Hot-pressed magnets in open-air presses

Abstract: This invention describes a practice for the hot-pressing and/or hot working of rare earth element-containing alloy powders (52) using open-to-the-air presses (10;100). The rare earth-containing powder (52) is pressed into a compact body (34) at ambient temperatures using a solid lubricant only on the die wall. This compact body (34) is then hot-pressed in an open-air press (100) utilizing a heated die (114) flooded with argon (130).

Description: This invention pertains to practices for the hot-pressing and subsequent hot-working of rare earth element-containing powder alloys as specified in the preamble of claim 1. Rare earth element-containing alloys composed so as to form a RE₂TM₁₄B tetragonal crystal phase have been melt-spun under carefully controlled processing to produce useful permanent magnet materials as disclosed in US-A-4,802,931 and US-A-4,851,058. Such melt-spun materials either as quenched or in an overquenched and annealed condition consist essentially and predominantly of the tetragonal crystal, prototype Nd₂Fe₁₄B phase. The tetragonal crystal-containing grains are very small, typically less than a few hundred nanometres on the average in grain size, and are surrounded by one or more secondary grain boundary phases which contribute to the permanent magnet characteristics of the composition. This fine-grain material is magnetically isotropic, and the melt-spun ribbon fragments can be pulverized to a suitable powder, combined with a suitable binder material and moulded into useful bonded permanent magnets as disclosed in US-A-4,902,361. Where permanent magnets of higher energy product are desired, it is known that the melt-spun powder material can be hot-pressed to form a fully densified permanent magnet body and that, where desired, such a fully densified body can be further hot-work-deformed. These practices are disclosed, for example, in US-A-4,792,367 and US-A-4,844,754. The fine-grained, melt-spun, rare earth element-containing material is initially in the form of ribbon particles or a powder produced by comminution of the ribbon fragments. In order to hot-press or otherwise hot-work the material, it is necessary that it be heated to a suitable hot-working temperature typically in the range of 700°C to 800°C. As disclosed in the aforesaid U.S. patents, it is prudent to heat the powder in vacuum or a suitable inert gas that provides a dry and substantially oxygen-free environment in order to prevent the powder from burning. In attempting to work with such readily oxidizable rare earth element-containing materials, it has been necessary to provide a suitable protective atmosphere in which the rare earth and other constituents are not oxidized and the permanent magnetic properties of the materials are not degraded. GB-A-2235700 discloses a method of pre-pelletizing an alloy powder using a lubricant on the die walls before compacting the pellets in a hot die to their final shape. In powder metallurgy practices, it is known to produce a compact body by pressing suitably ductile powder particles together at ambient conditions. This can be done (and is done with larger grain size rare earth element-transition metal-boron, RE-TM-B, materials in sintering processes) to produce a partly densified, porous body in air at room temperature. However, if such a Nd₂Fe₁₄B compact body is heated preparatory to hot-working, it must be protected from oxidation in order to avoid degradation of the permanent magnet properties thereof. Obviously, it is possible to enclose the operative portion of a press in such a non-oxidizing atmosphere, but it is expensive and impractical to adapt such apparatus for high-speed production if accurate powder feeding, powder heating, compaction and hot-working are to be carried out entirely within such a special atmosphere chamber. Such a press would be very expensive to construct and operate and cumbersome to operate and maintain. Accordingly, it is necessary to develop a practice for hot-pressing and, optionally, the additional hot-work-deformation of such rare earth element-containing powder alloy materials so that the rapid and efficient production of permanent magnets can be accomplished. It is a general object of this invention to provide a method for hot-pressing and additional hot-work-processing of RE-TM-B type powder materials on relatively inexpensive open-air type presses in a way that provides a suitable protection from oxidation or burning of the powder alloy materials. A method of consolidating a rare earth element-containing powder alloy of RE₂TM₁₄B precursor composition according to the present invention is characterised by the features specified in the characterising portion of claim 1. In accordance with preferred embodiments of the present invention, the above and other objects are accomplished as follows. The starting material for the practice of the invention is a melt-spun ribbon particle or powder composition composed so as ultimately to form a magnet body consisting essentially of the tetragonal phase RE₂TM₁₄B and a minor portion of a grain boundary phase(s) of higher rare earth element content. Whilst RE stands for rare earth elements generally, it is preferred that the rare earth constituent of this material be made up of at least 60 percent of neodymium and/or praseodymium. The transition metal element (TM) is preferably iron or mixtures of iron with cobalt and/or with minor portions of other metals. This rapidly-solidified starting material will preferably be of very fine grain size (e.g., less than 50 nm) or almost amorphous. The hot-pressing process and any additional hot-working process will then densify and work the material and simultaneously effect a growth in grain size such that the average grain size is larger but still less than about 500 nm in largest dimension. The product has useful permanent magnet properties. The practices of the present invention are suitably carried out in an open-air press of the type having a die(s) with a die wall defining a die cavity of suitable cross-sectional configuration. In such presses, the workpiece material or body is inserted in the die cavity and compacted i.e., worked by opposing machine members, typically lower and upper punches. Frequently the opposing press members are upper and lower punches and the die is of a uniform cross-section throughout its length. Sometimes the dies contain steps or shelves and the punch(es) is (are) configured accordingly. Sometimes a punch is cored. Sometimes a punch is replaced with a flat anvil surface. The invention may be practiced with all such press arrangements. Referring to the operation of a conventional two-punch press with uniform die cavity, the upper punch is initially raised out of the die cavity and the lower punch is initially in a low position so as to open the cavity to receive the material to be worked. The upper punch is then lowered to close the cavity, and the two punches are then mechanically or hydraulically actuated so as to press and compact the workpiece material between them. The punches closely fit the die wall so as to confine the material being worked but are slightly spaced from the die wall so as to reduce friction and wear therebetween. After the material is compacted, the upper punch is raised out of the cavity and the lower punch is raised so as to elevate the compacted workpiece above the top edge of the die or so that the worked piece can be removed. This process is repeated on a continuous basis. In accordance with the present invention, a hot-pressed, fully densified, permanent magnet body is produced in two pressing steps. Powder material of an above-described composition, in an amount based on the dimensions of the desired workpiece, is first compacted to a green compact body at ambient temperature and in air. This pressing can be called cold-pressing. The cold-pressed compact body suitably has a density of about five grams per cubic centimetre or higher, preferably about 5.3 to 5.5 grams per cubic centimetre. Such a compact body is formed, in part, to somewhat reduce the particle surface exposed to oxidation and to improve heat transfer to the overall mass. In this cold-pressing operation, a film of a solid die lubricant, such as Teflon tm powder, is formed on the die wall of the press. No lubricant or binder is mixed with the rare earth element-containing powder because the material is quite reactive and chemical changeof the powder degrades the magnetic properties thereof. The Teflon tm or like lubricant is preferably applied in the form of a liquid suspension of powder in a non-flammable, highly volatile liquid vehicle that helps to disperse the powder. In this regard, it is preferable to use a liquid comprising a suitable fluorocarbon(s). The fluid Teflon tm -containing mixture is preferably applied to the die cavity wall through suitable small holes in a punch, e.g., the lower punch, after the previously formed compact body has been ejected from the die and the punch is being moved to its lowest position to receive the next charge of melt-spun powder. The upper punch is actuated to cold-press the powder against the lower punch into a porous green compact body. The dried die wall lubricant film facilitates the compaction and the removal of the compact body in one piece from the die. This process can usually be repeated every one to six seconds or so depending upon the size of the compact body to be formed and the complexity of its shape. After the green compact body has been formed, it is then ready to be hot-worked in another open-air press. Usually, a different press is employed because it is adapted to heat the tools and the workpiece to facilitate the hot-pressing operation and requires heat-resistant tooling materials. In the operation of this hot press, the movements of the punches are co-ordinated with the introduction of a dry inert gas such as dry argon into the die cavity. Starting with the upper punch raised above the die cavity and the lower punch in its uppermost position for ejecting a previously hot-pressed workpiece, flooding of the die cavity with dry argon is commenced. A preferred mechanical arrangement for this practice will be described in detail below. As the lower punch is dropped to a position for receiving a cold-pressed workpiece, argon flooding continues so as to fill the enlarged cavity and to purge it of air. The die itself is preferably maintained at a temperature suitable for heating the workpiece and carrying out the hot-working operation, e.g., 870°C. The workpiece is dropped into the hot, relatively massive hot die cavity, and it lands on the hot lower punch. The upper punch is lowered, and the punches are then loaded so as to exert compaction pressure on the workpiece. The consolidated workpiece is almost instantaneously heated to a temperature (700°C to 800°C) that permits it to be rapidly pressed into a fully dense magnet body. After pressing, the upper punch is raised clear of the die cavity, and the lower punch is raised to eject the hot-pressed magnet from the die cavity. The fully dense body is cooled in the normal atmosphere. The process can then be repeated every 25 to 90 seconds or so, depending upon the size of the workpiece. The hot-worked body upon cooling is useful as is as a substantially isotropic permanent magnet. The hot-working produces a suitable grain size for permanent magnet properties. In the event an anisotropic permanent magnet is required, the hot-pressed body may be further hot-worked to deform it into a body in which the small 2-14-1 grains are flattened and aligned with one another. For example, this operation can be carried out in a larger hot die, i.e., the known process of die-upsetting, using an open-air hot press and practice. The resulting die-upset body upon cooling has a preferred direction of magnetization parallel to the direction of pressing and is an extremely strong permanent magnet. Both the hot-pressing operation described above and this die-upsetting operation yield permanent magnet bodies that require little further finishing operations. Other objects and advantages of the present invention will become apparent from a detailed description thereof, with reference to the accompanying drawings, in which: Figures 1a to 1d are schematic views, partly in section, of a cold-forming, open-air press illustrating the sequence of cold compact forming steps, including lubrication of the die cavity wall by spraying through the lower punch;and Figures 2a to 2d are schematic views, partly in section, of an open-to-the-air hot press illustrating the sequence of steps involved in the hot-pressing of a cold compact body, including the preferred practice for flooding the hot die cavity with dry inert gas. As summarized above, the process of the present invention involves two compaction steps for the manufacture of a fully-densified magnet body and a third manufacturing step where additional hot-working or hot-deformation of the fully-densified body is required in order to produce a more fully anisotropic magnet with stronger permanent magnet properties. The first two steps of the process are compacting or pressing steps, and they can be carried out on conventional presses for this purpose. Indeed, an advantage of the present invention is that both compaction steps can be carried out on open-to-the-air presses. During the description of the process, reference will be made to the drawings in which only a small portion of the press is depicted, namely that depicting the die and the upper and lower punches because it is in this region of the press that the special features of the process of the present invention are involved. A preferred embodiment of the invention illustrates the making of a sensor magnet in the shape of a circular right cylinder. However, it is to be understood that other magnet shapes can be produced by changing the die cross-section and punch shape. It is also to be understood that other press tooling constructions may be employed such as one punch anvil pressing, the pressing of ring shapes requiring cores, and the pressing of assemblies, i.e., magnets onto rotors or shunts, and the use of die shapes like shelf dies and step dies. Figures 1a to 1d thus depict a small portion only of an open-to-the-air operable-at-ambient-condition cold press 10. Cold press 10 has a die block 12 with a round cylindrical die cavity 14. Reciprocably operative in the die cavity 14 is a lower punch assembly 16. Also reciprocably operable in the die cavity is an upper punch 18. Upper punch 18 is slidably retained and guided by upper punch carrier 20. Upper punch 18 has a round, flat punch face 22. As shown in Figures 1a and 1b, upper punch 18 has been raised to its uppermost position to facilitate removal of a compacted product from the die of the cold press and the addition of a new particulate starting material. Lower punch 16 comprises a head 24, with a flat face 26, that is circular in cross section and adapted to closely fit the wall of die cavity 14. The lower punch 16 comprises a smaller diameter shank portion 28. Lower punch 16 also includes an enlarged base 30 that is below the die block 12. As shown in Figure 1a, the lower punch is elevated to its uppermost position with face 26 just flush with an upper surface 32 of die block 12. In this position, the lower punch 16 has raised a just-formed cold compact body of RE-TM-B particles 34. This cold compact body 34 is being moved aside by a rake or other mechanical means (not shown) at the end of the compaction cycle of the press operation. A typical such cold compact body is a still slightly porous green compact body of RE-TM-B particles of the type described above. It has a density in excess of 5 grams per cubic centimetre and is very useful in accordance with the process of the invention for the hot-pressing and, if necessary, further hot-working of this compact body into a fully-densified magnet body with exceptionally good permanent magnet properties. Following the ejection of the cold compact body 34, lower punch 16 is then lowered to its lowest position (as shown in Figure 1b) in the operation of the press. It is during this lowering process that this lower punch carries out an important part of the practice of the present invention. Formed in lower punch 16 is a central axial duct 36 that extends from the base 30 of the punch 16 the length of the shank 28 of the punch and into the head 24. Axial duct 36 can be formed by drilling a hole through the base 30 up through the shank 28 into the head 24 and then closing off the outlet in the base with a plug member 38. Plug member 38 is preferably flush with the bottom of the base member 30 so that the mechanically-actuated press can operate on the bottom of the base to raise and lower the lower punch 16. A transverse duct 40 is provided in the base member 30 that intersects axial duct 36. Duct 40 is threaded to receive fitting 42 and a supply tube 44 that is used for purposes that will soon be described. A small-diameter, second transverse duct 46 with respect to axial duct 36 is drilled in the head 24 of the punch. Small duct 46 extends diametrically across the head 24 of the punch and with outlets in a machined annular ring 48 that is parallel to the face 26 of the punch but slightly below it at the upper end of axial duct 36. Thus, lower punch 16 contains a continuous internal passage leading from tube 44 into cross duct 40 through axial duct 36 to the small outlet duct 46 in the head 24 of the punch. The purpose of this passage is to supply a suitable lubricant to the wall surface of die cavity 14. The selection of the lubricant system in the practice of the invention is important. The lubricant is not mixed with the rapidly-solidified particles that are to be consolidated into a green compact body in this step of the invention. The rare earth element constituent of the composition is reactive and susceptible to degradation by residual lubricant material, particularly during storage and/or hot-pressing of the compact body. The lubricant is applied to the die wall through the ductwork described above in the lower punch. The use of a solid lubricant is preferred. The solid lubricant preferably comprises Teflon tm particles. The Teflon tm particles are applied by the use of a liquid carrier vehicle. The mixture is suitably about 90 percent by volume of the liquid vehicle and 10 percent by volume of the Teflon tm particles. The liquid is a material that can suspend the Teflon tm particles if the mixture is agitated and carry them through the tube and ductwork of the lower punch. The vehicle must also be a material that is non-flammable and which will readily vaporize from the wall of the die. A suitable vehicle for use in the invention is a fully fluorinated derivative of an aliphatic hydrocarbon, preferably a hydrocarbon of 2 to 8 carbon atoms in the molecule. A perfluorinated hexane or octane is suitable. These molecules may be in the form of either molecular chains or cyclo compounds. It is preferable to use perfluorinated hexane. Such materials are able to suspend the lubricant powder and are non-reactive with the rare earth element-containing compact body. Thus, a mixture of about 90 percent by volume liquid fluorocarbon and 10 percent by volume Teflon tm powder is mixed and prepared in a separate container not shown in the drawings. The mixture is agitated and then delivered from the container through tube 44 and ducts 40, 36 and 46 to the die cavity wall 14 of die 12. The container or delivery system (not shown) is adapted to supply the fluid mixture under pressure as required. Referring now to Figures 1a and 1b, the lubricant mixture is pressurized at the time that the lower punch is at its uppermost point as depicted in Figure 1a. As the lower punch is lowered in the die cavity until it reaches its position in Figure 1b, pressure is applied to the fluid mixture and a coating film 50 of the fluid mixture is applied to the cavity wall 14 of the die as depicted in Figure 1b. The liquid vehicle of the lubricant mixture vaporizes very rapidly although there is a residual amount. Another important feature of the invention requiring the use of the perfluorinated compound is the fact that this material, if it remains on the surface of the cold compact body, does not adversely affect the permanent magnet properties of the body during any storage or hot-pressing thereof. Thus, with the lower punch 16 in its down position and the upper punch 18 in its upper position and the lubricant film applied to the wall of the die cavity (Figure 1b), the cavity 14 is now ready to receive the powdered, rapidly-solidified iron-neodymium-boron type material. This material is loaded into the lower die in loose particulate form. It is dropped into the die from a hopper (not shown), and it is measured by any suitable method into the die cavity 14, such as by volume. As seen in Figure 1c, the powdered material 52 is now in the die, filling the cavity above the lower punch. As soon as the particulate material 52 has been loaded into the die, the upper punch 18 is lowered to close the die cavity 14. The upper and lower punches are then loaded to consolidate the powder into the green compact 34. A compaction pressure of about 386.11 MPa (25 tons/in²) is employed in this example. Figure 1d illustrates the position of the upper and lower punches at the time that the particles have been consolidated into the green compact body 34 which is such an important aspect of the practice of the present invention. As soon as the compaction has been completed, the upper punch 18 is raised out of the way to its upper position as depicted in Figure 1a, the lower punch is raised to eject the compact body 34 from the die, the compact body 34 is removed, and the process is repeated. This cold-compaction process typically requires about one to six seconds per cycle and is carried out at ambient conditions. The cold compact body may have a trace of Teflon tm powder on its outer surfaces, it may also have a trace of the liquid vehicle, but the composition of the liquid vehicle is such that it does not adversely affect the permanent magnet properties of the iron-neodymium type material. This completes the first step of the process of the invention. It is important to note that the use of the lubricant on the die wall only and the selection of the liquid vehicle for application of the solid lubricant, preferably Teflon tm , is very important in the formation of a green compact body of a suitable density in a practical amount of time and of a material that will not degrade its properties upon aging. The green compact body of the iron-neodymium type particulate material serves as a pre-form for the hot-pressing step which is to follow. These pre-forms are coated with a die-release type lubricant preparatory to the hot-pressing operation. A suitable die-release lubricant for this practice is a suspension of boron nitride powder in an isopropyl alcohol carrier. This material is sprayed onto the compact bodies by any suitable method, and the compact bodies are dried to evaporate the isopropyl alcohol and to leave a coating of the finely-divided boron nitride particles on the outer surface of the pre-forms. Whilst the lubricant may be applied by any suitable application equipment such as conventional spray painting equipment, it has been found useful to place the pre-forms in trays with a plurality of cylindrical cavities sized to receive about half of the pre-form bodies. A tray of several of the pre-forms can thus be sprayed to coat them on one half, the tray is inverted into another like tray that receives the coated portions of the pre-forms, and the other half of each pre-form is sprayed with a lubricant material. The cold compact pre-forms have a density of approximately 70 percent of the density of a fully consolidated iron-neodymium-boron type composition that is useful as a permanent magnet material. Whilst much of the porosity of the loose powder has been removed by the cold compaction process, the pre-form is still porous and susceptible to oxidation, if not burning, when heated to an elevated temperature in air. However, one of the advantages of the use of the pre-form of the invention is that the material is dense enough to be fairly rapidly heated to a hot-pressing temperature. The practice of the hot-pressing step of method of the invention will illustrate how to accomplish the hot-pressing in a rapid compaction cycle whilst protecting the pre-form body from oxidation when it is at its hot-working temperature. An open-to-the-air hot press is used for the practice of the hot-pressing step of the method of the invention. Whilst presumably a single press could be used in sequence to do both the cold-compaction and the hot-pressing steps of the method of the invention, the use of separate presses is preferred because one press needs to be adapted for heating of the workpiece in the die and punches. However, both presses may be open-to-the-air presses. Referring now to Figures 2a to 2d, the operation of the hot-pressing practice in the method of the invention will be described. The totality of hot press 100 is not shown but just a die region 102 with a lower punch 104 and an upper punch 106 and an upper punch carrier 108. The upper punch 106 is heated via a guide member 110 by a suitable heating means such as a resistance heater 112. A die 114 is heated by a resistance heater 116 or other suitable heater and the lower punch 104 is heated through the die 114. Thus, the die and upper and lower punches are all capable of being heated so as to raise this region of the press to a suitable hot-pressing temperature. Figure 2a illustrates the position of the elements of the hot press 100 elements at the completion of a hot-pressing cycle. A fully-consolidated permanent magnet part 118 has just been ejected by action of the lower punch 104 from the die 114 and rests upon a die stack cover 120, having been pushed off a flat face 122 of lower punch 104 by a robot arm or a rake (not shown). Referring to the lower half of this operative portion of the hot press 100, the die stack cover 120 is carried on a manifold member 124 adapted for the delivery of argon or other suitable dry inert gas to the die cavity. Below the manifold member 124 is the heated die 114. Die 114 defines a circular, right-cylindrical die cavity 126 (Figure 2c) sized to receive the green compact 34. Since these press members are going to be operated at a hot-pressing temperature, suitably about 870°C, for heating and hot-pressing reactive rare earth element-containing compact bodies (e.g., compact 34), they must be formed of suitable temperature and reaction-resistive materials. The die 114 is preferably formed of nickel aluminide. The upper 106 and lower 104 punch members are suitably formed of Inconel 718 or other suitable high-temperature material. In Figure 2a, the lower punch 104 is seen at its uppermost position. The manifold member 124 comprises a duct 128 for the delivery of dry argon (illustrated as gas clouds 130 in Figures 2b and 2c) to the die cavity 126, and an annular ring 132 at a cavity portion 134 of the manifold 124 that is sized to receive the lower punch 104. The die stack cover 120 also has a round cylindrical opening 136 to receive alternately the upper punch 106 and the lower punch 104. It is slightly oversized with respect to the punches to permit and accommodate the flow of argon gas around the lower punch 104 and out the die stack member 120 for purging of the oxygen from the whole die cavity (see Figures 2b and 2c). Also seen in Figure 2a is the upper punch 106 in its uppermost retracted position. The punch 106 is carried by a suitable press support member 138 and the guide member 110. It is preferable that argon gas should be continually delivered into the manifold body 124. Thus, a small stream of argon is flowing around the lower punch 104, even in its upper position as shown in Figure 2a. The lower punch 104 is then dropped to a position just below the argon delivery duct 128. Argon continues to flow, purging the cavity of oxygen that may have been inducted by the down-stroke of the lower punch 104. A cold compact pre-form body 34 is dropped by a suitable automated arm (not shown) into the manifold cavity 134 as shown in Figure 2b. The flow of argon continues as indicated by the clouds 130 of the gas schematically depicted in Figures 2b and 2c. The lower punch 104 is then dropped further, the compact 34 riding on the lower punch 104 down into the cavity 126 of the nickel aluminide die body 114. Argon continues to flow into the manifold cavity 134 and die cavity 126 to purge them both of oxygen and moisture. The hot upper punch 106 is then lowered as illustrated in Figure 2d into a pressing position with the lower punch 104 and the compact 34. It is in this position, pressed between the hot punches and the heated dies, that most of the heat is transferred to the compact 34 to raise its temperature above about 700°C. The machine load on the punches is increased, and they exert a pressure of about 92.67 MPa (six tons/in²) on the compact body, which is consolidated in the hot die cavity into a fully-densified body 118 of a density of about 7.4 to 7.6 grams per cubic centimetre, depending upon the alloy composition. As soon as the compaction process has been completed as depicted in Figure 2d, the upper punch is raised, the lower punch follows, carrying the compacted body up to the top level of the die stack cover 120 as depicted in Figure 2a, and the fully-densified body 118 is pushed away from the die region 102. There are several features of this process which are believed to contribute substantially to the rapid hot-pressing operation. The whole procedure of purging the die with argon, loading the die cavity with a cold-formed precursor, heating the precursor by heat transfer from the heated die and punches, pressing the precursor into a fully-densified body, and the ejection of the hot-pressed body from the die all takes place in a period of about 25 to 90 seconds, depending upon part size (weight). The rapidity of this operation is facilitated both by the continual purging of the die from a manifold that surrounds the punch and continues to drive the oxygen from the die and the use of the cold compacted pre-form body which, although not fully densified and susceptible to oxidation, is dense enough to be rapidly heated in the hot die. Note that it is preferred not to heat the pre-form body before it is introduced into the die because this would require special protection of the pre-form body before it enters the die in order to prevent oxidation. The hot-pressed permanent magnet bodies produced as described above require little additional machining before use. It may be necessary to remove some flash, but very little grinding or other machining is required. The permanent magnet bodies upon magnetization display a maximum energy product of about 119318 AT/m (15 megaGaussOersted)* depending upon composition. They are fully densified, and they display some magnetic anisotropy although they are substantially isotropic in their magnetic properties. They are useful as is in many permanent magnet applications. The small cylindrical magnets 118 that were illustrated in Figures 2a and 2d during the hot-pressing operation are utilized, for example, in magneto-resistive speed sensors in anti-lock brake systems and the like. In many applications it is desired to further hot-work fully-densified magnetic bodies by hot-work-deforming them to produce a flow in the metallic material that aligns the 2-14-1 type grains and provides a substantially anisotropic magnetic body. Such bodies may display maximum energy products of the order of 238635 to 357952.5 AT/m (30 to 45 megaGaussOersted) depending upon composition and degree of hot-working. One suitable way for the additional hot-working of hot-pressed bodies such as those prepared by the process depicted in Figures 2a to 2d is a die-upsetting operation. In the hot die-upsetting operation, a fully-densified body is placed into a heated die that is larger than the body itself so that when the punch is pressed, the body flows laterally and is compressed in height to assume the shape of the cavity defined between the punches and by the die body. Utilizing suitably-shaped punches, suitably-shaped hot-pressed bodies and a die cavity of suitable configuration, considerable deformation of a fully-densified body can be obtained to achieve nearly complete alignment of the 2-14-1 grains in the body. The resulting product as stated above is a very strong permanent magnet. In accordance with the invention, the fully-densified bodies of the second step of the process may be subjected to any suitable form of hot-working such as die-upsetting, forging, hot-rolling and the like. In general, since deformation of the body itself is now going to occur, it may be useful to lubricate the body with a forging or hot-working type lubricant such as graphite powder. Since the starting point workpiece for the hot-upsetting or other hot-working operation is itself a fully-densified body, it is suitable to pre-heat the body in open air to some extent before it enters the hot-working equipment. Otherwise, the body can be added in an unheated condition to an open-air, hot die cavity such as that depicted in the hot-pressing operation. Thus, in conclusion and in summary, the present invention comprises at least two steps and optionally a third step. The first step of the method is a cold-compaction process in which rapidly-solidified particulate material is consolidated into a cold compact body in a solid lubricant-lined die cavity. Such solid lubricant may also be applied to cores if they are used. The solid lubricant is selected so as not to contaminate the cold compact body but to facilitate its compaction and removal from the die. In the second step of the process, the cold compact body is introduced into a dry inert gas-purged heated die cavity and is rapidly compressed at a suitable hot-working temperature into a fully-densified body. The resultant body is useful as a permanent magnet, and for many applications the two steps produce a wholly useful product. Where it is desired to achieve alignment of the 2-14-1 grains in the body, the fully-densified body may be further hot-worked to form an anisotropic permanent magnet. Whilst the present invention has been described in terms of a few specific embodiments thereof, it will be appreciated that other forms of the invention could readily be adapted by those skilled in the art. Accordingly, the scope of the invention is to be considered limited only by the scope of the following claims.

Claims: A method of consolidating a rare earth element-containing powder alloy of RE₂TM₁₄B precursor composition into a fully-densified permanent magnet body, characterised in that two pressing steps are utilised in at least one open-to-the-air press (10;100) of the type comprising a die member (12;114) defining a material-receiving die cavity (14;126) with a die wall defining a predetermined cross-sectional configuration and opposing pressing members (16,18;104,106), at least one of which is adapted to move reciprocally in the die cavity (14;126) to compress material placed therein; and in that said method comprises: applying a solid lubricant film (50) to the cavity-defining wall of a die (12) which is at substantially ambient temperature; charging a predetermined quantity of a lubricant-and binder-free rare earth element-containing metal alloy powder (52) into the lubricated die cavity (14); consolidating the powder (52) in the die cavity (14) by action of the pressing members (16,18) at ambient temperature to form a green compact body (34) of generally self-sustaining strength; flooding a die cavity (126) of a heated die press (100) with a dry inert gas (130) to displace air from the cavity (126), the die (114) being maintained at an elevated temperature for hot-pressing the compact body (34) and the cavity (126) being configured to receive a said green compact body (34); placing a said green compact body (34) in the heated, inert gas-containing die cavity (126) whilst continuing the flooding of the cavity (126); pressing the green compact body (34) to a substantially fully-densified body (118) by action of the pressing members (104,106) as the body is being heated to a hot-working temperature, and removing the hot, fully-dense body (118) from the cavity (126) into ambient air. A method according to claim 1, in which the green compact body (34) of generally self-sustaining strength formed in the pressing step at ambient temperature has a density of about five grams per cubic centimetre or higher. A method according to claim 1 or 2, in which the solid lubricant is applied to the die wall of the ambient temperature press (10) by spraying solid lubricant particles dispersed in a volatile non-flammable vehicle through a duct (36,40) in said one movable pressing member (16). A method according to claim 1 or 2, in which the opposing pressing members are upper and lower opposing punches (16,18;104,106) adapted to move reciprocally in the die cavity (14;126) to compress material placed therein; and the predetermined quantity of the lubricant-and binder-free rare earth element-containing metal alloy powder (52) is charged into the lubricated die cavity (14) onto the lower punch (16). A method according to claim 4, in which the hot-pressing operation is conducted by raising the lower punch (104) to an eject position for a hot-pressed part (118), and then introducing said dry inert gas (130) into the die cavity (126) whilst dropping the lower punch (104) to a workpiece-receiving position and continuing to flood the expanding cavity (126) with said dry inert gas (130). A method according to claim 4, in which the fully-densified body (118) is subsequently introduced into a heated die cavity that is larger in cross-sectional area than the hot-pressed body (118), and the body is heated to a die-upsetting temperature and deformed by die-upsetting to form a fully densified anisotropic magnet body.

Inventors: ESHELMAN LARRY JOE, KIRK DONALD SCOTT, WORDEN JOSEPH JAMES, YOUNG KEVIN ALLEN, ESHELMAN, LARRY JOE, KIRK, DONALD SCOTT, WORDEN, JOSEPH JAMES, YOUNG, KEVIN ALLEN
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: H01F1
IPC codes: B22F3, C22C33, B30B11, H01F1, H01F41, B30B15