A method for deep grain boundary modification of thick rare-earth permanent magnets using pulsed current electromigration effect

Abstract

This invention belongs to the field of rare earth permanent magnet material preparation technology, and discloses a method for achieving deep grain boundary modification of thick rare earth permanent magnets using the pulsed current electromigration effect. In the preparation process, the magnet is placed in a grain boundary liquefaction state and a high-density DC pulsed current is applied. Using "electron driving force" as an additional vector driving force, heavy rare earth ions are forcibly propelled to migrate directionally into the interior of the magnet along the liquid grain boundaries. This driving force is constant and does not decrease with depth, thereby achieving "full transparency" modification of the thick magnet.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet material preparation technology, specifically to a method for deep grain boundary modification of thick rare earth permanent magnets using the electromigration effect of pulsed current, and more particularly to a method for grain boundary modification preparation that utilizes the electromigration effect generated by DC pulsed current to achieve ultra-deep and highly uniform penetration of heavy rare earth elements (Tb / Dy) into sintered Nd-Fe-B magnets. Background Technology

[0002] Currently, the mainstream technology for manufacturing high-grade (e.g., 45UH) high-performance NdFeB magnets is grain boundary diffusion (GBD). This technology involves coating the magnet surface with a heavy rare earth source and then subjecting it to high-temperature heat treatment to allow the heavy rare earth elements to penetrate inward along the grain boundaries. However, this technology has several drawbacks, the main shortcomings and their causes being as follows: 1. Limited diffusion depth (physical limit): Traditional GBD relies on concentration gradient-driven natural diffusion (Fickian diffusion), and the driving force decreases exponentially with increasing depth. The effective diffusion depth in industry is usually limited to within 3mm on one side. For the 8mm~12mm thick magnets commonly used in new energy vehicles, the central region cannot obtain enough heavy rare earth elements, resulting in a "hard outer layer and soft inner core," and the overall coercivity cannot meet the standard.

[0003] 2. The Control Challenges of Heat-Assisted Electromigration (Core Difficulty): To overcome the Fick diffusion limitation, engineers attempted to introduce the electromigration effect, the basic principle of which is to use electron wind to propel metal ions. However, in practice, achieving "simultaneous electric pulse and efficient heating" presents a significant technological hurdle: (1) Joule thermal runaway: Rare earth permanent magnets are conductors and will generate intense Joule heat under high current density. If conventional direct current is used, the liquid grain boundaries inside the magnet will generate a severe current heat accumulation effect, causing the local temperature to far exceed the melting point of the matrix, resulting in abnormal grain growth, severely damaging the magnetic properties, and even causing the magnet to burn out and deform.

[0004] (2) Dynamic imbalance between temperature field and electric field: Grain boundary diffusion requires the magnet to be in a specific "grain boundary liquefaction window" (usually 850-950℃). At this high temperature, how to apply sufficient current driving force while ensuring that the magnet does not leave the temperature control window due to additional current heat generation is the main technical bottleneck of the current modification of thick magnets.

[0005] 3. Efficiency versus energy consumption contradiction: In order to forcibly increase the depth, existing processes often extend the high-temperature heat treatment time (20-50 hours), which not only leads to extremely low production efficiency, but also causes the main phase grains to coarsen due to prolonged high-temperature exposure, resulting in poor magnetic properties. Summary of the Invention

[0006] To overcome the shortcomings of the aforementioned background technology, this invention provides a method for deep modification of grain boundaries in thick rare-earth permanent magnets using the pulsed current electromigration effect. In the preparation process, the magnet is placed in a grain boundary liquefaction state and a high-density DC pulsed current is applied. Using "electron driving force" as an additional vector driving force, heavy rare-earth ions are forcibly propelled to migrate directionally into the magnet along the liquid grain boundaries. This driving force is constant and does not decrease with depth, thereby achieving "full transparency" modification of the thick magnet.

[0007] To achieve the above-mentioned electromigration-assisted modification, the method of the present invention needs to be carried out in a vacuum environment with the following conditions: 1. Dynamic flexible electrical contact: During the application of current, the contact is made with the magnet through a flexible conductive medium (such as graphite fiber or flexible contact) to compensate for the thermal deformation of the magnet at high temperature, ensure the uniformity of contact under high current density, and prevent the generation of electric arc.

[0008] 2. Asymmetric temperature control balance: Real-time monitoring of the temperature gradient at both ends of the magnet, and by adjusting the duty cycle of the pulse current or the auxiliary heating power, the temperature deviation caused by Joule heating and polarity effect is compensated to ensure that the magnet as a whole is within the preset grain boundary liquefaction temperature window.

[0009] 3. Atmosphere control: The process is carried out in a high-purity argon atmosphere of 500-1000 Pa to protect the magnet and conductive interface from oxidation at high temperatures.

[0010] Specifically, to achieve the objective of this invention, the method for deep grain boundary modification of thick rare-earth permanent magnets using pulsed current electromigration effect includes the following steps: Step A—Magnet Pretreatment and Asymmetric Diffusion Source Deposition: Surface cleaning of the sintered NdFeB substrate is performed (using magnetron sputtering or multi-arc ion plating, with sputtering power set to 1.0–3.0 kW and a base vacuum level better than 5 × 10⁻⁶). -4 Pa) A heavy rare earth eutectic alloy film is deposited on the cathode surface, and a conductive layer is deposited on the anode surface (to reduce contact resistance and guide current flow). Step B—Grain Boundary Liquefaction and High-Energy Pulse Electromigration: The magnet pretreated in Step A is connected to the circuit through a graphite fiber flexible contact and placed in an argon atmosphere. It is heated to 600°C at a rate of 5–15°C / min for degassing, and then rapidly heated to 880–980°C (so that the neodymium-rich grain boundary phase and diffusion source of the magnet are completely melted to form a liquid channel). Then, a directional current is applied, that is, after reaching the preset temperature, a unipolar rectangular pulse current is applied, with the current flowing from the cathode surface to the anode surface. Under the continuous drive of the electron wind, heavy rare earth ions are directionally transported along the liquid grain boundary. Step C—Controlled Cooling and Multi-level Structure Reconstruction: After Step B is completed, the current is cut off, and high-pressure argon gas is introduced for forced air cooling. Then, a first-stage heat treatment (homogenization) is performed, which involves holding at 800–900℃ for 1–5 hours to allow the infiltrated Tb / Dy elements to further diffuse to the surface of the main phase grains. A second-stage heat treatment (performance optimization) is then performed, which involves holding at 450–550℃ for 2–5 hours (by controlling the cooling rate, a thin and continuous grain boundary phase layer is obtained, forming a typical core-shell structure).

[0011] Furthermore, in some embodiments of the present invention, the thickness of the sintered NdFeB matrix is ​​8-15 mm.

[0012] Furthermore, in some embodiments of the present invention, the surface cleaning includes degreasing, acid washing and ultrasonic cleaning, and drying.

[0013] Furthermore, in some embodiments of the present invention, the pickling is performed using a nitric acid or hydrochloric acid solution, preferably, the mass fraction of the nitric acid or hydrochloric acid solution is 1%-3%.

[0014] Furthermore, in some embodiments of the present invention, the deposition thickness of the heavy rare earth eutectic alloy film is 10–50 μm.

[0015] Furthermore, in some embodiments of the present invention, the alloy composition of the heavy rare earth eutectic alloy film is selected from Tb. x M 100-x or Dy x M 100-x (M is one or more combinations of Cu, Al, Ga, and Zn, and x is 60-80).

[0016] Furthermore, in some embodiments of the present invention, the conductive layer is Cu, Ag, or Cr.

[0017] Furthermore, in some embodiments of the present invention, the deposition thickness of the conductive layer is 0.5–2 μm.

[0018] Furthermore, in some embodiments of the present invention, the argon atmosphere in step B is a high-purity argon atmosphere of 500-1000 Pa.

[0019] Furthermore, in some embodiments of the present invention, the temperature is rapidly increased to 910–940°C in step B.

[0020] Furthermore, in some embodiments of the present invention, the key parameter ranges for applying the unipolar rectangular pulse current in step B are as follows: Average current density (j avg ): 200~1500 A / cm 2(Preferred A / cm) 2 ); Pulse frequency: 100 Hz~10 kHz; Duty cycle: 10%~50% (by real-time monitoring of the potential and temperature feedback at both ends of the magnet, the duty cycle of the pulse current is dynamically adjusted, and the heat conduction effect during the pulse turn-off period is used to offset the instantaneous peak value of Joule heat generated by the current, thereby maintaining a high electromigration driving force while controlling the internal temperature difference of the magnet within ±5℃.

[0021] Furthermore, in some embodiments of the present invention, the time for applying the unipolar rectangular pulse current in step B is affected by the thickness of the magnet, and is usually 1 to 6 hours; preferably, when the thickness of the sintered NdFeB substrate is 9-11 mm, the time for applying the unipolar rectangular pulse current is 2 to 4 hours.

[0022] Furthermore, in some embodiments of the present invention, the cooling rate in step C is controlled at 50-100°C / min (to prevent coarse grain boundary phases and fix the distribution of heavy rare earth elements).

[0023] Compared with traditional coating diffusion processes, the advantages of this invention are as follows: (1) Achieve ultra-deep modification: Break through the physical limitation of 3mm, achieve full performance coverage on 10mm thick magnet, and improve the central coercivity by more than 40%.

[0024] (2) Rapid preparation: The traditional diffusion process, which often takes tens of hours, is shortened to less than 4 hours, which significantly improves production efficiency and reduces energy consumption.

[0025] (3) Precise material control: Relying on electric field force for directional transport, the ineffective accumulation of heavy rare earth on the surface is avoided, and the Tb utilization rate is nearly doubled.

[0026] (4) Performance uniformity: The obtained magnet has a very small heavy rare earth concentration gradient inside, and the magnet's squareness (H k / H cj It is significantly superior to traditional processes. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the magnet pretreatment and electrode connection structure in the method of the present invention; Figure 2 This is a schematic diagram of the pulse current waveform principle in the method of the present invention; Figure 3 These are the heavy rare earth element penetration depth distribution curves of Embodiment 1 and Comparative Example 1 of the present invention; Figure 4 This is a comparison chart of demagnetization curves under different processes in this invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Additional aspects and advantages of this invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. It should be understood that the following description is merely illustrative and not intended to limit the invention.

[0029] The terms “comprising,” “including,” “having,” “containing,” or any other variations thereof, as used herein, are intended to cover a non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0030] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1 to 5” is disclosed, the described range should be interpreted as including the ranges “1 to 4”, “1 to 3”, “1 to 2”, “1 to 2 and 4 to 5”, “1 to 3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0031] Furthermore, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., described below refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms are not necessarily directed at the same embodiment or example. Moreover, the technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0032] Example 1 Electromigration modification of Tb element in 10mm thick magnet Step A (Magnet Pretreatment and Asymmetric Diffusion Source Deposition): First, a 10mm thick 45SH sintered NdFeB substrate was selected and sequentially subjected to degreasing, acid washing with a 2% nitric acid solution for 30 seconds, and ultrasonic cleaning with deionized water. After drying, it was placed in a substrate with a background vacuum better than 5×10⁻⁶. 4 In the magnetron sputtering chamber of Pa, a 25 μm thick Tb layer was then deposited on the cathode surface of the magnet using a sputtering power of 2.0 kW. 70 Cu20 Ga 10 A heavy rare earth eutectic alloy film is formed, and a Cu conductive layer with a thickness of 1 μm is deposited on its opposite anode surface to reduce contact resistance and guide the directional flow of current in subsequent processes.

[0033] Step B (Grain Boundary Liquefaction and High-Energy Pulse Electromigration): The pretreated magnet is connected to the circuit via a graphite fiber flexible contact and placed in a high-purity argon atmosphere at 800 Pa. First, it is degassed by heating to 600 °C at a rate of 10 °C / min, then rapidly heated to 920 °C to completely melt the neodymium-rich grain boundary phase and diffusion source of the magnet, forming a liquid channel. While maintaining a constant temperature of 920 °C, an average current density j is applied to the magnet. avg A unipolar rectangular pulse current of 600 A / cm², 1 kHz frequency, and 30% duty cycle is continuously applied for 3 hours. The thermal conduction effect during the pulse turn-off period is used to offset the instantaneous peak value of Joule heating, thereby maintaining a high electromigration driving force while precisely controlling the internal temperature difference of the magnet within ±5℃.

[0034] Step C (Controlled Cooling and Multi-Level Structure Reconstruction): Immediately after pulsed electromigration, the current is cut off, and high-pressure argon gas is rapidly introduced for forced air cooling, ensuring the cooling rate is strictly controlled at 80℃ / min to effectively prevent coarse grain boundary phases and fix the depth distribution of heavy rare earth elements. After cooling to room temperature, a first-stage heat treatment of 850℃ for 3 hours and a second-stage heat treatment of 500℃ for 3 hours are performed sequentially to allow the infiltrated Tb elements to fully diffuse into the surface of the main phase grains. Finally, by controlling the cooling rate, a thin and continuous grain boundary phase layer is obtained, forming a typical core-shell structure to improve coercivity.

[0035] Example 2 High-intensity electromigration modification of 12mm thick magnets Step A (Magnet Pretreatment and Asymmetric Diffusion Source Deposition): For a 12mm thick 45SH substrate, after completing conventional degreasing and acid washing pretreatment, a 40μm thick Tb layer was deposited on the magnet cathode surface using magnetron sputtering. 75 Al 25 A heavy rare earth alloy film was formed, and a 1 μm thick Cu conductive layer was deposited on the anode surface.

[0036] Step B (Grain Boundary Liquefaction and High-Energy Pulse Electromigration): The pretreated magnet is connected to the circuit via a graphite fiber flexible contact and placed in a high-purity argon atmosphere at 800 Pa. First, it is degassed by heating to 600 °C at a rate of 10 °C / min, then rapidly heated to 920 °C to completely melt the neodymium-rich grain boundary phase and diffusion source of the magnet, forming a liquid channel. While maintaining a constant temperature of 920 °C, an average current density j is applied to the magnet. avgA unipolar rectangular pulse current of 600 A / cm², 1 kHz frequency, and 30% duty cycle was continuously processed for 5 hours.

[0037] Step C (Controlled Cooling and Multi-Level Structure Reconstruction): Immediately after pulsed electromigration, the current is cut off, and high-pressure argon gas is rapidly introduced for forced air cooling, ensuring the cooling rate is strictly controlled at 80℃ / min to effectively prevent coarse grain boundary phases and fix the depth distribution of heavy rare earth elements. After cooling to room temperature, a first-stage heat treatment of 850℃ for 3 hours and a second-stage heat treatment of 500℃ for 3 hours are performed sequentially to allow the infiltrated Tb elements to fully diffuse into the surface of the main phase grains. Finally, by controlling the cooling rate, a thin and continuous grain boundary phase layer is obtained, forming a typical core-shell structure to improve coercivity.

[0038] Example 3 Rapid electromigration modification of Dy element in 10mm thick magnet Step A (Magnet Pretreatment and Asymmetric Diffusion Source Deposition): First, a 10mm thick 45SH sintered NdFeB substrate was selected and sequentially subjected to degreasing, acid washing with a 2% nitric acid solution for 30 seconds, and ultrasonic cleaning with deionized water. After drying, it was placed in a substrate with a background vacuum better than 5×10⁻⁶. 4 In the magnetron sputtering chamber of Pa, a 25 μm thick Dy layer was then deposited on the cathode surface of the magnet using a sputtering power of 2.0 kW. 70 Cu 30 A heavy rare earth eutectic alloy film is formed, and a Cu conductive layer with a thickness of 1 μm is deposited on its opposite anode surface.

[0039] Step B (Grain Boundary Liquefaction and High-Energy Pulse Electromigration): The pretreated magnet is connected to the circuit via a graphite fiber flexible contact and placed in a high-purity argon atmosphere at 800 Pa. First, it is degassed by heating to 600 °C at a rate of 10 °C / min, then rapidly heated to 920 °C to completely melt the neodymium-rich grain boundary phase and diffusion source of the magnet, forming a liquid channel. While maintaining a constant temperature of 920 °C, an average current density j is applied to the magnet. avg A unipolar rectangular pulse current of 600 A / cm², 1 kHz frequency, and 30% duty cycle was continuously processed for 3 hours.

[0040] Step C (Controlled Cooling and Multi-Level Tissue Remodeling): Immediately after pulse electromigration is completed, the current is cut off, and high-pressure argon gas is rapidly introduced for forced air cooling, ensuring that the cooling rate is strictly controlled at 80℃ / min. After cooling to room temperature, a first-stage heat treatment of 850℃ for 3 hours and a second-stage heat treatment of 500℃ for 3 hours are performed sequentially to allow the infiltrated Dy element to fully diffuse into the surface of the main phase grains.

[0041] Comparative Example 1 Step A (Magnet Pretreatment and Asymmetric Diffusion Source Deposition): First, a 10mm thick 45SH sintered NdFeB substrate was selected and sequentially subjected to degreasing, acid washing with a 2% nitric acid solution for 30 seconds, and ultrasonic cleaning with deionized water. After drying, it was placed in a substrate with a background vacuum better than 5×10⁻⁶. 4 In the magnetron sputtering chamber of Pa, a 25 μm thick Tb layer was then deposited on the cathode surface of the magnet using a sputtering power of 2.0 kW. 70 Cu 20 Ga 10 A heavy rare earth eutectic alloy film is formed, and a Cu conductive layer with a thickness of 1 μm is deposited on its opposite anode surface to reduce contact resistance and guide the directional flow of current in subsequent processes.

[0042] Step B (simple heat treatment): The pretreated magnet is placed in a high-purity argon atmosphere of 800 Pa. First, the temperature is raised to 600 °C at a rate of 10 °C / min for degassing, and then the temperature is rapidly raised to 920 °C and the treatment is continued for 3 hours.

[0043] Step C (Controlled Cooling and Multi-Stage Tissue Reconstruction): After heat treatment, high-pressure argon gas is rapidly introduced for forced air cooling, ensuring the cooling rate is strictly controlled at 80℃ / min. After cooling to room temperature, a first-stage heat treatment is performed, holding at 850℃ for 3 hours, followed by a second-stage heat treatment, holding at 500℃ for 3 hours.

[0044] Comparative Example 2 Continuous DC-assisted diffusion (non-pulsed) Step A (Magnet Pretreatment and Asymmetric Diffusion Source Deposition): First, a 10mm thick 45SH sintered NdFeB substrate was selected and sequentially subjected to degreasing, acid washing with a 2% nitric acid solution for 30 seconds, and ultrasonic cleaning with deionized water. After drying, it was placed in a substrate with a background vacuum better than 5×10⁻⁶. 4 In the magnetron sputtering chamber of Pa, a 25 μm thick Tb layer was then deposited on the cathode surface of the magnet using a sputtering power of 2.0 kW. 70 Cu 20 Ga 10 A heavy rare earth eutectic alloy film is formed, and a Cu conductive layer with a thickness of 1 μm is deposited on its opposite anode surface to reduce contact resistance and guide the directional flow of current in subsequent processes.

[0045] Step B (Grain Boundary Liquefaction and High-Energy Pulse Electromigration): The pretreated magnet is connected to the circuit via a graphite fiber flexible contact and placed in a high-purity argon atmosphere at 800 Pa. First, it is degassed by heating to 600 °C at a rate of 10 °C / min, then rapidly heated to 920 °C to completely melt the neodymium-rich grain boundary phase and diffusion source of the magnet, forming a liquid channel. While maintaining a constant temperature of 920 °C, an average current density j is applied to the magnet. avg A constant DC current of 600 A / cm² was applied for 3 hours.

[0046] Step C (Controlled Cooling and Multi-Stage Structure Reconstruction): Immediately after Step B, the current is cut off, and high-pressure argon gas is rapidly introduced for forced air cooling, ensuring that the cooling rate is strictly controlled at 80℃ / min to effectively prevent coarse grain boundary phases and fix the deep distribution of heavy rare earth elements. After cooling to room temperature, a first-stage heat treatment is performed sequentially, holding at 850℃ for 3 hours and a second-stage heat treatment is performed, holding at 500℃ for 3 hours.

[0047] The background art mentions "Joule thermal runaway," and Comparative Example 2 proves that non-pulsed current can lead to "local overheating and abnormal grain growth," thus highlighting the superiority of the "pulsed current" in thermal management of this invention.

[0048] Comparative Example 3 Step A (Magnet Pretreatment and Asymmetric Diffusion Source Deposition): First, a 10mm thick 45SH sintered NdFeB substrate was selected and sequentially subjected to degreasing, acid washing with a 2% nitric acid solution for 30 seconds, and ultrasonic cleaning with deionized water. After drying, it was placed in a substrate with a background vacuum better than 5×10⁻⁶. 4 In the magnetron sputtering chamber of Pa, a 25 μm thick Tb layer was then deposited on the cathode surface of the magnet using a sputtering power of 2.0 kW. 70 Cu 20 Ga 10 A heavy rare earth eutectic alloy film is formed, and a Cu conductive layer with a thickness of 1 μm is deposited on its opposite anode surface.

[0049] Step B (Single-stage heat treatment): The pretreated magnet is placed in a high-purity argon atmosphere at 800 Pa and rapidly heated to 920℃. While maintaining a constant temperature of 920℃, an average current density j is applied to the magnet. avg A unipolar rectangular pulse current of 600 A / cm², 1 kHz frequency, and 30% duty cycle was continuously processed for 3 hours.

[0050] Step C (Controlled Cooling and Multi-Stage Tissue Remodeling): Immediately after pulsed electromigration, the current is cut off, and high-pressure argon gas is rapidly introduced for forced air cooling, ensuring that the cooling rate is strictly controlled at 80℃ / min. After cooling to room temperature, a first-stage heat treatment is performed sequentially, holding at 850℃ for 3 hours and a second-stage heat treatment is performed, holding at 500℃ for 3 hours.

[0051] The measured data of each embodiment and comparative example after processing (all magnets were processed to the core for sampling and testing) are shown in the table below.

[0052] Compared with the prior art, the method of the present invention solves the following technical problems: 1. Breakthrough in depth bottleneck: By using the controlled electron wind generated by high-energy pulsed current as a vector driving force, the "full penetration" of Tb / Dy elements in magnets with a thickness of more than 8mm is achieved.

[0053] 2. Solving the problem of "thermoelectric coupling" runaway: By introducing asymmetric temperature control balance technology and high-frequency pulse current control, and using the pulse gap for microsecond-level thermal relaxation, the local overheating caused by Joule heating is effectively suppressed, ensuring that the magnet grains do not grow abnormally while injecting high current density.

[0054] 3. Achieve ultra-fast homogenization modification: Utilize the high-speed transport characteristics of electromigration to shorten the traditional diffusion cycle of tens of hours to less than 6 hours, and form a highly homogenized heavy rare earth core-shell structure on the magnet cross-section, thereby improving the magnet's squareness.

[0055] Those skilled in the art will readily understand that the above description is merely a partial example of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for deep modification of grain boundaries in thick rare-earth permanent magnets using pulsed current electromigration effect, characterized in that, The method includes the following steps: Step A—Magnet Pretreatment and Asymmetric Diffusion Source Deposition: The sintered NdFeB substrate is surface cleaned, a heavy rare earth eutectic alloy film is deposited on the cathode surface, and a conductive layer is deposited on the anode surface; Step B—Grain Boundary Liquefaction and High-Energy Pulse Electromigration: The magnet pretreated in Step A is connected to the circuit through a graphite fiber flexible contact and placed in an argon atmosphere. It is heated to 600°C at a rate of 5–15 °C / min for degassing, and then rapidly heated to 880–980°C. A directional current is then applied, that is, after reaching the preset temperature, a unipolar rectangular pulse current is applied, with the current flowing from the cathode surface to the anode surface. Under the continuous drive of the electron wind, heavy rare earth ions are directionally transported along the liquid grain boundaries. Step C—Controlled Cooling and Multi-Stage Structure Reconstruction: After Step B is completed, the current is cut off, and high-pressure argon gas is introduced for forced air cooling. Then, a first-stage heat treatment is performed, which involves holding at 800–900℃ for 1–5 hours to allow the infiltrated Tb / Dy elements to further diffuse to the surface of the main phase grains. A second-stage heat treatment is then performed, which involves holding at 450–550℃ for 2–5 hours.

2. The method for deep modification of grain boundaries in thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, The thickness of the sintered NdFeB matrix is ​​8-15 mm.

3. The method for deep modification of grain boundaries in thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, The surface cleaning includes degreasing, pickling, ultrasonic cleaning, and drying; preferably, the pickling is done using a nitric acid or hydrochloric acid solution; preferably, the mass fraction of the nitric acid or hydrochloric acid solution is 1%-3%.

4. The method for deep grain boundary modification of thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, The deposition thickness of the heavy rare earth eutectic alloy film is 10–50 μm.

5. The method for deep modification of grain boundaries in thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, The alloy composition of the heavy rare earth eutectic alloy film is selected from Tb. x M 100-x or Dy x M 100-x M is one or more combinations of Cu, Al, Ga, and Zn, and x is 60-80.

6. The method for deep modification of grain boundaries in thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, The conductive layer is Cu, Ag, or Cr; preferably, the deposition thickness of the conductive layer is 0.5–2 μm.

7. The method for deep modification of grain boundaries in thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, In step B, the temperature is rapidly increased to 910–940°C.

8. The method for deep grain boundary modification of thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, The key parameter ranges for applying the unipolar rectangular pulse current in step B are as follows: Average current density: 200–1500 A / cm 2 ; Pulse frequency: 100 Hz~10 kHz; Duty cycle: 10% to 50%.

9. The method for deep grain boundary modification of thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, The duration of applying the unipolar rectangular pulse current in step B is affected by the thickness of the magnet and ranges from 1 to 6 hours; preferably, when the thickness of the sintered NdFeB substrate is 9-11 mm, the duration of applying the unipolar rectangular pulse current is 2 to 4 hours.

10. The method for deep modification of grain boundaries in thick rare-earth permanent magnets using pulsed current electromigration effect according to claim 1, characterized in that, In step C, the cooling rate is controlled at 50–100 °C / min.

Images