Manufacturing method for sintered compact

Abstract

A manufacturing method for a sintered compact includes a first step in which magnetic powder is fabricated by rapid solidification, a second step in which a mass of the magnetic powder is housed in a forming mold, and preliminary heating is performed by placing the mass of the magnetic powder in a preliminary heating part of the forming mold at first temperature that is lower than coarse crystal particle generation temperature, and a third step in which main heating is performed by placing the preliminarily heated mass of the magnetic powder at second temperature that is lower than the coarse crystal particle generation temperature and higher than the first temperature, and press forming is performed while keeping temperature of the magnetic powder at densification temperature or higher.

Claims

1. A manufacturing method for a sintered compact serving as a precursor of a rare earth magnet, comprising: a first step of fabricating magnetic powder having a microscopic crystal particle by rapid solidification; a second step of housing a mass of the magnetic powder in a forming mold having a preliminary heating part and a main heating part, and preliminary heating the mass of the magnetic powder by placing the mass of magnetic powder in the preliminary heating part at a first temperature that is lower than a coarse crystal particle generation temperature; and a third step of main heating the preliminarily heated mass of the magnetic powder by placing the preliminarily heated mass of the magnetic powder in the main heating part at a second temperature that is lower than the coarse crystal particle generation temperature and higher than the first temperature, and performing press forming while keeping a temperature of the magnetic powder at a densification temperature or higher, wherein the forming mold includes a lower die, a side die that is located above the lower die and forms a cavity with the lower die, and an upper die that is located above the side die and is able to enter and exit from the cavity, the preliminary heating part, which structures the forming mold, performs high frequency heating above the side die and on an outer periphery of the upper die, the main heating part, which structures the forming mold, is included in the side die, and, after the preliminary heating of the mass of the magnetic powder is performed in the preliminary heating part, the preliminarily heated mass of the magnetic powder is housed in the cavity and press-formed while main heating is performed in the main heating part.

2. A manufacturing method for a sintered compact serving as a precursor of a rare earth magnet, comprising: a first step of fabricating magnetic powder having a microscopic crystal particle by rapid solidification; a second step of housing a mass of the magnetic powder in a forming mold having a preliminary heating part and a main heating part, and preliminary heating the mass of the magnetic powder by placing the mass of magnetic powder in the preliminary heating part at a first temperature that is lower than a coarse crystal particle generation temperature; and a third step of main heating the preliminarily heated mass of the magnetic powder by placing the preliminarily heated mass of the magnetic powder in the main heating part at a second temperature that is lower than the coarse crystal particle generation temperature and higher than the first temperature, and performing press forming while keeping a temperature of the magnetic powder at a densification temperature or higher, wherein the forming mold includes a lower die, a side die that is located above the lower die and forms a cavity with the lower die, and an upper die that is located above the side die and is able to enter and exit from the cavity, and one of a lower region and an upper region of the side die is the preliminary heating part, the other one is the main heating part, and, after the mass of the magnetic powder is housed and preliminarily heated in a preliminary heating cavity space corresponding to the preliminary heating part in the cavity, the preliminarily heated mass of the magnetic powder is moved to a main heating cavity space corresponding to the main heating part and press-formed while performing main heating in the main heating part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

(2) FIG. 1 is a schematic view explaining a first step of a manufacturing method for a sintered compact according to the invention;

(3) FIG. 2A to FIG. 2C are schematic views showing a first embodiment of a second step and a third step of the manufacturing method;

(4) FIG. 3A to FIG. 3C are schematic views showing a second embodiment of the second step and the third step of the manufacturing method;

(5) FIG. 4 is a view explaining a microstructure of a manufactured sintered compact;

(6) FIG. 5 is a view explaining a microstructure of manufactured rare earth magnet;

(7) FIG. 6 is a view showing a result of a comparative example among experiment results that specify a relation between main heating time and temperature of magnetic powder;

(8) FIG. 7 is a view showing a result of an example among the experiment results that specify the relation between main heating time and temperature of magnetic powder;

(9) FIG. 8 is a schematic view showing dimensions of a mass of magnetic powder before press forming and a sintered compact after the press forming in the experiment;

(10) FIG. 9 is a view showing an experiment result that specifies a relation between temperature and a relative density of magnetic powder;

(11) FIG. 10 is a view showing an experiment result that specifies a relation between heating time of magnetic powder and a percentage of coarse crystal particles; and

(12) FIG. 11 is a SEM image of a section of a manufactured sintered compact.

DETAILED DESCRIPTION OF EMBODIMENTS

(13) Embodiments of a manufacturing method for a sintered compact according to the invention are explained with reference to the drawings.

(14) First and second embodiments of a manufacturing method for a sintered compact are explained below in order. Since a first step is common to the two embodiments of the manufacturing method, the first step is explained first, and then second and third steps in each of the embodiments are explained.

(15) (The First Step in the Manufacturing Method for a Sintered Compact)

(16) FIG. 1 is a schematic view explaining the first step of the manufacturing method for a sintered compact according to the invention.

(17) In the first step, a quenched thin belt, which is made of microscopic crystal particles, is fabricated by rapid solidification, and is then crushed. Thus, magnetic powder is fabricated. Specifically, as shown in FIG. 1, high frequency melting of an alloy ingot is performed in a melt spinning method by using a single roll in a furnace (not shown) in which pressure is reduced to, for example, 50 kPa or lower. Then, molten metal having a composition that is able to become rare earth magnet is injected on a copper roll R, thereby fabricating a quenched thin belt B (a quenched ribbon).

(18) A composition of the quenched ribbon B is made of a RE-FeB-based main phase (RE: at least one of Nd and Pr), and a RE-X alloy around the main phase (X: a metallic element and no heavy rare earth element is contained). For example, in the case where the composition is in a nanocrystal structure, the composition is made of a main phase having a crystal particle size of about 50 nm to 300 nm.

(19) A NdX alloy that structures a grain boundary phase is made of Nd and at least one or more of Co, Fe, Ga, Cu, Al, and so on. For example, the NdX alloy is any one of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa, or a mixture of two or more of them, making the alloy Nd-rich.

(20) The fabricated quenched ribbon B is collected and coarsely crushed, thereby fabricating magnetic powder. A particle size range of the coarsely crushed magnetic powder is adjusted to be within a range of, for example, 75 to 300 m (the end of the first step).

(21) Next, two methods for fabricating a sintered compact by using the magnetic powder fabricated in the first step are explained.

(22) (The First Embodiment of the Manufacturing Method for a Sintered Compact)

(23) FIG. 2A to FIG. 2C are schematic views showing in this order a second step and a third step according to the first embodiment of the manufacturing method for a sintered compact.

(24) First of all, a forming mold 10, which is used in the manufacturing method shown in the drawings, are explained. The forming mold 10 is made of a lower die 1, a side die 2 that is located above the lower die 1 and forms a cavity with the lower die 1, and an upper die 5 that is located above the side die 2 and is able to freely enter and exit from a cavity CV.

(25) A main heating part 3 such as a heater is built in the side die 2. A preliminary heating part 4 such as a high frequency coil, which preforms high frequency heating, is arranged above the side die 2 and on an outer periphery of the upper die 5.

(26) First, as shown in FIG. 2A, a mass of magnetic powder F is housed in a capsule CP, and the capsule CP is placed on the lower die 1 so that the preliminary heating part 4 is arranged around the capsule CP.

(27) Next, the preliminary heating part 4 is operated. The mass of the magnetic powder F is placed in the atmosphere at first temperature T.sub.0, which is lower than coarse crystal particle generation temperature, thereby performing preliminary heating for a given period of time (in Y1 directions). Thus, a preliminarily heated mass of the magnetic powder is fabricated (the second step).

(28) Once the preliminarily heated mass of the magnetic powder is fabricated, the side die 2 is moved upwardly (in a X1 direction) as shown in FIG. 2B so that the capsule CP is surrounded by the side die 2.

(29) In the state of FIG. 2B, the cavity CV is formed by the side die 2 and the lower die 1 due to the upward movement of the side die 2, and the capsule CP is automatically housed in the cavity CV. Then, the main heating part 3 is arranged around the capsule CP.

(30) The main heating part 3 is operated. The preliminarily heated mass of the magnetic powder F is placed in the atmosphere at second temperature T.sub.1, which is lower than the coarse crystal particle generation temperature and higher than the first temperature T.sub.0, thereby performing main heating for a given period of time (in Y2 directions). Thus, the temperature of the magnetic powder becomes densification temperature or higher.

(31) At a stage where both inner part and outer part of the mass of the magnetic powder F reach the densification temperature or higher, the upper die 5 is lowered as shown in FIG. 2C (in a X2 direction), and press forming is performed. Thus, a sintered compact S is manufactured (the third step). Here, the inner part of the mass of the magnetic powder F means 50% in a volume ratio of the mass on the central side, and the outer part of the mass of the magnetic powder F means 50% in a volume ratio of the mass on the outer side.

(32) By using the forming mold 10 as stated above, it is possible to carry out the preliminary heating to the main heating of the mass of the magnetic powder F and further to manufacturing of the sintered compact S by press forming in a series of flow. Thus, it is possible to manufacture the sintered compact S effectively while preventing coarsening of crystal particles.

(33) (The Second Embodiment of the Manufacturing Method for a Sintered Compact)

(34) FIG. 3A to FIG. 3C are schematic views showing in this order the second step and the third step according to the second embodiment of the manufacturing method for a sintered compact.

(35) A forming mold 10A used in the manufacturing method according to this embodiment is structured from a lower die 1, a side die 2A that is located above the lower die 1 and forms a cavity with the lower die 1, and an upper die 5 that is located above the side die 2A and is able to enter and exit freely from a cavity CV. A difference from the forming mold 10 shown in FIG. 2A to FIG. 2C is that a preliminary heating part 4A and a main heating part 3A are built in the side die 2A.

(36) The side die 2A is structured from an upper region 2a and a lower region 2b, the main heating part 3A such as a heater is built in the upper region 2a, and the preliminary heating part 4A such as a heater is built in the lower region 2b.

(37) First, as shown in FIG. 3A, a mass of magnetic powder F is housed in a capsule CP, the capsule CP is housed in the cavity CV formed by the lower die 1 and the side die 2A, and a lid 6 is placed on the capsule CP. In this state, the capsule CP is positioned in a preliminary heating cavity space in a lower part of the cavity, and the preliminary heating part 4A is arranged around the capsule CP.

(38) Next, the preliminary heating part 4A is operated. Then, the mass of the magnetic powder F is placed in an atmosphere at the first temperature T.sub.0, which is lower than the coarse crystal particle generation temperature, thereby performing preliminary heating for a given period of time (Y3 directions). Thus, a preliminarily heated mass of the magnetic powder is fabricated (the second step).

(39) Once the preliminarily heated mass of the magnetic powder is fabricated, the side die 2A is moved downwardly (in a X3 direction) as shown in FIG. 3B. Thus, the capsule CP is positioned in a main heating cavity space, which is an upper part of the cavity CV, and the main heating part 3A is arranged around the capsule CP.

(40) The main heating part 3A is operated. Then, the preliminarily heated mass of the magnetic powder F is placed in an atmosphere at second temperature T.sub.1, which is lower than the coarse crystal particle generation temperature and higher than the first temperature T.sub.0, thereby performing main heating for a given period of time (in Y4 directions). Thus, the temperature of the magnetic powder becomes densification temperature or higher.

(41) At a stage where both inner part and outer part of the mass of the magnetic powder F reaches the densification temperature or higher, the upper die 5 is lowered as shown in FIG. 3C (in a X4 direction) and press forming is performed. Thus, a sintered compact S is manufactured (the third step).

(42) In the case where the forming mold 10A is used as above, it is also possible to carry out the preliminary heating to the main heating of the mass of the magnetic powder F, and further to manufacturing of the sintered compact S by press forming in a series of flow. Thus, it is possible to manufacture a sintered compact effectively while preventing coarsening of coarsening of crystal particles.

(43) (Manufacturing of Rare Earth Magnet (Oriented Magnet) from a Sintered Compact)

(44) FIG. 4 shows a microstructure of the sintered compact S manufactured in the manufacturing method shown in FIG. 1, FIG. 2A to FIG. 2C, or the manufacturing method shown in FIG. 1 and FIG. 3A to FIG. 3C.

(45) As shown in FIG. 4, the sintered compact S has an isotropic crystal structure in which a grain boundary phase BP is filled between nanocrystal particles MP (the main phase).

(46) By performing hot plastic working of such an isotropic sintered compact S, rare earth magnet having a microstructure shown in FIG. 5, or rare earth magnet (oriented magnet) having magnetic anisotropy is manufactured. Extrusion such as backward extrusion and forward extrusion, and upsetting (forging) are applied to the hot plastic working.

(47) (Experiments and the Results that Specify a Relation Between Main Heating Time and Temperature of Magnetic Powder)

(48) The inventors and so on carried out experiments to specify a relation between main heating time and temperature of magnetic powder in the case of the manufacturing method in which main heating is performed after preliminary heating (example), and a manufacturing method in which main heating is performed without preliminary heating (comparative example). Coarse crystal particle generation temperature of magnetic powder to be used was 700 C., and densification temperature was 650 C. FIG. 6 shows the experiment result of the comparative example, and FIG. 7 shows the experiment result of the example. The coarse crystal particles mean crystal particles that are 400 nm or larger.

(49) As shown in FIG. 6, in the comparative example, heating time for the mass of the magnetic powder was 150 seconds, and pressure holding time thereafter was 1 second. From the drawing, in the comparative example, a temperature difference Ta between an inner region and an outer region of the mass of the magnetic powder was about 300 C. at time when the outer region reached the densification temperature. This resulted in that the outer region was exposed to an atmosphere at temperature that is equal to or higher than the coarse crystal particle generation temperature for about 80 seconds. As a result, a percentage of coarse crystal particles reached 2.7%.

(50) On the other hand, in the example, the preliminary heating time was 10 seconds. Heating time for the mass of the magnetic powder was 25 seconds as shown in FIG. 7, and pressure holding time thereafter was 1 second. According to the drawing, in the example, a temperature difference Tb between the inner region and the outer region of the mass of the magnetic powder was about 20 C. at time when the outer region reached the densification temperature. The temperature difference was improved considerably compared to the comparative example, and the inner region was densified sufficiently, not to mention the outer region. Further, the outer region was not placed in an atmosphere at the coarse crystal particle generation temperature or higher, not to mention the inner region. As a result, the percentage of coarse crystal particles was about 1.5%. This percentage of coarse crystal particles represents a ratio of raw material magnetic powder that is originally coarsened. Therefore, it is proved that the percentage of coarse crystal particles generated during the manufacturing processes of the sintered compact is substantially zero. It is also proved that forming time is reduced significantly in the example compared to the comparative example.

(51) (Experiments and Results that Specify a Relation Between Temperature and a Relative Density of Magnetic Powder that Structures a Sintered Compact, and a Relation Between the Magnetic Powder Heating Time and the Percentage of Coarse Crystal Particles)

(52) The inventors and so on also carried out experiments to specify a relation between temperature and a relative density of magnetic powder that structures a sintered compact, and a relation between magnetic powder heating time and a percentage of coarse crystal particles. FIG. 8 is a schematic view showing dimensions of a mass of magnetic powder before press forming and a sintered compact after the press forming.

(53) FIG. 8 does not show the forming mold used for the press forming. In the press forming, a rectangular parallelepiped mass of the magnetic powder was pressed from above at 500 MPa and press-formed to reduce a thickness to about . Thus, a testing body of a sintered compact was obtained. FIG. 9 is a view showing an experiment result that specifies a relation between temperature and a relative density of the magnetic powder, and FIG. 10 is a view showing an experiment result that specifies a relation between heating time of the magnetic powder and a percentage of coarse crystal particles. FIG. 11 is a SEM image of a section of the fabricated sintered compact.

(54) According to FIG. 9, it was found that the temperature of the powder needed to be 650 C. or higher in order to obtain a dense sintered compact with a target relative density of 98% or higher in one second of compression time of the magnetic powder.

(55) In FIG. 10, exposure time t of the magnetic powder to 700 C. without preliminary heating was 80 seconds. According to the drawing, it was found that the exposure time of the magnetic powder at 700 C. needed to be 30 seconds or shorter in order to achieve the target percentage of coarse crystal particle of 2% or lower.

(56) In FIG. 11, a measurement method for coarse crystal particles was SEM observation of a testing body that was etched with picral. In this drawing, it is possible to distinguish coarse crystal particles from a difference in contrast, and a black part shows coarse crystal particles. For calculation of a percentage of coarse crystal particles in FIG. 10, 10 visual fields were observed in each of upper, middle, lower, and outer parts of the sintered compact, and a percentage of coarse crystal particles was calculated from a width of coarsely crystalized part over a width of the ribbon.

(57) The embodiments of the invention have been explained in detail with reference to the drawings. However, the specific structure is not limited to the embodiments, and design changes and so on within the gist of the invention are included in the invention.