Patent Application
20080135721
This disclosure relates to ceramic molds and methods for manufacturing ceramic molds. In particular, this disclosure relates to ceramic molds for manufacturing turbine metal castings.
Investment casting is generally used for forming complex three-dimensional (3-D) components of a suitable material such as metal. The investment casting process is also termed the “lost wax process”. An exemplary cast component is the typical turbine rotor blade of gas turbine engine.
A turbine blade includes an airfoil integrally joined at its root with a blade platform. The blade platform communicates with and is integrally joined to a multi-lobed supporting dovetail. The airfoil is hollow and comprises one or more radial channels extending along the span thereof that commences inside the supporting dovetail, which has one or more inlets for receiving pressurized cooling air during operation of the engine.
The airfoil may have various forms of intricate cooling circuits therein for tailoring cooling of the different portions of the opposite pressure and suction sides of the airfoil between the leading and trailing edges and from the root at the platform to the radial outer tip.
Complex cooling circuits include a dedicated channel inside the airfoil along the leading edge for providing internal impingement cooling. A channel along the thin trailing edge of the airfoil provides dedicated cooling. Further, a multi-pass serpentine channel may be disposed in the middle of the airfoil between the leading and trailing edges. The three cooling circuits of the airfoil have corresponding inlets extending through the blade dovetail for separately receiving pressurized cooling air.
The cooling channels inside the airfoil may include features such as short turbulator ribs or pins for increasing the heat transfer between the heated sidewalls of the airfoil and the internal cooling air. The partitions or bridges which separate the radial channels of the airfoil may include small bypass holes extending through the forward bridge of the airfoil for impingement cooling the inside of the leading edge during operation.
Such turbine blades are generally manufactured from high strength, superalloy metal materials in a casting process. In an investment casting process, a ceramic core is first manufactured to conform to the intricate cooling passages desired inside the turbine blade. A precision die or mold is also created that defines the precise 3-D external surface of the turbine blade including its airfoil, platform, and integral dovetail.
The ceramic core is assembled inside two die halves, which form a space or void that defines the resulting metal portions of the blade. Wax is injected into the assembled dies to fill the void and surround the ceramic core encapsulated. The two die halves are split apart and removed from the molded wax. The molded wax has the configuration of the desired blade and is then coated with a ceramic material to form a surrounding ceramic shell.
The wax is melted and removed from the shell, leaving a corresponding void or space between the ceramic shell and the internal ceramic core. Molten metal is then poured into the shell to fill the void therein and again encapsulate the ceramic core contained in the shell.
The molten metal is cooled and solidifies, and then the external shell and internal core are suitably removed, leaving behind the desired metallic turbine blade in which the internal cooling passages are found.
The cast turbine blade may then undergo subsequent manufacturing processes such as the drilling of suitable rows of film cooling holes through the sidewalls of the airfoil for providing outlets for the internally channeled cooling air. In this manner, the air forms a protective cooling air film or blanket over the external surface of the airfoil during operation in the gas turbine engine.
Gas turbine engine efficiency is increased by increasing the temperature of the hot combustion gases generated during operation from which energy is extracted by the turbine blades. The turbine blades are formed of superalloy metals, such as nickel-based superalloys, for their enhanced strength at high temperature to increase the durability and useful life of the turbine blades.
The intricate cooling circuits provided inside the airfoils are instrumental in protecting the blades from the hot combustion gases for the desired long life of the blades in an operating turbine engine.
The cooling circuits inside turbine blades are therefore becoming more and more complex and intricate for tailoring the use of the limited pressurized cooling air and maximizing the cooling effectiveness thereof. Any such cooling air bled from the compressor during operation for cooling the turbine blades is not used in the combustion process and correspondingly decreases the overall efficiency of the engine.
Recent developments in improving turbine airfoil cooling include the introduction of double walls therein for enhancing local cooling of the airfoil where desired. The airfoil includes main channels such as the dedicated leading edge and trailing edge channels and the multi-pass serpentine channels that provide the primary cooling of the airfoil. These channels lie between the thin pressure and suction sidewalls of the airfoil that may be about 40 to about 50 mils (1 mil=10−3 inches) (about 1 millimeter to about 1.3 millimeters) thick.
In introducing double wall construction of the airfoil, a thin internal wall is provided between the main sidewalls of the airfoil and the main channels therein to define auxiliary or secondary channels that are relatively narrow. The secondary wall may include impingement holes therethrough for channeling from the main flow channels impingement cooling air against the inner surface of the main sidewalls.
The introduction of the double wall construction and the narrow secondary flow channels adds to the complexity of the already complex ceramic cores used in investment casting of turbine blades. Since the ceramic core identically matches the various internal voids in the airfoil which represent the various cooling channels and features thereof, it becomes correspondingly more complex as the cooling circuit increases in complexity.
Each radial channel of the airfoil requires a corresponding radial leg in the ceramic core, and the legs are suitably interconnected or otherwise supported inside the two dies during the casting process. As the ceramic core legs become thinner, such as for the secondary channels, their strength correspondingly decreases, which leads to a reduction in useful yield during the manufacture of the cores that are subject to brittle failure during handling.
Since the ceramic cores are separately manufactured and then assembled inside the two die halves, the relative positioning thereof is subject to corresponding assembly tolerances. The walls of the airfoil are relatively thin to begin with, and the features of the ceramic core are also small and precise. Therefore, the relative position of the ceramic core inside the die halves is subject to assembly tolerances that affect the final dimensions and relative position of the intricate cooling circuit inside the thin walls of the resulting airfoil.
In order to manufacture objects having complex internal passages and structures such as those described above, it is desirable to use casting compositions for the ceramic core that have suitable viscosities that can easily flow into such narrow passages and which, upon solidification, can retain the complex shapes that are desirable for the internal passages of the airfoil. It is also desirable for the casting composition to rapidly solidify so as to be load-bearing during rapid prototyping processes for manufacturing the intricate shapes required for turbine components.
Disclosed herein is a method comprising disposing a casting composition within a sacrificial die; wherein internal features of the sacrificial die provide a replica of a desired core; wherein the casting composition has a viscosity of about 1 to about 1,000 Pascal-seconds at room temperature when tested at a shear rate of up to 70 seconds−1; reacting the casting composition to form a gel matrix; removing the sacrificial die; extracting a solvent from the gel matrix to form a dried gel; and firing the dried gel to form a ceramic core.
Disclosed herein too is a casting composition comprising a monomer and/or a polymer; and a metal and/or ceramic powder; wherein the casting composition has a viscosity of about 1 to about 1,000 Pascal-seconds at room temperature when tested at a shear rate of up to 70 seconds−1 and a flow index of less than 0.6.
Disclosed herein are casting compositions that can be used for manufacturing the complex internal passages and structures of airfoils used in turbines. The casting composition advantageously has a viscosity of about 10 to about 1,000 Pa-s at room temperature when tested at a shear rate of 0.001 to 100 seconds−1, which permits the casting composition to flow easily into the complex internal passages and structures of airfoils. The casting composition also can undergo solidification upon heating and curing to become a load bearing ceramic core that can take the complex shape of the passages into which it is poured. The casting composition is rigid enough after curing to hold its own weight during a solvent/monomer removal process as well as to withstand shear stresses generated by the flow of viscous resin during the solvent or monomer removal.
In one embodiment, in a rapid prototyping process for manufacturing turbine components, a sacrificial die is first manufactured. The internal features of the sacrificial die are replicas of the desired complex internal passages and structures of the turbine blade. In order to manufacture a turbine blade it is desirable to replicate the complex internal passages and structures. In one embodiment, replication of the complex internal passages and structures is accomplished by pouring a casting composition of suitable viscosity into the complex internal passages and structures of the sacrificial die to form a ceramic core. After the casting composition is cast within the sacrificial die, the casting composition is cured to form an internal gel matrix. The internal gel matrix is sometimes referred to as the “green ceramic composite body”. After the internal gel matrix solidifies and can withstand its own weight, the sacrificial die can be removed. Removal of the sacrificial die is accomplished via thermal or chemical degradation, abrading, dissolution, or the like, or a combination of at least one of the foregoing. After removing the die, a freestanding, one-piece or monolithic gel matrix remains. The gel matrix is sintered to form a one-piece, three-dimensional ceramic core.
The three-dimensional ceramic core is suitable for further use in manufacturing investment casting multi-wall turbine airfoils or other components. In one embodiment, the ceramic core is used in an investment casting process to provide a metal casting comprising a multi-wall turbine airfoil that comprises hollow air-cooling passages. The air-cooling passages are replicas of the ceramic core. In this embodiment, the ceramic core is placed into a metal mold whose inner surface replicates the outer surface of a turbine blade, and molten wax is disposed between the ceramic core and the metal mold. The molten wax upon cooling has an outer surface that is a replica of the outer surface of the metal turbine blade. The wax pattern with the internally fixed ceramic core is then dipped repeatedly in a solvent/ceramic powder slurry (this is called “investing,” because the wax pattern is being encased in the ceramic slurry) to make an investment shell. After drying, the wax is melted out, and the investment shell along with the ceramic core is fired. The fired investment shell with the ceramic core is then used to mold turbine blades.
In another embodiment for manufacturing the turbine blade, the sacrificial die is used directly as a mold for manufacturing both the internal ceramic core and the external investment shell. In this embodiment, after the casting composition is poured in and cured, the sacrificial die with the internal gel matrix is used directly without a wax pattern and dipped into an investment slurry to make the investment shell mold. The resultant article comprises an outer investment shell that encloses the sacrificial die, which in turn, encloses the gel matrix.
In one embodiment, the entire article is heated to decompose the sacrificial die and to fire both the investment shell and the internal gel matrix simultaneously. In another embodiment, the sacrificial die is first removed, following which the investment shell and the internal gel matrix are subjected to a firing process to convert them into a final mold. This final mold is then used to manufacture turbine blades.
The aforementioned methods are advantageous in that they permit complex three-dimensional shapes and structures to be manufactured in a single step. Manufacturing a mold by using the casting composition also reduces the time required for making a mold, thereby improving productivity and reducing costs. In addition, the wall thickness of the mold can be carefully controlled. Another advantage is that the mold can be engineered to maximize strength, thereby reducing mold failures.
As noted above, the internal structure of the sacrificial die is a replica of the desired internal structure of the article to be molded. In one embodiment, the internal features of the sacrificial die are a replica of the hollow air-cooling passages of a turbine blade. The sacrificial die is generally made by a rapid prototyping process. In one embodiment, the sacrificial die is made according to a CAD file drawing. In one embodiment, the sacrificial die is made by a stereolithographic process.
The sacrificial die generally comprises a polymeric material. The polymeric material may be selected from a wide variety of thermoplastic polymers, thermosetting polymers, blends of thermoplastic polymers, or blends of thermoplastic polymers with thermosetting polymers. The polymeric material can comprise a homopolymer, a copolymer such as a star block copolymer, a graft copolymer, an alternating block copolymer or a random copolymer, ionomer, dendrimer, or a combination comprising at least one of the foregoing. The polymeric material may also be a blend of polymers, copolymers, terpolymers, or the like, or a combination comprising at least one of the foregoing.
Exemplary thermoplastic polymers for manufacturing the sacrificial die include polycarbonate, polypropylene, polyester, polyimide, polyarylate, polystyrene, polyethersulfone, polyamideimide, polyurethane, polyetheretherketone, or the like, or a combination comprising at least one of the foregoing. An exemplary polymer is LEXAN®, a polycarbonate, commercially available from General Electric Plastics. Another exemplary thermoplastic polymer is acrylonitrile butadiene styrene (ABS).
As noted above, the sacrificial die can be manufactured from thermosetting or crosslinked polymers. Examples of crosslinked polymers include radiation curable or photocurable polymers. Radiation curable compositions comprise a radiation curable material comprising a radiation curable functional group, for example an ethylenically unsaturated group, an epoxide, and the like. Suitable ethylenically unsaturated groups include acrylate, methacrylate, vinyl, allyl, or other ethylenically unsaturated functional groups. As used herein, “(meth)acrylate” is inclusive of both acrylate and methacrylate functional groups. The materials can be in the form of monomers, oligomers, and/or polymers, or mixtures thereof. The materials can also be monofunctional or polyfunctional, for example di-, tri-, tetra-, and higher functional materials. As used herein, mono-, di-, tri-, and tetrafunctional materials refers to compounds having one, two, three, and four radiation curable functional groups, respectively.
In one embodiment, it is desirable for the polymeric material used in the sacrificial die to not undergo any significant dimensional change during the curing of the casting composition. It is therefore generally desirable for the polymeric material used in the sacrificial die to have a glass transition temperature that is greater than the temperature at which the casting composition is cured.
The casting composition comprises an inorganic powder, a binder comprising a monomer and/or a polymer, a solvent, and other desired additives. The inorganic powder can be metallic or ceramic. Examples of suitable metal powders include steels, aluminum alloys, superalloys, titanium alloys, copper alloys, or a combination comprising at least one of the aforementioned metal powders. Examples of ceramic powders include alumina, silica, zirconia, magnesia, chromium oxide, iron oxide, zinc oxide, hydroxylapatite, silicon nitride, silicon carbide, boron nitride, refractory carbides (such as titanium carbide (TiC), tantalum carbide (TaC), or the like), refractory nitrides (such as titanium nitride (TiN), tantalum nitride (TaN), tungsten nitride (WN), or the like), refractory borides (such as titanium diboride (TiB2), zirconium diboride (ZrB2), or the like), clays, spinels, mullite, ferrites, titanates, zircon, glass frits, or a combination comprising at least one of the aforementioned materials.
In one embodiment, the metal or ceramic powder can have micrometer sized or nanometer sized (hereinafter nanosized) particles. Micrometer sized particles generally have particle sizes greater than or equal to about 1.2 micrometers (μm). In one embodiment, the micrometer-sized particles have particle sizes greater than or equal to about 1.5 μm. In another embodiment, the micrometer-sized particles have particle sizes greater than or equal to about 1.8 μm. In yet another embodiment, the micrometer-sized particles have particle sizes greater than or equal to about 2.5 μm. In yet another embodiment, the micrometer-sized particles have particle sizes greater than or equal to about 5.0 μm.
Exemplary nanosized particles include metal oxides, highly crosslinked siloxanes, polyhedral oligomeric silsesquioxanes (POSS) macromers, metal carbides, nanoclays and the like, which have maximum particle sizes less than or equal to about 1200 nanometers (nm). In general it is desirable to use nanosized particles wherein the particle sizes are less than or equal to about 500, preferably less than or equal to about 200, preferably less than or equal to about 100, and more preferably less than or equal to about 40 nm.
Nanosized metal oxides that may be used in the compositions are metal oxides of alkali earth metals, alkaline earth metals, transition metals and other commonly used metals. Suitable examples of metal oxides are calcium oxide, cerium oxide, magnesium oxide, titanium oxide, zinc oxide, silicon oxide, copper oxide, aluminum oxide, or the like, or combinations comprising at least one of the aforementioned metal oxides. Nanosized metal carbides such as silicon carbide, titanium carbide, tungsten carbide, iron carbide, or the like, or combinations comprising at least one of the aforementioned metal carbides may also be used in the compositions. The metal oxides and carbides are generally particles having surface areas in an amount of about 1 to about 1000 m2/gm. Within this range it is generally desirable for the metal oxides and carbides to have surface areas greater than or equal to about 5 square meter/gram (m2/gm), preferably greater than or equal to about 10 m2/gm, and more preferably greater than or equal to about 15 m2/gm. Also desirable within this range is a surface area less than or equal to about 950 m2/gm, preferably less than or equal to about 900 m2/gm, and more preferably less than or equal to about 875 m2/gm.
Commercially available examples of nanosized metal oxides are NANOACTIVE™ calcium oxide, NANOACTIVE™ calcium oxide plus, NANOACTIVE™ cerium oxide, NANOACTIVE™ magnesium oxide, NANOACTIVE™ magnesium oxide plus, NANOACTIVE™ titanium oxide, NANOACTIVE™ zinc oxide, NANOACTIVE™ silicon oxide, NANOACTIVE™ copper oxide, NANOACTIVE™ aluminum oxide, NANOACTIVE™ aluminum oxide plus, all commercially available from NanoScale Materials Incorporated. Commercially available examples of nanosized metal carbides are titanium carbonitride, silicon carbide, silicon carbide-silicon nitride, and tungsten carbide all commercially available from Pred Materials International Incorporated.
The casting composition comprises the ceramic or metal powder in an amount of about 5 to about 95 weight percent (wt %), based upon the weight of the casting composition. In one embodiment, the casting composition comprises the ceramic or metal powder in an amount of about 20 to about 85 wt % based upon the weight of the casting composition. In another embodiment, the casting composition comprises the ceramic or metal powder in an amount of about 30 to about 75 wt % based upon the weight of the casting composition. Commercially available ceramic powders (e.g., whitewares, alumina, mullite, zircon, silicon nitride, silicon carbide) can be used in an amount of about 40 to about 90 wt %, based upon the weight of the casting composition.
In one embodiment, a binder comprising a monomer and/or polymer is used in the casting composition to form a slurry mixture. The monomer and/or polymer may be used in conjunction with a solvent to form a monomer and/or polymer solution. The monomer and/or polymer solution provides a low viscosity vehicle for the inorganic powder. Additionally, when heated, the monomer solution polymerizes and gels to form a firm, strong polymer/solvent gel matrix. The gel matrix immobilizes the inorganic powder in the desired shape of the mold in which the slurry mixture is heated. The polymer can be an oligomer or a high molecular weight polymer. Exemplary monomers, oligomers or high molecular weight polymers include siloxanes as detailed below.
An exemplary siloxane monomer, oligomer or high molecular weight polymer is one having an alkenyl functionality as shown in the formula (I)
wherein R1, R2, and R3 each independently comprise hydrogen or a monovalent hydrocarbon radical, X a divalent hydrocarbon radical and a is a whole number having a value between 0 and 8, inclusive, and a hydride functionality consisting of silicon-hydrogen bonds; and a hydride functionality consisting of silicon-hydrogen bonds; adding a metallic catalyst compound to the ceramic slurry; and cross linking and/or polymerizing the siloxane monomers, oligomers and/or high molecular weight polymers to form the internal gel matrix. The terms “monovalent hydrocarbon radical” and “divalent hydrocarbon radical” as used herein are intended to designate straight chain alkyl, branched alkyl, aralkyl, cycloalkyl, and bicycloalkyl radicals.
The siloxane hydride monomers, oligomers and/or high molecular weight polymers are hydrosiloxanes having hydrogen directly bonded to one or more of the silicon atoms, and therefore contain a reactive Si—H functional group.
Cross-linking of the siloxane monomers, oligomers and/or high molecular weight polymers may be accomplished by utilizing a metal catalyzed reaction of the siloxane alkenyl groups and the silicon bonded hydrogen groups. The metal catalyst, preferably a platinum group metal catalyst, can be selected from such catalysts that are conventional and well known in the art. Suitable metallic catalysts include, but are not intended to be limited to, the Pt divinylsiloxane complexes as described by Karstedt in U.S. Pat. No. 3,715,334 and U.S. Pat. No. 3,775,452; Pt-octyl alcohol reaction products as taught by Lamoreaux in U.S. Pat. No. 3,220,972; the Pt-vinylcyclosiloxane compounds taught by Modic in U.S. Pat. No. 3,516,946; and Ashby's Pt-olefin complexes found in U.S. Pat. Nos. 4,288,345 and 4,421,903.
Exemplary alkenyl siloxanes useful in the present disclosure include polyfunctional olefinic substituted siloxanes of the following types:
wherein R is a monovalent hydrocarbon, R′ is an alkenyl radical such as vinyl, or other terminal olefinic group such as allyl, 1-butenyl, and the like. R″ may include R or R′, a=0 to 20, inclusive, and b=1 to 80, inclusive, and such that the ratio of b/a allows for at least three reactive olefinic moieties per mole of siloxane of formula (II) above.
Suitable alkyl/alkenyl cyclosiloxanes are of formula (III):
[RR′SiO]x, (III)
wherein R and R′ are as previously defined, and x is an integer 3 to 18 inclusive.
Other suitable functional unsaturated siloxanes may be of the formula (IV):
wherein R, R′, and R″ are as previously defined. Preferably, the ratio of the sum of (c+d+e+g)/f is ≧2.
Exemplary unsaturated siloxanes include 1,3-divinyl-tetramethyldisiloxane, hexavinyldisiloxane, 1,3-divinyltetraphenyldisiloxane, 1,1,3-trivinyltrimethyldisiloxane, 1,3-tetravinyldimethyldisiloxane, and the like. Exemplary cyclic alkyl-or arylvinylsiloxanes include 1,3,5-trivinyl-1,3,5-tri-methylcyclotrisiloxane, 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane, 1,3-divinyloctaphenylcyclopentasiloxane, and the like.
Suitable polyfunctional hydride siloxanes include compositions depicted below:
wherein R is as defined previously, R′″ may include R or H, and a and b are defined as above, and selected such that the ratio of b/a allows for at least three reactive Si—H moieties per mole of siloxane of formula (V) above.
Suitable alkyl/hydride cyclosiloxanes of formula:
[HRSiO]x, (VI)
wherein R is as previously defined, and x is an integer 3 to 18 inclusive.
Other suitable functional hydride siloxanes include:
wherein R and R′″ are as previously defined. In one embodiment, the ratio of the sum of (c+d+e+g)/f is ≧2.
Exemplary siloxane hydrides include poly(methylhydrogen)siloxane, poly[(methylhydrogen)-co-(dimethyl)]siloxane; 1,3,5,7-tetramethylcyclotetrasiloxane, 1,3,5,7,9-decamethylcyclopentasiloxane, and other cyclic methylhydrogen siloxanes; tetrakis(dimethylsiloxy)silane, and organically modified resinous hydride functional silicates corresponding to Formula (VII), with the composition [HSi(CH3)2O1/2]2 (SiO2).
The matrix for the slurry mixture may be selected so as to include at least one alkenyl and hydride siloxane as described above.
Additional terminally functional alkenyl or hydride siloxanes described below in formulas (VIII) and (IX), alone or in combination, may be added to augment the matrix composition in order to lower the viscosity of the uncrosslinked matrix, effect changes in the cured green body hardness and strength, and so on, as would be apparent to those skilled in the art in view of the present disclosure.
wherein R and R′ are as previously defined; and n=0 to 100, preferably 0 to 30, and most preferably 0 to 10.
It should also be apparent that a satisfactory crosslinked network may be effected in the course of this disclosure, by combining one component from each of A) a polyfunctional alkenyl or polyfunctional hydride siloxane, as defined in Formulas (II)-(IV) or Formulas (V)-(VII), respectively; and B) a terminally functional alkenyl or hydride siloxane as defined in Formulas (VIII) or (IX) respectively, restricted only such that the composition contains both an alkenyl and a hydride functional species to allow crosslinking between the complementary alkenyl and hydride reactive functional groups.
An exemplary acrylamide is hydroxymethylacrylamide, N-hydroxy-methacrylacrylamide (HMAM). Another exemplary acrylamide is N.N′-methylenebisacrylamide (MBAM).
In another embodiment, exemplary monomers for use in the casting composition are those that can be reacted to form epoxies, phenol-formaldehydes, epoxy-modified novolacs, furans, urea-aldehydes, melamine-aldehydes, polyester resins, alkyd resins, phenol formaldehyde novolacs, phenol formaldehyde resoles, phenol-aldehydes, resole and novolac resins, epoxy modified phenolics, polyacetals, polyurethanes, or a combination comprising at least one of the foregoing.
In one embodiment, the monomers used in the casting composition can be used to form cold setting resins. Cold setting resins are those that can react at room temperature without the use of additional heat. Cold setting resins generally cure at a temperature of less than or equal to about 65° C. Thus, for example, a thermosetting polymer that cures at 80° C. is not a cold setting resin. Examples of suitable cold setting resins include epoxies cured with an amine when used alone or with a polyurethane, alkaline modified resoles set by esters (e.g., ALPHASET® and BETASET®), furans, e.g., furfuryl alcohol-formaldehyde, urea-formaldehyde, and free methylol-containing melamines set with acid. For the purposes of this description, a cold set resin is any resin that can normally be cured at about room temperature.
The monomers used in the casting composition may be used in an amount of 1 to about 60 wt %, based on the total weight of the casting composition. In one embodiment, the monomers used in the casting composition may be used in an amount of about 2 to about 55 wt %, based on the total weight of the casting composition. In another embodiment, the monomers used in the casting composition may be used in an amount of about 3 to about 40 wt %, based on the total weight of the casting composition.
It is desirable for the monomers to have a viscosity of about 1 to about 1,000 Pascal-seconds (Pa-s) when measured at room temperature at a shear rate of up to 100 seconds−1. An exemplary viscosity for the monomers is about 2 to about 10 Pascal-seconds (Pa-s) when measured at room temperature at a shear rate of about 10 to about 70 seconds−1.
Thermoplastic polymers may optionally be used in the casting composition. As noted above, the thermoplastic polymers can be oligomers or high molecular weight polymers, or combinations thereof. While it is generally desirable for thermoplastic polymers to be used in the casting composition to have glass transition temperatures (Tg's) that are less than or equal to about 50° C., thermoplastic polymers having Tg's greater than or equal to about 50° C. may be used depending upon the solvents employed in the casting composition. Exemplary polymers having Tg's less than or equal to about 50° C., are polysiloxanes, polyalkylene glycols and polyacrylamides.
Examples of suitable polyalkylene glycols are polytetramethylene oxide, polyethylene glycol, or the like, or a combination comprising at least one of the aforementioned polyalkylene glycols.
Other thermoplastic polymers having glass transition temperatures below 50° C. that may be used in the casting composition are thermoplastic elastomers.
Examples of suitable thermoplastic elastomers are ionomers, block copolymers, or a combination comprising at least one of the aforementioned thermoplastic elastomers. An exemplary commercially available thermoplastic ionomers is SURLYN® manufactured by Du Pont. An exemplary thermoplastic block copolymer is a styrene butadiene rubber (SBR) comprising a diblock or triblock copolymer.
Thermoplastic polymers that have Tg's above 50° C., that may be used in the casting composition are polyethylene terephthalate, polyvinyl alcohol, polymethylmethacrylate, polystyrene, polycarbonate, polyvinyl chloride, cellulose, a cellulose derivative, polyacrylic acid, or the like, or a combination comprising at least one of the aforementioned thermoplastic polymers.
Other exemplary polymers that can be added to the casting composition are natural polymers. An example of a natural polymer that can be added to the casting composition is a polysaccharide. Exemplary polysaccharides are agar, xanthan gum, starch, locust bean gum, or the like, or a combination comprising one of the aforementioned polysaccharides. Another example of a natural polymer that can be added to the casting composition is a protein, for example, gelatin and albumin.
If a thermoplastic polymer is used in the casting composition, it is desirable for the polymer to have a viscosity of about 1 to about 1,000 Pascal-seconds (Pa-s) when measured at room temperature at a shear rate of up to 100 seconds−1. An exemplary viscosity for the monomers is about 2 to about 10 Pascal-seconds (Pa-s) when measured at room temperature at a shear rate of about 10 to about 70 seconds−1.
If a thermoplastic polymer used in the casting composition, it may be used in an amount of 1 to about 70 wt %, based on the total weight of the casting composition. In one embodiment, the thermoplastic polymer used in the casting composition may be used in an amount of about 2 to about 60 wt %, based on the total weight of the casting composition. In another embodiment, the thermoplastic polymer used in the casting composition may be used in an amount of about 3 to about 55 wt %, based on the total weight of the casting composition.
Solvents may also be added to the casting composition as desired. An exemplary solvent is water. Other organic solvents having a polar protic character, polar aprotic character or non-polar character may be used as desired. Solvents can generally be used in amounts of about 1 to about 50 wt %, based on the weight of the casting composition. In one embodiment, the solvent may be used in amounts of about 2 to about 45 wt %, based on the weight of the casting composition. In another embodiment, the solvent may be used in amounts of about 3 to about 40 wt %, based on the weight of the casting composition.
An initiator can be added to the casting composition in order to activate polymerization of the monomer. The initiator may be a free-radical initiator. Examples of suitable free-radical initiators include ammonium persulfate, ammonium persulfate and tetramethylethylenediamine mixtures, sodium persulfate, sodium persulfate and tetramethylethylenediamine mixtures, potassium persulfate, potassium persulfate and tetramethylethylenediamine mixtures, azobis[2-(2-imidazolin-2-yl) propane] HCl (AZIP), and azobis(2-amidinopropane) HCl (AZAP), 4,4′-azo-bis-4-cyanopentanoic acid, azobisisobutyramide, azobisisobutyramidine.2HCl, 2-2′-azo-bis-2-(methylcarboxy) propane, 2-hydroxy-1-[4-(hydroxyethoxy) phenyl]-2-methyl-1-propanone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, or the like, or a combination comprising at least one of the aforementioned free-radical initiators. Some additives or comonomers can also initiate polymerization, in which case a separate initiator may not be desired. The initiator can control the reaction in addition to initiating it. The initiator is used in amounts of about 0.005 wt % and about 0.5 wt %, based on the weight of the casting composition.
Other initiator systems, in addition to free-radical initiator systems, can also be used in the casting composition. These include ultraviolet (UV), x-ray, gamma-ray, electron beam, or other forms of radiation, which could serve as suitable polymerization initiators.
Dispersants, flocculants, and suspending agents can also be optionally added to casting composition to control the flow behavior of the composition. Dispersants make the composition flow more readily, flocculants make the composition flow less readily, and suspending agents prevent particles from settling out of composition. These additives are generally used in amounts of about 0.01 to about 10 wt %, based on the total weight of the ceramic or metal powder in the casting composition.
Various dispersants for inorganic powders can also be added to the casting composition. It is generally desirable for the dispersant not to interact with the initiator. Examples of suitable dispersants include inorganic acids, inorganic bases, organic acids, organic bases, polyacrylic acid, salts of polyacrylic acid, polymethacrylic acid, salts of polymethacrylic acid, copolymers of polyacrylic acid, salts of copolymers of polyacrylic acid, copolymers of polymethacrylic acid, salts of copolymers of polymethacrylic acid, polyethylene imine, polyvinylpyrrolidone, polyacrylamide, lignosulfonates, poly (ethylene oxide), adducts of ethylene oxide, adducts of propylene oxide, polycarboxylates, salts of polycarboxylates, naphthalene sulfonates, sulfosuccinates, polyphosphates, sodium silicates, phosphate esters, or the like, or a combination comprising at least one of the aforementioned dispersants.
Other additives can be included in order to modify the gel properties of the casting composition. Examples of suitable additives include plasticizers to modify the mechanical properties of the gel in the wet and dry states, electrolytes, defoamers, bactericides, fungicides, soluble functional polymers, inorganic particles or fibers. Soluble functional polymers are any polymeric species that are added to the gel or to the gel precursor to modify the properties of the gel or the gel precursor. These might include emulsifiers, dispersants, thickeners, polyeiectrolytes, chelating agents, foaming agents, suspending agents, or a combination comprising at least one of the aforementioned soluble functional properties.
The level of additive used in a particular casting composition can vary widely. It will depend directly on the role that the additive is playing in the casting composition. For example, one might add a plasticizer to the monomer solution to provide a more compliant polymer in the gelled and dried state. The plasticizer content would be added on the order of several percent, based on the weight of the dried gel (dried gels are obtained after the casting composition is dried). A bactericide can be added to the monomer solution to prevent growth of bacteria during storage. This would be added in parts per million, based on the weight of the casting composition. Foam control agents are added to casting compositions to either remove bubbles or form bubbles in the composition. Foam control agents are generally added at from about 0.01 to about 2 wt %, based on the weight of the casting composition.
It is desirable for the casting composition to have a viscosity of about 1 to about 1,000 Pascal-seconds (Pa-s), preferably about 3 to about 300 Pa-s and more preferably about 5 to about 100 Pa-s, when measured at room temperature at a shear rate up to 100 seconds−1. In one exemplary embodiment, it is desirable for the casting composition to have a viscosity of about 2 to 10 Pa-s, when measured at room temperature at a shear rate of up to about 10 to about 70 seconds−1.
It is also desirable for the casting composition to have a cure temperature that is less than the melting point or the degradation temperature of the sacrificial die. In one embodiment, it is desirable for the casting composition to undergo up to 90 mole percent (mol %) curing at a temperature that is less than 50° C., specifically less than 30° C., and more specifically less than or equal to about 20° C. In another embodiment, it is desirable for the casting composition to undergo up to 95 mol % curing at a temperature that is less than 50° C., specifically less than 30° C., and more specifically less than or equal to about 20° C. In yet another embodiment, it is desirable for the casting composition to undergo up to 98 mol % curing at a temperature that is less than 50° C., specifically less than 30° C., and more specifically less than or equal to about 20° C.
In one embodiment, it is desirable for the casting composition to have a shrinkage of less than or equal to about 1 volume percent (vol %), specifically less than or equal to about 0.75 vol %, more specifically less than or equal to about 0.5 vol %, upon undergoing up to 90 mol % curing. In another embodiment, it is desirable for the casting composition to have a shrinkage of less than or equal to about 1 vol %, specifically less than or equal to about 0.75 vol %, more specifically less than or equal to about 0.5 vol %, upon undergoing up to 95 mol % curing. In yet another embodiment, it is desirable for the casting composition to have a shrinkage of less than or equal to about 1 vol %, specifically less than or equal to about 0.75 vol %, more specifically less than or equal to about 0.5 vol %, upon undergoing up to 98 mol % curing.
Using a casting composition having a viscosity of about 10 to about 1,000 Pa-s permits the cavities of the die to be easily filled without using excessive pressure during an injection molding process. Excessive pressure may cause distortion of the sacrificial die during injection molding processes, depending upon the mechanical strength of the resin used in the sacrificial die. The casting composition has a viscosity low enough, that generally no pressure need be applied to the composition when it is used for manufacturing a ceramic core. In one embodiment, if pressure is to be used on the casting composition during an injection molding process it is desirable to use a pressure of about 1 to about 10 pounds per square inch (psi). In another embodiment, if pressure is to be used on the casting composition during an injection molding process it is desirable to use a pressure of about 2 to about 5 pounds per square inch (psi).
It is also desirable for the casting composition to be capable of curing to a gel that is capable of holding its own weight during deplasticization. In other words, the gel obtained from the casting composition has a strength that permits it to withstand capillary forces that occur within the gel during deplasticization. In one embodiment, the gel is capable of withstanding a pressure of about 0.2 to about 10 pounds per square inch (psi) during deplasticization. In another embodiment, the gel is capable of withstanding a pressure of about 0.5 to about 5 psi during deplasticization. In another embodiment, the gel is capable of withstanding a pressure of about 0.8 to about 2 psi during deplasticization.
In one embodiment, in one manner of manufacturing a ceramic core, the metallic or ceramic powder, the monomer and/or the polymer, the solvent, and optional additives may be combined in a suitable manner to form the casting composition. The casting composition formed by mixing the ceramic or metallic powder, the monomer and/or polymer, and the solvent is generally in the form of a slurry. In an exemplary embodiment, the casting composition is formed by dissolving a dispersant in the monomer and/or polymer to form a monomer and/or polymer solution followed by the addition of the inorganic powder and the solvent to the monomer and/or polymer solution. An initiator may be added to the casting composition if desired. The resultant casting composition is then poured or injected into the sacrificial die. As noted above, the internal features of the sacrificial die provide a replica of the desired metal casting. After the casting composition is formed into a desired shape, it is heated for a temperature and a time sufficient for the monomer and/or polymer and any optional comonomer to polymerize and gel to form a firm gel matrix.
The temperature at which the polymerization occurs depends on the initiator, as well as the monomers and/or comonomers that are used in the casting composition. The polymerization reaction is generally conducted at temperatures between the freezing point and the boiling point of the solvent being used. Heating activates the free-radical initiator, and a temperature of about 50° C. can be used to activate polymerization in many systems. Generally, polymerization temperatures of about 1° C. to about 100° C. are desirable. In one embodiment, the polymerization is conducted at a temperature of about 15° C. to about 80° C.
The gel time to form a firm gel matrix is dependent on the combination of monomers and/or polymers, the weight percent of the metallic or ceramic particles, the solvent and the type of the initiator. In general, it is desirable for the casting composition to be heated for a time period of greater than or equal to about 1 minute. In one embodiment, the slurry is heated for a period of from about 1 to about 120 minutes in order to polymerize the monomers and form a firm gel matrix. In another embodiment, the slurry is heated for a period of from about 2 to about 90 minutes in order to polymerize the monomers and form a firm gel matrix.
The gel matrix can be formed under vacuum, or at pressures greater than atmospheric, and as high as up to about 30 pounds per square inch (psi). The reaction can be carried out at atmospheric pressure, although other pressures can be utilized to produce gel matrices having different properties. After heating, the resultant shaped, solid gel matrix may be cooled to ambient temperature. The gel matrix is in a wet, green condition in that it contains plasticizer and/or solvent and is in the unfired form. The green product may subsequently be heated in order to substantially remove the solvent (undergo deplasticization) to provide a dried gel. The specific temperature and time suitable for producing the dried gel depends on the specific metallic or ceramic powder and monomer employed. Initially, drying should be conducted at a temperature such that evaporation is not too rapid. Consequently, the initial drying temperature will generally be closer to the melting point than to the boiling point of the solvent. As the drying process proceeds, the temperature may be raised to provide faster drying rates. In order to drive off the last traces of solvent from the gel matrix, temperatures in excess of the boiling point of water may be used. In general, drying can be conducted for at least one hour to about 30 hours to obtain the dried gel.
As noted above, at this stage, the sacrificial die may either be removed from the dried gel or may be dipped into a second slurry in an investment casting process. The sacrificial die is then removed after the investment casting process. These processes for further development of the sacrificial die and the gel matrix will not be detailed again here since they are already described above.
The sacrificial die can be removed by degrading it. In another embodiment, the sacrificial die may be removed by dissolution. In yet another embodiment, the sacrificial die may be removed by abrasion or etching. The dried gel may then be removed from the die and subjected to firing in order to decompose the polymer formed by the polymerization of monomer. The dried gel after firing will hereinafter be referred to as a ceramic core.
In one embodiment, the sacrificial die comprises a low-melting point thermoplastic wax and the sacrificial die can be removed by heating to a low temperature of less than about 180° C., specifically less than about 150° C., more specifically less than about 120° C. In one embodiment, the internal gel matrix undergoes substantially no chemical or physical changes during sacrificial die removal process.
In one embodiment, the sacrificial die comprises a high-melting point thermoplastic, a high-melting point thermosetting polymer, or a combination comprising at least one of the foregoing polymers. According to this embodiment, the sacrificial die removal process, curing of the casting composition, as well as pyrolysis of the organic component of the casting composition, is carried out in one continuous process through a series of ramp and hold steps, up to the final ceramic core firing/sintering temperature.
After the removal of the gel matrix, the firing of the gel matrix is generally conducted at a temperature of greater than or equal to about 300° C. In one embodiment, no shrinkage takes place during firing. As noted above, this method is advantageous in that the wall thickness of the die can be controlled. Another advantage is that the mold can be designed to maximize strength, thereby reducing mold failures.
In one advantageous embodiment, this method of making castings can be used to manufacture castings having cross-sectional areas less than or equal to about 2 square centimeters (cm2). In one embodiment, cross-sectional areas of less than or equal to about 1 cm2 can be manufactured by this method. In another embodiment, cross-sectional areas of less than or equal to about 0.5 cm2 can be manufactured by this method.
In another advantageous embodiment, castings manufactured from the casting composition can have a wall thickness of less than or equal to about 2,000 micrometers, specifically less than or equal to about 1,000 micrometers, more specifically less than or equal to about 800 micrometers. In one embodiment, wall thicknesses of greater than or equal to about 500 micrometers can be manufactured using the casting compositions described herein.
As noted above, molds manufactured from the casting composition may be used for molding metal castings. In one exemplary embodiment, the cast molds may be used for manufacturing turbine components. These turbine components can be used in either power generation turbines such as gas turbines, hydroelectric generation turbines, steam turbines, or the like, or they may be turbines that are used to facilitate propulsion in aircraft, locomotives, or ships. Examples of turbine components that may be manufactured using ceramic cores are stationary and/or rotating airfoils. Examples of other turbine components that may be manufactured using ceramic cores are seals, shrouds, splitters, or the like.
The following examples, which are meant to be exemplary, not limiting, illustrate compositions and methods of manufacturing of some of the various embodiments of the casting composition described herein.
This experiment was conducted to demonstrate the viscosity of a casting composition that may be advantageously used for manufacturing a cured gel and a ceramic core. A casting composition comprising a polysiloxane binder and a silica-zircon ceramic powder mixture was tested. The polysiloxane binder contained a hydride functional and a vinyl functional siloxane, both obtained from General Electric Silicones. The hydride functional siloxane corresponded to formula VII with c˜2 and f˜1, and the vinyl functional siloxane component corresponded to formula III with x˜4 (commercial designation TSLT465). The relative proportions of hydride and vinyl-functional siloxane reflect a H/Vi functional group molar ratio equal to one.
Fused silica (Pemco 325/F10), fumed silica (Degussa OX50) and zircon (325 mesh) were combined with the siloxane binder in quantities resulting in ceramic powder volume loadings of 37%, 1%, and 16%, respectively.
The polysiloxane binder was blended with the silica and zircon powders in a dual asymmetric centrifuge (Hauschild Model DAC-400FV). The blending was conducted at room temperature at 2750 rpm to produce the casting composition.
The viscosity of the casting composition was then determined in a Paar-Physica UDS 200 controlled stress rheometer at room temperature at a shear rate between 0 and 70 seconds−1 and the results are plotted in a graph depicted in
This example was conducted to demonstrate the advantages of the disclosed casting composition over comparative compositions disclosed in a technical publication titled “Rheological Properties of Alumina Injection Feedstocks”, Materials Research, Vol. 8, No. 2. 187-189, (2005) to Fredel et al. In the publication, formulations containing 55 to 59 vol % of alumina powder were prepared in order to study the effect of solid loads on the Theological behavior of alumina injection molding feedstocks. As disclosed in the publication, the binders used were polyethylene glycol (PEG), polyvinylbutyral (PVB) and stearic acid (SA). The publication discloses that the binder volume ratio was PEG:PVB::2:1 and Al2O3:SA::25:1. A composition from the publication comprising 55 volume percent filler (having feedstock abbreviation F55) was tested in a numerical simulation against the casting composition of Example 1. The simulation comprised pouring the respective compositions into a numerical model of a mold and computing the resulting pressures at the gate node. The gate node is the mouth of the mold. The data was plotted in the
The casting composition of this disclosure on the other hand flows uniformly through the mold. The gate node does not serve to hinder the flow of the casting composition. From the
The viscosity of a composition is generally mathematically represented by equation (I) as
η=k(γ′)n−1 (I)
where η is the viscosity, γ′ is the shear rate, k is a constant and n represents the flow index.
The viscosity-shear rate data from
η=9000(γ′)−0.6 (II)
while for the casting compositions of this disclosure, it was determined that the viscosity could be determined by equation (III)
η=10(γ′)−0.49 (III)
Thus it may be seen that the flow index for the casting compositions of this disclosure have a value of less than 0.6.
The compositions can advantageously be used to manufacture complex parts for turbines. In one embodiment, the flow index for the casting compositions of this disclosure have a value of less than or equal to about 0.55, more preferably less than or equal to about 0.50. These turbine components can be used in either power generation turbines such as gas turbines, hydroelectric generation turbines, steam turbines, or the like, or they may be turbines that are used to facilitate propulsion in aircraft, locomotives, or ships.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention.
