Abstract:
A method for making a turbine airfoil includes: (a) providing a mold having: (i) a core; (ii) an outer shell surrounding the core such that the core and the outer shell cooperatively define a cavity in the shape of an airfoil having at least one outer wall; and (iii) a core support extending from the core to the outer shell through a portion of the cavity that defines the at least one sidewall; (b) introducing molten metal alloy into the cavity and surrounding the core support; (c) solidifying the alloy to form an airfoil casting having at least one outer wall which has at least one core support opening passing therethrough; (d) removing the mold so as to expose the airfoil; and (e) sealing the at least one core support opening in the airfoil with a metal alloy metallurgically bonded to the at least one outer wall.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to the manufacture of gas turbine engine components and more particularly to methods for casting hollow turbine airfoils. 
         [0002]    Cast turbine airfoils for advanced gas turbine engines have internal features that challenge the capability of current casting technologies. The castings require complex ceramic cores to form the internal features and these cores are fragile during the casting process. The result is that casting yields of 50 percent to 70 percent are not uncommon. The 30 percent to 50 percent casting scrap factors into the cost of the useable castings. 
         [0003]    The issue is compounded by exotic alloys such as single crystal materials that drive up the cost to cast a part and thus drive up the cost caused by scrapping hardware. If a mere 5 percent to 10 percent casting yield improvement can be achieved, the impact to each gas turbine engine is in the millions of dollars per year, based on volume. 
         [0004]    One basic casting limitation is that the ceramic core that forms the internal structure of the airfoil can only be secured by the lower (i.e. root) portion with the majority of the core “floating” within the casting wax form. The forces of the molten metal and thermally induced forces during the cooling and solidification cycle result in movement and/or breakage of the ceramic core (referred to as “core shift”). The motion can be such that the cast component no longer meets drawing requirements, for example by violating minimum casting wall thicknesses. If the core fractures during the process this will also cause the component to fail requirements. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    These and other shortcomings of the prior art are addressed by the present invention, which provides a method for supporting an airfoil core during casting, while maintaining the metallurgical integrity of the finished component. 
         [0006]    According to an aspect of the invention, a method for making a turbine airfoil includes: (a) providing a mold having: (i) a core; (ii) an outer shell surrounding the core such that the core and the outer shell cooperatively define a cavity in the shape of an airfoil having at least one outer wall; and (iii) a core support extending from the core to the outer shell through a portion of the cavity that defines the at least one sidewall; (b) introducing molten metal alloy into the cavity and surrounding the core support; (c) solidifying the alloy to form an airfoil casting having at least one outer wall which has at least one core support opening passing therethrough; (d) removing the mold so as to expose the airfoil; and (e) sealing the at least one core support opening in the airfoil with a metal alloy metallurgically bonded to the at least one outer wall. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0008]      FIG. 1  is a perspective view of an exemplary turbine blade constructed in accordance with an aspect of the present invention; 
           [0009]      FIG. 2  is a perspective view of a mold core used in casting the blade shown in  FIG. 1 , with a core support carried therein; 
           [0010]      FIG. 3  is another perspective view of the mold core of  FIG. 2 ; 
           [0011]      FIG. 4  is a partial cross-sectional view of an assembled mold; 
           [0012]      FIG. 5  is a cross-sectional view of the mold of  FIG. 4  with a portion of a blade casting therein; 
           [0013]      FIG. 6  is a perspective view of an as-cast turbine blade, which includes an opening left by a core support; 
           [0014]      FIG. 7  is another perspective view of the turbine blade of  FIG. 6 ; 
           [0015]      FIG. 8  is a partial cross-sectional view of the turbine blade taken along lines  8 - 8  of  FIG. 7 ; 
           [0016]      FIG. 9  is a cross-sectional view taken along lines  9 - 9  of  FIG. 1 ; and 
           [0017]      FIG. 10  is a schematic view of an apparatus for closing the core support opening in the turbine blade. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates an exemplary turbine blade  10 . The turbine blade  10  includes a conventional dovetail  12 , which may have any suitable form including tangs that engage complementary tangs of a dovetail slot in a rotor disk (not shown) for radially retaining the blade  10  to the disk as it rotates during operation. A blade shank  14  extends radially upwardly from the dovetail  12  and terminates in a platform  16  that projects laterally outwardly from and surrounds the shank  14 . A hollow airfoil  18  extends radially outwardly from the platform  16 . The airfoil  18  has a concave pressure side outer wall  20  and a convex suction side outer wall  22  joined together at a leading edge  24  and at a trailing edge  26 . The airfoil  18  may take any configuration suitable for extracting energy from the hot gas stream and causing rotation of the rotor disk. The blade  10  is preferably formed as a one-piece casting of a suitable “superalloy” of a known type, such as a nickel-based superalloy (e.g. Rene 80, Rene 142, Rene N4, Rene N5) which has acceptable strength at the elevated temperatures of operation in a gas turbine engine. The airfoil  18  has a root  25  and a tip  27 , and incorporates a number of trailing edge bleed holes  28 . 
         [0019]    The interior of the turbine blade  10  is mostly hollow and includes a number of internal cooling features of a known type, such as walls defining serpentine passages, ribs, turbulence promoters (“turbulators”), etc. While the turbine blade  10  is a high pressure turbine blade, the principles of the present invention are applicable to any type of airfoil having a hollow interior. 
         [0020]    Components such as the turbine blade  10  are manufactured using a known investment casting process. The method includes shaping the turbine blade in wax by enveloping a conventional alumina or silica based ceramic core which defines internal coolant passages. The wax assembly then undergoes a series of dips in liquid ceramic solution. The part is allowed to dry after each dip, forming a hard outer shell, typically a conventional zirconia based ceramic shell. After all dips are complete, and the wax assembly is encased by several layers of hardened ceramic shell, the assembly is placed in a furnace where the wax in the shell is melted out. 
         [0021]    After wax removal, the mold comprises the internal ceramic core surrounded by the outer ceramic shell. The cavity between the core and the outer shell defines the component and its interior features. The mold is again placed in the furnace, and liquid metal is poured into an opening at the top of the mold. The molten metal enters the space between the ceramic core and the ceramic shell, previously filled by the wax. After the metal is allowed to cool and solidify, the external shell is broken and removed, exposing the casting which has taken the shape of the cavity created by removal of the wax, and which encases the internal ceramic core. The casting is then placed in a leeching tank, where the core is dissolved. The component now has the shape of the wax form, and an internal cavity which was previously filled by the internal ceramic core. 
         [0022]    The relative thermal growths of the ceramic outer shell and the ceramic core material are different, so that after the metal has been poured and is allowed to cool, the relative shrinking of the shell and core components are different. This can cause varying wall thicknesses at areas of the metal nozzle part where one side of the wall is defined by the external shell, and the other side of the wall is engaged by the internal core. Furthermore, the core is typically allowed to “float” and may thus shift its position relative to the outer shell during the casting process. This can cause the walls of a component such as an airfoil to be less than a required minimum thickness. 
         [0023]    To avoid core shift, the turbine blade  10  is cast by a modification of the above process, which incorporates one or more core supports.  FIGS. 2 and 3  are pre-casting views of a core  30  with a core support  32  captured therein. A surrounding outer shell  34  comprises first and second sidewalls  34 A and  34 , as shown in  FIG. 4 .  FIG. 4  also shows the core support  32  passing sequentially through the first sidewall  34 A, a first portion  36  of wax fill, the core  30 , a second portion of wax fill  38 , and the second sidewall  34 B. 
         [0024]    In the illustrated example, the core support  32  takes the form of a circular cross-section rod, but other cross-sectional shapes may be used to suit a particular application. 
         [0025]    The core support  32  is constructed from a suitable material having a melting point higher than the alloy used for the turbine blade  10 , which may be a known nickel- or cobalt-based “superalloy”. Examples of suitable core support materials include fused quartz, or a ceramic such as Yttria, (Y 2 O 3 ) or samarium oxide (Sm 2 O 3 ), as used to make the core  30 . 
         [0026]    The core support  32  remains in place during the casting process and resists motion of the core  30  during pouring and solidification. While any number of core supports  32  may be used and placed at any desired location, it is beneficial to support the core  30  in an area, denoted “A” in  FIG. 2 , which defines the airfoil  18 . This area of the core  30  is normally unsupported portion of the core  30 , and is a substantial distance from the part of the core  30  which defines the blade shank  14 . Support of the core helps maintain the core-to-outer shell spacing “S”, which directly affects the outer wall thickness of the finished turbine blade  10 . 
         [0027]      FIG. 5  is a post-casting partial cross-section which shows the core support  32  passing sequentially through the first sidewall  34 A of the outer shell  34 , the pressure side outer wall  20  of the turbine blade  10 , the core  30 , the suction side outer wall  22  of the turbine blade  10 , and the second sidewall  34 B of the outer shell  34 . 
         [0028]      FIGS. 6-8  illustrate the turbine blade  10  after casting and removal of the outer shell  34 , core  30 , and core support  32 . The turbine blade  10  includes core support openings  40  and  42  in the pressure and suction side outer walls  20  and  22 , respectively. The core support openings  40  and  42  must be sealed before the turbine blade  10  is usable. Although it is possible to seal them using brazing techniques, this is not a metallurgical bond and does not have the same properties as the basic turbine blade  10 , which has a directionally-solidified or single-crystal microstructure imparting enhanced high-temperature strength and creep resistance. 
         [0029]    An example of a suitable apparatus for sealing the core support openings  40  and  42  is disclosed in U.S. Pat. No. 5,622,638 to Schell et al., assigned to the assignee of this invention, and is schematically illustrated in  FIG. 10 . The apparatus includes a laser  44 , an enclosed beam delivery conduit  46 , laser focusing optics  48 , a part positioning system  50 , a vision system  52  for part location and laser path control, an optional preheat box (not shown), and a powder feed system  54  with a powder tube  56 . The working and coordination of the individual parts of the apparatus are controlled through a computerized system controller  58 . In a conventional manner, the powder enters the laser beam in close proximity to the blade  10  as it is manipulated to cause melting and weld build-up. 
         [0030]    The core support openings  40  and  42  may be sealed by using this apparatus to deposit molten alloy powder in one or more passes. Alternatively, powder can be deposited and then heated to melt and fuse it to the airfoil  18 . In either case, the power alloy composition is substantially the same as that of the basic turbine blade  10 . This process, sometimes referred to as “reverse machining”, produces a plug or patch that is metallurgically bonded to the core support openings  40  and  42 , effectively forming an integral structure with the turbine airfoil  10 . With proper control of the process parameters, this process can produce the same microstructure in the plug or patch (e.g. directionally solidified or single crystal) as that of the turbine blade  10 . The finished turbine blade  10  is shown in  FIGS. 1 and 9 . This process will result in substantially higher casting yields, because of the prevention of core shift, while maintaining the desired high-temperature properties of the turbine blade  10 . 
         [0031]    The foregoing has described a method for making gas turbine engine airfoils. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.