Patent Publication Number: US-9835035-B2

Title: Cast-in cooling features especially for turbine airfoils

Description:
FIELD OF THE INVENTION 
     The present invention relates to the casting of metal or alloy articles of manufacture and more particularly, to a method of making a ceramic core and cooperating integral ceramic mold, or mold portion, useful though not limited to, the casting a turbine airfoil with cast-in cooling features and enhanced external casting wall thickness control. 
     BACKGROUND OF THE INVENTION 
     Most manufacturers of gas turbine engines are evaluating advanced multi-wall, thin-wall turbine airfoils (i.e. turbine blade or vane) which include intricate air cooling channels to improve efficiency of airfoil internal cooling to permit greater engine thrust and provide satisfactory airfoil service life. However, cooling schemes for advanced high-thrust aircraft engines are complex, often involving multiple, thin walls and non-planar cooling features. The ceramic cores that define these advanced cooling schemes are conventionally formed by forcing ceramic compound into steel tooling, but core complexity is limited by the capabilities of tooling design/fabrication. Therefore, complex advanced cooling schemes often rely on the assembly of multiple ceramic core pieces after firing. Assembly requires specialized labor and results in core dimensional variability due to mismatch between assembled core components, while the fragile nature of fired cores results in elevated handling scrap, and compromises to the advanced cooling schemes are required to allow for assembly and positioning of the core assembly or multiple core pieces in the subsequent casting. 
     Some core geometries require the formation of multiple fugitive core inserts to define features that do not operate in common planes, including: (1) multiple skin core segments, (2) trailing edge features (e.g., pedestals and exits), (3) leading edge features (e.g., cross-overs), and (4) features that curve over the length of the airfoil. Forming multiple fugitive inserts and assembling them in a core die presents a similar problem to that created by core assembly. Intimate contact between inserts may not be insured when they are loaded into a core die, either due to dimensional variability in the individual inserts or poor locating schemes in the core die. Subsequent molding of the ceramic core material may result in formation of flash at the union of two fugitive insert segments. While flash is common in ceramic core molding and is removed as part of standard processing, flash around or between fugitive inserts may reside in hidden, internal cavities or as part of intricate features, where inspection and removal is not possible. Any such flash remaining in the fired ceramic core can alter air flow in the cast blade or vane. 
     U.S. Pat. Nos. 5,295,530 and 5,545,003 describe advanced multi-walled, thin-walled turbine blade or vane designs which include intricate air cooling channels to this end. 
     In U.S. Pat. No. 5,295,530, a multi-wall core assembly is made by coating a first thin wall ceramic core with wax or plastic, a second similar ceramic core is positioned on the first coated ceramic core using temporary locating pins, holes are drilled through the ceramic cores, a locating rod is inserted into each drilled hole and then the second core then is coated with wax or plastic. This sequence is repeated as necessary to build up the multi-wall ceramic core assembly. 
     This core assembly procedure is quite complex, time consuming and costly as a result of use of the multiple connecting and other rods and drilled holes in the cores to receive the rods. In addition, this core assembly procedure can result in a loss of dimensional accuracy and repeatability of the core assemblies and thus airfoil castings produced using such core assemblies. 
     U.S. Pat. No. 6,626,230 describes forming multiple fugitive (e.g. wax) thin wall pattern elements as one piece or as individual elements that are joined together by adhesive to form a pattern assembly that is placed in a ceramic core die for molding a one-piece core. 
     U.S. Pat. No. 7,258,156 describes the use of ceramic cores and refractory metal cores that are used to form trailing edge cooling passage exits or convoluted airfoil cast-in cooling features wherein the cores are removed to define internal cooling features. 
     Copending application U.S. Ser. No. 13/068,413 filed May 10, 2011, of common assignee herewith, describes a method of making multi-wall ceramic core wherein at least one fugitive core insert is pre-formed and then at least another fugitive core insert is formed in-situ connected to the pre-formed core insert to from complex cores with internal walls that cannot be readily inspected or repaired once the core is formed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method useful for, although not limited to, making a mold for casting of advanced turbine airfoils (e.g. gas turbine blade and vane castings) which can include complex cast-in internal and/or external cooling features to improve efficiency of airfoil cooling during operation in the gas turbine hot gas stream. 
     An illustrative method involves the steps of incorporating at least one fugitive insert in a ceramic material in a manner to form a core and at least a portion of an integral, cooperating mold wall wherein the core defines an internal feature to be imparted to the cast article and the at least portion of the mold wall has an inner surface that defines an external feature to be imparted to the cast article, selectively removing the fugitive insert, and incorporating the core and the at least portion of the integral, cooperating mold wall in a mold for receiving molten metal or alloy wherein the core defines an internal feature to be imparted to the cast article and the mold wall has an inner surface that defines an external feature to be imparted to the cast article. Solidification of molten metal or alloy in the mold produces such cast-in internal and external features of the cast article. 
     The present invention can be practiced to form a core with only a portion of an integral cooperating mold wall wherein the missing mold wall portions can be subsequently formed by conventional shell investment molding steps to provide a complete mold shell about the core. Alternately, the present invention can be practiced to form in one step in the first die a ceramic core and a substantially complete integral, cooperating ceramic mold for casting a turbine airfoil or other article of manufacture. 
     In practice of the present invention to cast a turbine airfoil, certain core surfaces can form cast-in internal cooling features, such as internal cooling air passages with turbulators to increase cooling efficiency, while the inner surface of the integral, cooperating mold wall can form cast-in external cooling air exit holes penetrating the adjacent external airfoil surface, and features on the casting external surface that enhance performance such as features that reduce aerodynamic drag or assist in coating adherance, when the molten metal or alloy is solidified. 
     Practice of the present invention is advantageous in that complex external cooling features, such as film cooling air exit holes and/or features that reduce aerodynamic drag or assist in coating adherance, can be cast-in external airfoil surfaces in locations and/or orientations that are not possible by post-cast machining operations, such as drilling, with shapes and tapers to improve cooling performance and with improved external and internal casting wall thickness control. Further, the thermal expansion characteristics of the core and cooperating mold wall are matched at least at the local region and can be tailored to provide desired thermal and/or mechanical properties in the mold as a whole or locally to reduce hot tearing in equiaxed castings, local recrystallization in DS/SC castings, and/or provide local grain size control. Moreover, practice of certain embodiments of the invention can be used to reduce or eliminate the extent of conventional investment shelling steps needed to form the mold. 
     Other advantages of the practice of the present invention will become more readily apparent from the following detailed description taken with the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a cast metal or alloy turbine blade having a pattern of cast-in cooling air exit holes penetrating the external airfoil surface and communicated to internal cast-in cooling air passages as shown in  FIG. 2 . 
         FIG. 2  is a sectional view along a single plane of the metal or alloy turbine blade taken normal to the stacking axis of the turbine blade of  FIG. 1  showing the cast-in cooling air exit holes connected to cast-in internal cooling air passages that are formed when the core is removed. 
         FIG. 3  is a sectional view of a transient (fugitive) insert residing in a first molding die in which ceramic material is injection or transfer molded to incorporate the transient insert into a ceramic component useful for casting after the insert is removed. 
         FIG. 3A  is an enlarged view of the region A of  FIG. 3 . 
         FIG. 3B  is an enlarged view of the region B of  FIG. 3 . 
         FIG. 4  is a sectional view of the transient (fugitive) insert after the ceramic core and integral, cooperating mold walls are formed. 
         FIG. 5  is a sectional view of the transient (fugitive) insert after the ceramic core and integral, cooperating mold walls are formed and after a mold shell is invested about regions of the core so as to provide a complete mold shell. 
         FIG. 6A through 6E  illustrate different types of cooling air hole configuration that can be formed pursuant to illustrative embodiments of the invention. 
         FIG. 7  is a sectional view of a transient (fugitive) insert residing in a first molding die which is designed to form a substantially complete mold shell and core about the insert when ceramic material is injection or transfer molded. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In order to make aero and/or industrial gas turbine engine airfoil cooling air schemes most effective, especially high pressure turbine blade and vanes (hereafter turbine airfoils), internal cooling features, such as air cooling passages, support pedestals, etc. as well as external cooling features, such as film cooling air exit holes, cooling-enhancing turbulators, etc. need to precisely partition and direct the cooling air such that its pressure is controlled and it is directed to the most needed regions of the blade or vane. Practice of the present invention permits production of complex airfoil geometries with complex cast-in internal and external cooling features and enhanced external casting wall thickness control. 
     Although the present invention will be described below in connection with the casting of advanced turbine airfoils (e.g. gas turbine blade and vane castings) which can include complex cast-in internal and external cooling air features to improve efficiency of airfoil cooling during operation in the gas turbine hot gas stream, the invention is not limited to turbine airfoils and can be practiced to produce other cast articles that include complex cast-in internal and/or external features pursuant to a particular design specification. 
     Referring to  FIGS. 1 and 2 , a cast gas turbine blade  10  is illustrated having an airfoil region  10   a , a root region  10   b , and a platform region  10   c  between the airfoil region and the root region. The airfoil region  10   a  is shown having a pattern of cast-in cooling air exit holes  20  communicated to the external airfoil surface and also communicated to cast-in internal cooling air passages  22  leading to and communicated with main cooling air passages  23  that receive cooling air. The particular spatial arrangement and number of cast-in cooling air exit holes  20  and air cooling passages  22 ,  23  are shown only for purposes of illustration and not limitation since each particular turbine airfoil design can be different in this regard. 
     The gas turbine blade  10  (or vane) can be cast using conventional nickel based superalloys, cobalt superalloys, titanium, titanium alloys, and other suitable metals or alloys including intermetallic materials. Practice of the present invention is not limited to any particular metal or alloy. Moreover, the turbine blade (or vane) can be cast using different conventional casting processes including, but not limited to, equiaxed casting processes to produce an equiaxed grain turbine blade or vane, directional solidification casting processes to produce a columnar grain turbine blade or vane, and single crystal casting processes to produce a single crystal turbine blade or vane. Practice of the present invention is not limited to any particular casting process. 
     Referring to  FIGS. 3, 4 and 5 , an illustrative method embodiment pursuant to the present invention is shown for purposes of illustration and not limitation. In this embodiment, a preformed transient (fugitive) insert  50  is provided for positioning in a core molding die D as shown best in  FIG. 3 , which illustrates the fugitive insert  50  as including internal insert main cavities  51  and internal insert passages  53  communicated to associated mold wall-forming cavities  55   a ,  55   b  formed as shown by cooperation of the insert surfaces and the inner surface recesses of the molding die D. The cavities  51 , passages  53 , and cavities  55   a ,  55   b  are subsequently filled with the ceramic material by injection or transfer molding, or pouring of a suitable ceramic material. The preformed fugitive insert  50  can be molded as one-piece, over-molded in two or more injections, or as multiple injection molded pieces or injection molded partial pieces, and assembled together. Over-molding to provide multi-piece fugitive insert is described in copending U.S. application Ser. No. 13/068,413, the teachings of which are incorporated herein by reference to this end. 
     Moreover, although the fugitive insert  50  is shown for convenience as a single piece in  FIGS. 3 and 4 , fugitive insert  50  can comprise multiple, preformed insert components or pieces molded individually and then assembled together and placed in the molding die D. The preformed multiple insert components or pieces can be assembled together in proper relationship using adhesive, interlocking between components, and/or over-molding to collectively form the desired final fugitive insert configuration. 
     The fugitive insert  50 , whether one-piece or multi-piece, can be molded from a fugitive material that can tolerate the temperature conditions typically employed to form ceramic cores using thermoplastic or thermosetting binders by injection or transfer molding, or pouring. Such temperature can range from 100 to 400 degrees F. For purposes of illustration and not limitation, the fugitive insert  50  can be made of soluble resins or high temperature liquid crystal polymers, that are soluble in water or other liquids such as alcohols, mild or strong acids, keytones and mineral spirits. 
       FIG. 3  shows the fugitive insert  50  placed in the core molding die D with  FIGS. 3A and 3B  showing enlarged views of the regions A and B, respectively, of  FIG. 3 . The fugitive insert  50  can be positioned in proper relationship in the cavity of the molding die using molded-on surface features of the insert  50  itself and/or by using positioning pins (not shown) otherwise known as locating pins or chaplets. The ceramic material is introduced into the molding die to fill the cavities  51 , passages  53 , and mold wall-forming cavities  55  and is allowed to cure and/or set for a time to reach a rigid ceramic state. To this end, for purposes of illustration and not limitation, the ceramic material can comprise silica based, alumina based, zircon based, zirconia based, yttria based, erbia based or other suitable core ceramic materials in slurry mixtures known to those skilled in the art containing a thermoplastic or thermosetting binder. Suitable ceramic core materials are described in U.S. Pat. No. 5,394,932, which is incorporated herein by reference. The core material is chosen to be chemically leachable from the cast turbine airfoil formed thereabout as is known. The ceramic material is initially fluid (e.g. a ceramic slurry) for injection or transfer molding, or pouring and cures and/or sets to the rigid state in the molding die. 
       FIG. 4  shows the ceramic core  100  and integral, cooperating mold wall portions  102   a ,  102   b  formed on the fugitive insert  50  as a result of the ceramic material filling the insert cavities  51 , passages  53 , and cavities  55   a ,  55   b  following removal of the assembly from the molding die D. In this embodiment of the invention, it is apparent that only a portion of the mold wall  102   a  is formed about the fugitive insert  50  in the preceding step shown in  FIG. 3 . According to one processing sequence, the fugitive insert  50  is selectively removed from the core  100  and the mold wall portions  102   a ,  102   b , which then are fired at elevated temperature as described herein to develop desired core/wall strength for further processing. A second fugitive pattern, such as wax or plastic, is formed on the fired core  100  and the mold wall portions  102   a ,  102   b  to provide a pattern assembly. For example, the fired core  100  with integral mold wall portions  102   a ,  102   b  are placed in a pattern injection die, and a desired fugitive pattern is formed on the fired core  100  and integral mold wall portions  102   a ,  102   b . The resulting pattern assembly resembles the assembly shown in  FIG. 4  with a second pattern replacing the fugitive insert  50 . To this end, the reference character P is shown immediately below the core insert reference numeral  50  in  FIG. 4 . Use of the second pattern may be advantageous to allow inclusion of further pattern root, platform or airfoil features at other section lines or planes of the turbine blade pattern that cannot be provided on the fugitive insert  50  due to core geometry complications and also allows selection and use of an easier-to-remove pattern material than insert material such that selective removal of the pattern from the final mold/core can be conducted more easily and completely than with the core insert material. The pattern assembly then is incorporated in a mold followed by removal of the pattern to yield a mold with internal integral core of the type shown as mold M and integral core  100  in  FIG. 5 . 
     In this processing sequence, the fugitive insert  50  or second pattern P can be selectively removed by dissolution if the insert or pattern comprises a soluble material, by thermal degradation if the insert or pattern comprises a thermal degradable material, or any other suitable means appropriate to the insert material being selectively. 
     According to another more direct processing sequence which may only be possible with some core geometries, the core  100  and the integral mold wall portions  102   a ,  102   b  on the fugitive insert  50 ,  FIG. 4 , are incorporated directly in the mold M followed by removal of the fugitive insert  50  to yield the mold M with internal core C of  FIG. 5 . The mold and integral core then are fired at elevated temperature as described herein to remove the core insert  50  and develop desired core/wall strength for casting of molten metal or alloy therein. This processing sequence eliminates the step of forming a second pattern P as described in the preceding two paragraphs. 
     In these processing sequences, the missing mold shell wall is formed in a further subsequent processing step where additional ceramic material is invested or otherwise formed about regions of the fired core  100  and integral mold wall portions  102   a ,  102   b  (first processing sequence) or about the unfired core  100  and mold wall portions  102   a ,  102   b  on fugitive insert  50  (second processing sequence) where missing the mold shell  102   a  as shown in  FIG. 5  in a manner to form a complete mold shell M (i.e. the remainder of the mold wall. In this investing step, the mold wall portions  102   b  also function to interlock with the mold shell M to lock the core  100  in position. The mold shell M is invested by processing pursuant to conventional investment shell molding processing by repeated dipped in ceramic slurry, drained of excess slurry, and stuccoed with coarse grain ceramic stucco particles until the mold shell M of desired mold wall thickness is built-up. 
     Alternately, referring to  FIG. 7 , the present invention can be practiced to form in one step a core  100 ′ and a substantially complete integral, cooperating mold shell M′ for casting a turbine airfoil or other article of manufacture. This embodiment is illustrated in  FIG. 7  where the core  100 ′ and mold shell M′ are formed in molding die D′. In  FIG. 7 , like features of previous figures are represented by like reference numerals primed. This embodiment of the invention greatly reduces or eliminates the need for the investment shelling operations discussed above to complete a mold shell about the core. 
     The present invention is capable of forming different types of cast-in cooling air passages/exit hole configurations as illustrated in  FIGS. 6A, 6B, 6C, 6D, and 6E , which illustrate a straight angled cooling passage  22  having external exit hole  20 , an end-flared cooling passage  22  having an external exit hole  20 , a convoluted cooling passage  22  having an external exit hole  20 , a converging (i.e. focusing conical) cooling passage  22  having an external exit hole  20 , and diverging (i.e. diverging conical) cooling passage  22  having an external exit hole  20 , respectively, which can be formed using the fugitive insert  50  appropriately shaped to this end. These cast-in cooling hole configurations are offered for purposes of illustration and not limitation as other configurations can be formed by practice of the invention. 
     Referring back to  FIG. 5 , the assembly shown can be subjected to an appropriate high temperature firing treatment, such as sintering, to impart a desired strength to the mold shell M, mold wall portions  102   a ,  102   b , and core  100  for casting. For casting a turbine blade  10 , molten superalloy then is introduced into the mold cavity MC defined between the mold wall  102 /mold shell M and the ceramic core  100  using conventional casting techniques. For example, molten superalloy can be poured into a pour cup (not shown) and gravity fed through a down sprue (not shown) to the mold cavity. The molten superalloy can be solidified in a manner to produce an equiaxed grain turbine blade, directionally solidified to form a columnar grain turbine blade, or solidified as a single crystal turbine blade casting. The mold wall  102 /mold shell M are removed from the solidified cast turbine blade using a mechanical knock-out operation followed by one or more known chemical leaching or mechanical grit blasting techniques. The core  100  is selectively removed from the solidified cast turbine blade by chemical leaching or other conventional core removal techniques, yielding the turbine blade of  FIG. 1  having the cast-in air cooling holes and passages shown wherein the core  100  forms internal cooling features such as cooling passages  22 ,  23  and the inner surface of the mold wall portions  102   a ,  102   b  form external features such as exit cooling holes  20  penetrating the adjacent external airfoil surface. 
     The present invention can produce core/mold wall geometries that require features that do not operate in common planes, including: (1) multiple skin core segments, (2) trailing edge features (e.g., pedestals and exits), (3) leading edge features (e.g., cross-overs), and (4) features that curve over the length of the airfoil. While one preformed fugitive insert  50  was over molded in the above description, in practice of the invention any number of preformed fugitive inserts can be preformed, assembled and over-molded with the ceramic material,  FIG. 3 . 
     Practice of the present invention is advantageous in that complex external cooling features, such as film cooling holes and/or cooling-enhancing turbulators, can be cast-in external cast airfoil surfaces in locations and/or orientations that are not possible by post-cast machining operations, such as drilling, with shapes and tapers to improve cooling performance and with improved external and internal casting wall thickness control. Further, the need for subsequent core pinning or locating is reduced or eliminated since the core not only forms the internal blade features, but also at least a portion of the external shell mold which more precisely locates the core with respect to the shell mold. The thermal expansion characteristics of the core and cooperating mold wall are matched at least at the local region and can be tailored to provide desired thermal and/or mechanical properties in the mold as a whole or locally to reduce hot tearing in equiaxed castings, local recrystallization in DS/SC castings, and/or provide local grain size control. Still further, a molten metal or alloy filter, such as a reticulated foam filter or lattice filter, can be molded into a down-sprue connected to the assembly of  FIG. 5  to improve cleanliness of molten metal or alloy being delivered to the mold cavity. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the embodiments of the present invention described above without departing from the spirit and scope of the invention as set forth in the appended claims.