Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2000-03-28
2002-03-26
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S038000, C075S231000, C252S062530, C148S103000, C148S302000, C524S404000
Reexamination Certificate
active
06361738
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to a method of producing an R—FE—B permanent magnet with which high crystal orientation is obtained and molded article strength is high and therefore, productivity is excellent. It relates to a method of producing an R—Fe—B magnet, wherein high crystal orientation is obtained, molded article strength is markedly improved, and an R—Fe—B magnet with a high Br can be mass-produced at a good yield by adding and mixing a lubricant for molding magnets, characterized in that specific amounts of methyl caproate and/or methyl caprylate, with which high crystal orientation can be obtained, and a lubricant comprising a depolymerized polymer for improving molded article strength are added, individually or as a combination, to R—Fe—B alloy fine powder, or by adding and mixing a lubricant for molding magnets, characterized in that a Ti coupling agent for improving crystal orientation is further combined with these lubricants for molding magnets, and then molding in a magnetic field.
BACKGROUND ART
In general, R—Fe—B (R is one or more rare earth element, including Y, and some of the Fe can be substituted with Co) permanent magnet starting powders are usually made by the following process 1) through 2) or 1a) through 2b):
1) A rare earth metal, electrolytic iron, and ferroboron alloy or, further, electrolytic Co, are melted under high-frequency as the starting material and cast into a mold or cast onto a quenching roll (strip casting method) to make ingots.
2)After crushing the ingots by the H
2
occlusion and comminution method, 1.5 to 5.0 &mgr;m fine powder starting material is obtained by wet comminution with a ball mill attritor, or by jet mill comminution using inert gas.
1a) Metallic Ca and CaCl
2
are mixed with a mixed powder of at least one type of rare earth oxide, iron powder and at least one of pure boron powder, ferroboron powder, and a boron oxide, or an alloy powder or a mixed oxide of the above-mentioned structural elements that have been combined to a specific composition and then reduced and diffused in an inert gas ambient atmosphere. The reaction product that is obtained is sprayed and treated with water (reduction-diffusion method).
2b) The above-mentioned treated product is made into a 1.5 to 5.0 &mgr;m fine stating powder by wet comminution with a ball mill attritor or by dry comminution with a jet mil.
As previously mentioned, pulverization of the starting powder for R—Fe—B permanent magnets is performed by wet comminution or dry comminution. However, there are problems with wet comminution in that the fine powder that is obtained contains C or O
2
from the organic solvent, impurities from abrasion of the balls are mixed in the fine powder, etc., and therefore, pulverization of starting powder for rare earth magnets is being changed to dry comminution.
However, N
2
gas or Ar gas with a purity of 95% or higher is being used as the gas that generates the jet mill for pulverization by a jet mill, which is dry comminution, in order to prevent oxidation, as well as to prevent ignition and combustion, of the fine powder, and there are problems in that when compared to wet comminution methods, such as attritor pulverization methods, etc., comminution efficiency is poor, and moldability is also poor because of the powder that is obtained.
In order to solve the problems with conventional dry comminution methods, methods are being tested whereby after adding and mixing stearic acid solid lubricant, such as zinc stearate, calcium stearate, etc., with a crushed powder before jet mill comminution, jet mill comminution is performed, or the above-mentioned stearic acid solid lubricant is added and mixed with fine powder after jet mill comminution, and then molding is performed.
However, it is extremely difficult to uniformly mix the above-mentioned solid lubricant with fine powder of a starting alloy for R—Fe—B permanent magnets, and there are fluctuations per unit weight during press molding, leading to defects such as cracking, etc.
Therefore, a method of producing an R—Fe—B magnet was previously presented by the present inventors (Japanese Patent Publication Laid-Open No. 8-111308) wherein after at least one fatty acid ester lubricant (lauric acid ester lubricant, oleic acid ester lubricant) is added and mixed with starting crushed powder of a specific composition, the powder is pulverized and the fine powder that is obtained is molded and sintered and aged as a method of producing an R—Fe—B magnet of markedly improved comminution efficiency during jet mill comminution as well as excellent press filling performance and excellent orientation performance. However, there are problems with methods that use the above-mentioned lubricants in that it is difficult to remove the binder and molded article strength is poor, leading to poor production yield.
Moreover, it is suggested that, in order to improve moldability, a binder with excellent binding force, such as PVA (polyvinyl alcohol), etc., be added during granulation of the magnet powder, but there are problems with removal of the binder during sintering, and there are problems in that special treatment, such as sintering in an H
2
reducing ambient atmosphere, etc., becomes necessary, there is a reduction in magnet properties with an increase in the amount of C remaining in the sintered compact, etc.
SUMMARY OF THE INVENTION
The present invention is based on problem points of lubricants that are added and mixed with R—Fe—B alloy powder that has been obtained by comminution of an alloy produced by the above-mentioned melting and casting method, strip casting, etc., or powders of an R—Fe—B permanent magnet composition obtained by Ca reduction, its purpose being to present a method of producing R—Fe—B permanent magnets and a lubricant for molding the same magnet with which binder removal is improved, high crystal orientation and an excellent Br are obtained during molding in a magnetic field, and molded article strength is high, making mass-productivity excellent and improvement of yield possible.
The inventors performed various studies of effective lubricants when mixed with starting fine powder for R—Fe—B magnets obtained by any of a variety of conventional methods in order to improve binder removal, improve orientation performance during molding in a magnetic field, improve molded article strength, and improve mass-productivity and yield of methods of producing R—Fe—B permanent magnets and as a result, they discovered lubricants with specific components that are at least one of methyl caproate lubricant or methyl caprylate lubricant, or a combination with Ti coupling agent further added and mixed.
Moreover, the inventors similarly discovered a lubricant comprising depolymerized polymer, as well as a lubricants wherein hydrocarbon solvent with a boiling point of 80 to 250° C. is combined with this lubricant, or said solvent and low-viscosity mineral oil with a boiling point of 200 to 500° C. and dynamic viscosity (40° C.) of 3 to 30 mm
2
/second is mixed with this lubricant.
Furthermore, the inventors discovered that when a combination lubricant for molding magnets consisting of a specific amount of at least one of methyl caproate and methyl caprylate and a lubricant comprising depolymerized polymer is used, each of the above-mentioned lubricants can be uniformly coated on the surface of the fine powder and when this kneaded product is molded in a magnetic field, each particle of the fine powder has high crystal orientation in the direction of the magnetic field, and molded article strength is markedly improved, leading to improved mass-productivity and yield. Moreover, the above-mentioned lubricant is released as a gas without reacting with this magnet powder during sintering and therefore, there is excellent removal of the binder. As a result, an R—Fe—B permanent magnet with high Br and iHc properties is obtained while controlling an increase in the amount of C remaining in the sintered compact.
Moreover, the inventors discovered that when a specific amount of Ti coupling agent is m
Baba Junichiro
Kaneko Yuji
Mori Shizuo
Tanaka Kazuo
Dykema Gossett PLLC
Mai Ngoclan
Sumitomo Special Metals Co. Ltd.
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