Gadolinium-yttrium activated recycled powder free isostatic pressing rare earth permanent magnet and preparation method thereof

Abstract

The application relates to the field of rare earth permanent magnet manufacturing technology, and discloses a gadolinium-yttrium activated recycled powder free isostatic pressing rare earth permanent magnet and a preparation method, the magnet is made of sintered neodymium-iron-boron grinding processing recycled powder, gadolinium-yttrium intermediate alloy and rapid solidification ingot, and a recycled material pre-wetting agent and an airflow mill online composite lubricant are further added in the raw material. The preparation method comprises the following steps: after the recycled material is pre-wetted in a liquid phase, the recycled material is mixed with new material and hydrogen is crushed; the composite lubricant is dynamically injected in the airflow mill grinding process to obtain modified fine powder; after the fine powder is oriented and pressed into shape, the fine powder is directly packed into a box without cold isostatic pressing treatment; the final product is obtained through negative pressure discharge, vacuum sintering and aging treatment. Through pre-wetting oxidation prevention and heavy rare earth grain boundary regulation, the application realizes high-value utilization of the recycled material; through in-situ lubrication modification of the airflow mill, the powder fluidity is improved, the free isostatic pressing short process manufacturing is realized, the manufacturing cost is reduced, and the excellent comprehensive magnetic performance and mechanical performance of the magnet are ensured.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet material manufacturing technology, specifically to a non-isostatic rare earth permanent magnet made from gadolinium-yttrium activated and recovered powder and its preparation method. Background Technology

[0002] Sintered NdFeB permanent magnets, with their excellent magnetic properties, are widely used in new energy vehicles, wind power generation, energy-saving home appliances, and industrial motors. With the explosive growth in downstream application demand, the supply and demand imbalance of rare earth raw materials, especially key elements such as praseodymium, neodymium, dysprosium, and terbium, has become increasingly prominent, leading to persistently high and volatile raw material prices. Machining is an essential step in the production of sintered NdFeB magnets. During the grinding, chamfering, or slicing of sintered magnet blanks, a large amount of grinding waste powder (commonly known as grinding material or grinding mud) is generated. Although this powder has a similar chemical composition to genuine magnets, its extremely large specific surface area and prolonged exposure during processing result in extremely high surface oxygen content.

[0003] However, applying recycled materials such as powder spilled from molding presses to the manufacture of high-performance magnets faces extremely severe technological challenges, especially in the pursuit of efficient, short-process manufacturing without isostatic pressing. In the traditional sintering NdFeB manufacturing process, the cold isostatic pressing (CIP) step is crucial for eliminating the density gradient of the green blank and ensuring sintering integrity. However, this step involves discontinuous batch production, involving cumbersome manual bagging, vacuuming, and ultra-high pressure treatment, which is the biggest bottleneck restricting production efficiency and increasing costs. Although the industry has begun to explore processes that eliminate cold isostatic pressing, this requires the powder to have excellent flowability and loose density so that uniform and dense packing can be achieved in a single molding process.

[0004] For powder systems containing recycled grinding powder, the irregular particle morphology (often flaky or needle-like fragments) and extremely high surface energy of these recycled powders result in a much stronger tendency to agglomerate than fresh powder, leading to extremely poor flowability (angle of repose often exceeding 45°). Directly applying a non-isostatic pressing process can easily result in uneven density within the green body. Forcing the application of such powders to a non-isostatic pressing process can easily cause severe density gradients within the green body due to uneven mold cavity filling and excessive internal friction, leading to defects such as macroscopic cracks, deformation, or insufficient density during subsequent sintering. Therefore, existing technologies often force the use of cold isostatic pressing to remedy green bodies containing recycled materials, failing to achieve truly short-process manufacturing. Furthermore, excessive addition of traditional lubricants to improve the flowability of recycled materials can cause new problems such as difficulty in debinding and high carbon residue. Therefore, how to endow difficult-to-process recycled materials with excellent rheological properties through innovative powder modification methods, so as to prepare high-density, defect-free high-performance magnets without the need for cold isostatic pressing, is a technical problem that urgently needs to be solved in the field of rare earth permanent magnet manufacturing. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a non-isostatic pressing rare earth permanent magnet based on gadolinium-yttrium activated recycled powder and its preparation method. This solves the problems of easy oxidation, low sintering density, and significant decrease in magnetic properties that occur during the direct utilization of existing neodymium-iron-boron recycled materials. It also solves the problems of long production processes and low efficiency caused by the reliance on cold isostatic pressing in traditional powder metallurgy processes.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a rare-earth permanent magnet without isostatic pressing based on gadolinium-yttrium activated and recovered powder, and its preparation method, comprising the following steps:

[0007] In a first aspect, the present invention provides a rare earth permanent magnet without isostatic pressing based on gadolinium-yttrium activated and recovered powder, employing the following technical solution:

[0008] A rare-earth permanent magnet without isostatic pressing based on gadolinium-yttrium activated and recovered powder is made from raw materials comprising the following weight percentages: 10.0-20.0 parts of sintered NdFeB grinding and processing recovered powder; 2.0-5.0 parts of gadolinium-yttrium master alloy; and 78.0-85.0 parts of rapidly solidified cast sheet. The raw materials also contain a recovered material pre-wetting agent and an online composite lubricant for air jet milling. The amount of the recovered material pre-wetting agent added is 0.2%-0.4% of the weight of the sintered NdFeB grinding and processing recovered powder; the amount of the online composite lubricant for air jet milling is 0.05%-0.10% of the total weight of the raw material powder.

[0009] Preferably, the magnet is made from raw materials comprising the following weight percentages: 15.0-20.0 parts of sintered NdFeB grinding and processing recycled powder; 2.0-3.0 parts of gadolinium-yttrium master alloy; and 78.0-82.0 parts of rapidly solidified cast sheets. By adopting the above technical solution, while maximizing the utilization of waste materials to reduce costs, the proportion of gadolinium-yttrium master alloy is adjusted to balance remanence and coercivity, ensuring that the overall performance of the magnet meets the high-performance grade standard.

[0010] Preferably, the pre-wetting agent for the recycled material is an emulsion formed by anhydrous ethanol and liquid paraffin, wherein the volume ratio of anhydrous ethanol to liquid paraffin is 6-10:1. By adopting the above technical solution, anhydrous ethanol is used as a dispersion carrier to highly disperse the liquid paraffin into an emulsion. During the mixing process, the high volatility of ethanol drives the liquid paraffin to quickly penetrate into the interior of the recycled powder agglomerates. As the ethanol evaporates, the liquid paraffin leaves a uniform and extremely thin antioxidant oil film on the surface of the powder, avoiding the local agglomeration phenomenon caused by directly adding paraffin.

[0011] Preferably, the online composite lubricant for the air jet mill is made of oleic acid and zinc stearate, wherein the weight ratio of zinc stearate to oleic acid is 1:1-1.2, and the online composite lubricant is a semi-transparent viscous liquid or a clear transparent liquid formed by dissolving zinc stearate in hot oleic acid and then cooling. By adopting the above technical solution, a homogeneous system formed by the thermal dissolution of zinc stearate with oleic acid is used, combining the steric hindrance effect of the long chain of oleic acid with the metal soap lubricating properties of zinc stearate. Under the high-pressure airflow of the air jet mill, this liquid system is atomized into micron-sized droplets, which are uniformly adsorbed onto the powder surface. Zinc stearate acts as an internal lubricant to reduce interparticle friction, while oleic acid acts as a dispersant to prevent agglomeration. The synergistic effect of the two significantly improves the flowability of the fine powder (manifested as a reduced angle of repose), which is the key material basis for achieving isostatic pressing-free molding.

[0012] Secondly, the present invention provides a method for preparing a rare earth permanent magnet without isostatic pressing using gadolinium-yttrium activated and recovered powder, employing the following technical solution:

[0013] A method for preparing a rare earth permanent magnet without isostatic pressing of gadolinium-yttrium activated and recovered powder includes the following steps: mixing a quick-solidified casting sheet with a gadolinium-yttrium master alloy, loading it into a hydrogen crushing furnace for hydrogen crushing and dehydrogenation, and obtaining fresh coarse powder.

[0014] The weighed sintered NdFeB grinding and recycling powder is placed in a powder mixing equipment, and the recycled material pre-wetting agent is sprayed in for wetting treatment. Then the fresh coarse powder is added and mixed to obtain mixed coarse powder.

[0015] The mixed coarse powder was ground using an air jet mill under inert gas protection. During the grinding process, the online composite lubricant of the air jet mill was dynamically injected into the grinding chamber through a nozzle to obtain modified fine powder.

[0016] The modified fine powder is pressed into a green body under an orientation magnetic field and molding pressure, and the green body is directly packaged without cold isostatic pressing treatment;

[0017] The packaged green blanks are sent into a vacuum sintering furnace, where they are first degassed under negative pressure, then heated to high vacuum for sintering, and finally cooled and aged to obtain isostatically pressed rare earth permanent magnets made from gadolinium-yttrium activated and recovered powder.

[0018] By adopting the above technical solution, the core of this invention lies in overcoming the problem that doped recycled material powder cannot be used in the isostatic pressing process. Its key innovative mechanism is as follows:

[0019] In-situ modification of gas-solid two-phase flow achieves isostatic pressing-free mechanism: Addressing the pain point of poor flowability of recycled materials, this invention abandons the traditional post-mixing lubrication method and innovatively establishes an online injection composite lubricant technology in air jet mills. Within the grinding chamber of the air jet mill, high-speed airflow atomizes a specific oleic acid-zinc stearate composite lubricant into micron-sized droplets. Utilizing the high activity of the fresh surface generated during the instantaneous crushing of alloy particles, instantaneous adsorption and in-situ coating of the lubricant are achieved. This crushing-coating mode minimizes the van der Waals forces and internal friction coefficient between particles, endowing the ultrafine powder with excellent fluid-like flow characteristics (angle of repose <35°). This allows the powder to achieve efficient rearrangement and close packing of particles with only low molding pressure during magnetic field orientation molding, resulting in uniform internal density of the green body, thus eliminating the dependence on secondary densification by cold isostatic pressing at the physical level.

[0020] Negative pressure gradient degassing mechanism to solve carbon residue: To achieve isostatic pressing-free magnets, sufficient composite lubricant must be introduced to ensure fluidity, but this introduces the potential risk of carbon residue. This invention utilizes the pore connectivity stage before sintering densification to construct a specific 20-50 Pa negative pressure gradient environment. This pressure gradient acts as a driving force, enabling small molecule gases generated by the thermal decomposition of organic components such as oleic acid, paraffin, and zinc stearate to overcome diffusion resistance and be directionally discharged from deep within the green body. This solves the problem of excessive carbon content caused by increased lubricant usage and ensures the microstructural purity of the isostatic pressing-free magnets.

[0021] Interface collaborative repair mechanism: Based on solving the molding process problem, by introducing gadolinium-yttrium master alloy and pre-wetting process, it is mainly used to compensate for the loss of grain boundary phase caused by the early process of recycled materials, so as to ensure that the magnetic properties of the magnet are not degraded under short process manufacturing.

[0022] Preferably, the specific process for hydrogen breaking and dehydrogenation is as follows: hydrogen is absorbed at a hydrogen pressure of 0.08-0.12 MPa, followed by dehydrogenation at a temperature of 500-600°C and a vacuum degree of less than 10 Pa for 3-5 hours. By adopting the above technical solution, the appropriate hydrogen absorption pressure ensures the full bursting of the main phase grains, while the high-temperature and high-vacuum dehydrogenation ensures the complete decomposition of hydrides, avoiding hydrogen-induced cracks, and providing the air jet mill with coarse powder raw materials of suitable particle size and good brittleness.

[0023] Preferably, the grinding pressure of the air jet mill is controlled at 0.59-0.61 MPa, and the average particle size D50 of the fine powder is adjusted to 4.1-4.5 μm. By adopting the above technical solution, the powder particle size is controlled within the single-domain particle size range (3-5 μm), and with the preset grinding pressure, the online injected lubricant can achieve the best dispersion coverage, avoiding insufficient or excessive lubricant usage due to excessive specific surface area.

[0024] Preferably, the orientation magnetic field strength for the magnetic field forming is 1.8-2.2T, and the forming pressure is 60-80MPa. By adopting the above technical solution, the friction of the powder is significantly reduced after online lubrication modification, allowing for close packing at a relatively low forming pressure (60-80MPa). Combined with strong magnetic field orientation, the particle orientation degree inside the green body is high, and due to uniform pressure transmission, the density gradient inside the green body is small, eliminating the need for subsequent cold isostatic pressing to relieve stress.

[0025] Preferably, the specific process for the degassing treatment is as follows: heating to 450-550℃ at a rate of 3-5℃ / min, controlling the furnace pressure at 20-50Pa, and holding at that temperature for 2-3 hours. By adopting the above technical solution, this temperature range covers the thermal decomposition temperature range of oleic acid and zinc stearate. The negative pressure environment of 20-50Pa creates a gas diffusion channel. If the pressure is too low (high vacuum), the surface of the green body is prone to premature sintering and sealing of the exhaust channel; if the pressure is too high, the resistance to gas escape is large. This preset pressure range achieves a balance between degassing efficiency and the structural integrity of the green body, effectively controlling the carbon content of the final magnet at a low level.

[0026] Preferably, the specific sintering conditions are: temperature 1040-1060℃, vacuum degree less than 0.1 Pa, and holding time 3-4 hours; the aging treatment includes: primary aging at 630-680℃ and secondary aging at 360-430℃. By adopting the above technical solution, high-vacuum sintering promotes the densification of the magnet, and the secondary aging treatment optimizes the distribution of grain boundary phases, so that the rare earth-rich phase is continuously distributed in thin layers between the main phase grains, cutting off the magnetic exchange coupling effect and maximizing the magnetic properties of the material.

[0027] This invention provides a non-isostatic pressing rare-earth permanent magnet based on gadolinium-yttrium activated and recovered powder, and its preparation method. It possesses the following beneficial effects:

[0028] 1. This invention realizes the high-value recycling of powders scattered from molding presses. By using a pre-wetting process to treat the surface of the recycled material with hydrophobicity, environmental oxidation is blocked. In addition, the grain boundary regulation effect of gadolinium-yttrium heavy rare earth alloy is used to repair the interruption of grain boundary continuity caused by the introduction of recycled material. While reducing the cost of raw materials, it ensures that key magnetic performance indicators such as coercivity and rectangularity ratio of the magnet are maintained at a high performance level. This solves the technical contradiction of cost advantage and performance quality that is difficult to balance in traditional recycling processes.

[0029] 2. This invention revolutionizes the traditional rare earth permanent magnet manufacturing process. By using online oil injection technology in air jet mills to improve the rheological properties of ultrafine powders, it successfully achieves a short-process manufacturing of isostatic pressing without cooling. The composite lubricant completes in-situ coating at the moment of particle crushing, which greatly reduces the internal friction coefficient between powder particles. This allows the powder to obtain high orientation and uniform density that meet sintering requirements during the magnetic field molding stage. This not only eliminates the cumbersome isostatic pressing process, greatly shortens the production cycle and reduces energy consumption, but also eliminates the risk of green blank cracking and deformation caused by pressing density gradient, thus improving the process yield.

[0030] 3. This invention solves the problem of carbon residue caused by the addition of lubricant in the non-isostatic pressing process by optimizing the negative pressure degassing process. By utilizing the negative pressure gradient environment before sintering densification, a directional diffusion channel for organic decomposition gas is established, ensuring that long-chain organic lubricant is completely extracted and discharged before the matrix is ​​closed. This avoids the formation of non-magnetic rare earth carbides and the destruction of grain boundary phases, reduces the carbon and oxygen impurity content in the final magnet, and ensures the purity of the magnet microstructure and effective magnetic isolation between grains. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments and comparative examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0032] Preparation Examples 1-3:

[0033] Preparation Example 1:

[0034] This preparation example provides a method for preparing a pre-wetting agent for recycled materials, including the following steps:

[0035] Measure 800 mL of anhydrous ethanol into a glass container. While stirring magnetically at 600 rpm, slowly add 100 mL of liquid paraffin. Continue stirring for 15 minutes, using mechanical shear force to disperse the liquid paraffin in the ethanol until a homogeneous milky white emulsion is formed. Seal and store. Shake or stir again before use. In this preparation example, the volume ratio of anhydrous ethanol to liquid paraffin is 8:1.

[0036] Preparation Example 2:

[0037] This preparation example provides a method for preparing an online composite lubricant A for an air jet mill, comprising the following steps:

[0038] 1000g of oleic acid was added to a stainless steel reactor equipped with a heating mantle and a stirrer. The temperature was heated and controlled to 65°C. While stirring at 250 rpm, a total of 1000g of zinc stearate powder was added in three batches, with a 10-minute interval between each addition to prevent agglomeration. After all additions were completed, the mixture was stirred continuously at a constant temperature for 40 minutes. The zinc stearate powder was completely dissolved using the thermal solubility of oleic acid, resulting in a semi-transparent, viscous liquid. Heating was stopped, and the mixture was allowed to cool naturally to room temperature while continuing to stir. The solution was then passed through a 200-mesh sieve to remove any trace amounts of insoluble particles, yielding the final product. In this preparation example, the weight ratio of zinc stearate to oleic acid was 1:1.

[0039] Preparation Example 3:

[0040] This preparation example provides a method for preparing an online composite lubricant B for air jet milling, comprising the following steps:

[0041] 1200g of oleic acid was added to a stainless steel reactor equipped with a heating mantle and a stirrer. The oil temperature was heated and controlled to 60°C. 1000g of zinc stearate powder was slowly added while stirring at 300rpm. The mixture was stirred continuously at a constant temperature for 45 minutes until the solution became clear and transparent. Heating was then stopped, and the mixture was allowed to cool naturally to room temperature. In this preparation example, the weight ratio of zinc stearate to oleic acid was 1:1.2.

[0042] Examples 1-5:

[0043] Example 1: This example provides a non-isostatic pressing rare earth permanent magnet based on gadolinium-yttrium activated and recovered powder and its preparation method, including the following steps:

[0044] Weigh out 15.0 parts by weight of sintered NdFeB grinding recovery powder, 3.0 parts by weight of gadolinium-yttrium master alloy and 82.0 parts by weight of rapid solidification casting sheet; place the weighed grinding recovery powder in a powder mixer, spray in 0.3% by weight of the recovery material pre-wetting agent obtained in Preparation Example 1, mix evenly and then load it into the hydrogen crushing furnace together with the gadolinium-yttrium master alloy and rapid solidification casting sheet.

[0045] After evacuation, hydrogen gas was introduced, and the mixture was fully dehydrogenated under a hydrogen pressure of 0.10 MPa. Then, the mixture was heated to 550°C and dehydrogenated for 4 hours under a vacuum of less than 10 Pa. After cooling, coarse powder was obtained. The coarse powder was then ground using a fluidized bed jet mill under nitrogen protection. The grinding pressure was controlled at 0.60 MPa, the powder output speed was 330 kg / h, and the speed of the classifier wheel was adjusted to make the fine powder D50 4.3 μm. During the grinding process, 0.08% of the total powder weight of the online composite lubricant A obtained in Preparation Example 2 was dynamically injected into the grinding chamber through a nozzle.

[0046] The obtained fine powder was fed into a magnetic field forming machine under nitrogen protection and pressed into a green body under a 2.0T orientation magnetic field and a forming pressure of 70MPa. The green body was directly packaged without cold isostatic pressing. The packaged green body was sent to a vacuum sintering furnace, first heated to 500℃ at 4℃ / min, and held at a negative pressure of 30Pa for 2.5 hours for degassing. Then, it was heated to 1050℃ and sintered for 3.5 hours under a vacuum of less than 0.1Pa, followed by gas quenching. Finally, the sintered body was held at 650℃ for 3 hours for first-stage aging, air-cooled, and then held at 400℃ for 3.5 hours for second-stage aging. Finally, it was rapidly cooled to room temperature to obtain the final product.

[0047] Example 2: This example provides a non-isostatic pressing rare earth permanent magnet based on gadolinium-yttrium activated and recovered powder and its preparation method, including the following steps:

[0048] Weigh 20.0 parts by weight of sintered NdFeB grinding recovery powder, 2.0 parts by weight of gadolinium-yttrium master alloy and 78.0 parts by weight of rapid solidification casting sheet; place the weighed grinding recovery powder in a powder mixer, spray in 0.4% by weight of the recovery material pre-wetting agent obtained in Preparation Example 1, mix evenly and then load it into the hydrogen crushing furnace together with the gadolinium-yttrium master alloy and rapid solidification casting sheet.

[0049] After evacuation, hydrogen gas was introduced, and the mixture was fully dehydrogenated under a hydrogen pressure of 0.12 MPa. Then, the mixture was heated to 600℃ and dehydrogenated for 5 hours under a vacuum of less than 10 Pa. After cooling, coarse powder was obtained. The coarse powder was then ground using a fluidized bed jet mill under nitrogen protection. The grinding pressure was controlled at 0.60 MPa, the powder output speed was 310 kg / h, and the speed of the classifier wheel was adjusted to make the fine powder D50 4.2 μm. During the grinding process, 0.10% of the total powder weight of the online composite lubricant A obtained in Preparation Example 2 was dynamically injected into the grinding chamber through a nozzle.

[0050] The obtained fine powder was fed into a magnetic field forming machine under nitrogen protection and pressed into a green body under a 2.2T orientation magnetic field and a forming pressure of 80MPa. The green body was directly packaged without cold isostatic pressing. The packaged green body was sent into a vacuum sintering furnace, first heated to 520℃ at 3℃ / min, and held for 3 hours under a negative pressure of 40Pa in the furnace for degassing treatment. Then, it was heated to 1055℃ and sintered for 4 hours under a vacuum of less than 0.1Pa, followed by gas quenching. Finally, the sintered body was held at 660℃ for 3 hours for first-stage aging, air-cooled, and then held at 410℃ for 3 hours for second-stage aging. Finally, it was rapidly cooled to room temperature to obtain the final product.

[0051] Example 3: This example provides a non-isostatic pressing rare earth permanent magnet based on gadolinium-yttrium activated and recovered powder and its preparation method, including the following steps:

[0052] Weigh out 10.0 parts by weight of sintered NdFeB grinding recovery powder, 5.0 parts by weight of gadolinium-yttrium master alloy and 85.0 parts by weight of rapid solidification casting sheet; place the weighed grinding recovery powder in a powder mixer, spray in 0.2% by weight of the recovery material pre-wetting agent obtained in Preparation Example 1, mix evenly and then load it into the hydrogen crushing furnace together with the gadolinium-yttrium master alloy and rapid solidification casting sheet.

[0053] After evacuation, hydrogen gas was introduced, and the mixture was fully dehydrogenated under a hydrogen pressure of 0.08 MPa. Then, the mixture was heated to 500°C and dehydrogenated for 3 hours under a vacuum of less than 10 Pa. After cooling, coarse powder was obtained. The coarse powder was then ground using a fluidized bed jet mill under nitrogen protection. The grinding pressure was controlled at 0.59 MPa, the powder output speed was 350 kg / h, and the speed of the classifier wheel was adjusted to make the fine powder D50 4.5 μm. During the grinding process, 0.05% of the total powder weight of the online composite lubricant B obtained in Preparation Example 3 was dynamically injected into the grinding chamber through a nozzle.

[0054] The obtained fine powder was fed into a magnetic field forming machine under nitrogen protection and pressed into a green body under a 1.8T orientation magnetic field and a forming pressure of 60MPa. The green body was directly packaged without cold isostatic pressing. The packaged green body was sent to a vacuum sintering furnace, first heated to 450℃ at 5℃ / min, and held at a negative pressure of 20Pa in the furnace for 2 hours for degassing treatment. Then, it was heated to 1040℃ and sintered for 3 hours under a vacuum of less than 0.1Pa, followed by gas quenching. Finally, the sintered body was held at 630℃ for 3.5 hours for first-stage aging, air-cooled, and then held at 360℃ for 4 hours for second-stage aging. Finally, it was rapidly cooled to room temperature to obtain the final product.

[0055] Example 4: This example provides a non-isostatic pressing rare earth permanent magnet based on gadolinium-yttrium activated and recovered powder and its preparation method, including the following steps:

[0056] Weigh out 15.0 parts by weight of sintered NdFeB grinding and recycling powder, 3.0 parts of gadolinium-yttrium master alloy, and 82.0 parts of rapid solidification casting sheet; the raw material pretreatment and hydrogen crushing steps are the same as in Example 1; the coarse powder is ground using a fluidized bed jet mill under nitrogen protection, with the grinding pressure controlled at 0.61 MPa, the powder output speed at 320 kg / h, and the classifier wheel speed adjusted to a fine powder D50 of 4.1 μm. During the grinding process, 0.08% of the total powder weight of the online composite lubricant B obtained in Preparation Example 3 is dynamically injected into the grinding chamber through a nozzle; the molding and subsequent processes are the same as in Example 1, except that the sintering temperature is adjusted to 1045℃.

[0057] Example 5: This example provides a non-isostatic pressing rare earth permanent magnet based on gadolinium-yttrium activated and recovered powder and its preparation method, including the following steps:

[0058] Weigh out 15.0 parts by weight of sintered NdFeB grinding and recycling powder, 3.0 parts of gadolinium-yttrium master alloy, and 82.0 parts of rapid solidification casting sheet; the raw material pretreatment, hydrogen crushing, and powder preparation processes are the same as in Example 1; the obtained fine powder is fed into a magnetic field forming machine under nitrogen protection and pressed into a green blank under a 2.0T orientation magnetic field and a forming pressure of 75MPa. The green blank is directly packaged without cold isostatic pressing treatment; the packaged green blank is sent into a vacuum sintering furnace, first heated to 550℃ at 3℃ / min, held at a negative pressure of 50Pa in the furnace for 2 hours for degassing treatment, then heated to 1060℃ and sintered for 3 hours under a vacuum of less than 0.1Pa, and then gas quenched; finally, the sintered body is held at 680℃ for 2.5 hours for first-stage aging, air-cooled, held at 430℃ for 3 hours for second-stage aging, and then rapidly cooled to room temperature to obtain the final product.

[0059] Comparative Examples 1-5:

[0060] Comparative Example 1:

[0061] Compared with Example 1, the difference is that the online oiling step is cancelled in the air jet milling process. Instead, after the air jet milling is completed, the fine powder and an equal amount of zinc stearate / oleic acid mixture are placed in a powder mixer and mixed evenly. In the molding step, the green body after magnetic pressing is subjected to an additional 200MPa cold isostatic pressing densification process before being packaged and sintered. The remaining raw material ratios and process steps are the same.

[0062] Comparative Example 2:

[0063] Compared with Example 1, the difference is that the online oiling step is cancelled in the air jet milling process. Instead, after the air jet milling is completed, the fine powder and an equal amount of zinc stearate / oleic acid mixture are placed in a powder mixer and mixed evenly. The rest are the same (i.e. the green body does not undergo cold isostatic pressing).

[0064] Comparative Example 3:

[0065] Compared with Example 1, the difference is that the sintered NdFeB grinding and recycling powder is not pre-wetting treated, but directly mixed with gadolinium-yttrium master alloy and rapid solidification casting sheet in proportion and loaded into the hydrogen crushing furnace; all other aspects are the same.

[0066] Comparative Example 4:

[0067] Compared with Example 1, the difference is that the negative pressure degassing section at 500°C and 30Pa is omitted in the sintering process, and the green billet is directly heated to 1050°C at a heating rate of 5°C / min for high vacuum sintering after being loaded into the furnace; all other aspects are the same.

[0068] Comparative Example 5:

[0069] Compared with Example 1, the difference is that the rotation speed of the classifying wheel of the air jet mill was adjusted to control the fine powder D50 to 5.2μm (which exceeds the range of 4.1-4.5μm defined in this invention); all other aspects are the same.

[0070] Test Examples 1-4:

[0071] Test Example 1: Powder Flow Characteristics and Green Body Density Test

[0072] This test case aims to verify the effect of the online oil injection modification process of air jet mill on the microscopic physical state of powder and to evaluate its contribution to the density of green bodies without isostatic pressing. The air jet mill fine powders prepared in Examples 1-5 and Comparative Examples 1-3, as well as the green bodies (before sintering) after magnetic field forming, were selected as test objects.

[0073] Experimental steps:

[0074] Inside a nitrogen-protected glove box, 500g powder samples were randomly selected from each group of air-jet mill powder collection tanks and tested using a Hogarth powder property tester. The angle of repose of the powder was determined using the injection method. Each sample was tested three times, and the arithmetic mean was recorded.

[0075] A quantitative amount of powder sample is placed into a graduated cylinder of calibrated volume and subjected to 3000 vibrations using a tap density meter to determine its tap density.

[0076] For the green sample that has been oriented and pressed by a magnetic field but has not yet been sintered (for Comparative Example 1, it has not yet been cold isostatically pressed), its geometric dimensions (length, width, and height) are measured using a high-precision vernier caliper, and it is weighed using a precision electronic balance. The density of the green sample is calculated using the mass / volume formula.

[0077] Observe the appearance of the green body and record whether there is powdering, cracks or delamination.

[0078] Test results:

[0079] Table 1. Test results of physical properties of powder and green body density for each group

[0080]

[0081] Results analysis:

[0082] Analysis of the data in Table 1 shows that the powder repose angles of Examples 1-5 are concentrated in the range of 31.8°-33.5°, lower than the 46°-47° range of Comparative Examples 1 and 2. This indicates that the online oil injection process of the air jet mill used in this invention utilizes high-speed turbulent airflow to uniformly coat the atomized lubricant onto the fresh surface generated by powder breakage, significantly reducing the van der Waals forces and friction coefficient between particles. In contrast, Comparative Examples 1 and 2, which use mechanical mixing of lubricant after air jet milling, struggle to achieve monodisperse coating of micron-sized particles, resulting in severe powder agglomeration and poor flowability.

[0083] Differences in powder flowability directly affect the particle rearrangement efficiency during the molding process. The green body density of the example group generally reached 4.12 g / cm³. 3 The above-mentioned tap density, close to that of the powder, indicates that during magnetic field orientation and molding, well-lubricated particles effectively filled the pores, achieving a high initial density. This consistent density level meets the critical requirements for sintering densification, eliminating the need for subsequent cold isostatic pressing.

[0084] In contrast, in Comparative Example 2, the lack of effective in-situ lubrication resulted in high frictional resistance between powder particles and uneven pressure transmission during molding, leading to a green body density of only 3.65 g / cm³. 3 Furthermore, the presence of microcracks confirms that directly eliminating the isostatic pressing process without online modification technology will lead to structural defects in the green body. Although Comparative Example 1 had a low green body density, its density could be forcibly increased through subsequent cold isostatic pressing (CIP), indirectly demonstrating the dependence of traditional processes on CIP. The angle of repose of Comparative Example 3 was slightly higher than that of the Example, presumably because the recycled material was not pre-wetted, and the difference in the state of the surface oxide layer affected the subsequent adsorption effect of the lubricant. In summary, the online oil injection technology of air jet mill improves the rheological properties of powder and is the physical basis for achieving high-density isostatic pressing-free molding.

[0085] Test Example 2: Impurity Content Analysis Test

[0086] This test aims to quantitatively analyze the carbon, oxygen, and hydrogen impurity content in sintered magnets to verify the contribution of the raw material pre-wetting process to antioxidant properties and the effectiveness of the negative pressure degassing process in removing organic additives. The final sintered magnets from Examples 1-5 and Comparative Examples 3 and 4 were selected as test subjects.

[0087] Experimental steps:

[0088] Each group of sintered magnet samples was cut into granular specimens with a size of approximately 3mm × 3mm × 3mm using a diamond wire cutter, and three parallel samples were prepared for each group of samples.

[0089] The cut particle sample was placed in a beaker containing anhydrous ethanol and cleaned in an ultrasonic cleaner for 10 minutes to remove the cutting fluid and oil adhering to the surface. After removal, it was placed in a vacuum drying oven at 80°C for 2 hours to dry.

[0090] The carbon content of the sample was determined using a high-frequency infrared carbon-sulfur analyzer. Approximately 0.5g of the sample was weighed, and pure iron flux and tungsten-tin flux were added. The sample was then burned in a high-frequency induction furnace, and the amount of CO2 released was measured using an infrared detector to calculate the carbon content.

[0091] The oxygen and hydrogen content of the sample was determined using an oxygen, nitrogen, and hydrogen analyzer. Approximately 0.1 g of the sample was weighed and placed in a graphite crucible, then melted at high temperature in a pulse furnace. Oxygen reacted with graphite to produce CO, which was detected by infrared spectroscopy, while hydrogen was measured using a thermal conductivity detector.

[0092] Record the arithmetic mean of each group of data.

[0093] Test results:

[0094] Table 2. Test results of impurity element content in sintered magnets

[0095]

[0096] Results analysis:

[0097] According to the data in Table 2, the carbon content of Comparative Example 4 is as high as 1458 ppm, which is much higher than the 655-712 ppm range of Examples 1-5. This difference confirms the necessity of the negative pressure degassing process in the isostatic pressing-free technology route. This invention introduces long-chain organic compounds (oleic acid, zinc stearate, and liquid paraffin) during powder preparation. If high-vacuum sintering is directly performed (as in Comparative Example 4), the pore-closing rate is too fast in the early stages of sintering, resulting in carbon residues generated by the decomposition of organic compounds remaining in the matrix and unable to be discharged. However, the 450-550℃ negative pressure degassing section used in the examples, utilizing the matrix still in the interconnected pore stage and combined with micro-negative pressure airflow suction, effectively removes most of the organic residues.

[0098] Regarding the oxygen content, Comparative Example 3 showed a value of 2845 ppm, approximately twice that of Example 1 (1420 ppm). Comparative Example 3 did not pre-wet the recycled material, indicating that the powder escaping from the molding press is highly susceptible to secondary oxidation during subsequent mixing and hydrogen crushing. In the examples, pre-wetting with ethanol-liquid paraffin created a hydrophobic protective film on the surface of the recycled powder, blocking contact between ambient oxygen and adsorbed water, thus controlling the oxygen content of the final magnet within the range required for high-performance magnets.

[0099] Furthermore, the hydrogen content of Comparative Example 4 (18.5 ppm) was significantly higher than that of the Example Group (approximately 6 ppm). High residual hydrogen can lead to increased magnet brittleness and lattice distortion. The negative pressure dehydrogenation process also serves to assist in dehydrogenation, ensuring that hydrogen absorbed during hydrogen crushing and hydrogen generated from the decomposition of organic matter can be completely discharged. Considering all impurity indicators, the process parameters of this invention achieve the goal of maintaining the purity of the magnet composition while utilizing waste materials.

[0100] Test Example 3: Magnetic Property Test

[0101] This test case aims to comprehensively evaluate the actual performance of the process of the present invention under different raw material ratios by measuring the key magnetic properties of the final sintered magnet, and to compare the impact of different process paths on magnet performance. Sintered magnets prepared in Examples 1-5 and Comparative Examples 1-5 were selected as test objects.

[0102] Experimental steps:

[0103] The sintered magnet blanks of each group were processed into cylindrical standard specimens with a diameter of 10.0 mm and a height of 10.0 mm using a wire EDM machine. During the processing, the dimensional tolerances were strictly controlled, and the upper and lower end faces of the specimens were ground and polished to ensure that the parallelism of the end faces was within 0.01 mm.

[0104] The processed sample is placed in a pulsed magnetic field magnetizer and saturated magnetized by applying a 5T pulsed magnetic field at room temperature to ensure that the magnetic domains inside the magnet are completely oriented along the easy magnetization axis.

[0105] Using the NIM-10000 high-temperature permanent magnet measuring instrument system, the closed-circuit magnetic properties of the magnetized sample were tested in a constant temperature environment of 20℃. The demagnetization curve was recorded, and the software automatically calculated the remanence (Br), intrinsic coercivity (Hcj), maximum energy product ((BH)max), and knee magnetic field (Hk). The rectangle ratio of the hysteresis loop was calculated based on Hk / Hcj.

[0106] Five samples were tested in each group of samples. The maximum and minimum values ​​were removed, and the arithmetic mean was taken as the final recorded data.

[0107] Test results:

[0108] Table 3. Room temperature magnetic property test data of sintered magnets

[0109]

[0110] Results analysis:

[0111] According to the data in Table 3, Example 1, using 15% recycled material and completely eliminating the cold isostatic pressing (CIP) process, exhibited remanent magnetization (Br) (13.62 kGs) and a rectangularity ratio (0.97) that were essentially on par with Comparative Example 1, which employed a traditional CIP long-process. This strongly demonstrates that the online composite lubrication modification technology proposed in this invention is key to achieving isostatic pressing-free operation. The uniform coating layer formed through online injection significantly improves the rearrangement ability of the powder within the mold cavity, enabling Example 1 to achieve green density and uniformity comparable to that after 200 MPa cold isostatic pressing, relying solely on molding. In contrast, Comparative Example 2, lacking effective online lubrication modification and directly eliminating CIP, resulted in a sharp drop in the rectangularity ratio to 0.65, further demonstrating the decisive role of the lubrication modification process described in this invention in achieving isostatic pressing-free operation.

[0112] The magnetic properties of Comparative Example 2 (without online oiling and CIP) showed a precipitous drop, especially with a rectangularity ratio of only 0.65 and remanence decreasing to 11.23 kGs. This is mainly attributed to excessive friction between powder particles, making it difficult for particles to tumble during magnetic field orientation, resulting in disordered c-axis orientation. Furthermore, due to the low density of the green body, it failed to achieve complete densification after sintering, resulting in a large number of internal pores.

[0113] The coercivity Hcj of Comparative Example 3 (without pre-wetting) was 18.25 kOe, significantly lower than the 21.45 kOe of Example 1. Because the recycled material was not pre-wetted using an alcohol-oil system, its high surface oxygen content consumed rare earth elements (especially expensive heavy rare earth Gd / Y) in the rare earth-rich phase during sintering, leading to a reduction in grain boundary phases and a weakening of grain decoupling effect, thus lowering the coercivity.

[0114] In Comparative Example 4 (without negative pressure degassing), the rectangularity ratio decreased to 0.84, and the coercivity decreased to 16.48 kOe. Combined with Test Example 2, it can be seen that this is because the residual carbon reacts with rare earth elements to form non-magnetic rare earth carbides, or carbon elements segregate at the grain boundaries, which disrupts the continuity of the grain boundary phase and has a negative effect of pinning the reversed domain nuclei.

[0115] Although Example 3 used a relatively high proportion of heavy rare earth alloy (5%), the coercivity increased to 23.68 kOe due to the introduction of 10% recycled material. While the remanence decreased slightly (12.85 kGs) due to the substitution of light rare earth by heavy rare earth and the influence of recycled material, it still maintained an excellent rectangularity ratio (0.98), indicating that the process system of this invention has good universality for different formulations. In summary, this invention, through the synergy of multiple processes, significantly reduces manufacturing costs (CIP-free, utilizing recycled materials) while ensuring excellent overall magnetic properties of the magnet.

[0116] Test Example 4: Mechanical Properties and Appearance Yield Test

[0117] This test case aims to evaluate the practicality of the process of the present invention from the perspective of industrial production, focusing on whether the cold-free isostatic pressing process will introduce macroscopic defects, and whether the addition of recycled materials will weaken the mechanical strength of the magnet. Example 1, Comparative Example 1 (conventional CIP process), and Comparative Example 2 (unmodified CIP-free) were selected as the main test objects.

[0118] Experimental steps:

[0119] 200 magnet blanks that have undergone vacuum sintering and secondary aging treatment were randomly selected from each group as samples for appearance inspection. Visual inspection was carried out on a high-intensity lamp inspection table with the assistance of a magnifying glass. The number of samples with visible cracks (including microcracks), chipped corners, peeling, or delamination defects was counted, and the appearance yield was calculated.

[0120] Standard bending test specimens with dimensions of 3mm × 4mm × 25mm were cut from the samples that passed the appearance test, and 10 specimens were prepared for each group. Three-point bending strength tests were performed using a universal testing machine with a span of 20mm and an indenter loading speed of 0.5mm / min until the specimen broke. The maximum load at the time of fracture was recorded and the bending strength was calculated.

[0121] Based on the production of a single batch (50kg) of powder, the total time spent on each process from the completion of pressing by the magnetic field forming machine to the packaging and delivery into the sintering furnace was recorded. For Comparative Example 1, this time includes the time for vacuum sealing of the green body, cold isostatic pressing, and unpacking; for Example 1 and Comparative Example 2, it is the direct packaging time.

[0122] Test results:

[0123] Table 4. Comparison of Appearance Yield, Mechanical Properties and Process Time

[0124]

[0125] Results analysis:

[0126] According to the data in Table 4, Example 1, by omitting the cold isostatic pressing (CIP) process, achieved a 98.5% appearance yield, which is basically the same as Comparative Example 1 (99.0%) using the traditional CIP process. Furthermore, there was no statistically significant difference in the average flexural strength between the two examples (275.4 MPa and 278.1 MPa, respectively). This demonstrates that the present invention, through the synergistic effect of liquid-phase pre-wetting and online air jet milling modification, ensures uniform particle packing and consistent stress distribution within the green body, effectively eliminating the risk of sintering cracking caused by density gradients during pressing. The addition of recycled material did not damage the continuity and mechanical properties of the matrix, indicating that the purification and dispersion process effectively solved the brittleness problem caused by impurities easily introduced from recycled material.

[0127] Data from Comparative Example 2 shows that if CIP is directly eliminated without employing the modification technology of this invention, the yield rate is only 45.5%, and the flexural strength drops significantly to 185.2 MPa. This indicates that unmodified powder exhibits severe density inhomogeneity under simple molding, and the internal stress generated during sintering shrinkage leads to numerous macroscopic cracks and internal micro-defects. Regarding process time, the single-batch processing time in Example 1 is only 1.2 hours, compared to 5.8 hours in Comparative Example 1, reducing process flow time by nearly 80% and significantly lowering energy consumption and labor costs. Considering both yield rate and efficiency indicators, the solution of this invention achieves a balance between high efficiency and high quality, possessing industrial application value.

Claims

1. A rare-earth permanent magnet without isostatic pressing based on gadolinium-yttrium activated and recovered powder, characterized in that, Made from the following ingredients in parts by weight: Sintered NdFeB grinding and processing recycled powder: 10.0-20.0 parts; Gadolinium-yttrium master alloy: 2.0-5.0 parts; Rapid-setting cast sheets: 78.0-85.0 parts; The raw materials also contain recycled material pre-wetting agent and air jet mill online composite lubricant; The pre-wetting agent for the recycled material is a mixture of anhydrous ethanol and liquid paraffin, and the amount added is 0.2%-0.4% of the weight of the recycled powder from the grinding of sintered NdFeB metals. The online composite lubricant for the air jet mill is a mixture of oleic acid and zinc stearate, and the amount added is 0.05%-0.10% of the total weight of the raw material powder. The pre-wetting agent for the recycled material is an emulsion, wherein the volume ratio of anhydrous ethanol to liquid paraffin is 6-10:1; The weight ratio of zinc stearate to oleic acid in the online composite lubricant for air jet mills is 1:1-1.2, and the online composite lubricant for air jet mills is a semi-transparent viscous liquid or a clear transparent liquid formed by dissolving zinc stearate in hot oleic acid and then cooling it. Furthermore, the online composite lubricant for the air jet mill is dynamically injected into the grinding chamber through a nozzle during the grinding process.

2. The isostatic pressing-free rare earth permanent magnet of gadolinium-yttrium activated and recovered powder according to claim 1, characterized in that, Made from the following ingredients in parts by weight: Sintered NdFeB grinding and processing recycled powder: 15.0-20.0 parts; Gadolinium-yttrium master alloy: 2.0-3.0 parts; Rapidly solidifying cast sheets: 78.0-82.0 parts.

3. A method for preparing a rare-earth permanent magnet without isostatic pressing using gadolinium-yttrium activated and recovered powder, characterized in that, The method for preparing a non-isostatic rare-earth permanent magnet of gadolinium-yttrium activated recovery powder according to any one of claims 1-2 includes the following steps: The rapidly solidified cast sheets were mixed with gadolinium-yttrium master alloy and loaded into a hydrogen crushing furnace for hydrogen crushing and dehydrogenation to obtain fresh coarse powder. The weighed sintered NdFeB grinding and recycling powder is placed in a powder mixing equipment, and the recycled material pre-wetting agent is sprayed in for wetting treatment. Then the fresh coarse powder is added and mixed to obtain mixed coarse powder. The mixed coarse powder was ground using an air jet mill under inert gas protection. During the grinding process, the online composite lubricant of the air jet mill was dynamically injected into the grinding chamber through a nozzle to obtain modified fine powder. The modified fine powder is pressed into a green body under an orientation magnetic field and molding pressure, and the green body is directly packaged without cold isostatic pressing treatment; The packaged green blanks are sent into a vacuum sintering furnace, where they are first degassed under negative pressure, then heated to high vacuum for sintering, and finally cooled and aged to obtain isostatically pressed rare earth permanent magnets made from gadolinium-yttrium activated and recovered powder.

4. The method for preparing a rare earth permanent magnet without isostatic pressing from gadolinium-yttrium activated and recovered powder according to claim 3, characterized in that, The specific process for hydrogen destruction and dehydrogenation is as follows: Hydrogen is absorbed under a hydrogen pressure of 0.08-0.12 MPa, and then dehydrogenated for 3-5 hours under conditions of 500-600℃ and a vacuum of less than 10 Pa.

5. The method for preparing a rare earth permanent magnet without isostatic pressing from gadolinium-yttrium activated and recovered powder according to claim 3, characterized in that, The grinding pressure of the air jet mill is controlled at 0.59-0.61 MPa, and the average particle size D50 of the fine powder is adjusted to 4.1-4.5 μm.

6. The method for preparing a rare earth permanent magnet without isostatic pressing from gadolinium-yttrium activated and recovered powder according to claim 3, characterized in that, The orientation magnetic field strength for magnetic field forming is 1.8-2.2T, and the forming pressure is 60-80MPa.

7. The method for preparing a rare earth permanent magnet without isostatic pressing from gadolinium-yttrium activated and recovered powder according to claim 3, characterized in that, The specific process for the desiccation treatment is as follows: Heat to 450-550℃ at a rate of 3-5℃ / min, control the pressure inside the furnace at 20-50Pa, and hold for 2-3 hours.

8. The method for preparing a rare earth permanent magnet without isostatic pressing from gadolinium-yttrium activated and recovered powder according to claim 3, characterized in that, The specific conditions for sintering are as follows: Temperature 1040-1060℃, vacuum degree less than 0.1Pa, heat preservation for 3-4 hours; The timeliness processing includes: First-level aging at 630-680℃ and second-level aging at 360-430℃.

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