Sintering method for preparing neodymium-iron-boron magnet

Abstract

The invention discloses a sintering method for neodymium-iron-boron magnet preparation, and belongs to the technical field of neodymium-iron-boron magnet preparation. Pulsed magnetic field sintering is that a pulsed magnetic field is applied in the sintering process, and during auxiliary sintering, the pulsed magnetic field can generate multiple positive effects, on one hand, the sintering temperature can be reduced, and the sintering efficiency can be improved; the magnetic field can influence the diffusion process of atoms and ions in the material, so that the atoms can obtain enough energy to migrate at a lower temperature so as to promote the densification of the material, on the other hand, the pulsed magnetic field can refine crystal grains of the material, and the magnetic field plays a role in the movement process of a crystal boundary and the like, so that the growth of the crystal grains is inhibited, and the densification of the material is promoted. In the sintering process of the magnetic material, the advantages are obvious, and the magnetic performance of the material can be effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of NdFeB magnets, in particular to a sintering method for preparing NdFeB magnets. Background Art

[0002] NdFeB magnets are made of neodymium, iron, and boron (Nd2Fe 14 B) formed tetragonal crystals. The magnetic energy product of this magnet is greater than that of samarium cobalt magnets, and it was the material with the largest magnetic energy product in the world at that time. Later, Sumitomo Special Metals successfully developed powder metallurgy and General Motors successfully developed the rotary spray melting method to produce neodymium iron boron magnets. This magnet is the permanent magnet with magnetic properties second only to absolute zero holmium magnets, and is also the most commonly used rare earth magnet. Neodymium iron boron magnets are widely used in electronic products such as hard drives, mobile phones, headphones, and battery-powered tools.

[0003] In the prior art, the Chinese invention content of publication number: CN117334460A discloses a method for processing waste sintered NdFeB magnets. The preparation method comprises: degreasing and vibrating the waste sintered NdFeB magnets in sequence to obtain pre-processed waste; mixing the pre-processed waste with NdFeB magnet raw materials to obtain a mixture; vacuum melting and slicing the mixture to obtain NdFeB alloy sheets; hydrogen crushing the NdFeB alloy sheets to obtain coarse powder; mixing the coarse powder with an antioxidant, and then air-jet milling to obtain fine powder; mixing the fine powder with an additive to obtain a modified powder; orienting and pressing the modified powder to obtain a NdFeB compact; isostatic pressing the NdFeB compact to obtain a NdFeB green compact; sintering and tempering the NdFeB green compact in sequence to obtain a sintered NdFeB magnet. This invention improves the recycling efficiency of waste sintered NdFeB magnets and obtains high-performance magnets that can be used in different fields.

[0004] In the above technical solution, the microstructure of the NdFeB magnet is improved by sintering and tempering to enhance the magnetic properties of the magnet. However, the tempering method can only reduce the growth of grains but cannot stop their growth, which has little effect on improving the magnetic properties of the magnet. Summary of the invention

[0005] The object of the present invention is to provide a sintering method for preparing NdFeB magnets, XX, to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solution: a sintering method for preparing NdFeB magnets, comprising the following steps:

[0007] Step 1: Preparation before sintering

[0008] (1) Preparation of raw materials

[0009] Selecting main raw materials such as neodymium (Nd), iron (Fe), and boron (B), and mixing them according to a certain chemical measurement weight ratio;

[0010] (2) Smelting of raw materials

[0011] Put the prepared raw materials into a vacuum induction furnace or an electric arc furnace for smelting. During the smelting process, the temperature and time must be strictly controlled to fully melt the raw materials and mix them evenly to form an alloy liquid;

[0012] (3) Flour making

[0013] After the alloy is cooled, it is made into fine powder by mechanical crushing, air jet milling, etc.;

[0014] (4) Magnetic field orientation

[0015] Placing the powder in a strong magnetic field so that the easy magnetization axis of the powder particles is aligned along the direction of the magnetic field, this process can improve the remanence and magnetic energy product of the magnet;

[0016] (5) Pressing

[0017] The powder oriented by the magnetic field is placed in a mold and pressed into a three-dimensional shape under a certain pressure;

[0018] Step 2: Sintering

[0019] (1) Inspection and maintenance of equipment;

[0020] (2) Preparation for furnace loading;

[0021] The pressed green body is placed in a vacuum sintering furnace for sintering; the sintering temperature is usually around 1000-1100°C. At this temperature, the powder particles diffuse and fuse, increasing the density of the magnet and improving its performance.

[0022] (3) Pulse magnetic field sintering assistance

[0023] During the sintering process, a pulsed magnetic field is applied with a magnetic field strength of 2-5 Tesla and a pulse frequency of 5-20 Hz.

[0024] Step 3: Post-sintering treatment

[0025] Heat treatment: the sintered magnets are subjected to aging heat treatment, including tempering and other treatments;

[0026] Rapid cooling and cryogenic treatment: After sintering, rapid cooling is used, such as rapidly lowering the magnet from the sintering temperature and then performing cryogenic treatment.

[0027] Preferably, when preparing the raw materials in step 1, a small amount of other elements, such as dysprosium, terbium, etc., can be added to the raw materials such as neodymium (Nd), iron (Fe), boron (B) in a certain stoichiometric ratio. Generally, the content of neodymium is about 29%-35% (mass fraction), the content of boron is about 0.9%-1.2%, and the rest is mainly iron. The selection of raw materials requires metals and metal alloys with higher purity.

[0028] Preferably, in the step one, the ingredients weighed according to a certain ratio are added to the three-dimensional mixer, the mixing time is not less than 2 hours, and the three-dimensional mixer maintains a constant speed. At the same time, lubricants (such as zinc stearate) and additives can be added to the three-dimensional mixer to improve the molding and fluidity of the powder.

[0029] Preferably, in the step 1, during the compression molding stage, the honey is placed in a mold and molded by compression, and the pressure is usually around several hundred MPa, so that the powder becomes an embryo with a certain shape and density, and the density of the embryo generally reaches 50%-70% of the theoretical density.

[0030] Preferably, in the step 2, during the inspection and maintenance phase of the equipment, the vacuum system and the heating system need to be inspected. Vacuum system inspection: check whether the vacuum pump is working properly, including the mechanical pump and the diffusion pump (if any), and ensure that the vacuum system is well sealed and has no leaks. It can be detected by a helium mass spectrometer leak detector. For example, observe whether the reading of the vacuum gauge can reach the required vacuum degree within a reasonable time; heating system inspection: check whether the heating elements, such as graphite heaters or molybdenum wire heaters, are damaged. Debug the heating control cabinet to ensure that the temperature control accuracy is within the allowable range. Normally, the temperature control accuracy should reach ±5°C.

[0031] Preferably, in the furnace loading preparation stage of step 2, mold selection: select a suitable graphite or ceramic mold according to the shape and size of the NdFeB magnet, the mold must be able to withstand the sintering temperature and cannot chemically react with the magnet material. For example, for small block magnets, a simple square or round graphite mold can be selected; magnet blank placement: place the NdFeB magnet blanks to be sintered neatly in the mold, and keep a certain distance between the blanks to avoid mutual adhesion during the sintering process. At the same time, the uniformity of heating should be considered to ensure that the blanks are heated evenly in the furnace.

[0032] Preferably, during the sintering in step 2, it is necessary to set the heating rate, sintering temperature and holding time of the vacuum sintering furnace. Usually, the heating rate can be set to 10-20°C / min, the sintering temperature is about 1000-1100°C, and the holding time is 2-5 hours. At the same time, the sintering treatment can be carried out in a distributed sintering manner. First, the embryo is pre-sintered at a lower temperature (800-900°C) for 1-2 hours. At this stage, impurities such as lubricants in the embryo can be preliminarily removed, reducing the influence of volatiles on the quality of the magnet during subsequent high-temperature sintering; then the temperature is raised to the conventional sintering temperature (1050-1100°C) for formal sintering.

[0033] Preferably, in the post-sintering treatment process of step three, the heat treatment includes tempering treatment at a temperature of about 500-600° C. for about 1-3 hours.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. The pulse magnetic field sintering in the present invention is to apply a pulse magnetic field during the sintering process. When assisting sintering, the pulse magnetic field can produce a variety of positive effects. On the one hand, it can reduce the sintering temperature. This is because the magnetic field can affect the diffusion process of atoms and ions inside the material, allowing atoms to obtain sufficient energy to migrate at a lower temperature, thereby promoting the densification of the material. On the other hand, the pulse magnetic field can also refine the grains of the material. The magnetic field affects the movement of grain boundaries and other processes, which inhibits grain growth, thereby improving the mechanical properties and other comprehensive properties of the material. This advantage is more obvious in the sintering process of magnetic materials, and can effectively improve the magnetic properties of the material.

[0036] 2. The present invention uses rapid cooling and cryogenic treatment. Rapid cooling can fix the microstructure in the magnet, and cryogenic treatment can further improve the crystal structure and reduce internal stress, thereby improving the comprehensive performance of the magnet. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 The figure is a flow chart of the sintering method of the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] The present invention provides Figure 1 A sintering method for preparing a NdFeB magnet is shown, comprising the following steps:

[0040] Step 1: Preparation before sintering

[0041] (1) Preparation of raw materials

[0042] Selecting main raw materials such as neodymium (Nd), iron (Fe), and boron (B), and mixing them according to a certain chemical measurement weight ratio;

[0043] (2) Smelting of raw materials

[0044] Put the prepared raw materials into a vacuum induction furnace or an electric arc furnace for smelting. During the smelting process, the temperature and time must be strictly controlled to fully melt the raw materials and mix them evenly to form an alloy liquid;

[0045] (3) Flour making

[0046] After the alloy is cooled, it is made into fine powder by mechanical crushing, air jet milling, etc.;

[0047] (4) Magnetic field orientation

[0048] Placing the powder in a strong magnetic field so that the easy magnetization axis of the powder particles is aligned along the direction of the magnetic field, this process can improve the remanence and magnetic energy product of the magnet;

[0049] (5) Pressing

[0050] The powder oriented by the magnetic field is placed in a mold and pressed into a three-dimensional shape under a certain pressure;

[0051] Step 2: Sintering

[0052] (1) Inspection and maintenance of equipment;

[0053] (2) Preparation for furnace loading;

[0054] The pressed green body is placed in a vacuum sintering furnace for sintering; the sintering temperature is usually around 1000-1100°C. At this temperature, the powder particles diffuse and fuse, increasing the density of the magnet and improving its performance.

[0055] (3) Pulse magnetic field sintering assistance

[0056] During the sintering process, a pulsed magnetic field is applied with a magnetic field strength of 2-5 Tesla and a pulse frequency of 5-20 Hz.

[0057] Step 3: Post-sintering treatment

[0058] Heat treatment: the sintered magnets are subjected to aging heat treatment, including tempering and other treatments;

[0059] Rapid cooling and cryogenic treatment: After sintering, rapid cooling is used, such as rapidly lowering the magnet from the sintering temperature and then performing cryogenic treatment;

[0060] Pulsed magnetic field sintering is the application of a pulsed magnetic field during the sintering process. When assisting sintering, the pulsed magnetic field can produce a variety of positive effects. On the one hand, it can reduce the sintering temperature. This is because the magnetic field can affect the diffusion process of atoms and ions inside the material, allowing atoms to obtain enough energy to migrate at a lower temperature, thereby promoting the densification of the material. On the other hand, the pulsed magnetic field can also refine the grains of the material. The magnetic field affects processes such as the movement of grain boundaries, which inhibits grain growth, thereby improving the mechanical properties and other comprehensive properties of the material. This advantage is more obvious in the sintering process of magnetic materials and can effectively improve the magnetic properties of the material.

[0061] When preparing the raw materials in step 1, a small amount of other elements, such as dysprosium and terbium, can be added to the raw materials such as neodymium (Nd), iron (Fe), boron (B) in a certain stoichiometric ratio. Generally, the content of neodymium is about 29%-35% (mass fraction), the content of boron is about 0.9%-1.2%, and the rest is mainly iron. The selection of raw materials requires metals and metal alloys with higher purity. These added elements can improve the performance of the magnet. For example, dysprosium and terbium can improve the coercive force of the magnet.

[0062] In the step 1, the ingredients weighed according to a certain ratio are added into the three-dimensional mixer, the mixing time is not less than 2 hours, and the three-dimensional mixer maintains a constant speed. At the same time, lubricants (such as zinc stearate) and additives can be added to the three-dimensional mixer to improve the molding and fluidity of the powder, and fully mix to make various raw materials evenly distributed. The mixing time is determined according to factors such as the amount of powder and the power of the equipment, generally several hours.

[0063] In the step 1, during the pressing and molding stage, honey is placed in a mold and molded by pressing. The pressure is usually around several hundred MPa, so that the powder becomes an embryo with a certain shape and density. The density of the embryo generally reaches 50%-70% of the theoretical density. By pressing, the alloy originally in powder form is made dense under the action of pressure, which is convenient for subsequent sintering processing.

[0064] In the second step of the equipment inspection and maintenance phase, the vacuum system and heating system need to be inspected. Vacuum system inspection: Check whether the vacuum pump is working properly, including the mechanical pump and diffusion pump (if any), and ensure that the vacuum system is well sealed and leak-free. It can be detected by a helium mass spectrometer leak detector. For example, observe whether the reading of the vacuum gauge can reach the required vacuum degree within a reasonable time; heating system inspection: Check whether the heating elements, such as graphite heaters or molybdenum wire heaters, are damaged. Debug the heating control cabinet to ensure that the temperature control accuracy is within the allowable range. Normally, the temperature control accuracy should reach ±5°C. Vacuum degree setting: According to the sintering requirements of the NdFeB magnet, set the appropriate vacuum degree. Generally, the vacuum degree is required to reach 1×10-3 Pa or less to prevent the magnet from oxidizing at high temperatures.

[0065] In the furnace preparation stage of step 2, mold selection: select a suitable graphite or ceramic mold according to the shape and size of the NdFeB magnet. The mold must be able to withstand the sintering temperature and cannot react chemically with the magnet material. For example, for small block magnets, a simple square or round graphite mold can be selected; magnet blank placement: place the NdFeB magnet blanks to be sintered neatly in the mold, and keep a certain distance between the blanks to avoid mutual adhesion during the sintering process. At the same time, the uniformity of heating should be considered to ensure that the blanks are heated evenly in the furnace.

[0066] During the sintering in the second step, it is necessary to set the heating rate, sintering temperature and holding time of the vacuum sintering furnace. Usually, the heating rate can be set to 10-20°C / min, the sintering temperature is about 1000-1100°C, and the holding time is 2-5 hours. At the same time, the sintering treatment can be carried out in a distributed sintering manner. First, the embryo body is pre-sintered at a relatively low temperature (800-900°C) for 1-2 hours. At this stage, impurities such as lubricants in the embryo body can be preliminarily removed, reducing the influence of volatiles on the quality of the magnet during subsequent high-temperature sintering; then the temperature is raised to the conventional sintering temperature (1050-1100°C) for formal sintering, and the sintering time is shortened to 1.5-3 hours. This can not only ensure the densification of the magnet, but also reduce grain growth, which is beneficial to improving the magnetic properties.

[0067] In the post-sintering treatment process of step 3, the heat treatment includes tempering treatment, the temperature is about 500-600°C, and the time is about 1-3 hours. The heat treatment can optimize the microstructure of the magnet and improve the magnetic properties. After sintering, a rapid cooling method is adopted, such as cooling the magnet from the sintering temperature to 400-500°C at a cooling rate of 50-100°C / second, and then performing a deep cooling treatment at a deep cooling temperature of -150--180°C for 1-3 hours. The microstructure in the magnet can be quickly fixed, and the deep cooling treatment further improves the crystal structure and reduces internal stress, thereby improving the comprehensive performance of the magnet.

[0068] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A sintering method for preparing NdFeB magnets, characterized in that: The steps include: Step 1: Preparation before sintering (1) Preparation of raw materials Selecting main raw materials such as neodymium (Nd), iron (Fe), and boron (B), and mixing them according to a certain chemical measurement weight ratio; (2) Smelting of raw materials Put the prepared raw materials into a vacuum induction furnace or an electric arc furnace for smelting. During the smelting process, the temperature and time must be strictly controlled to fully melt the raw materials and mix them evenly to form an alloy liquid; (3) Flour making After the alloy is cooled, it is made into fine powder by mechanical crushing, air jet milling, etc.; (4) Magnetic field orientation Placing the powder in a strong magnetic field allows the easy magnetization axis of the powder particles to be aligned along the magnetic field. This process can improve the remanence and magnetic energy product of the magnet. (5) Pressing The powder oriented by the magnetic field is placed in a mold and pressed into a three-dimensional shape under a certain pressure; Step 2: Sintering (1) Inspection and maintenance of equipment; (2) Preparation for furnace loading; The pressed green body is placed in a vacuum sintering furnace for sintering; the sintering temperature is usually around 1000-1100°C. At this temperature, the powder particles diffuse and fuse, increasing the density of the magnet and improving its performance. (3) Pulse magnetic field sintering assistance During the sintering process, a pulsed magnetic field is applied with a magnetic field strength of 2-5 Tesla and a pulse frequency of 5-20 Hz. Step 3: Post-sintering treatment Heat treatment: the sintered magnets are subjected to aging heat treatment, including tempering and other treatments; Rapid cooling and cryogenic treatment: After sintering, rapid cooling is used, such as rapidly lowering the magnet from the sintering temperature and then performing cryogenic treatment.

2. A sintering method for preparing NdFeB magnets according to claim 1, characterized in that: When preparing the raw materials in step 1, a small amount of other elements, such as dysprosium and terbium, can be added to the raw materials such as neodymium (Nd), iron (Fe), and boron (B) prepared in a certain stoichiometric ratio. Generally, the content of neodymium is about 29%-35% (mass fraction), the content of boron is about 0.9%-1.2%, and the rest is mainly iron. The selection of raw materials requires metals and metal alloys of higher purity.

3. A sintering method for preparing NdFeB magnets according to claim 1, characterized in that: In the step 1, the ingredients weighed according to a certain ratio are added into the three-dimensional mixer, the mixing time is not less than 2 hours, and the three-dimensional mixer maintains a constant speed. At the same time, lubricants (such as zinc stearate) and additives can be added to the three-dimensional mixer to improve the molding and fluidity of the powder.

4. A sintering method for preparing NdFeB magnets according to claim 1, characterized in that: In the step 1, during the compression molding stage, the honey is placed in a mold and molded by compression. The pressure is usually around several hundred MPa, so that the powder becomes an embryo with a certain shape and density. The density of the embryo generally reaches 50%-70% of the theoretical density.

5. A sintering method for preparing NdFeB magnets according to claim 1, characterized in that: In the second step of the equipment inspection and maintenance phase, the vacuum system and heating system need to be inspected. Vacuum system inspection: Check whether the vacuum pump is working properly, including the mechanical pump and diffusion pump (if any), and ensure that the vacuum system is well sealed and leak-free. It can be detected by a helium mass spectrometer leak detector, for example, observe whether the reading of the vacuum gauge can reach the required vacuum degree within a reasonable time; Heating system inspection: Check whether the heating elements, such as graphite heating elements or molybdenum wire heating elements, are damaged. Debug the heating control cabinet to ensure that the temperature control accuracy is within the allowable range. Usually, the temperature control accuracy should reach ±5°C.

6. A sintering method for preparing NdFeB magnets according to claim 1, characterized in that: In the furnace preparation stage of step 2, mold selection: according to the shape and size of the NdFeB magnet, select a suitable graphite or ceramic mold. The mold should be able to withstand the sintering temperature and cannot react chemically with the magnet material. For example, for small block magnets, a simple square or round graphite mold can be used; Magnet blank placement: Place the NdFeB magnet blanks to be sintered neatly in the mold, and keep a certain distance between the blanks to avoid sticking to each other during the sintering process. At the same time, consider the uniformity of heating to ensure that the blanks are heated evenly in the furnace.

7. A sintering method for preparing NdFeB magnets according to claim 1, characterized in that: During the sintering in the second step, it is necessary to set the heating rate, sintering temperature and holding time of the vacuum sintering furnace. Usually, the heating rate can be set to 10-20°C / min, the sintering temperature is about 1000-1100°C, and the holding time is 2-5 hours. At the same time, the sintering treatment can be carried out in a distributed sintering manner. First, the embryo is pre-sintered at a relatively low temperature (800-900°C) for 1-2 hours. At this stage, impurities such as lubricants in the embryo can be preliminarily removed, reducing the influence of volatiles on the quality of the magnet during subsequent high-temperature sintering; and then the temperature is raised to the conventional sintering temperature (1050-1100°C) for formal sintering.

8. A sintering method for preparing NdFeB magnets according to claim 1, characterized in that: In the post-sintering treatment process of step 3, the heat treatment includes tempering treatment at a temperature of about 500-600° C. for about 1-3 hours.

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