Metal injection molding process and components formed therewith   

This application claims the benefit of U.S. Provisional Application No. 61/385,805, filed Sep. 23, 2010, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to gas turbine engine components. More particularly, embodiments herein generally relate to processes for producing gas turbine engine components from alloys that are difficult to cast, for example, combustor heat shields that have nonuniform sections.

FIG. 1 schematically represents a high-bypass turbofan engine 10 of a type known in the art. The engine 10 is schematically represented as including a fan assembly 12 and a core engine 14. The fan assembly 12 is shown as including a composite fan casing 16 and a spinner nose 20 projecting forwardly from an array of fan blades 18. Both the spinner nose 20 and fan blades 18 are supported by a fan disc (not shown). The core engine 14 is represented as including a high-pressure compressor 22, a combustor 24, a high-pressure turbine 26 and a low-pressure turbine 28. Air is drawn into the inlet duct 16 of the engine 10 and then compressed by the compressor 22 before being transported to the combustor 24, where the compressed air is mixed with fuel and ignited to produce hot combustion gases that pass through the turbines 26 and 28. An exhaust nozzle 30 and a centerbody 32 extend afterward from the core engine 14 to define the outer and inner boundaries of the engine exhaust flowpath.

Fuel is transported to the combustor 24 by a fuel distribution system (not shown), where it is introduced at the front end of a burner in a highly atomized spray from a fuel nozzle. An annular-shaped dome defines the upstream end of the combustion chamber where combustion occurs. A number of circumferentially-spaced contoured cups are formed in the dome, and each cup defines an opening in which an air/fuel mixer (swirler assembly) is mounted for introducing the air/fuel mixture into the combustion chamber. The ignited air/fuel mixture can reach temperatures in excess of about 3500° F. (about 1930° C.). Due to these high temperatures, heat shields are typically placed around each air/fuel mixer to protect other combustor components from the ignited air/fuel mixture.

Heat shields have been fabricated from various materials, a particularly notable example of which is a cobalt-based alloy known as HS188, commercially available from Haynes International, Inc. HS188 has a nominal composition of, by weight, Co-22Ni-22Cr-14W-0.35Si-0.10C-0.03La-3Fe(max)-1.25Mn(max) and has a melting range of about 2400° F. to about 2570° F. (about 1315° C. to about 1410° C.). Importantly, HS188 is characterized by mechanical and environmental properties that are particularly well suited for its use as a heat shield in the combustor environment of a gas turbine engine. However, HS188 has been limited to wrought forms (e.g., bar and sheet) because it contains reactive elements such as lanthanum, which result in the alloy being incompatible with casting processes. In particular, lanthanum can react preferentially with a casting mold when the molten alloy is poured into the mold. This reaction can result in the formation of undesired oxide particles in the casting and rob the alloy of the desired properties of the lanthanum addition.

Increasing pressure variations associated with next generation lean-burning combustors have led to the need for improved heat shield designs that are capable of improving stiffness and heat transfer efficiency in higher temperature applications. Such designs have included, for example, ribs and tapered geometries that increase stiffness and/or turbulators that promote heat transfer. Such features of the heat shield define nonuniform sections, which as used herein means that the thickness dimension of the shield varies gradually (e.g., tapered sections) or abruptly (e.g., ribs and turbulators) in directions transverse to the thickness. These nonuniform sections can be difficult to produce from sheet metal processes of the type required by HS188, and are otherwise often impractical from the standpoint of costs due to machining and significant raw material waste. As a result, more advanced heat shield designs have made use of other cobalt-based alloys and nickel-based alloys that can be readily cast using conventional castings techniques, including investment casting. However, alternative alloys that are currently available and castable have certain drawbacks, for example, reduced environmental properties or higher costs as compared to HS188.

Accordingly, there remains a need for alternative methods for manufacturing components used in high temperature applications, such as heat shields, that would allow such components to be produced with nonuniform sections, including tapered geometries and/or turbulators.

The present invention provides a process of producing a metallic component having a desired shape that includes at least one nonuniform section, as well as metallic components produced by such a process.

According to a first aspect of the invention, the process includes providing a composition comprising a mixture of a metal powder and a polymeric binder, the metal powder comprising particles of an alloy that contains at least one reactive element that renders the alloy uncastable. The composition is metal injection molded within a mold cavity having a shape corresponding to the shape of the metallic component so as to yield a green compact having a shape corresponding to the shape of the metallic component that includes the at least one nonuniform section of the metal component. A majority of the binder is then removed from the green compact, and then the green compact is sintered to remove a remainder of the binder and fuse particles of the metal powder together to form the metallic component and the at least one nonuniform section thereof.

According to a particular aspect of the invention, the process results in the metal component having a microstructure that contains networks of agglomerated carbide precipitates at grain boundaries thereof, and the process further comprises a solution heat treatment performed on the metal component to reduce the agglomerated carbide precipitates to discrete carbides at the grain boundaries.

According to another aspect of the invention, the metallic component is a combustor heat shield of a gas turbine engine, and the heat shield comprises a hot-side surface adapted to face hot combustion gases within a combustor of the gas turbine engine, a cold-side surface adapted to face away from the hot combustion gases within the combustor, and at least one nonuniform section chosen from the group consisting of at least one tapered wall region between a midportion and an end portion of the heat shield and turbulators on the cold-side surface of the heat shield. The heat shield is formed of a cobalt-based alloy composition that contains at least one reactive element that renders the heat shield uncastable, and has a polycrystalline microstructure formed by a metal injection molding process to contain discrete carbides at grain boundaries of the microstructure.

According to another particular aspect of the invention, the cobalt-based alloy of the combustor heat shield consists essentially of, by weight, 20-24% Ni, 20-24% Cr, 13-15% W, 0.2-0.5% Si, 0.05-0.15% C, 0.02-0.12% La, up to 3% Fe, up to 1.25% Mn, up to 0.015% B, and the balance cobalt and incidental impurities.

A technical effect of the invention is the ability to manufacture components capable of being used in high temperature applications, such as heat shields, and having nonuniform sections that render the components difficult if not impossible to produce using conventional casting techniques. In addition, the invention permits the use of an alloy that exhibits excellent environmental and mechanical properties for use in the combustor section of a gas turbine engine, yet cannot be cast using conventional casting techniques due to the alloy containing one or more reactive elements.

Other aspects and advantages of this invention will be better appreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents a cross-sectional view of a high-bypass turbofan engine.

FIG. 2 is a scanned image of a heat shield adapted for use in a gas turbine engine and having turbulators on a cold-side surface thereof.

FIG. 3 is a scanned image showing some of the turbulators of FIG. 2 in more detail.

FIG. 4 is a schematic representation of a perspective view of the heat shield of FIGS. 2 and 3.

FIG. 5 represents a partial cross-sectional view of the heat shield of FIG. 4 taken along section line 5-5.

FIGS. 6 through 21 are a series of photomicrographs evidencing the effects of solution heat treatments and simulated brazing treatments performed on HS188 specimens produced using a metal injection molding process of the present invention.

DETAILED DESCRIPTION

The present invention provides a process for producing components using a metal injection molding process. The process is particularly well suited for producing heat shields that, as discussed previously, protect combustor components of gas turbine engines from the high temperatures associated with hot combustion gases. Furthermore, the process can be employed to produce heat shields from the HS188 alloy (and other alloys) that are typically or only available in wrought forms, for example, bar and sheet stock, or are otherwise difficult to manufacture with nonuniform sections, nonlimiting examples of which include tapered geometries and turbulators.

FIGS. 2 and 3 are scanned images showing a heat shield 34 of a type used in gas turbine engines, such as the engine 10 represented in FIG. 1. A cold-side surface 36 is visible in FIGS. 2 and 3, as are a large number of turbulators 40 that, when the heat shield 34 is subjected to backside cooling by directing cooling air at the surface 36, improves the heat transfer efficiency from the shield 34. Though the turbulators 40 are represented as forming a uniform pattern and having uniform cylindrical shapes, various shapes, dimensions and patterns are possible. As evident from FIGS. 2 and 3, the turbulators 40 result in abrupt changes in the section thickness of the heat shield 34, and are difficult and costly to produce by machining.

FIGS. 4 and 5 further represent the heat shield 34 as comprising a hot-side surface 38 (adapted to face combustion gases) oppositely disposed from the cold-side surface 36, and an opening 42 for receiving a fuel nozzle (not shown). The heat shield 34 has a length between its oppositely-disposed ends 44 and 46, which are defined by end portions 48 and 50 of the shield 34 that diverge from a plane defined by a midportion 52 of the shield 34. The shield 34 has a section thickness between its surfaces 36 and 38 that varies along its length. In particular, FIG. 5 represents the heat shield 34 as having a greater thickness (t1) within its midportion 52 than the thickness (t2) within its end portion 48. As a result, the heat shield 34 can be seen in FIG. 5 to have a tapered wall region 54 that increases the stiffness of the heat shield 34. While the thicknesses (t1 and t2) and the degree of taper will vary depending on the particular configuration of the heat shield 34, one example of the heat shield 34 has a thickness t1 of about 0.050 to about 0.070 inch (about 1.3 mm to about 1.8 mm) and a thickness t2 of about 0.075 to about 0.095 inch (about 1.9 mm to about 2.4 mm), resulting in a nominal change in thickness of about 25% from the midportion 52 to the end portion 48.

The turbulators 40 and variable section thicknesses t1 and t2 of the heat shield 34 are nonlimiting examples of nonuniform sections that may be desired for the heat shield 34 in order to promote its mechanical and thermal properties. However, other types of nonuniform sections are also possible and within the meaning of the term “nonuniform section” as defined herein. For example, nonuniform sections can include stiffening ribs, larger fillets, and other asymmetrical cross-sectional features that are capable of promoting mechanical, thermal or other desirable properties of the heat shield 34. Furthermore, while the following discussion will be directed to the heat shield 34 represented in FIGS. 2 through 5, the invention is not limited to heat shields, and instead the benefits of the invention can be adapted for a variety of components. Though the discussion will also focus on manufacturing the shield 34 from the HS188 cobalt-based alloy, it should be understood that the benefits of the invention can be obtained with a variety of other alloys, and particularly cobalt-based alloys that cannot or otherwise typically are not produced by casting techniques.

According to a preferred aspect of the invention, the heat shield 34 described above is manufactured using a metal injection molding (MIM) technique. According to a preferred embodiment of the invention, the MIM technique involves compounding a composition suitable for injection molding. The composition preferably contains a metal powder containing or consisting of particles of the HS188 alloy (or other alloy desired for the shield 34) and a polymeric binder, and is injected into a suitable mold to produce a green compact. The green compact then undergoes debinding to remove a portion of the binder and yield a brown compact, which then undergoes sintering at a temperature that is below the melting point of the metal powder yet sufficiently high to cause the metal powder particles to fuse (agglomerate). The remainder of the binder is also removed during sintering to yield the desired heat shield, including any desired nonuniform sections such as of the type described in reference to FIGS. 2 through 5. Following sintering, the heat shield can optionally undergo hot isostatic pressing (HIP). According to a preferred aspect of the invention, the heat shield 34 further undergoes a high-temperature solution heat treatment, particular if the metal powder consists of or otherwise contains the HS188 alloy.

The injection molding composition may contain, by weight, about 60% to about 70% of the metal powder and about 30% to about 40% of the binder. The particles of the metal powder can have any desired particle size suitable for use in a MIM process, for example, a mesh size of −270 (less than 53 micrometers), with a preferred particle size being −325 mesh (less than 45 micrometers). In a particular embodiment, the metal powder consists entirely of the HS188 alloy. The HS188 alloy can be described as a reactive cobalt-based alloy, in that the HS188 alloy contains more cobalt than any other constituent, and one or more constituents of the HS188 alloy tend to adversely react with molds typically used to cast high-temperature alloys. The HS188 alloy has been reported to contain, by weight, Also reported to have a composition of, by weight, 20-24% Ni, 20-24% Cr, 13-15% W, 0.2-0.5% Si, 0.05-0.15% C, 0.02-0.12% La, up to 3% Fe, up to 1.25% Mn, up to 0.015% B, and the balance (about 39%) cobalt and incidental impurities. In the HS188 alloy, lanthanum is considered to be a reactive element that can result in undesired properties in the alloy if attempting to produce castings of the alloy using traditional casting techniques. More generally, in addition to lanthanum, other reactive elements are known to impair the casting of various types of alloys. A particularly notable group includes reactive metals such as yttrium, zirconium and hafnium, whose additions to certain types of alloys can improve oxidation resistance.

A wide variety of polymer materials can be used as the binder. Preferred characteristics of the binder include chemical compatibility with the metal powder, the ability to provide adequate green strength to the green compact, and the ability to burn off cleanly during the debinding and sintering steps so as not to leave any amount of residue within the heat shield 34 that would reduce its mechanical properties. Another desired characteristic of the binder is to contribute a consistency to the composition that enables the composition to be injected into a mold under pressure without leaking through the parting lines of the mold.

The metal injection mold may be of any suitable type that can be used in MIM processes. Typical MIM molds are constructed of steel or other comparable material. The mold defines a mold cavity whose shape corresponds to the external shape of the heat shield to be fabricated. According to a preferred aspect of the invention, the shape of the mold cavity is also configured to produce the desired nonuniform sections for the heat shield, for example, the tapered wall regions 54 and turbulators 40 as represented in FIGS. 2 through 5. The injection molding composition can be injected into the mold according to conventional practices, for example, at a pressure of about 200 psi to about 400 psi (about 1.4 to about 2.8 MPa). If desired, the mold may be preheated, for example, to a temperature of about 200° F. (about 90° C.), to facilitate injection and dispersal of the injection molding composition within the mold.

Once injected, the injection molding composition is preferably allowed to cool and solidify within the mold, for example, due to solidification of the binder, resulting in the green compact noted previously. The time necessary for this to occur will vary depending on the particular composition of the molding composition, including the compositions of the metal powder and binder and their relative amounts. The rigid green compact can then be removed from the mold and, if desired, undergo drying and/or further cooling to facilitate handling.

The green compact is then preferably subjected to the debinding process to remove at least a portion of the binder and produce a porous yet still rigid brown compact. Complete removal of the binder generally does not occur until completion of the sintering cycle, discussed in more detail below. Debinding may be carried out by solvent extraction, thermal treatment, or a combination thereof. If solvent extraction is used, a solvent is selected that is capable of dissolving the polymeric binder within the green compact. In some instances, the solvent may be water or a suitable hydrocarbon solvent. If a thermal treatment is used, the temperature to which the green compact can be heated to carry out the debinding process will vary depending on the particular metal powder and binder used. For example, if the metal powder is the HS188 alloy and the binder is a wax or polymer resin, the green compact can be held at a temperature of about 150° F. to about 500° F. (about 65° C. to about 260° C.) for about one to two hours to remove most but not all of the binder, and thereby yield the brown compact.

As previously noted, sintering involves heating the brown compact to a temperature below the melting point of the metal powder, yet sufficiently high to remove the remaining binder and cause the metal powder particles to fuse together (agglomerate), thereby yielding the desired heat shield and its desired shape, or at least its near-net shape so that minimal machining is required to obtain the desired shape for the heat shield. As noted above, the as-molded shape of the heat shield includes any desired nonuniform sections, which may include the tapered wall region 54 and turbulators 40 represented in FIGS. 2 through 5. The sintering process tends to densify the brown compact by eliminating voids created during removal of the binder during the debinding process. Densification and void elimination generally results in shrinkage of the heat shield, for example, by about 3% to about 20% by volume of the brown compact. Those skilled in the art will understand that it may be desirable to control the amount of shrinkage to provide dimensional reproducibility and help minimize variation between components made using the methods set forth herein.

Preferred heating and cooling cycles used during the sintering process will tend to vary, depending on the particular compositions of the metal powder and binder. As a nonlimiting example in which the metal powder is the HS188 alloy and the binder is a wax or polymer resin, sintering may be carried out in a series of cycles over a temperature range of about 700° F. to about 2300° F. (about 370° C. to about 1260° C.) using conventional sintering practices. Furthermore, sintering may be carried out in a vacuum furnace having a partial pressure capability. For example, the furnace may be evacuated and then backfilled with hydrogen gas or an inert gas such as argon to attain a backfilled pressure of about 600 micrometers of mercury (about 80 Pa). The backfill gas may be intermittently or continuously flowed through the furnace to purge any volatiles that evolve during removal of the binder at the elevated sintering temperatures.

Heat shields produced by the process described above are capable of being about 95% to about 99% dense, which as used herein refers to the percent of the finished heat shield that is nonporous and can be measured using conventional image analysis techniques. Optionally, the heat shield may be further densified using a hot isostatic pressing (HI) technique involving the application of both heat and pressure. During HIPing, it may be possible to eliminate most if not all of any remaining voids within the heat shield resulting from removal of the binder from the green and brown compacts. A suitable HIPing temperature is believed to be in a range of about 2100° F. to about 2200° F. (about 1150° C. to about 1200° C.), for example, about 2125° F. (about 1160° C.), and a suitable HIPing pressure is believed to be about 10 ksi to about 20 ksi (about 70 to about 140 MPa), for example, about 15 ksi (about 100 MPa), in an inert (e.g., argon) atmosphere. The HIPing temperature and pressure are preferably held for a duration of about four hours. The end result of the HIPing process is a densified heat shield that is preferably at least about 99.9% dense.

According to another preferred aspect of the invention, the heat shield may be subjected to a solution heat treatment following the sintering process and any HIPing process. The solution heat treatment can be performed to develop the microstructure of the heat shield and/or dissolve any undesired phases present in the microstructure. As known in the art, suitable temperatures and durations for a solution heat treatment depend on the particular chemistry and microstructure of the metal article being solutioned. Conventional solution heat treatments performed on the HS188 alloy after conventional wrought processing are typically less than about 2150° F. (about 1180° C.), which results in a fully recrystallized equiaxed microstructure. Due to its carbon content, the solutioned HS188 alloy contains predominantly intragranular carbide precipitates that are sufficiently fine and discrete to prevent significant grain growth. Unexpectedly, carbide precipitates have been observed to preferentially agglomerate at grain boundaries of the polycrystalline microstructures produced by the MIM process described above. The carbide precipitates are sufficiently continuous to promote intergranular (brittle) fracture and reduce the fatigue properties of the heat shield. Conventional solution heat treatments at temperatures up to 2150° F. (about 1180° C.) were demonstrated to be incapable of eliminating the agglomerated carbides. During investigations leading to the present invention, higher solution temperatures were investigated. From these investigations, it was concluded that solutioning temperatures on the order of about 2200° F. to about 2325° F. (about 1200° C. to about 1275° C.), more preferably about 2225° F. to about 2275° F. (about 1220° C. to about 1250° C.), for example, about 2250° F. (about 1230° C.), were able to reduce the agglomerated carbide precipitates to discrete carbides at grain boundaries. These higher solutioning temperatures eliminate or at least reduce the amount of agglomerated carbides at the grain boundaries by solutioning (dissolving) the carbides, which then re-precipitate during cooling as smaller discrete precipitates at the grain boundaries. The ductility and fatigue properties of the heat shield are promoted by breaking up the agglomerated and continuous grain boundary carbide precipitates into finer and more discrete precipitates.

FIGS. 6 through 21 are illustrative of the above, and show microphotographs of specimens produced of the HS188 alloy using the MIM process described above. The microphotographs of FIGS. 6 and 7 show the microstructure of an as-HIPed specimen containing networks of carbides at grain boundaries. FIGS. 8 and 9 are microphotographs showing a specimen that underwent the same HIPing process as the previous specimen, and then further underwent solutioning at about 2150° F. (about 1180° C.) for about 45 minutes, followed by a helium quench. In comparison to the images of FIGS. 6 and 7, the specimen of FIGS. 8 and 9 exhibited a lower volume fraction (Vf) of carbides and a slightly coarsened grain structure. FIGS. 10 and 11 are microphotographs showing another specimen that underwent the same HIPing and solutioning treatments as the specimen of FIGS. 8 and 9, and then further underwent a simulated braze cycle at about 2150° F. (about 1180° C.) for about 10 minutes, followed by vacuum furnace cooling. From the images of FIGS. 10 and 11, it can be seen that the simulated braze cycle significantly coarsened the carbide precipitates to form a network of carbides at the grain boundaries. From these images, it can be appreciated that a heat shield that has been HIPed and solutioned according to the specimens of FIGS. 6 through 9 would exhibit undesirable carbide coarsening if the heat shield were to be brazed, as is often the case during fabrication and/or repair of a heat shield

FIGS. 12 and 13 are microphotographs showing another specimen that underwent the same HIPing process as the previous specimens, but then underwent solutioning at about 2200° F. (about 1200° C.) for about 45 minutes, followed by a helium quench. In comparison to the images of FIGS. 6 through 9, a considerable reduction in the carbide volume fraction can be seen in FIGS. 12 and 13. FIGS. 14 and 15 are microphotographs showing another specimen that underwent the same HIPing and solutioning treatments as the specimen of FIGS. 12 and 13, and then further underwent the same simulated braze cycle that had been performed on the specimen of FIGS. 10 and 11. In contrast to the latter specimen, the microstructure of the specimen seen in FIGS. 14 and 15 has a reduced carbide volume fraction similar to that of the specimen of FIGS. 12 and 13, which had undergone only HIPing and solutioning. These results evidenced that a heat shield that has been solutioned at about 2200° F. (about 1200° C.) is capable of retaining a desirable microstructure following brazing of the heat shield.

FIGS. 16 and 17 are microphotographs showing yet another specimen that underwent the same HIPing process as the previous specimens, but then underwent solutioning at about 2250° F. (about 1230° C.) for about 45 minutes, followed by a helium quench. Again, FIGS. 16 and 17 evidence that solutioning at temperatures above 2150° F. (about 1180° C.) result in a considerable reduction in the carbide volume fraction. In FIGS. 18 and 19, a specimen that underwent the same HIPing and solutioning treatments as the specimen of FIGS. 16 and 17 has undergone the same simulated braze cycle described for the specimen of FIGS. 10, 11, 14 and 15, and the microstructures seen in FIGS. 18 and 19 evidence that solutioning at about 2250° F. (about 1230° C.) would enable a heat shield to retain a desirable microstructure following brazing.

Finally, FIGS. 20 and 21 are microphotographs showing a specimen that underwent the same HIPing process as the previous specimens, but underwent solutioning at about 2325° F. (about 1275° C.) for about 45 minutes, followed by a helium quench. The microstructures seen in FIGS. 20 and 21 evidence a slightly lower carbide volume fraction in comparison to the specimens solutioned at 2250° F. (about 1230° C.), but otherwise do not significantly differ.

From these results, it was concluded that a MIM heat shield produced from the HS188 alloy requires solutioning at a temperature above the conventional solutioning temperature of 2150° F. (about 1180° C.) for HS188, for example, on the order of about 2200° F. to about 2325° F. (about 1200° C. to about 1275° C.), and more preferably about 2225° F. to about 2275° F. (about 1220° C. to about 1250° C.). These temperatures are necessary to not only solution carbide precipitates, but also inhibit their reprecipitation during a subsequent brazing operation or other equivalent thermal treatment.

Heat shields produced by the MIM process described above will consist essentially of the alloy of the metal powder used in the MIM process. While the heat shield may also comprise trace amounts of the binder, such traces are in such small amounts that they do not adversely impact the properties of the alloy or heat shield. Moreover, the MIM process is capable of producing gas turbine engine hot section components, and specifically heat shields, and further capable of overcoming a variety of issues that can arise when attempting to cast reactive cobalt-based alloys, such as HS188 and others comprising lanthanum. In particular, because metal injection molding is a solid-state process that does not require melting of the metal alloy being molded, reactive elements of the alloy do not melt and, therefore, do not react with the mold to form undesirable contaminates. Advantageously, the metal injection molding process allows for the creation of components having complex geometries, including various desired nonuniform sections of the types represented in FIGS. 2 through 5, namely, tapered regions 54 and turbulators 40, both of which are difficult or cost-prohibitive to produce from wrought stock.

While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configurations of the heat shield 34 represented in FIGS. 2 through 5 could differ from those shown, and materials and processes other than those noted could be used to produce a wide variety of components. Therefore, the scope of the invention is to be limited only by the following claims.