Patent Application
20120073303
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.
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.
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.
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
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
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
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
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.
Finally,
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
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
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.
| Number | Date | Country | |
|---|---|---|---|
| 61385805 | Sep 2010 | US |
