INTRODUCTION

The present disclosure generally relates to graded investment casting two piece core-shell mold components and processes utilizing these components. The two piece core-shell mold is assembled from a partial mold including a first core and shell portion. The two piece core-shell mold is assembled by attaching the first core and shell portion to at least a second core and shell portion of a second partial mold. The two piece core-shell mold made in accordance with the present invention may also include integrated ceramic filaments between the core and shell of the mold that can be utilized to form holes, i.e., effusion cooling holes, in the cast component made from these two-piece molds. 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.

BACKGROUND

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 inFIG. 1. 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 3-D external surface of the turbine blade including its airfoil, platform, and integral dovetail. A schematic view of such a mold structure is shown inFIG. 2. The ceramic core200is 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. Then, the wax is melted and removed from the shell202leaving a corresponding void or space201between the ceramic shell202and the internal ceramic core200and tip plenum204. Molten superalloy metal is then poured into the shell to fill the void therein and again encapsulate the ceramic core200and tip plenum204contained in the shell202. The molten metal is cooled and solidifies, and then the external shell202and internal core200and tip plenum204are 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 chute203and tip pins205are provided, which upon leaching form a ball chute and tip holes within the turbine blade that must subsequently be 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 chute203of the ceramic core200forms a passageway that is later welded 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 in 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 inFIG. 3, the core portion301and shell portion302of the integrated core-shell mold is held together via a series of tie structures303provided at the bottom edge of the mold. Cooling passages are proposed in the '151 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 '151 patent, and incorporated herein by reference. 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.

SUMMARY

The present invention relates to a novel casting mold that is formed as a two piece core-shell mold consisting of a first ceramic mold portion comprising a first shell portion and optionally a first core portion and a second ceramic mold portion comprising a second shell portion and optionally a second core portion, the first ceramic mold portion being adapted to interface with the second ceramic mold portion to form a two piece ceramic mold comprising a cavity between the first and/or second core portions and the first and second shell portions, the cavity adapted to define a cast component upon casting and removal of the ceramic mold. The first and/or second ceramic mold portion may be provided with at least one attachment point to facilitate assembly into a complete core-shell mold. Non-limiting examples of suitable attachment points include, for example, an interlocking arrangement, such as for example a tongue and groove or dovetail type attachment. Another non-limiting example of a suitable attachment point is a rabbet joint.

