Patent Publication Number: US-7905273-B2

Title: Method of forming a cast metal article

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of forming a cast metal article using a core assembly having a base core with a core element disposed in an opening formed in the base core. 
     Ceramic cores have previously been utilized to form openings or passages in cast metal articles, such as turbine engine components. The turbine engine components may be blades or vanes. In U.S. Pat. No. 6,929,054, it is suggested that a refractory metal article, such as a wire or sheet, can be cut and utilized as a core element in association with a ceramic base core. 
     When a core element is used in combination with a ceramic base core, difficulty may be encountered due to the ceramic base core having a different coefficient of thermal expansion than the core element. For example, the core element may be a refractory metal article having a coefficient of thermal expansion of approximately 7.0×10 −6  inches per inch per degree centigrade. The ceramic base core may be formed of silica and have a coefficient of thermal expansion of approximately 0.5×10 −6  inches per inch per degree centigrade. The relatively high coefficient of thermal expansion of the core element can result in a cracking of the ceramic base core during firing. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new and improved method of forming a cast metal article. The method includes providing a ceramic base core having a first coefficient of thermal expansion. A core element having a second coefficient of thermal expansion is positioned in an opening formed in the ceramic base core. The core element may, for example, be formed of a refractory metal or a ceramic material. Of course, the core element may be formed of other materials. 
     The opening in the ceramic base core is filled with filler material having a third coefficient of thermal expansion. The third coefficient of thermal expansion may be greater than the first coefficient of thermal expansion and less than the second coefficient of thermal expansion. 
     Although it is contemplated that the ceramic base core may have many different compositions, the ceramic base core may contain silica and zircon. It is also contemplated that the filler material may have many different compositions. However, the filler material may contain silica and zircon. Alternatively, the filler material may contain mullite. Regardless of its composition, the filler material may advantageously be formed of particles having substantially the same particle size. 
     The present invention includes many different features which may advantageously be utilized together as disclosed herein. Alternatively, the features may be utilized separately or in various combinations with each other and/or with features from the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the present invention will become more apparent upon a consideration of the following description taken in connection with the accompanying drawings wherein: 
         FIG. 1  is an enlarged fragmentary schematic illustration depicting the relationship between a ceramic base core, a refractory metal core element which is received in an opening in the base core, and filler material which is received in the opening in the base core; 
         FIG. 2  is a schematic illustration, generally similar to  FIG. 1 , illustrating the relationship between the base core, refractory metal core element and a covering of wax; 
         FIG. 3  is a fragmentary schematic illustration, generally similar to  FIGS. 1 and 2 , illustrating the relationship of a layer of ceramic mold material to the covering of wax, the base core and the refractory metal core element; 
         FIG. 4  is a fragmentary schematic illustration, generally similar to  FIG. 3 , illustrating a space formed in a mold which at least partially encloses the base core and refractory metal core element, by removal of the covering of wax; 
         FIG. 5  is a fragmentary schematic illustration, generally similar to  FIG. 4 , illustrating the manner in which the mold is filled with molten metal; 
         FIG. 6  is a fragmentary schematic illustration, generally similar to  FIG. 1 , depicting the manner in which the refractory metal core element may be positioned in an opening in the base core with filler material located between opposite sides of the refractory metal core element and surfaces of the opening in the base core; 
         FIG. 7  is a fragmentary schematic illustration, generally similar to  FIG. 1 , depicting the manner in which a ceramic core element may be positioned in the base core so that the filler material is located between one side of the ceramic core element and surfaces of the opening in the base core; and 
         FIG. 8  is a fragmentary schematic illustration, generally similar to  FIG. 6 , depicting the manner in which a ceramic core element may be positioned in an opening in the base core so that the filler material is located between opposite sides of the ceramic core element and surfaces of the opening in the base core. 
     
    
    
     DESCRIPTION OF SPECIFIC PREFERRED EMBODIMENTS OF THE INVENTION 
     A method of forming a cast metal article is illustrated schematically in  FIGS. 1-5 . An unfired ceramic base core  10  ( FIG. 1 ) has an opening  12  in which a core element  14  is positioned. The opening  12  in the base core  10  is filled with filler material  16 . The base core  10 , filler material  16 , and core element  14  form a core assembly  20  having a configuration corresponding to a configuration of a desired space in a cast metal article. 
     The cast metal article may be a blade or vane for use in a turbine engine. Alternatively, the cast metal article may be a housing or other portion of a turbine engine. For example, the cast metal article may be a blade outer air seal. It should be understood that the core assembly  20  may be utilized in the making of cast metal articles other than components of a turbine engine. 
     In the embodiment of the invention illustrated in  FIG. 1 , the opening  12  is a linear, longitudinally extending slot having a rectangular cross sectional configuration. However, the opening  12  may have a different configuration if desired. For example, the opening  12  may have an arcuate longitudinal and/or transverse cross sectional configuration. The opening  12  may be molded into the base core  10  during formation of the base core or may be cut into the base core after formation of the base core. 
     The core element  14  is formed of a refractory metal and has the configuration of a flat plate. The opening  12  has a width which is greater than the thickness of the core element  14  so that a portion of the core element can be positioned in the opening. The core element  14  may be formed of different materials and with a different configuration if desired. 
     Holes may be formed in and projections or tabs may extend from the core element  14 . For example the core element  14  may have an arcuate configuration with radially and/or axially extending flanges. The core element  14  may be formed as a wire. The core element  14  may be formed of a refractory metal and may have any one of the configurations disclosed in U.S. Pat. No. 6,929,054 or 6,637,500. Of course, the core element  14  may be formed of a different material and may have a different configuration. 
     The core element  14  may be formed as one piece or a plurality of pieces. When the core element  14  is formed of a refractory metal, the core element may be formed of molybdenum, tantalum, niobium, tungsten and/or alloys thereof. The refractory metal core element  14  may be an intermetallic compound based on any of the foregoing refractory metals or similar metals. 
     The core assembly  20  is fired to dry the material of the ceramic base core  10  and the filler material  16 . As this occurs, a secure bond is formed between the filler material  16  and both the base core  10  and the refractory metal core element  14 . This results in the refractory metal core element  14  being securely held in a desired orientation relative to the base core  10 . 
     Once the core assembly  20  has been fired, a wax covering  22  ( FIG. 2 ) is applied over the core assembly  20 . The wax covering may be applied to the core assembly  20  by positioning the core assembly in a die and injecting hot wax around the core assembly. Of course the covering  22  of wax may be applied to the core assembly  20  in a different manner if desired. 
     The covering  22  of wax cooperates with the core assembly  20  to form a pattern assembly  24 . The pattern assembly  24  has a configuration corresponding to the configuration of an article to be cast. The core assembly  20  has a configuration corresponding to the configuration of a desired space or passage within the cast article. 
     Once the pattern assembly  24  has been formed in the manner previously explained, a layer  28  ( FIG. 3 ) of ceramic mold material is applied over the pattern assembly  24 . The layer  28  of ceramic mold material encloses the pattern assembly  24  and is formed with suitable gating. 
     The layer  28  of wet ceramic mold material is fired or dried to form a mold  32  ( FIG. 4 ) which encloses the core assembly  20 . During firing of the layer  28  ( FIG. 3 ) of wet ceramic mold material, the covering  22  of wax is melted and a cavity or space  34  ( FIG. 4 ) is formed within the mold  32 . The mold cavity or space  34  has a configuration corresponding to the desired configuration of a cast metal article  38  ( FIG. 5 ). The core assembly  20  which is enclosed by the mold  32  ( FIG. 4 ), has a configuration corresponding to the configuration of a desired space within the cast metal article  38 . 
     To form a cast metal article  38  ( FIG. 5 ), molten metal is poured into the mold cavity  34  ( FIG. 4 ) and solidified around the core assembly  20 . This results in the formation of the cast metal article  38  with a configuration corresponding to the configuration of the mold cavity  34 . The mold  32  is removed from around the cast metal article  38 . The core assembly  20  is then removed from within the cast metal article  38 . 
     Although the cast metal article  38  may be formed of many different metals, the cast metal article  38  is formed of a nickel-chrome superalloy. However, the cast metal article may be formed of titanium, or other metals. 
     During formation of the cast metal article  38 , difficulty has previously been encountered due to cracking of the ceramic base core  10  and/or the filler material  16  during firing of the core assembly  20 . This cracking is due, in part at least, to differences in the coefficients of thermal expansion of ceramic base core  10 , filler material  16 , and refractory metal core element  14  ( FIG. 1 ). The ceramic base core  10  and filler material  16  have previously been formed of silica which has a coefficient of thermal expansion of approximately 0.5×10 −6  inches per inch per degree centigrade. The refractory metal core element  14  may have been formed of molybdenum which has a coefficient of thermal expansion of approximately 7.0×10 −6  inches per inch per degree centigrade. It is believed that the different coefficients of thermal expansion of the components of the core assembly  20  have resulted in cracking of the filler material  16  and/or the ceramic base core  10 . 
     In accordance with one of the features of the present invention, the filler material  16  has a coefficient of thermal expansion which is greater than the coefficient of thermal expansion of the ceramic base core  10  and less than the coefficient of thermal expansion of the refractory metal core element  14 . This results in two interfaces being established between the components of the core assembly  20 . The first interface  44  ( FIG. 1 ) is formed where the filler material  16  engages the refractory metal core element  14 . At the interface  44 , a surface  48  of the filler material  16  engages a surface  50  on the refractory metal core element  14 . During firing of the core assembly  20 , force is transmitted between the refractory metal core element  14  and the filler material  16  across the interface  44 . This force results from the different coefficients of thermal expansion of the ceramic base core  10 , refractory metal core element  14 , and the filler material  16 . 
     A second interface  54  ( FIG. 1 ) is formed where the filler material  16  engages the ceramic base core  10 . At the second interface  54 , a surface  58  of the filler material  16  engages a surface  60  on the ceramic base core  10 . The surface  60  forms one side of the opening  12  and faces toward and is spaced from the surface  50  on the refractory metal core element  14 . 
     The surface  60  extends parallel to an opposite side surface  62  of the opening  12  in the ceramic base core  10 . A flat side surface  64  ( FIG. 1 ) on the refractory metal core element  14  is disposed in abutting engagement with the side surface  62  of the opening  12 . The side surface  62  locates the refractory metal core element  14  relative to the opening  12  and base core  10 . It should be understood that the opening  12  and the core element  14  may have a different configuration if desired. 
     During firing of the core assembly  20 , force is transmitted between the filler material  16  and the ceramic base core  10  across the interface  54 . Force is also transmitted between the filler material  16  and the refractory metal core element  14  across the interface  44 . These forces result from the different coefficients of thermal expansion of the refractory metal core element  14 , the filler material  16 , and the base core  10 . By providing two interfaces, that is, the interface  44  and the interface  54 , the magnitude of the force transmitted at either one of the interfaces is reduced. This results in a reduction in stress on the material of the ceramic base core  10  and on the filler material  16 . 
     To provide the two interfaces  44  and  54  between materials having different coefficients of thermal expansion, the coefficient of thermal expansion of the filler material  16  is less than the coefficient of thermal expansion of refractory metal core element  14 . In addition, the coefficient of thermal expansion of the filler material  16  is greater than the coefficient of thermal expansion of the ceramic base core  10 . If the filler material  16  was to have a coefficient of thermal expansion which was substantially the same as the coefficient of thermal expansion of the ceramic base core  10 , only a single interface would be established, that is, the interface  44  between the filler material  16  and the refractory metal core element  14 . By forming the core assembly  20  of components having three distinctly different coefficients of thermal expansion, two interfaces  44  and  54  are established between the components of the core assembly. This results in a reduction in stress on the material of the core assembly  20  at any one of the interfaces  44  and  54 . 
     In accordance with another feature of the present invention, the coefficient of thermal expansion of the base core  10  is increased so that it approaches the coefficient of thermal expansion of the refractory metal core element  14 . The refractory metal core element  14  may have a coefficient of thermal expansion of approximately 7×10 −6  inches per inch per degree centigrade. The base core  10  is formed of a mixture of silica and zircon. Silica has a coefficient of thermal expansion of approximately 0.5×10 −6  inches per inch per inch per degree centigrade. Zircon has a coefficient of thermal expansion of approximately 4.2×10 −6  inches per inch per degree centigrade. However, it should be understood that the base core  10  may be formed of different materials having different coefficients of thermal expansion. 
     To increase the coefficient of thermal expansion of the base core  10 , the base core is formed of a mixture of silica and zircon. The greater the amount of zircon provided in the base core  10 , the greater will be the coefficient of thermal expansion of the base core  10 . By increasing the coefficient of thermal expansion of the base core  10 , the magnitude of force transmitted between the components of the core assembly  20 , during firing of the core assembly, is decreased. 
     Although it is contemplated that the base core  10  may be composed of different mixtures of silica and zircon, it is believed that the base core  10  should have a silica content of 70% or less and a zircon content of 30% or more. For example, the base core  10  may be formed of a mixture which is 50% silica and 50% zircon. Alternatively, the base core  10  may be formed of a mixture of 60% silica and 40% zircon. The greater the amount of zircon in the ceramic base core  10 , the greater will be the coefficient of thermal expansion of the base core. However, core removal problems may occur if to much zircon is utilized in the base core  10 . 
     The silica and zircon forming ceramic base core  10  may have the same particle size. By providing silica and zircon with substantially with same particle size, voids between relatively large particles of one material are not filled by relatively small particles of the other material. This enables a mixture of zircon and silica particles to have a relatively large coefficient of thermal expansion. 
     Although it is believed that it may be desired to provide a base core  10  containing silica and zircon, the base core may have a different composition if desired. If the base core  10  contains silica and zircon, these materials may be present in percentages different than the specific percentages previously set forth. 
     The filler material  16  has a coefficient of thermal expansion which is greater than the coefficient of thermal expansion of the ceramic base core  10  and less than the coefficient of thermal expansion of the refractory metal core element  14 . The filler material  16 , when it is positioned in the opening  12 , may be a slurry which is a water based mixture of silica and zircon. If this is the case, the ceramic filler material  16  may have a greater zircon content than the ceramic base core  10 . 
     When the core assembly  20  is dried, water is removed from the silica and zircon forming the filler material  16 . This results in shrinkage of the ceramic filler material  16 . When the core assembly  20  is fired, additional water is removed from the ceramic filler material  16 . This results in additional shrinkage of the ceramic filler material  16 . 
     The silica and zircon in the filler material  16  may have the same particle size. By providing silica and zircon with substantially the same particle size, voids between particles are not filled with other particles. This enables the silica and zircon ceramic filler material  16  to have a relatively large coefficient of thermal expansion. 
     Alternatively, the filler material  16  may be formed of mullite (3Al 2 O 3 .2SiO 2 ) with a binder. Mullite has a coefficient of thermal expansion of approximately 5.3 inches per inch per degree Centigrade. In one embodiment of the invention, the ceramic filler material  16  was 60% silica and 40% mullite. The resulting mixture had a coefficient of thermal expansion of approximately 3.0×10 −6  inches per inch per degree centigrade. 
     The silica and mullite forming the ceramic filler material  16  may have the same particle size. By providing silica and mullite with substantially the same particle size, voids between particles are not filled with other particles. This enables the silica and mullite ceramic filler material  16  to have a relatively large coefficient of thermal expansion. 
     The silica and mullite ceramic filler material  16  has a coefficient of thermal expansion which is greater than the coefficient of thermal expansion of the ceramic base core  10  and less than the coefficient of thermal expansion of the refractory metal core element  14 . The silica and mullite ceramic filler material  16  may form a slurry which is a water based mixture of silica and mullite. This slurry is used to fill the opening  12  after the core element  14  is positioned in the opening. 
     When the core assembly  20  is dried, water is removed from the silica and mullite of the ceramic filler material  16 . This results in shrinkage of the ceramic filler material  16 . When the core assembly is fired, additional water is removed from the silica and mullite of the ceramic filler material  16 . This results in additional shrinkage of the silica and mullite of the ceramic filler material  16 . 
     It is believed that the base core  10  can be formed of a mixture of silica and zircon and will advantageously have a coefficient of thermal expansion of approximately 2.0×10 −6  inches per inch per degree centigrade. The filler material  16  may be formed of a mixture of silica and mullite and have a coefficient of thermal expansion of approximately 3.0×10 −6  inches per inch per degree centigrade. If desired, the filler material  16  may be formed of a mixture of silica, zircon, mullite and/or other materials. 
     The foregoing specific percentages of silica, zircon, and/or mullite for use in the base core  10  and filler material  16  have been set forth herein for purposes of clarity of description. It is not intended to limit the invention to a specific percentage of silica, zircon and/or mullite in either the base core  10  or the filler material  16 . In addition, the foregoing specific coefficients of thermal expansion for the base core  10 , filler material  16 , and refractory metal core element  14  have been set forth herein for purposes of clarity of description. It is not intended to limit the invention to specific coefficients of thermal expansion. It should be understood that the coefficients of thermal expansion of the base core  10  and filler material  16  will vary with variations in the silica and/or mullite content of the base core and filler material. 
     In the embodiment of the invention illustrated in  FIGS. 1-5 , two interfaces  44  and  54  have been formed between the refractory metal core element  14 , the filler material  16  and the ceramic base core  10 . In the embodiment of the invention illustrated in  FIG. 6 , four interfaces are formed between the ceramic base core, filler material and refractory metal core element. Since the embodiment of the invention illustrated in  FIG. 6  is generally similar to the embodiment of the invention illustrated in  FIGS. 1-5 , similar numerals will be utilized to identify similar components, the suffix letter “a” being associated with the numerals of  FIG. 6  to avoid confusion. 
     A core assembly  20   a  includes a ceramic base core  10   a  ( FIG. 6 ). A refractory metal core element  14   a  is received in an opening  12   a  formed in the ceramic base core  10   a . Filler material  16   a  is disposed on opposite sides of the refractory metal core element  14   a . Thus, a first body  70  of filler material is disposed on the left (as viewed in  FIG. 6 ) side of the refractory metal core element  14   a . Similarly, a second body  72  of filler material is disposed on the right side of the refractory metal core element  14   a.    
     The two bodies  70  and  72  of filler material cooperate with the ceramic base core  10   a  and refractory metal core element  14   a  to form four interfaces. Thus, a first interface  44   a  is formed where the body  70  of filler material engages a side surface  50   a  of the refractory metal core element  14   a . A second interface  54   a  is formed between the first body  70  of filler material and a side surface  60   a  of the ceramic base core  10   a.    
     In accordance with a feature of the embodiment of the invention illustrated in  FIG. 6 , a third interface  76  is formed between the second body  72  of filler material and a side surface  64   a  of the refractory metal core element  14   a . A fourth interface  82  is formed between the second body  72  of filler material and a side surface  84  of the opening  12   a  in the base core  10   a.    
     By forming four separate interfaces  44   a ,  54   a ,  76  and  82  between the ceramic base core  10   a , filler material  16   a  and refractory metal core element  14   a , the amount of force which is transmitted across any one of the interfaces is reduced with a resulting reduction in the stress applied to the ceramic base core  10   a  and the filler material  16   a . Of course, reducing the stress applied to the ceramic base core  10   a  and filler material  16   a  is effective to reduce any tendency for these components of the core assembly  20   a  to crack during firing of the core assembly. 
     The base core  10   a  ( FIG. 6 ) may be composed of different mixtures of silica and zircon. It is believed that the base core  10   a  may have a silica content of 70% or less and a zircon content of 30% or more. For example, the base core  10   a  may be formed of a mixture which is 50% silica and 50% zircon. Alternatively, the base core  10   a  may be formed of a mixture of 60% silica and 40% zircon. The greater the amount of zircon in the base core  10   a , the greater of coefficient of thermal expansion of the base core. However, core removal problems may occur if to much zircon is utilized in the base core  10   a.    
     Although it is believed that it may be desired to provide a base core  10   a  containing silica and zircon, the base core may have a different composition if desired. If the base core  10   a  contains silica and zircon, these materials may be present in percentages different than the specific percentages previously set forth. 
     The filler material  16   a  has a coefficient of thermal expansion which is greater than the coefficient of thermal expansion of the ceramic base core  10   a  and less than the coefficient of thermal expansion of the refractory metal core element  14   a . The filler material  16   a  may be a slurry which is a water based mixture of silica and zircon. If this is the case, the filler material  16   a  may have a greater zircon content than the ceramic base core  10   a . The filler material slurry fills the opening  12   a  after the core element  14   a  has been positioned in the opening. 
     When the core assembly  20   a  is dried, water is removed from the silica and zircon filler materials  16   a . This results in shrinkage of the filler material  16   a . When the core assembly  20   a  is fired, additional water is removed from the ceramic filler material  16   a . This results in additional shrinkage of the ceramic filler material  16   a.    
     The silica and zircon in the filler material  16   a  may have the same particle size. By providing the silica and zircon with substantially the same particle size, voids between particles are not filled with other particles. This enables the silica and zircon filler material  16   a  to have a relatively large coefficient of thermal expansion. 
     Alternatively, the filler material  16   a  may be formed of mullite (3Al 2 O 3 .2SiO 2 ) with a binder. Mullite has a coefficient of thermal expansion of approximately 5.3 inches per inch per degree centigrade. In one embodiment of the invention, the filler material  16   a  was approximately 60% silica and 40% mullite. The resulting mixture had a coefficient had a coefficient of thermal expansion of approximately 3.0×10 −6  inches per inch per degrees centigrade. Of course, the filler material  16   a  may contain silica and mullite in percentages other than the foregoing percentages. 
     The silica and mullite forming the filler material  16   a  may have the same particle size. By providing silica and mullite with substantially the same particle size, voids between particles are not filled with other particles. This enables the silica and mullite of the ceramic filler material  16   a  to have a relatively large coefficient of thermal expansion. If desired, the silica may be omitted from the filler material  16   a  if this is done, a different material may or may not be substituted for the silica. 
     The silica and mullite filler material  16   a  has a coefficient of thermal expansion which is greater than the coefficient of thermal expansion of the ceramic base core  10   a  and less than the coefficient of thermal expansion of the refractory metal core element  14   a . The silica and mullite filler material  16   a  may be a slurry which is a water based mixture of silica and mullite. Of course, the filler material may contain materials other than silica and mullite. 
     It is believed that the base core  10   a  may be formed of a mixture of silica and zircon and may have a coefficient of thermal expansion of approximately 2.0×10 −6  inches per inch per degree centigrade. The filler material  16   a  may be formed of a mixture of silica and mullite and may have a coefficient of thermal expansion of approximately 3.0×10 −6  inches per inch per degrees centigrade. If desired, the filler material  16   a  may be formed of a mixture of a silica, zircon, mullite and/or other materials. 
     In the embodiments of the invention illustrated in  FIGS. 1-6 , the core element  14  is formed of a refractory metal. In the embodiment of the invention illustrated in  FIG. 7 , the core element is formed of a ceramic material. Since the embodiment of the invention illustrated in  FIG. 7  is generally similar to the embodiment of the invention illustrated in  FIGS. 1-6 , similar numerals will be utilized to identify similar components, the suffix letter “b” being associated with the numerals of  FIG. 7  to avoid confusion. 
     A core assembly  20   b  includes a ceramic base core  10   b  ( FIG. 7 ). A ceramic core element  14   b  is received in an opening  12   b  formed in the ceramic base core  10   b . Filler material  16   b  fills the opening  10   b . The base core  10   b , filler material  16   b  and ceramic core element  14   b  form the core assembly  20   b . The core assembly  20   b  has a configuration corresponding to a configuration of a desired space in a cast metal article. In the embodiment of  FIG. 7 , the filler material  16   b  is disposed at only one side of the ceramic core element  14   b.    
     The core element  14   b  is formed of a ceramic material and has the configuration of a flat plate. Of course, the ceramic core element may be formed with a different configuration. The ceramic core element  14   b  may be formed with holes, projections, and/or tabs. For example, the core element  14   b  may have an arcuate configuration with radially and/or axially extending flanges. The ceramic core element  14   b  may be formed as a wire. The ceramic core element  14   b  may be formed as one piece or a plurality of pieces. 
     The ceramic core element  14   b  may be formed of many different ceramic materials. However, it is believed that it may be preferred to form the ceramic core element  14   b  of alumina (Al 2 O 3 ). The core element  14   b  has a coefficient of thermal expansion of approximately 8.8×10 −6  inches per inch per degree centigrade. Of course, the ceramic core element  14   b  may be formed of a material other than alumina and have a different coefficient of thermal expansion. The filler material  16   b  has a coefficient of thermal expansion which is greater than the coefficient of thermal expansion of the ceramic base core  10   b  and less than the coefficient of thermal expansion of the ceramic core element  14   b.    
     There are two interfaces between the ceramic filler material  16   b  and other components of the core assembly  20   b . The first interface  44   b  ( FIG. 7 ) is formed where the filler material  16   b  and engages the ceramic core element  14   b . At the interface  44   b , a surface  48   b  of the ceramic filler material  16   b  engages a surface  50   b  on the ceramic core element  14   b . During firing of the core assembly  20   b , force is transmitted between the ceramic core element  14   b  and the ceramic filler material  16   b  across the interface  44   b . This force results from the different coefficients of thermal expansion of the ceramic core element  14   b  and the ceramic filler material  16   b.    
     A second interface  54   b  is formed where the filler material  16   b  engages the ceramic base core  10   b . At the second interface  54   b , a surface  58   b  of the filler material  16   b  engages a surface  60   b  on the ceramic base core  10   b . The surface  60   b  forms one side of the opening  12   b  and faces toward and is spaced from the surface  50   b  on the ceramic core element  14   b.    
     During firing of the core assembly  20   b  force is transmitted between the ceramic filler material  16   b  and the ceramic base core  10   b  across the interface  54   b . Force is also transmitted between the ceramic filler materials  16   b  and the ceramic core element  14   b  across the interface  44   b . These forces result from different coefficients of thermal expansion of the ceramic core element  14   b , the filler material  16   b , and the base core  10   b . By providing two interfaces, that is, the interface  44   b  and the interface  54   b , the magnitude of the force transmitted at either one of the interfaces is reduced. This results in a reduction in stress on the material of the ceramic base core  10   b  and the ceramic filler material  16   b.    
     The coefficient of thermal expansion of the base core  10  is increased so that it approaches the coefficient of thermal expansion of the ceramic core element  14   b . The alumina of the ceramic core element  14   b  may have a coefficient of thermal expansion of approximately 8.8×10 −6  inches per inch per degree centigrade. The base core  10   b  is formed of a mixture of silica and zircon. Silica has a coefficient of thermal expansion of approximately 0.5×10 −6  inches per inch per degree centigrade. Zircon has a coefficient of thermal expansion of approximately 4.2×10 −6  inches per inch per degree centigrade. 
     To increase the thermal expansion of the base core  10   b , the base core is formed of a mixture of silica and zircon. The greater the amount of zircon provided in the base core  10   b , the greater will be the coefficient of thermal expansion of the base core  10   b . It is contemplated that the base core  10   b  may have a silica content of 70% or less and a zircon content of 30% or more. However, the base core  10   b  may have a different composition if desired. 
     The silica and zircon forming the ceramic base core  10   b  may have the same particle size. By providing silica and zircon with substantially the same particle size, voids between relatively large particles of one material are not filled by relatively small particles of the other material. This enables a mixture of zircon and silica particles to have a relatively large coefficient of thermal expansion. 
     The filler material  16   b  may be a slurry which is a water based mixture of silica and zircon. If this is the case, the filler material  16   b  may have a greater zircon content than the base core  10   b . The slurry of silica and zircon is used to fill the opening  12   b  after the ceramic core element has been positioned in the opening. 
     When the core assembly  20   b  is dried, water is removed from the silica and zircon of the ceramic filler material  16   b . This results in shrinkage of the filler material  16   b . When the core assembly is fired, additional water is removed from the ceramic filler material  16   b . This results in additional shrinkage of the ceramic filler material  16   b.    
     The silica and zircon in the filler material  16   b  may have the same particle size. By providing silica and zircon with substantially the same particle size, voids between particles are not filled with other particles. This enables the silica and zircon ceramic filler material  16   b  to have a relatively large coefficient of thermal expansion. 
     Alternatively, the ceramic filler material  16   b  may be formed of mullite (3Al 2 O 3 .2SiO 2 ) with a binder. Mullite has a coefficient of thermal expansion of 5.3 inches per inch per degree centigrade. In one embodiment of the invention, the ceramic filler material  16   b  was approximately 60% silica and 40% mullite. The resulting mixture had a coefficient of thermal expansion of approximately 3.0×10 −6  inches per inch per degree centigrade. Of course different percentages of silica and mullite may be used. This may result in the filler material  16   b  having a different coefficient of thermal expansion. 
     The silica and mullite filler material  16   b  has a coefficient of thermal expansion which is greater than the coefficient of thermal expansion of the ceramic base core  10   b  and less than the coefficient of thermal expansion of the ceramic core element  14   b . The silica and mullite filler material  16   b  may be a slurry which is a water based mixture of silica and mullite. 
     The foregoing percentages of silica, zircon, and/or mullite for use in the base core  10   b  and/or filler material  16   b  have been set forth herein for purposes of clarity of description. It is not intended to limit the invention to a specific percentage of silica, zircon, and/or mullite in either the base core  10   b  or the ceramic filler material  16   b . In addition, the foregoing specific coefficients of thermal expansion for the base core  10   b , filler material  16   b  and ceramic core element  14   b  have been set forth herein for purposes of clarity of description. It is not intended to limit the invention to specific coefficients of thermal expansion. It should be understood that the coefficients of thermal expansion of the base core  10   b  and filler material  16   b  will vary with variations in the silica and/or mullite content of the base core and filler material. 
     In the embodiment of the invention illustrated in  FIG. 7 , two interfaces  44   b  and  54   b  have been formed between the ceramic core element  14   b , the filler material  16   b  and the ceramic base core  10   b . In the embodiment of the invention illustrated in  FIG. 8 , four interfaces are formed between the ceramic base core, filler material and ceramic core element. Since the embodiment of the invention illustrated in  FIG. 8  is generally similar to the embodiments of the invention in  FIGS. 1-7 , similar numerals will be utilized to identify similar components. The suffix letter “c” being associated with the numerals of  FIG. 8  to avoid confusion. 
     A core assembly  20   c  includes a ceramic base core  10   c  ( FIG. 8 ). A ceramic core element  14   c  is received in an opening  12   c  formed in the ceramic base core  10   c . The ceramic filler material  16   c  is disposed on opposite sides of the ceramic core element  14   c . Thus, a first body  70   c  of ceramic filler material is disposed on the left (as viewed in  FIG. 8 ) side of the ceramic core element  14   c . Similarly, a second body  72   c  of ceramic filler material is disposed on the right side of the ceramic core element  14   c . The ceramic filler material  16   c  has a coefficient of thermal expansion which is greater than coefficient of thermal expansion of the base core  10   c  and less than the coefficient of thermal expansion of the ceramic core element  14   c.    
     The two bodies  70   c  and  72   c  of ceramic filler material cooperate with the ceramic base core  10   c  and ceramic core element  14   c  to form four interfaces. Thus, a first interface  44   c  is formed where the body  70   c  of ceramic filler material  16   c  engages a side surface  50   c  of the ceramic core element  14   c . A second interface  54   c  is formed between the first body  70   c  of ceramic filler material and a side surface  60   c  of the ceramic base core  10   c . A third interface  76   c  is formed between the second body  72   c  of ceramic filler material and a side surface  64   c  of the ceramic core element  14   c . A fourth interface  82   c  is formed between the second body  72   c  of ceramic filler material and a side surface  84   c  of the opening  12   c  in the base core  10   c.    
     The ceramic core element  14   c  is formed of alumina. The alumina core element  14   c  has a coefficient of thermal expansion of approximately 8.8×10 −6  inches per inch per degree centigrade. The core element  14   c  may be formed of a different material and have a different coefficient of thermal expansion. 
     CONCLUSION 
     The present invention provides a new and improved method of forming a cast metal article  38 . The method includes providing a ceramic base core  10  having a first coefficient of thermal expansion. A core element  14  having a second coefficient of thermal expansion is positioned in an opening  12  formed in the ceramic base core  10 . The core element  14  may be formed of a refractory metal or a ceramic material. The opening  12  in the ceramic base core  10  is filled with ceramic filler material  16  having a third coefficient of thermal expansion. The third coefficient of thermal expansion may be greater than the first coefficient of thermal expansion and less than the second coefficient of thermal expansion. 
     Although it is contemplated that the ceramic base core  10  may have many different compositions, the ceramic base core may contain silica and zircon. The silica content may be 70% or less and the zircon content may be 30% or more. It is also contemplated that the filler material  16  may have many different compositions. However, the filler material  16  may contain silica and zircon. Alternatively, the filler material  16  may contain silica and mullite. The silica, zircon and/or mullite forming the filler material  16  may advantageously have substantially the same particle size. 
     The present invention includes many different features which may advantageously be utilized together as disclosed herein. Alternatively, the features may be utilized separately or in various combinations with each other and/or with features from the prior art. For example, the filler material  16  having a coefficient of thermal expansion which is greater than the coefficient of thermal expansion of the ceramic base core may not contain zircon and/or mullite and may be used with a base core  10  which does not contain zircon. As another example, a base core  10  containing zircon may be used with filler material  16  which is free of zircon and/or mullite.