Patent Application
20180161852
The present disclosure generally relates to investment casting core-shell mold components and processes utilizing these components. The core-shell mold made in accordance with the present invention includes integrated hollow ceramic core tubes that are used to define at least a portion of an interior region of a cast article. In the case of a turbine blade, the tubes may define the inner core region, and/or may define a portion of a ceramic core filament for producing a cooling hole in the turbine blade. The use of sufficient ceramic filaments between core and shell to both locate and provide leaching pathways for the core serpentine also enables the elimination of ball braze chutes. Ceramic filaments between the tip plenum core and the shell may also be provided to support a floating tip plenum, eliminating the need for traditional tip pins, and their subsequent closure by brazing. The integrated core-shell molds provide useful properties in casting operations, such as in the casting of superalloys used to make turbine blades and stator vanes for jet aircraft engines or power generation turbine components.
Many modern engines and next generation turbine engines require components and parts having intricate and complex geometries, which require new types of materials and manufacturing techniques. Conventional techniques for manufacturing engine parts and components involve the laborious process of investment or lost-wax casting. One example of investment casting involves the manufacture of a typical rotor blade used in a gas turbine engine. A turbine blade typically includes hollow airfoils that have radial channels extending along the span of a blade having at least one or more inlets for receiving pressurized cooling air during operation in the engine. The various cooling passages in a blade typically include a serpentine channel disposed in the middle of the airfoil between the leading and trailing edges. The airfoil typically includes inlets extending through the blade for receiving pressurized cooling air, which include local 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 manufacture of these turbine blades, typically from high strength, superalloy metal materials, involves numerous steps shown in
The cast turbine blade may then undergo additional post-casting modifications, such as but not limited to drilling of suitable rows of film cooling holes through the sidewalls of the airfoil as desired for providing outlets for the internally channeled cooling air which then forms a protective cooling air film or blanket over the external surface of the airfoil during operation in the gas turbine engine. After the turbine blade is removed from the ceramic mold, the ball chute 203 of the ceramic core 200 forms a passageway that is later brazed shut to provide the desired pathway of air through the internal voids of the cast turbine blade. However, these post-casting modifications are limited and given the ever increasing complexity of turbine engines and the recognized efficiencies of certain cooling circuits inside turbine blades, more complicated and intricate internal geometries are required. While investment casting is capable of manufacturing these parts, positional precision and intricate internal geometries become more complex to manufacture using these conventional manufacturing processes. Accordingly, it is desired to provide an improved casting method for three dimensional components having intricate internal voids.
Methods for using 3-D printing to produce a ceramic core-shell mold are described in U.S. Pat. No. 8,851,151 assigned to Rolls-Royce Corporation. The methods for making the molds include powder bed ceramic processes such as disclosed U.S. Pat. No. 5,387,380 assigned to Massachusetts Institute of Technology, and selective laser activation (SLA) such as disclosed in U.S. Pat. No. 5,256,340 assigned to 3D Systems, Inc. The ceramic core-shell molds according to the '151 patent are limited by the printing resolution capabilities of these processes. As shown in
There remains a need to prepare ceramic core-shell molds produced using higher resolution methods that are capable of providing fine detail cast features in the end-product of the casting process.
In one embodiment, the invention relates to a method of making a ceramic mold having a core and shell. The method having steps of (a) contacting a cured portion of a workpiece with a liquid ceramic photopolymer; (b) irradiating a portion of the liquid ceramic photopolymer adjacent to the cured portion through a window contacting the liquid ceramic photopolymer; (c) removing the workpiece from the uncured liquid ceramic photopolymer; and (d) repeating steps (a)-(c) until a ceramic mold is formed, the ceramic mold comprising: (1) a core portion and a shell portion with at least one cavity between the core portion and the shell portion, the cavity adapted to define the shape of a cast component upon casting and removal of the ceramic mold, and (2) a plurality of filaments joining the core portion and the shell portion where each filament spans between the core and shell and defines a hole in the cast component upon removal of the mold, wherein at least a portion of the filament and/or the core portion is in the shape of a hollow tube. After step (d), the process may further include a step (e) of pouring a liquid metal into a casting mold and solidifying the liquid metal to form the cast component. After step (e), the process may further include a step (f) comprising removing the mold from the cast component, and this step preferably involves a combination of mechanical force and chemical leaching in an alkaline bath.
In another aspect, the invention relates to a method of preparing a cast component. The method includes steps of pouring a liquid metal into a ceramic casting mold and solidifying the liquid metal to form the cast component, the ceramic casting mold comprising (1) a core portion and a shell portion with at least one cavity between the core portion and the shell portion, the cavity adapted to define the shape of a cast component upon casting and removal of the ceramic mold, and (2) a plurality of filaments joining the core portion and the shell portion where each filament spans between the core and shell and defines a hole in the cast component upon removal of the mold, wherein at least a portion of the filament and/or the core portion is in the shape of a hollow tube; and removing the ceramic casting mold from the cast component by leaching at least a portion of the ceramic core through the holes in the cast component provided by the filaments.
In one aspect, the cast component is a turbine blade or stator vane. Preferably the turbine blade or stator vane is used in a gas turbine engine in, for example, an aircraft engine or power generation. The turbine blade or stator vane is preferably a single crystal cast turbine blade or stator vane having a cooling hole pattern defined by the ceramic filaments mentioned above. Preferably, the filaments join the core portion and shell portion where each filament spans between the core and shell, the filaments having a cross sectional area ranging from 0.01 to 2 mm2.
The large number of filaments used to form a cooling hole pattern may provide sufficient strength to support the tip core. If the tip filaments are made to support tip plenum core, they may be made larger, i.e., >2 mm cross section area, and a much lower number of filaments, or a single filament, could be used. Although two to four of these larger filaments is a desirable number. After casting, any holes or notches remaining in the tip plenum sidewalls as a result of the filaments may be brazed shut or incorporated into the turbine blade or stator vane design, or the filaments may be placed outside the finish machined shape of the component to prevent the need for this.
In another aspect, the invention relates to a ceramic casting mold having (1) a core portion and a shell portion with at least one cavity between the core portion and the shell portion, the cavity adapted to define the shape of a cast component upon casting and removal of the ceramic mold, and (2) a plurality of filaments joining the core portion and the shell portion where each filament spans between the core and shell and defines a hole in the cast component upon removal of the mold, wherein at least a portion of the filament and/or the core portion is in the shape of a hollow tube. Preferably, the cast component is a turbine blade or stator vane and the plurality of filaments joining the core portion and shell portion define a plurality of cooling holes in the turbine blade upon removal of the mold. Preferably, the plurality of filaments joining the core portion and shell portion have a cross sectional area ranging from 0.01 to 2 mm2. The ceramic may be a photopolymerized ceramic or a cured photopolymerized ceramic.
The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. For example, the present invention provides a preferred method for making cast metal parts, and preferably those cast metal parts used in the manufacture of jet aircraft engines. Specifically, the production of single crystal, nickel-based superalloy cast parts such as turbine blades, vanes, and shroud components can be advantageously produced in accordance with this invention. However, other cast metal components may be prepared using the techniques and integrated ceramic molds of the present invention.
The present inventors recognized that prior processes known for making integrated core-shell molds lacked the fine resolution capability necessary to print filaments extending between the core and shell portion of the mold of sufficiently small size and quantity to result in effusion cooling holes in the finished turbine blade. In the case of earlier powder bed processes, such as disclosed in U.S. Pat. No. 5,387,380 assigned to Massachusetts Institute of Technology, the action of the powder bed recoater arm precludes formation of sufficiently fine filaments extending between the core and shell to provide an effusion cooling hole pattern in the cast part. Other known techniques such as selective laser activation (SLA) such as disclosed in U.S. Pat. No. 5,256,340 assigned to 3D Systems, Inc. that employ a top-down irradiation technique may be utilized in producing an integrated core-shell mold in accordance with the present invention. However, the available printing resolution of these systems significantly limit the ability to make filaments of sufficiently small size to serve as effective cooling holes in the cast final product.
The present inventors have found that the integrated core-shell mold of the present invention can be manufactured using direct light processing (DLP). DLP differs from the above discussed powder bed and SLA processes in that the light curing of the polymer occurs through a window at the bottom of a resin tank that projects light upon a build platform that is raised as the process is conducted. With DLP an entire layer of cured polymer is produced simultaneously, and the need to scan a pattern using a laser is eliminated. Further, the polymerization occurs between the underlying window and the last cured layer of the object being built. The underlying window provides support allowing thin filaments of material to be produced without the need for a separate support structure. In other words, producing a thin filament of material bridging two portions of the build object is difficult and was typically avoided in the prior art. For example, the '151 patent discussed above in the background section of this application used vertical plate structures connected with short cylinders, the length of which was on the order of their diameter. Staggered vertical cavities are necessitated by the fact that the powder bed and SLA techniques disclosed in the '151 patent require vertically supported ceramic structures and the techniques are incapable of reliably producing filaments. In addition, the available resolution within a powder bed is on the order of ⅛″ making the production of traditional cooling holes impracticable. For example, round cooling holes generally have a diameter of less than 2 mm corresponding to a cooling hole area below 3.2 mm2. Production of a hole of such dimensions requires a resolution far below the size of the actual hole given the need to produce the hole from several voxels. This resolution is simply not available in a powder bed process. Similarly, stereolithography is limited in its ability to produce such filaments due to lack of support and resolution problems associated with laser scattering. But the fact that DLP exposes the entire length of the filament and supports it between the window and the build plate enables producing sufficiently thin filaments spanning the entire length between the core and shell to form a ceramic object having the desired cooling hole pattern. Although powder bed and SLA may be used to produce filaments, their ability to produce sufficiently fine filaments as discussed above is limited.
One suitable DLP process is disclosed in U.S. Pat. No. 9,079,357 assigned to Ivoclar Vivadent AG and Technische Universitat Wien, as well as WO 2010/045950 A1 and US 2011310370, each of which are hereby incorporated by reference and discussed below with reference to
Opposite the exposure unit 410, a production platform 412 is provided above the tank 404; it is supported by a lifting mechanism (not shown) so that it is held in a height-adjustable way over the tank bottom 406 in the region above the exposure unit 410. The production platform 412 may likewise be transparent or translucent in order that light can be shone in by a further exposure unit above the production platform in such a way that, at least when forming the first layer on the lower side of the production platform 412, it can also be exposed from above so that the layer cured first on the production platform adheres thereto with even greater reliability.
The tank 404 contains a filling of highly viscous photopolymerizable material 420. The material level of the filling is much higher than the thickness of the layers which are intended to be defined for position-selective exposure. In order to define a layer of photopolymerizable material, the following procedure is adopted. The production platform 412 is lowered by the lifting mechanism in a controlled way so that (before the first exposure step) its lower side is immersed in the filling of photopolymerizable material 420 and approaches the tank bottom 406 to such an extent that precisely the desired layer thickness Δ (see
These steps are subsequently repeated several times, the distance from the lower side of the layer 422 formed last to the tank bottom 406 respectively being set to the desired layer thickness Δ and the next layer thereupon being cured position-selectively in the desired way.
After the production platform 412 has been raised following an exposure step, there is a material deficit in the exposed region as indicated in
In order to replenish the exposure region with photopolymerizable material, an elongate mixing element 432 is moved through the filling of photopolymerizable material 420 in the tank. In the exemplary embodiment represented in
The movement of the elongate mixing element 432 relative to the tank may firstly, with a stationary tank 404, be carried out by a linear drive which moves the support arms 430 along the guide slots 434 in order to achieve the desired movement of the elongate mixing element 432 through the exposed region between the production platform 412 and the exposure unit 410. As shown in
Other alternative methods of DLP may be used to prepare the integrated core-shell molds of the present invention. For example, the tank may be positioned on a rotatable platform. When the workpiece is withdrawn from the viscous polymer between successive build steps, the tank may be rotated relative to the platform and light source to provide a fresh layer of viscous polymer in which to dip the build platform for building the successive layers.
The mold core 900 may also be a hollow mold core in accordance with certain aspects of the invention. The hollow core has an inside diameter 908 and an outside diameter 909. In general, the cross sectional area defined by the inner diameter is greater than 80% of the cross sectional area of the outer diameter, preferably greater than 90%. In the case where the mold core does not have a cylindrical shape, the wall thickness of the hollow core is equivalent to that of the cylinder where the inner diameter is greater than 80% of the cross sectional area of the outer diameter.
The filaments 902 are preferably cylindrical or oval shape but may be curved or non-linear. Their exact dimensions may be varied according to a desired film cooling scheme for a particular cast metal part. For example cooling holes may have a cross sectional area ranging from 0.01 to 2 mm2. In a turbine blade or stator vane, the cross sectional area may range from 0.01 to 0.15 mm2, more preferably from 0.05 to 0.1 mm2, and most preferably about 0.07 mm2. In the case of a vane, the cooling holes may have a cross sectional area ranging from 0.05 to 0.2 mm2, more preferably 0.1 to 0.18 mm2, and most preferably about 0.16 mm2. The spacing of the cooling holes is typically a multiple of the diameter of the cooling holes ranging from 2× to 10× the diameter of the cooling holes, most preferably about 4-7× the diameter of the holes.
The length of the filament 902 is dictated by the thickness of the cast component, e.g., turbine blade or stator vane wall thickness, and the angle at which the cooling hole is disposed relative to the surface of the cast component. The typical lengths range from 0.5 to 5 mm, more preferably between 0.7 to 1 mm, and most preferably about 0.9 mm. The angle at which a cooling hole is disposed is approximately 5 to 35° relative to the surface, more preferably between 10 to 20°, and most preferably approximately 12°. It should be appreciated that the methods of casting according to the present invention allow for formation of cooling holes having a lower angle relative to the surface of the cast component than currently available using conventional machining techniques.
The cross-sectional area defined by the inner diameter 906 of the hollow filament should be at least 50% of the outer diameter 907 of the filament. For thinner tubes this cross-sectional area can be increased, for example, to 60%, 70%, or 75% of the outer diameter of the filament. In some cases one or more of the filaments connecting the core 900 and the shell 901 of the turbine blade may be solid.
Upon leaching of the ceramic core-shell, the resulting cast object is a turbine blade having a cooling hole pattern in the surface of the blade. It should be appreciated that although
After leaching, the resulting holes in the turbine blade from the core print filaments may be brazed shut if desired. Otherwise the holes left by the core print filaments may be incorporated into the design of the internal cooling passages. Alternatively, cooling hole filaments may be provided to connect the tip plenum core to the shell in a sufficient quantity to hold the tip plenum core in place during the metal casting step.
After printing the core-shell mold structures in accordance with the invention, the core-shell mold may be cured and/or fired depending upon the requirements of the ceramic core photopolymer material. Molten metal may be poured into the mold to form a cast object in the shape and having the features provided by the integrated core-shell mold. In the case of a turbine blade or stator vane, the molten metal is preferably a superalloy metal that is formed into a single crystal superalloy turbine blade or stator vane using techniques known to be used with conventional investment casting molds.
In an aspect, the present invention relates to the core-shell mold structures of the present invention incorporated or combined with features of other core-shell molds produced in a similar manner. The following patent applications include disclosure of these various aspects and their use:
U.S. patent application Ser. No. ______, titled “INTEGRATED CASTING CORE-SHELL STRUCTURE” with attorney docket number 037216.00036/284976, and filed Dec. 13, 2016;
U.S. patent application Ser. No. ______, titled “INTEGRATED CASTING CORE-SHELL STRUCTURE WITH FLOATING TIP PLENUM” with attorney docket number 037216.00037/284997, and filed Dec. 13, 2016;
U.S. patent application Ser. No. ______, titled “MULTI-PIECE INTEGRATED CORE-SHELL STRUCTURE FOR MAKING CAST COMPONENT” with attorney docket number 037216.00033/284909, and filed Dec. 13, 2016;
U.S. patent application Ser. No. ______, titled “MULTI-PIECE INTEGRATED CORE-SHELL STRUCTURE WITH STANDOFF AND/OR BUMPER FOR MAKING CAST COMPONENT” with attorney docket number 037216.00042/284909A, and filed Dec. 13, 2016;
U.S. patent application Ser. No. ______, titled “INTEGRATED CASTING CORE-SHELL STRUCTURE AND FILTER FOR MAKING CAST COMPONENT” with attorney docket number 037216.00039/285021, and filed Dec. 13, 2016;
U.S. patent application Ser. No. ______, titled “INTEGRATED CASTING CORE SHELL STRUCTURE FOR MAKING CAST COMPONENT WITH NON-LINEAR HOLES” with attorney docket number 037216.00041/285064, and filed Dec. 13, 2016;
U.S. patent application Ser. No. ______, titled “INTEGRATED CASTING CORE SHELL STRUCTURE FOR MAKING CAST COMPONENT WITH COOLING HOLES IN INACCESSIBLE LOCATIONS” with attorney docket number 037216.00055/285064A, and filed Dec. 13, 2016;
U.S. patent application Ser. No. ______, titled “INTEGRATED CASTING CORE SHELL STRUCTURE FOR MAKING CAST COMPONENT HAVING THIN ROOT COMPONENTS” with attorney docket number 037216.00053/285064B, and filed Dec. 13, 2016.
The disclosures of each of these applications are incorporated herein in their entirety to the extent they disclose additional aspects of core-shell molds and methods of making that can be used in conjunction with the core-shell molds disclosed herein.
This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. Aspects from the various embodiments described, as well as other known equivalents for each such aspect, can be mixed and matched by one of ordinary skill in the art to construct additional embodiments and techniques in accordance with principles of this application.
