JP4283802B2 - Manufacturing method of NdFeB-based sintered magnet - Google Patents

Description

  The present invention relates to a method for producing a NdFeB-based sintered magnet, and in particular, an NdFeB-based sintered magnet alloy powder (hereinafter referred to as an alloy powder) is a container designed according to the shape and dimensions of a product (hereinafter referred to as this). To obtain a NdFeB sintered magnet of the desired shape by applying a magnetic field to the alloy powder, aligning the crystal orientation of the powder, and heating and sintering the entire container with the alloy powder in place. It is about. Hereinafter, this method is referred to as a pressless process.

In no conventional press process, the alloy powder having an average particle size of 2 to 5 [mu] m, the filling density is charged into the mold so as to ^{2.7g / cm 3 ~3.5g / cm 3} , and placing the lid on the mold upper surface, The powder was oriented by applying a magnetic field, then sintered, and the sintered body was taken out of the mold and subjected to an aging treatment. The contents are shown in Patent Document 1. Here, although the measuring method of the particle size of the alloy powder is not described in Patent Document 1, it is considered that the method is based on the Fisher method generally used at the time of patent application related to the document (1993).

Japanese Unexamined Patent Publication No. 7-13612

The inventor has noticed a significant problem with this technology in the process of implementing the technology of the pressless process. That is, if a mold is made of a common metal material such as stainless steel, which is easy to process precisely, the mold cannot be used over and over again. In the sintering process, the mold is deformed or welded with the alloy powder, resulting in large irregularities on the inner surface of the mold, making it unusable. If a mold is made of a heat-resistant metal such as molybdenum or tungsten, it is expensive, and it becomes brittle due to a temperature rise during sintering, so it cannot be used many times. The technology of the pressless process is not used much as a mass production process of NdFeB sintered magnet. This was partly due to the short life of this mold. On the other hand, it is conceivable to use ceramic in terms of heat resistance, but ceramic is unsuitable as a mold material because precise molding is difficult and mechanically brittle.
The subject of this invention is providing the manufacturing method of the NdFeB sintered magnet which can use the mold produced with the metal which is easy to process in this technique many times, such as stainless steel.

  The above-described deformation of the mold and damage to the inner surface of the mold are caused by the sintering temperature being too high. In the example of Patent Document 1, the sintering temperature is described as 1100 ° C. When a mold made of an iron-based alloy such as stainless steel is heated to a high temperature of 1100 ° C, it cannot be deformed or welded to the alloy powder. From the time NdFeB sintered magnets were born until recently, sintering temperatures were generally around 1100 ° C. Recently, the use of a strip cast alloy as an alloy raw material has led to a decrease in the sintering temperature of the NdFeB sintered magnet, and the temperature around 1050 ° C. to 1070 ° C. is becoming common. However, when a metal mold is used in the pressless process technology, even if the sintering temperature is 1050 ° C., the degree of deformation and damage of the mold is not improved so much. In the pressless process, the sintering temperature needs to be 1000 ° C. or less in order to extend the life of the metal mold.

The present inventor has found that the sintering temperature can be greatly reduced by sufficiently reducing the particle size of the alloy powder and sufficiently reducing the oxygen content in the powder, and has applied it to the pressless process. NdFeB sintered magnets are manufactured by changing the sintering temperature for various types of alloy powders with different alloy composition, powder particle size, and oxygen content in the non-pressing process, and the density, magnetic properties and mold of the sintered magnet are produced. The degree of welding of the sintered body and the degree of deformation of the mold were examined. As a result, when the particle size of the alloy powder evaluated by the median value (D ₅₀ ) of the particle size distribution curve measured with a laser type particle size distribution measuring instrument is 3 μm or less and the oxygen content is 2000 ppm or less, the sintering temperature is 1000 ° C. We found out that we can: That is, if this particle size and oxygen content are satisfied, the sintering density is 7.45 g / cm ³ or more and the coercive force H _cJ (as a rare earth) even if the sintering temperature is 900 to 1000 ° C., which is lower than the conventional temperature. The present invention has been completed by finding that it can be increased to 12 kOe or higher (even in an alloy composition mainly containing only Nd) and can greatly extend the life of the mold.
Here, the oxygen content of the alloy powder can be measured by quantitatively analyzing the gas released by heating the alloy powder at a high temperature by infrared spectroscopy. This measurement can be performed, for example, with a LECO oxygen quantitative analyzer.

Under the manufacturing conditions described above, even if the amount of oxygen in the powder is 2000 ppm or less, the sintering temperature cannot be lowered to 1000 ° C. or less if the particle diameter D ₅₀ of the powder is 3 μm or more. If the sintering temperature is lowered to 1000 ° C. or lower, the density of the sintered body cannot be increased to 7.45 g / cm ³ or more, and the magnetic properties cannot be improved. Even if the D _{50 of} the powder is 3 μm or less, if the oxygen content in the powder is 2000 ppm or more, the density of the sintered body after sintering cannot be made 7.45 g / cm ³ or more, and the magnetic properties cannot be improved.

By the way, Patent Document 1 describes the range of 2 μm to 5 μm as the powder particle size, and describes the use of powder having a particle size of 3 μm in the examples. Since there was no reliable laser type particle size distribution measuring instrument at the time of the application according to Patent Document 1, the particle size of the powder was evaluated by an air transmission type particle size measuring instrument called the Fisher method. In the case of NdFeB sintered magnet alloy powder, the powder having a particle size of 2 μm measured by the Fisher method is estimated to be a powder having a D ₅₀ of about 3 μm when measured by a laser particle size distribution meter. Patent Document 1 discloses an example in which a powder having a particle size of 3 μm is sintered by a pressless process, and the sintering temperature at this time is 1100 ° C. This indicates that the particle size measurement of the powder in this example was measured by the Fisher method. This is because the optimum sintering temperature of the powder evaluated as 3 μm particle size by the Fisher method is about 1100 ° C. On the other hand, when a powder with a particle size D ₅₀ of 3 μm as measured with a laser particle size distribution meter is sintered at 1100 ° C., it can be easily proved experimentally that it becomes an extremely oversintered state. . The oversintered state refers to a state in which giant crystal grains that reach 50 to 100 times the average grain size appear in the sintered body (referred to as abnormal grain growth). An NdFeB sintered magnet having such a structure is extremely low in coercive force and squareness of the magnetization curve, and may be said to have no value as an industrial material. In the example of Patent Document 1, since the sintered magnet produced by sintering powder having a particle size of 3 μm at 1100 ° C. exhibits high magnetic properties, the value of 3 μm described in Patent Document 1 is obtained by the Fisher method. It is clear that it was evaluated.

  In addition to Fe-Ni alloys such as stainless steel and permalloy, there are various iron-based, nickel-based, or cobalt-based alloys with improved heat resistance as mold materials. When a mold is made of these heat-resistant alloys, thermal deformation is improved in the case of sintering at 1000 ° C. or higher than that of stainless steel. However, according to experiments, no improvement was observed in the welding with the alloy powder. Therefore, from the viewpoint of the life of the mold, even if a mold is made of these heat-resistant alloys, it is an essential condition for industrialization of the pressless process that the sintering temperature is 1000 ° C. or less.

  According to the present invention, the sintering temperature can be reduced to 1000 ° C. or less in the NdFeB sintered magnet pressless process, the problem that the mold life in the conventional pressless process is short, and the mold and alloy during sintering The problem that the powder was welded and the mold was broken was solved. As a result, the pressless process can be used industrially.

An alloy of 31.5% Nd, 1% B, 0.2% Al, and 0.1% Cu balance Fe by weight ratio was prepared by strip casting, and laser particle size distribution analyzer (manufactured by SYMPATECH) with hydrogen crushing and jet mill. Three types of alloy powders having a measured median D ₅₀ of 2.0 μm, 2.9 μm, and 5 μm were prepared. These powders were mixed with a mixing stirrer equipped with a rotating blade at a rotation speed of the blade of 500 rpm and 0.5% of the lubricant methyl caproate was added. When the oxygen content of the alloy powder was measured, it was 1600 ppm for the D ₅₀ = 2.0 μm powder, 1300 ppm for the D ₅₀ = 2.9 μm powder, and 980 ppm for the D ₅₀ = 5 μm powder.

A stainless container made by deep drawing was used as a mold for filling the alloy powder. This stainless steel container is manufactured as a battery case for a mobile phone, and has a rectangular parallelepiped cavity of depth 47.7 mm, length 7 mm, width 33 mm, and is made of nonmagnetic stainless steel having a thickness of 0.3 mm. Coating was applied to the inner surface of the container as follows. First, wax having a melting point of 50 ° C. and BN powder having an average particle diameter of 5 μm were mixed at a weight ratio of 60:40, and 2.5 g of this mixture was added to 1 kg of zirconia balls having a diameter of 0.5 mm. This zirconia ball is called impact media. A mixture obtained by adding wax and BN to impact media was placed in a 500 cc beaker and heated to 80 ° C., and the beaker was shaken to stir the mixture. Thereafter, the mixture was placed in the stainless steel mold and the mold was rocked for 3 minutes, and then the mixture was taken out of the mold to form a film of a mixture of powder and wax on the inner surface of the mold. The alloy powder thus coated is filled with the alloy powder so that the packing density is 3.7 g / cm ³ , and a magnetic field is applied in the longitudinal direction of the mold (direction parallel to the shortest side of the rectangular parallelepiped). After orientation of the alloy powder, the entire mold was heated to produce a sintered body of the alloy powder. In this example, all the steps for handling the alloy powder were performed in high-purity Ar, and sintering was performed in a high vacuum to prevent the alloy powder from being oxidized as much as possible.

For the three types of alloy powders having different D ₅₀ , the sintering temperature was changed and the density after sintering was measured. As a result, the temperature at which the density exceeded 7.45 g / cm ³ for each alloy powder was as follows.
D ₅₀ = 2.0μm alloy powder: sintering temperature 800 ° C
D ₅₀ = 2.9μm alloy powder: sintering temperature 950 ° C
D ₅₀ = 5.0 μm alloy powder: sintering temperature 1040 ° C
A density of 7.45 g / cm ³ or more is one of the essential requirements for NdFeB sintered magnets as industrial products. If the density is below this value, such magnets have poor magnetic properties and are susceptible to corrosion. A density of 7.45 g / cm ³ or more is an essential requirement. When a sintered magnet is made by a press-free process using the above-mentioned three types of alloy powder, the lower limit of the sintering temperature is the above-mentioned respective temperature of 800 ° C., 950 ° C and 1040 ° C.

In addition to the fact that the density of the sintered body is 7.45 g / cm ³ or more, the optimum value of the sintering temperature is also determined from the viewpoint of the temperature at which the magnetic properties are maximized. It was confirmed that the temperature at which the magnetic properties, particularly the coercive force H _cJ, was the highest, was slightly higher than the temperature at which the density of the sintered body reached the upper limit and no longer increased.

From the above results, the optimum sintering temperature for the above-mentioned three types of alloy powders was as follows.
D ₅₀ = 2.0 μm alloy powder: sintering temperature 940 ° C
D ₅₀ = 2.9μm alloy powder: sintering temperature 975 ° C
D ₅₀ = 5.0 μm alloy powder: sintering temperature 1050 ° C.
Each alloy powder filled in the mold at these sintering temperatures was sintered while being put in the mold, and after cooling, the sintered body was taken out of the mold and subjected to heat treatment to produce a sintered magnet.

As a result of measuring the magnetic properties of the three types of sintered magnets, the residual magnetic flux density and the maximum energy product were _almost the same for all magnets, but there was a difference in the coercive force H _cJ . The coercive force of the magnet made from D ₅₀ = 5.0 μm powder was 13.2 kOe, but the one made from D ₅₀ = 2.9 μm powder was 16.5 kOe, and the magnet from D ₅₀ = 2.0 μm powder was also It had a coercive force of 17.3 kOe.

Thus, powder with smaller particle size can be sintered at a lower temperature and the coercive force becomes higher. However, if the sintering temperature is too low, the coercive force may not reach a sufficiently large value of 12 kOe even if the sintering density is sufficiently high. For example, in the case of the above-mentioned alloy powder of D ₅₀ = 2.0 μm, the sintering density reaches a high density of 7.45 g / cm ³ or more even when the sintering temperature is as low as 800 ° C., but the coercive force is 12 kOe. Does not reach. The coercive force reaches 12 kOe or higher when the sintering temperature is 900 ° C. or higher. This lower limit of 900 ° C. hardly changes when the D _{50 of} the alloy powder is 3.0 μm or less and the oxygen content is in the range of 2000 ppm. Therefore, it is desirable that the sintering temperature in the present invention is 900 ° C. or higher and 1000 ° C. or lower as described above.

An experiment was repeated in which the same mold was coated each time with the above-mentioned three types of powders and sintered at the respective sintering temperatures described above. As a result, for alloy powders with D ₅₀ = 2.0 μm and D ₅₀ = 2.9 μm, the sintering temperature can be reduced to 1000 ° C or lower, so there is no deformation or damage to the mold even after 30 or more cycles. Furthermore, it was in a state that could withstand many uses. However, since the mold used to sinter the alloy powder with D ₅₀ = 5.0 μm must have a sintering temperature of 1000 ° C. or higher, the sintered body is welded to the inner surface of the mold in one sintering process. The mold could be deformed and the mold could not be reused, and a single mold could not withstand more than 5 cycles. The sintering temperature of 1050 ° C in the powder of D ₅₀ = 5.0 μm is severe for industrially used alloys such as stainless steel and other Fe-based and Ni-based alloys. It has been demonstrated that the deformation of the mold progresses due to stress and creep, making the mold unusable.

For comparison, by exposing the above-mentioned alloy powder of D ₅₀ = 2.9 μm (oxygen content 1300 ppm) to Ar containing 0.1% oxygen, three types of samples with oxygen content of 2500 ppm, 3100 ppm and 3500 ppm Was made. As a result of producing sintered magnets with these sample powders by the same method as described above, in any case, the sintering density could not be increased to 7.45 g / cm ³ or more at a sintering temperature of 1000 ° C. or less. .

Claims (1)

An NdFeB magnet alloy powder (hereinafter referred to as alloy powder) is filled in a filling container designed for the shape and dimensions of the product, and this alloy powder is oriented by applying a magnetic field. In the manufacturing method of the NdFeB series sintered magnet to obtain a sintered body having a desired shape and size by heating the whole filled container , the median particle diameter of the alloy powder measured by the laser method is 3 μm or less, and the alloy powder NdFeB oxygen content in the is at 2000ppm or less, Ri temperature 900 ° C. or higher 1000 ° C. less der during the heating, and the sintered density and wherein der Rukoto 7.45 g / cm ³ or more Manufacturing method of sintered magnet.