Patent Description:
Neodymium iron boron material, also known as neodymium iron boron magnet, is composed of tetragonal Nd<NUM>Fe<NUM>B crystal as main body, and is the most widely used rare earth magnet. It is widely used in electronic products, such as hard disks, mobile phones, headsets and battery powered tools.

Neodymium iron boron material has excellent properties. At present, neodymium iron boron material is often used in high-temperature environment, and the performance thereof under high-temperature conditions is very important. The high-temperature performance of neodymium iron boron material is improved by attaching elements, alloys and compounds of heavy rare earths such as Dy and Tb on the surface of the magnet, in combination with the grain boundary diffusion technology of heavy rare earths, so as to diffuse the heavy rare earths through the grain boundary into the interior of the magnet under high-temperature heat treatment condition. The coercivity of the magnet is enhanced by the high magnetic anisotropy field of heavy rare earth, while the remanence is hardly reduced. Chinese patent publication no. <CIT> discloses a method for the grain boundary diffusion of a rare earth neodymium iron boron magnet including the following steps: preparing RTM alloy cast pieces containing heavy rare earth elements; preparing neodymium iron boron magnet blanks and processing them into blank squares; surface activation of blank square sheets; laying the RTM alloy cast piece on the bottom of the graphite box and placing the RTM alloy cast piece and the surface-activated blank square on top of each other until the graphite box is filled until the graphite box is filled; and the graphite box filled with the product is subjected to vacuum heat treatment to obtain the neodymium iron boron magnet. US patent publication no. <CIT> discloses a bulk high performance permanent magnet including a neodymium-iron-boron core having an outer surface, and a coercivity-enhancing element residing on at least a portion of said outer surface, with an interior portion of said neodymium-iron-boron core not having said coercivity-enhancing element therein. Also described herein is a method for producing the high-coercivity bulk permanent magnet, the method including: (i) depositing a coercivity-enhancing element on at least a portion of an outer surface of a neodymium-iron-boron core substrate to form a coated permanent magnet; and (ii) subjecting the coated permanent magnet to a pulse thermal process that heats said outer surface to a substantially higher temperature than an interior portion of said neodymium-iron-boron core substrate, wherein said substantially higher temperature is at least <NUM>, higher than said interior portion and is of sufficient magnitude to induce diffusion of said coercivity-enhancing element below said outer surface but outside of said interior portion.

However, the grain boundary diffusion technology is limited by the diffusion degree of heavy rare earth. For large thickness magnet with a thickness greater than <NUM>, the diffusion effect is very poor, rendering it difficult to apply the grain boundary diffusion technology to large thickness magnets. In view of the above problems, after attaching heavy rare earths on the surface of small thickness (less than <NUM>) neodymium iron boron single sheet, the laminated blank is obtained by gap free stacking, then the heavy rare earth is diffused, and the neodymium iron boron single sheets are weld to give a large thickness neodymium iron boron material.

For the neodymium iron boron magnet obtained in the above related technologies, the welding strength between adjacent neodymium iron boron single sheets is very low, resulting in poor overall mechanical properties and service properties of the neodymium iron boron magnet, which hinders the present application of the product.

In order to improve the welding strength between neodymium iron boron single sheets, and in turn improve the overall mechanical properties and service properties thereof, this application provides a preparation method of neodymium iron boron products.

In a first aspect, the present application provides the following technical solutions: a preparation method of neodymium iron boron products, including the following steps:.

By adopting the above technical solutions, the blank magnet is obtained by purchase or self-made, a plurality of blank magnets are stacked after coating with heavy rare earth layers, to give magnets greater than <NUM> under the grain boundary diffusion treatment, so as to realize the application of grain boundary diffusion on large thickness magnets. Such technical solution adopts the diffusion step of primary heat treatment, then heat treatment again and final tempering. In addition, an induction heat treatment or electric spark sintering is specifically used in the heat treatment. In particular, the induction heat treatment is a surface heat treatment process for locally heating the workpiece by using induced current; and the electric spark sintering is a sintering method which performs sintering at the high temperature produced by spark discharge between powders while subjecting to an external stress. In addition, this technical solution is applicable to the welding of blank magnets with a single sheet thickness of <NUM>-<NUM>. It can be seen from the test that it can not only improve the coercivity, remanence and squareness of neodymium iron boron products made of <NUM>-<NUM> blank magnets, but also realize the diffusion to blank magnets with a thickness of <NUM>-<NUM>.

The above technical solution greatly improves the welding strength between blank magnets, and can slightly improve the coercivity, remanence and squareness of magnets. At the same time, it is also found that it can greatly improve the anti magnetic attenuation performance of magnets.

In the process of heat treatment, heavy rare earth diffuses along the grain boundary to the interior of two magnets it contacts. In the process of diffusion, the use of rare earth enhances the effect of liquid phase mass transfer and achieves a good welding effect.

Further, in Step S6, the stacked magnet is subjected to a secondary heat treatment under a pressure of <NUM>-50MPa.

In the above technical solution, it can be seen from the test that, in combination with pressurization, the coercivity and remanence of the magnet can be slightly improved, and the anti magnetic attenuation performance and welding strength of the magnet can be further improved.

Further, in Step S6, a later stage of the primary heat treatment accounting for <NUM>-<NUM>% of the total time of the primary heat treatment is carried out under a pressure of <NUM>-50MPa.

In the above technical solution, It can be seen from the test that, in combination with pressurization, the coercivity, remanence and squareness of the magnet can be stabilized and slightly improved, and the anti magnetic attenuation performance and welding strength of the magnet can be further improved.

Further, the pressurization treatment in Step S6 is a pressurization in an atmosphere of <NUM>-30MPa.

In the above technical solution, nitrogen or inert gas is generally selected for atmosphere pressurization, which is combined with high temperature to achieve heat treatment, improving the efficiency of surface heating. It can be seen from the test that the remanence, coercivity and squareness of the magnet can be slightly improved.

Further, the induction heat treatment frequency is <NUM>-<NUM>.

In the above technical solution, on the basis of induction heat treatment, the current is further limited to a medium frequency band. It can be seen from the test that, the positive effect on magnet performance, welding strength and anti magnetic attenuation performance can be further achieved in a small extent on the basis of stabilizing the squareness.

Further, in Step S6, during the primary heat treatment, the heating mode of stacked magnets is transverse flux heating.

In the above technical solution, when performing the transverse magnetic flux heating, the welding surface is perpendicular to the direction of the induced magnetic field, which, cooperated with the medium-frequency frequency, achieves a better heating effect on the magnet with a small thickness, so as to improve the uniformity of surface heating. It can be seen from the test that the positive effect on coercivity and remanence of the magnet can be further achieved on the basis of stabilizing squareness, and the anti magnetic attenuation performance of the magnet can also be improved at the same time.

Further, in Step S2, the area of a single welding surface of the blank magnet is <NUM>-<NUM><NUM>.

In the above technical solution, the difficulty of operation due to too small welding surface of blank magnet is avoided, ensuring the firmness of the welding.

Further, in Step S6, a temperature for the secondary heat treatment is <NUM>-<NUM> and a temperature for the tempering is <NUM>-<NUM>.

In the above technical solution, it can be seen from the test that after further limiting the heat treatment temperature and tempering temperature, the heat treatment effect can be improved, and the coercivity, remanence and squareness of the magnet can be improved in a small extent on the basis of stabilizing the squareness.

Further, when the primary heat treatment is electric spark sintering, a temperature for the sintering is <NUM>-<NUM>, a current is <NUM>-5000A and a voltage is <NUM>-<NUM> V.

In the above technical solution, it can be seen from the test that the electric spark sintering process within such parameters can slightly improve the coercivity, remanence and squareness of the magnet.

In a second aspect, the present application provides the following technical solutions: a neodymium iron boron product, which is prepared by the above preparation method.

In the above technical solution, a neodymium iron boron product with excellent coercivity, remanence and squareness and excellent anti magnetic attenuation performance can be obtained.

In summary, this application has the following beneficial effects.

Examples <NUM>, <NUM>, <NUM> and <NUM> are part of the invention. Examples <NUM>-<NUM> and <NUM>-<NUM> are outside the scope of the invention.

Example <NUM>: a preparation method of neodymium iron boron products, including the following steps:.

Before use, the hot pressing furnace, the diffusion furnace and the tempering furnace were vacuumed with an air pump, and then introduced therein with argon to protect the magnets.

Before use, the electric spark sintering machine, the diffusion furnace and the tempering furnace were introduced with argon to evacuate air thereinside, so as to protect the magnets.

Before use, the hot pressing furnace, the diffusion furnace and the tempering furnace were introduced with argon to evacuate the air thereinside, so as to protect the magnets.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that:
in step S4: terbium was coated on the two welding surfaces of ① and ②; and, in Step S6, after being transferred to the diffusion furnace, a secondary heat treatment was carried out at a temperature of <NUM>, in which the thickness of the heavy rare earth layer was <NUM>ο.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that:.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, <NUM> iron block was pressed onto the <NUM>* <NUM> surface of the stacked magnet during the secondary heat treatment.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, after transferring the stacked magnet to the diffusion furnace, argon was first introduced into the diffusion furnace until the pressure in the furnace reached 10MPa, and the secondary heat treatment was carried out at <NUM> for <NUM>.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, after transferring the stacked magnet to the diffusion furnace, argon was first introduced into the diffusion furnace until the pressure in the furnace reached 30MPa, and the secondary heat treatment was carried out at <NUM> for <NUM>.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, after placing the stacked magnet into the hot pressing furnace, argon was first introduced into the diffusion furnace until the pressure in the furnace reaches 30MPa, and then induction heat treatment was carried out.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, after placing the stacked magnets into the hot pressing furnace, argon was first introduced into the diffusion furnace until the pressure in the furnace reached 10MPa, and then induction heat treatment was carried out.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, after placing the stacked magnet into the hot pressing furnace, induction heat treatment was carried out for after <NUM>, <NUM> iron block was pressed onto the stacked magnet, and then the induction heat treatment was continued.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, after placing the stacked magnets into the hot pressing furnace, induction heat treatment was carried out for <NUM>, argon was introduced into the diffusion furnace until the pressure in the furnace reached 10MPa, and induction heat treatment was continued.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the induction heat treatment frequency was <NUM>.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, transverse magnetic flux heating was carried out during the induction heat treatment, in which the welding surface was perpendicular to the magnetic field direction of the induction coil.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the secondary heat treatment temperature was <NUM>, and the treatment time was <NUM>; the tempering temperature was <NUM> and the tempering time was <NUM>.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the secondary heat treatment temperature was <NUM> and the treatment time was <NUM>; the tempering temperature was <NUM> and the tempering time was <NUM>.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the parameters of electric spark sintering were: voltage 8V, current 3500A, sintering temperature <NUM>, and pressure 20MPa.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the parameters of electric spark sintering were: voltage 10V, current 4100A, sintering temperature <NUM>, and pressure 6MPa.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the parameters of electric spark sintering were: voltage 15V, current 5000A, sintering temperature <NUM>, and pressure 50MPa.

Example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S2, the blank magnet was machined to a size of <NUM>*<NUM>*<NUM>, and marked as ①, ② respectively, to give the preprocessed sheets, in which the surface of <NUM>*<NUM> was the welding surface.

Comparative example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the stacked magnets were put into the diffusion furnace, heat treated at <NUM> for <NUM> under 1atm condition, and then transferred to the tempering furnace for tempering.

Comparative example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S2, the blank magnet was machined to a size of <NUM>*<NUM>*<NUM>; and, in Step S6, the stacked magnets were placed into the diffusion furnace, heat treated at <NUM> for <NUM> under 1atm condition, and then transferred to the tempering furnace for tempering.

Comparative example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the secondary heat treatment was directly carried out without the primary heat treatment.

Comparative example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the primary heat treatment was carry out in a diffusion furnace, at a temperature of <NUM> for <NUM>.

Comparative example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the stacked magnets were placed into the diffusion furnace, heat treated at <NUM> for <NUM> at 1atm condition, and then transferred to the tempering furnace for tempering.

Comparative example <NUM>: a preparation method of neodymium iron boron products, which was different from example <NUM> in that: in Step S6, the stacked magnet was put into the diffusion furnace, heat treated at <NUM> for <NUM> at 1atm condition, and then transferred to the tempering furnace for tempering.

Test object: the neodymium iron boron products obtained in examples <NUM>-<NUM> and comparative examples <NUM>-<NUM>, a total of <NUM> groups of test samples.

Test method: the remanence (Br), coercivity (Hcj), maximum magnetic energy product ((BH) max) and squareness (Q) of the test samples were tested. The size of the test sample during the test was <NUM> wide*<NUM> length*<NUM> height, of which <NUM> thickness was composed of adjacent single sheets, including the heavy rare earth layer. According to GB/T <NUM>-<NUM> permanent magnet (hard magnetic) materials - magnetic test method, the remanence (Br), coercivity (Hcj) and maximum magnetic energy product ((BH) max) of the test samples were tested, and the squareness (Q) was obtained as the ratio of knee-point coercivity Hk on the demagnetization curve to intrinsic coercivity Hcj.

Test results: the records of basic performance test results were shown in Table <NUM>.

The following units are used: <NUM> kGs = <NUM> T; <NUM> kOe = <NUM>/4π kA/m; <NUM> MGOe = <NUM>/4π kJ/m<NUM>.

Data analysis: for magnets with different brands, since the basic performances thereof are different, and the increase and decrease of performance after diffusion are different, they can not be compared with each other. Therefore, the examples and comparative examples with the same brand are compared. Examples <NUM>, <NUM>-<NUM>, <NUM>-<NUM> and comparative examples <NUM>-<NUM> are group I; example <NUM> and comparative example <NUM> are group II; example <NUM>, examples <NUM>-<NUM> and comparative example <NUM> are group III.

It can be seen from the data in Table <NUM> that, in individual groups, the remanence (Br), coercivity (Hcj), maximum magnetic energy product ((BH) max) and squareness (Q) of the examples are all better than those of the comparative examples. In particular, the order of remanence (Br), coercivity (Hcj) and maximum magnetic energy product ((BH) max) in group I from poor to excellent is: comparative examples <NUM>-<NUM>, examples <NUM> and <NUM>-<NUM>, examples <NUM>-<NUM>, examples <NUM>-<NUM> and <NUM>, examples <NUM>-<NUM>, examples <NUM>-<NUM>, examples <NUM> and examples <NUM>-<NUM>; and the order of squareness (Q) from poor to excellent is: comparative example <NUM> and comparative examples <NUM>-<NUM>, examples <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM>. Example <NUM> and comparative example <NUM> are large thickness products, which need to be compared separately.

In group I, example <NUM> and comparative example <NUM>, as well as example <NUM> and comparative example <NUM> are compared with each other. Two blank magnets with a thickness of <NUM> in comparative example <NUM> are welded by conventional heating, and two blank magnets with a thickness of <NUM> in comparative example <NUM> are welded by the welding method in comparative example <NUM>. It can be seen from the test results that individual performances in example <NUM> are better than those in comparative example <NUM>, and individual performances in example <NUM> are better than those in comparative example <NUM>. On the one hand, it indicates that the primary heat treatment and secondary heat treatment adopted in this solution and the specific methods of the primary heat treatment can effectively improve the magnetic attenuation (Br), coercivity (Hcj), maximum magnetic energy product ((BH) max) and squareness (Q). On the other hand, the magnet with a thickness of <NUM> can be treated by adopting this solution, and performance thereof can be improved to a certain extent. In comparison, the squareness and coercivity of magnet in comparative example <NUM> are very low, indicating that the existing preparation method in comparative example <NUM> is not suitable for the welding of large thickness blank magnets.

Example <NUM> is compared with comparative examples <NUM>-<NUM>. Primary heat treatment is omitted in comparative example <NUM>, and that in comparative example <NUM> is replaced by high-temperature heat treatment at <NUM> for equal time. It can be seen from the test that, the four performances in comparative examples <NUM>-<NUM> are worse than those in example <NUM>. The reason may be that the combination of primary heat treatment and secondary heat treatment in the technical solution and the limitation of primary heat treatment improve the welding effect and the diffusion effect of heavy rare earth, and the two kinds of primary heat treatment methods can better concentrate the heating area on the magnet surface, so as to achieve the purpose of effectively heating the heavy rare earth layer.

In examples <NUM> and <NUM>-<NUM>, the thickness of the blank magnets in examples <NUM>-<NUM> are changed on the basis of example <NUM>. It can be seen from the test that magnets in examples <NUM>-<NUM> can achieve the same level of performance as that in example <NUM>. In addition, corresponding methods in example <NUM> and comparative example <NUM> are diffusion process for large thickness blank magnets of <NUM>. Example <NUM> has better magnet performance than comparative example <NUM>, which shows that the technical solution has diffusion effect on large thickness blank magnets of <NUM> and improve the performance of the magnet. In addition, compared with examples <NUM>-<NUM>, the increase in example <NUM> is smaller, and the increase is small due to its large thickness.

In examples <NUM>-<NUM>, on the basis of example <NUM>, a pressurization treatment is added in the secondary heat treatment process. It can be seen from the test that the coercivity and remanence of the magnet can be improved in a small extent on the basis of improving the squareness in combination with pressurization. The reason may be that pressurization can better promote the further diffusion of heavy rare earth during the secondary heat treatment, so as to improve the diffusion effect of heavy rare earth layer.

In example <NUM>, the pressurization treatment is added during primary heat treatment on the basis of example <NUM>, which can effectively improve the diffusion efficiency of heavy rare earth on the basis of example <NUM>, so as to improve the performance of magnets.

In examples <NUM>-<NUM>, the pressurization treatment during the primary heat treatment is limited on the basis of example <NUM>, which can stabilize and slightly improve the coercivity, remanence and squareness of the magnet. Moreover, in examples <NUM>-<NUM>, the performances of examples <NUM>-<NUM> are better than that of example <NUM>. On the one hand, it indicates that the pressurization treatment in the primary heat treatment can slightly improve the magnet performance, on the other hand, it indicates that the pressurization treatment in the later stage of the primary heat treatment can further effectively improve the magnet performance and reduce energy consumption in the same time.

In examples <NUM>-<NUM>, the frequency of induction heat treatment is limited to <NUM>-<NUM> on the basis of example <NUM>. It can be seen from the test that the remanence, coercivity and squareness of the magnet can be slightly improved on the basis of stabilizing the squareness. In example <NUM>, the mode of induction heat treatment is limited to transverse flux heating on the basis of example <NUM>, which further realizes the positive effect on the coercivity and remanence of the magnet on the basis of stabilizing the squareness. The possible reason is that both of the limitation of induction heat treatment frequency and the heating mode of transverse flux heating improve the diffusion effect of heavy rare earth. In example <NUM>, the mode of transverse magnetic flux heating is limited on the basis of example <NUM>, which also improves the performance of the magnet on the basis of example <NUM>, and the improvement extent is smaller than that of examples <NUM>-<NUM>. On the one hand, it indicates the positive effect of transverse flux heating on the magnet performance, on the other hand, it indicates that the combination of transverse flux heating, induction heat treatment frequency and pressurization treatment can achieve a better diffusion effect of heavy rare earth.

In examples <NUM>-<NUM>, the temperature and time of secondary heat treatment are further limited on the basis of example <NUM>. It achieves a positive effect on the coercivity, remanence and squareness of the magnet in a small extent.

In group II, example <NUM> is compared with comparative example <NUM>. The blank magnets of N54SH in comparative example <NUM> are welded by conventional heating. Individual performances of magnet in example <NUM> are better than those in comparative example <NUM>, indicating that the primary heat treatment and secondary heat treatment adopted in this solution can effectively improve the performance of the magnet.

In group III, example <NUM> is compared with comparative example <NUM>. The blank magnets of N50SH in comparative example <NUM> are welded by conventional heating. Individual performances of magnet in example <NUM> are better than those in comparative example <NUM>, indicating that the primary heat treatment and secondary heat treatment adopted in this solution, and the mode of electric spark sintering adopted in primary heat treatment can effectively improve the performance of the magnet. In examples <NUM>-<NUM>, the parameters of electric spark sintering are further limited on the basis of example <NUM>, which slightly improves the performance of the magnets on the basis of stabilizing the squareness.

Test method: four parallel samples were prepared for each group of test samples, three of which shall be tested for welding strength, and the remaining samples shall be tested for anti magnetic attenuation performance.

Welding strength test: the parallel samples were cut in the direction perpendicular to the heavy rare earth layer to a size of <NUM> length*<NUM> wide*<NUM> height, of which the heavy rare earth layer in the middle was included in the <NUM>. The test was carried out on the universal material testing machine, in which the bending test fixture was installed on the workbench, and the spacing between the two pressure rollers was adjusted to <NUM>; and the bending punch was installed at the lower end of the moving beam, and the bending punch was kept parallel to the two pressing rollers and located between the two pressing rollers. Two ends of the parallel sample were placed on two pressing rollers, and the heavy rare earth layer was directly below the punch. The oil delivery valve was opened to start loading. After the sample being damaged, the oil delivery valve was closed and the shear strength (MPa) test data was recorded as the welding strength value.

Anti magnetic attenuation performance test: examples <NUM>, <NUM>-<NUM>, <NUM>-<NUM> and comparative examples <NUM>-<NUM> were recorded as group I; example <NUM> and comparative example <NUM> were recorded as group II; example <NUM>, examples <NUM>-<NUM> and comparative example <NUM> were recorded as group III. The samples with the same brand were divided into the same group for comparison.

The magnetic fluxes of the samples in individual groups were first tested to obtain the original magnetic fluxes, the the samples were placed in the oven at <NUM> for <NUM>, then taken out and cooled to room temperature, the magnetic fluxes of the parallel samples were tested to obtain the final magnetic fluxes, and the magnetic attenuation (%) = [(original magnetic flux - final magnetic flux) / original magnetic flux] * <NUM>%.

Test results: the test result records of welding strength and anti magnetic attenuation performance test were shown in Table <NUM>.

Data analysis: the greater the shear strength is, the greater the force required to disconnect the weld is, indicating that the better the welding effect; and the smaller the magnetic attenuation is, the smaller the magnetic flux loss is, and the better the anti magnetic attenuation effect is.

It can be seen from the data in Table <NUM> that, in group I, the order of welding strength and anti magnetic attenuation performance from good to bad is: example <NUM> and examples <NUM>-<NUM>; examples <NUM>-<NUM>; examples <NUM>-<NUM>; examples <NUM>-<NUM> and example <NUM>; example <NUM> and examples <NUM>-<NUM> and example <NUM>-<NUM>; comparative example <NUM> and comparative examples <NUM>-<NUM>. Example <NUM> and comparative example <NUM> are compared separately.

Example <NUM> is compared with comparative examples <NUM>-<NUM>. The conventional heating treatment is used in comparative examples <NUM>-<NUM> to realize grain boundary diffusion and welding, and in contrast, the methods of primary heat treatment and secondary heat treatment are adopted in example <NUM>, in which the primary heat treatment is limited to induction heat treatment, and the electric spark sintering is adopted in example <NUM>, thereby greatly improving the anti magnetic attenuation performance of the magnets. In addition, the welding strength between blank magnets is also greatly improved.

In addition, it can be seen from the data of examples <NUM>-<NUM> and the comparison between the data of comparative example <NUM> and example <NUM> that, after welding the blank magnet with a thickness of <NUM>-<NUM>, the anti magnetic attenuation performance and welding strength can also be improved. The reasons may lie in that: the diffusion efficiency of heavy rare earth is improved, such that the heavy rare earth layer goes deeper into the interior of the magnet, which improves the overall anti magnetic attenuation performance of the magnets, and, at the same time, achieves full diffusion effect. Although the conventional heat treatment is adopted in comparative example <NUM> for the large thickness blank magnets, the welding strength and anti magnetic attenuation performance are very poor, which indicates that this technical solution can effectively realize the diffusion welding of large thickness magnets.

In examples <NUM>-<NUM>, a pressurization treatment is added in the secondary heat treatment process on the basis of example <NUM>; and in example <NUM>, the pressurization treatment is added during primary heat treatment on the basis of example <NUM>, both of which improve the anti magnetic attenuation performance and welding strength of the magnets. The reason may be that, under the action of pressure, the blank magnets are tightly bonded and the heavy rare earth layer is more dense, which improves the diffusion efficiency of heavy rare earth, makes the heavy rare earth layer go deeper into the interior of the magnet and achieves the effect of full diffusion.

In examples <NUM>-<NUM>, the pressurization treatment is added in the primary heat treatment on the basis of example <NUM>, which further improves the welding strength of the magnets; and examples <NUM>-<NUM> achieve better effect; indicating that pressurization treatment in the later stage of primary heat treatment can further improve the diffusion effect of heavy rare earth in a single magnet and improve the internal performance of the magnet, so as to improve the overall anti magnetic attenuation performance of the magnet and reduce energy consumption at the same time.

In examples <NUM>-<NUM>, the induction heat treatment frequency is limited on the basis of example <NUM>, which further improves the welding effect and anti magnetic attenuation performance. It indicates that the frequency of induction heat treatment can directly affect the diffusion effect, thus affecting the magnet welding effect and the final performance of the product.

In example <NUM>, the transverse magnetic flux heating is added on the basis of example <NUM>. Combined with the medium-frequency frequency, it achieves a better heating effect on the magnet with a small thickness and improves the uniformity of surface heating, so as to promote the diffusion of heavy rare earth into the interior of magnet and improve the anti magnetic attenuation performance of the magnet. In example <NUM>, the transverse magnetic flux heating is added on the basis of example <NUM>, which achieves a better anti magnetic attenuation performance than example <NUM>.

In group II, example <NUM> is compared with comparative example <NUM>. In comparative example <NUM>, conventional heating is used to weld the blank magnet of N54SH. The welding strength and anti magnetic attenuation performance of example <NUM> are better than those of comparative example <NUM>, indicating that the primary heat treatment and secondary heat treatment adopted in this solution can effectively improve the welding strength and anti magnetic attenuation performance.

In group III, example <NUM> is compared with comparative example <NUM>. In comparative example <NUM>, conventional heating is used to weld the blank magnet of N50SH. Each performance in example <NUM> is better than that in comparative example <NUM>, indicating that the primary heat treatment and secondary heat treatment adopted in this solution, and the limitation of the primary heat treatment to electric spark sintering, can effectively improve the welding strength and anti magnetic attenuation performance. In examples <NUM>-<NUM>, the parameters of electric spark sintering are further limited on the basis of example <NUM>, which slightly improves the performance of the magnet on the basis of stabilizing the anti magnetic attenuation performance.

Claim 1:
A preparation method of neodymium iron boron products, characterized by comprising the following steps:
Step S1: preparing blank magnet;
Step S2: obtaining preprocessed sheets: forming at least one group of welding surface from two opposite sides of the blank magnets, wherein the side of the blank magnet acts as a welding surface, and the distance between two welding surfaces in each group of welding surface is <NUM>-<NUM>; and magnetizing the blank magnet in a direction forming an angle relative to the welding surface to obtain the preprocessed sheets;
Step S3: surface treating: cleaning the preprocessed sheets;
Step S4: heavy rare earth coating: after surface treating, coating a heavy rare earth layer on the welding surface of the preprocessed sheet, wherein the heavy rare earth layer is one selected from a group consisting of heavy rare earth element, heavy rare earth alloy and heavy rare earth compound;
Step S5: stacking: stacking a plurality of preprocessed sheets to give stacked magnets, wherein there is at least one heavy rare earth layer between adjacent preprocessed sheets; and
Step S6: grain boundary diffusion: successively subjecting the stacked magnets to a primary heat treatment for <NUM>-<NUM>, a secondary heat treatment at <NUM>-<NUM> for <NUM>-<NUM>, and then tempering at <NUM>-<NUM>, characterised in that the primary heat treatment is one selected from a group consisting of induction heat treatment and electric spark sintering; and in that, in Step S6, the heating mode of stacked magnets is transverse flux heating during the primary heat treatment.