Patent Application
20190111472
The disclosure generally relates to high temperature engineering stiffness core-shell mold components for casting and processes utilizing these components. The core-shell mold made in accordance with the present invention utilizes structural supports to accommodate ceramic pieces made via an additive process. The high temperature structural supports may be utilized during a liquid metal pouring phase of a casting process to improve casting cooling rates. As such, the 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.
A gas turbine engine generally includes at least one compressor to pressurize air to be channeled into a combustor. The engine may include at least one combustor in which at least a portion of the channeled pressurized air is mixed with fuel and ignited. Hot gasses from the compressor flow downstream through at least one turbine section. Each turbine section has rotating blades rotating about an axis and contained within an engine housing. The turbine section or sections can power any one of the compressor, a fan, a shaft, and/or may provide thrust through expansion through a nozzle.
The turbine blades and/or stator vanes in the turbine portions must be able to withstand thermal stresses due to high temperatures and large temperature fluctuations as well as forces due to the high rotational speed experienced during normal operation of the turbine. As the pressure ratio and efficiency of turbines have increased, the thermal stresses the high pressure and low pressure turbine portion are exposed to have also increased. Accordingly, in combination with manufacturing components of the turbine (for example, turbine blades and stator vanes) from a high-temperature resistant material, effective cooling of the turbine blades, stator vanes and other components have become increasingly important and challenging. To counteract the radiation and convection of heat to the turbine section, several heat removal techniques have been employed in the past; fluid cooling is generally employed to prolong the life of the turbine components. Further, small cooling holes have been drilled though the blade at angles optimized to remove heat and provide a thermal barrier on the surface of each airfoil surface of the turbine blades and stator vanes. Passages are also formed within the turbine and/or stator vanes to provide convection cooling of the surface of each airfoil.
The desire for increased cooling efficiency within turbine engine has led to complex internal cooling passages within turbine components. Conventional techniques for manufacturing engine parts and components involve the process of investment or lost-wax casting. One example of investment casting involves the manufacture of a typical blade used in a gas turbine engine. A turbine blade and/or stator vane 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 of the engine. 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, as shown in
In the above-mentioned process, a precision ceramic core 200 is manufactured to conform to the serpentine cooling passages desired inside the turbine blade. A precision die or mold is also created which defines the precise 3-D external surface of the turbine blade including its airfoil, platform, and integral dovetail. The ceramic core 200 is assembled inside two die halves which form a space or void therebetween that define the resulting metal portions of the blade. A relatively rigid wax and/or plastic is injected into the assembled dies to fill the void and surround the ceramic core 200, at which point the ceramic core 200 is encapsulated within the wax. The two die halves are split apart and removed to expose and remove the rigid wax and/or plastic that has the precise configuration of the desired blade formed of a molded wax 211. The molded wax blade 211 with encapsulated ceramic core 200 is then attached to a wax tree structure 212. The wax tree structure 212 is formed of a paraffin wax or any wax that is less rigid than the wax used to form the molded wax blade 211. Because the wax of the wax tree 212 will ultimately define a flowpath for molten metal into the ceramic mold, the dimensional accuracy of the outer surface of the wax used to form the tree structure 212 is less crucial. Thus, a softer wax is generally used to form the individual paths of the wax tree 212 than for the precisely molded wax blade 211 of the desired wax blade. The wax blade 211 requires pins 205 for holding the core in place. The tree structure 212 may include a funnel shaped portion 214 for adding molten metal to the mold. As shown in
After wax injection and the attachment of wax passageways 212 which form the wax tree structure, the entire wax tree structure 212, ceramic filter 213, and wax turbine blade 211 is then coated with a ceramic material to form a ceramic shell 206, 204 as shown in
The cast turbine blade 208 typically undergoes additional post-casting modifications, such as 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 (generally referred to as film cooling) over the external surface of the airfoil during operation in the gas turbine engine. After the turbine blade is removed from the ceramic mold, pins 205 which held the ceramic core 200 form 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 efficiency improvements provided by 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 and thus increase manufacturing time and expense significantly. Accordingly, it was desirable to provide an improved casting method for three dimensional components having intricate internal voids and cooling circuits.
Additive manufacturing techniques and 3-D printing allowed molds to be manufactured without the toolpath and/or molding limitations associated with subtractive manufacturing. For example, 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 significantly limited by the printing resolution capabilities of these processes. As shown in
There still remains the 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 such as 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, for example. 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 limits the ability to make filaments of sufficiently small size to serve as effective cooling holes in the cast final product.
While the above-mentioned processes can be used to form integrated core-shell mold, it is advantageous to manufacture a core-shell mold 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. 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.
Further, in employing the above-mentioned DLP method of manufacturing a core-shell of the embodiment described above, various difficulties arise in integrating the use of a core-shell mold into an efficient manufacturing process. For example, the time required to form a core-shell mold having sufficient dimensional stability (e.g., wall thickness) using a DLP process may delay the manufacturing process and require the use of excess material. Further, in the molding process it may be desired to efficiently produce portions of a mold that do not require the same dimensional accuracy as is required in portions of the core-shell mold itself. For example, it may be desirable to produce passages for directing the flow of molten superalloy into a single or plurality of core-shell molds. Further, when forming a core-shell mold using a DLP process it may be desirable to improve the ease of removing the core-shell once the casting is completed. For example, the knockout process may be improved by producing a thinner core-shell, to reduce the likeliness that the cast product is damaged upon removal of the core-shell. It may also be desirable to control the thermal conductivity of the core-shell mold to control crystal growth and/or tailor the material properties of the cast component and/or manage the thermal strains in the ceramic material.
The following presents a simplified summary of one or more aspects of the present disclosure in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.
The foregoing and/or other aspects of the present invention may be achieved by a method of fabricating a ceramic mold. In one aspect, the method includes (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 includes a first opening for creating a cast article and a second opening for receiving a support member.
The foregoing and/or aspects of the present invention may also be achieved by a method of preparing a cast component. In one aspect, the method includes forming a printed ceramic mold, the ceramic mold including a first and second opening. The first opening may be for creating the cast component and the second opening may be for receiving a support member.
The foregoing and/or aspects of the present invention may also be achieved by an apparatus for preparing a cast component including a ceramic casting mold, the ceramic casting mold including a first and second opening. The first opening may be for creating the cast component and the second opening may be for receiving a support member.
Other features and aspects may be apparent from the following detailed description, the drawings, and the claims.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more example aspects of the present disclosure and, together with the detailed description, serve to explain their principles and implementations.
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.
According to an example embodiment, the present invention provides a methodology of using high temperature engineered support mechanisms during the liquid metal pouring phase of the casting process. As such, the amount of additive material and print time of the additive ceramic process is minimized.
As described above, the present invention may provide internal support features such as, for example, high temperature spheres and metal sheets. In other exemplary embodiments, the core-shell mold may include external supports features such as, for example, ceramic containment blocks, metal clips, and metal bands on an outer portion of the core-shell mold having internal support features. It may be appreciated that the internal supports and the external supports may be made of a ceramic refractory metal having a melting temperature higher that is higher than the melting temperature of the metal used in casting the cast component.
In accordance with the above-described example embodiments, the present invention provides structural supports to accommodate ceramic pieces made via an additive process. An aspect of the present invention provides a methodology of using high temperature supports during the liquid pouring phase of the casting process. As such, the material and print time of the additive ceramic process is minimized and support needed during the casting process is provided. Engineered features such as, for example, high temperature spheres and metal sheets provide stiffness at selective areas while allowing the use of thin ceramic shells and improved casting cooling rates. Additionally, the present invention may utilize external engineered support features such as, for example, ceramic containment blocks, metal clips, and metal bands along with the spheres and metal sheets to further provide structural support during the additive process.
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. 15/377,728, titled “INTEGRATED CASTING CORE-SHELL STRUCTURE” with attorney docket number 037216.00036/284976, and filed Dec. 13, 2016;
U.S. patent application Ser. No. 15/377,711, 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. 15/377,796, 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. 15/377,746, 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. 15/377,673, titled “INTEGRATED CASTING CORE SHELL STRUCTURE WITH PRINTED TUBES FOR MAKING CAST COMPONENT” with attorney docket number 037216.00032/284917, and filed Dec. 13, 2016;
U.S. patent application Ser. No. 15/377,787, 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. 15/377,783, 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. 15/377766, 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.
