Die-upset manufacture to produce high volume fractions of RE-Fe-B type magnetically-aligned material

Abstract: A method of increasing the volume fraction of magnetically-aligned material in rare-earth (RE), iron, boron type anisotropic permanently magnetic material includes forming an adaptively-shaped, fully-dense, substantially magnetically-isotropic pre-form (64) from relatively coarse powder particles of melt-spun alloy containing a very fine-grain RE₂Fe₁₄B phase. The pre-form (64) is heated and die-upset to provide uniformity of strain in the pre-form (64) as it is conformed to the die walls (82), thereby causing an increased percentage of the crystallites in the pre-form (64) to be oriented along a crystallographically-preferred magnetic axis which increases the energy product of a resultant magnet formed therefrom.

Description: This invention relates to a method of processing magnetically-isotropic alloy material based on rare-earth elements, iron and boron to make magnetically-anisotropic material as specified in the preamble of claim 1, for example as disclosed in EP-A-O 133 758. Background of the Invention Permanent magnet compositions based on the rare-earth (RE) elements neodymium (Nd) or praseodymium (Pr) 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. The maximum magnetic energy product to date for such quenched Nd-Fe-B based alloy is about 1.59 x 10⁵ AT/m (20 megaGaussOersted). It is also known that anisotropic permanent magnetic properties can be introduced into rapidly solidified RE-Fe-B based isotropic alloys by hot-working. Alloys with overquenched, substantially amorphous microstructures are worked 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 alloy 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. However, the volume fraction of the workpiece which is in the higher energy product range has been limited by tool friction effects and unsuitable metal flow produced during the hot-working steps. 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. The preferred rare-earth elements are Nd and Pr, and the preferred transition metal (TM) is iron or mixtures of iron and cobalt. EP-A-0 174 735 discloses a method for producing from such alloys a hot-pressed permanent magnet having high and low coercivity regions in which a hot-pressed pre-form is deformed by hot-working in a heated die by contoured punches which deform the pre-form so that it is non-uniformly magnetically aligned. The method disclosed in this European patent application is ideally suited for producing arcuate permanent magnets for electric motors, since in such arcuate permanent magnets one edge thereof is more susceptible to demagnetization and thus needs to be formed with a higher magnetic coercivity than the remainder of the permanent magnet. EP-A-0 231 620 discloses a method of obtaining a magnetically anisotropic material by the hot-working of a rare earth-transition metal-boron alloy formed by directionally chill-casting the molten alloy. Such a chill-cast material has relatively large, directionally-solidified grains and is essentially a soft magnetic material as produced. Hot-working of this material aligns the grains therein more closely with one another to produce a body which, upon magnetisation, is magnetically anisotropic. The present invention enables highly magnetically-anisotropic permanent magnets to be formed. The starting material is formed by initial rapid solidification of the molten alloy but without the fine grinding step of conventional orient, press and sinter processes used in the manufacture of samarium, cobalt and other rare-earth permanent magnets. Furthermore, the present invention enables near net-shape magnets to be formed which require less finish-grinding. The present invention uses rapid solidification and subsequent hot compaction to form an initial pre-form with a magnetically-isotropic intermetallic phase of Nd-Fe-B. Suitable pre-forms have basically spherically-shaped RE₂-Fe₁₄-B grains which are randomly oriented in an optimum relationship with rare-earth-rich grain boundaries. It is known that die-upsetting improves the maximum energy product of the magnetic material in such pre-forms by causing the individual particles to orient along a crystallographically-preferred axis. While such die-upsetting is suitable for its intended purpose, it has been observed that die-upset orientation of the particles often produces a less than expected high-energy product. The highest alignment (and resulting energy product) occurs only in the volume centre of the compact. This problem is believed to be attributable to substantial friction which develops between the die-upset tools and the pre-form during upsetting thereof with a resultant unsuitable metal flow. The frictional contact between hot upset rams and die and a workpiece produces a barrelling effect in the grain directionality in which the spread of the material at the top, bottom and outer edges of the workpiece is restricted. As a consequence, the material in the workpiece adjacent to the die-upset tools undergoes little or no deformation and this effect extends into the workpiece from the opposite ends thereof. As a consequence, there is less strain in parts of the compact than in other parts thereof, and the lesser strained regions produce a lesser volume fraction of the final product with magnetically aligned higher energy products in the range of 2.78 x 10⁵ AT/m to 3.58 x 10⁵ AT/m (35MGOe to 45MGOe). A method of processing magnetically-isotropic alloy material according to the present invention is characterised by the features specified in the characterising portion of claim 1. In one preferred form of the precursor of the present invention, the pre-forms of magnetically-isotropic alloy material with an intermetallic Nd₂Fe₁₄B phase (hereinafter referred to as substantially isotropic 2-14-1 grains) are adaptively configured with respect to the die-upset tools such that unsuitable metal flow effects are reduced and a greater volume percent of the precursor experiences a required strain to induce crystallographic alignment as the height of the workpiece is reduced and its shape is altered to conform to the configuration of the die-upsetting tool. A resultant product with anisotropic permanent magnetic properties is formed having crystallographically-aligned platelet-shaped RE₂-Fe₁₄-B grains in an optimum compositional relationship with rare-earth-rich grain boundaries. Such grains, on average, are no greater than about 500 nm in the greatest dimension. Another precursor configuration which is contemplated by the invention is formed from hot die-upsettable material of dense, substantially isotropic 2-14-1 grains. The precursor has a surface configuration adapted to the shape of a hot-working die to cause a greater volume percent of the precursor to experience a strain capable of inducing desired crystallographic alignment to produce higher energy products in the resultant product. Yet another precursor contemplated by the invention is formed of such dense material adaptively-configured at surface regions thereon between the opposite ends thereof to provide uniform lateral material flow between the surface regions and the containment die for compressing the precursor during hot die-upsetting of the precursor. Yet another precursor contemplated by the present invention is shaped as an hour-glass configuration between opposite ends thereof and which configuration is uniformly laterally deformed during hot die-upsetting to conform to a larger diameter cylindrical die to magnetically align the 2-14-1 grains therein parallel to the press direction. The invention further contemplates a method of hot-working such precursors to magnetically align most of the particles or crystallites in the resultant product. The invention also features adaptively-shaping a fully-dense pre-form of isotropic 2-14-1 grains into a precursor that conforms to hot-working dies to limit friction effects and resultant unsuitable metal flow. The invention further contemplates an improved method for processing alloy material based on rare-earth elements, iron and boron to make isotropic ribbon particles of amorphous or finely crystalline material having grains of RE₂TM₁₄B. RE is one or more rare-earth elements containing neodymium and/or praseodymium, TM is iron or iron-cobalt combinations and B is the element boron. The improvement comprises compressing the ribbon particles to a fully dense state to form a substantially magnetically-isotropic pre-form and thereafter adaptively-shaping the pre-form to form a precursor with compression-relief regions therein and a height-to-diameter ratio to prevent buckling. The adaptively-shaped precursor is then hot die-upset to cause the material of the precursor to flow into and fill the compression-relief regions whilst maintaining the precursor at an elevated temperature so as to produce uniform strain patterns in the precursor as the precursor is reduced in height and conformed to the die walls. The particles or crystallites thereby become aligned along a crystallographically-preferred magnetic axis to increase the magnetic energy-product fraction of the total volume of the compressed product. In one preferred method, such a preferred magnetic axis is parallel to the press direction. In another preferred method, compression-relief regions are formed from a fully-dense pre-form having 2-14-1 grains by shaping a precursor therefrom as a plurality of discs. The discs are stacked end-to-end in a die cylinder having its containment walls spaced from the outer surface of the discs. A compression force is imposed by plungers against end surfaces of the outermost stacked discs to reduce the height of the discs, causing the outer surfaces thereof to expand uniformly against the die cylinder whilst compressing the discs, to cause the diameter thereof to correspond to that of the die. Yet another preferred method is to provide hot die-upsetting of stacked discs as set forth above, in which the fully-dense starting material has a high Nd content. The method includes maintaining a hot pressing temperature during the die-upsetting which causes a Nd phase to diffuse to the exterior surfaces of the discs so as to form an in situ lubricant between the discs thereby to produce uniformity of deformation therein during compression thereof. Another preferred method includes modifying any of the above stated disc-stacking methods by shaping the pre-form of dense isotropic NdFeB material as a right-circular cylinder; and thereafter slicing the pre-form into a plurality of discs. The plurality of discs are then adaptively configured by stacking them with end surfaces thereon in juxtaposed relationship in a die cavity of a diameter greater than that of the stacked discs. The discs are then hot-upset to compress the discs to reduce their height and to conform them to the shape of the die cavity so as to uniformly deform and strain the discs to orient 2-14-1 grains therein along the crystallographicallypreferred magnetic axis. Yet another method of the present invention includes the step of adaptively-shaping an hour-glass precursor to provide desired relief for lateral flow of material. In more specific methods, the hour-glass shape is formed either by shaping two conical components each having a small-diameter end and a large-diameter end and wherein the small-diameter ends are stacked with their surfaces in contact at a mid-line or by shaping the hour-glass-shaped precursor by subjecting a right-circular cylinder to etching at the centre girth thereof. 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 of the transition metal component. 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 present. 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 be expressed by the 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 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 preferably in an amount of about 1 to 10 atomic percent (y = 0.01 to 0.11) of the total composition. This 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 of the invention contains at least fifty percent by weight of this tetragonal 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 melted to form alloy ingots. The ingots are re-melted and sprayed through a discharge nozzle having a small-diameter outlet onto a rotating chill surface. The resultant product is a directly-quenched or overquenched alloy ribbon with crystallites or grains within the microstructure having a fairly regular shape. The Nd-Fe-B intermetallic phase has high magnetic symmetry and the directly-quenched material (as well as annealed forms of the overquenched material which causes growth of the crystallites) are magnetically-isotropic as formed. Depending on the rate of cooling, molten transition metal-rare-earth-boron compositions can be solidified to have microstructures ranging from: (a) amorphous (glassy) and extremely fine-grained microstructures (e.g., less than 20 nanometres in largest dimension) through (b) very fine (micro)-grained microstructures (e.g., 20 nm to about 400 or 500 nm) to (c) larger grained microstructures. Thus far, large-grained microstructure melt-spun materials have not been produced with useful permanent magnet properties. Fine-grain microstructures, where the grains have a maximum dimension of about 20 to 500 nanometres, have useful permanent magnet properties. Amorphous materials do not. However, some of the glassy microstructure 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. In accordance with the present invention, such ribbon-formed alloy is broken into coarse powder particles and hot-precompacted (e.g. at 725°C) to full density by use of a standard plunger press. The grain size after hot-pressing is of the order of 150nm. In the past, pre-forms of such precompacted, fully-dense ribbon material have been placed in a die-upsetting tool and compressed to conform to the die shape under elevated temperature conditions at which the Nd-Fe-B phase is plastically deformed to cause particles or the crystallites themselves to be oriented along a crystallographically-preferred magnetic axis with a resultant production of magnetically-anisotropic material having greater magnetic energy products than the parent isotropic material. However, known hot-working processes produce substantial friction at the interface between the pre-form and the hot-work tooling. Such friction restricts lateral deformation at the surfaces of the pre-form and through a portion of the axial length thereof. A resultant barrelling effect has been observed which reduces the volume fraction of the resultant magnet in which the material is oriented on a desired crystallographically-preferred magnetic axis. In accordance with the present invention, an increased volume percentage of magnetically-aligned material is obtained by adaptively-shaping a pre-form to reduce hot-working friction. This precursor is then placed in a die and upset to more uniformly deform the precursor whilst maintaining an equalized lateral strain in the material to produce a high volume fraction of high-energy products in the resultant product. In one embodiment, the pre-form is adaptively shaped as a ring-shaped dough-nut with its outer diameter slightly less than the diameter of a die cylinder having an upset-die plunger therein. The pre-form is hot-upset to compress the dough-nut to a 50% height reduction. Such adaptive shaping shifts poorly-aligned material toward the centre of the dough-nut and produces greater orientation at the outer diameter of the resultant product. In another embodiment, the pre-form is adaptively-shaped by removing material from the upper and lower edges of a right-circular cylindrical pre-form to form frustoconical ends thereon. The shaped pre-form is hot-upset by a die tool with a die-cylinder diameter greater than the precursor diameter. Resultant relief provides a uniform lateral flow of the precursor as it is compressed. This causes increased percentages of high-energy products in the resultant product. In yet another embodiment, the pre-form is adaptively-shaped by removing material from the centre of a right-circular cylinder to form an hour-glass shaped precursor with ends engageable by the hot die-upset plungers and with a diameter less than that of the die cylinder. The resultant product produced after die-upsetting was found to have increased volume fractions with high energy products reflecting desired crystallographic magnetic alignment in the precursor. An increased volume percentage of magnetically-aligned material is also obtained by adaptively-shaping the pre-form as a plurality of stacked discs having the interfaces thereof lubricated by diffusion of an Nd phase to the disc interfaces and wherein the dimensions of the discs are selected with reference to the dimensions of the die-upset tooling to prevent buckling of the stacked discs as compressive loading is applied thereagainst by the die plungers. An advantage of the present invention is that magnetically-anisotropic permanent magnets can be hot-worked to final shape without resorting to finish-machining. Moreover the resultant product produced will have a high percentage of properly magnetically-aligned particles therein to increase the high-energy product content in predictable regions of the finished product. These and other objects and advantages of the invention will become more apparent from a detailed description thereof which follows when taken in conjunction with the accompanying drawings wherein: Detailed Description of the Drawings Figure 1 is a diagrammatic view of a system for producing melt-spun magnetically-isotropic ribbon material of Nd-Fe-B alloy; Figure 2 is a cross-sectional view of a hot-pressing die for compressing the isotropic ribbon material to a fully-dense state; Figure 3 is a second quadrant, room temperature, 4PiM versus H plot of a sample produced by the Figure 2 press; Figure 4 is a second quadrant, room temperature 4PiM versus H plot of a hot die-upset cylindrical precursor. Figure 5 is a perspective view of a standard precursor of substantially isotropic permanent magnet material used in hot-press die-upsetting methods; Figure 6 is a diagrammatic view of a barrelling effect produced in the standard precursor as it is compressed during hot-press die-upsetting; Figure 7 is a diagrammatic view of the standard precursor in a hot-press die before and after compression of the precursor; Figure 8 is a chart of the distribution pattern of high-energy products in a resultant product formed from the precursor of Figure 5; Figure 9 is a perspective view of one embodiment of the invention shown as a precursor adaptively-shaped as a ring-shaped dough-nut; Figure 10 is a cross-sectional view of a hot-working die used to hot-work the precursor of Figure 9; Figure 11 is a cross-sectional view of the die and pre-form of Figure 10 after hot-working the precursor; Figure 12 is a chart of the distribution pattern of high-energy products in a resultant product formed from the dough-nut pre-form of Figure 9; Figure 13 is a perspective view of another embodiment of an inventive precursor adaptively-shaped as a right-circular cylinder having frusto-conical ends; Figure 14 is a cross-sectional view of a hot-press upset die including the precursor of Figure 13; Figure 15 is a chart of the distribution pattern of high-energy products in a resultant product formed from the precursor of Figure 13; Figure 16 is a perspective view of another embodiment of an inventive precursor adaptively-shaped as a right-circular cylinder having an hour-glass-shaped centre region; Figure 17 is a cross-sectional view of a hot-press upset die including the precursor of Figure 16; Figure 18 is a chart of the distribution pattern of high-energy products in a resultant product formed from the pre-form of Figure 16; Figure 19 is a perspective view of another embodiment of an inventive precursor adaptively-shaped as a plurality of right-circular cylindrical discs having a height to diameter ratio chosen to prevent buckling; Figure 20 is a cross-sectional view of a hot-press upset die including the precursor of Figure 19; and Figure 21 is a chart of the distribution pattern of high-energy products in a resultant product formed from the precursor of Figure 19. Detailed Description As stated above, the present invention is applicable to high-coercivity, fine-grain materials comprised of basically spherically-shaped, randomly-oriented Nd₂-Fe₁₄-B grains with rare-earth-rich grain boundaries. Suitable compositions can be made by melt-spinning apparatus 2 as shown in Figure 1. The Nd-Fe-B starting material is contained in a suitable vessel, such as a quartz crucible 4. The composition is melted by an induction or resistance heater 6. The melt is pressurized by a source 8 of inert gas, such as argon. A small, circular ejection orifice 10 about 500 micrometres in diameter is provided at the bottom of the crucible 4. A closure 12 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 14. The molten stream 14 is directed onto a moving chill surface 16 located about 6 mm below the ejection orifice. In examples described herein, the chill surface is a 25 cm diameter, 1.3 cm thick copper wheel 18. The circumferential surface is chrome-plated. The wheel is not cooled 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. When the melt hits the turning wheel, it flattens, almost instantaneously solidifies and is thrown off as a ribbon 20 or ribbon fragments. The thickness of the ribbon 20 and the rate of cooling are largely determined by the circumferential speed of the wheel. In this work, the speed can be varied to produce a desired fine-grained ribbon for practicing the present invention. The cooling rate or 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. Summary of the Prior Art A fully-dense isotropic magnet formed from ribbon alloy broken into coarse-size powder particles 20a, of the order of 150 um, may be compacted to full density. The particles 20a are placed in a pre-heated high temperature die 22. The die 22 is heated by an induction heater 24 in vacuum or an inert atmosphere. Uni-axial pressure is applied when the particles are heated. A pre-form results having full density. A suitable high temperature press process has an operating time, temperature and pressure which produces sufficient plasticity of the ribbon alloy for full densification of the pre-form to occur. The pre-form has typical room temperature magnetic characteristics shown in Figure 3. Curve 3a therein shows room temperature demagnetization characteristics of the particles in a direction parallel to the press direction. Curve 3b shows the room temperature demagnetization characteristics in a direction perpendicular to the press direction. Whilst the material is substantially isotropic, it has a slight magnetic alignment in the press direction. Such starting material may be formed as a right-circular cylindrically-shaped standard precursor 26 as shown in Figure 5. Such a standard precursor 26 has opposite ends 28, 30 thereof engaged by hot upset plungers 32, 34 of a hot upset-die apparatus. The plungers 32, 34 are driven into a die cylinder 36 to compress the precursor 26 so that it conforms to the walls 38 thereof. The plungers 32, 34 compress the precursor 26 to a resultant product 40 having the shape shown in broken outline in Figure 7. In this example, the standard precursor 26 has a diameter of 13mm and a height of 13mm. The die cylinder diameter is 16mm and the compressed resultant product 40 has a height of 6mm and a diameter of 16mm. In the past, pre-forms of such pre-compacted material have been placed in a hot-press upset-die apparatus of a diameter greater than that of the pre-form. Such an apparatus compresses the pre-form so that it conforms to the die-shape under elevated temperature conditions produced by an induction heater 41. In this case, crystallites are strained and oriented along a crystallographically-preferred magnetic axis with a resultant production of magnetically-anisotropic material having higher value magnetic energy products than in the parent isotropic material, as shown in Figure 4. Curve 4a therein shows room temperature demagnetization characteristics of hot-worked material in a direction parallel to the hot-upset-press direction. Curve 4b shows room temperature demagnetization characteristics of the hot-worked material in a direction perpendicular to the hot-upset-press direction. While Figure 4 indicates an improved alignment of particles, in practice it has been observed that a substantial volume percentage of the resultant product 40 has lower energy products than the precursor 26. Such reduction is attributed to undesirable metal flow patterns caused by substantial friction effects at the interface between the plungers 32, 34 and the precursor 26. Such friction effects prevent lateral deformation at the ends of the precursor and through a portion of the axial length thereof and results in a barrelling effect shown in Figure 6. Such a barrelling effect is an example of unsuitable metal flow which can reduce the volume fraction of the resultant product in which the material becomes oriented on a desired crystallographically-preferred magnetic axis. More specifically, Figure 6 shows that only a small central region 42 of the precursor 40 is free of such lateral restraint. Lateral deformation at each end of the precursor 26 adjacent to the surfaces of the plungers 32, 34 is restrained by the tool friction so that the spread of the material is constrained at the ends of the precursor 26 and barrelled at the midsection thereof. The result is a pair of cone-shaped zones 44, 46 in the compressed resultant product 40 which are deformed to a lesser degree than the material in free-flow barrelled zones 48, 50 on either side of the central region 42. The barrelling is, of course, limited by the inside diameter of the wall 36. As the precursor is compressed from the original height (broken outline) shown in Figure 6 to the compressed height, the zones 44, 46 are more resistant to deformation than the free-flow zones 48, 50. Consequently, the material adjacent to the plungers is not subject to the same strain as at the middle, central region 42. As shown in Figure 8, only a small central region (approximately 5 volume %) of the resultant product reached maximum energy product levels in the order of BHmax of 3.18 x 10⁵ AT/m (40MGOe). The outer extremities of the compressed resultant product 40 have energy products which fall off to values less than 1.59 x 10⁵ AT/m (20MGOe). Accordingly, there is a lesser volume fraction of the desired high-energy products in the resultant product 40. The following examples illustrate the practice of the present invention. Each of said examples demonstrates that adapting the precursor shape to a metal-forming tool can promote higher lateral strain over a larger volume of precursor and thereby result in increased volume fractions of high-energy products in a resultant product. As a variation of the invention, it is also demonstrated (e.g. Example 1) that the highest energy product regions can be moved from the centre to further out in the resultant product. In other words, one can choose where the maximum energy product regions occur. In all of the following examples (as well as in the case of resultant product 40 above), room temperature demagnetization loops were measured in the press-direction on cube segments of the resultant product. The examples demonstrate that adaptively-shaped precursors of fully-dense isotropic permanent magnet material with a Nd-Fe-B phase, can promote higher lateral strain over increased percentages of the volume of the resultant product so as to produce desired results. Specifically, the desired results are an increased percentage of high-energy products in the resultant product due to improved alignment of grains of the Nd-Fe-B phase in a preferred direction transverse to the press direction. As previously discussed such alignment is along a crystallographically-preferred magnetic axis which produces the resultant high-energy product material. In all of the examples, a pre-form of fully-dense, substantially isotropic, permanent magnet material is shaped to have a height to diameter ratio less than 3:1 which will prevent buckling of the precursor as it is pressed into a reduced height configuration. Furthermore, the precursor is adaptively-shaped to provide compression relief that will improve lateral flow of the precursor to overcome metal flow patterns that otherwise inhibit equal lateral strain over increased volume fractions of the resultant product. Example 1 Fully-dense, isotropic magnet material is shaped as a dough-nut 54 (precursor) as shown in Figure 9. The outer diameter of the dough-nut is 14mm and the height of the dough-nut is 14mm. The central hole 56 has a diameter of 8mm. The hot-upset-die cylinder has a diameter of 16mm. The dough-nut 54 is die-upset in a heated cylindrical upset-die 58 to one half of its original height to produce a resultant product shown at 60 in Figure 11. The resultant product 60 has an improved smoothness at the outer surface thereof. A volume fraction of 16% with a magnetic energy of greater than 2.63 x 10⁵ AT/m (33MGOe) was attained in the resultant product 60. The demagnetization curves of measured cubes had the energy product distribution as shown in Figure 12. In contrast to the pre-form of the first example the dough-nut shaped pre-form provides a compression-relief space at the centre thereof to adaptively conform the precursor to the shape of a hollow die cylinder to produce predictable particle alignments in a preferred direction parallel to the press direction. Whilst the total gain in the volume fraction of high-energy product is less than in other examples to follow, it affords the advantage of predictable particle flow and an improved surface finish which may be of value in the production of certain kinds of finished permanent magnet products. It also produces higher energy products near the circumference but at the expense of lower energy product values in the volume centre -- a desirable configuration in some magnet geometries. Example 2 Figure 13 shows a fully-dense, isotropic magnet pre-form 64 adaptively shaped by removing material from the upper and lower ends 66, 68 of a right-circular cylindrical part (like 26 in Figure 5) to form frusto-conical segments 72, 74 thereon. The precursor 64 is hot-worked in a heated cylindrical upset-die shown in Figure 14. The maximum diameter of the pre-form is 13mm and the interior diameter of the die cylinder 76a is 16mm. The arrangement provides toroidally-shaped compression-relief spaces 78, 80 adjacent the frusto-conical segments 72, 74. The precursor material expands into the spaces 78, 80 without restraint to conform with the wall 82 of the die cylinder 76a. This provides for a uniform lateral flow of the precursor to occur as it is compressed, resulting in even greater percentages of high-energy product in the resultant product. Specifically, as shown in Figure 15, high-energy product values occur at both ends of a compressed resultant product 84 to define an anisotropic permanent magnet with a high volume fraction of Nd-Fe-B type, magnetically-aligned ribbon particles. A volume fraction of 30% having a magnetic energy greater than 3.02 x 10⁵ AT/m (38MGOe) was attained in the resultant product 84. Such increased volume fraction reflects increased ribbon alignment along the press direction from side to side of the compressed precursor in deformation patterns which are more uniform than in standard precursors subject to metal flow restraints. Example 3 Another embodiment of the present invention is shown in Figure 16 as a precursor 90 having an hour-glass-shaped centre segment 92 formed between generally flat circular discs 94, 96 at either end of the precursor 90. The precursor 90 is hour-glass shaped from a right-circular cylinder pre-form (like 26 in Figure 5) by controlled etching of the central girth 100 of the cylinder in 50% nitric acid (HNO₃). Alternatively, as shown in Figure 17, a precursor 90a is defined by two generally conical portions 102, 104, each having their smaller-diameter flat surfaces 108, 110 in contact at the mid-line of the precursor. The precursor 90a is shown mounted in a hot-upset-die 106 prior to upsetting. The precursor 90 in this example is dimensioned to have a height of 13mm and a maximum end diameter of 13mm. The hour-glass shape has a height of 7mm and a minimum centre diameter of 7mm. It is placed in a hollow die cylinder 106a of 16mm and is heated to a temperature of 750°C and subjected to a pressure of 75 MPa and die-upset 60% in height by die-plungers. An annular compression-relief space 112 of a hemispherical-like cross-section is provided between the die cylinder 106a and the precursor 90a for allowing uniform deformation thereof during hot die-upsetting. The resultant product 114 in Figure 18 is formed by a substantially unrestrained plastic metal flow. The resulting demagnetization values of the resultant product 114, shown in the chart of Figure 18, reflect a commensurate increase in maximum energy product which in this example produced a volume fraction of 35% of the resultant product having energy products greater than 3.18 x 10⁵ AT/m (40MGOe). This example has a reverse metal flow pattern in that the central volume of the precursor compensates for the metal flow restraint problems previously discussed. The following example of adaptive-shaping is provided to accommodate a wider variety of final magnet product shapes. Example 4 This example includes an adaptively-shaped precursor suited for production of permanent magnetically-anisotropic magnets of both circular and rectangular shapes. A precursor 120 is formed from a plurality of individual discs 122 having a height to diameter ratio less than 3:1 which will prevent buckling of the precursor during hot-upsetting thereof. A right-circular cylinder of isotropic permanent magnet material with an intermetallic phase of Nd-Fe-B is sliced into 5 discs. Alternatively, one may start with thin discs pressed as such. The discs 122 are re-stacked and loaded into a hollow die cylinder 124 and hot-pressed at 750°C and 75 MPa by plungers 125 and an induction heater 127. The individual discs have an initial height of 3mm; the stacked discs have a total initial height of 15mm and a diameter of 10mm. Exterior end surfaces 130, 132 of the discs 122 form compression-relief regions therebetween when the discs 122 are re-stacked. The die cylinder 124 has an inside diameter of 16mm. The dimensional relationships result in a reduction in height of the stack of 64% when the stack is fully hot-upset. A resultant product 126 (shown in broken outline in Figure 20) is fully dense and completely fills a hollow cylindrical compression-relief space 128 formed between the stacked discs 122 and the inside wall of the cylinder 124. It has been observed that a high Nd content phase (93% Nd) becomes molten and migrates to the exterior juxtaposed end surfaces 130, 132 of the discs 122 (two such surfaces are identified in Figure 19). The migrated molten phase acts as a natural lubricant to prevent frictional restraint of the lateral flow of material and consequently more uniform deformation of the ribbon layers is achieved. Energy products of equal to or greater than 3.18 x 10⁵ AT/m (40MGOe) were measured in a volume fraction of 48% of the resultant product 126. Cubes made from the end surfaces of the resultant product 126 (50mg cubes) were also found to have reasonably uniform ribbon deformation with energy products of 1.99 x 10⁵ AT/m (25MGOe) or greater. The aforesaid precursor shape and method of manufacture is specially suited to the manufacture of magnets of complex shapes with a variety of cross-sections including triangles, squares, rectangles or other shapes. The use of the stacked disc precursor configuration produces desired uniform deformation which is a function of the ratio of the surface areas of the precursor 120 and the surface area of the resultant product 126. The improved distribution of high-energy product is shown in the chart of Figure 21. Summary The aforesaid examples are select examples of the invention. It is clear that other precursor shapes are possible which will provide a desired compression-relief space for the flow of metal to overcome unsuitable metal flow patterns. An advantage of the present invention is that magnetically-anisotropic permanent magnets can be formed in a final shape without resorting to finish-machining. Moreover the resultant product will have a high percentage of properly-aligned particles therein to increase the high-energy product content either in predictable regions of the finished product or more uniformly throughout the body of the finished product.

Claims: A method of processing magnetically-isotropic alloy material based on rare-earth elements, iron and boron to make magnetically-anisotropic material, in which the magnetically-isotropic alloy material includes fine-grained crystalline material having grains of RE₂TM₁₄B where RE is one or more rare-earth elements, at least sixty percent of which RE is neodymium and/or praseodymium, TM is iron or iron-cobalt combinations and B is the element boron, the method comprising compressing particles (20,20a) of said magnetically-isotropic material to form a fully-dense precursor, placing said precursor in a hot-working die (58;106;124), and hot-working the precursor in said hot-working die (58;106;124) whilst maintaining the precursor at an elevated temperature so as to align particles or crystallites of the precursor along a crystallographically-preferred magnetic axis to form a magnetically-anisotropic resultant product (60;84;114;126), characterised in that the method includes the steps of pre-compressing particles (20,20a) of said magnetically-isotropic material to form a fully-dense pre-form (26); shaping the pre-form (26) to form said precursor (54;64;90;120) having compression-relief regions (56;78;80;112;128,130,132) formed therein or defined therewith when said precursor (54;64;90;120) is placed in said hot-working die (58;106;124); and hot-working said precursor (54;64;90;120) so that the material of the precursor is displaced into and fills the compression-relief regions (56;78;80;112;128,130,132) as the precursor (54;64;90;120) is being conformed to a hot-working tool, to more uniformly distribute a high-energy product fraction of the total volume of said magnetically-anisotropic resultant product (60;84;114; 126) throughout said magnetically-anisotropic resultant product (60;84;114;126). A method of processing magnetically-isotropic alloy material according to claim 1, said alloy including a high Nd content intermetallic phase of Nd-Fe-B, in which the particles are pre-compressed into a plurality of separate discs (122) having compression-relief regions (130,132) therebetween when stacked together; and the discs (122) are hot-worked by applying compression forces thereagainst so as to reduce the height of the discs (122) whilst causing the outer surfaces thereof to expand uniformly in a die (124) having a lateral dimension greater than the greatest lateral dimension of the discs (122) and compressing the discs (122) to cause the lateral dimension thereof to correspond substantially to that of the die (124). A method of processing magnetically-isotropic alloy material according to claim 1, said alloy including a high Nd content intermetallic phase of Nd-Fe-B, in which said preform is in the shape of a right-circular cylinder; the pre-form is sliced into a plurality of discs (122); the discs (122) are re-stacked to locate end surfaces (130,132) thereon in juxtaposed relationship within a die cavity having a diameter greater than that of said discs (122); and the discs (122) are hot-pressed to conform to the die cavity so as to uniformly deform and strain the discs (122) to form said magnetically-anisotropic resultant product (126). A method of processing magnetically-isotropic alloy material according to claim 2 or 3, in which the stacked discs (122) are hot-pressed at a temperature at which the high Nd content intermetallic phase in the discs becomes molten and migrates to the exterior surfaces of said discs (122) including the juxtaposed end surfaces (130,132) therebetween, to provide an in situ lubricant between said discs (122) which aids in producing uniform deformation therein and a maximum deformation thereof exceeding 50 percent of the total initial height of said stacked discs (122). A method of processing magnetically-isotropic alloy material according to claim 1, in which the step of shaping said pre-form creates compression-relief spaces (78,80;112) which geometrically compensate for tool restraint of the volume of the precursor (64;90) during compression of the precursor (64;90) to fill the hot-working die (106). A method of processing magnetically-isotropic alloy material according to claim 5, in which the pre-form is shaped by removing material at surface regions (72,74;92) thereon between the opposite ends thereof; and the hot-working of the precursor (64;90) produces unrestrained lateral material flow between the surface regions (72,74;92) and said hot-working tool. A method of processing magnetically-isotropic alloy material according to claim 6, in which the pre-form is shaped to form an hour-glass precursor configuration (92) between opposite ends thereof, and the precursor (90) is placed in a hollow containment cylinder (106) and is uniformly deformed by hot-working so as to fill the cylinder. A method of processing magnetically-isotropic alloy material according to claim 7, in which the hour-glass shape is formed from two conical components (102,104), each having a small-diameter end (108,110) and a large-diameter end, and the small-diameter ends (108,110) are stacked with their surfaces in contact at a mid-line. A method of processing magnetically-isotropic alloy material according to claim 7, in which the hour-glass-shaped precursor (90) is shaped by etching a right-circular cylinder at the centre girth (100) thereof. A method of processing magnetically-isotropic alloy material according to claim 6, characterised in that said pre-form is shaped to form a precursor (64) having frusto-conical ends (72,74) thereon.

Inventors: BREWER EARL G, LEE ROBERT W, BREWER, EARL G, LEE, ROBERT W.
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1989
Main classification: H01F41
IPC codes: B22F3, H01F1, C22C33, H01F41, C21D6, C22C38, B21J5