Patent Description:
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 <FIG>. First, a precision ceramic core is manufactured to conform to the intricate cooling passages desired inside the turbine blade. A precision die or mold is also created which defines the precise <NUM>-D external surface of the turbine blade including its airfoil, platform, and integral dovetail. A schematic view of such a mold structure is shown in <FIG>. The ceramic core <NUM> is assembled inside two die halves which form a space or void therebetween 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 therein. The two die halves are split apart and removed from the molded wax. The molded wax has the precise configuration of the desired blade and is then coated with a ceramic material to form a surrounding ceramic shell <NUM>. Then, the wax is melted and removed from the shell <NUM> leaving a corresponding void or space <NUM> between the ceramic shell <NUM> and the internal ceramic core <NUM> and tip plenum <NUM>. Molten superalloy metal is then poured into the shell to fill the void therein and again encapsulate the ceramic core <NUM> and tip plenum <NUM> contained in the shell <NUM>. The molten metal is cooled and solidifies, and then the external shell <NUM> and internal core <NUM> and tip plenum <NUM> are suitably removed leaving behind the desired metallic turbine blade in which the internal cooling passages are found. In order to provide a pathway for removing ceramic core material via a leaching process, a ball chute <NUM> and tip pins <NUM> are provided, which upon leaching form a ball chute and tip holes within the turbine blade that must subsequently brazed shut.

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 <NUM> of the ceramic core <NUM> 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 <NUM>-D printing to produce a ceramic core-shell mold are described in <CIT> assigned to Rolls-Royce Corporation. The methods for making the molds include powder bed ceramic processes such as disclosed <CIT> assigned to Massachusetts Institute of Technology, and selective laser activation (SLA) such as disclosed in <CIT> assigned to 3D Systems, Inc. The ceramic core-shell molds according to the '<NUM> patent are limited by the printing resolution capabilities of these processes. As shown in <FIG>, the core portion <NUM> and shell portion <NUM> of the integrated core-shell mold is held together via a series of tie structures <NUM> provided at the bottom edge of the mold. Cooling passages are proposed in the '<NUM> patent that include staggered vertical cavities joined by short cylinders, the length of which is nearly the same as its diameter. A superalloy turbine blade is then formed in the core-shell mold using known techniques disclosed in the '<NUM> patent.

After a turbine blade is cast in one of these core-shell molds, the mold is removed to reveal a cast superalloy turbine blade.

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.

<CIT> discloses implementing the LAMP process for additive manufacturing.

The invention is set out by the appended claims. In one embodiment, the invention relates to a method of making a ceramic casting mold in accordance with claim <NUM>. After step (d), the method 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 method 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 in accordance with claim <NUM>.

In one aspect, the cast component is a turbine blade or a stator vane. Preferably the turbine blade or vane is used in a gas turbine engine in, for example, an aircraft engine or power generation. The turbine blade or vane is preferably a single crystal cast turbine blade or 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 <NUM> to <NUM><NUM>.

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., ><NUM> 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 vane design, or the filaments may be placed outside the finish machined shape of the component to prevent the need for this.

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 or stator vane. In the case of earlier powder bed processes, such as disclosed in <CIT> 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 <CIT> 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 '<NUM> 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 '<NUM> 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 <NUM>/<NUM>" (<NUM>) making the production of traditional cooling holes impracticable. For example, round cooling holes generally have a diameter of less than <NUM> corresponding to a cooling hole area below <NUM><NUM>. 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 <CIT> assigned to Ivoclar Vivadent AG and Technishe Universitat Wien, as well as <CIT> and <CIT>, each of which are discussed below with reference to <FIG>. The apparatus includes a tank <NUM> having at least one translucent bottom portion <NUM> covering at least a portion of the exposure unit <NUM>. The exposure unit <NUM> comprises a light source and modulator with which the intensity can be adjusted position-selectively under the control of a control unit, in order to produce an exposure field on the tank bottom <NUM> with the geometry desired for the layer currently to be formed. As an alternative, a laser may be used in the exposure unit, the light beam of which successively scans the exposure field with the desired intensity pattern by means of a mobile mirror, which is controlled by a control unit.

Opposite the exposure unit <NUM>, a production platform <NUM> is provided above the tank <NUM>; it is supported by a lifting mechanism (not shown) so that it is held in a height-adjustable way over the tank bottom <NUM> in the region above the exposure unit <NUM>. The production platform <NUM> 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 <NUM>, 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 <NUM> contains a filling of highly viscous photopolymerizable material <NUM>. 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 <NUM> 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 <NUM> and approaches the tank bottom <NUM> to such an extent that precisely the desired layer thickness Δ (see <FIG>) remains between the lower side of the production platform <NUM> and the tank bottom <NUM>. During this immersion process, photopolymerizable material is displaced from the gap between the lower side of the production platform <NUM> and the tank bottom <NUM>. After the layer thickness Δ has been set, the desired position-selective layer exposure is carried out for this layer, in order to cure it in the desired shape. Particularly when forming the first layer, exposure from above may also take place through the transparent or translucent production platform <NUM>, so that reliable and complete curing takes place particularly in the contact region between the lower side of the production platform <NUM> and the photopolymerizable material, and therefore good adhesion of the first layer to the production platform <NUM> is ensured. After the layer has been formed, the production platform is raised again by means of the lifting mechanism.

These steps are subsequently repeated several times, the distance from the lower side of the layer <NUM> formed last to the tank bottom <NUM> 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 <NUM> has been raised following an exposure step, there is a material deficit in the exposed region as indicated in <FIG>. This is because after curing the layer set with the thickness Δ, the material of this layer is cured and raised with the production platform and the part of the shaped body already formed thereon. The photopolymerizable material therefore missing between the lower side of the already formed shaped body part and the tank bottom <NUM> must be filled from the filling of photopolymerizable material <NUM> from the region surrounding the exposed region. Owing to the high viscosity of the material, however, it does not flow by itself back into the exposed region between the lower side of the shaped body part and the tank bottom, so that material depressions or "holes" can remain here.

In order to replenish the exposure region with photopolymerizable material, an elongate mixing element <NUM> is moved through the filling of photopolymerizable material <NUM> in the tank. In the exemplary embodiment represented in <FIG>, the mixing element <NUM> comprises an elongate wire which is tensioned between two support arms <NUM> mounted movably on the side walls of the tank <NUM>. The support arms <NUM> may be mounted movably in guide slots <NUM> in the side walls of the tank <NUM>, so that the wire <NUM> tensioned between the support arms <NUM> can be moved relative to the tank <NUM>, parallel to the tank bottom <NUM>, by moving the support arms <NUM> in the guide slots <NUM>. The elongate mixing element <NUM> has dimensions, and its movement is guided relative to the tank bottom, such that the upper edge of the elongate mixing element <NUM> remains below the material level of the filling of photopolymerizable material <NUM> in the tank outside the exposed region. As can be seen in the sectional view of <FIG>, the mixing element <NUM> is below the material level in the tank over the entire length of the wire, and only the support arms <NUM> protrude beyond the material level in the tank. The effect of arranging the elongate mixing element below the material level in the tank <NUM> is not that the elongate mixing element <NUM> substantially moves material in front of it during its movement relative to the tank through the exposed region, but rather this material flows over the mixing element <NUM> while executing a slight upward movement. The movement of the mixing element <NUM> from the position shown in <FIG>, to, for example, a new position in the direction indicated by the arrow A, is shown in <FIG>. It has been found that by this type of action on the photopolymerizable material in the tank, the material is effectively stimulated to flow back into the material-depleted exposed region between the production platform <NUM> and the exposure unit <NUM>.

The movement of the elongate mixing element <NUM> relative to the tank may firstly, with a stationary tank <NUM>, be carried out by a linear drive which moves the support arms <NUM> along the guide slots <NUM> in order to achieve the desired movement of the elongate mixing element <NUM> through the exposed region between the production platform <NUM> and the exposure unit <NUM>. As shown in <FIG>, the tank bottom <NUM> has recesses <NUM>' on both sides. The support arms <NUM> project with their lower ends into these recesses <NUM>'. This makes it possible for the elongate mixing element <NUM> to be held at the height of the tank bottom <NUM>, without interfering with the movement of the lower ends of the support arms <NUM> through the tank bottom <NUM>.

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.

<FIG> shows a schematic side view of an integrated core-shell mold with filaments <NUM> connecting the core <NUM> and shell portions <NUM>. By printing the ceramic mold using the above DLP printing process, the mold can be made in a way that allows the point of connections between the core and shell to be provided through filaments <NUM>. Once the core-shell mold is printed, it may be subject to a post-heat treatment step to cure the printed ceramic polymer material. The cured ceramic mold may then be used similar to the traditional casting process used in the production of superalloy turbine blades or vanes. Notably because the filaments <NUM> are provided in a large quantity consistent with formation of a pattern of effusion cooling holes in the surface of a turbine blade or vane, the need for a ball chute structure as shown in <FIG> may be eliminated. In this embodiment, the tip pins <NUM> connecting the tip plenum core <NUM> to the core <NUM> are retained, and a void <NUM> exists between the shell portion <NUM> and the tip plenum core <NUM>. After removal of the ceramic mold, tip holes exist between the core <NUM> and tip plenum core <NUM> that may be subsequently brazed shut. However, the tip pins <NUM> may be eliminated, avoiding the need to braze shut tip holes connecting the core cavity with the tip plenum.

The filaments <NUM> are preferably cylindrical or oval shape but may also be curved or nonlinear. 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 <NUM> to <NUM><NUM>. In a turbine blade, the cross sectional area may range from <NUM> to <NUM><NUM>, more preferably from <NUM> to <NUM><NUM>, and most preferably about <NUM><NUM>. In the case of a vane, the cooling holes may have a cross sectional area ranging from <NUM> to <NUM><NUM>, more preferably <NUM> to <NUM><NUM>, and most preferably about <NUM><NUM>. The spacing of the cooling holes is typically a multiple of the diameter of the cooling holes ranging from <NUM>× to <NUM>× the diameter of the cooling holes, most preferably about <NUM>-<NUM>× the diameter of the holes.

The length of the filament <NUM> 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 <NUM> to <NUM>, more preferably between <NUM> to <NUM>, and most preferably about <NUM>. The angle at which a cooling hole is disposed is approximately <NUM> to <NUM>° relative to the surface, more preferably between <NUM> to <NUM>°, and most preferably approximately <NUM>°. 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.

<FIG> shows a side view of an integrated core-shell mold according to an embodiment of the present invention. As with the schematic shown in <FIG>, the core <NUM> is connected to the shell <NUM> through several filaments <NUM>. The core-shell mold <NUM>/<NUM> defines a cavity <NUM> for investment casting a turbine blade. <FIG> shows the cavity <NUM> filled with a metal <NUM>, such as a nickel based alloy, i.e., Inconel. 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 <FIG> provide a cross sectional view showing cooling holes at the leading and trailing edge of the turbine blade, that additional cooling holes may be provided where desired including on the sides of the turbine blades or any other location desired. In particular, the present invention may be used to form cooling holes within the casting process in any particular design. In other words, one would be able to produce conventional cooling holes in any pattern where drilling was used previously to form the cooling holes. However, the present invention will allow for cooling hole patterns previously unattainable due to the limitations of conventional technologies for creating cooling holes within cast components, i.e., drilling.

<FIG> is a schematic view of an integrated core-shell mold having a floating tip plenum core <NUM>. The tip plenum core <NUM> is held in place relative to the shell with core print filaments <NUM>, eliminating the need for tip pins as shown in <FIG> or a shell lock to attach the tip plenum core to the shell as used in conventional investment casting. In this embodiment, the core print filaments <NUM> exit at or near the top of the tip plenum core <NUM>. The core print filaments <NUM> provide additional support holding tip plenum core <NUM> in place, and have a cross sectional area that is sufficiently large to support the weight of the tip plenum core during the casting process. For example the cross sectional area of a core print filament is generally larger than the cross sectional area of the cooling hole filaments. In one case, the cross sectional area of the core print filament is greater than <NUM><NUM>, and may have an area on the order of a square centimeter.

The core print filaments <NUM> may be necessary if there are no cooling hole filaments between the tip plenum core and the shell, or if the amount or size of the filaments are insufficient to hold the tip plenum core in place during the metal casting step. The provision of core print filaments <NUM> allow the tip plenum core <NUM> to float above and be disconnected from the main core. This eliminates the need for tip pins that result in tip holes connecting the surface of the turbine blade exposed through the tip plenum to the main core cavity of the turbine blade. The elimination of the tip holes is advantageous since it eliminates the post-casting step of brazing tip holes shut. This design provides a novel core-shell structure and eliminates conventional structures such as tip pins and/or a shell lock to hole the tip core relative to the shell.

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. <FIG> provides an alternative design where core filaments exit to the side of the blade tip plenum <NUM>.

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 formed into a single crystal superalloy turbine blade or stator vane using techniques known to be used with conventional investment casting molds.

Claim 1:
A method for fabricating a ceramic casting mold, comprising:
(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, the 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 casting mold is formed, the ceramic casting mold comprising a core portion (<NUM>) and a shell portion (<NUM>) with at least one cavity between the core portion (<NUM>) and the shell portion (<NUM>), the at least one cavity adapted to define the shape of a cast component upon casting and removal of the ceramic casting mold.