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Method for making molybdenum parts using metal injection molding

Abstract: Embodiments of a method for making molybdenum parts using metal injection molding are disclosed. In general, molybdenum powder is mixed or blended with a binder to form a feedstock, which is injection molded to form a green-state part. The green-state part is then sintered, such as by heating the part in a furnace for a predetermined period of time to effect consolidation and densification of the part. Desirably, the green-state part can be debound to remove at least a portion of the binder prior to sintering. In exemplary embodiments, sintering produces the final molybdenum article, and therefore the process does not require, but optionally may include, further processing of the sintered part, such as machining, cold-working, and/or hot-working. ...

Agent: - ,
USPTO Applicaton #: #20080075619 - Class: 419 36 (USPTO) - 03/27/08 - Class 419

The Patent Description & Claims data below is from USPTO Patent Application 20080075619, Method for making molybdenum parts using metal injection molding.

FIELD

[0001]The present invention concerns embodiments of a method for making molybdenum parts or shapes using metal injection molding.

BACKGROUND

[0002]Molybdenum parts, such as may be used in semiconductor devices or aerospace structures, are difficult to manufacture using conventional furnace melting techniques because of the high melting temperature of the element. Typically, molybdenum parts instead are made using powder metallurgy. A typical conventional manufacturing process involves several steps including forming a green-state part by powder cold compaction, sintering the green-state part, and employing metal-working techniques such as swaging, rolling, or drawing to make sheets, rods, bars or wires. The semi-finished parts are then further processed such as by machining, hot-working, and/or cold-working to form parts having complex three-dimensional shapes; that is, shapes other than the common sheet, rod, bar, or wire. As can be appreciated, this process is complicated and the use of machining often makes such processing uneconomical.

SUMMARY

[0003]The present disclosure concerns embodiments of a method for making molybdenum parts using metal injection molding. In general, molybdenum powder is mixed or blended with a binder to form a feedstock, which is injection molded to form a green-state part. The green-state part is then sintered, such as by heating the part in a furnace for a predetermined period of time to effect consolidation and densification of the part. Desirably, the green-state part can be debound to remove at least a portion of the binder prior to sintering. In exemplary embodiments, sintering produces the final molybdenum article, and therefore the process does not require further processing of the sintered part, such as machining, cold-working, and/or hot-working. Advantageously, the process can be used to form molybdenum parts having complex three-dimensional shapes more easily and is more economical than known processes. The process also can be used to form molybdenum alloy parts.

[0004]In one representative embodiment, a method for making a molybdenum part comprises injection molding a feedstock to form an unsintered part. The feedstock comprises molybdenum powder and at least 45% by volume of a binder. The method further includes sintering the unsintered part.

[0005]In another representative embodiment, a method for making a molybdenum part comprises injection molding a feedstock to form a green-state part, wherein the feedstock comprises molybdenum powder and at least one binder. The method further comprises debinding the green-state part to remove at least 40% of the binder to form a brown-state part. The brown-state part is then sintered at a peak temperature of no greater than about 3000.degree. F.

[0006]In another representative embodiment, a method for making a molybdenum part comprises injection molding a feedstock to form an unsintered part, wherein the feedstock comprising molybdenum powder and at least one binder. The molybdenum powder desirably has a particle size ranging between about 0.1 micron and about 4 microns, and more desirably in the range of about 0.1 micron to about 0.5 micron. The method further includes sintering the unsintered part.

[0007]In another representative embodiment, a method for making a molybdenum part comprises forming a feedstock by mixing a metal powder and a binder, and heating the mixture to a temperature sufficient to cause the binder to melt. The metal powder consists of molybdenum and has a particle size of no greater than about 5 microns, and the binder comprises at least 60% by volume of the feedstock. The feedstock is injection molded to form an unsintered part, which is then placed on an inner surface of a first setter. The inner surface of the setter is generally contoured to the shape of an adjacent surface of the part in contact with the inner surface. The first setter can be formed from a feedstock formed from molybdenum powder and a binder, which can be the same feedstock used to form the unsintered part. The unsintered part can be debound such as by placing the part (supported on the first setter) in a bath of at least one solvent. After the debinding step, a second setter is placed on the debound part so as to sandwich the part between the first and second setters. The second setter, like the first setter, also can be formed from a feedstock formed from molybdenum powder and a binder, which can be the same feedstock used to form the unsintered part. Finally, the debound part is sintered, such as by placing the part, along with the setters, in a sintering furnace for a predetermined period of time. The setters can be formed by any suitable method(s), such as metal injection molding.

[0008]The foregoing and other features and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a flowchart illustrating a method for making a molybdenum part using metal injection molding, according to one embodiment.

[0010]FIG. 2 is a perspective view of one example of a molybdenum part that was formed by metal injection molding.

[0011]FIG. 3 is a perspective view of another example of a molybdenum part that can be formed by metal injection molding.

[0012]FIG. 4 is a perspective view of first and second setters that can be used to maintain the shape of the part shown in FIG. 3 during debinding and/or sintering of the part.

DETAILED DESCRIPTION

[0013]As used herein, the singular forms "a," "an," and "the" refer to one or more than one, unless the context clearly dictates otherwise.

[0014]As used herein, the term "includes" means "comprises." For example, a device that includes or comprises A and B contains A and B but may optionally contain C or other components other than A and B. A device that includes or comprises A or B may contain A or B or A and B, and optionally one or more other components such as C.

[0015]FIG. 1 shows a flowchart, indicated generally at 10, that illustrates a method for making a molybdenum part using metal injection molding, according to one embodiment. As shown, the method in exemplary embodiments generally includes forming a feedstock from a metal powder comprising molybdenum and at least one binder (indicated at 12), forming a green-state part (indicated at 14), optionally debinding the green-state part to form a brown-state part (indicated at 16), and sintering the brown-state or green-state part (indicated at 18). The metal powder optionally may include other metals or metal alloys.

[0016]The part can have any of various complex three-dimensional shapes, which can include apertures, curves, recesses and/or other features that are not easily and readily formed using conventional manufacturing processes. FIGS. 2 and 3 show two examples of molybdenum parts indicated at 50 (FIG. 2) and 60 (FIG. 3), respectively, that can be made using metal injection molding.

[0017]The metal powder can be manufactured using conventional techniques. The molybdenum powder desirably has a relatively small particle size, such as about 5 microns or less. In one specific implementation, the molybdenum powder has a particle size in the range of from about 0.1 to about 0.5 micron. In another implementation, the molybdenum powder has a particle size in the range of from about 3 to about 5 microns. In yet another implementation, the molybdenum powder comprises a mixture of at least two molybdenum powders having different particle sizes, such as a first powder having a particle size of about 0.1 to 0.5 micron and a second molybdenum powder having a particle size of about 3 to about 5 microns. The composition of the powder mix can be, for example, about 10-90% by volume of the 0.1-0.5 micron powder and about 90-10% by volume of the 3-5 micron powder.

[0018]Any suitable binder can be used to form the feedstock. For example, the binder generally can comprise a plasticizer, an oil, or combinations thereof. Also, various water-soluble binders can be used. In certain embodiments, the binder comprises a plasticizer, a strengthener, a compatibilizer for the plasticizer and strengthener, and a surfactant. Without limitation, examples of plasticizers include paraffin wax, carnauba wax, polyethylene wax, or microcrystalline wax; examples of strengtheners include polypropylene, polystyrene, and polyacetal; examples compatibilizers include styrene-butadiene block copolymer (e.g., Kraton.RTM. commercially available from Shell) and ethyl vinyl acetate copolymer; and examples of surfactants include stearic acid, and zinc stearate.

[0019]In one embodiment, a binder typically has a composition in weight percent of about 45% to 55% plasticizer, 45% to 55% strengthener, 3% to 6% compatibilizer, and 0.25% to 0.5% surfactant. A particular working embodiment comprised 48.5% paraffin wax, 48.5% polypropylene, 3% styrene-butadiene, and 0.25% stearic acid being a specific example.

[0020]In particular embodiments, the concentration of the binder in the feedstock can vary between about 45% to about 80% by volume and the concentration of the molybdenum powder in the feedstock can vary between about 20% to about 55% by volume, although different concentrations can be used in other embodiments. In an exemplary embodiment, the composition of the feedstock comprised about 63% to about 78% by volume of a binder and about 22% to about 37% by volume of molybdenum powder. In another exemplary embodiment, the composition of the feedstock comprised about 60% to about 71% by volume of a binder and about 29% to about 40% by volume of molybdenum powder.

[0021]To prepare the feedstock, molybdenum powder is mixed with at least one binder. This mixture is heated to a temperature sufficient to cause the binder to melt and form a paste-like mixture. Any of various conventional mixers, such as a planetary mixer or equivalent mechanism, can be used to mix the metal powder and the binder. The temperature at which the mixture is heated depends on the composition of the binder. Generally, any temperature greater than room temperature may be sufficient to melt the binder. In one example, the binder composition described above is heated to a temperature of about 300.degree. F. to 400.degree. F., and more preferably 325.degree. F. to 350.degree. F. In particular embodiments, the feedstock is allowed to cool and form a solidified mass, which is then pelletized or otherwise fractionated to form a plurality of smaller, feedstock particles or pellets with thermoplastic properties.

[0022]The green-state part is formed by injecting the feedstock, in the form of a moldable paste or slurry, under pressure into a mold. For example, in one specific implementation, the feedstock particles are loaded into the hopper of a conventional injection molding machine, and the particles are heated at a temperature sufficient to cause the binder to melt and form a feedstock slurry. The temperature of the feedstock slurry can vary depending on the composition of the binder used. For example, feedstock particles comprising a binder having the composition described above generally can be heated to a temperature of about 300.degree. F. to 450.degree. F., and more preferably 325.degree. F. to 350.degree. F. to form a moldable slurry.

[0023]In an alternative embodiment, the feedstock can be transferred directly from the mixer to the injection molding machine without the intermediate steps of solidifying and fractionating the feedstock into smaller particles. In still another embodiment, an injection molding machine having mixing and molding capabilities can be used. Thus, in the latter embodiment, the feedstock is formed by mixing the metal powder and binder in the injection molding machine itself prior to forming the green-state part.

[0024]In any event, the feedstock is injected at a pressure greater than ambient pressure, such as a high pressure (e.g., 2,000 psi) into a mold of any desired shape to form a green-state part. Since sintering generally will cause the part to densify, the size of the mold is slightly greater than the required final size of the part after sintering.

[0025]In particular embodiments, the unsintered, green-state part (or multiple parts) is debound to form a brown-state part (indicated at 16 in FIG. 1). Debinding can be accomplished by immersing the part in a bath of a suitable solvent to dissolve at least a portion of the binder in the part. The solvent can be, for example, water or a halogenated (e.g., chlorinated, brominated, etc.) lower aliphatic, such as ethylene or propylene, with one example being trichlorinated trichloroethylene.

[0026]In lieu of or in addition to extracting the binder with solvent, the binder can be removed by thermal treatment. In one implementation, for example, the part can be placed in a bath of a heated solvent. In another implementation, the binder can be removed by heat treating the part in a furnace in lieu of or in addition to chemically treating the part with a solvent. In the context of the present disclosure, "debinding" means to remove or extract at least a portion of the binder from a part. Hence, debinding can include, but does not require, removal of the entire binder phase from a part. In some embodiments, for example, the solvent is effective to extract about 30% to 60% of the binder from the part, and more desirably about 40% to 60% of the binder is removed.

[0027]The green-state part can be placed on a bottom, first setter to minimize distortion of the green-state part during debinding, for example to maintain a curved shape of the part. If the part is generally flat, the part can be placed on a flat tray for further processing (debinding and/or sintering). The setter has an inner surface that contacts an adjacent surface of the part. The inner surface is generally contoured to the shape of the adjacent surface of the part. During debinding, the part, supported on the setter, can be placed in the solvent bath.

[0028]The setter can be formed from any of various suitable materials using any suitable techniques or mechanisms. In exemplary embodiments, the setter is injection molded from a feedstock of molybdenum powder and a binder, which can be the same feedstock used to form the part. The setter can be formed from materials other than molybdenum, such as (without limitation) carbonyl iron, stainless steel, or any of various ceramics. In addition, other metal forming methods also can be used to form the setter, such as (without limitation), slurry casting, powder pressing, or ramming.

[0029]After debinding, the brown-state part is placed in a furnace or similar device for sintering. To minimize distortion of the part during sintering, the part can be kept on the bottom setter, and a top, second setter can be placed on top of the part so that it is sandwiched between the two setters in a stacked configuration. The top setter has a lower, inner surface that is generally contoured to the shape of the adjacent surface of the part that contacts the inner surface of the setter. The weight of the top setter bearing down on the part is sufficient to retain the setters and the part in a stacked configuration and prevent, or at least minimize, distortion of the part during processing of the part. If desired, however, a fixture or equivalent mechanism, such as a clamp, can be used to hold the components together and optionally apply pressure to one or both of the setters so as to apply pressure to the part beyond the weight of the top setter.

[0030]The top setter, like the bottom setter, can be injection molded from a feedstock of molybdenum powder and a binder, which can be the same feedstock used to form the part. The top setter alternatively can be formed from other materials and by other processes as described above for the bottom setter.

[0031]Although only a bottom setter is used for debinding in the embodiment described above, in an alternative embodiment, both the top and bottom setters can be used during debinding and sintering.

[0032]In alternative embodiments, one or two setters can be used for debinding and/or sintering a part that is injection molded from metals other than molybdenum. Accordingly, a bottom setter or a bottom and top setter can be used for debinding and/or sintering a part that is injection molded from any of various metals or metal alloys (including superalloys), including (without limitation), tungsten, steel, silver, and cobalt-based, titanium-based, iron-based, and nickel-based superalloys, to name just a few. The materials used to make the setters can be selected based on the specific debinding and/or sintering conditions required for the particular part being processed.

[0033]FIG. 4 shows one example of a pair of setters that can be used for the part 60 (FIG. 3). As shown in FIG. 3, the exemplary part 60 has opposed, first and second curved surfaces 62, 64, respectively. As shown in FIG. 4, a first setter 70 includes an inner surface 72 that is generally contoured to the shape of the first surface 62 of the part 60 (FIG. 3). A second setter 74 (FIG. 4) includes an inner surface 76 that is generally contoured to the second surface 64 of the part 60 (FIG. 3). Either setter 70, 74 can be used as the "top" or "bottom" setter during processing of the part.

[0034]The specific sintering conditions can vary depending on the binder used. However, in general, sintering is carried out at a temperature of about 2200.degree. F. to about 3000.degree. F. for a period of time to effective to sinter a part as desired, such as from about 2 to 10 about hours. In addition, the sintering temperature can be varied to achieve a desired density. The part desirably (although not necessarily) is sintered to densify the part to at least about 90% of the theoretical density of molybdenum, and more desirably to at least about 95% of the theoretical density of molybdenum.

[0035]In certain embodiments, the part can be pre-heated at one or more temperature levels less than the final or peak sintering temperature. In addition, the part desirably is sintered under conditions that minimize oxidation of the part. Such conditions can include, for example, sintering in a partial vacuum, in an atmosphere of an inert gas (e.g., argon or nitrogen), in a reducing atmosphere (e.g., a hydrogen atmosphere), or a combination of any of the foregoing conditions. In exemplary embodiments, for example, the part is initially heated in a hydrogen atmosphere and then in an inert gas atmosphere.

[0036]Sintering is effective to remove most, if not all, of the binder remaining in the part after the debinding step. After sintering, the part is cooled. For example, the part can be cooled in the furnace to a temperature of about 100.degree. F., after which the part can be removed from the furnace. If desired, an inert gas (e.g., argon) can be introduced into the furnace to facilitate cooling of the part.

[0037]In an alternative embodiment, the green-state part can be sintered without first subjecting the part to a separate debinding step (e.g., the debinding step indicated at 16 in FIG. 1).

[0038]In certain embodiments, the sintered part is in its final form and does not require further processing (e.g., machining, hot-working, and/or cold-working) to achieve the desired, final form. However, if desired, the sintered part can be subjected to further processing. For example, the part can be further densified by, for example, pressing processes including conventional hot isostatic pressing or conventional cold isostatic pressing. Also, the surfaces of the part can be finished using conventional surface-finishing techniques, such as centerless grinding.

[0039]In one specific embodiment, a feedstock comprises about 63-78% by volume of a binder and 22-37% by volume of molybdenum powder. The binder had a composition in weight percent of about 48.5% wax, about 48.5% polypropylene, about 3% styrene-buta-diene, and about 0.25% stearic acid. The molybdenum powder has a particle size in the range of about 0.1 micron to about 0.5 micron. The feedstock is injection molded to form a green-state part that is debound in a bath of trichloroethylene for a predetermined period of time, for example about 60 minutes. The trichloroethylene can be maintained at an elevated temperature above room temperature (e.g., about 155.degree. F.) to facilitate the debinding process. The brown-state part then can be placed in a furnace for sintering. Sintering can be accomplished by pre-sintering at one or more progressively increasing temperatures before sintering at a final or peak sintering temperature. In exemplary embodiments, the part is heated at about 500.degree. F. for about 30 minutes, at about 1200.degree. F. for about 30 minutes, at about 2450.degree. F. for about 30 minutes, and finally at about 2550.degree. F. for about 120 minutes.

EXAMPLE 1

[0040]The part shown in FIG. 2 was formed from a feedstock comprising about 63-78% by volume of a binder and 22-37% by volume of molybdenum powder. The binder had a composition in weight percent of about 48.5% wax, about 48.5% polypropylene, about 3% styrene-buta-diene, and about 0.25% stearic acid. The molybdenum powder had a particle size in the range of about 0.1 micron to about 0.5 micron.

[0041]The powder and binder were heated to about 325.degree. F. to 350.degree. F. and blended in a planetary mixer. The blend was allowed to cool to form a solidified mass, which was pelletized into a plurality of feedstock pellets. Pellets were loaded into the hopper of an injection molding machine and heated to a temperature of about 375.degree. F. to form a moldable paste. The injection molding machine had a 22-mm screw and barrel. The injection molding machine was used to form three parts having different thicknesses.

[0042]The green-state parts were placed on a tray, which were then placed in a bath of trichloroethylene at a temperature of about 155.degree. F. for about 60 minutes, which removed about 40-50% of the binder. After debinding in the trichloroethylene, the tray with the parts were placed in a furnace for sintering. The conditions for sintering were as follows. The atmosphere inside furnace was evacuated using a vacuum pump, after which hydrogen was introduced into the furnace until the pressure inside the furnace was about 6 torr. The temperature inside the furnace was increased from room temperature to about 500.degree. F. at a rate of about 5.degree. F./minute and held at about 500.degree. F. for about 30 minutes, increased from 500.degree. F. to about 1200.degree. F. at a rate of about 2.degree. F./minute and held at about 1200.degree. F. for about 30 minutes, and increased from about 1200.degree. F. to about 2450.degree. F. at about 6.degree. F./minute and held at about 2450.degree. F. for about 30 minutes. At about 1650.degree. F., the hydrogen atmosphere in the furnace was evacuated and replaced with an argon atmosphere at about 10 torr. The temperature of the furnace was further increased from about 2450.degree. F. to about 2550.degree. F. at a rate of about 4.degree. F./minute and held at about 2550.degree. F. for about 120 minutes to complete sintering of the parts.

[0043]Thereafter, the parts were allowed to cool. The sintered parts exhibited a theoretical density of about 95-99%. The final thicknesses of the sintered parts were 0.020 inch, 0.030 inch, and 0.040 inch.

EXAMPLE 2

[0044]A green-state part was injection molded from the feedstock described above in Example 1. The green-state part was removed from the injection molding machine and positioned on a bottom setter formed from the same feedstock. The part, supported on the setter, was debound in a solvent bath as described above in Example 1. A top setter, formed form the same feedstock as the part, was placed on top of the debound, brown-state part, which was then placed in a sintering furnace along with the setters sandwiching the part. The part was sintered according to the steps described above in Example 1. The sintered part exhibited a theoretical density of about 97%.

EXAMPLE 3

[0045]The part shown in FIG. 2 was injection molded from a feedstock formed from molybdenum powder having a particle size in the range of about 3 to 5 microns and a binder having the composition described above in Example 1. The feedstock comprised about 60-71% by volume of the binder and about 29-40% by volume of the molybdenum powder. The green-state part was debound by placing the part in a bath of trichloroethylene at a temperature of about 155.degree. F. for about 60 minutes. The debound, brown-state part was subjected to the sintering steps described above in Example 1, except that final densification was achieved by heating the furnace from about 2450.degree. F. to about 3000.degree. F. at a rate of about 4.degree. F./minute and heating the part at about 3000.degree. F. for about 300 minutes. The sintered part exhibited a theoretical density greater than 95%.

EXAMPLE 4

[0046]The part shown in FIG. 2 was formed from a feedstock comprising a binder having the composition described above in Example 1 and a powder mixture comprising about 30% 3-5 micron molybdenum powder and about 70% 0.1-0.5 micron molybdenum powder. The feedstock was processed as described in Example 1. An injection molding machine was used to form a green-state part from the feedstock. The green-state part was debound by placing the part in a bath of trichloroethylene at a temperature of about 155.degree. F. for about 60 minutes. The debound, brown-state part was subjected to the sintering steps described above in Example 1, except that final densification was achieved by heating the furnace from about 2450.degree. F. to about 3000.degree. F. at a rate of about 4.degree. F./minute and heating the part at about 3000.degree. F. for about 300 minutes. The sintered part exhibited a theoretical density of about 95%.

[0047]In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.

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Method for making molybdenum parts using metal injection molding