Classifications

• • 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
• • 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
  • B22F3/10—Sintering only
  • B22F3/1003—Use of special medium during sintering, e.g. sintering aid
  • B22F3/1007—Atmosphere
  • B22F3/101—Changing atmosphere
• • 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
• • 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
  • H01F1/0577—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 sintered

Definitions

• This invention relates to iron based permanent magnets and a method of making such magnets.
• Major permanent magnet materials currently in use today are AlNiCo, hard ferrite, rare earth-cobalt stabilized with iron, and rare earth-iron magnets which have been stabilized with boron.
• the AlNiCo and ferrite materials (such as strontium or barium ferrite) have proved to be inexpensive with costs typically about $1.50 per pound of material, but their magnetic properties do not provide a sufficiently high energy product or coercivity that would be suitable for applications such as an automotive starter motor or a magnetic imagining device (cat scan).
• Such magnets have exhibited energy products at the level of about 4 MGOe, and a coercivity at the level of less than 4 KOe.
• rare earth-cobalt-iron and rare earth -iron-boron magnets each of which have proven to be highly expensive with a cost factor of at least about $80.00 per pound of material.
• Technology for making iron stabilized rare earth-cobalt magnets is taught in U.S. patents 4,081,297; 4,369,075; and 4,131,495. This type of magnet usually contains rare earths and cobalt in amounts of up to 50-60% by weight.
• Cobalt is strategic material and the main source of cobalt for the United States is from Southern Africa, particularly Zaire. Thus, political considerations frequently affect the availability and price of cobalt.
• This type of magnet also contains intermetallic compounds of rare earth and cobalt which exhibit extremely large magnetocrystalline anisotropy.
• An energy product of at least about 24 MGOe (several times over the conventional AlNiCo or ferrite permanent magnets) is obtainable with a samarium-cobalt permanent magnet.
• Even higher saturation magnetization, a higher energy product, and a higher operating temperature can be obtained with the presence of the intermetallic compound Sm2Co17.
• Misch metal is a more concentrated form of rare earth metals obtained in the early stages of the chemical treatment of the ores containing naturally occurring rare earths.
• the specific composition of the misch metal will depend on the ore employed (the ore content will vary somewhat between different geographical locations but will exhibit essentially the same primary rare earths in essentially similar amounts).
• the most common misch metal composition consists of 52% cerium, 20% La, 15.7% Nd, 4.8% Pr, and about 6% other rare earths. Misch metal will have a content that reflects to a large extent the combination of rare earths found in the ore from which it is refined.
• Didymium is sometimes referred to as misch metal, but it is more expensive because it is extracted from the original misch metal through additional chemical processes. Cerium and lanthanum are the easiest rare earths to remove from misch metal and thus Didymium reflects this ease of elimination in varying degrees.
• the most common compositions of Didymium consist essentially of: (i) 90% Nd-10% Pr, (ii) 80% Nd-5% Pr, (iii) 50% Nd-40% Ce-10% Pr, and (iv) 60% Pr-20%Nd-20% La.
• the invention is a method for producing a permanent magnet of the Fe-B-MM type, comprising: (a) preparing a metallic powder having an average particle size of about, but above, 2 microns and having a composition consisting essentially of, by atomic weight percent: 12-22% misch metal, 1-8% Al, 6-10% B, less than .43% oxygen, substitutional agents of up to 5% Dy as a substitute or adjunct of misch metal and/or up to 10% Ni as a partial substitute or adjunct of Al, up to 3-10% of a modifying agent for modifying physical properties other than magnetic properties, and the remainder Fe, said substitutional and/or modifying agents only being present in an amount that would not prevent Fe from being present in an amount of substantially at least 60%, the powder having essentially a crystalline structure dominated by at least two R2Fe14B phases (the misch metal preferably comprising at least four naturally occurring insoluble cerium earth metals and at least 20% of such earth metals consisting of cerium and/or lanthanum); (b
• the resultant permanent magnet cylinders will be characterized by magnetic properties intermediate that of commercial ferrite magnets and commercial R-FE-B type magnets (where R consists essentially of Nd in high amounts).
• This invention replaces Nd by a combination of aluminum, misch metal, and/or small amounts of Dy and/or Ni. It has been found that by this substitution and by the process control parameters herein, a permanent magnet of intermediate range magnetic properties can be produced at low cost.
• the rare earth Dy is added as an oxide (Dy2O3), it is done so in an amount of 2-5% with the total content of Dy in the mixture being in the restricted range of 1-1.5%.
• This invention provides a more economical permanent magnet of the Fe-B-MM type which has controlled magnetic coercivities much more effectively tailored to the needs of automotive applications;
• the permanent magnet is advantageously made from an alloyed powder designed to have a controlled coercive field in the range of 4-8 KOe, an energy produce in the range of 10-17 MGOe, and M s of at least 65 Emu/gram, and desirably a T c of at least 250°C.
• the permanent magnet embodying the invention employs dual magnetic phases in a new, advantageous manner, redistributes the nonmagnetic phases in a novel manner, is much less costly than the known iron-boron-­rare earth (Fe-B-R) or rare earth-cobalt type magnets, and yet exhibits, desirably, an energy product in the range of 14-17 MGOe and a coercivity in the range of 5-8 KOe.
• the magnet includes a phase redistributing the agent that increases the coercivity and energy product of such material and promotes impediment phases for greater domain wall pinning.
• the magnets of this invention are made by powder metallurgy techniques and are iron-misch metal based (more than 70% by weight). Iron is one of the most abundant elements on earth and is very inexpensive. Misch metal is also very abundant and because it does not need the purification process of purer rare earth elements (like neodymium) it is much less expensive. Boron acts as a fluxing agent for sintering.
• the coercive fields are significantly increased with the addition of controlled amounts of Al which promotes impediment phases or locking of dual iron-rich intermetallic phases and redistributes the nonmagnetic phases.
• Al can be added as an element or as ferro aluminum to the melt for the ingot.
• Dy for MM can be made either to the melt for the as-cast samples, i.e., through the addition of elemental Dy, or by blending the as-cast powder with Dy2O3 (which is much less expensive) and then proceeding with the aligning, compacting and sintering procedure.
• Ni and other modifying agents such as up to 3% Cr for corrosion resistance or up to 10% cobalt, can be added as elements to the melt for the as-cast ingot.
• an arc melted ingot (under an argon atmosphere) is formed from a starting melt consisting essentially of, by atomic weight percent: 12-22% MM, 1-8% Al, 6-10% B, up to 5% Dy and/or 10% Ni as a substitute or adjunct of MM or Fe, respectively, up to 3-10% of an agent for modifying physical properties other than magnetic properties, .1-.43% oxygen, and the remainder Fe, with iron being present in an amount of at least 60%.
• Misch metal as used herein, means a material having at least four naturally occurring insoluble cerium earth metals with at least 20% of such earth materials consisting of cerium and/or lanthanum.
• Cerium earth metals is a group consisting of La, Ce, Pr, Nd, Sm, and Sc; such metals are often referred to as light rare earth metals.
• the misch metal will contain typically 3-4% heavy rare earth metals.
• Such misch metal may be derived from Indian sources which have been analyzed to contain 52% Ce, 20% La, 15.7% Nd, 4.8% Pr, 3-4% heavy rare earth metals.
• Synthetic misch metals (labeled Didymium) have been devised to contain 60% Pr, 0% Ce, 20% La, and 20% Nd; 40% Ce, 0% La, 50% Nd, and 10% Pr; 90% Nd, 0% Ce, 0%, La, and 10% Pr; 5% Ce, 0% La, 80% Nd, and 15% Pr.
• Such synthetic misch metals result from the elimination of Ce and/or La.
• Didymiums which have Ce and/or La in an amount of 20% will suffice for use in the process described herein.
• This invention advantageously and preferably uses misch metal that results from an early stage of beneficiating rare earth ores, prior to the removal of Ce or La.
• certain of the Didymiums can be used as indicated (resulting from a later stage of rare earth processing) as long as Ce and/or La constitute at least 20% of the Didymium.
• Misch metal is often produced by beneficiating monazite ore to a concentrated combination of rare earth salts; these commercial salts are then converted to chlorides and subjected to electrolysis of such fused anhydrous rare earth chlorides in NaCl or KCl (see the cited references in U.S. patent 4,597,938, or "The Metallurgy of Non-Ferrous Metals", The Rare Earth Metals, by W.H. Dennis, Chapter 21 (1954), or Rare Metals Handbook , by Clifford Hampel, Chapter 16 (1954)).
• the introduction of aluminum is important to this invention because it promotes redistribution of nonmagnetic phases (such as a rare earth-rich phase designated R-Fe-O which forms at the temperature level of about 600°C), primarily to the grain boundaries, and promotes impediment between dual magnetic phases as a result of such redistribution. If aluminum is present in an amount less than 1%, the coercivity will fail to increase after sintering. If aluminum is added in an amount greater than 8%, the remanence and T c will be adversely affected. Boron, if used in an amount less than about 6%, will fail to produce R2Fe14B phases, and if added in an amount greater than 10%, will adversely affect the magnetic properties of the shape.
• nonmagnetic phases such as a rare earth-rich phase designated R-Fe-O which forms at the temperature level of about 600°C
• R-Fe-O which forms at the temperature level of about 600°C
• Dy is added as a substitute or adjunct for Fe and functions to increase coercivity when using Al or misch metal at the low end of the prescribed ranges.
• the chemical system can function with or without the presence of Dy or Ni. It has been found that if Dy is added in an amount less than 1%, the desired benefits of an increase in anisotropy and an increase in coercive field will not be obtained. If added in an amount greater than 5%, there may be a reduction in remanence. Ni can be used in a similar fashion but with less effectiveness.
• Modifying agents such as 0-3% chromium, can be added to the melt to enhance corrosion resistance of the magnet material, or 0-10% cobalt my be added to enhance the Curie temperature. Such modifying agents are added to influence the physical properties of the material that are not magnetic properties.
• the ingots resulting from arc melting have a purity of at least 99.8% and the ingredients thereof are first crushed under argon using a micro-mill.
• the powder resulting is then ball milled under moisture-free toluene using an attritor to achieve uniform particle size and homogeneous composition.
• the milling time is controlled to produce an average particle size that is preferably in the range of 2-5 microns, but may not be smaller than about 2 ⁇ 's, and specifically a critical particle size of 1.72 micrometers for certain types of chemistry.
• Sintering, particle size, and oxygen concentration are interrelated factors for the development of the better hard magnetic properties. Improper processing of the ingot can result in the oxidation of the measured rare earths leading to a decrease of magnetic properties.
• no vacuum is used during milling to dry; the powder should not be dried. It appears that the decrease in remanence and coercivity after sintering corresponds to a critical particle size at which the Fe-MM-B crystallites are affected significantly by the formation of bulk oxides, which size is 1.7 microns. For powder particles larger than the critical powder size, all the oxygen is chemisorbed and does not tend to reduce the rare earth content.
• the oxygen is partially chemisorbed and partially in the form of bulk oxides. It has been found that the corresponding critical maximum oxygen concentration for the type of alloys under consideration for this invention, to promote excellent properties, is 0.43 weight percent.
• the alloyed powder mixture must be protected against oxidation; it is preferably protected by a liquid cover or toluene, of other inorganic solvent protector, that is added in amounts that thoroughly wet the powder without much excess liquid.
• the wet powder is measured in predetermined quantities and aligned in magnetic fields in the range of 5-10 KOe. These measured quantities are then subsequently and slowly pressed with about 5000 pounds in a compaction die in a direction perpendicular to the applied field, allowing the toluene to escape.
• the shapes are pressed as blocks, cubes, cylinders or arcs.
• the pressure under which the powder mixture is compacted is about 50,000 psi (40,000-60-000 psi) to render a presintering density of 70-80%.
• the shapes are green bodies characterized by dominant magnetic phases designated R2Fe14B which is present in an amount of about 65-75%.
• the remainder of the green body is comprised of nonmagnetic phases RFe4B4 and Rx-Fe-O (rare earth-rich) where x is at least 30-40.
• the magnetic properties of such green bodies comprise an energy product of 10-15 MGOe, a remanence of 65-80 Emu/gram, and a coercivity of 4-8 KOe.
• the intrinsic coercivity of such green bodies is about 7.0 KOe.
• the pressed shapes are heated in a high vacuum (i.e., 10 ⁇ 4 Torr or more), preferably in stages, first to about 400°C for 8-15 minutes to de-gas all of the toluene, and then to the temperature range of 1000-1100°C for .5-9 hours (preferably one hour).
• the shapes are then quenched to room temperature with a quenching rate of 100-200°C/minute, using flashing of argon (high purity).
• Sintering is carried out in a high vacuum of about 10 ⁇ 4 Torr or more. Cooling is carried out until the shapes are below at least 50°C and preferably 25°C.
• the hard magnetic properties can be obtained with or without a post-sintering heat treatment. Without the heat treatment, the process is easier and therefore less expensive, but H c is slightly lower.
• the heat treatment consists of heating to a temperature level of 550-650°C for about 1-4 hours (preferably 190 minutes) in a high vacuum, then cooled by flashing argon.
• the resulting permanent magnets will have a crystalline microstructure characterized by a matrix dominated by at least two R2Fe14B phases; for example, one being with a light rare earth and the other with a heavy rare earth, resulting in distinctly different magnetic properties. Since each of the R2Fe14B phases have different magnetic properties, their coexistence tends to create domain wall pinning that leads to an increase in coercivity. This is clearly reflected on the "effective initial curves" of A.C. demagnetized samples where there is a critical field below which the domain walls are unpinned leading to an increase in magnetization.
• the nonmagnetic phases will constitute 15-20% by volume of the final material with the rare earth-rich phase predominating in the grain boundaries.
• the increased redistribution of the nonmagnetic phases to the grain boundaries is a unique aspect of this invention.
• One of the interesting aspects of such resulting permanent magnet is the thermal demagnetization characteristic.
• a generally square or rectangular hysteresis loop tends to suggest that uniformity in coercivity is related to the presence of two R2Fe14B phases causing a locking in domain walls leading to such hysteresis squareness. This microstructure tends to lock the domain walls against sliding and thus maintains a high level of coercivity and remanence.
• Crystal size of the resulting permanent magnet will be in the range of 5-10 microns. Essentially, by volume, 70-75% of the magnet material will be the iron-rich intermetallic ferromagnetic phases, 1-5% boron-rich phase, and about 15-19% rare earth-rich phase. The latter two nonmagnetic phases will reside essentially in the grain boundary.
• an arc melted ingot of approximately 100 grams was first crushed under argon using a micro mill and then ball milled for varying periods of time to give an average particle size of about 2 microns and an oxygen content of about .4%. Ball milling was carried out under moisture-free toluene using an attritor to achieve uniform particle size and homogeneous composition. The particle size and oxygen concentration was measured using micrographs obtained with Jeol 100 C scanning electron microscope. In all cases, iron was present in the range of 60-80% atomic weight as the remainder for the chemistry.
• Table I the rare earth constituency was modified and varied throughout the samples utilizing misch metal in various combinations along with varying aluminum amounts and the presence or absence of Dy or Ni. The use of a post-heat treatment (annealing) was generally used but sometimes eliminated as indicated. The effectiveness of these variables are reflected in the level of H c (KOe), M s (Emu/gram), and T c (°C).
• samples 1-8 illustrate the effect of varying Al while maintaining B at 8-10% and MM at the high end of the inventive range (18-22%).
• Sample 5 demonstrates the effect of not using a post heat-treatment (annealing).
• Samples 9-21 illustrate the addition by Dy (added as an oxide after milling of the alloy powder) along with variations in the amount of MM and/or Al.
• Samples 22-27 depict the use of Ni as an adjunct to Al, MM and/or Dy.
• sample 1 shows the properties of a standard Fe-Nd-B magnet material.
• Sample 2 shows the effect of using Nd and MM.
• Samples 3 and 5-10 illustrate the elimination of aluminum.
• Samples 5-8 and 11 show the effect of exceeding or underusing the limits for MM or B.

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• 1988
  • 1988-04-01 KR KR1019880003727A patent/KR880013194A/ko not_active Withdrawn
  • 1988-04-05 EP EP88303018A patent/EP0286357A3/fr not_active Withdrawn
  • 1988-04-05 BR BR8801585A patent/BR8801585A/pt unknown
  • 1988-04-05 JP JP63083968A patent/JPS63262805A/ja active Pending
  • 1988-04-05 AU AU14154/88A patent/AU608309B2/en not_active Ceased

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