Technology for preparing neodymium-iron-boron magnet through ultrasonic-assisted screen printing-gradient temperature control

Abstract

The invention relates to the technical field of rare earth permanent magnet material preparation, in particular to a method for preparing a neodymium-iron-boron magnet through ultrasonic-assisted screen printing and gradient temperature control. According to the preparation method, the nano aluminum oxide coating is prepared by adopting an ultrasonic-assisted screen printing technology, and the grain orientation degree and the microstructure uniformity are improved through the synergistic effect of an acoustic streaming effect, a heat effect and mechanical vibration energy generated by ultrasonic waves with the frequency of 20-30 kHz and the power density of 30-60 W / cm < 2 >; according to the invention, an Al-O-Si three-network structure is formed at a pre-sintering stage of 800 DEG C through nano aluminum oxide, so that the heat-resistant shrinkage stress is improved by 3 times; according to the temperature zone gradient temperature control diffusion process, the grain boundary structure is optimized, and the treatment time is shortened by more than 50%. The motor can be widely applied to new energy automobile driving motors and wind driven generators.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet material preparation, specifically to the ultrasonic-assisted screen printing-gradient temperature control process for preparing neodymium iron boron magnets. Background Technology

[0002] Neodymium iron boron (NdFeB) permanent magnets occupy a core position in modern industries such as new energy vehicles, wind power generation, and precision motors due to their ultra-high energy product and strong coercivity. However, NdFeB magnets are prone to a sharp decline in coercivity at high temperatures. The traditional solution is to diffuse heavy rare earth elements (Tb / Dy) onto the magnet surface, but this approach has several drawbacks.

[0003] First, during high-temperature sintering, especially when the temperature exceeds 1100℃, abnormal grain growth is likely to occur. This is because at high temperatures, a large amount of Nd-rich phase precipitates to form a liquid phase, which, through the Ostwald maturation mechanism, causes large grains to engulf small grains, increasing the grain size from the ideal 3-5 μm to 10 μm or even higher, significantly reducing the coercivity of the magnet.

[0004] Secondly, to improve the high-temperature stability and corrosion resistance of magnets, a protective coating is usually prepared on the magnet surface. Traditional coating preparation methods, such as electroplating and electroless plating, pose environmental pollution problems, while methods such as physical vapor deposition are costly. Although screen printing technology is lower in cost, ordinary screen-printed coatings are prone to structural instability problems such as cracking and peeling under high-temperature sintering environments of 800–1000℃, and the mismatch of the thermal expansion coefficients between the coating and the substrate leads to thermal stress concentration.

[0005] Third, to obtain high coercivity, traditional processes require the addition of large amounts of heavy rare earth elements Dy and Tb, or the use of complex grain boundary diffusion processes. On the one hand, heavy rare earth resources are scarce and expensive, limiting the development of the industry; on the other hand, grain boundary diffusion processes require multiple heat treatments, with a total processing time of 20 to 30 hours, resulting in high energy consumption and low efficiency.

[0006] Although researchers have attempted to use ultrasound-assisted techniques to improve the microstructure of magnets in recent years, high-power ultrasound, while improving grain orientation, can also generate excessive thermal effects and mechanical stress on the magnet surface, which in turn promotes abnormal growth of surface grains.

[0007] Therefore, developing a novel NdFeB magnet preparation technology that can effectively control grain growth, improve the high-temperature stability of coatings, simplify the process, and reduce costs is of great significance for promoting the development of the new energy industry. Summary of the Invention

[0008] Based on the problems summarized above, this invention provides an ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets. Its main feature is that through the synergistic effect of the ultrasonic vibration energy field and screen printing coating technology, combined with an innovative gradient temperature control diffusion process, significant improvements in magnet performance and substantial reductions in process costs are achieved. The specific technical solution is as follows:

[0009] Ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets. It comprises the following components (by weight percentage): NdFeB magnetic powder (92%–96%), binder (3.5%–4.5%), nano-alumina (γ phase, 0.8%–1.8%), dispersant (0.3%–0.8%), and grain boundary diffusion source (0.5%–2.0%).

[0010] S1: Accurately weigh the target component and place it in the furnace for melting. Evacuate to 10... -2 Below Pa, high-purity argon gas is introduced for protection while melting is carried out to ensure complete liquefaction and uniform composition of the alloy. The molten alloy is then poured onto the surface of a high-speed rotating water-cooled copper roller for cooling.

[0011] The cooled alloy sheet is placed in a reaction vessel, evacuated, and then filled with high-purity hydrogen. The temperature is then raised to 200–300°C and held for 1–2 hours. The resulting coarse powder is then crushed by impact in a grinding chamber using a high-speed inert gas flow to obtain flaky powder.

[0012] S2: The product obtained in S1 is filled into a specially designed hot press mold and pre-cooled under an inert atmosphere. A step-by-step pressurization method is used: an initial pressure of 50–60 MPa is maintained for 30–60 seconds to initially compact the powder; a second pressurization is applied to 150–180 MPa and held for 1–2 minutes to form a compact with a certain strength. The cold-pressed compact then enters the hot-pressing stage, where it is held at 700℃ and 300 MPa for 1–2 minutes. During this process, the magnetic powder particles achieve densification through plastic flow and diffusion creep, forming the hot-pressed compact.

[0013] The hot-pressed blank is transferred to a high-precision hot-deformation mold equipped with an ultrasonic vibration system. The mold features a special structure that allows the ultrasonic transducer to directly transmit mechanical vibration to the deformation area via the vibrating indenter. Vibration parameters are set to a frequency of 20 kHz and an intensity of 12–47 W / cm². 2 Phased adjustment within the range: Initial stage using 12W / cm 2 Low strength promotes grain slip; the main deformation stage increases to 37 W / cm 2 To enhance dislocation movement and dynamic recrystallization; the final stage uses 47W / cm 2 The orientation degree is further optimized for high strength. The hot deformation process is carried out in a vacuum or argon environment, with the temperature controlled at 900℃ and the strain rate at 0.001 / s.

[0014] S3: A piezoelectric ceramic transducer is installed on the back of the squeegee, with the vibration direction at a 20° angle to the substrate normal, using a pulsed operating mode. The treated alumina paste is coated onto the substrate via screen printing. The screen printing parameters are: screen mesh size 300–400 mesh, squeegee pressure 0.25–0.35 MPa, substrate preheating temperature 55–65°C, and printing speed 8–15 mm / s. After printing, the substrate is placed in an 80°C hot air circulating oven for 10–20 minutes, and finally pre-fired.

[0015] S4: Stepped temperature controlled sintering. The diffusion treatment is carried out under vacuum or argon protection, using a three-stage gradient temperature controlled process, and finally a secondary tempering treatment.

[0016] Furthermore, the smelting requirements in S1 are: a temperature ≥1450℃ and an oxygen content ≤500ppm; the high-speed rotating water-cooled copper roller requires a rotation speed of 2-5m / s and a cooling rate of 10... 5 ~10 6 K / s;

[0017] The reactor described in S1 is a pressure-resistant reactor with a hydrogen pressure requirement of 0.1–0.3 MPa; the inert gas flow is N2 or Ar with an oxygen content ≤50 ppm; the classifying wheel is required to run at 3000–6000 rpm during grinding; the flaky powder has a thickness of 0.1–0.5 μm and a diameter of 3–5 μm.

[0018] Furthermore, the hot press mold described in S2 requires the use of dual heating zones with independent temperature control, and the temperature difference is controlled within ±5℃; the pressure is applied using a hydraulic servo system, and the pressure fluctuation during the pressure holding stage does not exceed ±1%.

[0019] Furthermore, the scraper material described in S3 is polyurethane with a hardness of 80; the electro-ceramic converter has a frequency of 27–29 kHz and a power of 200 W. The pulse operation mode is 2 seconds on / 1 second off, with a power density of 1.2 W / cm³. 2 The action time is 4 to 6 minutes.

[0020] The molar ratio of alumina in S3 to silica in the preform in S3 is controlled between 1:1 and 3:1.

[0021] The first stage of pre-firing, as described in S3, involves raising the temperature to 300-400°C at a rate of 3-5°C / min under N2 protection and holding for 30-40 minutes. The second stage switches to an Ar atmosphere, raising the temperature to 600-800°C at a rate of 5-10°C / min and holding for 60-80 minutes. The third stage continues to use a high vacuum atmosphere (<10). -3 (Pa), raise to 900-1000℃ at 5-10℃ / min, and keep warm for 2-4 hours.

[0022] Furthermore, the temperature range of the first-stage high-temperature diffusion period described in S4 is 900–950°C, the holding time is 2–3 hours, and the atmosphere requirement is high vacuum (<10). -3 The second stage, the intermediate temperature aging period, has a temperature range of 700–800℃ and a holding time of 3–4 hours, requiring an Ar atmosphere. The third stage, the slow cooling period, has a temperature range of 450–500℃ and a holding time of 1–3 hours, requiring an Ar atmosphere.

[0023] The secondary tempering process described in S4 is carried out at a temperature of 500–550°C for 2–3 hours, followed by furnace cooling to room temperature to obtain a high-performance neodymium iron boron magnet.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. Effective control of grain growth: Through the synergistic effect of ultrasonic vibration's acoustic flow, thermal effect, and mechanical vibration energy, abnormal growth is suppressed while promoting grain orientation. Ultrasonic treatment improves grain orientation while maintaining grain size within the ideal range.

[0026] 2. Improve the high-temperature stability of the coating: The Al-O-Si three-network structure formed by nano-alumina during the pre-calcination stage not only improves the chemical bonding strength between the coating and the substrate, but also improves the thermal expansion matching, enabling the coating to maintain structural stability at high temperatures.

[0027] 3. Optimized grain boundary structure and magnetic properties: Gradient temperature-controlled diffusion technology achieves uniform distribution of grain boundary phases and optimized core-shell structure by precisely controlling the diffusion kinetics in different temperature regions. Compared with traditional processes, it enhances coercivity and improves remanence.

[0028] 4. The process flow is simplified, the total processing time is shortened, and energy consumption is reduced. At the same time, by optimizing the grain boundary structure, the amount of heavy rare earth elements used is reduced, thus lowering raw material costs. Attached Figure Description

[0029] Figure 1 This is a flowchart of the ultrasonic-assisted screen printing-gradient temperature-controlled NdFeB magnet process of the present invention.

[0030] Figure 2 This is a comparison diagram of the coercivity and remanence of the neodymium iron boron magnet prepared in this invention.

[0031] Figure 3 This is a comparison chart of the raw material cost and energy consumption of the neodymium iron boron magnet prepared according to the present invention. Detailed Implementation

[0032] The following embodiments further explain and illustrate the technical solutions of the present invention. It should be specifically noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention.

[0033] This invention proposes an ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets, comprising the following components (by weight percentage): NdFeB magnetic powder (92%–96%), binder (3.5%–4.5%) (e.g., polyvinylpyrrolidone), nano-alumina (γ phase, 0.8%–1.8%), dispersant (0.3%–0.8%) (e.g., ammonium polyacrylate), and grain boundary diffusion source (0.5%–2.0%). The solvent percentage is relative to the total solids content; in the actual formulation, the final evaporation of the solvent is not included in the weight of the finished product. (See attached...) Figure 1 The diagram shows the fabrication process of neodymium iron boron magnets using ultrasound-assisted screen printing and gradient temperature control. The detailed fabrication steps are as follows:

[0034] 1. Preparation of NdFeB magnetic powder

[0035] 1.1 Raw material proportioning and smelting

[0036] Metallic Nd (purity ≥ 99.5%), metallic Pr (purity ≥ 99.5%), industrial pure iron (purity ≥ 99.8%), ferroborone alloy (B content 18-20%), DyH x Powder and trace elements. Ingredients are formulated according to the designed composition and melted in a vacuum induction furnace (≤10). -2 The alloy is smelted in a solution of Pa at 1450–1500℃ for 4 hours, with an oxygen content ≤500ppm. The molten alloy is then poured onto the surface of a high-speed rotating, water-cooled copper roller at a speed of 2–5 m / s and a cooling rate of 10 m / s. 5 ~10 6 K / s.

[0037] 1.2 Magnetic Powder Pretreatment

[0038] The smelting product is placed in a pressure-resistant reactor, evacuated, and then filled with high-purity hydrogen (≥99.99%). The hydrogen pressure is required to be 0.1–0.3 MPa, and the oxygen content is <10 ppm. The temperature is then raised to 200–300℃ and held for 1–2 hours. The resulting coarse powder is then crushed in a grinding chamber by a high-speed inert gas flow (such as N2), with an oxygen content ≤50 ppm and a classifying wheel designed for 3000–6000 rpm. Finally, flake-like powder with a thickness of 0.1–0.5 μm and a diameter of 3–5 μm is obtained.

[0039] 2. Hot pressing

[0040] 2.1 Preparation of hot-pressed embryo

[0041] Pretreated magnetic powder is filled into a specially designed hot press mold and pre-cooled under an inert atmosphere. A step-by-step pressurization method is used: an initial pressure of 50–60 MPa is maintained for 30–60 seconds to initially compact the powder; a second pressurization is applied to 150–180 MPa and held for 1–2 minutes to form a compact with a certain strength. The cold-pressed compact then enters the hot-pressing stage, where it is held at 700℃ and 300 MPa for 1–2 minutes. During this process, the magnetic powder particles achieve densification through plastic flow and diffusion creep, forming the hot-pressed compact.

[0042] The hot press mold adopts a dual heating zone with independent temperature control, and the temperature difference is controlled within ±5℃; the pressure is applied by a hydraulic servo system, and the pressure fluctuation during the pressure holding stage does not exceed ±1%.

[0043] 2.2 Ultrasonic-assisted thermal deformation

[0044] The hot-pressed blank is transferred to a high-precision hot deformation mold equipped with an ultrasonic vibration system. The mold features a special structure that allows the ultrasonic transducer to directly transmit mechanical vibration to the deformation area via the vibrating indenter. Vibration parameters are set to a frequency of 20 kHz and an intensity of 12–47 W / cm². 2 Phased adjustment within the range: Initial stage using 12W / cm 2 Low strength promotes grain slip; the main deformation stage increases to 37 W / cm 2 To enhance dislocation movement and dynamic recrystallization; the final stage uses 47W / cm 2 The high strength further optimizes the orientation.

[0045] The hot deformation process is carried out in a vacuum or argon atmosphere, with the temperature controlled at 900℃ and the strain rate at 0.001 / s.

[0046] 3. Passivated γ-alumina nanoparticles

[0047] 3.1 Raw material selection

[0048] The raw material composition and mass ratio are selected as follows: 15-25% by mass of γ-alumina (particle size 10-30nm, specific surface area >150m²). 2 / g), 70-80% by mass of dispersion medium (e.g., anhydrous ethanol, ethylene glycol diether, water content <50ppm), 2-4% by mass of electrostatic dispersant (e.g., ammonium polyacrylate, molecular weight 2000-5000Da), 1-3% by mass of surface passivator (e.g., diethanolamine phosphate), and 0.5-1.5% by mass of binder (e.g., polyvinylpyrrolidone).

[0049] 3.2 Surface passivation

[0050] First, the γ-alumina nanopowder is vacuum dried at a temperature of 80℃, with a heating rate of 3-5℃ / min and a holding time of 2-3h.

[0051] Then, γ-alumina nanoparticles and surface passivating agent diethanolamine phosphate were mixed in anhydrous ethanol, and then ultrasonically treated in an ice bath at a controlled temperature (<25℃) with a treatment frequency of 30-40kHz, an intensity of 150-200W, and a duration of 20-30min.

[0052] Finally, it is vacuum dried at 60℃ with a heating rate of 3-5℃ / min.

[0053] 3.3 Slurry Preparation

[0054] Anhydrous ethanol and ammonium polyacrylate were mixed and magnetically stirred at 400–600 rpm for 10–20 minutes until completely dissolved. Then, the γ-alumina nanopowder prepared above was added and stirred until no dry powder remained.

[0055] Shear emulsification is performed using a high-speed shear emulsifier with a rotation speed of 10,000 to 12,000 rpm for 10 to 20 minutes. At the same time, an ice bath is used to control the temperature of the slurry to keep it below 25°C.

[0056] Insert the ultrasonic probe (titanium alloy, φ6mm), power 400W, pulse mode: 3s on / 1s off. Total working time is 30-50 minutes. The slurry temperature should be below 30℃. If it exceeds 30℃, stop stirring.

[0057] Add polyvinylpyrrolidone to the ethanol premix and stir magnetically at 200–300 rpm for 5–10 min.

[0058] Transfer the slurry to a vacuum container, evacuate to -0.1 MPa, maintain for 15–30 min, and then pass it through a 0.22 μm nylon filter to remove undispersed aggregates.

[0059] 4. Ultrasonic-assisted screen printing

[0060] A piezoelectric ceramic transducer is mounted on the back of the squeegee, with the vibration direction at a 20° angle to the substrate normal, employing a pulsed operating mode. The screen printing parameters are: screen mesh count 300–400 mesh, squeegee pressure 0.25–0.35 MPa, substrate preheating temperature 55–65°C, and printing speed 8–15 mm / s. After printing, the substrate is placed in an 80°C hot air circulating oven for 10–20 minutes, followed by pre-firing. The squeegee material is polyurethane with a hardness of 80. The piezoelectric ceramic transducer has a frequency of 27–29 kHz and a power of 200 W. Furthermore, the pulsed operating mode is 2s on / 1s off, with a power density of 1.2 W / cm³. 2 The action time is 4 to 6 minutes.

[0061] The first stage of pre-firing involves raising the temperature to 300–400°C at a rate of 3–5°C / min under N2 protection and holding for 30–40 min. The second stage switches to an Ar atmosphere, raising the temperature to 600–800°C at a rate of 5–10°C / min and holding for 60–80 min. The third stage continues to use a high vacuum atmosphere (<10⁻⁶ ppm). -3 (Pa), raise to 900-1000℃ at 5-10℃ / min, and keep warm for 2-4 hours.

[0062] 5. Stepped temperature control and secondary tempering

[0063] The diffusion process is carried out under vacuum or argon protection, using a three-stage gradient temperature control process, and finally undergoes a secondary tempering treatment.

[0064] The first stage, the high-temperature diffusion period, has a temperature range of 900–950℃ and a holding time of 2–3 hours. The required atmosphere is a high vacuum (<10). -3 Pa).

[0065] The second stage of medium-temperature aging involves a temperature range of 700–800℃, a holding time of 3–4 hours, and an Ar atmosphere.

[0066] The third stage, a slow cooling period, has a temperature range of 450–500℃ and a holding time of 1–3 hours. The atmosphere required is Ar.

[0067] The secondary tempering treatment temperature is 500-550℃, the tempering time is 2-3 hours, and then the furnace is cooled to room temperature to obtain high-performance neodymium iron boron magnets.

[0068] Example 1

[0069] The process for fabricating NdFeB magnets using ultrasound-assisted screen printing and gradient temperature control is as follows:

[0070] The raw material composition includes:

[0071] Neodymium iron boron magnetic powder 94.5%, polyvinylpyrrolidone 4%, nano-alumina (γ phase, 30-35nm) 1.2%, ammonium polyacrylate 0.5%, DyH x powder( Hydrogen content ≥99.5% 0.5%, terpineol 35% (solvent, percentage by weight of NdFeB magnetic powder, final volatilization not included in the finished product weight). Preparation process is as follows:

[0072] S1: Selected metals: Nd (purity ≥ 99.5%), Pr (purity ≥ 99.5%), industrial pure iron (purity ≥ 99.8%), ferroborone alloy (B content 18-20%), and DyH. x Powder and trace elements added. In a vacuum induction melting furnace (≤10... -2 The alloy is smelted at 1450℃ for 4 hours in a solution containing ≤500ppm oxygen. The molten alloy is then poured onto the surface of a high-speed rotating, water-cooled copper roller at a speed of 4m / s and a cooling rate of 10... 6 K / s.

[0073] S2: Place the product obtained in S1 into a pressure-resistant reactor, evacuate it, and then fill it with high-purity hydrogen gas at a pressure of 0.2 MPa. Then heat it to 250℃ and hold it at that temperature for 1.5 hours. The resulting coarse powder is then crushed by impact in a grinding chamber through an N2 gas flow. The classifying wheel is required to run at 5000 rpm during grinding. Finally, a flaky powder with a thickness of 0.3 μm is obtained.

[0074] S3: The product obtained in S2 is filled into a specially designed hot press mold and pre-cooled and pressed under an inert atmosphere. A step-by-step pressurization method is used: an initial pressure of 50 MPa is maintained for 30 seconds to initially compact the powder; a second pressurization is applied to 150 MPa and maintained for 60 seconds to form a compact with a certain strength. The cold-pressed compact then enters the hot-pressing stage, where it is held at 700℃ and 300 MPa for 90 seconds. During this process, the magnetic powder particles achieve densification through plastic flow and diffusion creep, forming the hot-pressed compact.

[0075] The hot press mold requires independent temperature control in dual heating zones, with the temperature difference controlled within ±5℃; pressure application uses a hydraulic servo system, and pressure fluctuation during the holding stage does not exceed ±1%.

[0076] S4: Transfer the product from S3 to a high-precision heat deformation mold equipped with an ultrasonic vibration system. The mold is designed with a special structure that allows the ultrasonic transducer to directly transmit mechanical vibration to the deformation area through the vibrating indenter. The vibration parameters are set to a frequency of 20kHz and an intensity of 12–47 W / cm². 2 Phased adjustment within the range: Initial stage using 12W / cm 2 Low strength promotes grain slip; the main deformation stage increases to 37 W / cm2 To enhance dislocation movement and dynamic recrystallization; the final stage uses 47W / cm 2 The orientation degree is further optimized for high strength. The hot deformation process is carried out in a vacuum or argon environment, with the temperature controlled at 900℃ and the strain rate at 0.001 / s.

[0077] S5: Preparation of nano-alumina slurry. The raw material components and mass ratios are selected as follows: 25% by mass of γ-alumina (particle size 10–30 nm, specific surface area > 150 m²). 2 / g), 73% anhydrous ethanol by mass, 4% ammonium polyacrylate (molecular weight 2000-5000 Da) relative to γ-alumina by mass, 1% diethanolamine phosphate relative to γ-alumina by mass, and 0.5-1.5% polyvinylpyrrolidone by mass.

[0078] The γ-alumina nanoparticles were vacuum dried at a temperature of 80℃, a heating rate of 5℃ / min, and a holding time of 2h.

[0079] γ-alumina nanoparticles were mixed with diethanolamine phosphate in anhydrous ethanol, and then subjected to ultrasonic treatment in an ice bath at a controlled temperature (<25℃) with a treatment frequency of 35kHz, an intensity of 150W, and a duration of 20min.

[0080] Vacuum drying at 60℃ with a heating rate of 5℃ / min.

[0081] Anhydrous ethanol and ammonium polyacrylate were mixed and magnetically stirred at 400 rpm for 10 minutes until completely dissolved. Then, the γ-alumina nanopowder prepared above was added and stirred until no dry powder remained.

[0082] Shear emulsification was performed using a high-speed shear emulsifier at a speed of 11,000 rpm for 15 minutes, while an ice bath was used to control the temperature of the slurry to keep it below 25°C.

[0083] Insert the ultrasonic probe (titanium alloy, φ6mm), power 400W, pulse mode: 3s on / 1s off. Total working time is 35 minutes. The slurry temperature must be below 30℃. If it exceeds 30℃, stop stirring.

[0084] Add polyvinylpyrrolidone to the ethanol premix and stir magnetically at 200 rpm for 10 min.

[0085] The slurry was transferred to a vacuum container, evacuated to -0.1 MPa, and held for 20 minutes. Then it was passed through a 0.22 μm nylon filter to remove undispersed aggregates.

[0086] S6: A piezoelectric ceramic transducer is mounted on the back of the squeegee, with the vibration direction at a 20° angle to the substrate normal, operating in a pulse mode. Screen printing parameters are: 350 mesh screen, squeegee pressure 0.3 MPa, substrate preheating temperature 60°C, and printing speed 10 mm / s. After printing, the substrate is left to stand in an 80°C hot air circulating oven for 15 minutes, followed by pre-firing. The squeegee material is polyurethane with a hardness of 80. The piezoelectric ceramic transducer frequency is 27 kHz, with a pulse operating mode of 2 seconds on / 1 second off, and a power density of 1.2 W / cm³. 2 The action time is 6 minutes.

[0087] The first stage of pre-firing involves raising the temperature to 300℃ at a rate of 5℃ / min under N2 protection and holding for 30 min. The second stage switches to an Ar atmosphere, raising the temperature to 600–800℃ at a rate of 10℃ / min and holding for 70 min. The third stage continues to use a high vacuum atmosphere (<10). -3 The temperature was increased to 1000℃ at a rate of 5℃ / min and held for 3 hours.

[0088] S7: Step temperature control is carried out under vacuum or argon protection, using a three-stage gradient temperature control process, and finally undergoes a second tempering treatment.

[0089] The first stage, the high-temperature diffusion period, has a temperature range of 930℃ and a holding time of 2.5 hours. The required atmosphere is high vacuum (<10). - 3 Pa).

[0090] The second stage of medium-temperature aging involves a temperature range of 750℃, a holding time of 3.5 hours, and an Ar atmosphere.

[0091] The third stage, a slow cooling period, has a temperature range of 480℃ and a holding time of 2 hours. The atmosphere required is Ar.

[0092] The secondary tempering treatment temperature is 520℃, the tempering time is 2.5h, and then the furnace is cooled to room temperature to obtain high-performance neodymium iron boron magnets.

[0093] Example 2

[0094] The process for fabricating NdFeB magnets using ultrasound-assisted screen printing and gradient temperature control is as follows:

[0095] The preparation method is the same as in Example 1, except that the raw material composition includes: 92% neodymium iron boron magnetic powder, 4.5% polyvinylpyrrolidone, 1.8% nano-alumina (γ phase, 30-35 nm), 0.8% ammonium polyacrylate, and DyH. x powder( Hydrogen content ≥99.5%) 2.0%, terpineol 40%.

[0096] In step S2 of the preparation process, the hydrogen crushing pressure is 0.3 MPa, the hydrogen crushing temperature is 300℃, and the heat deformation ultrasonic strength is 47 W / cm. 2 The other steps are the same.

[0097] In step S4 of the preparation process, the ultrasonic strength during heat deformation is 47 W / cm. 2 The other steps are the same.

[0098] In step S7 of the preparation process, the first stage of sintering is 950℃ / 3h, and the tempering is 550℃ / 3h. The other steps are the same.

[0099] Example 3

[0100] The process for fabricating NdFeB magnets using ultrasound-assisted screen printing and gradient temperature control is as follows:

[0101] The preparation method is the same as in Example 1, except that the raw material composition includes: 96% neodymium iron boron magnetic powder, 3.5% polyvinylpyrrolidone, 0.8% nano-alumina (γ phase, 30-35 nm), 0.3% ammonium polyacrylate, and DyH. x powder( Hydrogen content ≥99.5% 0.5%, terpineol 30%.

[0102] In step S1 of the preparation process, the melting cooling rate is 10. 5 K / s, the other steps are the same.

[0103] In step S3 of the preparation process, the hot pressing pressure is 250 MPa, and the other steps are the same.

[0104] In step S6 of the preparation process, the printing speed is 15 mm / s, and the other steps are the same.

[0105] In step S7 of the preparation process, the first stage of sintering is 900℃ / 2h.

[0106] Example 4

[0107] The process for fabricating NdFeB magnets using ultrasound-assisted screen printing and gradient temperature control is as follows:

[0108] The preparation method is the same as in Example 1, except that:

[0109] In step S2 of the preparation process, the hydrogen crushing time is shortened to 1 hour, while the other steps remain the same.

[0110] In step S3 of the preparation process, hot pressing and holding pressure is performed for 60 seconds, while the other steps are the same.

[0111] In step S6 of the preparation process, the pre-calcination time is set to 300℃ / 30min, 700℃ / 60min, and 900℃ / 1.5h, and the other steps are the same.

[0112] Comparative Example 1

[0113] The preparation steps are the same as in Example 1, except that the formulation does not contain nano-alumina.

[0114] In step S4 of the preparation process, conventional hot pressing is used, while the other steps are the same.

[0115] In step S7 of the preparation process, isothermal sintering is adopted, and the temperature is set to 1050℃ / 4h.

[0116] Comparative Example 2

[0117] The preparation steps are the same as in Example 1, except that step S7 uses a single-stage 950℃ / 5h process, while the other steps are the same.

[0118] Comparative Example 3

[0119] The preparation steps are the same as in Example 1, except that ultrasonic vibration is turned off in step S6.

[0120] The NdFeB magnets prepared in Examples 1-4 and Comparative Examples 1-3 were compared in terms of their overall performance. Coercivity and remanence were tested according to GB / T 13560-2017 "Sintered NdFeB Permanent Magnet Materials", and energy consumption was calculated according to GB / T2589-2020 "General Rules for Calculating Comprehensive Energy Consumption". Specific test comparison results are shown in Tables 1 and 2.

[0121] Table 1. Comparison of core performance between Examples 1-4 and Comparative Examples 1-3

[0122]

[0123]

[0124] Table 2. Cost and Energy Consumption Comparison of Examples 1-4 and Comparative Examples 1-3

[0125]

[0126] As can be seen from the above comparison results, Example 1, through staged ultrasound (12→37→47W / cm), 2 The proportion of abnormal grains was reduced to less than 5%, far lower than the 30% in Comparative Example 1; nano-alumina and gradient pre-sintering resulted in a high-temperature failure rate of less than 3% in Example 1, compared to as high as 40% in Comparative Example 3; Example 1 reduced costs by 16% compared to Comparative Example 1 by reducing heavy rare earth elements and solvent recovery; gradient temperature-controlled sintering reduced energy consumption to 55 kWh / kg, while traditional industry processes are ≥100 kWh / kg; the decrease in coercivity in Example 3 was due to DyH xThe reduced powder content caused the coating strength in Example 4 to be slightly lower due to the rapid process. Comparative Example 1 used traditional multi-step sintering, resulting in higher energy consumption. Comparative Example 3 did not use ultrasonic assistance, so the high-temperature failure rate was higher and the coating strength was lower.

Claims

1. A process for fabricating NdFeB magnets using ultrasound-assisted screen printing and gradient temperature control, characterized by: Raw material composition and formulation: NdFeB magnetic powder 92%–96%, binder 3.5%–4.5%, nano-alumina 0.8%–1.8%, dispersant 0.3%–0.8%, grain boundary diffusion source 0.5%–2.0%; The raw materials for neodymium iron boron magnetic powder are first weighed, smelted, and cooled, and then subjected to hydrogen crushing and air jet milling to obtain flake powder. The powder is first pre-cooled and pressed, and then hot-pressed to form a dense blank. Alumina slurry is coated onto the blank by screen printing, and then left to stand, pre-fired, and then subjected to staged temperature-controlled sintering and secondary tempering to obtain high-performance neodymium iron boron magnets.

2. The ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets according to claim 1, characterized in that, Includes the following steps: S1: The target component is accurately weighed and melted in a vacuum furnace, protected by high-purity argon gas. The molten alloy is poured onto the surface of a high-speed rotating water-cooled copper roller. The resulting product is placed in a reaction vessel, evacuated, and then filled with high-purity hydrogen gas. The temperature is then raised to 200-300°C and held for 1-2 hours. Flake powder is obtained by air jet milling. S2: The flake powder obtained in S1 is filled into a hot press mold and pre-cooled and pressed using a step-by-step pressing method. Then the cold-pressed blank is transferred to the hot pressing stage to finally achieve densification and form a hot-pressed blank. The hot-pressed blank is transferred to a hot deformation mold, which allows the vibrating press head to directly transmit mechanical vibration to the deformation area. S3: Apply the prepared alumina paste to the surface of the preform obtained in S2 using a scraper; after screen printing, the substrate is placed in a hot air circulating oven and then pre-fired. S4: The neodymium iron boron magnet is obtained by three-stage stepped temperature control sintering and a second tempering treatment.

3. The ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets according to claim 2, characterized in that: The smelting parameters described in S1 are: temperature ≥ 1450℃, oxygen content ≤ 500ppm; the parameters of the high-speed rotating water-cooled copper roller described in S1 are: rotation speed 2~5m / s, cooling rate 10 5 ~10 6 K / s; The high-purity hydrogen gas mentioned in S1 has the following parameters: hydrogen pressure 0.1~0.3MPa; The air jet mill mentioned in S1 has the following parameters: classifier speed 3000~6000rpm.

4. The ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets according to claim 2, characterized in that: The step-by-step pressurization described in S2 has the following parameter settings: initial pressure of 50-60 MPa held for 30-60 seconds, followed by a second pressurization to 150-180 MPa held for 1-2 minutes; the hot pressing stage described in S2 has the following parameter settings: heat and pressure held at 700℃ and 300 MPa for 1-2 minutes; the hot pressing mold described in S2 has the following features and parameter settings: it adopts independent temperature control in dual heating zones, with the temperature difference controlled within ±5℃; The hot deformation mold described in S2 has the following parameter settings: frequency 20kHz, temperature 900℃, strain rate 0.001 / s, and strength adjusted in stages: initial stage 12W / cm². 2 37W / cm during the main deformation stage 2 The final stage was 47 W / cm 2 .

5. The ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets according to claim 2, characterized in that: The squeegee described in S3 has the following characteristics and parameter settings: a piezoelectric ceramic transducer is installed on the back, the vibration direction is at a 20° angle to the substrate normal, and it adopts a pulse working mode: 2s on / 1s off; the screen printing described in S3 has the following parameters: screen mesh number 300-400 mesh, squeegee pressure 0.25-0.35MPa, substrate preheating temperature 55-65℃, and printing speed 8-15mm / s.

6. The ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets according to claim 5, characterized in that: The piezoelectric ceramic transducer has the following parameters: frequency 27–29 kHz, maximum power 200 W, and power density 1.2 W / cm³. 2 The action time is 4-6 minutes; the scraper is made of polyurethane with a hardness of 80.

7. The ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets according to claim 2, characterized in that: The pre-firing described in S3 has the following parameter settings: First stage: Under N2 protection, heat to 300-400℃ at a rate of 3-5℃ / min and hold for 30-40 minutes; Second stage: Under Ar atmosphere, heat to 600-800℃ at a rate of 5-10℃ / min and hold for 60-80 minutes; Third stage: High vacuum less than 10... -3 Increase the temperature at 5-10℃ / min to 900-1000℃ and hold for 2-4 hours.

8. The process for preparing NdFeB magnets using ultrasound-assisted screen printing and gradient temperature control according to claim 2, characterized in that: The three-stage stepped temperature-controlled sintering described in S4 has the following parameter settings: the first stage is performed under high vacuum of less than 10... -3 The second stage involves holding the material at 900–950℃ for 2–3 hours under an Ar atmosphere; the third stage involves holding the material at 700–800℃ for 3–4 hours under an Ar atmosphere; the fourth stage involves holding the material at 450–500℃ for 1–3 hours under an Ar atmosphere; and the second tempering treatment described in S4 has the following parameters: temperature of 500–550℃ and tempering time of 2–3 hours.

9. The ultrasonic-assisted screen printing-gradient temperature control process for preparing NdFeB magnets according to claim 2, characterized in that: The molar ratio of alumina in S3 to silica in the preform in S3 is controlled between 1:1 and 3:

1.

10. A method for preparing neodymium iron boron magnets using ultrasonic-assisted screen printing and gradient temperature control according to any one of claims 1-9, characterized in that, It has a coercivity ≥2000kA / m, remanence ≥1.32T, grain size of 3~5μm, and a stable coating with an Al-O-Si three-network structure.

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