Method and apparatus for making flakes of RE-Fe-B-type magnetically-aligned material

Abstract: A method and apparatus is disclosed for hot-working particles of magnetically-isotropic material comprising iron, neodymium/praseodymium and boron. The method includes heating the particles (38) to a hot-working temperature; impelling the heated particles individually against co-operating working surfaces (66,68) of a hot-working device (70,72), pressing the individual particles between said working surfaces (66,68) to produce plastic flow in the particles that flattens the grains therein and thereby makes the flattened particles magnetically-­ anisotropic, and removing and cooling the individual flattened particles, the flattened particles (76b) having an average grain size no greater than about 500 nm. The particles (38) are contained in a feed hopper (44) and are discharged into a spray pattern (64) of a plasma spray gun (40) by means of a pressurized gas source (48), so as to be heated to said hot-working temperature immediately prior to the heated particles reaching the working surfaces (66,68) of the hot-working device (70,72).

Description: This invention relates to methods and apparatus for forming anisotropic permanently magnetic material from particles of magnetically isotropic preforms of finely crystalline alloys containing one or more light rare-earth (RE) elements, one or more transition metals (TM) and boron with a Nd-Fe-B type intermetallic phase and more particularly to methods and apparatus for hot-working such isotropic particles so as to magnetically align most of the grains or crystallites therein as specified in the preamble of claim 1, for example as disclosed in EP-A-0 133 758. An apparatus for hot-working a Nd-based alloy powder so as to magnetically align the crystalline grains therein is disclosed in JP-A-63-33801. The apparatus disclosed in this Japanese patent application comprises a container for the Nd-based alloy powder which feeds the alloy powder into a cascade furnace, and a pair of counter-rotating rolls, fed from the cascade furnace, which roll the heated alloy powder flat to produce the desired magnetic alignment in the alloy powder. Background of the Invention Permanent magnet compositions based on the rare-earth (RE) elements neodymium or praseodymium or both, the transition metal iron or mixtures of iron and cobalt, and boron are known. Preferred compositions contain a large proportion of a RE₂TM₁₄B phase where TM is one or more transition metal elements including iron. A preferred method of processing such alloys involves rapidly solidifying molten alloy to achieve a substantially amorphous to very finely crystalline microstructure that has isotropic, permanently magnetic properties. In another preferred method, overquenched alloys without appreciable coercivity can be annealed at suitable temperatures to cause grain growth and thereby induce magnetic coercivity in a material having isotropic permanently magnetic properties. It is also known that particles of rapidly solidified RE-Fe-B based isotropic alloys can be hot-pressed into a substantially fully-densified body and that such a body can be further hot-worked and plastically deformed to make an excellent anisotropic permanent magnet. Thus, alloys with overquenched, substantially amorphous microstructures are worked and plastically deformed at elevated temperatures to cause grain growth and crystallite orientation which result in substantially higher energy products than in the best as-rapidly-solidified alloys. The maximum energy product to date for hot-worked, melt-spun Nd-Fe-B magnet bodies is up to about 3.98 x 10⁵ AT/m (50MGOe), although energy products as high as 5.09 x 10⁵ AT/m (64MGOe) are theoretically possible. As stated above, the preferred rare-earth (RE)-transition metal (TM)-boron (B) permanent magnet composition consists predominantly of RE₂TM₁₄B grains with a RE-containing minor phase(s) present as a layer at the grain boundaries. It is particularly preferred that, on the average, the RE₂TM₁₄B grains be no greater than about 500 nm in greatest dimension in the permanent magnet product. Whilst such a hot-pressing process using a hot die-upsetting procedure is suitable for its intended purpose, in certain manufacturing processes it would be desirable to directly convert the isotropic particles to anisotropic permanently magnetic particles. Such anisotropic particles can then be mixed with a suitable matrix material and shaped to form a bonded permanent magnet having magnetically-anisotropic properties. A method of making a magnetically-anisotropic composition comprising iron, neodymium/praseodymium and boron according to the present invention is characterised by the features specified in the characterising portion of claim 1. The present invention contemplates a method and apparatus for making flakes of permanent magnetically-anisotropic material from, e.g., melt-spun ribbon particles of amorphous or finely crystalline material having grains of RE₂TM₁₄B where RE is one or more rare-earth elements, at least sixty percent of which is rare-earth material such as neodymium and/or praseodymium, TM is iron or iron-cobalt combinations and B is the element boron. The ribbon is fragmented, if necessary, into individual particles of such isotropic material. The individual particles are then heated to a plastic state and individually worked to deform each particle to align crystallites or grains therein along a magnetically-preferred axis and to form flakes of material which are not fused to one another. The flakes with such aligned crystallites are then individually cooled and collected for use in the manufacture of permanent magnets having magnetically-anisotropic properties. A feature of the present invention is to provide a method wherein the individual particles of magnetically-isotropic material are passed through a heat source to heat the individual particles to a plastic state; and thereafter the particles are impelled whilst in their plastic state against spaced surfaces of a hot-working device; thereafter the individual particles are shaped into individual flakes by deforming the particles between the spaced surfaces whilst still in their plastic state. The method contemplates maintaining a controlled separation of the individual particles during such shaping to prevent fusion of the resultant individual flakes together, whilst producing a crystallite grain structure therein which is aligned along a crystallographically-preferred magnetic axis. A feature of the method of the present invention is to provide a method of the type set forth in the preceding objects and features wherein the isotropic particles are heated to a plastic state by heating them by directing them with respect to a plasma torch and impelling such particles against the shaping die surfaces by plasma spraying. Yet another feature of the present invention is that the isotropic particles are processed whilst in their plastic state by a continuous process which includes shaping the plastic particles by directing them through a gap between hot-working rolls. Still another feature of the present invention is to provide methods of the type set-forth above including sizing the individual particles in the range of from 1 to 350 µm to form a resultant anisotropic flake material suitable for mixing with matrix material from which different shaped anisotropic permanent magnets can be subsequently processed. Yet another object is to provide apparatus to practice the aforesaid methods wherein the apparatus includes a plasma spray system and a pair of counter-rotating rollers to shape particles sprayed from a plasma spray system as individual flakes of magnetically-anisotropic material. Brief Summary of the Preferred Embodiment The method of the present invention is applicable to compositions comprising a suitable transition metal component, a suitable rare-earth component, and boron. The transition metal component is iron or iron and (one or more of) cobalt, nickel, chromium or manganese. Cobalt is interchangeable with iron up to about 40 atomic percent. Chromium, manganese and nickel are interchangeable in lower amounts, preferably less than about 10 atomic percent. Zirconium and/or titanium in small amounts (up to about 2 atomic percent of the iron) can be substituted for iron. Very small amounts of carbon and silicon can be tolerated where low-carbon steel is the source of iron for the composition. The composition preferably comprises about 50 atomic percent to about 90 atomic percent transition metal component -- largely iron. The composition also comprises from about 10 atomic percent to about 50 atomic percent rare-earth component. Neodymium and/or praseodymium are the essential rare-earth constituents. As indicated, they may be used interchangeably. Relatively small amounts of other rare-earth elements, such as samarium, lanthanum, cerium, terbium and dysprosium, may be mixed with neodymium and praseodymium without substantial loss of the desirable magnetic properties. Preferably, they make up no more than about 40 atomic percent of the rare-earth component. It is expected that there will be small amounts of impurity elements with the rare-earth component. The composition contains at least 1 atomic percent boron and preferably about 1 to 10 atomic percent boron. The overall composition may also be expressed in the general formula RE 1-x (TM 1-y B y )x . The rare-earth (RE) component makes up 10 to 50 atomic percent of the composition (x = 0.5 to 0.9), with at least 60 atomic percent of the rare-earth component being neodymium and/or praseodymium. The transition metal (TM) as used herein makes up about 50 to 90 atomic percent of the overall composition, with iron representing at least 60 to 80 atomic percent of the transition metal content. The other constituents, such as cobalt, nickel, chromium or manganese, are called "transition metals" insofar as the above empirical formula is concerned. Boron is present in an amount of about 1 to 10 atomic percent (y = 0.01 to 0.11) of the total composition. The practice of the present invention is applicable to a family of iron-neodymium and/or praseodymium-boron-containing compositions which are further characterized by the presence or formation of the tetragonal crystal phase specified above, illustrated by the atomic formula RE₂TM₁₄B, as the predominant constituent of the material. In other words, the hot-worked permanent magnet product contains at least fifty percent by weight of this tetragonal phase. Here RE means principally Nd or Pr and the easy magnetic direction is parallel to the "c" axis of the tetragonal crystal. The suitable composition also contains at least one additional phase, typically a minor phase at the grain boundaries of the RE₂TM₁₄B phase. The minor phase contains the rare-earth constituent and is richer in content of said constituent than is the major phase. For convenience, the compositions have been expressed in terms of atomic proportions. Obviously these specifications can be readily converted to weight proportions for preparing the composition mixtures. For purposes of illustration, the invention will be described using compositions of approximately the following proportions: Nd 0.13 (Fe 0.95 B 0.05 ) 0.87 However, it is to be understood that the method of the invention is applicable to a family of compositions as described above. Such compositions are arc-melted to form alloy ingots. The ingots are re-melted and rapidly solidified, e.g., melt-spun, i.e., discharged, through a nozzle having a small diameter outlet onto a rotating chill surface. The molten metal alloy is thus solidified almost instantaneously and comes off the rotating surface in the form of small, ribbon-like particles. The resultant product may be amorphous or it may be a very finely crystalline material. If the material is crystalline, it contains the Nd₂Fe₁₄B type intermetallic phase which has high magnetic symmetry. The quenched material is magnetically-isotropic as formed. Depending on the rate of cooling, molten transition metal-rare-earth-boron compositions can be solidified to have a wide range of microstructures. Thus far, however, melt-spun materials with grain sizes greater than several micrometres do not yield preferred permanent magnet properties. Fine-grain microstructures, where the grains have a maximum dimension of about 20 to 500 nanometres, have coercivity and other useful permanent magnet properties. Amorphous materials do not. However, some of the glassy microstucture materials can be annealed to convert them to fine-grain permanent magnets having isotropic magnetic properties. The present invention is applicable to such overquenched, glassy materials. It is also applicable to "as-quenched" high coercivity, fine-grain materials. Care must be taken to avoid excessive time at high temperature to avoid coercivity loss through excessive grain growth. In accordance with the present invention such ribbon-formed alloy is broken into coarse powder particles. Individual particles of such rapidly-solidified material are then heated and directed onto a hot working surface of a suitable deforming apparatus. The individual particles are deformed by the apparatus while in a plastic state (approx.750°C). Each Nd-Fe-B particle is plastically deformed to cause generally spherically-configured grains in the individual particles to be flattened so as to cause the grains or crystallites to be oriented along a crystallographically-preferred magnetic axis and thereby produce magnetically-anisotropic material. In a preferred embodiment of the invention apparatus is provided to feed the magnetically-isotropic particles from a feed hopper by means of a carrier gas. The particles are heated by a plasma arc and are discharged from a plasma spray gun against two counter-rotating rollers spaced to form a deforming gap therebetween. The gap is sized to be about half the size of the minor dimension of the ribbon particles so as to provide the required amount of deformation. The particles are discharged from the plasma spray gun against the roller surfaces upstream of the gap. The process of shaping the particles takes place while the particles are in a plastic state (approximately 750°C.). In apparatus for practice of the invention, the plastic particles are splattered across the rollers upstream of the gap so that a substantial percentage of the particles are separately deformed in the roller gap without being fused into larger particles. The dimension of the gap can be varied to control the amount of deformation. The resultant deformed particles are flattened from a spheroidal shape to a flake form. The flakes are cooled and ejected from the downstream end of the gap as individual flakes. During such deformation, the individual isotropic grains in the plastic spheroid are rotated such that their "c" axis of the (Nd,Pr)₂TM₁₄B phase becomes normal to the direction of the plastic flow imparted by the rotating rollers. Such orientation along a crystallographically-preferred magnetic axis produces magnetically-anisotropic material in the resultant individual flakes. The aforesaid objects and advantages of the invention will be better understood from the succeeding detailed description of the invention and the accompanying drawings thereof. Detailed Description of the Drawings Figure 1 is a chart showing a preferred practice of the present invention; Figure 2 is a diagrammatic view of apparatus for making magnetically-isotropic ribbon particles; Figure 3 is a diagrammatic view of apparatus for plasma spraying and hot-working the ribbon particles of Figure 2; Figure 4 is an enlarged region of the view of Figure 3 showing the upstream end of a deforming gap in the apparatus of Figure 3; Figure 5 is a diagrammatic representation of spherically-configured isotropic grains; Figure 6 is a diagrammatic representation of such grains deformed to produce anisotropic grains; and Figure 7 is a diagrammatic view of another process for deforming such isotropic grains. Detailed Description of the Invention Referring now to Figure 1, the inventive method of the present invention includes the following generalized steps: 1. Forming step 10, in which ribbon particles of magnetically-isotropic material are formed. 2. Heating step 12, in which each of the individual particles is heated to a temperature at which the particle is in a plastic state. 3. Impelling step 14, in which the plastic particles are impelled onto the surfaces of a hot-working apparatus; 4. Shaping step 16, in which each of the particles is shaped to form a resultant flake of magnetically-anisotropic material. 5. Cooling and extracting step 18, in which the particles in flake form are removed from the hot-working apparatus without fusing the individual flakes. The forming step 10 of the invention is applicable to magnetically-isotropic, amorphous or fine-grain materials that are comprised basically of spherically-shaped, randomly-oriented Nd₂-Fe₁₄-B grains with rare-earth-rich grain boundaries. Suitable compositions can be made by melt-spinning apparatus 20 as shown in Figure 2. The Nd-Fe-B starting material is contained in a suitable vessel, such as a quartz crucible 22. The composition is melted by an induction or resistance heater 24. The melt is pressurized by a source 8 of inert gas, such as argon. A small, circular ejection orifice 26, e.g., about 500 micrometres in diameter, is provided at the bottom of the crucible 22. A closure 28 is provided at the top of the crucible so that the argon can be pressurized to eject the melt from the vessel in a very fine stream 30. The molten stream 30 is directed onto a moving chill surface 32 located about 6.35 mm below the ejection orifice. In examples described herein, the chill surface is a 25 cm diameter, 1.3 cm thick copper wheel 34. The circumferential surface is chrome-plated. The wheel does not need to be cooled in small runs since its mass is so much greater than the amount of melt impinging on it in any run that its temperature does not appreciably change. Alternatively, a water-cooled wheel can be used. When the melt hits the turning wheel, it flattens, almost instantaneously solidifies and is thrown off as a ribbon or as ribbon particles 36. The thickness of the ribbon particles 36 and the rate of cooling are largely determined by the circumferential speed of the wheel. In this work, the speed of the wheel can be varied to produce a desired fine-grained ribbon for practicing the present invention. The cooling rate, i.e., speed of the chill wheel, preferably is such that a fine crystal structure is produced which, on the average, has Re₂TM₁₄B grains no greater than about 500 nm in greatest dimension and preferably less than 200 nm in greatest dimension. The ribbon alloy is broken or pulverized into coarse size powder particles 36, of the order of an average size of 150 µm at the greatest dimension. The starting material size can be selected from a range of from 1 to 350 µm particles from the broken or fragmented ribbon 36. Figure 3 shows plasma spray apparatus 40 and rolls 70, 72 for carrying out the aforesaid steps of heating 12; impelling 14; shaping 16 and cooling and extracting 18. Specifically the apparatus includes a plasma spray gun 40 which is connected to a feed hopper 44 by a carrier tube 46. The feed hopper 44 has particles 38 of the magnetically-isotropic ribbon therein. The feed hopper is pressurized by a suitable inert carrier gas from a source 48. The carrier gas directs the particles 38 into plasma spray pattern 64 at a point downstream of the plasma torch 40. The plasma is formed between an electrode 52 and a conductive housing segment 54. The electrode 52 and the housing segment 54 are connected across a suitable arc-current generator 56. Arc gas is directed through passages 58, 60 to produce the plasma spray pattern 64 into which the particles are injected by the carrier gas. The temperature of the spray pattern 64 at the particle entry point must be such as to heat the particles to the plastic state (approximately 750°C.) without melting. The spray pattern 64 is impelled against surfaces 66, 68 of a pair of counter-rotating rollers 70, 72 arranged and operative to hot-work each of the individual particles. As best shown in Figure 4 the rollers 70, 72 are supported on drive axes which define a gap 74 therebetween. The gap 74 has a dimension less than the size of individual particles 76 impelled against the rollers 70, 72. The impelled particles 76 are generally platelet-shaped and will deform to a slightly globular form as they impact on the roller segments 70a, 72a upstream of the gap 74. The impacted globules 76a are drawn by rotation of the rollers 70, 72 into a gap 74 which is sized to reduce the shape of the globule 76a to a very shallow profile platelet 76b. The platelet-shaped particles 76a, 76b remain in a plastic state during such deformation and the splatter pattern of the particles against the roller segments 70a, 72a is selected so that the greatest number of the impacted particles remain separated without fusion therebetween. Consequently, the majority of the platelets 76b are not fused to one another. The platelets 76b are cooled as they pass from the outlet, downstream end of the gap 74. The resultant product is a number of individual platelets of material which have been deformed. As shown in Figure 5, before the particles 76 are deformed they have spherical grains or crystallites 78 therein of magnetically-isotropic material. As illustrated the "c" axis of the RE₂TM₁₄B grains are arranged in random direction to cause such isotropic properties. Obviously, the grains are illustrated at a very large magnification and the thickness of an inter-granular phase 82 is exaggerated. As the particles 76 are reshaped by hot-working from the substantially spherical shape 76a to the flake shape 76b, the grains 78 are formed as platelets 80 (see Figure 6) having their "c" axes rotated into a direction which is normal to the hot-deforming or flattening action described above. Such alignment of the grains along a crystallographically-preferred magnetic axis results in the formation of flakes 76b with good permanent magnetically-anisotropic characteristics. The rollers 70, 72 can have coolant directed therethrough to regulate the rate at which the flakes 76b are cooled within gap 74. For the process to work, the plasma-sprayed particles must pass between the rollers whilst above their plastic state. Any cooling of the particles below their plastic state can result in crushing of the particles which will prevent hot-working crystallographic orientation in the particles. While calendering-type rollers are shown in the apparatus of Figure 3, it should be understood that other roll-forming apparatus is equally suited for use in practicing the invention. Likewise other heat sources and impelling systems can be used to direct the isotropic starting material into a deformation gap. For example, as shown in Figure 7 the particles can be directed from a spray nozzle 90 through an arc formed between a heating electrode 92 and a centrifuge bowl 94. The bowl 94 has an inner surface 96 which receives the impelled heated particles in a plastic state and to which the particles adhere. The bowl is rotated with respect to a roller 98 which forms a gap 100 with the inner surface 96 which is dimensioned to flatten platelets of isotropic material to a flake form of anisotropic material. A scraper 102 is provided to remove the flakes from the inner surface 96 for collection in a hopper 104. The deformation of the particles produces the same desired crystallographic orientation of the magnetic axes of the grains in each of the individual particles. The particles are separated by the splatter pattern against the inner surface 96 to prevent fusion of the individual particles during the deformation at gap 100 and subsequent extraction from the apparatus. Other embodiments of the practice of the present invention could be devised. For example, particles of magnetically-isotropic material could be suitably heated as they are dropped down a vertically-disposed tube onto a gap between a pair or horizontally-disposed hot-working rolls.

Claims: A method of making a magnetically-anisotropic composition comprising iron, neodymium/praseodymium and boron, said composition either having appreciable coercivity as processed or being heat-treatable to acquire such coercivity, said method comprising: preparing a molten mixture comprising a transition metal (TM) taken from the group consisting of iron and mixtures of iron and cobalt, one or more rare-earth metals (RE) including neodymium and praseodymium, and boron, the proportions of such constituents being sufficient to form a product that consists essentially of the tetragonal crystalline compound having the empirical formula RE₂TM₁₄B, rapidly solidifying said mixture to form magnetically-isotropic particles (38) of an amorphous material or of a very finely crystalline material containing said compound and having small, generally spherical grains of an average size no greater than about 200 nm, and then hot-working said magnetically-isotropic material to convert it into said magnetically-anisotropic composition, characterised in that the particles (38) of magnetically-isotropic material are heated by discharging them into a spray pattern (64) formed by a plasma spray gun (40); impelling the heated particles (76) individually against co-operating working surfaces (66,68) of a hot-working device (70,72) by plasma spraying, pressing the individual particles (76a) between said working surfaces (66,68) to produce plastic flow in the particles (76a) that flattens the grains therein and thereby makes the flattened particles (76b) magnetically-anisotropic, and removing and cooling the individual flattened particles (76b), the flattened particles (76b) having an average grain size no greater than about 500 nm. A method of making a magnetically-anisotropic composition according to claim 1, characterised in that the heated particles (76) are directed into a gap (74) between the co-operating surfaces (66,68) of the hot-working device (70,72) to pressure-shape the individual particles (76a) into individual flakes (76b). A method of making a magnetically-anisotropic composition according to claim 2, characterised in that the gap (74) is formed between a pair of rotatable rolls (70,72). A method of making a magnetically-anisotropic composition according to claim 3, characterised in that the rolls (70,72) are counter-rotating rolls. A method of making a magnetically-anisotropic composition according to any one of the preceding claims, characterised in that the method includes sizing the magnetically-isotropic particles (38) to obtain particle sizes in a range of from 1 to 350 µm. A method of making a magnetically-anisotropic composition according to claim 5, characterised in that the sizing of the magnetically-isotropic particles (38) yields individual particles (38) having a nominal average size of 150 µm. Apparatus for processing permanent magnetically-isotropic material based on rare-earth elements, iron and boron to make permanent magnetically-anisotropic material and wherein the magnetically-isotropic alloy material is formed as broken ribbon particles (38) of material having a fine-grained structure of RE₂TM₁₄B where RE is one or more rare earth elements, at least 60 atomic percent of which RE is neodymium and/or praseodymium, TM is iron or iron-cobalt combinations and B is the element boron, said apparatus comprising heating means (40) for heating the particles (38) to a plastic state and hot-working means (70,72), said hot-working means including movable surfaces (66,68) thereon for shaping said plastic particles (76a) thereon by deformation whilst the particles (76a) are in said plastic state, characterised in that said heating means includes a plasma spray gun (40); the apparatus includes impelling means (44,46,48) arranged to discharge the particles (38) of magnetically-isotropic material into the spray pattern (64) of said plasma spray gun (40) formed for heating said particles to said plastic state, prior to impelling said plastic particles (76a) individually against said movable surfaces (66,68); said movable surfaces (66,68) are movable with respect to the impelled particles (76a) to maintain a controlled separation between the individual plastic particles (76a) during said shaping so as to align the grain structure of each shaped particle (76b) along a crystallographically-preferred magnetic axis whilst preventing fusion of the individual shaped particles (76b) together; and the apparatus also includes means for cooling and removing said individual shaped particles (76b) from said hot-working means (70,72) as separate flakes of permanent magnetically-anisotropic material. Apparatus according to claim 7, characterised in that said hot-working means includes hot-working rollers (70,72) having a gap (74) therebetween, the plastic particles (76a) being shaped by directing them through said gap (74). Apparatus according to claim 8, characterised in that said hot-working rolls are counter-rotating calendering rolls (70,72).

Inventors: HAVERSTICK JERRY EDWARD, HAVERSTICK, JERRY EDWARD
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1989
Main classification: H01F41
IPC codes: B30B11, B22F9, H01F1, C22C33, B22F1