Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
  • B22F3/12—Both compacting and sintering
  • B22F3/14—Both compacting and sintering simultaneously
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
  • H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
  • H01F1/047—Alloys characterised by their composition
  • H01F1/053—Alloys characterised by their composition containing rare earth metals
  • H01F1/055—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5
  • H01F1/057—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B
  • H01F1/0571—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes
  • H01F1/0575—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes pressed, sintered or bonded together
  • H01F1/0576—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes pressed, sintered or bonded together pressed, e.g. hot working

Definitions

• the present invention relates to hot-worked permanent magnets consisting substantially of rare-earth elements, transition metals and boron and provided with magnetic anisotropy by hot-working, and more particularly to hot-worked magnets having improved crystal grain orientation and thus having good magnetic properties.
• the present invention especially relates also to a method of producing such hot-worked magnets without cracking by adding proper amounts of additives such as graphite powder and glass material having a low melting point to improve workability, and to compositions of matter for use in this method.
• R-T-B Permanent magnets consisting essentially of rare-earth elements, transition metals and boron
• R-T-B permanent magnets
• R2T14B having a tetragonal crystal structure
• the R-T-B permanent magnets are usually classified into two groups: Sintered magnets and rapidly quenched magnets. Whichever production method is utilized, it is necessary to form them to desired shapes. In this sense, they should have good workability. In order to improve the workability of the magnets, the addition of lubricating agents has conventionally been conducted.
• the lubricants are classified into external lubricants which are applied to die surfaces or surfaces of magnet products to be formed to reduce friction between the die surfaces and the magnet products being formed, and internal lubricants which are in the form of powder, liquid, solid, etc. and are added to the magnet products to be formed to reduce friction between the powder particles.
• EP-A-133 758 discloses the coating of a die surface with graphite as an external lubricant for hot die-upsetting, to improve the workability of magnets in the hot-working process, thereby obtaining hot-worked magnets free from cracks.
• the effects of graphite on the inner lubrication of the magnets are not referred to.
• US-A-4 780 226 discloses a method of producing hot-worked magnets wherein there is used a complex additive of graphite and glass material as an external lubricant for hot die-upsetting, to improve the workability of magnets in the hot-working process. In the method, a glass powder material having a melting point which is lower than the hot-working temperature, or a mixture of glass powder and graphite powder is sprayed on the surfaces of punches and dies to form a green body of magnet material.
• stearic acid is widely used as an internal lubricant (JP-A-61-34101).
• Stearic acid is a saturated aliphatic acid having the formula CH3(CH2)16COOH. It is also known to suppress the growth of crystal grains and simultaneously increase the density of the resulting magnet in the sintering step by adding carbon powder or a powder of carbide-forming components such as Ti, Zr, Hf, etc. to form metal carbides (JP-A-63-98105).
• the individual thin ribbons or flakes produced by such a rapid quenching method usually contain innumerable fine crystal grains. Even though the thin ribbons or flakes produced by rapid quenching are in various planar shapes of 30 ⁇ m in thickness and 500 ⁇ m or less in length, the crystal grains contained therein are as fine as 0.02-1.0 ⁇ m as an average grain size, which is smaller than the average grain size of 1-90 ⁇ m in the case of sintered magnets (see, for instance, EP-A-126 179).
• the average grain size of the rapidly quenched magnets is close to 0.3 ⁇ m, the critical size of a single domain of the R-T-B magnet, which means that it provides essentially excellent magnetic properties.
• EP-A-195 219 discloses a rapidly quenched hot-worked permanent magnet of the R-T-B type in which each particle of the powder material used for the preform may be coated with an inorganic or organic lubricant.
• suitable lubricants are graphite and molybdenum disulfide.
• JP-A-60-184 602 discloses the use of polyethylene glycol monolaurate to increase the formability of sintered magnets.
• external lubricants such as graphite and/or glass applied to the die surface for die lubrication to reduce friction between a work body and surfaces of tools (dies and punches) only partly, if at all, attach to the thin ribbons or flakes produced by a rapid quenching method, which are 30 ⁇ m or so in thickness and 500 ⁇ m or less in length, much less to the innumerable fine crystal grains inside the thin flakes.
• external lubricants do not play a role as an inner lubricant to reduce occurance of cracks in a magnet produced by hot-working.
• the method according to the present invention for producing fine-grained magnetically anisotropic hot-worked magnets comprises the following steps:
• the component R of the R-T-B type alloys is substantially Nd, and the boundary layers include neodymium carbide.
• an internal lubricant is used in step III of the method comprising a glass component having a low melting point and particularly a softening point below the temperature at which precipitation of an R-rich phase occurs in the R-T-B alloy, and having a low oxygen content.
• the softening point of the glass component is advantageously about 500 to about 800 °C, preferably less than about 650 °C, and more preferably about 550 °C.
• the fine-grained magnetically anisotropic hot-worked magnets according to the present invention comprise an R-T-B type alloy, wherein R is selected from the rare-earth elements including yttrium, and mixtures thereof, T is a transition metal, and B is boron, each of the grains of which is at least partially surrounded by a boundary layer comprising a carbide material dispersed in a glass material.
• the magnets of the present invention preferably have an average grain size of the fine crystal grains of 0.02 to 1.0 ⁇ m, the carbon content is ⁇ 0.5 mass-%, and the oxygen content is ⁇ 0.3 mass-%, the transition metal T preferably being the main alloy component.
• the grains of the magnets preferably have an average aspect ratio greater than about 2.0.
• the hot-working in step V of the method of the present invention is preferably carried out at a temperature of about 600 to about 850 °C and more preferably at about 720 to about 760 °C.
• compositions of matter of the present invention particulary for use in the above-defined method, comprise accordingly
• the glass component B1 consists of at least one glass material selected from water glasses PbO-B2O3-SiO2 type glasses, B2O3-SiO2-Bi2O3 type glasses, and Deltaglaze (R) conventionally used in casting processes for Ti metal or extrusion processes for Ti metal at room temperature.
• the Deltaglaze (R) is applied in a powder form together with trichloroethylene.
• Magnets provided with improved magnetic properties compared with conventional hot-worked magnets can be produced as explained below according to the method of the present invention.
• the graphite powder affects the residual magnetic flux density of the hot-worked magnets produced according to the present invention.
• graphite powder alone mixed with flakes of magnet material tends to reduce the iHc value of the magnet produced by hot-working.
• plastic deformation of the grains in the magnet flakes tends to be hindered and even prevented because of the lumps produced as the graphite content increases.
• the boundaries can be apparently observed in a magnet containing 0.1 mass-% of glass and 0.3 mass-% of graphite, and the boundaries are remarkably distinct in a magnet containing 0.1 mass-% of glass and 0.5 mass-% of graphite.
• magnets containing 0.3 mass-% of graphite and various amounts of a glass material in a range of 0.1 to 0.5 mass-% By observation of magnets containing 0.3 mass-% of graphite and various amounts of a glass material in a range of 0.1 to 0.5 mass-%, the following statement can be made.
• the flow shape of the rapid-quench magnet material varies depending on the glass content, with the boundaries of the individual flakes being clearer in magnets containing graphite than in magnets having no graphite additive.
• the magnets containing 0.3 mass-% of graphite and 0.3 mass-% of glass are provided with a more uniform shape of flake-flow than ones comprising 0.3-% mass of graphite and 0.1 mass-% of glass. However, some irregular flows which are not penpendicular to the die-upsetting direction are observed in magnets containing 0.5 mass-% of glass and 0.3 mass-% of graphite.
• Figs. 6 and 7 show the microstructures of the fracture planes observed in a direction perpendicular to the hot-­ compression direction.
• Fig. 7 shows the microstructures of the fracture planes observed in a direction parallel to the hot-compression direction.
• the photomicrographs in the upper column are magnified by 2,000 times, and the photo­ micrographs in the lower column are magnified by 30,000 times in Figs. 6 and 7.
• Fig. 3 shows the residual carbon content and the residual oxygen content in magnets containing glass material for various amounts of added graphite. It is believed that the slight increase of residual oxygen content according to the increase of the added graphite content is caused by absorption of water from the air during mixing of the flakes and the graphite. The residual carbon content increases linearly with the increase of added graphite and independently of the added glass content.
• the preferable carbon content and oxygen content remaining in a magnet are, respectively, ⁇ 0.5 mass-% and ⁇ 0.3 mass-% (3000 ppm) in a magnet provided with good magnetic characteristics as taught by Figs. 4 and 5 which will be explained later in relation to Example 3.
• the strain rate affects the magnetic characteristics of a magnet which is hot-worked according to the present invention.
• the deformation resistance does depend on the strain rate even in the range of 0.5 to 0.1 mm/s. This tendency is pronounced when the strain rate is relatively fast.
• the coercive force tends to decrease somewhat as the deformation rate is reduced.
• the residual magnetic flux density and the saturation magnetization are sensitive to the deformation rate. These properties decrease with an increase in the deformation rate, and increase as the deformation rate is reduced. In particular, the rate of increase is enhanced in cases of a deformation rate of 0.006 (l/s) or less.
• a magnet can be provided with a high saturation magnetization and a high residual magnetic flux density, resulting in a maximum energy product as high as 40 MOe, without lowering the coercive force appreciably when it is hot-worked at a low strain rate.
• isothermal forging makes such a preferable hot-working step easy.
• the high degree of orientation of the grains which causes the magnetic anisotropy contributes to improve the magnetic characteristics significantly according to the present invention.
• the high degree of orientation of grains is observed by X-ray diffraction analysis.
• Organic lubricants in liquid form are inferior to the dry powder additives disclosed in this specification because of the following problem. Segregation of oxygen and carbon would occur by virtue of the time lag of the vaporization of oxygen and carbon depending on the speed of heat transfer during the hot-compression process, particularly in cases of large hot-worked magnets. In such cases the characteristics, especially the coercive force of the magnet, are not uniform in the magnet. It is a problem to produce large hot-worked magnets in an industrial scale using liquid lubricants. However, a proper amount of such liquid lubricants can be used in the present invention along with a glass material to produce magnets having excellent characteristics.
• the upper limit of the average grain size in a magnet produced according to the present invention is about 1 ⁇ m, but a smaller grain size is preferable to provide the excellent magnetic characteristics.
• the average grain size in a magnet according to the present invention is about 0.5 ⁇ m.
• a magnet having an average grain size of more than 1 ⁇ m suffers from a reduction in its coercive force.
• An excess addition of graphite powder (about 0.5 mass-% or more) can form gross grains distributed in the magnet.
• the determination of the average grain size can be accomplished by a "cut-method" of microphotography.
• the average grain size can be calculated by taking an average of about twenty or more values which are obtained using lines arbitrarily marked on the photomicrograph. Each line length is divided by the number of grain particles in that line length to obtain the value for that line, and the values are then averaged. It should be noted that the grain has a flat shape which is shorter in a direction parallel to the C-axis of the crystal, and the above stated average grain size is measured in a plane perpendicular to the C-axis of the crystal.
• an average grain size (a) measured on a plane parallel to the C-axis of the crystal in addition to the average grain size (c) measured on a plane perpendicular to the C-axis.
• (c) is about 0.2-0.3 ⁇ m
• (a) is about 0.1 ⁇ m, giving an aspect ratio c/a of 2 or more, in cases where excellent characteristics of anisotropic bonded magnets are produced, as described in JP-A-62-37378.
• An excess addition of graphite causes a severe reduction in the aspect ratio of magnets produced according to the present invention.
• the excess graphite results in an excess amount of carbon remaining in the magnet of more than 0.5 mass-% which reduces the magnetic properties of the magnet substantially.
• an excess amount of oxygen remaining in a magnet causes enhanced deformation resistance which, in turn, results in a severe reduction in the workability of the magnet.
• the magnets according to the present invention comprise as main components, a transition element T, a rare-earth element R, and boron B.
• the compositions of the magnets are similar to the compositions disclosed in JP-A-­ 60-100 402 which discloses known hot-worked magnets.
• the transition metal element T can be Co, Ni, Ru, Rh, Pd, Os, Ir and Pt in the narrow sense of the transition element definition, and also an element having an atomic number of 21-29, 39-47, 72-79 and 89 or more, in accordance with a broad definition of transition elements.
• Ga is effective to enhance the magnetic properties of hot-worked magnets produced according to the present invention.
• R can be Nd, Pr as the main constituent, Ce or Didymium can be used to partially substitute for Nd or Pr, and Dy or Tb can be added to enhance thermal stability.
• An alloy having the composition of 14.5 at.-% of Nd, 6 at.-% of B, 7.5 at.-% of Co, 0.75 at.-% of Ga and the balance Fe was produced by arc melting.
• This alloy melt was ejected onto a single roll rotating at a surface velocity of 30 m/s in an Ar atmosphere to produce irregularly shaped thin flakes of about 30 ⁇ m in thickness.
• the thin flakes contained a mixture of amorphous phases and crystalline phases.
• the thin flakes were then pulverized to produce magnetic powder of 500 ⁇ m (32 mesh) or less in particle size, and then spherically shaped particles were removed by a classifier.
• 150 g of the separated particles were mixed with 0.2 mass-% of graphite powder and 0.3 mass-% of a low melting point glass material in a V-shaped mixer for 10 min.
• the graphite was flake shaped, and the glass material was an amorphous B2O3-SiO2-Bi2O3 type glass.
• the characteristics of the above-mentioned glass are shown in the following table. Coefficient of linear expansion 72 ⁇ 10 ⁇ 7 cm/cm ⁇ °C Glass trans. temp. 470 °C Yielding point 502 °C Softening point 550 °C.
• the mixture was pressed in a die under a pressure of 3 to/cm2 without applying a magnetic field, yielding green bodies having a density of 5.8 g/cm3, a diameter of 28.5 mm and a height of 40.5 mm.
• Each of the resulting green bodies was hot-pressed and subjected to die-upsetting at 740 °C and a compression ratio of 3.90 in a hot-working machine having a capacity of 30 to, to provide magnetic anisotropy.
• the obtained magnet samples were evaluated by various methods on the basis of test specimens each having a 0.5 mm x 10.5 mm rectangular shape cut out from each sample. The following are the evaluation methods and apparatus therefore.
• the hysteresis loop in the second quadrant of the iHc v. 4 I curve was measured by a B-H tracer.
• a mean value was calculated as a representative value based on five samples which were cut out from a magnet.
• a layout of the cut portions used to obtain the samples and their dimensions are shown in Fig. 5 (depicting a prior art specimen which developed peripheral cracks), each sample having a 10.5 mm x 10.5 mm rectangular shape.
• the numerals indicate the cut portions, and the portions numbered as 1, 3, 5, 7, and 9 were actually used as samples. Observations by an optical microscope were conducted for sample 8.
• Measurements of the carbon content, oxygen content and glass content remaining in the hot-worked magnet were conducted on the basis of magnet powder produced by pulverizing the center portion of the sample magnet using concentration analyzers. The mean value of these measurements on each sample is the representative value of the content.
• the distribution of glass was estimated on the basis of the distributions of the element Si and the element Bi which are contained in the low melting point glass used in the experiment.
• the analysis of Bi and Si was conducted by an EPMA, measuring linearly the values in a plane oriented perpendicular to the die-upsetting direction.
• the observation of the microstructure was conducted on a surface direction, the surface being first ground with emery paper and then mirror polished by buffing.
• the hardness of the hot-worked samples cut out from a magnet were measured by a Micro-Vickers testing apparatus after mirror polishing the surface to be observed.
• the hardness was estimated on basis of a correlation table of hardness and the length of a diagonal line of a compressed mark formed by a compression pin made of diamond under a load of 1000 g.
• the measurements were conducted on two surfaces parallel to the die-upsetting direction and two surfaces perpendicular to the die-upsetting direction.
• the mean often values measured on ten points each comprising five points in the two parallel planes is the represen­ tative value of the hardness in the respective direction.
• the evaluation results of the examples are as follows:
• the stress value was 0.48 to/cm2 for the case of a strain rate of 0.1 s ⁇ 1. It is understood that the deformation resistance decreases with a decrease in the strain rate.
• Carbon content remaining 0.32 mass-%
• Oxygen content remaining 1700 ppm.
• a microstructure having a uniform composition flow was observed.
• the Vicker's hardness of the magnet was 650 Hv.
• the hardness of the magnet made in accordance with the present invention was 650 Hv measured by Vicker's hardness test.
• the hardness of a magnet comprising no glass and no graphite was 580 Hv.
• the magnet according to the present invention is provided with a higher hardness, it does not become brittle.
• Magnetic characteristics were measured on magnets produced using only low melting point glasses for internal lubricant additives, that is, without graphite additives.
• the experimental results are shown in Fig. 1.
• the figure shows magnetic properties of magnets in dependence of the glass amount added.
• the residual magnetic flux density and the maximum energy product increase as the content of the glass additive increases.
• the peak value of 4 ⁇ I r and BH max can be observed at a glass amount of 0.3 mass-%.
• the 4 ⁇ I r value and the BH max value are, respectively, 320 G and 2 MOe higher than the corresponding properties of a magnet having no additives.
• the intrinsic coercive force decreases only slightly as the glass amount increases, the iHc value remaining as high as 10900 Oe at the glass amount of 0.5 mass-%.
• Example 1 was repeated except that various amounts of graphite powder were used with various amounts of a low melting point glass material. With respect to each of the resulting magnetically anisotropic hot-worked magnets, magnetic properties were measured to evaluate the effects of the additives. In Fig. 2, the dependence of the magnetic characteristics on the glass amount are shown, measured at 0.1, 0.3 and 0.5 mass-% of glass amount, each with various amounts of graphite powder.
• the 4 ⁇ I r value is improved by 910 G, and BH max is improved by 5.9 MOe as compared with the case of no additive when 0.3 mass-% of graphite and 0.3 mass-% of glass is added.
• the amount of glass added was 0.5 mass-%, the residual magnetic flux density rapidly increased with increasing amounts of added graphite, but remarkably decreased when graphite amounts reached about 0.5 mass-%.
• the iHc value decreases rapidly as graphite content increases.
• the tendency for iHc to decrease is pronounced when the graphite additive and glass additive amounts each were 0.5 mass-%.
• the iHc value was 15430 Oe in case of 0.3 mass-% of glass and 0.3 mass-% of graphite, lower by about 2590 Oe compared with the case of no additive.
• the amount of graphite powder is preferably less than 0.5 mass-% because an iHc value of at least 10 kOe is necessary for a practical magnet having sufficient heat resistance.
• a maximum of BH max can be obtained with addition of 0.3 mass-% of graphite powder.
• Fig. 4 shows correlations between the residual oxygen content, the residual carbon content and the amounts of graphite and glass added to the magnet.
• the residual oxygen content does not strongly depend on the graphite content, the increase of oxygen content by graphite additive can be neglected, which is in contrast to the case of a complex additive of organic lubricant and glass.
• the residual carbon content is a strong function just of the graphite amount added. As stated above, the oxygen content is considered to depend only on the glass amount added.
• the iHc value of a hot-worked magnet decreases as the graphite amount added or glass amount added increases.
• the tendency of iHc to decrease with increasing amounts of added graphite is not as pronounced in the case of concurrent glass addition as in case of the sole addition of graphite.
• the decrease in iHc by carbon or oxygen can not be avoided because these elements react with e.g. the Nd component which is necessary to increase the coercive force.
• the maximum amount of glass additive is 0.4 mass-% in a case where a coercive force of 16 kOe is necessary and the graphite amount is 0.2 mass-%, according to the data in Fig. 3.

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