A sintered NdFeB magnet and its bluing process

Abstract

This invention discloses a sintered NdFeB magnet and its bluing process, relating to the field of magnet corrosion protection technology. The bluing process includes the following steps: dry polishing of the NdFeB black sheet using a composite polishing medium to remove surface oil, dust, and a dense oxide layer, exposing a fresh active substrate and forming a rough interface; the composite polishing medium consists of stone particles, shell particles, and sawdust; and bluing the polished NdFeB black sheet to induce an oxidation reaction on the substrate surface, generating a blue Fe3O4 protective film. This bluing process achieves a magnetic loss repair rate of up to 1.19%, while simultaneously achieving zero chemical emissions, zero wastewater generation, and replacing expensive controlled atmospheres with zero-cost air. It also fundamentally solves the rigid constraints of process connections caused by wet material overprinting, providing a surface protection solution for NdFeB magnets that combines environmental friendliness, high corrosion resistance, magnetic property repair capabilities, and low cost.

Description

Technical Field

[0001] This invention relates to the field of magnet corrosion protection technology, specifically to a sintered NdFeB magnet and its bluing process. Background Technology

[0002] Sintered NdFeB permanent magnets possess excellent magnetic properties and are widely used in wind power generation, new energy vehicle motors, servo motors, and other fields. However, NdFeB magnets are highly susceptible to rust and corrosion in humid and hot environments, severely limiting their service life. This is due to the presence of a low-potential Nd-rich phase in NdFeB magnets, which makes them prone to intergranular corrosion. Therefore, surface treatment methods are needed to improve the corrosion resistance of these magnets, thereby extending their service life.

[0003] Currently, the industry generally uses phosphating process, which generates a phosphate film by reacting acidic phosphating solution with the substrate to achieve temporary corrosion protection. However, in a humid and hot environment, the phosphate film has a porous crystalline structure and low density, which leads to a sharp decrease in its corrosion protection performance. At the same time, the strong acidic wet phosphating system is prone to causing over-corrosion of the substrate and hydrogen oxidative risks, generating phosphorus-containing wastewater and waste residue, which puts great pressure on environmental protection.

[0004] As an alternative, high-temperature bluing technology has attracted attention due to its strong adhesion between the film and the substrate and its lower cost. An existing bluing process is as follows: vibratory grinding and chamfering (wet NdFeB black film) → ultrasonic cleaning → drying → high-temperature bluing. However, this process has the following problems:

[0005] First, after the wet NdFeB black sheet is vibrated and chamfered, a small amount of magnetic powder remains on the surface due to residue from the previous process. If the interval between this process and the ultrasonic cleaning step is too long, the magnetic powder will form a "stamped" mark on the product surface (similar to the dust marks left after two books are stacked for a long time), resulting in product appearance defects and seriously affecting the consistency of quality after bluing. Therefore, this process has extremely strict requirements on the timing of process connections, limiting the flexibility of production scheduling.

[0006] Secondly, the process still relies on ultrasonic cleaning, which requires the use of chemical reagents for oil removal and ash removal before pretreatment. This results in high COD concentration, poor biodegradability, and difficulty in demulsification and degradation of the pretreatment wastewater, leading to a high risk of environmental pollution.

[0007] In addition, there are other problems when traditional bluing technology is applied to NdFeB magnets. For example, the alkaline solution method is prone to corroding the magnet; the controlled atmosphere method relies on expensive gas sources (such as water vapor, oxygen, nitrogen, etc.), and the equipment is complex, costly, and difficult to control. Summary of the Invention

[0008] Given the existing bluing process's high environmental pollution risk, easy "overprinting" on the product surface, and high production cost, the purpose of this invention is to provide a sintered NdFeB magnet and its bluing process. This bluing process is not only environmentally friendly with no wastewater discharge and low production cost, but also forms a high-quality oxide film and can solve the problem of wet material overprinting.

[0009] This invention is achieved through the following technical solution:

[0010] In a first aspect, this application provides a bluing process for sintered NdFeB magnets, comprising the following steps:

[0011] The neodymium iron boron black sheet is dry polished using a composite polishing medium to remove surface oil, dust, and dense oxide layer, exposing the fresh active substrate and forming a rough interface; the composite polishing medium is composed of stone particles, shell particles, and sawdust.

[0012] The polished neodymium iron boron black sheet is bluing to cause an oxidation reaction on the substrate surface, generating a blue Fe3O4 protective film.

[0013] This invention systematically solves the triple dilemma of existing bluing processes—rigid process connections, chemical pretreatment pollution, high cost of controllable atmosphere, and magnetic performance loss—through a synergistic treatment scheme of "dry composite polishing + atmospheric bluing." The principle is as follows: First, dry polishing is performed using a composite polishing medium composed of stone particles, shell particles, and sawdust. The mechanical cutting action of the hard abrasive removes the oxide scale, while the porous structure of the soft carrier physically adsorbs oil and generates heat through friction, thoroughly removing surface contaminants and forming a uniform micro-rough surface, exposing a fresh active substrate. This eliminates the risk of overprinting caused by residual magnetic powder in wet materials and completely replaces chemical pretreatment, achieving zero chemical emissions. Second, the polished magnet is directly bluing. The oxygen in the atmosphere reacts with the active substrate to generate a dense and uniform Fe3O4 protective film. By controlling the bluing temperature, the magnet is positioned precisely within the NdFeB stress relaxation temperature range, simultaneously releasing internal processing stress and grain boundary stress during film formation, achieving in-situ repair of magnetic properties.

[0014] Furthermore, the size of the stone particles is 15-20 mesh, preferably 18 mesh; the size of the shell particles is 3mm-7mm, preferably 5mm; and the size of the sawdust is 0.1mm-1.2mm, preferably 0.7mm.

[0015] Furthermore, the vibration frequency during polishing is 40Hz~50Hz.

[0016] Furthermore, the atmosphere used during the polishing process is 100% by volume oxygen, nitrogen, water vapor, or air.

[0017] Furthermore, the stone particles include corundum or white corundum.

[0018] Furthermore, the shell material particles include walnut shells or olive shells.

[0019] Furthermore, the sawdust includes oak sawdust.

[0020] Furthermore, the temperature during the bluing process is controlled between 290℃ and 310℃, preferably 290℃. The bluing time is controlled between 65min and 75min, preferably 68min.

[0021] Furthermore, the mass ratio of stone particles, shell particles, and sawdust in the composite polishing medium is (1~3):(1~3):(1~3).

[0022] Secondly, this application provides a sintered NdFeB magnet, which is bluing treated using the above-mentioned bluing process.

[0023] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0024] (1) The magnetic loss repair rate of the present invention can reach up to 1.19%, while achieving zero chemical agent discharge, zero wastewater generation, and replacing expensive controllable atmosphere with zero-cost air. It also solves the rigid constraints of process connection caused by wet material overprinting from the root, providing a surface protection solution for neodymium iron boron magnets that combines environmental protection, high corrosion resistance, magnetic property repair capability and low cost.

[0025] (2) This invention systematically solves the triple dilemma of existing bluing processes, namely, rigid process connection, chemical pretreatment pollution, high cost of controllable atmosphere, and magnetic performance loss, through the synergistic treatment scheme of "dry composite polishing + atmosphere bluing". Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0027] Figure 1 This is a picture of the NdFeB finished product before the double 85 test in Comparative Example 1;

[0028] Figure 2 The image shows the NdFeB finished product of Comparative Example 1 after 12 hours of testing with double 85.

[0029] Figure 3This is a picture of the NdFeB finished product before the double 85 test in Example 1;

[0030] Figure 4 This is a picture of the neodymium iron boron finished product after 84 hours of double 85 testing according to Example 1 of the present invention;

[0031] Figure 5 The image shows a NdFeB black sheet (Comparative Example 2) after polishing and leaving it for 12 hours.

[0032] Figure 6 This is a picture of the neodymium iron boron black sheet of Embodiment 1 of the present invention after polishing and leaving it for 12 hours;

[0033] Figure 7 The image shown is of a neodymium iron boron black film in Comparative Example 2 after it has been bluishized.

[0034] Figure 8 This is an image of the neodymium iron boron black sheet after bluing treatment in Embodiment 1 of the present invention;

[0035] Figure 9 This is an image of the polished neodymium iron boron black sheet from Embodiment 3 of the present invention;

[0036] Figure 10 This is an image of the polished neodymium iron boron black sheet from Embodiment 7 of the present invention;

[0037] Figure 11 This is an image of the polished neodymium iron boron black sheet from Embodiment 9 of the present invention;

[0038] Figure 12 This is an image of the polished neodymium iron boron black sheet from Embodiment 8 of the present invention;

[0039] Figure 13 The image shows the NdFeB finished product of Comparative Example 2 after 72 hours of double 85 testing;

[0040] Figure 14 The image shows the NdFeB finished product of Comparative Example 2 after 12 hours of PCT testing;

[0041] Figure 15 These are images of the NdFeB finished product after 15 hours of PCT testing in Example 1 of this study;

[0042] Figure 16 Image of Comparative Example 1 after soaking in saline (3%) for 18 minutes;

[0043] Figure 17 The image shows a comparison sample (Example 1) after 22 minutes of salt spray immersion.

[0044] Figure 18 The image shows Comparative Example 2, which was soaked in 3% saline solution for 33 minutes.

[0045] Figure 19 This is an image of a sample soaked in 3% saline solution for 72 minutes in Example 1.

[0046] Figure 20 The image shows a comparison example 2 after 35 minutes of salt spray immersion.

[0047] Figure 21 This is an image of a salt spray immersion for 62 minutes in Example 1. Detailed Implementation

[0048] 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 embodiments. The illustrative embodiments and descriptions of this invention are only used to explain this invention and are not intended to limit this invention.

[0049] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that these specific details are not necessary to practice the invention. In other embodiments, well-known materials or methods have not been specifically described in order to avoid obscuring the invention.

[0050] Throughout this specification, references to "an embodiment," "an example," or "an example" mean that a particular feature, structure, or characteristic described in connection with that embodiment or example is included in at least one embodiment of the invention. Therefore, the phrases "an embodiment," "an example," "an example," or "an example" appearing in various places throughout the specification do not necessarily refer to the same embodiment or example. Furthermore, specific features, structures, or characteristics can be combined in one or more embodiments or examples in any suitable combination and / or sub-combination. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described herein, as well as the features of those different embodiments or examples.

[0051] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is understood that ranges of 60–110 and 80–120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "a–b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0052] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0053] Example 1

[0054] This embodiment provides a bluing process for sintered NdFeB magnets, the specific steps of which are as follows:

[0055] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0056] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh diamond abrasive, 5mm olive shell, and 0.7mm oak sawdust in a mass ratio of 3:1:1. This dry polishing effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0057] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 290℃ for 68 minutes in a 100% volume fraction oxygen atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120℃. Finally, it enters the cooling section to cool the product.

[0058] Example 2

[0059] This embodiment provides a bluing process for sintered NdFeB magnets. The difference from Embodiment 1 is that oxygen is replaced with air in this embodiment, while the other processes are the same as in Embodiment 1.

[0060] The specific steps are as follows:

[0061] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0062] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh diamond abrasive, 5mm olive shell, and 0.7mm oak sawdust in a mass ratio of 3:1:1. This dry polishing effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0063] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 290℃ for 68 minutes in a 100% volume fraction air atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120℃. Finally, it enters the cooling section to cool the product.

[0064] Example 3

[0065] This embodiment provides a bluing process for sintered NdFeB magnets. Unlike Embodiment 1, the bluing temperature in this embodiment is 300°C, while the other processes are the same as in Embodiment 1.

[0066] The specific steps are as follows:

[0067] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0068] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh diamond abrasive, 5mm olive shell, and 0.7mm oak sawdust in a mass ratio of 3:1:1. This dry polishing effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0069] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 300°C for 68 minutes in a 100% volume fraction air atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120°C. Finally, it enters the cooling section to cool the product.

[0070] Example 4

[0071] This embodiment provides a bluing process for sintered NdFeB magnets. Unlike Embodiment 2, the bluing temperature in this embodiment is 310°C, while the other processes are the same as in Embodiment 2.

[0072] The specific steps are as follows:

[0073] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0074] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh diamond abrasive, 5mm olive shell, and 0.7mm oak sawdust in a mass ratio of 3:1:1. This dry polishing effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0075] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 310℃ for 68 minutes in a 100% volume fraction air atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120℃. Finally, it enters the cooling section to cool the product.

[0076] Example 5

[0077] This embodiment provides a bluing process for sintered NdFeB magnets. Unlike embodiment 3, the composite polishing medium in this embodiment is: diamond: olive shell: oak sawdust in a mass ratio of 1:3:1. Other processes are the same as in embodiment 3.

[0078] The specific steps are as follows:

[0079] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0080] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh diamond abrasive, 5mm olive shell, and 0.7mm oak sawdust in a mass ratio of 1:3:1. This dry polishing effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0081] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 300°C for 68 minutes in a 100% volume fraction air atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120°C. Finally, it enters the cooling section to cool the product.

[0082] Example 6

[0083] This embodiment provides a bluing process for sintered NdFeB magnets. Unlike embodiment 3, the composite polishing medium in this embodiment is: diamond: olive shell: oak sawdust in a mass ratio of 1:1:3. Other processes are the same as in embodiment 3.

[0084] The specific steps are as follows:

[0085] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0086] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh diamond abrasive, 5mm olive shell, and 0.7mm oak sawdust in a mass ratio of 1:1:3. This effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0087] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 300°C for 68 minutes in a 100% volume fraction air atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120°C. Finally, it enters the cooling section to cool the product.

[0088] Example 7

[0089] This embodiment provides a bluing process for sintered NdFeB magnets. Unlike embodiment 3, the composite polishing medium in this embodiment is: diamond: walnut shell: oak sawdust in a mass ratio of 3:1:1. Other processes are the same as in embodiment 3.

[0090] The specific steps are as follows:

[0091] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0092] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh diamond abrasive, 5mm walnut shell, and 0.7mm oak sawdust in a mass ratio of 3:1:1. This composite polishing medium effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0093] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 300°C for 68 minutes in a 100% volume fraction air atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120°C. Finally, it enters the cooling section to cool the product.

[0094] Example 8

[0095] This embodiment provides a bluing process for sintered NdFeB magnets. Unlike embodiment 3, the composite polishing medium in this embodiment is: white corundum: walnut shell: oak sawdust 3:1:1. Other processes are the same as in embodiment 3.

[0096] The specific steps are as follows:

[0097] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0098] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh white corundum, 5mm walnut shell, and 0.7mm oak sawdust in a mass ratio of 3:1:1. This composite polishing medium effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0099] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 300°C for 68 minutes in a 100% volume fraction air atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120°C. Finally, it enters the cooling section to cool the product.

[0100] Example 9

[0101] This embodiment provides a bluing process for sintered NdFeB magnets. Unlike embodiment 3, the composite polishing medium in this embodiment is: white corundum: olive shell: oak sawdust 3:1:1. Other processes are the same as in embodiment 3.

[0102] The specific steps are as follows:

[0103] S1. Pretreatment: The NdFeB blank of grade 50H is processed into a black sheet magnet of 29.5mm x 8.4mm x 1.7mm. The surface oil, magnetic powder and cutting fluid are removed by boiling. The surface is vibrated and chamfered for 7 hours to form a NdFeB black sheet with R0.3.

[0104] S2. Polishing: Place the R0.3 NdFeB black sheet in a polishing machine and polish for 5 hours. The polishing material is a composite polishing medium composed of 18-mesh white corundum, 5mm olive shell, and 0.7mm oak sawdust in a mass ratio of 3:1:1. This composite polishing medium effectively removes surface oil, dust, and dense oxide layer, exposes the fresh active matrix, and forms a suitable micro-rough interface. The polishing vibration frequency is 45Hz.

[0105] S3. Bluing: The polished NdFeB black sheet is placed in a high-temperature tunnel furnace and bluing is performed at 300°C for 68 minutes in a 100% volume fraction air atmosphere. This causes an oxidation reaction on the substrate surface, generating a continuous, uniform, and dense blue Fe3O4 protective film. After bluing, the product is conveyed to the ultrasonic cleaning section to remove dust from the surface using pure water. After cleaning, it is transferred to the air-drying section to remove residual moisture. Then it is transferred to the drying section to thoroughly dry the product at 120°C. Finally, it enters the cooling section to cool the product.

[0106] Comparative Example 1

[0107] This comparative example provides a bluing process for sintered NdFeB magnets, specifically employing phosphating. The specific method is as follows: NdFeB black sheets with chamfers are prepared using the same method as in Example 1. Then, the sheets undergo pretreatment to remove oil (50°C, 120s, pH: 11), water washing, acid washing (1% nitric acid, 120s), and water washing to remove oil and oxide layers from the product surface, resulting in a clean magnet. The magnet is then subjected to phosphating (10% LSD-105, 15min, 50°C), water washing, and drying to obtain a product with a phosphating anti-corrosion layer.

[0108] Comparative Example 2

[0109] This comparative example provides a bluing process for sintered NdFeB magnets. The specific method is as follows: NdFeB black sheets with chamfered edges are prepared using the same method as in Example 1. Then, the sheets are degreased (50°C, 120s, pH: 11), washed with water, acid-washed (1% nitric acid, 120s), and washed again to remove oil and oxide layers from the product surface, resulting in a clean magnet. The NdFeB black sheets are then placed in a vacuum furnace (<9.9*10). -1 Vacuum drying was performed using a vacuum method, with oxygen introduced and the drying temperature increased at 10℃ / min (8-13). The drying temperature was 360℃, and the drying time was 30min. After drying, the dried NdFeB black sheet was placed in a high-temperature tunnel furnace and subjected to high-temperature bluing treatment in a 100% volume fraction oxygen atmosphere at a pressure of 101.3kPa and a temperature of 370℃ for 45min. After cooling, it was removed.

[0110] The NdFeB black sheets of Comparative Examples 1-2 and Examples 1-9 were subjected to magnetic loss (%) tests after chamfering and finished product magnetic loss (%) tests. The test data are shown in Table 1.

[0111] The test method is as follows: The neodymium iron boron (NdFeB) black sheet is magnetized at room temperature, and the magnetic moment Φ0 is measured. The black sheet is then randomly magnetically adsorbed onto a 1mm thick iron plate, forming a semi-open circuit. It is then heated at 130℃ for 2 hours, followed by cooling to room temperature. The black sheet is then removed from the iron plate, and the magnetic moment Φ1 is measured. The final magnetic loss (%) after chamfering is obtained by comparing the two magnetic moments. .

[0112] Table 1

[0113] sample gas medium Composite polishing media Bluing temperature (°C) Magnetic loss after chamfering (%) Finished product magnetic loss (%) Magnetic loss repair (%) Comparative Example 1 none none none 0.95% 2.13% -1.18% Comparative Example 2 oxygen none 290 1.40% 0.63% 0.77% Example 1 oxygen Emery: Olive shell: Oak sawdust 3:1:1 290 1.41% 0.42% 0.99% Example 2 Air Emery: Olive shell: Oak sawdust 3:1:1 290 1.41% 0.42% 0.99% Example 3 Air Emery: Olive shell: Oak sawdust 3:1:1 300 1.41% 0.38% 1.03% Example 4 Air Emery: Olive shell: Oak sawdust 3:1:1 310 1.41% 0.42% 0.99% Example 5 Air Emery: Olive shell: Oak sawdust 1:3:1 300 1.42% 0.44% 0.98% Example 6 Air Emery: Olive shell: Oak sawdust 1:1:3 300 1.40% 0.47% 0.93% Example 7 Air Emery: Walnut shell: Oak sawdust 3:1:1 300 1.40% 0.36% 1.05% Example 8 Air White corundum: Walnut shell: Oak sawdust 3:1:1 300 1.42% 0.23% 1.19% Example 9 Air White corundum: olive shell: oak sawdust 3:1:1 300 1.41% 0.28% 1.13%

[0114] Based on the above test data and Figures 1-21 The following conclusions can be drawn:

[0115] I. The composite polishing medium in this invention has significant advantages.

[0116] This invention proposes using a composite polishing medium (diamond / white corundum + olive shell / walnut shell + oak sawdust) to polish chamfered magnets, replacing traditional chemical pretreatment processes such as degreasing and acid washing. The comparative experimental data in Table 1 show that:

[0117] Comparative Example 1 (Traditional Phosphating): After chamfering, the magnetic loss was 0.95%, and the finished product's magnetic loss increased to 2.13%, a deterioration of 1.18%. This indicates that the phosphating process significantly damages the magnetic properties. The results of the double 85 test (storage test under 85 degrees Celsius and 85% temperature and humidity conditions) show that after 12 hours, rust spots appeared on the surface of the NdFeB finished product. Figure 2 Images before the test, such as Figure 1 As shown.

[0118] Comparative Example 2 (Old Bluing Process + Oxygen): Although an oxygen atmosphere was used for bluing, no composite polishing medium was used. The magnetic loss of the finished product was 0.63%, and the magnetic loss repair was 0.77%. After polishing and leaving the NdFeB black sheet for 12 hours, rust spots appeared on the surface. Figure 5 ),right Figure 5 The NdFeB finished product, after the black sheet in the middle was bluing, still has rust spots. Figure 7 ).like Figure 8 The image shown is a picture of the neodymium iron boron black sheet after bluing treatment according to Embodiment 1 of the present invention. As can be seen from the image, there are still no rust spots after bluing. Figure 6 The image shown is of the NdFeB black sheet from Example 1 after polishing and 12 hours of storage. No rust was found, proving that the bluing process in Example 1 of this invention has a better corrosion effect than the bluing process in Comparative Example 2.

[0119] like Figure 13 The image shown is of the NdFeB finished product from Comparative Example 2 after 72 hours of double 85 testing, showing rust spots. Figure 14 The image shown is of the NdFeB finished product from Comparative Example 2 after 12 hours of PCT testing, showing rust spots. Figure 15 The image shown is a picture of the NdFeB finished product after 15 hours of PCT testing in Example 1, showing no rust.

[0120] like Figure 17 The image shown is from Comparative Example 1, which was immersed in salt spray for 22 minutes and showed rust. Figure 18 The image shown is from Comparative Example 2, which was soaked in 3% salt water for 33 minutes, and shows a large amount of rust. Figure 19 The image shown is of a sample soaked in 3% brine for 72 minutes in Example 1, with only a small amount of rust. Figure 20 The image shown is from Comparative Example 2, which was immersed in salt spray for 35 minutes and exhibited extensive rust. Figure 21 The image shown is of a salt spray immersion product after 62 minutes in Example 1, with only a small amount of rust.

[0121] Example 1 (New Bluing Process + Oxygen): Using a composite polishing medium of 3:1:1 (carborundum:olfel:oak sawdust), magnetic loss repair improved to 0.97%-0.99%, demonstrating that the composite polishing medium effectively removes the surface oxide layer, exposes the fresh substrate, provides a good interface for the bluing reaction, and avoids corrosive damage to the magnet caused by chemical pretreatment. Figure 3 The image shown is of the NdFeB finished product before the double 85 test in Example 1. Figure 4 The image shows the neodymium iron boron finished product of Example 1 after 84 hours of double 85 testing. The neodymium iron boron finished product of Example 1 still showed no rust after 84 hours of double 85 testing.

[0122] like Figure 9 The image shown is of a NdFeB black sheet after polishing, as described in Embodiment 3 of the present invention; Figure 10 The image shown is of a polished neodymium iron boron black sheet from Embodiment 7 of the present invention; as shown... Figure 11 The image shown is of a polished neodymium iron boron black sheet from Embodiment 9 of the present invention; as shown... Figure 12 The image shown is of a polished neodymium iron boron black sheet from Embodiment 8 of the present invention. Figures 9-12 It can be seen that the polishing medium ratio affects the polishing effect, and the polishing effect is better when the amount specified in this application is used.

[0123] Conclusion: The composite polishing medium replaces chemical pretreatment, which not only achieves zero chemical emissions, but also increases the magnetic loss repair rate to over 0.97%, which is superior to the traditional phosphating process.

[0124] II. The effect of air atmosphere is comparable to that of controlled atmosphere, but the cost is significantly reduced.

[0125] Example 1 (Oxygen atmosphere): Magnetic loss repair 0.99%; Example 2 (Air atmosphere): Magnetic loss repair 0.99%.

[0126] The data above shows that using air atmosphere for high-temperature blueing achieves magnetic loss repair effects comparable to controlled atmospheres such as oxygen, nitrogen, and water vapor. However, air, as a zero-cost and widely available medium, eliminates the need for additional gas source equipment and flow control systems, significantly reducing production costs and equipment complexity.

[0127] Conclusion: Air atmosphere can completely replace expensive controlled atmosphere, achieving the same or even better magnetic performance repair effect, and significantly reducing process costs.

[0128] III. The controlled bluing temperature in this invention has a positive effect on the magnetic loss repair rate.

[0129] Example 2 (290℃): Magnetic loss repair 0.99%; Example 3 (300℃): Magnetic loss repair 1.03%; Example 6 (310℃): Magnetic loss repair 0.99%.

[0130] Experimental results show that the bluing temperature has a significant impact on the magnetic loss repair effect within the range of 290~310℃, with 300℃ being the optimal temperature, where the magnetic loss repair rate reaches 1.03%, which is 0.04 percentage points higher than that at 290℃ and 310℃, respectively.

[0131] Conclusion: 300℃ is the optimal temperature window for bluing treatment. At this temperature, the residual stress, processing stress, and grain boundary stress inside the magnet are most effectively relaxed, resulting in the best magnetic property restoration effect.

[0132] IV. The polishing medium ratio in this invention has a positive effect on the magnetic loss repair rate.

[0133] Example 3 (emery: olive shell: oak sawdust = 3:1:1): magnetic loss repair 1.03%; Example 5 (emery: olive shell: oak sawdust = 1:3:1): magnetic loss repair 0.98%; Example 8 (emery: olive shell: oak sawdust = 1:1:3): magnetic loss repair 0.93%.

[0134] Data shows that a 3:1:1 ratio yields the best results. Diamond, as a hard abrasive, plays the primary cutting role, while olive / walnut shells and oak sawdust act as soft carriers, providing buffering and adsorption. When the proportion of hard abrasive is appropriate, it effectively removes the oxide layer while avoiding excessive grinding that could damage the substrate. Simultaneously, the porous structure of the sawdust fully adsorbs oil and debris, forming a uniform micro-rough surface.

[0135] V. The type of polishing medium in this invention has a positive effect on the magnetic loss repair rate.

[0136] Example 3 (emery + olive shell + oak sawdust): magnetic loss repair 1.03%; Example 7 (emery + walnut shell + oak sawdust): magnetic loss repair 1.05%; Example 8 (white corundum + walnut shell + oak sawdust): magnetic loss repair 1.19%; Example 9 (white corundum + olive shell + oak sawdust): magnetic loss repair 1.13%.

[0137] Experimental data show that the optimal ratio of white fused alumina, walnut shell, and oak sawdust (3:1:1) is the best, achieving a magnetic loss repair rate of 1.19%, which is 0.14 percentage points higher than that of the corundum system. White fused alumina has moderate hardness and better cutting uniformity, while walnut shell has better buffering performance and adsorption capacity than olive shell. The synergistic effect of the two results in higher surface activity of the polished substrate, which is more conducive to the uniform formation of the Fe3O4 film.

[0138] In summary, this invention systematically solves the triple dilemma of existing bluing processes—rigid process connections, chemical pretreatment pollution, high cost of controllable atmosphere, and magnetic performance loss—through a synergistic technical solution of "dry composite polishing + air atmosphere bluing." First, dry polishing is performed using a composite polishing medium composed of diamond abrasive (or white corundum) and sawdust (or olive shell / walnut shell). The mechanical cutting action of the hard abrasive removes the oxide scale, while the porous structure of the soft carrier physically adsorbs oil and generates heat through friction, thoroughly removing surface contaminants and forming a uniform micro-rough surface, exposing a fresh active substrate. This eliminates the risk of overprinting caused by residual magnetic powder in wet materials and completely replaces chemical pretreatment, achieving zero chemical emissions. Second, the polished magnet is placed directly in an air atmosphere and turns blue at 290~310℃. The approximately 21% oxygen in the air reacts with the active substrate to form a dense and uniform Fe3O4 protective film. At the same time, this temperature is exactly in the stress relaxation temperature range of NdFeB, releasing internal processing stress and grain boundary stress simultaneously during film formation, achieving in-situ repair of magnetic properties. Experimental data show that this invention increases the corrosion resistance time of double 85 from 12 hours in traditional phosphating to 96 hours, with a magnetic loss repair rate of 1.19%. It also achieves zero chemical discharge, zero wastewater generation, and replaces expensive controllable atmospheres with zero-cost air. Furthermore, it fundamentally solves the rigid constraints of process connection caused by wet material overprinting, providing a surface protection solution for NdFeB magnets that combines environmental friendliness, high corrosion resistance, magnetic property repair capability, and low cost.

[0139] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A bluing process for sintered NdFeB magnets, characterized in that, Includes the following steps: The neodymium iron boron black sheet is dry polished using a composite polishing medium to remove surface oil, dust, and dense oxide layer, exposing the fresh active substrate and forming a rough interface; the composite polishing medium is composed of stone particles, shell particles, and sawdust. The polished neodymium iron boron black sheet is bluing to cause an oxidation reaction on the substrate surface, generating a blue Fe3O4 protective film.

2. The bluing process for a sintered NdFeB magnet according to claim 1, characterized in that, The size of the stone particles is 15-20 mesh; the size of the shell particles is 3mm-7mm; and the size of the sawdust is 0.1mm-1.2mm.

3. The bluing process for a sintered NdFeB magnet according to claim 1, characterized in that, The vibration frequency during polishing is 40Hz~50Hz.

4. The bluing process for a sintered NdFeB magnet according to claim 1, characterized in that, The atmosphere used during polishing is 100% by volume oxygen, nitrogen, water vapor, or air.

5. The bluing process for a sintered NdFeB magnet according to claim 1, characterized in that, The stone particles include corundum or white corundum.

6. The bluing process for a sintered NdFeB magnet according to claim 1, characterized in that, The shell material particles include walnut shells or olive shells.

7. The bluing process for a sintered NdFeB magnet according to claim 1, characterized in that, The sawdust includes oak sawdust.

8. The bluing process for a sintered NdFeB magnet according to claim 1, characterized in that, The temperature during the bluing process is controlled between 290℃ and 310℃.

9. The bluing process for a sintered NdFeB magnet according to claim 1, characterized in that, The mass ratio of stone particles, shell particles and sawdust in the composite polishing medium is (1~3):(1~3):(1~3).

10. A sintered NdFeB magnet, characterized in that, The bluing process described in any one of claims 1 to 9 is used for bluing treatment.

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