Control method for inhibiting cracking of large sintered neodymium-iron-boron permanent magnet
By designing a multi-stage differentiated heating rate and controlling the vacuum level, the cracking problem of sintered NdFeB permanent magnets was solved, thereby improving the stability and performance of the magnets and reducing production costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-02-05
- Publication Date
- 2026-04-10
AI Technical Summary
Existing technologies pose a risk of cracking during the production of sintered NdFeB permanent magnets, especially for large magnets. Furthermore, existing methods, such as sintering with specific heating rates and inert gas partial pressure, suffer from high costs and difficulties in process control.
By adopting a multi-stage differentiated heating rate design, combined with the control of temperature ranges in each stage, including vacuum management and gas discharge, the design achieves uniform growth of magnet grains, effective release of internal stress, and full discharge of volatiles, thus suppressing the cracking risk of blanks with a single unit weight greater than 1kg.
It significantly improves the structural stability of the magnet, reduces the risk of cracking, increases production efficiency and magnet performance, and reduces production costs.
Smart Images
Figure CN121839408A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of neodymium iron boron magnet production technology, and in particular to a control method for suppressing cracking in bulk sintered neodymium iron boron permanent magnets. Background Technology
[0002] As is well known, Nd2Fe 14 R-Fe-B type rare earth sintered magnets with B-type compounds as the main phase are permanent magnets and are the highest-performing magnets among all magnetic materials. They are widely used in voice coil motors (VCMs) for hard disk drives, servo motors, variable frequency air conditioning motors, and electric motors for hybrid vehicles. They are prepared from sintered NdFeB blanks. The preparation process mainly includes batching, melting, hydrogen crushing, air jet milling, stirring, orientation molding, cold isostatic pressing, and sintering. However, after cold isostatic pressing, the contact between particles is mechanical, resulting in low bonding strength. When the pressing pressure is high, some of the powder particles that have already come into contact may deform. Such powder compacts do not have the microstructure of high permanent magnetic properties. Therefore, in order to further improve the density, improve the contact properties between powder particles, increase the strength, and give the magnet the microstructure characteristics of high permanent magnetic properties, it is necessary to heat the compact to a temperature below the melting point of the powder matrix phase and perform heat treatment for a period of time. This process is called sintering.
[0003] During the sintering and heating process, neodymium iron boron magnets release a certain amount of gas. These gases originate from compounds such as Nd₂Fe₂ after hydrogen sintering. 14 Residual hydrogen in the forms of BHx and NdHy, along with organic solvents such as gasoline, antioxidants, release agents, and lubricants added during air jet milling, stirring, and forming processes for oxidation prevention and lubrication, will evaporate or decompose in a molecular state during heating to produce single-atom C gas and H2 gas. N2 is a common gas used in air jet milling, and N2 gas is also used to protect the NdFeB magnetic powder during stirring and orientation forming stages. These production processes inevitably result in the adsorption of residual nitrogen gas and the solid solution gas within the 2:14:1 phase of the NdFeB magnetic powder. The porosity of the Nd-Fe-B billet after cold isostatic pressing reaches 34.5%~27.6%. The pores are filled with gas, which is also discharged during the sintering process. The research on sintering heating process aims to accelerate the heating process, shorten the heating time, and save production costs while ensuring that the magnet does not crack. Currently, there are two main methods to adjust the sintering heating process. The first method is to set specific heating rates and heating steps, which will prolong the sintering time and increase costs. The second method is to introduce a certain amount of inert gas for partial pressure sintering during the sintering heating process. However, this method has high requirements for equipment, requires precise coordination of pressure and temperature, is difficult to control, and has high technical requirements for operators. Summary of the Invention
[0004] To overcome the above shortcomings, the purpose of this invention is to provide a control method for suppressing cracking in bulk sintered NdFeB permanent magnets. By designing a multi-stage differentiated heating rate and controlling the temperature range of each stage, the method achieves uniform growth of magnet grains, effective release of internal stress, and full discharge of volatiles, thereby suppressing the cracking risk of blanks with a single unit weight greater than 1kg and significantly improving the structural stability of the magnet.
[0005] The technical solution of this invention to solve its technical problem is: A method for controlling cracking in bulk sintered NdFeB permanent magnets, characterized by comprising the following steps: Step S1: Place the prepared neodymium iron boron magnet blank with a single weight greater than 1 kg into a sintering furnace for sintering, wherein the vacuum degree in the sintering furnace is evacuated to less than 1 Pa. Step S2: In the sintering furnace, heat the temperature to 200℃~300℃ at a heating rate of 6℃ / min~10℃ / min, and hold for 20min~60min. Step S3: In the sintering furnace, heat the temperature to 350℃~500℃ at a heating rate of 0.2℃ / min~1℃ / min, and hold for 120min~240min. Step S4: In the sintering furnace, heat the temperature to 630℃~780℃ at a heating rate of 4℃ / min~8℃ / min, and hold for 20min~60min. Step S5: In the sintering furnace, heat the temperature to 850℃~900℃ at a heating rate of 1℃ / min~4℃ / min, and hold for 90min~210min. Step S6: In the sintering furnace, heat the temperature to sintering temperature of 1020℃~1100℃ at a heating rate of 6℃ / min~10℃ / min, and hold for 120min~600min. Step S7: In the sintering furnace, Ar gas is introduced, and after cooling to 900°C, it is air-cooled to less than 100°C.
[0006] As an improvement of the present invention, in step S5, when the heat preservation ends, the vacuum degree inside the sintering furnace is controlled to be less than 5 Pa.
[0007] As a further improvement of the present invention, in step S5, the temperature is increased to 850°C to 900°C at a heating rate of 1°C / min to 3.5°C / min, and held at that temperature for 90 min to 210 min.
[0008] As a further improvement of the present invention, in step S6, the temperature is increased to the sintering temperature of 1020℃~1100℃ at a heating rate of 7℃ / min~10℃ / min, and held for 120min~600min.
[0009] As a further improvement of the present invention, in step S2, the temperature is increased to 250°C to 300°C at a heating rate of 6°C / min to 8°C / min, and held at that temperature for 20 min to 40 min.
[0010] As a further improvement of the present invention, in step S3, the temperature is increased to 450°C to 500°C at a heating rate of 0.2°C / min to 0.4°C / min, and held at that temperature for 120 min to 200 min.
[0011] As a further improvement of the present invention, in step S4, the temperature is increased to 750°C to 780°C at a heating rate of 4°C / min to 6°C / min, and held at that temperature for 20 min to 60 min.
[0012] As a further improvement of the present invention, in step S5, the temperature is increased to 890°C to 900°C at a heating rate of 1°C / min to 3°C / min, and held for 120 min to 210 min. At the end of the holding period, the vacuum degree is controlled to be less than 1 Pa.
[0013] As a further improvement of the present invention, in step S6, the temperature is increased to the sintering temperature of 1080℃~1100℃ at a heating rate of 6℃ / min~7℃ / min, and held at that temperature for 300min~600min.
[0014] As a further improvement of the present invention, the neodymium iron boron magnet blank comprises 31 wt% PrNd, 0.18 wt% Ti, 0.2 wt% Cu, 1.5 wt% Co, 0.95 wt% B, 0.05 wt% Al and the balance Fe by weight ratio, wherein the weight ratio of Pr to Nd is 1:3.
[0015] In this invention, by designing a multi-stage differentiated heating rate and controlling the temperature range of each stage, uniform growth of magnet grains, effective release of internal stress, and full discharge of volatiles are achieved, suppressing the risk of cracking in blanks with a single weight greater than 1kg and significantly improving the structural stability of the magnet. Attached Figure Description
[0016] For ease of explanation, the present invention will be described in detail below with reference to the preferred embodiments and accompanying drawings.
[0017] Figure 1 This is a comparison table of the heating process parameters for the embodiments and comparative examples of the present invention; Figure 2 This is a comparison table of magnet performance data and cracking rates for embodiments and comparative examples of the present invention. Figure 3 This is a comparison image of a normal blank and a blank SEM image (1000x magnification) of Embodiment 1 of the present invention; Figure 4 This is a comparison image of the normal blank and the blank SEM image (1000x) of Comparative Example 1 of the present invention. Figure 5 This is a comparison image of the normal blank and the blank SEM image (1000x) of Comparative Example 6 of the present invention. Detailed Implementation
[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.
[0020] Currently, the sintering temperature rise process is mainly adjusted through two methods, as follows: 1. Setting a specific heating rate and heating step will prolong the sintering time and increase the cost. For example, in the published patent technology, CN115602401 discloses a method to reduce the cracks in NdFeB magnets by setting a specific heating rate and heat preservation step. However, this method significantly prolongs the sintering time. 2. During the sintering heating process, a certain amount of inert gas is introduced for partial pressure sintering. For example, in the published patent technology, CN112670048 proposes a partial pressure sintering method for manufacturing neodymium iron boron magnets. The magnetic steel products produced by this method have a high yield, stable product quality, good mechanical and magnetic properties, good machinability, and are not prone to cracking. However, this method has high requirements for equipment heating, and the pressure and temperature need to be precisely matched. The process control is difficult and the technical requirements for operators are high.
[0021] like Figures 1 to 5 As shown, a method for controlling cracking in bulk sintered NdFeB permanent magnets according to the present invention includes the following steps: Step S1: Place the prepared neodymium iron boron magnet blank with a single weight greater than 1 kg into a sintering furnace for sintering, wherein the vacuum degree in the sintering furnace is evacuated to less than 1 Pa. Step S2: In the sintering furnace, heat the temperature to 200℃~300℃ at a heating rate of 6℃ / min~10℃ / min, and hold for 20min~60min. Step S3: In the sintering furnace, heat the temperature to 350℃~500℃ at a heating rate of 0.2℃ / min~1℃ / min, and hold for 120min~240min. Step S4: In the sintering furnace, heat the temperature to 630℃~780℃ at a heating rate of 4℃ / min~8℃ / min, and hold for 20min~60min. Step S5: In the sintering furnace, heat the temperature to 850℃~900℃ at a heating rate of 1℃ / min~4℃ / min, and hold for 90min~210min. Step S6: In the sintering furnace, heat the temperature to sintering temperature of 1020℃~1100℃ at a heating rate of 6℃ / min~10℃ / min, and hold for 120min~600min. Step S7: In the sintering furnace, Ar gas is introduced, and after cooling to 900°C, it is air-cooled to less than 100°C.
[0022] In this invention, during step S5, at the end of the heat preservation, the vacuum degree inside the sintering furnace is controlled to be less than 5 Pa.
[0023] Specifically, this invention places the prepared NdFeB green blanks with a single weight >1kg into a sintering furnace for sintering, and divides the sintering heating process into the following five stages: 1. First stage: After the vacuum degree in the sintering furnace is reduced to less than 1 Pa, the temperature is raised from room temperature to the first step of 200-300℃ at a rate of 6-10℃ / min and held for 20-60 min. Specifically, the temperature is raised to 250-300℃ at a rate of 6-8℃ / min and held for 20-40 min. This stage is mainly a physical dehydration process. The residual moisture, adsorbed gases and some low-boiling-point molding release agent inside the green body evaporate in this temperature range. The amount of the above-mentioned adsorbed gases is relatively small and can be quickly transferred. The purpose of rapid heating is to efficiently pass through this non-reactive sensitive zone and avoid staying in the low-temperature zone for too long, which would lead to low production efficiency. At the same time, sufficient time window is reserved for the subsequent critical second stage.
[0024] 2. Second Stage: After the first stage of heat preservation, the temperature is increased to the second stage (350-500℃) at a rate of 0.2-1℃ / min, and held for 120-240 minutes; preferably, the temperature is increased to 450-500℃ at a rate of 0.2-0.4℃ / min, and held for 120-200 minutes. This stage is crucial for completely resolving the cracking problem. This stage is the window period for the vigorous decomposition and vaporization of the organic binder, and it also allows the hydrides remaining from the hydrogen crushing process to decompose slowly and evenly, giving hydrogen atoms sufficient time to diffuse and combine into hydrogen molecules for slow release. If the temperature is increased too quickly, these gases will remain inside the billet. The rapid expansion of the powder creates enormous internal pressure. When this internal pressure exceeds the ultimate strength of the green compact (which has extremely low strength at this point), it will burst, causing blistering, macroscopic cracks, or even overall cracking. Therefore, the purpose of this stage is to reduce the heating rate to an extremely slow 0.2~1℃ / min in order to control the gas production rate and ensure that the amount of gas produced per unit time is small enough. This small amount of gas has enough time to slowly and steadily diffuse out through the tiny pores between the powder particles, rather than accumulating violently and generating pressure. This eliminates the source of internal stress that leads to cracking, removes internal gas, and lays a solid foundation for subsequent densification.
[0025] 3. Third stage: The second-stage heat-insulating bundle is heated to the third stage of 630-780℃ at a rate of 4-8℃ / min and held for 20-60min. Preferably, the temperature is increased to 750-780℃ at a rate of 4-6℃ / min and held for 20-60min. In this stage, after the exhaust of the second stage, the amount of gas released is small, which can quickly transition the process and speed up production efficiency.
[0026] 4. Fourth stage: After the third stage of heat preservation, the temperature is increased to 850-900℃ at a rate of 1-4℃ / min, and held for 90-210min. The vacuum degree at the end of the heat preservation is required to be <5Pa. Preferably, the temperature is increased to 850-900℃ at a rate of 1-3.5℃ / min and held for 90-210min. The gas degassing in this stage is N2 in solid solution, and the amount of degassing is relatively large. The heating rate is lower to reduce the risk of cracking. After this stage, there will be almost no more gas venting from the billet. Therefore, it is necessary to ensure that the vacuum degree at the end of the heat preservation in this stage is <5Pa to ensure complete degassing and prepare for the subsequent vacuum sintering stage.
[0027] 5. Fifth stage: After the fourth stage of heat preservation, the temperature is increased to the sintering temperature of 1020~1100℃ at a rate of 6~10℃ / min, and held for 120~600min; preferably, the temperature is increased to the sintering temperature of 1020~1100℃ at a rate of 7~10℃ / min, and held for 120~600min; the binder removal process is basically completed, and the green body enters the temperature range dominated by sintering and densification. At this time, the rapid heating is to improve efficiency, quickly reach the sintering temperature, shorten the total production cycle, reduce energy consumption, and at the same time inhibit grain growth. Under the premise of ensuring complete densification, the total residence time at high temperature is shortened as much as possible, which helps to obtain fine grains.
[0028] In this invention, by using three rapid heating stages with low gas release in NdFeB green blanks and two slow heating stages with high gas release, the risk of green blank cracking is reduced while sintering efficiency is greatly improved.
[0029] This invention provides Examples 1-6 and Comparative Examples 1-5, as detailed below: Example 1 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 6℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 0.4℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 6℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 1℃ / min, hold for 120min, and the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 6℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0030] Example 2 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 10℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 0.4℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 8℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 1℃ / min, hold for 120min, and the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 10℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0031] Example 3 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 6℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 0.8℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 6℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 1℃ / min, hold for 120min, and the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 6℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0032] Example 4 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 6℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 1.0℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 6℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 1℃ / min, hold for 120min, and the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 6℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0033] Example 5 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 6℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 0.4℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 6℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 2.5℃ / min, hold for 120min, and ensure that the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 6℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0034] Example 6 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 6℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 0.4℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 6℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 3.5℃ / min, hold for 120min, and ensure that the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 6℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0035] Comparative Example 1 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 10℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 1.2℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 10℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 4℃ / min, hold for 120min, and the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 10℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0036] Comparative Example 2 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 10℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 1.2℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 10℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 3.5℃ / min, hold for 120min, and ensure that the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 10℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0037] Comparative Example 3 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1400g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 10℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 1.2℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 10℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 3.5℃ / min, hold for 120min, and ensure that the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 10℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0038] Comparative Example 4 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1600g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 10℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 1.2℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 10℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 3.5℃ / min, hold for 120min, and ensure that the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 10℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0039] Comparative Example 5 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 6℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 2.5℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 6℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 1℃ / min, hold for 120min, and the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 6℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0040] Comparative Example 6 includes the following steps: (1) Provide a cube-shaped neodymium iron boron magnet blank with a single weight of 1200g (31wt% PrNd, 0.18wt% Ti, 0.2wt% Cu, 1.5wt% Co, 0.95wt% B, 0.05wt% Al and balance Fe), with the weight ratio of Pr to Nd being 1:3; (2) Place the blank into a vacuum sintering furnace and control the vacuum degree to less than 1 Pa; (3) Heat to 250℃ at a heating rate of 6℃ / min and hold for 20min; (4) Heat to 450℃ at a heating rate of 5℃ / min and hold for 120min; (5) Heat to 750℃ at a heating rate of 6℃ / min and hold for 20min; (6) Heat to 890℃ at a heating rate of 1℃ / min, hold for 120min, and the vacuum degree is less than 1Pa at the end of the holding period; (7) Heat to 1080℃ at a heating rate of 6℃ / min and hold for 300min; (8) Fill with Ar gas, cool to 900°C and then air cool to less than 100°C.
[0041] A detailed comparison of the embodiments and comparative examples, such as the raw material images and SEM images of Embodiment 1, Comparative Example 1, and Comparative Example 6, and... Figure 1 and Figure 2 Chinese table Figure 3 , Figure 4 , Figure 5 As shown, macroscopic observation of Example 1 revealed that the sintered blank surface was smooth, without visible cracks, with intact edges and stable dimensions. Microscopic observation (SEM, ×1000) showed uniform grain size, with an average grain size of approximately 5.5 μm, clear and intact grain boundaries, no abnormal growth or local aggregation, low porosity, and dense structure. Macroscopic observation of Comparative Example 1 revealed slight cracks on the sintered blank surface, mostly distributed at the edges and cross-sectional changes. Microscopic observation (SEM, ×1000, crack area) showed that the cracks extended along the grain boundaries, and the grain size near the cracks was significantly larger than the average size of the matrix (some grains reached 7–11 μm). The grain size distribution was uneven, with local abnormal grain growth, indicating that the rapid heating during the exhaust stage led to local stress concentration and caused uneven grain growth, ultimately forming microcracks. Furthermore, the magnetic properties were slightly reduced. Macroscopic observations of Comparative Example 6 revealed severe through-cracks and even localized fragmentation in the blank, primarily distributed at edges and areas of cross-sectional change. Microscopic analysis (SEM, ×1000, fracture surface region) showed significant oxidation and a dark color on the fracture surface, exhibiting brittle fracture characteristics. No distinct, intact grain morphology was observed. This indicates that during the critical venting stage, intense gas escape led to a rapid expansion and oxidation of internal defects (porosity, microcracks), resulting in severe structural damage. Furthermore, the magnetic properties were severely degraded.
[0042] This invention improves the density and uniformity of the magnet, enhances its magnetic properties, and slows down the heating rate during the stage of high gas release in the sintering process, which helps reduce the risk of blank cracking during sintering. At the same time, it accelerates the heating rate during the stage of low gas release in the sintering process, which helps reduce sintering time, increase sintering efficiency, and save costs.
[0043] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A method for controlling cracking in bulk sintered NdFeB permanent magnets, characterized in that, Includes the following steps: Step S1: Place the prepared neodymium iron boron magnet blank with a single weight greater than 1 kg into a sintering furnace for sintering, wherein the vacuum degree in the sintering furnace is evacuated to less than 1 Pa. Step S2: In the sintering furnace, heat the temperature to 200℃~300℃ at a heating rate of 6℃ / min~10℃ / min, and hold for 20min~60min. Step S3: In the sintering furnace, heat the temperature to 350℃~500℃ at a heating rate of 0.2℃ / min~1℃ / min, and hold for 120min~240min. Step S4: In the sintering furnace, heat the temperature to 630℃~780℃ at a heating rate of 4℃ / min~8℃ / min, and hold for 20min~60min. Step S5: In the sintering furnace, heat the temperature to 850℃~900℃ at a heating rate of 1℃ / min~4℃ / min, and hold for 90min~210min. Step S6: In the sintering furnace, heat the temperature to sintering temperature of 1020℃~1100℃ at a heating rate of 6℃ / min~10℃ / min, and hold for 120min~600min. Step S7: In the sintering furnace, Ar gas is introduced, and after cooling to 900°C, it is air-cooled to less than 100°C.
2. The method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 1, characterized in that, In step S5, at the end of the heat preservation, the vacuum degree inside the sintering furnace is controlled to be less than 5 Pa.
3. The method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 2, characterized in that, In step S5, the temperature is increased to 850℃~900℃ at a heating rate of 1℃ / min~3.5℃ / min, and held at that temperature for 90min~210min.
4. A method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 2 or 3, characterized in that, In step S6, the temperature is increased to the sintering temperature of 1020℃~1100℃ at a heating rate of 7℃ / min~10℃ / min, and held for 120min~600min.
5. The method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 1, characterized in that, In step S2, the temperature is increased to 250℃~300℃ at a heating rate of 6℃ / min~8℃ / min, and held at that temperature for 20min~40min.
6. The method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 1, characterized in that, In step S3, the temperature is increased to 450℃~500℃ at a heating rate of 0.2℃ / min~0.4℃ / min, and held at that temperature for 120min~200min.
7. The method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 1, characterized in that, In step S4, the temperature is increased to 750℃~780℃ at a heating rate of 4℃ / min~6℃ / min, and held at that temperature for 20min~60min.
8. The method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 1, characterized in that, In step S5, the temperature is increased to 890℃~900℃ at a heating rate of 1℃ / min~3℃ / min, and held for 120min~210min. At the end of the holding period, the vacuum degree is controlled to be less than 1Pa.
9. The method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 1, characterized in that, In step S6, the temperature is increased to the sintering temperature of 1080℃~1100℃ at a heating rate of 6℃ / min~7℃ / min, and held for 300min~600min.
10. The method for controlling cracking of bulk sintered NdFeB permanent magnets according to claim 1, characterized in that, The neodymium iron boron magnet blank comprises, by weight, 31 wt% PrNd, 0.18 wt% Ti, 0.2 wt% Cu, 1.5 wt% Co, 0.95 wt% B, 0.05 wt% Al and the balance Fe, wherein the weight ratio of Pr to Nd is 1:3.