Neodymium-iron-boron magnet without heavy rare earth gradient structure and low-temperature preparation process thereof

Abstract

The invention relates to the technical field of neodymium-iron-boron magnetic materials, in particular to a neodymium-iron-boron magnet without a heavy rare earth gradient structure and a low-temperature preparation process of the neodymium-iron-boron magnet. According to the technical scheme, the neodymium-iron-boron magnet without the heavy rare earth gradient structure comprises the steps that firstly, heavy-rare-earth-free neodymium-iron-boron matrix powder is prepared; step 2, preparing the matrix powder into a prefabricated body with a gradient pore structure by adopting a laser 3D printing process; step 3, placing the preform and a grain boundary diffusion source in a totally closed inert atmosphere furnace; and step 4, after cooling, cutting the sintered body by adopting low-temperature ultrahigh-pressure water jet, the innovative low-temperature integrated process combines the traditional two-step method of'high-temperature sintering and high-temperature diffusion ', the total energy consumption is reduced by more than 35%, and the total treatment time is shortened from more than 12 hours to less than 5 hours. The low-temperature environment effectively inhibits growth of main phase grains, the average grain size is controlled to be 3 microns or below, and magnetic energy product loss caused by grain coarsening is avoided.

Description

Technical Field

[0001] This invention relates to the field of neodymium iron boron magnetic materials technology, and in particular to a heavy rare earth-free gradient structure neodymium iron boron magnet and its low-temperature preparation process. Background Technology

[0002] Neodymium iron boron magnets are the third generation of rare earth permanent magnet materials, composed of neodymium, iron and boron. They have high magnetic energy product and coercivity, are resistant to high temperatures, have a long lifespan and strong magnetism, and are the most commonly used rare earth magnets today.

[0003] The preparation of traditional NdFeB magnets mainly relies on the addition of heavy rare earth elements (such as dysprosium (Dy) and terbium (Tb) to improve coercivity. It usually adopts high-temperature sintering and two-step grain boundary diffusion process to form a homogeneous structure. It is necessary to add 2-5 wt.% of heavy rare earth elements, which makes the raw material cost account for more than 40%. Moreover, heavy rare earth resources are scarce and the supply is unstable. The sintering temperature is as high as 1050-1100℃, which consumes a lot of energy and is prone to grain coarsening (particle size > 5μm), resulting in a decrease in magnetic energy product of ≥8%. The two-step diffusion process takes more than 10 hours, resulting in low overall preparation efficiency. The magnet is a homogeneous sintered body, which cannot achieve the gradient performance matching of "high coercivity on the surface" and "high magnetic energy product in the core", making it difficult to adapt to the complex working conditions of wind power, new energy vehicle motors and so on. At the same time, the oxygen content is not well controlled (>0.5%) during sintering and transportation, which makes the magnet easy to oxidize and corrode. Afterwards, it is necessary to rely on a thick coating (such as nickel plating) for protection, but it is still difficult to meet the requirements of long-term corrosion resistance. Summary of the Invention

[0004] Overcoming the dependence on heavy rare earth elements has driven up costs. The high-temperature process used to maintain coercivity has led to increased energy consumption and grain coarsening, which in turn affects the overall performance and structural adaptability of the magnet. Furthermore, the lack of homogeneous structure design and oxidation control further limits its reliable application in high-temperature and highly corrosive environments.

[0005] The technical solution of this invention is: a neodymium iron boron magnet with a weightless rare earth gradient structure, comprising the following steps: Step 1: Prepare heavy rare earth-free NdFeB matrix powder with the following atomic percentage composition: (Pr 17-19%, Ho 0.8-1.2%, Co 1.8-2.2%, Al 0.4-0.6%, Fe 81-83%, B 5-7%), with the balance being unavoidable impurities; Step two: The matrix powder is prepared into a preform with a gradient pore structure using laser 3D printing technology, wherein: The surface porosity is 20%–30%, and the thickness is 0.5–1 mm; The transition layer has a porosity of 10%–20% and a thickness of 1–1.5 mm; The core layer has a porosity of 3%–8% and a thickness of 2–3 mm; Step 3: Place the preform and the grain boundary diffusion source together in a fully enclosed inert atmosphere furnace, control the oxygen content in the furnace to ≤0.1%, and hold at 640-660℃ for 2.8-3.2h to carry out a low-temperature grain boundary diffusion and sintering. Step four: After cooling, the sintered body is cut using a low-temperature ultra-high pressure water jet. The water jet pressure is 300-400 MPa, and the cutting medium temperature is controlled between -6℃ and -4℃.

[0006] Preferably, the laser 3D printing process parameters in step two are: Surface layer: Laser power 55-65W, scanning speed 45-55mm / s; Transition layer: laser power 45-55W, scanning speed 70-80mm / s; Core layer: Laser power 35-45W, scanning speed 95-105mm / s.

[0007] Preferably, the grain boundary diffusion source is a mixture of Nd70Cu30 alloy powder and Nd60Al10Ni10Cu20 alloy powder in a mass ratio of 2.8:1 to 3.2:1, and the particle size of the alloy powder is 5 to 10 μm.

[0008] Preferably, the Nd60Al10Ni10Cu20 alloy powder in the grain boundary diffusion source can be replaced with Nd65Cu25Zr10 alloy powder, while the mixing mass ratio of Nd65Cu25Zr10 alloy powder to Nd70Cu30 alloy powder remains unchanged.

[0009] Preferably, the fully enclosed inert atmosphere furnace adopts a magnetic fluid sealing structure, and the atmosphere is high-purity argon with a purity ≥99.999%.

[0010] A neodymium iron boron magnet was prepared according to a low-temperature preparation process for a neodymium iron boron magnet with a weightless rare earth gradient structure. The magnet includes a surface layer with a coercivity of 18-20 kOe. The transition layer has a coercivity of 15–17 kOe. The core layer has a magnetic energy product of 38–42 MGOe.

[0011] Preferably, the coercivity decreases by ≤10% at 180℃.

[0012] Preferably, after 1000 hours of neutral salt spray testing, the weight loss per unit area is ≤0.5 mg / cm².

[0013] The beneficial effects of this invention are: By adopting a multi-element synergistic formulation of Pr-Ho-Co-Al, the expensive heavy rare earth element Dy / Tb is completely eliminated. Raw material costs are reduced by 20%-30%, fundamentally solving the dependence on strategically scarce resources and ensuring supply chain security and cost controllability. Under conditions without heavy rare earth elements, it achieves excellent comprehensive magnetic properties with room temperature coercivity ≥18 kOe and magnetic energy product ≥38 MGOe. In particular, it has outstanding high-temperature stability, with coercivity attenuation ≤10% at 180℃, which is far superior to traditional magnets containing heavy rare earth elements and meets the requirements of high-temperature application scenarios. The innovative low-temperature integrated process combines the traditional two-step method of "high-temperature sintering and high-temperature diffusion," reducing total energy consumption by more than 35% and shortening the total processing time from over 12 hours to 5 hours. The low-temperature environment effectively inhibits the growth of the main phase grains, controlling the average grain size to below 3μm, thus avoiding magnetic energy product loss caused by grain coarsening. The highly porous surface-transition layer-dense core gradient prefabrication, precisely constructed using laser 3D printing technology, provides directional channels for subsequent grain boundary diffusion. This allows Nd-rich phase diffusion sources to preferentially and selectively accumulate at grain boundaries, especially in the surface and transition layers, thus naturally creating an ideal performance gradient within a single magnet: high coercivity (anti-demagnetization) on the surface and high magnetic energy product (high magnetic moment) in the core. This integrated structure eliminates the need for subsequent mechanical composites or additional protective structures and can be directly adapted to components such as motor rotors with complex magnetic field distribution requirements. Detailed Implementation

[0014] The present invention will be further described below with reference to embodiments.

[0015] Example 1: Fabrication of a general-purpose gradient magnet using standard parameters Includes the following steps: Step 1: Preparation of matrix powder and diffusion source The raw materials were weighed according to atomic percentage: Pr 18.0%, Ho 1.0%, Co 2.0%, Al 0.5%, Fe 82.0%, B 6.5%. The master alloy was prepared by vacuum induction melting, and then the matrix powder with an average particle size of 3.5 μm was obtained by hydrogen crushing and air jet milling. The grain boundary diffusion source was prepared by mixing Nd70Cu30 alloy powder and Nd60Al10Ni10Cu20 alloy powder at a mass ratio of 3.0:1, and both had a particle size of 6 μm. Step 2: Laser 3D printing to prepare gradient pore preforms The matrix powder was placed in a sealed printing chamber filled with high-purity argon gas (O2 content <50ppm), and printing was performed using a fiber laser selective melting system. The printing parameters were set layer by layer as follows: Surface layer (set thickness 0.8mm): Laser power 60W, scanning speed 50mm / s, scanning spacing 0.08mm, powder layer thickness 0.03mm. This parameter combination results in high energy density, incomplete powder melting, and the formation of a porous structure with a porosity of approximately 25%.

[0016] Transition layer (set thickness 1.2mm): laser power 50W, scanning speed 75mm / s, scanning spacing 0.08mm, powder layer thickness 0.03mm, moderate energy density, forming a transition structure with a porosity of about 15%.

[0017] Core layer (set thickness 2.5mm): laser power 40W, scanning speed 100mm / s, scanning spacing 0.08mm, powder layer thickness 0.03mm, low energy density, powder fully melted and densed, forming a dense core with a porosity of about 5%.

[0018] After printing, a cylindrical preform (φ10mm×4.5mm) with a clearly defined three-layer gradient pore structure is obtained. Step 3: Integrated processing of low-temperature grain boundary diffusion and sintering The preform is embedded in the grain boundary diffusion source powder prepared in step one, and then loaded into a fully enclosed tubular sintering furnace. The furnace body adopts a combination of magnetic fluid rotary sealing (the sealing fluid is ester-based magnetic fluid with a saturation magnetization of about 400 Gs) and metal "C" ring static sealing to ensure dynamic and static sealing. First, the furnace chamber is evacuated to below 5 Pa, and then high-purity argon gas with a purity ≥99.999% is introduced to a slight positive pressure (0.05 MPa). The oxygen content in the furnace is monitored in real time using a Siemens ULTRAMAT23 micro oxygen analyzer (measurement range 0-1000 ppm) to ensure that the oxygen content in the furnace is stable at ≤0.1% (i.e., 100 ppm). The temperature is increased to 650℃ at a rate of 3℃ / min, and then precisely held at this temperature for 3.0 hours. After the heat preservation is completed, the temperature is programmed to be cooled to 200°C at a rate of 10°C / min, and then cooled to room temperature with the furnace. During this process, the diffusion source forms a liquid phase, which preferentially penetrates into the surface and transition layers along the gradient pore network of the preform, so as to achieve the simultaneous completion of grain boundary diffusion and compaction of the preform.

[0019] Step 4: Post-processing and performance testing Sintered magnets were cut using a low-temperature, ultra-high-pressure waterjet cutting machine. The cutting conditions were: waterjet pressure 350 MPa, 120-mesh garnet sand abrasive, and the cutting medium (a mixture of water and ethylene glycol) temperature controlled at -5°C. Performance tests were then performed on the cut magnet samples. Magnetic properties: Measured using a pulse magnetometer, the surface coercivity (Hcj) is 19.5 kOe, and the core magnetic energy product ((BH)max) is 40.5 MGOe.

[0020] High temperature performance: After being kept at 180℃ for 1 hour, the coercivity decay rate was 8.5%.

[0021] Corrosion resistance: After 1000 hours of neutral salt spray testing according to GB / T10125, no red rust was observed on the sample surface, and the weight loss per unit area was 0.35 mg / cm².

[0022] Microstructure: Scanning electron microscopy revealed that the average grain size of the magnet was 2.8 μm, and the surface grain boundary phase was continuous and significantly thicker than that of the core region.

[0023] Example 2; Preparation example for optimizing high-temperature performance.

[0024] Step 1: Preparation of matrix powder and diffusion source The atomic percentage of the matrix powder was adjusted to: Pr 17.5%, Ho 1.2%, Co 2.2%, Al 0.6%, Fe 81.5%, B 6.0%. The Ho content was taken as the upper limit, and the Co and Al contents were slightly increased to further enhance the thermal stability of the grain boundary phase. The diffusion source ratio was the same as in Example 1.

[0025] Step 2: Laser 3D printing to prepare gradient pore preforms The printing strategy is similar to that of Example 1, but the surface thickness and porosity are slightly increased to improve the surface diffusion effect: surface thickness 1.0 mm, laser power 62 W, ​​scanning speed 48 mm / s, target porosity 28%; the parameters of the transition layer and core layer are the same as those of Example 1.

[0026] Step 3: Integrated processing of low-temperature grain boundary diffusion and sintering The process is the same as in Example 1, but the holding temperature is slightly increased to 660℃ and the holding time is extended to 3.2 hours, in order to improve the high-temperature coercivity through more sufficient grain boundary diffusion. The oxygen content in the furnace is controlled using a SERVOTOUGHOxyber5100 micro oxygen analyzer.

[0027] Step 4: Post-processing and performance testing The cutting parameters are the same as in Example 1, and the performance test results are as follows: Magnetic properties: surface coercivity is 20.1 kOe, and core magnetic energy product is 39.8 MGOe.

[0028] High-temperature performance: The coercivity decay rate at 180℃ was significantly reduced to 7.2%, which reflects the improvement in high-temperature stability brought about by the optimization of composition and process.

[0029] Corrosion resistance: Weight loss of 0.28 mg / cm² after 1000 hours of neutral salt spray test.

[0030] Example 3: Preparation example using an alternative diffusion source and adjusted structure.

[0031] Step 1: Preparation of matrix powder and diffusion source The matrix powder used the same standard formulation as in Example 1. The Nd60Al10Ni10Cu20 alloy powder in the grain boundary diffusion source was replaced with Nd65Cu25Zr10 alloy powder, which was still mixed with Nd70Cu30 alloy powder at a mass ratio of 3.0:1. The introduction of Zr element was intended to further refine the grain boundary phase.

[0032] Step 2: Laser 3D printing to prepare gradient pore preforms To accommodate potentially different motor design requirements, the gradient structure was adjusted: the overall magnet design is thinner. The surface layer thickness is 0.5mm (laser power 58W, scanning speed 52mm / s), the transition layer thickness is 1.0mm, and the core layer thickness is 2.0mm. The core layer uses a higher scanning speed (110mm / s) and power (42W) to ensure high density is maintained at a thinner thickness.

[0033] Step 3: Integrated processing of low-temperature grain boundary diffusion and sintering The process is the same as in Example 1, with the holding temperature set at 640°C and the holding time at 2.8 hours. Using a different diffusion source may result in slightly different eutectic characteristics; a slightly lower processing temperature helps control the diffusion process.

[0034] Step 4: Post-processing and performance testing The cutting parameters are the same as in Example 1. Performance test results show that: Magnetic properties: surface coercivity is 18.8 kOe, and core energy product is 41.2 MGOe.

[0035] High temperature performance: The coercivity decay rate at 180℃ is 9.0%.

[0036] Corrosion resistance: Weight loss of 0.40 mg / cm² after 1000 hours of neutral salt spray testing. Microscopic analysis showed a uniform distribution of grain boundary phases, confirming the effectiveness of the alternative diffusion source.

[0037] The above working principles can be summarized as follows: Through the four-dimensional synergy of component design, structural design, process integration and environmental control, high-performance heavy rare earth-free gradient magnets are prepared at low temperature and with high efficiency. The core logic is: First, through the Pr-Ho-Co-Al multi-element synergistic formulation, grain boundary magnetic enhancement and domain wall pinning are achieved under heavy rare earth-free conditions. Secondly, laser 3D printing is used to precisely manufacture preforms with decreasing porosity from the surface to the interior. This gradient pore structure serves as a "guide channel" for subsequent diffusion. Then, a low-temperature integrated process is adopted, at a temperature far below the traditional sintering temperature (640-660℃), to melt a specific ratio of grain boundary diffusion source and preferentially penetrate into the surface and transition layer grain boundaries along the pore channels, achieving selective grain boundary modification and densification at the same time. This low-temperature environment effectively inhibits the coarsening of the main phase grains. Finally, the ultra-low oxygen (≤0.1%) inert atmosphere protection throughout the process fundamentally prevents the oxidation of active components and improves the intrinsic properties and corrosion resistance of the magnet.

[0038] The present invention has demonstrated the feasibility and superiority of its technical solution through the above specific embodiments. Example 1 shows that the magnet prepared under standard parameters has fully met or even exceeded the performance of traditional heavy rare earth magnets in terms of comprehensive performance (coercivity 19.5 kOe, magnetic energy product 40.5 MGOe), high temperature stability (attenuation 8.5%) and corrosion resistance (weight loss 0.35 mg / cm²). Example 2, by increasing the Ho content and optimizing the process (holding at 660℃ for 3.2h), further reduced the high-temperature coercivity decay rate to 7.2%, making it particularly suitable for application scenarios with extremely stringent requirements for high-temperature working stability. Example 3 successfully verified the feasibility of using Nd65Cu25Zr10 as an alternative diffusion source, and by adjusting the gradient structure parameters, it proved that the process of the present invention has good flexibility and adaptability, and can "tailor" the magnet performance by adjusting the parameters to meet different application requirements.

[0039] The three embodiments collectively demonstrate the effectiveness of the core inventive points of this invention: the specific heavy rare earth-free composition system is the foundation of performance; the gradient porosity constructed by laser 3D printing is the physical guarantee and key means for forming performance gradients; the low-temperature integrated diffusion sintering process is the core step for achieving high performance, low energy consumption, and fine grain refinement; and the ultra-low oxygen control throughout the process (supplemented by specific sensors such as ULTRAMAT 23 and sealing materials such as magnetorheological fluid) is a necessary condition for ensuring process reproducibility and high reliability of the magnet. These links are interconnected and work synergistically to jointly solve a series of technical problems pointed out in the background art, such as resource dependence, high energy consumption, single performance, and easy oxidation and corrosion.

[0040] Example Performance Comparison Data Table The embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A low-temperature fabrication process for a heavy rare-earth gradient structure NdFeB magnet, characterized in that, Includes the following steps: Step 1: Prepare heavy rare earth-free NdFeB matrix powder with the following atomic percentage composition: (Pr 17-19%, Ho 0.8-1.2%, Co 1.8-2.2%, Al 0.4-0.6%, Fe 81-83%, B 5-7%), with the balance being unavoidable impurities; Step 2: The matrix powder is prepared into a preform with a gradient pore structure using laser 3D printing technology, wherein: the surface layer has a porosity of 20% to 30% and a thickness of 0.5 to 1 mm; the transition layer has a porosity of 10% to 20% and a thickness of 1 to 1.5 mm; and the core layer has a porosity of 3% to 8% and a thickness of 2 to 3 mm. Step 3: Place the preform and the grain boundary diffusion source together in a fully enclosed inert atmosphere furnace, control the oxygen content in the furnace to ≤0.1%, and hold at 640-660℃ for 2.8-3.2h to carry out a low-temperature grain boundary diffusion and sintering. Step four: After cooling, the sintered body is cut using a low-temperature ultra-high pressure water jet. The water jet pressure is 300-400 MPa, and the cutting medium temperature is controlled between -6℃ and -4℃.

2. The low-temperature preparation process of a heavy rare-earth gradient structure NdFeB magnet according to claim 1, characterized in that, The laser 3D printing process parameters in step two are as follows: Surface layer: Laser power 55-65W, scanning speed 45-55mm / s; Transition layer: laser power 45-55W, scanning speed 70-80mm / s; Core layer: Laser power 35-45W, scanning speed 95-105mm / s.

3. The low-temperature preparation process of a heavy rare-earth gradient structure NdFeB magnet according to claim 1, characterized in that: The grain boundary diffusion source is a mixture of Nd70Cu30 alloy powder and Nd60Al10Ni10Cu20 alloy powder in a mass ratio of 2.8:1 to 3.2:1, with the alloy powder particle size being 5 to 10 μm.

4. The low-temperature preparation process of a heavy rare-earth gradient structure NdFeB magnet according to claim 1, characterized in that: In the grain boundary diffusion source, Nd60Al10Ni10Cu20 alloy powder can be replaced with Nd65Cu25Zr10 alloy powder, while the mixing mass ratio of Nd65Cu25Zr10 alloy powder to Nd70Cu30 alloy powder remains unchanged.

5. The low-temperature preparation process of a heavy rare-earth gradient structure NdFeB magnet according to claim 1, characterized in that: The fully enclosed inert atmosphere furnace adopts a magnetic fluid sealing structure, and the atmosphere is high-purity argon with a purity of ≥99.999%.

6. A neodymium iron boron magnet with a weightless rare earth gradient structure, characterized in that, The magnet is manufactured using a low-temperature fabrication process according to claims 1-4, wherein the magnet comprises: The surface layer has a coercivity of 18–20 kOe; The transition layer has a coercivity of 15–17 kOe. The core layer has a magnetic energy product of 38–42 MGOe.

7. A heavy rare-earth-free gradient structure NdFeB magnet according to claim 6, characterized in that: The coercivity decreases by ≤10% at 180℃.

8. A heavy rare-earth gradient structure NdFeB magnet according to claim 6, characterized in that: After 1000 hours of neutral salt spray testing, the weight loss per unit area is ≤0.5 mg / cm².

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