Abstract:
Concepts for fabricating improved cores for investment casting are described. The cores are composite which include refractory metal elements and ceramic elements. The refractory metal elements are provided to enhance the mechanical properties of the core and/or to permit the fabrication of cores having shapes and geometries that could not otherwise be achieved. In one embodiment, the entire core may be made of refractory metal components. The cores may be used to investment cast gas turbine superalloy components.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to investment casting cores, and in particular to investment casting cores which are formed at least in part from refractory metals. 
     2. Background Information 
     Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components. The invention will be described in respect to the production of superalloy castings, however it will be understood that the invention is not so limited. 
     Gas turbine engines are widely used in aircraft propulsion, electric power generation, and ship propulsion. In all gas turbine engine applications, efficiency is a prime objective. 
     Improved gas turbine engine efficiency can be obtained by operating at higher temperatures, however current operating temperatures are at such a level that, in the turbine section, the superalloy materials used have limited mechanical properties. Consequently, it is a general practice to provide air cooling for components in the hottest portions of gas turbine engines, typically in the turbine section. Cooling is provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. It will be appreciated that cooling comes with an associated cost in engine efficiency, consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air. 
     Referring to FIG. 1, a gas turbine engine  10  includes a compressor  12 , a combustor  14 , and a turbine  16 . Air  18  flows axially through the sections  12 ,  14 , and  16  of the engine  10 . As is well known in the art, air  18 , compressed in the compressor  12 , is mixed with fuel which is burned in the combustor  14  and expanded in the turbine  16 , thereby rotating the turbine  16  and driving the compressor  12 . 
     Both the compressor  12  and the turbine  16  are comprised of rotating and stationary airfoils  20 ,  22 , respectively. The airfoils, especially those disposed in the turbine  16 , are subjected to repetitive thermal cycling under widely ranging temperatures and pressures. To avoid thermal damage to the airfoils, each airfoil  20  includes internal cooling. 
     Referring to FIG. 2, the airfoil  20  includes a leading edge  26  and a trailing edge  28  extending from a root end  30  to a tip  32  thereof and a platform  34 . A leading edge cooling passage  40  is formed within the leading edge  26  of the airfoil  20  having radially extending, connected channels  42 - 44  and a leading edge inlet  46 , formed within the platform  34  and in fluid communication with the channel  42 . A plurality of leading edge crossover holes  48  formed within a leading edge passage wall  50  separating the channel  44  from a leading edge exhaust passage  52 , allow the cooling air from the channel  44  to flow into the leading edge exhaust passage  52 . A trailing edge cooling passage  56  is formed within the trailing edge  28  of the airfoil  20  having radially extending connected channels  58 - 60  and a trailing edge inlet  62  formed within the platform  34  and in fluid communication with the channel  58 . A first plurality of trailing edge crossover holes  66  is formed within a first trailing edge wall  68  and a second plurality of trailing edge crossover holes  72  is formed within a second trailing edge wall  74  to allow cooling air from channel  58  to flow through an intermediate passage  78  to a plurality of trailing edge slots  80 . 
     A ceramic core  120 , as depicted in FIGS. 3 and 4, is used in the manufacturing process of the airfoils  20  and defines the hollow cavities therein. A ceramic core leading edge  126  and a ceramic core trailing edge  128  correspond to the leading edge  26  and trailing edge  28  in the airfoil  20 , respectively. A ceramic core root  130  and a tip  132  correspond to the airfoil root  30  and tip  32 , respectively. Ceramic core passages  140 ,  156  with channels  142 - 144 ,  158 - 160 , and inlets  146 ,  162  respectively, correspond to passages  40 ,  56  with channels  42 - 44 ,  58 - 60  and inlets  46 ,  62 , of the airfoil, respectively. Passages  52  and  78  of the airfoil correspond to channels  152  and  178  in the ceramic core. Pluralities of fingers  148 ,  166 ,  172  in the core  120  correspond to the plurality of crossover holes  48 ,  66 ,  72  in the airfoil  20 , respectively. A core tip  190  is attached to the core passages  140 ,  156  by means of fingers  182 - 185 , to stabilize the core  120  at the tip  132 . An external ceramic handle  194  is attached at the core trailing edge  128  for handling purposes. A core extension  196  defines a cooling passage at the root to the airfoil  20 . Centerlines  197 - 199  extend radially through each row of fingers  148 ,  166 ,  172 , respectively. 
     While turbine blades and vanes are some of the most important components that are cooled, other components such as combustion chambers and blade outer air seals also require cooling, and the invention has application to all cooled turbine hardware, and in fact to all complex cast articles. 
     Currently cores such as that shown in FIGS. 3 and 4 are fabricated from ceramic materials but such ceramic cores are fragile, especially the advanced cores used to fabricate small intricate cooling passages in advanced hardware. Current ceramic cores are prone to warpage and fracture during fabrication and during casting. In some advanced experimental blade designs casting yields of less than 10% are achieved, principally because of core failure. 
     Conventional ceramic cores are produced by a molding process using a ceramic slurry and a shaped die; both injection molding and transfer-molding techniques may be employed. The pattern material is most commonly wax although plastics, low melting-point metals, and organic compounds such as urea, have also been employed. The shell mold is formed using a colloidal silica binder to bind together ceramic particles which may be alumina, silica, zirconia and alumina silicates. 
     The investment casting process to produce a turbine blade, using a ceramic core, will be explained briefly here. A ceramic core having the geometry desired for the internal cooling passages is placed in a metal die whose walls surround but are generally spaced away from the core. The die is filled with a disposable pattern material such as wax. The die is removed leaving the ceramic core embedded in a wax pattern. The outer shell mold is then formed about the wax pattern by dipping the pattern in a ceramic slurry and then applying larger, dry ceramic particles to the slurry. This process is termed stuccoing. The stuccoed wax pattern, containing the core, is then dried and the stuccoing process repeated to provide the desired shell mold wall thickness. At this point the mold is thoroughly dried and heated to an elevated temperature to remove the wax material and strengthen the ceramic material. 
     The result is a ceramic mold containing a ceramic core which in combination define a mold cavity. It will be understood that the exterior of the core defines the passageway to be formed in the casting and the interior of the shell mold defines the external dimensions of the superalloy casting to be made. The core and shell may also define casting portions such as gates and risers which are necessary for the casting process but are not a part of the finished cast component. 
     After the removal of the wax, molten superalloy material is poured into the cavity defined by the shell mold and core assembly and solidified. The mold and core are than removed from the superalloy casting by a combination of mechanical and chemical means. 
     As previously noted, the currently used ceramic cores limit casting designs because of their fragility and because cores with dimensions of less than about 0.012-0.015 inches cannot currently be produced with acceptable casting yields. 
     Accordingly, it is an object of this invention to provide cores for investment casting which have improved mechanical properties. 
     It is another object of the invention to provide cores which can be made in thinner thicknesses than current ceramic cores. 
     It is another object of the invention to provide cores which are resistant to thermal shock during the casting process. 
     It is another object of the invention to provide cores which have geometries and features which cannot be achieved in ceramic cores. 
     It is another object of the invention to provide cores which allow rapid implementation of complex design changes without the need to employ costly tooling and processes. 
     DISCLOSURE OF INVENTION 
     To achieve the foregoing objectives and to provide other benefits, in accordance with the present invention, cores are described which include refractory metal elements. 
     Refractory metals include molybdenum, tantalum, niobium, tungsten, and alloys thereof. For purposes of this invention, the term “refractory metals” will also be understood to include intermetallic compounds based on the foregoing refractory metals. 
     According to one embodiment of the invention, wires of these refractory metals are embedded in ceramic cores to provide improved mechanical properties. 
     In accordance with another embodiment of the invention, a ceramic core may be formed about a sheet of refractory material which has previously been cut and shaped to conform to at least a portion of the required core geometry. 
     In accordance with another embodiment of the invention a refractory wire or sheet metal element may form a portion of a core and may be exposed to the molten metal during the casting process. 
     In accordance with embodiments of the invention, the refractory metal core components may be coated with one or more layers of protective material to prevent the refractory constituents from interacting with the molten metal during casting. 
     In accordance with another embodiment of the invention, investment casting cores may be fabricated from multiple ceramic and refractory metal components. 
     The present invention may be understood by reference to the following drawings taken with the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified, broken away elevation of a gas turbine engine; 
     FIG. 2 is an enlarged, cross-sectional elevation of an airfoil of the gas turbine engine of FIG. 1; 
     FIG. 3 is an elevation of a ceramic core defining cooling passages for manufacturing of the airfoil of FIG. 2 according to the present invention; and 
     FIG. 4 is a cross-sectional elevation of the ceramic core taken in the direction of  4 — 4  in FIG.  3 . 
     FIG. 5 shows a cross sectional elevation of a ceramic core taken in the direction of  4 — 4 , illustrating embodiments of the invention 
     FIG. 6 shows mechanical attachment schemes; 
     FIG. 7 shows a refractory metal core detail for forming a convoluted cooling passage. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As previously noted, conventional ceramic cores are currently a limiting factor in the design of advanced complex superalloy articles because they impose dimensional limitations on casting design. FIG. 5 illustrates various embodiments of the present invention. FIG. 5 shows the cross sectional elevation as in FIG. 4 with various illustrative refractor metal elements. 
     Referring now to FIG. 5 which illustrates embodiments of the invention, one or more refractory metal wires  200  may be embedded within the ceramic core to provide strength and resistance to cracking and warping. Although shown as circular in cross section, other wire cross sections may be employed. 
     Wire  202  may also be located adjacent the surface ceramic of core  120  and may provide a core surface contour. 
     Refractory metal sheet elements may also be utilized. Refractory metal sheet elements  204  may be located at the surface of a core element; or a shaped refractory sheet element  206  may be shaped to form a radius and corner of a core element; similarly, a refractory metal element  208  may form three sides and two corners of a ceramic core element. Refractory sheet metal element  210  may be located largely within a core element, extending from one surface to another, or refractory core element  212  may be located entirely within a core element. 
     The trailing edge  128  or any one or more core elements of the core  120  may be formed entirely from a refractory metal sheet to provide a thinner core element with usable properties than could otherwise be produced from ceramic. 
     Core elements or entire cores may also be built up from multiple shaped sheets  216  of refractory metals joined using various methods including resistance welding, T1G welding, brazing, and diffusion bonding. 
     The previously described embodiments are illustrative. The core designer may use any one or more of these embodiments in a core design, utilizing them as appropriate in view of the specific core design. 
     FIG. 6 shows how a thin refractory sheet metal trailing edge core component can be used to form a part of an overall investment casting core. The thin refractory metal element  220  can be attached to the ceramic portion  222  by providing a refractory metal component with regions  224  which protrude or recessed pockets  226  injecting the ceramic around this protruding element, and/or into the pockets to provide a mechanical lock between the ceramic element and the refractory metal element. 
     FIG. 7 illustrates how refractory metal core elements  230  can be used to form small diameter cooling holes within the wall of an airfoil. In FIG. 7, refractory element  230  extends between the core  232  and the shell  234 . Refractory element  230  will form a convoluted cooling passage in the wall of a turbine component, a cooling passage which could not be formed by casting using conventional core technology. 
     The refractory alloys of Mo, Cb, Ta and W are commercially available in standard shapes such as wire and sheet which can be cut as needed to form cores using processes such as laser cutting, shearing, piercing and photo etching. The cut shapes can be deformed by bending and twisting. The standard shapes can be corrugated or dimpled to produce passages which induce turbulent airflow. Holes can be punched into sheet to produce posts or turning vanes in passageways. 
     Refractory metals are generally prone to oxidize at elevated temperatures and are also somewhat soluble in molten superalloys. Accordingly, refractory metal cores require a protective coating to prevent oxidation and erosion by molten metal. Refractory metal core elements can be coated with one or more thin continuous adherent ceramic layers for protection. Suitable ceramics include silica, alumina, zirconia, chromia, mullite and hafnia. Preferably, the coefficient of thermal expansion (C.T.E.) of the refractory metal and the ceramic are similar. Ceramic layers may be applied by CVD, PVD, electrophoresis, and sol gel techniques. 
     Multiple layers of different ceramics may be employed. Individual layers will typically be 0.1 to 1 mil thick. 
     Metallic layers of Pt, other noble metals, Cr and Al may be applied to the refractory metal elements for oxidation protection, in combination with a ceramic coating for protection from molten metal erosion. 
     Refractory metal alloys and intermetallics such as Mo alloys and MoSi2, respectively, which form protective SiO2 layers may also be preferred. Such materials are expected to allow good adherence of a non-reactive oxide such as alumina. It is understood that silica though an oxide is very reactive in the presence of nickel based alloys and must be coated with a thin layer of other non-reactive oxide. However, by the same token silica readily diffusion bonds with other oxides such as alumina forming mullite. 
     For purposes of the invention, metals containing solid solution strengtheners, precipitation strengtheners and dispersion strengtheners are classed as alloys. 
     Alloys of Mo include TZM (0.5% Ti, 0.08% Zr, 0.04% C, bal Mo), and lanthanated Molybdenum Alloys of W include W-38% Re. 
     The previously noted alloys are by way of example and are not intended to be limiting. 
     After the casting process is complete the shell and core are removed. The shell is external and can be removed by mechanical means to break the ceramic away from the casting, followed as necessary by chemical means usually involving immersion in a caustic solution. 
     In the prior art, ceramic cores are usually removed using caustic solutions, often under conditions of elevated temperatures and pressures in an autoclave. 
     To the extent that the invention cores are partially ceramic, the same caustic solution core removal techniques may be employed. 
     The refractory metal portion of the invention cores may be removed from superalloy castings by acid treatments. For example, to remove Mo cores from a nickel superalloy, we have used 40 parts HNO 3  30 parts H 2 S0 4 , bal H 2 0 at temperatures of 60-100° C. 
     For refractory metal cores of relatively large cross sectional dimensions thermal oxidation can be used to remove Mo which forms a volatile oxide. In Mo cores of small cross sections, we have found thermal oxidation to be ineffective. 
     As noted, cores based on the metals Mo, Nb, W and Te and alloys thereof, along with intermetallic compounds based on these metals are preferred.