CN116190089A - Step-by-step grain boundary diffusion process of high-coercivity and high-corrosion-resistance sintered neodymium-iron-boron magnet - Google Patents
Step-by-step grain boundary diffusion process of high-coercivity and high-corrosion-resistance sintered neodymium-iron-boron magnet Download PDFInfo
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- H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
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- H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
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- H01F1/057—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B
- H01F1/0571—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes
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Abstract
The invention belongs to the field of rare earth permanent magnet materials, and particularly relates to a step grain boundary diffusion process of a high-coercivity high-corrosion-resistance sintered neodymium-iron-boron magnet. The invention carries heavy rare earth element metal flakes or compound flakes containing heavy rare earth elements on the surface of an acidified commercial N50 sintered NdFeB magnet, and diffuses the heavy rare earth elements along grain boundaries into the magnet through primary vacuum heat treatment to obtain a primary grain boundary diffusion sample; and carrying out secondary vacuum heat treatment process on the primary grain boundary diffusion sample by carrying out secondary pretreatment on the low-melting-point metal sheet on the surface of the sample, so as to obtain the sintered NdFeB magnet with high coercivity and strong corrosion resistance.
Description
Technical Field
The invention belongs to the field of rare earth permanent magnet materials, and particularly relates to a step grain boundary diffusion process of a high-coercivity high-corrosion-resistance sintered neodymium-iron-boron magnet.
Background
The neodymium-iron-boron permanent magnet has the advantages of high remanence, high coercivity and low raw material cost, so that the neodymium-iron-boron permanent magnet becomes one of permanent magnet materials with highest cost performance, and is currently the first-choice material for the industries of rail transit, aerospace, new energy automobiles, nuclear magnetic resonance, wind power generation and the like. However, the sintered neodymium-iron-boron magnet with a multiphase structure mainly comprises an Nd-Fe-B main phase and a rare earth-rich grain boundary phase with high chemical activity, and the high potential difference between the Nd-Fe-B main phase and the rare earth-rich grain boundary phase enables the magnet to be extremely easy to generate intergranular corrosion in corrosive media and damp and hot environments and finally cause the magnet to be pulverized and demagnetized, and particularly, the heavy rare earth-metal grain boundary diffusion type neodymium-iron-boron magnet effectively improves the coercive force of the magnet at room temperature, but the increase of the quantity of the rare earth-rich grain boundary phase leads to the deterioration of the magnet corrosion resistance, and seriously affects the application field and service life of the magnet.
At present, corrosion resistance of the sintered NdFeB magnet is improved mainly through surface protection technologies such as electroplating, chemical plating and spraying. Although the method improves the corrosion resistance of the magnet to a certain extent, because the sintered NdFeB magnet prepared by adopting the powder metallurgy process has a micropore structure, the electroplating solution often permeates into the magnet along the pores so as to damage the magnetic performance of the magnet; meanwhile, the defects of poor stability of the chemical plating solution, low mechanical property of the spray coating and the like limit the application of the magnet surface protection technology to a certain extent. The intergranular doping of trace non-rare earth elements can effectively stabilize the rare earth-rich grain boundary phase and improve the corrosion resistance of the magnet. Researches by Zeng et Al compare the comprehensive properties of Pr-Al-Cu and Al-Cu two-crystal boundary diffusion neodymium-iron-boron magnets, and find that the crystal boundary diffusion Pr-Al-Cu rare earth alloy can remarkably improve the coercive force of the magnet, but the corrosion resistance of the magnet is poor due to multiple crystal boundary phases caused by diffusion; the Al-Cu diffusion magnet has enhanced corrosion resistance compared with the original magnet, but has very limited coercivity improvement. (Zeng H. Et al. Journal of Materials Science & technology.2020, 36:50-54.). Chinese patent application CN201911029280.8 discloses a method for improving coercive force and wear resistance and corrosion resistance of a neodymium-iron-boron magnet, wherein the corrosion resistance of a magnet formed by depositing an Al-Cr film on the surface of the magnet and performing one-step grain boundary diffusion is improved, but the coercive force of the magnet is smaller than that of the original magnet and is less than 15kOe; in addition, the magnetron sputtering process is complex in operation, high in coating cost and low in cost performance. Therefore, the traditional grain boundary diffusion process cannot simultaneously meet the requirements of the high-end market field on the magnetic performance and the corrosion resistance of the sintered NdFeB magnet. The Chinese patent application CN202210742061.X discloses a preparation method of a high-performance sintered NdFeB magnet, which emphasizes that the coercivity of the magnet is enhanced by a step grain boundary diffusion method, but the research on the corrosion resistance of the magnet is not involved, and the change of the corrosion resistance of the magnet before and after grain boundary diffusion cannot be clarified.
Disclosure of Invention
Aiming at the technical problems, the invention provides a grain boundary diffusion process of a high-coercivity high-corrosion-resistance sintered NdFeB magnet, which aims to improve the corrosion resistance of the grain boundary diffusion type sintered NdFeB magnet and sequentially comprises the following steps:
(1) And carrying heavy rare earth element metal flakes or compound flakes containing heavy rare earth elements on the surface of the acidified commercial N50 sintered NdFeB magnet, wherein the commercial N50 sintered NdFeB magnet does not contain the heavy rare earth elements Dy and Tb. Diffusing heavy rare earth elements into the magnet along the grain boundary by primary vacuum heat treatment to obtain a primary grain boundary diffusion sample;
(2) And carrying out secondary vacuum heat treatment process on the primary grain boundary diffusion sample by carrying out secondary pretreatment on the low-melting-point metal sheet on the surface of the sample, so as to obtain the sintered NdFeB magnet with high coercivity and strong corrosion resistance.
The acidification step of the commercial N50 sintered NdFeB magnet comprises the following steps:
(1) Cutting a large N50 sintered NdFeB magnet into a cube sample of 10mm multiplied by 3-6 mm, wherein the size of the sample along the c-axis direction is 3-6 mm;
(2) Sequentially polishing the cube sample with 800, 1500, 2000, 3000 and 5000 mesh sand paper until the surface is mirror-surface;
(3) Sequentially carrying out ultrasonic treatment on the polished sample by using ultrapure water for 5min and absolute ethyl alcohol for 5min to obtain a clean surface; thereafter, 3wt.% HNO is used 3 The solution is continuously ultrasonic for 60s and absolute ethanol is superSound for 5min;
(4) And (5) drying the magnet in vacuum to obtain the acidified N50 sintered NdFeB magnet.
The primary grain boundary diffusion sample is pretreated again by the following steps:
(1) Sequentially polishing the primary grain boundary diffusion magnet with 800, 1500, 2000, 3000 and 5000 mesh sand paper until the surface is mirror surface;
(2) Sequentially carrying out ultrasonic treatment on the polished sample by using ultrapure water for 5min and absolute ethyl alcohol for 5min to obtain a clean surface;
(3) And (5) drying the magnet in vacuum to obtain the primary grain boundary diffusion pretreatment magnet.
Preferably, the heavy rare earth element metal sheet is one of Dy and Tb, and the heavy rare earth element-containing compound sheet is DyF 3 、TbF 3 One of them.
Preferably, the low-melting-point metal sheet is one of Al, cu and an alloy thereof, and the content of Cu atoms in the alloy is lower than 30%.
Preferably, the thickness of the heavy rare earth element metal flake, the heavy rare earth element-containing compound flake and the low melting point metal flake is 1-2 mm.
Preferably, the heavy rare earth element metal sheet, the heavy rare earth element-containing compound sheet and the low-melting metal sheet are all carried out double-sided entrainment along the upper and lower surfaces of the c-axis of the magnet.
Preferably, the heavy rare earth metal-entrained thin sheet, the compound thin sheet containing heavy rare earth elements and the metal thin sheet with low melting point are all wrapped by molybdenum foil and then subjected to primary vacuum heat treatment and secondary vacuum heat treatment. Preferably, the primary vacuum heat treatment process parameters are as follows: single temperature zone tube furnace vacuum: 6X 10 -4 Pa or less, diffusion temperature: 800-950 ℃, diffusion time: 5-8 h, annealing temperature: 450-650 ℃, and annealing time: 2-6 h; the technological parameters of the secondary vacuum heat treatment are as follows: single temperature zone tube furnace vacuum: 6X 10 -4 Pa or less, tempering temperature: 570-660 ℃ and tempering time: and 5-9 h.
Compared with the prior art, the grain boundary diffusion type sintered NdFeB magnet has the following advantages: firstly, heavy rare earth elements are diffused into a magnet along a grain boundary through a primary vacuum heat treatment diffusion process, a rare earth-rich grain boundary phase is optimized while a heavy rare earth-rich shell-core structure is formed, and the coercivity of the magnet is greatly improved; secondly, a large amount of rare earth-rich grain boundary phases in the primary grain boundary diffusion sample provide effective diffusion channels for low-melting-point metals, so that the phenomenon that diffusion elements are enriched on the surface layer of a magnet in the grain boundary diffusion process is greatly improved, and the diffusion depth and the uniformity of the magnet are enhanced; thirdly, the chemical activity of a new rare earth-rich grain boundary phase formed by a secondary vacuum heat treatment process is reduced, so that the electrode potential difference between the main phase and the grain boundary phase is reduced, the intergranular corrosion is weakened, and the corrosion resistance of the sintered NdFeB magnet is enhanced. Fourthly, a layer of low-melting-point metal film is generated on the surface of the magnet through a secondary vacuum heat treatment process, so that a physical shielding or passivation protection effect can be provided for a matrix, and the corrosion resistance of the magnet is obviously improved; fifth, the entraining grain boundary diffusion process is simple to operate, and the low-melting-point metal has low cost, can be repeatedly utilized, and is easy for industrial production.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments described in the present application, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a graph showing the demagnetizing curves of sintered NdFeB magnet and initial magnet obtained in example 1 of the present invention;
FIG. 2 is a graph showing the demagnetizing curves of sintered NdFeB magnet and initial magnet obtained in example 2 of the present invention;
FIG. 3 is a graph showing the demagnetizing curves of sintered NdFeB magnet and initial magnet obtained in example 3 of the present invention;
FIG. 4 is a graph showing the demagnetizing curves of the sintered NdFeB magnet and the initial magnet obtained in example 4 of the present invention;
FIG. 5 is a graph showing the distribution of heavy rare earth elements in the cross section of sintered NdFeB magnet obtained in example 4 of the present invention;
FIG. 6 is a potentiodynamic polarization curve of the sintered NdFeB magnet obtained in comparative example 1 of the present invention with the initial magnet in 3.5wt.% NaCl solution;
FIG. 7 is a potentiodynamic polarization curve of sintered NdFeB magnets obtained in example 1 and comparative example 1 of the present invention in 3.5wt.% NaCl solution;
FIG. 8 is a potentiodynamic polarization curve of sintered NdFeB magnets obtained in example 2 and comparative example 1 of the present invention in 3.5wt.% NaCl solution;
FIG. 9 is a potentiodynamic polarization curve of sintered NdFeB magnets obtained in example 3 and comparative example 1 of the present invention in 3.5wt.% NaCl solution;
FIG. 10 is a potentiodynamic polarization curve of sintered NdFeB magnets and original magnets obtained in example 4, comparative example 1 of the present invention in 3.5wt.% NaCl solution;
Detailed Description
The invention will be further illustrated with reference to specific examples. It is to be understood that these examples are illustrative of the present invention and are not intended to limit the scope of the present invention. Further, it is understood that various changes and modifications of the invention will become apparent to those skilled in the art upon reading the description herein, and such equivalents are intended to fall within the scope of the invention as defined by the appended claims.
Example 1
The embodiment provides a method for preparing a high-coercivity high-corrosion-resistance sintered neodymium-iron-boron magnet through a grain boundary diffusion process, which specifically comprises an acidification treatment step of an initial magnet, a Dy sheet entrained magnet step, a vacuum heat treatment step, a re-pretreatment step of the obtained magnet, an Al sheet entrained magnet step and a re-vacuum heat treatment step:
1. acidification treatment procedure for commercial N50 magnet:
(1) Cutting a large sintered NdFeB magnet into a cubic sample of 10mm×10mm×6mm, wherein the sample size along the c-axis direction is 6mm;
(2) Sequentially polishing the cube sample with 800, 1500, 2000, 3000 and 5000 mesh sand paper until the surface is mirror-surface;
(3) Sequentially using ultrapure samplesUltrasonic treatment with water for 5min, ultrasonic treatment with absolute ethanol for 5min, and HNO of 3wt.% 3 Ultrasonic treatment of the solution for 60s and ultrasonic treatment of absolute ethyl alcohol for 5min to obtain a clean surface;
(4) And (5) drying the magnet in vacuum to obtain the sintered NdFeB acidified magnet.
Dy flake entrainment magnet step: 99.9wt.% high-purity Dy metal sheets with the dimensions of 10mm multiplied by 2mm are placed on the upper surface and the lower surface of the magnet in the c-axis direction, and are tightly wrapped by 99.99wt.% molybdenum foil to obtain a Dy-entrained magnet;
3. primary vacuum heat treatment: setting a diffusion temperature: the diffusion time is 5 hours at 900 ℃; annealing temperature is 500 ℃, and annealing time is 3 hours. Placing the Dy-entrained magnet into a quartz boat, placing into a single-temperature zone tube furnace, and vacuumizing to 6.0X10 -4 And (3) under Pa, starting to run a program, and after the program is finished, waiting for the furnace chamber to be naturally cooled to room temperature and taking out the furnace chamber to obtain the primary grain boundary diffusion magnet.
4. The primary grain boundary diffusion magnet obtained was again pretreated in substantially the same manner as in step 1, except that 3wt.% HNO was not required to be cut again and performed 3 Solution ultrasonic treatment is carried out to remove Dy element and oxide layer remained on the surface.
Al flake entrainment magnet step: placing 99.999wt.% high-purity Al metal sheets with the dimensions of 10mm multiplied by 2mm on the upper and lower surfaces of the c-axis direction of the primary grain boundary diffusion magnet subjected to the secondary pretreatment, and tightly wrapping the primary grain boundary diffusion magnet with 99.99wt.% molybdenum foil to obtain an Al-carried magnet;
6. and a secondary vacuum heat treatment step: setting a tempering temperature: 600 ℃ and tempering time: 5h. Placing the Al-entrained magnet into a quartz boat, placing into a single-temperature zone tube furnace, and vacuumizing to 6.0X10 -4 And (3) starting to run the program under Pa, and after the program is finished, waiting for the furnace chamber to naturally cool to room temperature and taking out the furnace chamber to obtain the high-coercivity and high-corrosion-resistance magnet.
Example 2
The preparation process of this example is basically the same as that of example 1, except that: the tempering temperature of the Al-entrained magnet during the secondary vacuum heat treatment is 570 ℃.
Example 3
The preparation process of this example is basically the same as that of example 1, except that: the tempering temperature of the Al-entrained magnet during the secondary vacuum heat treatment is 630 ℃.
Example 4
The preparation process of this example is basically the same as that of example 1, except that: tempering time is 7h during the secondary vacuum heat treatment of the Al-carried magnet.
Comparative example 1
The preparation method of the neodymium-iron-boron magnet of the comparative example is different from that of the example 1 in that: specific steps only include acidification treatment of the initial magnet, dy flake entrainment magnet, and primary vacuum heat treatment, and the specific operations thereof are completely identical to those of the corresponding steps in example 1.
The original magnet, the NdFeB magnet treated by the method of the above example and comparative example 1 were subjected to magnetic test and electrochemical test, respectively.
Table 1: magnetic performance test data sheet
| Sample of | Coercivity (kOe) | Residual magnetism (kG) |
| Original magnet | 13.86 | 14.37 |
| Example 1 | 19.39 | 11.99 |
| Example 2 | 18.77 | 11.73 |
| Example 3 | 17.66 | 11.36 |
| Example 4 | 19.10 | 13.29 |
| Comparative example 1 | 19.77 | 12.14 |
As can be seen from analysis of examples 1-4 and comparative example 1, the coercivity of the grain boundary diffusion magnet provided by the application is greatly improved compared with that of the original magnet, wherein the coercivity of example 1 is as high as 19.39kOe, the amplitude is 39.9%, the coercivity value of the grain boundary diffusion magnet is only 380Oe different from that of comparative example 1, and the proper secondary vacuum thermal annealing treatment has little influence on the magnetic performance of the primary grain boundary diffusion magnet.
Table 2: electrochemical test data sheet
| Sample of | Self-corrosion potential (V) | Self-etching current density (. Mu.A/cm) 2 ) |
| Original magnet | -0.64 | 3.83 |
| Example 1 | -0.64 | 2.40 |
| Example 2 | -0.65 | 3.17 |
| Example 3 | -0.64 | 3.32 |
| Example 4 | -0.61 | 1.23 |
| Comparative example 1 | -0.68 | 6.02 |
As can be seen from analysis of the original magnet and comparative example 1, the self-corrosion potential of the original magnet after primary heavy rare earth grain boundary diffusion treatment is reduced from-0.64V to-0.68V, and the self-corrosion current density is reduced from 3.83 mu A/cm 2 Increase to 6.02. Mu.A/cm 2 The corrosion resistance of the magnet is drastically deteriorated.
As can be seen from the analysis of examples 1 to 4 and comparative example 1, the magnet has an increased self-corrosion potential and a decreased self-corrosion current density after the secondary vacuum heat treatment, and has an enhanced corrosion resistance compared with the initial magnet; wherein the self-etching current density was reduced to 1.23. Mu.A/cm in example 4 2 The corrosion resistance is improved by 67.89 percent compared with the original magnet.
In conclusion, the neodymium-iron-boron magnet prepared by the method has the characteristics of high coercivity and high corrosion resistance.
Claims (10)
1. The step-by-step grain boundary diffusion process of the high-coercivity high-corrosion-resistance sintered NdFeB magnet is characterized by comprising the following specific steps of:
(1) Carrying heavy rare earth element metal flakes or compound flakes containing heavy rare earth elements on the surface of an acidified commercial N50 sintered NdFeB magnet, and carrying out primary vacuum heat treatment to diffuse the heavy rare earth elements along grain boundaries into the magnet to obtain a primary grain boundary diffusion sample;
(2) And carrying out secondary vacuum heat treatment process on the primary grain boundary diffusion sample by carrying out secondary pretreatment on the low-melting-point metal sheet on the surface of the sample, so as to obtain the sintered NdFeB magnet with high coercivity and strong corrosion resistance.
2. The step grain boundary diffusion process of a high coercivity and high corrosion resistance sintered neodymium-iron-boron magnet according to claim 1, wherein the commercial N50 sintered neodymium-iron-boron magnet does not contain heavy rare earth elements Dy and Tb, and the acidification step of the commercial N50 sintered neodymium-iron-boron magnet is as follows:
(1) Cutting a large N50 sintered NdFeB magnet into a cube sample of 10mm multiplied by 3-6 mm, wherein the size of the sample along the c-axis direction is 3-6 mm;
(2) Sequentially polishing the cube sample with 800, 1500, 2000, 3000 and 5000 mesh sand paper until the surface is mirror-surface;
(3) Sequentially carrying out ultrasonic treatment on the polished sample by using ultrapure water for 5min and absolute ethyl alcohol for 5min to obtain a clean surface; thereafter, 3wt.% HNO is used 3 The solution is continuously subjected to ultrasonic treatment for 60 seconds and absolute ethyl alcohol ultrasonic treatment for 5 minutes;
(4) And (5) drying the magnet in vacuum to obtain the acidified N50 sintered NdFeB magnet.
3. The step grain boundary diffusion process of a high coercivity and high corrosion resistant sintered neodymium-iron-boron magnet according to claim 1, wherein the step of re-preprocessing the primary grain boundary diffusion sample is as follows:
(1) Sequentially polishing the primary grain boundary diffusion magnet with 800, 1500, 2000, 3000 and 5000 mesh sand paper until the surface is mirror surface;
(2) Sequentially carrying out ultrasonic treatment on the polished sample by using ultrapure water for 5min and absolute ethyl alcohol for 5min to obtain a clean surface;
(3) And (5) drying the magnet in vacuum to obtain the primary grain boundary diffusion pretreatment magnet.
4. The step grain boundary diffusion process of a high coercivity and high corrosion resistant sintered NdFeB magnet according to claim 1, wherein the heavy rare earth metal flake is one of Dy and Tb, and the heavy rare earth-containing compound flake is DyF 3 、TbF 3 One of them.
5. The step grain boundary diffusion process of a high coercivity and high corrosion resistance sintered neodymium-iron-boron magnet according to claim 1, wherein the low melting point metal sheet is one of Al, cu and alloys thereof, and the content of Cu atoms in the alloys is lower than 30%.
6. The process for stepwise grain boundary diffusion of a high coercivity and high corrosion resistant sintered neodymium-iron-boron magnet according to claim 1, wherein the thickness of each of the heavy rare earth element metal sheet, the heavy rare earth element-containing compound sheet and the low melting point metal sheet is 1-2 mm.
7. The step grain boundary diffusion process of the high-coercivity and high-corrosion-resistance sintered neodymium-iron-boron magnet according to claim 1, wherein the heavy rare earth element metal sheet, the compound sheet containing the heavy rare earth element and the low-melting point metal sheet are all carried out double-sided entrainment along the upper surface and the lower surface of the c-axis of the magnet.
8. The process for diffusing the high-coercivity and high-corrosion-resistance sintered neodymium-iron-boron magnet by step grain boundaries according to claim 1, wherein the heavy rare earth metal-carrying sheet, the compound sheet containing the heavy rare earth element and the low-melting metal-carrying sheet are all wrapped by molybdenum foils and then subjected to primary vacuum heat treatment and secondary vacuum heat treatment.
9. The step grain boundary diffusion process of the high-coercivity and high-corrosion-resistance sintered neodymium-iron-boron magnet according to claim 1, wherein the primary vacuum heat treatment process parameters are as follows: single temperature zone tube furnace vacuum: 6X 10 -4 Pa or less, diffusion temperature: 800-950 DEG CDiffusion time: 5-8 h, annealing temperature: 450-650 ℃, and annealing time: 2-6 h.
10. The step grain boundary diffusion process of the high-coercivity and high-corrosion-resistance sintered neodymium-iron-boron magnet according to claim 1, wherein the secondary vacuum heat treatment process parameters are as follows: single temperature zone tube furnace vacuum: 6X 10 -4 Pa or less, tempering temperature: 570-660 ℃ and tempering time: and 5-9 h.
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