A method for improving the three-point bending strength of neodymium-iron-boron by grain boundary diffusion

Abstract

The present application relates to a method for improving the three-point bending strength of neodymium-iron-boron by grain boundary diffusion. The method is as follows: polishing and cutting the surface of a neodymium-iron-boron magnet blank, then ultrasonic cleaning in a degreasing agent solution, followed by acid pickling, water washing and drying to obtain a dry neodymium-iron-boron base material; placing the neodymium-iron-boron base material in a copper plating solution and applying current to carry out copper plating to obtain a copper-coated neodymium-iron-boron base material; physically vapor depositing a heavy rare earth terbium film on the surface of the copper-coated neodymium-iron-boron base material in a direction parallel to the easy magnetization axis to obtain a terbium-copper-coated neodymium-iron-boron base material; and then sequentially carrying out heat treatment and tempering treatment under vacuum conditions. The present application coats a copper film on the neodymium-iron-boron base material by electroplating, then physically vapor deposits a heavy rare earth element terbium on the copper-coated neodymium-iron-boron base material in a direction parallel to the easy magnetization axis, carries out grain boundary diffusion heat treatment, and obtains excellent comprehensive magnetic properties while improving the three-point bending strength of neodymium-iron-boron.

Description

A method for improving the three-point bending strength of NdFeB through grain boundary diffusion Technical Field

[0001] This invention belongs to the field of neodymium iron boron magnets, specifically relating to a method for improving the three-point bending strength of neodymium iron boron magnets through grain boundary diffusion. Background Technology

[0002] Neodymium iron boron (NdFeB) magnets, renowned for their high remanence and energy product, are widely used in automotive motors, 3C electronic products, and other fields. Applications of NdFeB magnets are trending towards miniaturization and weight reduction. However, as NdFeB magnets become smaller, their temperature stability deteriorates and their three-point bending strength weakens. Improving temperature stability requires increasing coercivity. Current methods for improving coercivity include grain refinement, controlling oxygen content, adding heavy rare earth elements, and grain boundary diffusion. However, NdFeB magnets are manufactured using powder metallurgy, resulting in numerous porosity defects, making machining difficult, reducing bending strength, and predominantly causing intergranular fracture. Therefore, improving bending strength should begin with strengthening the grain boundary phase.

[0003] Grain boundary diffusion technology is widely used. For example, patent CN113394017A uses a method of electroplating and electrophoresis combined deposition diffusion to deposit a composite diffusion layer on the surface of sintered NdFeB magnets, which improves the coercivity of NdFeB magnets after heat treatment. Patent CN108305772A selects diffusion during the dehydrogenation treatment of ternary alloy hydride powder, which can avoid oxidation and thus significantly improve coercivity, facilitating production. Patent CN112802677A uses a spherical heavy rare earth alloy diffusion source mixed with NdFeB magnets in a certain proportion, combined with a rotational diffusion process, to simultaneously improve the coercivity and mechanical properties of the magnets, suitable for improving the coercivity of small-sized magnets and industrial production. However, the above patents only improve the coercivity of NdFeB magnets, and it is unknown whether the bending strength is improved. Through grain boundary diffusion treatment, the diffusion of heavy rare earth elements into the main phase can be effectively reduced, reducing the decrease in remanence and magnetic energy product, thereby improving coercivity, but the bending strength problem is not solved.

[0004] Therefore, based on this, the present invention proposes a technical solution that specifically improves the three-point bending strength of NdFeB magnets through grain boundary diffusion technology. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method for improving the three-point bending strength of NdFeB magnets through grain boundary diffusion. The aim is to improve the coercivity of sintered NdFeB magnets while simultaneously enhancing their three-point bending strength. In this invention, a copper thin film is coated onto a NdFeB substrate using electroplating. Subsequently, the copper-coated NdFeB substrate undergoes physical vapor deposition of terbium (a heavy rare earth element) parallel to the easy magnetization axis, followed by grain boundary diffusion heat treatment. This process yields excellent comprehensive magnetic properties while simultaneously improving the three-point bending strength of the NdFeB magnet.

[0006] The present invention provides a method for improving the three-point bending strength of NdFeB magnets through grain boundary diffusion, the method comprising the following steps:

[0007] (1) Grind and cut the surface of the NdFeB magnet blank, then put it into the degreasing agent solution for ultrasonic cleaning, and then pickle, wash with water and dry in sequence to obtain a dry NdFeB substrate.

[0008] (2) The neodymium iron boron substrate is placed in a copper plating solution and an electric current is applied to perform copper plating to obtain a copper-coated neodymium iron boron substrate;

[0009] (3) A heavy rare earth terbium film is physically vapor-deposited on the surface of the copper-coated NdFeB substrate in a direction parallel to the easy magnetization axis to obtain a terbium-copper-coated NdFeB substrate.

[0010] (4) The terbium-copper coated neodymium iron boron substrate is heat-treated and tempered under vacuum conditions to obtain a neodymium iron boron magnet with improved three-point bending strength.

[0011] To facilitate understanding of this invention, the principle is explained as follows: This invention utilizes grain boundary diffusion technology to physically vapor-deposit a thin film of heavy rare earth elements on the surface of NdFeB magnets. Subsequently, through sintering, tempering, and heat treatment processes, the heavy rare earth elements diffuse along the grain edges into the interior of the magnet, forming a core-shell structure encapsulated by heavy rare earth iron boron in the epitaxial layer at the NdFeB main phase boundary. This enhances the anisotropy field of the main phase grains and strengthens the resistance to demagnetization during demagnetization. Compared to adding heavy rare earth elements during existing magnet manufacturing processes, both magnetic properties and heavy rare earth utilization are significantly improved. The heavy rare earth elements form a core-shell structure encapsulated by heavy rare earth iron boron in the epitaxial layer of the NdFeB main phase. While ensuring improved coercivity, remanence is slightly reduced. Furthermore, sintered NdFeB magnets are produced using powder metallurgy, resulting in a porous magnet with Cu elements located in the grain boundary phase, which improves the magnet's toughness.

[0012] Preferably, in step (1), the drying temperature is 55-75°C.

[0013] Preferably, in step (2), the current density is 0.45–0.55 A / dm³. 2 .

[0014] Preferably, in step (4), the pressure of the vacuum condition is 0.001 to 0.01 Pa.

[0015] Preferably, in step (4), the heat treatment temperature is 850-950°C and the heat treatment time is 5 hours.

[0016] Preferably, in step (4), the tempering temperature is 480-550°C and the tempering time is 2 hours.

[0017] The beneficial effects of this invention are as follows:

[0018] The method for improving the three-point bending strength of NdFeB magnets through grain boundary diffusion described in this invention employs grain boundary diffusion technology, which improves the utilization rate of heavy rare earth elements. Furthermore, it combines electroplating with physical vapor deposition to deposit a composite thin film, enhancing the coercivity of NdFeB while improving its three-point bending strength. It also improves the uniformity and continuity of the NdFeB-rich phase and enhances the ductility and toughness of the grain boundary phase, comprehensively overcoming the difficulties faced in the miniaturization of NdFeB magnets. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0020] Example 1

[0021] This embodiment provides a method for improving the three-point bending strength of NdFeB magnets through grain boundary diffusion, the method comprising the following steps:

[0022] (1) Select a sintered NdFeB bulk blank (grade 54M) containing heavy rare earth terbium, polish it on all six sides to a smooth finish, and cut it to (length) 47mm × (width) 8mm × (height) 3.5mm. Then, put it into a degreasing agent solution for 30s ultrasonic cleaning to remove oil. Then, clean the NdFeB magnet with a 3% volume fraction dilute nitric acid solution. The acid pickling thickness is 3 filaments of the NdFeB substrate. Then, wash with water and ultrasonically vibrate to clean. Finally, dry at 55℃ to obtain a dry NdFeB substrate.

[0023] (2) The dried NdFeB substrate is placed in an electroplating solution as the cathode, and the anode is a copper wire. The temperature in the plating bath is 25℃~85℃, and the current density is 0.45A / dm³. 2 The impact electroplating time is 8 to 10 minutes until the copper plating layer is dense and uniform, thus obtaining a copper-coated NdFeB substrate;

[0024] (3) The copper-coated NdFeB substrate is placed in a magnetron sputtering device, and heavy rare earth terbium thin films are deposited in both directions parallel to the easy magnetization axis. The total thickness of the copper and terbium composite film is 20 μm, and terbium-copper coated NdFeB substrate is obtained.

[0025] (4) The terbium-copper coated neodymium iron boron substrate is placed in a vacuum heat treatment equipment, the vacuum is drawn to 0.001 Pa, the heat treatment temperature is raised to 850°C and held for 5 hours, the tempering treatment temperature is raised to 480°C and held for 2 hours. After completion, a neodymium iron boron magnet with improved three-point bending strength (i.e., the diffused magnet) is obtained.

[0026] The diffused magnets were sliced ​​into 0.385mm thick slices along a 47mm direction using a slicer. They were then degreased, ultrasonically cleaned, and dried, and their three-point bending strength was tested. Simultaneously, the undiffused NdFeB blanks were also tested for comparison.

[0027] Example 2

[0028] This embodiment provides a method for improving the three-point bending strength of NdFeB magnets through grain boundary diffusion, the method comprising the following steps:

[0029] (1) Select a sintered NdFeB bulk blank (grade N58) that does not contain heavy rare earth terbium, polish it on all six sides until it is smooth, and cut it to (length) 47mm × (width) 8mm × (height) 3.5mm. Then, put it into the degreasing agent solution for 60s ultrasonic cleaning to remove oil. Then clean the NdFeB magnet with a 6% volume fraction dilute nitric acid solution. The acid pickling thickness is 5 filaments of the NdFeB substrate. Then wash with water and ultrasonic vibration cleaning. Finally, dry it at 75℃ to obtain a dry NdFeB substrate.

[0030] (2) The dried NdFeB substrate is placed in an electroplating solution as the cathode, and the anode is a copper wire. The temperature in the plating bath is 25℃~85℃, and the current density is 0.55A / dm³. 2 The impact electroplating time is 8 to 10 minutes until the copper plating layer is dense and uniform, thus obtaining a copper-coated NdFeB substrate;

[0031] (3) The copper-coated NdFeB substrate is placed in a magnetron sputtering apparatus, and heavy rare earth terbium thin films are deposited in both directions parallel to the easy magnetization axis. The total thickness of the copper and terbium composite film is 21.5 μm, thus obtaining a terbium-copper-coated NdFeB substrate.

[0032] (4) The terbium-copper coated neodymium iron boron substrate is placed in a vacuum heat treatment equipment, vacuumed to 0.01 Pa, heated to 950°C and held for 5 hours, tempered to 550°C and held for 2 hours. After completion, a neodymium iron boron magnet with improved three-point bending strength (i.e., the diffused magnet) is obtained.

[0033] The diffused magnets were sliced ​​into 0.385mm thick slices along a 47mm direction using a slicer. They were then degreased, ultrasonically cleaned, and dried, and their three-point bending strength was tested. Simultaneously, the undiffused NdFeB blanks were also tested for comparison.

[0034] Comparative Example

[0035] This comparative example provides a method for improving the three-point bending strength of NdFeB magnets through grain boundary diffusion. The difference between this comparative example and Example 2 is that the electroplated copper film is deposited perpendicular to the easy magnetization direction. Specifically, the copper-coated NdFeB substrate is placed in a magnetron sputtering apparatus. In this comparative example, a heavy rare earth terbium film is deposited in both directions perpendicular to the easy magnetization axis using physical vapor deposition. The total thickness of the copper and terbium composite film is 21.5 μm. The remaining process parameters and operating steps are completely consistent with Example 2.

[0036] The diffused magnets were sliced ​​into 0.385mm thick slices along a 47mm direction using a slicer. They were then degreased, ultrasonically cleaned, and dried, and their three-point bending strength was tested. Simultaneously, the undiffused NdFeB blanks were also tested for comparison.

[0037] The neodymium iron boron magnets of Examples 1, 2 and the comparative examples were subjected to relevant tests, and the test results are shown in Table 1.

[0038] Table 1 Test Results

[0039]

[0040] As shown in Table 1, Examples 1 and 2 differ only in the terbium content of the NdFeB blanks; however, both the coercivity and the maximum three-point bending strength are significantly improved after diffusion. Example 2 uses the same NdFeB blanks, process parameters, and operating steps as the comparative example, but the direction of terbium diffusion is different. The results show that although the coercivity of the comparative example is improved, the three-point bending strength is not significantly improved. Therefore, diffusion of the composite film parallel to the easy magnetization axis is beneficial for improving the three-point bending strength.

[0041] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for improving the three-point bending strength of NdFeB magnets through grain boundary diffusion, characterized in that, The method includes the following steps: (1) grinding and cutting the surface of the NdFeB magnet blank, then ultrasonically cleaning it in a degreasing agent solution, followed by acid washing, water washing, and drying to obtain a dry NdFeB substrate; (2) placing the NdFeB substrate in a copper plating solution and applying current to perform copper plating to obtain a copper-coated NdFeB substrate; (3) physically vapor depositing a heavy rare earth terbium film on the surface of the copper-coated NdFeB substrate in a direction parallel to the easy magnetization axis to obtain a terbium-copper-coated NdFeB substrate; (4) subjecting the terbium-copper-coated NdFeB substrate to heat treatment and tempering under vacuum conditions to obtain a NdFeB magnet with improved three-point bending strength; wherein: the drying temperature is 55~75℃; the current density is 0.45~0.55A / dm 2 The heat treatment temperature is 850~950℃ and the heat treatment time is 5h; the tempering temperature is 480~550℃ and the tempering time is 2h; the vacuum pressure is 0.001~0.01Pa.

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