropically pressed into a blank at a pressure of 250 MPa. The blank density is approximately 65%TD (theoretical density).

[0029] Two layers of diffusion paste are screen-printed on the surface of the preform: the first layer (outer layer) is the first diffusion alloy paste with a thickness of ≈20μm; the second layer is the second diffusion alloy paste inside (close to the preform) with a thickness of ≈10μm.

[0030] Step E: Sintering and First-Stage High-Temperature Short-Time Diffusion (Shell Construction) Vacuum or inert atmosphere sintering: pure Ar protection, pre-vacuumed to 10 °C -1 -10 -3 Pa is followed by the protective gas Ar.

[0031] Sintering temperature and holding time: Heat to 1050℃ and hold for 45 minutes; the temperature can be adjusted within this range for different materials and sizes to obtain a dense structure (relative density ≥7.3 g / cm³). 3 Target).

[0032] High-temperature short-time diffusion (completed in the sintering and subsequent high-temperature treatment stages): holding at 1000℃ for 1 hour allows heavy rare earth elements (Dy / Ho) to accumulate along grain boundaries and grain surfaces, forming a hard magnetic shell; controlling the diffusion time and temperature restricts the entry of heavy rare earth elements into the main phase core, thereby improving utilization and reducing Br loss.

[0033] Cooling rate: After a short period of high temperature, it is recommended to use a medium cooling rate (50℃ / min) to 500-600℃ to lock in the shell distribution, and then slowly cool to room temperature.

[0034] Step F: Second-stage low-temperature grain boundary reconstruction and in-situ self-passivation (passivation layer formation) Low-temperature tempering / grain boundary reconstruction treatment: This will be carried out under a protective atmosphere or controlled oxygen partial pressure, at a temperature range of 450-650℃, for a holding time of 4 hours. During this stage, a trace amount of O2 (approximately 500 ppm) is introduced into Ar, which enriches elements such as Ga / Al at the grain boundaries and forms a dense Nd(Ga,Al)O with a small amount of residual oxygen. x A passivation layer is formed to improve grain boundary corrosion resistance and wettability. After the low-temperature stage, the material is slowly cooled to room temperature (<50℃ / h) to facilitate densification of the passivation layer and release of internal stress.

[0035] Step G: Surface Treatment To remove residual diffusion sources and impurities from the surface, mechanical peeling, chemical etching (short-term immersion in dilute acid), or low-temperature vacuum heat treatment can be used.

[0036] Surface coatings to improve corrosion resistance: such as epoxy / polymer coatings or conventional protective layers such as nickel / zinc plating.

[0037] Performance testing Detection methods Magnetic properties: measured by VSM or hysteresis loop measuring instrument (room temperature, maximum field 2T).

[0038] Microstructure: SEM (backscattered image + EDS mapping) and cross-sectional SEM are used for shell / grain boundary observation; TEM is used for nanoscale shell analysis when necessary.

[0039] Elemental depth distribution: GD-OES or EPMA (section scan) to obtain depth concentration curves of Dy, Ga, and Al.

[0040] Surface chemical states: XPS analysis of the oxidation states of Nd, Ga, and Al.

[0041] Corrosion resistance: Salt spray test (according to ASTM B117) and 3.5% NaCl electrochemical test (EIS, Tafel).

[0042] Test results Coercivity Hcj (room temperature): 1650 kA / m.

[0043] Remanence Br: 1.22T.

[0044] Maximum magnetic energy product (BH)max: 41 MGOe.

[0045] Shell thickness (SEM / TEM): 0.5-1.2 μm (Dy enriched); Grain boundary passivation layer thickness (XPS / TEM): 10-50 nm (as Nd(Ga,Al)O) x (Mainly).

[0046] Mass loss after 48 h of salt spray: 0.9% (compared to approximately 3.8% for the control sample); EIS showed a significant increase in polarization resistance of the passivated sample.

[0047] Example 2 (Ga content is high) Raw material formula Same as Example 1.

[0048] First diffusion alloy: Dy 40 wt%, Nd 30 wt%, Fe 30 wt%.

[0049] Second diffusion alloy: Ga 18 wt%, Al 4 wt%, Nd 38 wt%, Fe 40 wt%.

[0050] The total addition amount of the first and second diffusion alloys: 1.0 wt% (slightly higher than in Example 1). Preparation process. The heat preservation time in step F is extended to 6 hours, and the rest is the same as in Example 1.

[0051] Test results Coercivity Hcj (room temperature): 1500 kA / m.

[0052] Remanence Br: 1.20T.

[0053] Maximum magnetic energy product (BH)max: 40 MGOe.

[0054] Shell thickness (SEM / TEM): 0.5-1.2 μm (Dy enriched); Grain boundary passivation layer thickness (XPS / TEM): 10-50 nm (as Nd(Ga,Al)O) x (Mainly).

[0055] Mass loss after 48 h of salt spray: 0.6% (better than Example 1, indicating that increased Ga content helps to further enhance corrosion resistance); EIS showed that the polarization resistance of the passivated sample was significantly improved.

[0056] Increasing the Ga content can further improve corrosion resistance, but this may come at the cost of slightly sacrificing the Br / magnetic energy product.

[0057] Example 3 Raw material composition Total addition of the first and second diffusion alloys: 1.0 wt%. The rest is the same as in Example 1. Preparation process When pressing the embryo into a blank in step D, the blank is a block with a thickness of 20 mm; In step E, the sintering temperature is 1050 °C and the time is 60 min; the high-temperature diffusion temperature is 1000 °C and the time is 1.5 h; and the low-temperature passivation temperature is 550 °C and the time is 6 h. The rest is the same as in Example 1.

[0058] Result detection Detection methods Cross-sectional sampling: Cross-sectional samples are cut every 2 mm along the thickness direction for EPMA or GD-OES depth analysis and local magnetic property measurement.

[0059] result Hcj measured along the thickness: center ~1480 kA / m, surface ~1620 kA / m, with an overall gradient of less than 10-12% (indicating that the hierarchical diffusion strategy can still maintain good uniformity on large-sized parts, which is better than the strong surface attenuation phenomenon of traditional GBD on thick parts).

[0060] XRD / SEM observation: The central region still maintains the integrity of the Nd2Fe14B main phase, and a Dy-enriched shell appears on the surface. EPMA mapping shows that Dy enrichment gradually decreases from the surface inwards but is effective in the range of 1.0-2.0 μm.

[0061] Salt spray test: The overall mass loss was significantly lower than that of Comparative Example 2 (the 20 mm thick part in the control showed obvious rapid pulverization after 7 days of salt spray, while this example only showed slight surface changes after 7 days).

[0062] Comparative Example 1 (The comparative example is conventional Dy diffusion, but without Ga / Al passivation components) Raw material composition The same applies to the main phase alloy.

[0063] Diffusion alloy: Dy 50 wt%, Nd 25 wt%, Fe 25 wt%, addition amount 1.0 wt%.

[0064] Preparation process Step F is omitted; the rest is the same as in Example 1. Result detection Hcj: 920 kA / m (compared to the initial value of the matrix, there is an increase after diffusion but it is much lower than that of the embodiment of the present invention).

[0065] Br: 1.25 T (maintaining relatively high Br).

[0066] (BH)max: 44 MGOe.

[0067] The mass loss after 48 hours of salt spray was 3.8%, and intergranular corrosion and pulverization of grain boundaries were observed (SEM observation).

[0068] Note: Although Dy diffusion improves local coercivity, the lack of grain boundary passivation leads to poor corrosion resistance and low utilization of heavy rare earth elements (requiring higher Dy addition to obtain higher Hcj), which is precisely the problem that this invention aims to solve.

[0069] 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 neodymium iron boron magnet with ultra-high coercivity, characterized in that, Including the main phase alloy, the first diffusion alloy, and the second diffusion alloy. The main phase alloy consists of 10.0-16.0 wt% Nd, 0-3.0 wt% Pr, 0.8-1.5 wt% B, with the balance being Fe; The first diffusion alloy consists of 20-70 wt% Dy and / or Ho, 10-40 wt% Nd and / or Pr, with the balance being Fe; The second diffusion alloy consists of 5-30 wt% Ga, 1-10 wt% Al, 0-5 wt% Cu, with the balance being Nd or Fe. The total mass of the first diffusion alloy and the second diffusion alloy is 0.1-5 wt% of the mass of the main phase alloy, and the mass ratio of the first diffusion alloy to the second diffusion alloy is 2:

1.

2. The ultra-high coercivity neodymium iron boron magnet according to claim 1, characterized in that, The main phase alloy consists of 12.0-14.0 wt% Nd, 0-3.0 wt% Pr, 0.8-1.5 wt% B, 0-3 wt% Co, 0-1 wt% Cu, with the balance being Fe; The first diffusion alloy is 30-60 wt% Dy and / or Ho, 10-40 wt% Nd and / or Pr, with the balance being Fe; The second diffusion alloy consists of 8-20 wt% Ga, 2-6 wt% Al, 0-5 wt% Cu, with the balance being Nd or Fe.

3. The method for preparing an ultra-high coercivity neodymium iron boron magnet according to any one of claims 1-2, characterized in that, Includes the following steps: S1 is smelted into a main phase alloy, crushed into powder, and pressed into a blank. The first diffusion alloy and the second diffusion alloy are smelted separately and made into alloy particles or organic solvent slurry respectively. S2 is coated with a first diffusion alloy and a second diffusion alloy in sequence on the surface of the blank; S3 Sintering: The preform treated in step S2 is heated to 1040-1080℃ in an inert atmosphere and held for 15-120 min. High-temperature short-time diffusion: It is held at 900-1050℃ for 0.5-6 h, then cooled at a medium speed to 500-600℃, and then slowly cooled to room temperature. S4 involves heating the preform, which has undergone the thickness treatment in step S3, to 450-650℃ under controlled oxygen partial pressure or an inert atmosphere containing ppm-level O2, holding it at that temperature for 0.5-12 hours, and then slowly cooling it to room temperature, wherein the oxygen partial pressure is 10. -6 -10 -3 Pa.

4. The method for preparing an ultra-high coercivity NdFeB magnet according to claim 3, characterized in that: In step S2, the thickness of the first diffusion alloy is 20 μm and the thickness of the second diffusion alloy is 10 μm.

5. The method for preparing an ultra-high coercivity NdFeB magnet according to claim 3, characterized in that: In step S2, the holding time for heating to 1040-1080℃ is 30-60 min, and the holding time for 900-1050℃ is 0.5-3 h. The medium-speed cooling is 10-100℃ / min, and the slow cooling in steps S3 and S4 is <50℃ / h.

6. The method for preparing an ultra-high coercivity NdFeB magnet according to claim 3, characterized in that, The specific steps of melting the main phase alloy and then pulverizing it into powder in step S1 are as follows: melting the main phase alloy in an induction furnace or vacuum arc furnace, melting and alloying high-purity metals according to the proportion, preferably at a melting temperature of 1200-1400℃, holding for 10-60 minutes, then clarifying and casting into alloy ingots, and mechanically pulverizing or ball milling under nitrogen / argon protection until the powder particle size d50 is 3-10μm.

7. The method for preparing an ultra-high coercivity NdFeB magnet according to claim 3, characterized in that, In step S1, the pressing process to form the preform involves isotropic pressing or magnetic field-oriented pressing under inert atmosphere or dry conditions, wherein the pressure of isotropic pressing is 100-400 MPa, and the pressure of magnetic field-oriented pressing is 50-200 MPa.

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