Patent Publication Number: US-2004052671-A1

Title: Composite structural body, method of manufacturing the structural body, and motor

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to a structure in which, for example, a yoke or a support member and a magnet are integrated, and more particularly to a composite structure using a magnetic material, to a method for the manufacture thereof, and to a motor.  
       BACKGROUND ART  
       [0002] In motors using magnets, a magnet and a yoke or a magnet and a part fixing such are assembled as a rotor or a stator.  
       [0003] Further, using a magnet and a yoke in a motor allows for more effective utilization of magnetism, and yokes of complex shape, such as IPM (Interior Parmanent Magnet) rotors have recently been suggested to increase further the energy efficiency of motors.  
       [0004] In IPM rotors, a method for opening a gap for placing a magnet into a yoke and inserting and adhesively securing the magnet therein, and a method for inserting a flowable mixture of a magnet powder and a resin and curing it thereafter (Japanese patent Application Laid-open H11-215746) have been studied, the latter method being adapted for more complex shapes.  
       [0005] Further, a method for obtaining an integrated joint structure by inserting a sintered magnet or the like into a metal container and conducting hot isostatic pressing has been suggested (WO98/31497) as a method for obtaining a high strength of the structure itself.  
       [0006] Fixing a magnet and a yoke in an assembly with such a complex structure has mainly been conducted by using an adhesive, which created problems in terms of reliability, for example, strength and heat resistance. Because IPM structures also use an adhesive and a resin, high strength and heat resistance cannot be obtained.  
       [0007] Further, the following problems are associated with the above-mentioned hot isostatic pressing. Because a sintered magnet is inserted into a metal container, a complex shape cannot be obtained, or if a magnet is inserted into a metal container, a gap is formed. As a result, when the gap is eliminated by hot isostatic pressing, the metal container can be deformed.  
       DISCLOSURE OF THE INVENTION  
       [0008] It is an object of the present invention to obtain a composite structure using a magnetic material, for example, the above-mentioned rotor, stator, IPM structure, and the like, and to provide a composite structure making it possible to obtain excellent strength and heat resistance and having a configuration such that a metal container is not deformed during hot isostatic pressing, a method for the manufacture of such a structure, and a motor.  
       [0009] The inventors have conducted a comprehensive study of treatment conditions and structures of containers with the object of finding a configuration such that a metal container is not deformed during hot isostatic pressing. The results obtained demonstrated that this object can be attained by employing a high-strength part made of a material or having a thickness such that it is difficult to deform under a pressure during hot isostatic pressing as a required part and setting a low-strength part made of a material or having a thickness such that it is easy to deform under the pressure when the metal container is constructed, introducing and sealing a powder in the metal container, and placing the container in the prescribed high-temperature and high-pressure atmosphere. Deformation of the part with a low resistance to pressure reduces deformation of the part with a high resistance to pressure, that is, a preset high-strength part which requires precision and shape characteristic, thereby making it possible to form integrally, with good tightness, the metal container and the powder placed inside thereof. This finding laid the foundation for the present invention.  
       [0010] Thus, the present invention provides a composite structure comprising a metal container having a high-strength part with a high resistance to deformation under a required pressure and a low-strength part which can be deformed and a powder formed body integrated with the container by filling the container with the powder, sealing the container, and conducting hot isostatic pressing.  
       [0011] Further, the present invention also provides the following composite structures of the above-described configuration:  
       [0012] a composite structure in which the high-strength part and low-strength part of the metal container are composed of different materials or of the same material, but with different thickness;  
       [0013] a composite structure in which the powder is a magnetic powder or a magnetic powder containing a non-magnetic powder added thereto;  
       [0014] a composite structure in which the powder comprises a powder with a different melting point added thereto; and  
       [0015] a composite structure in which the magnetic powder is at least one from among a Nd—Fe—B magnet powder, a Sm—Co magnet powder, a Pr—Fe—B magnet powder, an exchange-spring magnet powder, an Alnico magnet powder, and a ferrite magnet powder.  
       [0016] The present invention also relates to a method for the manufacture of a composite structure, comprising the steps of:  
       [0017] filling a metal container having a high-strength part with a high resistance to deformation under a required pressure and a low-strength part which can be deformed with a powder and sealing the container, integrally forming the metal container and the powder by hot isostatic pressing, and optionally further processing, for example, removing a required part from the integrally formed product by mechanical processing such as slicing.  
       [0018] Further, the present invention also provides the following methods for the manufacture of a composite structure, these methods having the above-described configuration:  
       [0019] a manufacturing method in which the high-strength part and low-strength part of the metal container are composed of different materials or of the same material, but with different thickness; and  
       [0020] a manufacturing method in which the hot isostatic pressing is conducted under the conditions of temperature of no lower than 600° C. and no higher than 1000° C. and pressure of 1˜200 MPa.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0021]FIG. 1 is an explanatory view of a metal container illustrating the process in accordance with the present invention, in which FIG. 1A is a longitudinal sectional view prior to hot isostatic pressing, FIG. 1B is a longitudinal sectional view after hot isostatic pressing, and FIG. 1C is a perspective view illustrating the product subjected to slicing;  
     [0022]FIG. 2 is a lateral sectional view of the rotor in accordance with the present invention;  
     [0023]FIG. 3A is a graph illustrating the relationship between the treatment temperature and powder density in the embodiment of the present invention, and FIG. 3B is a graph illustrating the relationship between the treatment temperature and magnetic properties; and  
     [0024]FIG. 4A is a graph illustrating the relationship between the treatment pressure and powder density in the embodiment of the present invention, and FIG. 4B is a graph illustrating the relationship between the treatment pressure and magnetic properties.  
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     [0025] A specific feature of the present invention is that a metal container is provided with a part having a high resistance and a part having a low resistance to pressure of hot isostatic pressing. Deformation of the part with a low resistance to pressure in the prescribed high-temperature and high-pressure atmosphere reduces deformation of the part with a high resistance to pressure, thereby making it possible to form integrally, with good tightness, the metal container and the powder.  
     [0026]FIG. 1 is a cross-sectional explanatory view of a metal container illustrating the process in accordance with the present invention. A metal container  1  shown in FIG. 1A has a configuration in which disk-like lids  3 ,  4  are provided on the upper and lower ends of a cylindrical container body  2 , thereby sealing the powder placed inside the body  2 . The disk-like lids  3 ,  4  are constructed so that they can be deformed by the required pressure during hot isostatic pressing, and the container body  2  is constructed so as to withstand the pressure.  
     [0027] For example, the container body  2  brought in contact with the lower disk-like lid  3  is filled with the powder  5 , then the upper disk-like lid  4  is placed, and the inside of metal container  1  is evacuated, followed by sealing, as shown in FIG. 1A.  
     [0028] If the metal container  1  is then placed in a hot isostatic pressing apparatus and the temperature and pressure are raised by using a gas as a pressure medium, then the parts of metal container  1 , which has a low resistance to pressure, that is, disk-like lids  3 ,  4 , will be deformed squashed by the gas pressure. As a result, the powder will be compressed inside the part of metal container  1 , which has a high resistance to pressure, that is, the container body  2 , and if the temperature is further raised, sintering of the sample powder will proceed and, at the same time, the metal container  1  and sample powder will be formed integrally and will assume a state shown in FIG. 1B.  
     [0029] Then, a composite structure  6  shown in FIG. 1C in which the cylindrical metal material is integrally formed with the powder located inside thereof is formed by slicing the upper and lower portions of the integrally formed metal container  1  shown in FIG. 1B.  
     [0030] For example, when iron is employed for the container body  2 , a different metal such as aluminum or copper can be used for the disk-like lids  3 ,  4  to form a part that can be deformed under the required pressure. Furthermore, when the same metal material is used, it is possible to cause the deformation only of the required part by the pressure if the thickness of disk-like lids  3 ,  4  is made less than that of container body  2 .  
     [0031] In accordance with the present invention, yokes and magnets of a variety of complex shapes can be formed integrally by the above-described mechanism which is realized during hot isostatic pressing and, for example, a IPM rotor shown in FIG. 2 relating to the embodiment can be manufactured.  
     [0032] In accordance with the present invention, materials with a high magnetic permeability such as iron and permalloy, which are used, for example, for yokes, can be employed for the metal container.  
     [0033] When magnetic parts used in a motor or the like are fabricated, a variety of powders, for example, a Nd—Fe—B magnet powder, a Sm—Co magnet powder, a Pr—Fe—B magnet powder, an exchange-spring magnet powder, an Alnico magnet powder, and a ferrite magnet powder can be used as the powder placed in the metal container.  
     [0034] In addition to magnetic powders, a variety of metal alloys (Ti, Co, Cu, permalloy), ceramics (PZT, barium titanate), and the like can be used as the container and the powder inserted therein in order to obtain a combination of strength, thermal conductivity, corrosion resistance, and functionality.  
     [0035] Furthermore, heat generation due to eddy current can be avoided by introducing a nonmagnetic powder with a high electric resistance, such as SiO2, A 1 2O3, and the like, in the above-described filling powder. In order to improve dispersivity and increase electric resistance, it is preferred that those powders have a small average particle size. The particle size is preferably no more than 100 m, more preferably no more than 1 m.  
     [0036] Furthermore, in order to fix the powder and increase bonding strength with the container, a material with a melting point different from that of the powder, for example, a low-melting glass such as borosilicate glass and the like and a low-melting metal such as Zn, Pb, Sn, and the like can be added, thereby making it possible to decrease temperature and pressure of hot isostatic pressing. As a result, deformation of the metal container and modification of the inserted powder can be prevented.  
     [0037] Any method and apparatus for hot isostatic pressing which have the conventional configuration can be used in accordance with the present invention, and specific method and apparatus can be appropriately selected according to the selected type of container and powder or shape and application of the product. Furthermore, the treatment conditions can be appropriately selected according to shape or application of the product, but raising the temperature and pressure in excess is undesirable from the standpoint of productivity, and it is preferred that the temperature be 600˜900° C. and the pressure be 10˜100 MPa. Further, when improvement of magnetic properties is given top priority, the temperature is preferably 500˜900° C. and the pressure is preferably 10˜100 MPa.  
     [0038] Embodiments  
     [0039] Embodiment 1  
     [0040] A cylindrical container obtained by joining a SUS304 tube and a copper disk was used as a metal container, a Ne—Fe—B magnet powder (average particle size no more than 200 m) was used as a filling powder, and the powder was vacuum sealed in the metal container under 8 Pa. Hot isostatic pressing was conducted in a hot isostatic pressing apparatus with argon gas as a pressure medium under a variety of conditions with a temperature of 300˜1000° C. and a pressure of 1˜200 MPa.  
     [0041] Hot isostatic pressing was conducted by changing the temperature from 300° C. to 1000° C. at a constant pressure of 50 MPa. The relationship between the treatment temperature during forming and the density of the obtained powder compact is shown in FIG. 3A. FIG. 3A clearly demonstrates that the density increases as the treatment temperature rises, but the difference is small at a temperature of 700° C. or higher.  
     [0042] The relationship between the treatment temperature and magnetic properties (residual magnetic flux density, coercive force) of the obtained powder compact during hot isostatic pressing similarly conducted by changing the temperature from 300° C. to 1000° C. at a constant pressure of 50 MPa is shown in FIG. 3B. FIG. 3B clearly demonstrates that the residual magnetic flux density increases as the treatment temperature rises, but the difference is small at a temperature of no less than 700° C. Further, the coercive force decreases as the temperature rises, but the decrease is small at a temperature below 700° C.  
     [0043] Hot isostatic pressing was then conducted by changing the pressure from 1 MPa to 200 MPa at a constant temperature of 700° C. The relationship between the treatment pressure during forming and the density of the obtained powder compact is shown in FIG. 4A. FIG. 4A clearly demonstrates that the density increases as the treatment pressure rises, but the difference is small at a pressure of no less than 50 MPa. From the standpoint of productivity, the unnecessary increase of temperature and pressure is undesirable.  
     [0044] The relationship between the treatment pressure and magnetic properties (residual magnetic flux density, coercive force) of the obtained powder compact during hot isostatic pressing similarly conducted by changing the pressure from 1 MPa to 200 MPa at a constant temperature of 700° C. is shown in FIG. 4B. FIG. 4B clearly demonstrates that the residual magnetic flux density increases as the treatment pressure rises, but the difference is small at a pressure of 50 MPa or greater. Further, the coercive force practically does not change as the pressure changes. Therefore, when magnetic properties are given top priority, the temperature is preferably 500˜900° C. and the pressure is preferably 10˜100 MPa.  
     [0045] The circumference of the composite structure obtained under the above-described hot isostatic pressing conditions was measured and the presence of deformations was checked. Deformations of the circumference were found in none of the composite structures.  
     [0046] Embodiment 2  
     [0047] A metal container was formed by using a cylindrical iron material, arranging rectangular through holes in the axial direction to obtain a rotor, as shown in FIG. 2, vacuum filling the rectangular holes with a Ne—Fe—B magnet powder (average particle size no more than 200 m), and using copper plates as lids on both ends. Hot isostatic pressing was conducted in a hot isostatic pressing apparatus with argon gas as a pressure medium under conditions of a temperature of 800° C. and a pressure of 100 MPa.  
     [0048] The rotor obtained was found to demonstrate no deformations even at a temperature of 300° C. When used in a motor, the rotor was found to demonstrate excellent heat resistance and strength.  
     INDUSTRIAL APPLICABILITY  
     [0049] In accordance with the present invention, a high-strength part made of a material or having a thickness such that it is difficult to deform under a pressure during hot isostatic pressing and a low-strength part which is easier to deform than the hard-strength part are set in a metal container. Therefore, a portion with a low resistance to pressure is deformed during the aforesaid treatment, thereby making it possible to reduce the deformation of the high-strength portion and to form integrally, with good tightness, the metal container and the powder.  
     [0050] Therefore, the present invention can provide a magnet element with a strength, heat resistance, and complex structure superior to those of the conventional composite structures. Another advantage is that because the metal container is not deformed, processing after forming is facilitated and productivity is greatly improved.