PROCESSES FOR EXTRUDING POWDERED METALS INCLUDING TANTALUM AND NIOBIUM

Abstract: The process for extruding tantalum or niobium includes cold isostatic pressing (12) a charge of powdered metal (10) to form a green compact. The compact is then placed in a metal capsule, which is sealed (14), heated (16), and thereafter cold isostatically pressed (17) to form a pressed capsule. The pressed capsule is then heated (18) and extruded (19) to form an extruded product (20). In another embodiment, powdered metals are cold isostatically pressed (12) to form a billet, which is thereafter placed in a sealed container (14). The sealed container is then placed in a second metal container (25), and a powdered metal (26) is placed within the gap between the containers. The second metal container is thereafter cold isostatically pressed (17), heated (18), and extruded (19) to form the final product.

Description: PROCESSES FOR EXTRUDING POWDERED METALS INCLUDING TANTALUM AND NIOBIUM This invention relates to processes for extruding powdered metals including tantalum, niobium, molybdenum, tungsten carbide cobalt and tungsten heavy alloy. BACKGROUND As is known, refractory metals such as tantalum and niobium are important metals due to their high melting temperatures, good corrosion properties and special electrical properties. In the pure state, tantalum and niobium are very ductile but are sensitive to even small levels of interstitial impurities such as oxygen, nitrogen and carbon. Due to this factor and the extremely high melting temperatures of these metals, for example, tantalum melts at 3000°C, the production of tantalum wrought products has been very costly due to the need to have many production steps, intermediate annealing steps and yield losses. As is also known, one technique for producing mill products such as rod, sheet and tubing of iron, nickel and cobalt-based alloys is to extrude the metal from powder. For example, as described in U.S. Patent 4,050, 143 one known process known as the Anval process uses a metal powder wherein the powder has spherical grains. In addition, the spherical powder is introduced into a thin carbon steel capsule and cold isostatically pressed to a density of over 80 percent and subsequently extruded into tubes, bars and other shapes. The pressing of the powder to a density over 80% is required in order to avoid wrinkling of the capsule during the extrusion step. Other processes use hot pressed blanks, for example using hot isostatic pressing followed by extrusion. U.S. Patent 4,599,214 describes a method of extruding dispersion strengthened metallic materials in which a billet of dispersion strengthened metallic powdered material comprised of one or more metals and one or more refractory compounds is extruded through a die having an internal contour such that the material is subjected to a natural strain rate which is substantially constant as the material passes through the die. As described, the dispersion strengthened materials are those wherein a hard phase is present with one or more metals. The preferred materials are described as being alloys containing two or more metals. European Patent 0 305 766 describes a method of making a reinforced refractory metal composite which employs niobium powders. In one embodiment, a paniculate dispersoid and a matrix material are mixed together. Thereafter, the mixture is mechanically alloyed and cold pressed to reduce the volume of the composition and to form a low density green compact of 50% to 70% of theoretical density. Next, the green compact is enclosed in a can which is evacuated and then heated to temperatures in excess of 1000°C and pressures in excess of 680 atmospheres to be hot isostatically pressed to a density greater than 90%. After consolidation, the densifϊed compact is extruded through a die to form a required shape. In another embodiment, short fibers are added to the matrix to strengthen the composite. U.S. Patent 4,646,197 describes a method for making a tantalum capacitor lead wire. As described, the product is formed by wrapping a tantalum foil around a metal billet which may be made of niobium to provide at least one layer of tantalum around the billet. The compacted body is then inserted into an extrusion billet and the resulting composite extruded and further reduced by rolling and/or drawing to a wire of the requisite size. As is known, refractory metals such as tantalum and niobium can be produced efficiently by mechanical or chemical means. However, these production means provide an irregularly shaped powder. Generally, the irregularly shaped powder can be characterized as being of very low filling density, normally between 40% to 50%, with poor flowability. Accordingly, when using such irregularly shaped powder in canned billets which are subsequently extruded, the low filling density of the powder causes the billet to collapse causing wrinkling and distortion in the extruded product. Further, even if such a billet were to be cold isostatically pressed, the low initial density does not permit the final density to exceed approximately 60%. This, in turn, also causes problems in extrusion especially when extruding tubes or shapes. When hot isostatic pressing is performed, the flow density causes wrinkling and often makes machining necessary before extrusion. In the past, it has been difficult to obtain products made of tantalum and niobium with a high density at a low cost. Hot extrusion of metal powder is a feasible technique to consolidate metal powders to fully dense products. Extrusion offers distinct advantages for long products like bars, rods, tubes and profiles not only because of high yields but also because "difficult to make" metals can be produced directly to a certain shape. The specific deformation pattern characterized by hot extrusion also tends to break up surface films, such as oxides and other impurities on the powders, thereby giving a much better quality to the finished product. Refractory metals, such as tantalum and niobium and their alloys can be extruded. However, one of the characteristics of these metals is that their raw material cost is extremely high. This makes it very important to optimize the yield in each operation. In the case of extrusion, it is important to minimize the losses caused by imperfections and especially also to minimize losses in the front and back ends of the extrusion body. As described below, a normal appearance of an extruded billet from a solid material is shown in Fig. 8 and the same for a powder billet is shown in Fig. 10. The total product yield after extrusion is very seldom over 85% for a powder capsule and seldom over 90% for a solid material. The reasons for the lower yield for the powder billet is that a front plate is drawn over the powder at extrusion and a back end plate is sucked into the powder at the other end. Tantalum powder can be made from tantalum solid metal which is hydrided, crushed, and dehydrided to powder form or produced from potassium tantalum fluoride by sodium reduction or by other means. This irregular powder is then pressed to a small bar which is sintered in vacuum at a high temperature to form a rod which is then processed in several steps down to finer dimensions, for example, through cold rolling. Alternatively, down to finer dimensions, for example, through cold rolling. Alternatively, tantalum is electron beam melted or arc cast melted to produce ingots which are processed into rod, sheet or tubing, for example, by forging, swaging, rolling, etc. In powder form, tantalum picks up oxygen at the hydriding, dehydriding and melting stages. Subsequent sintering in vacuum is also a refining step where not only oxygen but also other impurities are removed as a result of high temperature and a high vacuum. Tantalum forms extremely stable oxides. High purity, low oxygen tantalum powder which is exposed to air, even at room temperature, can be a safety risk; the finer the powder is, the higher the risk. The conventional way of processing tantalum is a very costly process. Not only the many process steps but also production of the rod, sheet or tubing provide low product yield which affect the final production costs dramatically. Accordingly, it is an object of the invention to be able to produce products made of tantalum and/or niobium at a relatively low cost and with a high density. It is another object of the invention to provide a relatively simple technique for extruding tantalum, niobium, molybdenum, tungsten carbide cobalt and tungsten heavy alloy powders into extruded products of high density. It is another object of the invention to provide an improved method of forming high density tantalum and niobium products. Briefly, the invention provides a process of forming extruded products of tantalum and/or niobium. In accordance with the process, a charge of powdered metal selected from the group consisting of tantalum and niobium and having a density in the range of 40% is placed in a container and cold isostatically pressed at a predetermined pressure to a density sufficient to form a green compact with a sufficient strength to be handled outside the container. In accordance with the process, the charge of powdered metal is pressed at a pressure of 400 Mpa. However, a range of from 200 to 500 Mpa may be used. Of note, the term "density" used herein is the theoretical density of the metal. Thereafter, the green compact is placed in a capsule, for example, of carbon steel. The capsule is then sealed and then preferably evacuated to remove air or any gases within the compact. Next, the capsule is heated to a predetermined temperature for a time sufficient to anneal the metal of the compact thereby reducing the hardness of the metal. Further, the capsule is heated at a temperature of 1250 a C for 30 minutes. However, the capsule may be heated to a range of from 1150°C to 1400°C for a time of from 10 to 30 minutes. Thereafter, the capsule and encapsulated compact are cold isostatically pressed at a pressure which, at the minimum is 200 Mpa, to a density of from 70% to 85%. The pressed capsule and capsulated compact is then heated and extruded to form an extruded product, for example in the form of a rod. The isostatic pressing of the capsule and encapsulated compact is carried out at a pressure of 400 Mpa. However, the pressure used may be in the range of from 200 to 500 Mpa. Likewise, the heating of the capsule and encapsulated compact is carried out at a temperature of 1200°C. However, the temperature may be in the range of from 1150°C to 1400°C. The extruded product may be of any suitable shape such as a bar shape, rod shape, tube shape or the like. After extrusion, the layer carbon steel existing on the outside surface of the extruded product is removed, for example, by pickling. The event a capsule of another material is used, other techniques may be employed to remove the layer of the material from the extruded product. The extruded product may also be made in the form of a hollow tube. In this respect, a green compact of the powdered material can be formed around a solid mandrel which is subsequently removed with the green compact then being placed within a hollow billet which is subsequently sealed, evacuated, heated and cold, isostatically pressed to a density of at least 80%. Thereafter, the pressed billet can be heated and extruded to form an extruded hollow tube. The extruded product may also be made to have a layer of different material on the outside, or on the inside surface where the extruded product is hollow. In this embodiment, when the green compact is placed in a metal capsule, a gap existing between the outside of the green compact and the inside of the capsule can be filled with a spherical powder of suitable material, for example, Inconel 625 which is a corrosion resistant nickel-based alloy which can be gas atomized to a spherical shape. For example, the filling density of the nickel-alloy may approximate 65% which is near the density of the compact. The capsule with the two materials therein can then be sealed, evacuated, and heated, as above, at 1250°C for 30 minutes. After the heating (annealing) treatment, the capsule is cold isostatically pressed as above. However, even in this case, it is extremely difficult to increase the yield from the powder in the billet to the extruded product to more than 85%. Even if this is a normal yield and far higher than the yield obtained in a conventional process to produce tantalum, it is of important economical value to reach as high a yield as possible. Furthermore, during the final cold isostatic pressing of the metal capsule, there is always a risk of a leakage when a water emulsion subjects the powder billet to a pressure of up to 500 MPa (75,000 psi). In such a case, the water represents a risk during the following heating of the billet. That is to say, the water which has penetrated the billet can transform into steam during heating and create an explosive situation. There are techniques to detect such water but if the water-extrusion penetrates the tantalum powder, the powder is in principle destroyed and must be scrapped. A normal yield loss in cold isostatic pressing is between 1-5%, thus, further decreasing the efficiency of the process. One way to ensure that no water exists during the final heating before extrusion is to vacuum pump the billet, preferably under combined vacuum and heat In accordance with an improved process, a charge of powdered metal selected from the group consisting of tantalum and niobium is cold isostatically pressed to a density sufficient to form a green compact or billet with a sufficient strength to be handled. Preferably, the compressed billet of tantalum is made by the known so-called wet bag process. The cold isostatic pressure is chosen so that the obtained density is between 70-85% of the theoretical density. The compressed billet is then released from the wet bag and placed in a container or capsule which is then sealed with end caps by welding. The material of this container could consist of carbon steel or of tantalum or niobium, i.e. the same material as the compressed powder billet. The carbon steel used, could typically be of low carbon content to avoid segregation during the following heating. For example, such carbon content could be less than 0.005% and typically in the range of 0.002- 0.003%. With such low carbon content, carbon pick up is avoided, for example, into the tantalum, which is very sensitive even for small amounts of impurities like carbon. In the first process described above, if a wet bag operation is made and the compressed powder billet is placed in a metal container, a second heating operation is necessary before the second cold isostatic pressing reaches a sufficiently good extrusion result. The commonly accepted reason is that in order to extrude thin-walled capsules, the density of the powder must be approximately a minimum of 80% of the theoretical density to avoid wrinkling of the capsule thereby causing imperfections. In accordance with the improved process, the wet bag compressed billet is placed in a metal container with narrow tolerances between the billet and the container. No annealing is made but other operations can be done, for example, in the case of tantalum, evacuation of the container or dehydriding of the tantalum powder can be performed while subjecting the tantalum in the container to a vacuum at moderate temperature (600° to 1000°C). The metal container is then placed in a second metal container with an annular gap between the two containers. This gap is filled with a carbon steel powder or another type of metal produced with a spherical shape which gives a high filling density after vibration, i.e. approximately 70%, and with a yield strength (flow stress) substantially lower than the enclosed tantalum or niobium at the extrusion temperature. It is important before filling to place the inner metal container concentrically in the outer container in order to ensure a satisfactory extrusion result. Accordingly, spacers may be provided to maintain an annular gap between the containers. This double container is then sealed, for example, using end caps, and cold isostatically pressed in order to avoid segregation of the metal powder in the cap between the containers during the subsequent handling before extrusion. The isostatic pressing may be performed at a pressure of at least 200 mpa for this purpose. As the tantalum or niobium powder is already cold isostatically pressed once, the density of this material will not be affected. The hardness of the carbon steel is (in the atomized condition) so high that the density of this surrounding material, also after cold isostatic pressing, is increased very little to just slightly over 70%. Thereafter, the compressed double container is heated and extruded to form an extruded product, for example, of bar shape. Typically, the front and rear ends of the bar are primarily made of the material of the end caps which serve to seal the containers and the compacted powder in the outer container. Hence, the front and rear ends of the extruded product can be cut off and removed leaving a bar which is primarily made of tantalum or niobium, as the case may be. Typically, the extruded rod at this point has a yield of from 93% to 96% of the beginning powder. The above described double container process may also be used for the encapsulation and extrusion of other metals such as molybdenum, tungsten carbide cobalt and tungsten heavy alloys. In the past, it has been known that sintering is a relatively long and expensive process which permits the dilution of dissimilar materials which can cause failure of a bimetallic component. Encapsulation and extrusion eliminates the need to sinter the powder, which in the case of molybdenum, tungsten carbide cobalt and tungsten heavy alloys is performed at 1300°C to 1500°C. In this respect, extrusion exerts forces which can enhance the modulus of elasticity of these materials. The molybdenum particles range in size from 1 to 8 microns and the same process parameters as the tantalum parameters are used. The tungsten carbide particles which are used have an average particle size ranging from submicron to twelve (12) microns with a cobalt binder of from 6 to 30% by weight while the cobalt particles range from submicron to 7 microns average particle size. Tungsten heavy alloy is defined as 89 to 98% tungsten powder ranging in size from 1 to 8 microns with a binder phase of nickel, iron, copper, cobalt or a combination thereof. These and other objects and advantages of the invention will become more apparently from the following detailed description taken in conjunction with the accompany drawings wherein: Fig. 1 schematically illustrates the steps in a process in accordance with the invention; Fig. 2 illustrates a cross-sectional view of an extruded product in accordance with the process of Fig. 1; Fig. 3 schematically illustrates the steps of a process for producing an extruded tube in accordance with the invention; Fig. 4 illustrates a cross-sectional view of an extruded tube produced by the process in Fig. 3; Fig. 5 schematically illustrates the steps of a further modified process in accordance with the invention for producing a multi-layer extruded product; Fig. 6 illustrates a cross-sectional view of a multi-layer extruded product made in accordance with the process of Fig. 5; Fig. 7 illustrates a side view of a solid billet, for example, of wrought metal prior to extrusion in a conventional extrusion process; Fig. 8 illustrates a view of an extruded rod made from the solid billet of Fig. 7 in accordance with known techniques; Fig. 9 illustrates a cross-sectional view of a powder billet in accordance with the prior art prior to extrusion; Fig. 10 illustrates a rod extruded from the powder billet of Fig. 9 in accordance with known techniques; Fig. 11 illustrates a cross-sectional view of a double container formed in accordance with the invention; and Fig. 12 illustrates a view of an extruded product in accordance with the invention. Referring to Figs. 1 and 2, the process of forming an extruded product of a powdered metal material selected from the group consisting of tantalum and niobium is initiated with the placement of a charge 10 of the powdered metal having a density in the range of 40% in a container 11. The charge in the container is then placed in a cold isostatic press and subjected to a step 12 of cold isostatic pressing at a predetermined pressure to a density sufficient to form a green compact with sufficient strength to be handled outside the container. Cold isostatic presses are well-known constructions typically used to compact metal powders, ceramics and graphite, usually to produce a green body for further sintering and the like. One known supplier of such presses is ABB Metallurgy and, in the United States, ABB Autoclave. The capacity of such presses are determined by the volume of a pressure chamber which is filled with water. The water is thereafter pressurized and acts directly as a pressure medium on the capsule. Several capsules can be pressed at the same time. Further, the sizes of the presses can range in terms of diameter and length of from 2 inches by 4 inches up to 60 inches by 120 inches. The irregularly shaped powder of tantalum or niobium can be cold pressed and has enough green strength so as to be handled after pressing. One method of cold pressing such powders is to use cold isostatic pressing in a rubber bag (the so-called wet bag process). The compacted powder can thereafter be released from the rubber bag and handled for subsequent steps. Generally, the density of the compact which is achieved at pressures of from 300 to 800 Mpa is approximately 60%. Higher densities cannot usually be obtained as the work hardening of the powder prevents the powder form achieving a density of a higher degree. Heating such compacts at temperatures over 1200°C reduces the hardness and thereby allows the possibility of further cold pressing. In one test, irregular tantalum powder was filled in a container 11 in the form of a rubber bag with a length of 550 millimeters and a diameter of 165 millimeters. The filling density was approximately 46%. The bag was then cold isostatically pressed in the press 12 at 500 Mpa to a density of 62%. After releasing the pressure, the green compact was removed from the rubber bag and was measured to have a length of 500 millimeters and a diameter of 150 millimeters. The green compact was then encapsulated 13 by being placed in a low carbon steel capsule with a gap of 1 millimeter between the compact and the capsule in order to be able to introduce the compact into the capsule. The capsule was then sealed by welding end closures at the ends and evacuated in suitable steps of sealing 14 and evacuation 15. Thereafter, the capsule was subjected to a heating step 16 in a suitable heater or oven to a temperature of 1250°C in a salt bath to avoid heavy oxidation of the capsule for a time of 30 minutes. Subsequently, the capsule with the compact was subjected to a cold isostatic pressing step 17 in the same cold press and pressed 400 Mpa to a density of 80%. The resulting billet had a length of 460 millimeters and a diameter of 138 millimeters. The billet was thereafter subjected to a heating step 18 again to 1250°C and subsequently subjected to a step of extrusion 19 in a suitable extruder and extruded to a bar 20 of 40 millimeters diameter. This extruded bar 20 was then pickled to remove the thin capsule layer 21 from the core 22. The resulting extruded bar 22 showed a smooth and regular surface with no marks or indentations. Both steps of heating 16, 18 before and after the second cold isostatic pressing step 17 can be performed in several ways. One way is to preheat in a gas furnace for a time of one hour to 700°C (to avoid oxidation). Other heating techniques such as direct heating in a protective gas atmosphere or vacuum may also be used. In another test, the same type of tantalum powder was filled directly in the same carbon steel capsule as above with the same filling density of 46%. The capsule with the powder was then cold isostatically pressed to a density of 60% heated and extruded to the same diameter bar. After removal of the extruded layer of carbon steel, the extruded bar showed an imperfect surface due to wrinkling and indentations caused by the low initial density. In this respect, when using such powders in can billets which are subsequently extruded, the low filling density causes the can to collapse thereby causing wrinkling and distortion. Eve if such as billet were cold isostatically pressed, the low initial density does not permit the final density after cold isostatic pressing to exceed approximately 60%. This, in turn, also causes problems in extrusion especially when extruding tubes or shapes. When hot isostatic pressing is carried out, the low density causes wrinkling and often makes machining necessary before extrusion. Referring to Figs. 3 and 4, wherein like reference characters indicated like parts, as above, in a third test a rubber bag with a diameter of 100 millimeters and a length of 550 millimeters was charged with powdered tantalum as above, with the powdered metal being placed about a tapered mandrel 23 with a diameter of 60 millimeters in the bag. After cold isostatic pressing 12, as above, the tapered mandrel 23 was removed and the hollow compact encapsulated 13 in a hollow billet having an outer wall with an external diameter of 150 millimeters and an inner wall with an outer diameter of 58 millimeters. The gap between the outside diameter of the compact and the inside diameter of the billet was then filled with a spherical powder of quality Inconel 625 with the filling density of this nickel-alloy being approximately 65%, that is, at a density near the density of the compact. The resultant compound capsule was then sealed, evacuated and annealed, as above, at 1250°C for 30 minutes. After the annealing treatment 16, the capsule was cold isostatically pressed 17 at 400 Mpa and subsequently heated and extruded 19 to a compound tube 24 with an outside diameter of 65 millimeters and an inside diameter of 50 millimeters. The bond between the two layers of metal was excellent and the thickness of the two layers was homogeneous along the length of the tube. In this respect, as indicated in Fig. 4, the extruded tube 24 formed an internal layer 25 of pure tantalum and an outer layer 26 of nickel alloy. In another trial, the two metals, as above, were directly filled in a capsule, cold isostatically pressed, heated and extruded to the same two dimensions. Due to the low filling density of the tantalum powder, and the difference relative to the nickel alloy, the extruded tube showed very large variations in wall thickness and many imperfections. Referring to Figs. 5 and 6, wherein like reference characters indicate like parts as above, an extruded product 27 of multi-layered construction may be formed in a manner similar to the product of Figs. 3 and 4. That is, the extruded product 27 may have a solid core 28 of a tantalum and/or niobium surrounded by an outer layer 29 of another material. In the embodiment where a hollow extruded product is to be made, the hollow green compact as formed above, is placed in a hollow billet or capsule without the introduction of the added powdered material of another metal. In this case, after extrusion and removal of the billet materia, as by pickling, the resultant hollow extruded product is made solely of tantalum and/or niobium. The evacuation of the green compact with the capsule can be accomplished by using a vacuum or by backfilling with a suitable gas after evacuation. The extrusion of the billet may be carried out so that the extrusion ratio exceeds five (5) times. Generally, where a compounded (multilayered) extruded product is to be produced, a spherical powder is used for the second alloy which is to be filled into the capsule after the cold pressed compact has been placed in the capsule. Further, the spherical powder may be made of any suitable alloy such as a gas atomized powder of iron - nickel - or cobalt base alloys. The tantalum or niobium powder which is used in the process is usually less than -12 mesh size with the preferred range being from minus 12 to +100 mesh size. Therefore, the metal powder is made of particles which are of coarse size rather than being of finer or smaller size so that there is less chance of picking up nitrogen, oxygen or the like. This, in turn, reduces the risk of explosion which exists should very fine particles be used. Generally, the gas content of the original powder should be under 300 PPM of oxygen with a preferred range of less than 200 PPM. Referring to Figs. 7 and 8, it has been known that a solid billet 10 of wrought metal, such as tantalum, may be extruded in to an extruded bar 11 using conventional extrusion techniques. As indicated, the front end of the extruded bar 11 has a rounded shape due to the extrusion process while the rear end has an enlarged shape. Typically, the ends are cut off and removed so that the yield in the conventional extrusion process is between 85% and 92% of the original billet 10. Referring to Figs. 9 and 10, the conventional powder billet 12 is formed of a cylindrical container 13, for example, of carbon steel which is filled with powder 14, such as a tantalum powder and which is sealed at the respective ends by two plates 15,16, each of which is welded as by welds 17 to the end of the container 13. The resulting billet 12 is then extruded using known techniques so as to form an extruded rod 18 as indicated in Fig. 10. Referring to Fig. 10, the extruded rod 18 typically has a front end 19 which is formed primarily by the metal of the front plate 15 of the billet 12 while the rear end 20 is formed primarily of the metal of the back plate 16. As in Fig. 8, the rear end of the extruded rod 18 is typically enlarged as this is a part which remains after pressing in an extrusion press. As in the previous technique, the front end and rear end of the extruded rod 18 are cut off and discarded so that only the central portion A which is formed of consolidated powder is used as the extruded product. In this technique the yield is usually between 83% and 88% of the original powder charge. Referring to Figs. 11 and 12, in accordance with the invention, an extrusion process is performed which is able to obtain a high yield, for example, in a range of from 93% to 96% of the original powder charge. By way of example, a charge of tantalum powder (e.g. tantalum hydride) is cold isostatically pressed using the wet bag process to form a billet 21 of 102 mm diameter and a length of 500 mm. The billet 21 was then placed in a container 22 of cylinder shape and of low carbon steel with tight tolerances. The billet 21 had a density of 72% of the theoretical density. The container 22 was then sealed by securing end caps 23, 24 of metal at each end and placed in another container 25 with a diameter of 126 mm and with a wall thickness of 2 mm. The two containers 22, 25 were spaced apart, e.g. by spacers (not shown), to maintain an annular gap between the two containers of approximately 10 mm. The gap was then filled with a carbon steel powder 26 with a carbon content of 0.85% by weight. The carbon steel powder 26 was atomized with high hardness and was spherical in shape. The filling density of this surrounding powder was 68%. The outer container 25 was then sealed by securing metal front and back plates 27, 28 to the cylinder 25, as by welding. Next, the sealed container 25 was subjected to a cold isostatic pressure of 435 Mpa (approximately 65,000 psi). In spite of this high pressure, the density of the carbon steel only increased to 75%. After this cold isostatic pressing, a hole was drilled in one of the end plates 27, 28 which gave access to the surrounding carbon steel powder 26. This powder was then checked for leakage, eliminating the need for testing and exposing the tantalum 21 to air. No water was detected and the hole was closed again. The container 25 was then heated at a temperature of 1,200°C and extruded as a force of 1,350 tons to a bar 29 of 42 mm diameter. Referring to Fig. 12, the extruded bar 29, as is conventional, has a rounded shape at the front end 30 and an enlarged shape at the rear end 31. In addition, the front end 30 is formed primarily of the metal of the front plate 27 secured to the container 25 mixed with the compacted carbon steel powder while the rear end 31 is formed primarily of the metal of the back plate 28 mixed with the compacted carbon steel powder. In addition, the front end 30 has a section which is primarily formed of the consolidated carbon steel powder. This is followed by the consolidated tantalum powder 26. Likewise, the rear end 31 has a section of consolidated carbon steel powder 26 between the enlarged rear end 31 and the rear end of the consolidated tantalum powder. In accordance with the invention, the front end and rear end of the extruded bar 29 can be cut off at points so as to produce an elongated rod of tantalum which has a yield of from 93% to 96% of the original powdered tantalum product. The extruded rod of the tantalum also has a "skin" which is formed of the outer container 25, the consolidated carbon steel powder 26 and the inner container 22. This "skin" can be removed using any suitable conventional technique. After removal of the carbon steel consolidated powder 26 surrounding the extruded tantalum, which can be done by machining, pickling or mechanical means, it was found that the extruded tantalum had an excellent surface in spite of the low density of the container 25 before extrusion. Apparently, the carbon steel 26 functioned as a lubricant during extrusion and prevented the carbon steel container 22 surrounding the extruded tantalum from wrinkling. Note was also made that the outer carbon steel container 25 surrounding the carbon steel powder 26 had wrinkled here and there during extrusion but this surface problem had not been transferred to the inner container 22. In another case, water was found in the carbon steel powder 26 between the containers 22, 25 after the cold isostatic pressing. In this case, the container 25 and the powder 26 were removed and replaced with a new outer container and with new carbon steel powder and cold isostatically pressured again. Through this operation, the extremely expensive tantalum powder could be saved and only the inexpensive carbon steel powder needed to be replaced. In this case, it is possible but extremely unlikely that water had also penetrated the tantalum billet 21. Hence, there was no need for this billet to be checked for water. Another surprising result of this double container was that the extrusion force was substantially lower than the extrusion force of 1,800 tons when extruding a full container of tantalum. It is possible to calculate the theoretical extrusion force for a combined billet of two materials by considering their respective hot strength relative to their cross section at extrusion. It was here found that the flow stress of the double container with the two powders had a flow stress at extrusion temperature approximately 20% lower than what could be expected. This helps to extrude thinner sections or bigger billets as the extrusion force is a limiting factor for tantalum. Another factor of importance is the fact that the double container can be optimized in shape to give a high product yield of tantalum. A yield of approximately 95% of product (see Fig. 12) and a yield of 98% of material can be obtained. This is of course very important when dealing with extremely expensive materials like tantalum. As a summary, the result of using the double container obtains several advantages when used on materials like tantalum or niobium. First, there is no need to make an intermediate anneal after the wet bag operation. Also, the material can be extruded to a good product without surface defects. Second, the container can be safely inspected for water after the second cold isostatic pressing while saving the expensive tantalum if leakage occurs. It is extremely unlikely that both containers leak at the same time. Third, unexpectedly, the flow stress is lower than expected for the double container giving the possibility to extrude thinner dimensions safely. Fourth, a higher product yield can be obtained. The double container process may be employed with tungsten carbide cobalt and tungsten heavy alloys. In this respect, a powdered charge of tungsten carbide cobalt or of tungsten heavy alloy may be processed as above to produce a rod of tungsten carbide cobalt or a rod of tungsten heavy alloy. In one example, the extrusion temperature was 1200°C and the extrusion force was 1400 metric tons. The outer diameter of the capsule was 150 millimeters and the outer diameter of the core capsule was 50 millimeters. As above, the space between the two capsules was filled with a carbon steel powder. The extrusion resulted in a rod of tungsten carbide cobalt of 18 millimeters. The double container process may be used to produce a roll of solid tungsten carbide cobalt. Such a roll may be used for the rolling of metals to convert the metals from bar or sheet-bar to rod, sheet or foil. In accordance with the invention, a roll may be produced having a tungsten carbide cobalt or tungsten heavy alloy surface with a core of a different material. That is to say, the double container extrusion technique allows combinations of materials to be made, particularly where a harder exterior or outer diameter encases a more ductile, lower cost core. The double container process may be used to make a roll as small as one having a diameter of 2 1/2" and a length of 12" or a roll as large as 8" in diameter and 4' in length with the outer 20% (or a range of from 5 to 50%) made of tungsten carbide cobalt with the inner core made of steel. Such combinations have been produced previously by brazing a tungsten carbide sleeve over a steel core but such composite structures usually fail due to cracking caused by impact stresses or by failure of the brazed bond. A coextruded composite structure would have a metallurgical bond which would mitigate or eliminate such values and would be less costly to produce. It is generally recommended that the steel core be made of a nickel steel, such as Invar which has a low coefficient of expansion. This characteristic makes the core more compatible with tungsten carbide cobalt which also has a very low coefficient of expansion and minimizes stress cracking from differential coefficients of expansion. The tungsten carbide cobalt powder which is used is typically one composed essentially of 6% carbon powder, 6% to 25% cobalt powder and the remainder tungsten powder. The particular size of the cobalt powder is 2 1/2 microns and the size of the tungsten powder is 3 microns. Another example of a composite structure would be a micrograin (submicron or less than one micron) size tungsten carbide cobalt powder coextruded with a relatively coarse (3.0 micron average particle size or greater) tungsten carbide cobalt core. The submicron powder is harder and more wear resistant but more costly than the coarser tungsten carbide cobalt powder core which, conversely, is more ductile, shock resistant and is less costly than the micrograin powder. The hardness of sintered micrograin tungsten carbide cobalt is 90.5 to 94 Rockwell A, compared with standard tungsten carbide cobalt which has a Rockwell A hardness of 86.5 to 90.5. A third example of a composite structure made in accordance with the invention would be an armor piercing penetrator with a core of tungsten carbide cobalt and an outer shell of tungsten heavy alloy. One of the principle advantages of tungsten carbide is its hardness. A disadvantage of tungsten carbide is its relative brittleness which causes the tungsten carbide to shatter when impacting an initial layer of armor. A tungsten heavy alloy, while softer than tungsten carbide cobalt, has a higher modulus of elasticity and would act as an effective sheath for the tungsten carbide cobalt. The double container process may also be used to process other powdered metals such as molybdenum. Typically, the particle size of the molybdenum powder would be in the range of from 2 to 5 microns. The invention thus provides a process of achieving extruded products of pure metals and alloys of tantalum, niobium, molybdenum and tungsten such as tungsten carbide cobalt and/or tungsten heavy alloy in a relatively simple inexpensive manner. Further, the invention provides a process which is able to use coarse powdered metal particles thereby reducing the risk of explosions and of incorporating unwanted gases in the intermediate products and final product made from the powdered metal.

Claims: WHAT IS CLAIMED IS: 1. A process comprising the steps of placing a charge of powdered metal having a density in the range of 40 percent in a container, said metal being selected from the group consisting of tantalum and niobium; Cold isostatically pressing the charge at a predetermined pressure to a density sufficient to form a green compact with sufficient strength to be handled outside the container; placing the green compact in a metal capsule; thereafter sealing the capsule; heating the capsule to a predetermined temperature and for a time sufficient to effect annealing of the green compact; thereafter cold isostatically pressing the capsule and encapsulated compact at a predetermined pressure of at least 200 Mpa. thereafter heating the pressed capsule and encapsulated compact; and extruding the heated capsule and encapsulated compact to form an extruded product. 2. A process as set forth in claim 1 wherein the charge of powdered metal is pressed at a pressure of 400 Mpa. 3. A process as set forth in claim 1 wherein the capsule is heated to a temperature of 1250°C for 30 minutes to effect said annealing. 4. A process as set forth in claim 1 wherein the capsule and encapsulated compact are pressed at a pressure of 400 Mpa. 5. A process as set forth in claim 1 wherein encapsulated compact is pressed at a pressure sufficient to achieve a density of from 70% to 85%. 6. A process as set forth in claim 4 wherein the pressed capsule and encapsulated compact are heated to a temperature of 1200°C. 7. A process as set forth in claim 1 wherein the extruded product is of bar shape. 8. A process as set forth in claim 1 wherein the extruded product has an outer layer of metal different from the remainder of the product and which further comprises the step of removing said outer layer. 9. A process as set forth in claim 8 wherein the capsule is made of carbon steel and said step of removing said layer of carbon steel from the extruded product includes pickling of the extruded product to remove said carbon steel layer. 10. A process as set forth in claim 9 which further includes the step of evacuating the capsule after sealing thereof. 11. A process as set forth in claim 1 which further comprises the step of placing spherical particles of a powdered metal about said green compact in said capsule prior to sealing of said capsule to effect extrusion of a multilayer extruded product having a layer of said metal about a core of tantalum or niobium. 12. A process comprising the steps of placing a charge of powdered metal having a density in the range of 40% about a tapered mandrel in a container, said metal being selected from the group consisting of tantalum or niobium; cold isostatically pressing the charge at a predetermined pressure to a density sufficient to form a hollow green compact with sufficient strength to be handled outside the container; placing the hollow green compact in a tubular metal capsule having an outer wall and an inner wall; thereafter sealing the capsule; heating the capsule to a predetermined temperature and for a time sufficient to effect annealing of the green compact; thereafter cold isostatically pressing the capsule and encapsulated compact at a predetermined pressure of at least 200 Mpa; thereafter heating the pressed capsule and encapsulated compact; and extruding the heated capsule and encapsulated compact to form a hollow extruded product. 13. A process as set forth in claim 12 wherein the charge of powdered metal is cold isostatically pressed at a pressure of 400 Mpa and the capsule is heated to a temperature of 1250°C for 30 minutes to effect said annealing. 14. A process as set forth in claim 13 wherein the capsule and encapsulated compact are pressed at a pressure of 400 Mpa and heated to a temperature of 1200°C. 15. A process as set forth in claim 12 which further comprises the step of placing spherical particles of powdered metal about said green compact in said capsule prior to sealing of said capsule to effect extrusion of a multilayer extruded product having a layer of said metal about a core of tantalum or niobium. 16. An extruded product made in accordance with the process of claim 1. 17. An hollow extruded product made in accordance with the process of claim 12. 18. A process comprising the steps of cold isostatically pressing a charge of powdered metal to a density sufficient to form a billet with sufficient strength to be handled; placing the billet in a first metal container; thereafter sealing the container; placing the container in a second metal container with an annular gap defined between said containers; filling said gap with a metal powder having a spherical shape; thereafter sealing said second container; thereafter cold isostatically pressing said second container and encapsulated billet at a predetermined pressure of at least 200 Mpa; heating the compressed second container and encapsulated compact; and extruding the heated capsule and encapsulated compact to form an extruded product. 19. A process as set for in claim 18 wherein the second container and encapsulated billet are isostatically pressed at a pressure of 435 Mpa. 20. A process as set forth in claim 18 wherein the charge of powdered metal is selected from the group consisting of tantalum, niobium, molybdenum, tungsten carbide cobalt and tungsten heavy alloy and cold isostatically pressed at a pressure sufficient to achieve a density of from 70% to 85% of theoretical density. 21. A process as set forth in claim 18 wherein the first container and encapsulated billet are subjected to a vacuum to remove air therefrom. 22. A process as set forth in claim 18 wherein the metal powder in said gap is a carbon steel powder with a carbon content of 0.85%. 23. A process as set forth in claim 18 wherein the extruded product has an outer layer of metal different in composition from the remainder of the product and which further comprises the step of removing said outer layer. 24. A process as set forth in claim 23 wherein the second container is made of carbon steel and said step of removing said layer of carbon steel from the extruded product includes pickling of the extruded product to remove said carbon steel layer. 25. A process as set forth in claim 18 wherein said powdered metal is tantalum in the form of tantalum hydride and which further comprises the step of evacuating hydrogen from the first container during sealing thereof. 26. A process as set forth in claim 18 wherein said second container is cylindrical and wherein said step of sealing said second container includes securing a first metal plate across a front end of said second container and a second metal plate across a rear end of said second container. 27. A process as set forth in claim 26 wherein the extruded product has a forward end formed primarily of the metal of said first plate and the metal powder on said gap and a rear end formed primarily of the metal of said second plate and the metal powder in said gap, said method further comprising the steps of removing said forward end and said rear end of the extruded product to obtain a consolidated product having a yield of from 93% to 96%. 28. A process comprising the steps of cold isostatically pressing a charge of tantalum powder to a density of 72% of theoretical density; sealing the compressed charge in a first metal container; sealing the first container in a second container with a metal powder having a spherical shape disposed in an annular gap between said containers; thereafter cold isostatically pressing the second container and encapsulated compressed charge at a predetermined pressure of at least 200 Mpa; and heating and extruding the second container and encapsulated compressed charge to form an extruded product. 29. A process as set forth in claim 30 wherein the second container is heated to a temperature of 1,200C° after said isostatic pressing thereof and is isostatically pressed at a pressure of 435 Mpa. 30. A process as set forth in claim 28 which further comprises the step of venting said gap between said containers after said step of isostatically pressing the second container to eliminate water vapor from said gap and the powder therein. 31. An extruded product made in accordance with the process of claim 18. 32. An extruded product made in accordance with the process of claim 28.

Inventors: FRIEDMAN IRA, FRIEDMAN, IRA
Applicants: ASLUND CHRISTER, FRIEDMAN IRA, ASLUND, CHRISTER, FRIEDMAN, IRA
Application year: 1995
Main classification: B22F3
IPC codes: B22F3, C22C1