Grain boundary diffusion method for high-Ga low-B neodymium-iron-boron magnet

Abstract

The invention discloses a high-Ga low-B neodymium-iron-boron magnet grain boundary diffusion method which comprises the following steps: (1) performing vacuum melting on a high-Ga low-B neodymium-iron-boron magnet raw material to obtain a quick-setting throwing sheet, performing hydrogen decrepitation on the quick-setting throwing sheet to obtain medium powder, performing jet milling on the medium powder to obtain fine powder, adding silicon powder into the fine powder, uniformly mixing, performing oriented pressing and isostatic pressing to obtain a pressed blank, and performing vacuum sintering on the pressed blank to obtain a high-Ga low-B neodymium-iron-boron magnet; performing vacuum sintering on the pressed blank to obtain a magnet blank, and performing heat treatment to obtain a magnet substrate; and (2) the surface of the magnet matrix is covered with a diffusion source, grain boundary diffusion and tempering treatment are carried out, and the diffused high-Ga low-B neodymium-iron-boron magnet is prepared. A small amount of silicon element is introduced into a magnet matrix, and heat treatment is performed after sintering, so that Si and Fe are combined, the content of free-state Fe in a grain boundary phase is reduced, and the grain boundary diffusion effect is improved. According to the magnet added with the silicon powder, the coercive force increment after grain boundary diffusion is improved by 2-3 kOe compared with a magnet without the silicon powder, the squareness is improved, and the magnetic performance is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of sintered NdFeB magnet preparation, specifically relating to a grain boundary diffusion method for high Ga, low B NdFeB magnets. Background Technology

[0002] In the engineering practice of sintered NdFeB, the main methods to improve coercivity include grain boundary optimization, grain refinement, high Ga and low B, and grain boundary diffusion. Among these, grain boundary diffusion is the most effective way to improve the coercivity of NdFeB.

[0003] However, the grain boundary diffusion effect varies among different types of magnets. Generally speaking, the diffusion effect of high-Ga, low-B magnets (generally considered to have a B content of less than 0.92 wt.%) is worse than that of magnets with normal B content (e.g., B content of 0.96 wt.%). This is manifested in the fact that, when printed with the same 1 wt.% Tb diffusion source, the coercivity increase of high-Ga, low-B magnets will be 2-4 kOe lower than that of normal B magnets after diffusion.

[0004] The reason for this phenomenon is that after sintering, the main phase of low-B class magnets is not fully formed, and a large number of Fe atoms do not participate in the construction of the main phase lattice, but remain in a free state in the grain boundary region. During the grain boundary diffusion heat treatment, the free Fe atoms at the grain boundary easily react with diffused Tb and other heavy rare earth elements to form binary / ternary phases. This reaction consumes heavy rare earth elements, reduces the effective penetration of heavy rare earth elements into the surface of the main phase and the binding efficiency with the main phase, thereby weakening the effect of grain boundary diffusion on improving the coercivity of the magnet. Therefore, how to improve the grain boundary diffusion effect of heavy rare earth elements in NdFeB magnets with high gallium and low boron composition has become an urgent technical problem to be solved in this field.

[0005] Some studies have attempted to improve diffusion by increasing the diffusion temperature to enhance the diffusion driving force, but the reproducibility is poor and the drawback of excessive Fe content in grain boundaries has not been completely resolved. Summary of the Invention

[0006] To address the problem of poor grain boundary diffusion and low coercivity improvement in existing high-gallium, low-boron magnets, this application provides a method for improving the diffusion effect of high-gallium, low-boron magnets by adding metallic Si, thereby increasing the coercivity increment after diffusion.

[0007] The technical solution adopted in this invention is: A method for grain boundary diffusion in a high-Ga, low-B NdFeB magnet, the method comprising the following steps: (1) The high Ga and low B NdFeB magnet raw materials are vacuum melted to obtain rapid solidification spun sheets, the rapid solidification spun sheets are hydrogen broken to obtain medium powder, the medium powder is air jet milled to obtain fine powder, silicon powder is added to the fine powder, mixed and then oriented and pressed and isostatically pressed to obtain a compact, the compact is vacuum sintered to obtain a magnet blank, and then heat treated to obtain a magnet matrix. (2) A diffusion source is covered on the surface of the magnet substrate to carry out grain boundary diffusion, and then tempering is performed to obtain a high Ga and low B NdFeB magnet after diffusion.

[0008] Furthermore, the particle size of the silicon powder is 1~5μm, preferably 1~3μm. The particle size of the silicon powder is preferably close to the particle size of the fine powder after air jet milling; the smaller the silicon powder particle size, the more complete the reaction with Fe.

[0009] The amount of silicon powder used is 0.2-0.5% of the mass of the fine powder, preferably 0.3-0.4%.

[0010] Furthermore, in step (1), the heat treatment temperature is 950-980℃, and the holding time is 3-6h. In this invention, after sintering, a heat treatment at 950-980℃ is performed to allow Si and Fe to react and form a binary phase, thereby consuming Fe.

[0011] The high-Ga, low-B NdFeB magnet comprises the following components by mass percentage: Re: 29.5~35%, where Re is one or more of Pr, Nd, Dy, Tb, Gd, Ho, and Y; B: 0.8~0.9%; M is at least one or more of Al, Cu, Zr, Ti, and Nb, and the mass content of M is greater than 0 and less than or equal to 3.5%. Ga: 0.~0.% The balance element is Fe, or Fe and Co, with Fe accounting for more than 90% of the balance element.

[0012] This invention adds Si to the fine powder after air jet milling and then tempers it at 950-980℃ after sintering to consume Fe. Si can form β-FeSi2 (melting point 982℃) with Fe at 950-980℃. On the one hand, FeSi2 is an antiferromagnetic phase, which helps with grain boundary isolation. On the other hand, FeSi2 partially fixes Fe, making the channels for heavy rare earth elements to diffuse into the substrate smoother.

[0013] In this invention, Si is added after air jet milling. During subsequent sintering, the silicon powder mainly enters the grain boundaries and more easily forms the β-FeSi2 phase with Fe. Therefore, this invention preferably adds Si powder after air jet milling.

[0014] The vacuum melting, hydrogen breaking, air jet milling, orientation forming, isostatic pressing, and vacuum sintering of this invention can all be performed using conventional processes in the art. Commonly used processes are as follows: In the vacuum melting process, the refining temperature is 1480~1500℃ and the casting temperature is 1410~1430℃.

[0015] In the hydrogen breakdown step, the hydrogen pressure is typically 0.05~0.1 MPa. During the hydrogen absorption reaction, the pressure change inside the reactor should not exceed 0.5% within 10 minutes, indicating the end of the hydrogen absorption process. After the hydrogen absorption reaction, the temperature is raised to 400~600℃ while simultaneously evacuating the vacuum, and held at this temperature for 2~6 hours to remove the hydrogen from the alloy sheet. The sheet is then cooled to obtain intermediate powder. The particle size of the intermediate powder is typically 80-300 μm.

[0016] In the air jet milling step, nitrogen is used as the protective gas, and the powder particle size is controlled by adjusting the separating wheel and cyclone separator of the air jet mill. The average particle size of the fine powder obtained by air jet milling is 2~5μm, preferably 3~4μm.

[0017] The fine powder produced after air jet milling is typically mixed with lubricant and antioxidant, and then molded under an orientation magnetic field. Commercially available magnetic powder lubricant or antioxidant is sufficient. The antioxidant can also be added before air jet milling. The amount of lubricant added can be 0.03~0.2% of the powder mass, and the amount of antioxidant can be 0.03~0.15% of the powder mass.

[0018] The orientation magnetic field is typically 1.8~2T, and the forming pressure is 5~15MPa. The oriented compact can then undergo further cold isostatic pressing at a pressure of 180~220MPa. The density of the oriented compact is 3.8~4.2g / cm³. 3 The density of the compact after cold isostatic pressing is approximately 4.4~4.6 g / cm³. 3 .

[0019] Vacuum sintering process is generally: 10 -2 ~10 -3 Under a vacuum of Pa, the sintering temperature is 1050–1150℃, and the holding time is 3–24 hours. Then, it is cooled to room temperature.

[0020] In step (2), the diffusion source for grain boundary diffusion is a heavy rare earth diffusion source, which is a pure heavy rare earth element metal, a heavy rare earth element hydride, or an alloy of heavy rare earth element and other metal elements; the heavy rare earth element is at least one of Dy and Tb, and the mass content of heavy rare earth element in the heavy rare earth diffusion source is ≥20wt.%.

[0021] Furthermore, the diffusion source can be deposited onto the magnet substrate surface using methods such as vapor deposition, magnetron sputtering, multi-arc ion plating, and screen printing.

[0022] The weight gain of the diffusion source is generally controlled according to the design requirements of the magnet performance, typically ranging from 0.1 to 3 wt%.

[0023] Generally, a diffusion source is covered on two surfaces perpendicular to the orientation direction of the magnet substrate.

[0024] The temperature for grain boundary diffusion is generally 880~920℃, and the preferred holding time is 15~30h; The tempering temperature is 400~650℃, and the holding time is 4~6h.

[0025] The present invention also provides a high-Ga, low-B NdFeB magnet prepared by the above method.

[0026] The beneficial effects of this invention are as follows: This invention introduces a small amount of silicon into a high-Ga, low-B magnet matrix. After sintering, the matrix is ​​tempered at 950-980℃, which allows the Si and Fe elements to combine, reducing the content of free Fe in the grain boundary phase. This makes the diffusion channels for heavy rare earth elements smoother during subsequent grain boundary diffusion, and they are not consumed by Fe, thus improving the grain boundary diffusion effect. The magnet with added silicon powder has a 2-3 kOe increase in coercivity after grain boundary diffusion compared to the magnet without added silicon powder, improved squareness, and a minimal decrease in remanence, resulting in a significant improvement in magnetic properties. Detailed Implementation

[0027] The technical solution of the present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0028] Example 1 Preparation of magnet substrate A: according to the formula (PrNd) 30 Co 0.5 Cu 0.4 Ga 0.5 B 0.9 Zr 0.1 Ti 0.1 Fe bal The formula (mass percentage) was used for feeding, and vacuum melting was used to obtain rapidly solidified flakes with an average thickness of 0.3 mm. The refining temperature was 1500℃, and the casting temperature was 1430℃. The flakes were hydrogen-crushed to obtain medium powder with a particle size of 80-300 μm. The hydrogen absorption pressure during hydrogen crushing was 0.096 MPa, the dehydrogenation temperature was 500℃, the dehydrogenation time was 3 h, and the powder was cooled for 3 h after dehydrogenation. The medium powder after hydrogen crushing was then subjected to air jet milling to obtain fine powder with an SMD of 3.0 μm and X90 / X10 = 4.5.

[0029] To 200 kg of fine powder, Si powder with an average particle size of 3 μm was added at a feeding ratio of 0.2 wt.%, along with 0.1% antioxidant and 0.1% lubricant. The mixture was then stirred for 3 hours. The resulting fine powder was then oriented and pressed into shape under a magnetic field strength of 1.8 T, achieving a pressing density of 3.9 g / cm³. 3 Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billet is sintered in a sintering furnace with a vacuum level of 10.-3 The magnet blank was sintered at 1095℃ for 4 hours, then heated to 1100℃ and held for 4 hours to obtain a magnet blank. The magnet blank was then heat-treated at 960℃ for 5 hours, cooled, and machined to obtain a 3.5mm thick sheet, which served as the magnet substrate. The thickness direction corresponds to the magnet orientation direction.

[0030] Tb diffusion sources were deposited on two surfaces of the magnet substrate perpendicular to the orientation direction, with a weight gain of 1 wt.%. The substrate was heated to 910°C in a vacuum sintering furnace for grain boundary diffusion and held for 24 hours. After holding, the substrate was cooled to a temperature below 200°C and then tempered at 475°C for 5 hours.

[0031] Example 2 The procedure was carried out according to the method of Example 1, except that Si powder with an average particle size of 3μm was added to 200kg of fine powder at a feeding ratio of 0.4wt.%, and the remaining steps were the same as in Example 1.

[0032] Example 3 The procedure was the same as in Example 1, except that Si powder with an average particle size of 3 μm was added to 200 kg of fine powder at a feeding ratio of 0.6 wt.%. The remaining steps were the same as in Example 1.

[0033] Comparative Example 1 Preparation of magnet substrate A: according to the formula (PrNd) 30 Co 0.5 Cu 0.4 Ga 0.5 B 0.9 Zr 0.1 Ti 0.1 Fe bal The formula (mass percentage) was used for feeding, and vacuum melting was used to obtain rapidly solidified flakes with an average thickness of 0.3 mm. The refining temperature was 1500℃, and the casting temperature was 1430℃. The flakes were hydrogen-crushed to obtain medium powder with a particle size of 80-300 μm. The hydrogen absorption pressure during hydrogen crushing was 0.096 MPa, the dehydrogenation temperature was 500℃, the dehydrogenation time was 3 h, and the powder was cooled for 3 h after dehydrogenation. The medium powder after hydrogen crushing was then subjected to air jet milling to obtain fine powder with an SMD of 3.0 μm and X90 / X10 = 4.5.

[0034] 0.1% antioxidant and 0.1% lubricant were added to 200 kg of fine powder and mixed for 3 hours. The mixed fine powder was then oriented and pressed into shape under a magnetic field strength of 1.8 T, resulting in a pressing density of 3.9 g / cm³. 3 Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billet is sintered in a sintering furnace with a vacuum level of 10. -3The magnet blank was sintered at 1095℃ for 4 hours, then heated to 1100℃ and held for 4 hours to obtain a 3.5mm thick sheet, which served as the magnet substrate. The thickness direction was the magnet orientation direction.

[0035] A Tb diffusion source was deposited on the surface of the magnet substrate, with a weight gain of 1 wt.%. The substrate was heated to 910°C in a vacuum sintering furnace for grain boundary diffusion and held at that temperature for 28 hours. After holding, the substrate was cooled at a rate of not less than 80°C / min until the magnet temperature was below 200°C. The substrate was then tempered at 475°C for 5 hours.

[0036] Comparative Example 2 The procedure is the same as in Example 1, except that the magnet blank is not heat-treated and is directly subjected to the subsequent grain boundary diffusion step.

[0037] The properties of the substrates and magnets before and after diffusion in Comparative Examples 1 and 2 and Examples 1-3 were measured using NIM magnetic testing equipment. The magnetic properties of the substrates are shown in Table 1, and the properties of the magnets after diffusion are shown in Table 2.

[0038] Table 1 Magnetic properties of the magnet matrix Table 2 Magnetic properties of the magnet after diffusion: The results showed that after adding silicon powder, the remanence, coercivity, maximum energy product, and squareness of the magnet matrix all decreased, and increased proportionally with the proportion of silicon powder added. This indicates that the addition of silicon powder reduces the magnetic properties of the magnet matrix. However, after diffusion, the coercivity of Examples 1-2 increased by more than 2 kOE compared to Comparative Example 1, and the squareness also improved significantly, indicating that the combination of silicon powder addition and heat treatment significantly improved the diffusion effect. Furthermore, the decrease in remanence was not significant. The coercivity of Example 3 increased by more than 3 kOE compared to Comparative Example 1, indicating that the higher the Si content, the more Fe is consumed, and the more significant the effect of heavy rare earth diffusion on improving coercivity. However, due to excessively high Si content, remanence, BH, and squareness all decreased significantly, resulting in poor overall magnet performance. Therefore, the amount of Si added should not be too high, ≤0.5% is sufficient.

[0039] Comparative Example 2 added silicon powder, but did not undergo heat treatment after sintering. The coercivity after diffusion increased by approximately 1 kOe compared to Comparative Example 1, but not as much as in Example 1. This is because during grain boundary diffusion after sintering, the temperature was 910℃ and the holding time was 24 hours. At this time, Si could partially combine with Fe, but the magnet surface was already covered by the diffusion source. Heavy rare earth elements had already diffused into the magnet grain boundaries and reacted with Fe, consuming some of the heavy rare earth elements, resulting in a less significant improvement in diffusion effect. Therefore, heat treatment should be performed before covering the diffusion source so that the silicon powder can better consume Fe.

Claims

1. A method for grain boundary diffusion in a high-Ga, low-B NdFeB magnet, characterized in that... The method includes the following steps: (1) The high Ga and low B NdFeB magnet raw materials are vacuum melted to obtain rapid solidification spun sheets, the rapid solidification spun sheets are hydrogen broken to obtain medium powder, the medium powder is air jet milled to obtain fine powder, silicon powder is added to the fine powder, mixed and then oriented and pressed and isostatically pressed to obtain a compact, the compact is vacuum sintered to obtain a magnet blank, and then heat treated to obtain a magnet matrix. (2) A diffusion source is covered on the surface of the magnet substrate to carry out grain boundary diffusion, and then tempering is performed to obtain a high Ga and low B neodymium iron boron magnet after diffusion.

2. The method as described in claim 1, characterized in that... The particle size of the silicon powder is 1~5μm.

3. The method as described in claim 1, characterized in that... The amount of silicon powder used is 0.2 to 0.5% of the mass of the fine powder.

4. The method as described in claim 1, characterized in that... In step (1), the heat treatment temperature is 950-980℃ and the holding time is 3-6h.

5. The method as described in claim 1, characterized in that... The raw materials for the high-Ga, low-B NdFeB magnets comprise the following components by mass percentage: Re: 29.5%~35%, Re is one or more of Pr, Nd, Dy, Tb, Gd, Ho, and Y; B：0.85~0.92％； M is at least one or more of Al, Cu, Zr, Ti, and Nb, and the mass content of M is greater than 0 and less than or equal to 3.5%. Ga: 0.4~0.5% The balance element is Fe, or Fe and Co, with Fe accounting for more than 90% of the balance element.

6. The method as described in claim 1, characterized in that... In step (2), the diffusion source for grain boundary diffusion is a heavy rare earth diffusion source, which is a pure heavy rare earth element metal, a heavy rare earth element hydride, or an alloy of heavy rare earth element and other metal elements; the heavy rare earth element is at least one of Dy and Tb, and the mass content of heavy rare earth element in the heavy rare earth diffusion source is ≥20wt.%.

7. The method as described in claim 1, characterized in that... In step (2), the weight gain of the diffusion source is 0.1~3wt%.

8. The method as described in claim 1, characterized in that... In step (2), the temperature for grain boundary diffusion is 880~920℃ and the holding time is 15~30h; the temperature for tempering is 400~650℃ and the holding time is 4~6h.

Images