Patent Publication Number: US-2012043046-A1

Title: Process for casting columnar grain airfoil with preferential primary orientation

Description:
BACKGROUND OF THE INVENTION 
     This application is a divisional of U.S. application Ser. No. 12/244,203 filed on Oct. 2, 2008. 
     This disclosure generally relates a preferential grain casting process. More particularly, this disclosure describes a method and device for preferential grain casting utilizing a seed to control orientation of a columnar grain casting. 
     Processes for orientating a grain structure in a cast article are known to provide either a columnar structure or a single crystal structure. In such processes, a ceramic casting mold that is heated to a temperature above the melting point of the alloy is filled with molten material within a furnace. This mold is subsequently withdrawn from a hot chamber to a cold chamber of the casting furnace in a controlled manner to control solidification and obtain directional alignment of the crystals. A starter block provides a starting point for the solidification of a molten material. For a columnar grain structure, the starter block facilitates the nucleation and growth of several columns that will grow in the direction opposite of heat extraction as opposed to the formation of a random grain structure without directionality. Although a columnar grain structure is an improvement over random grain structures, the column grain structure is only orientated in one direction identified as the primary orientation. Adjacent columns would possess the same primary orientation but possess different secondary orientations denoting a crystal structure that is rotated about the primary growth axis. 
     Accordingly, it is desirable to design and develop a method and device for providing a preferential columnar grain structure for a cast article. A process that promotes the nucleation and growth of many directional columns is preferred to optimize mechanical properties within a cast component. Because of component and mold geometries or furnace design limitations, obtaining many directional columns that progress from the bottom to the top of the casting is not always easily achievable. In many instances, only a few columns will nucleate and grow compromising part integrity. 
     It is desirable to develop a method that promotes the nucleation and growth of as many directional columns as possible to optimize part integrity. 
     SUMMARY OF THE INVENTION 
     A disclosed example molding process utilizes a seed with a predetermined columnar grain structure for defining a desired grain structure in a completed casting. 
     The example process utilizes a mold that defines the geometric features of a molded part, such as for example an airfoil. The seed is received or embedded within a seed cavity of the ceramic mold having a plurality of preferentially orientated columns. The molten material is poured into the ceramic mold, melts a small portion of the prefabricated metal seed, and solidifies adopting the orientation of the columns defined within the seed during solidification to form a completed molded article having a desired columnar grain structure. 
     The example method and device provides for the preferential orientation of columnar grain structures within a completed molded article. The example columnar grain structures are defined within a seed that is received within each mold cavity prior to receiving molten metal material. The preferential defined columnar grain structure provides for tailoring of grain structure to obtain desired mechanical characteristics and properties in the completed molded article. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an example casting process. 
         FIG. 2  is a sectional view of an example columnar grain structure. 
         FIG. 3  is schematic view of an orientation of an example columnar grain. 
         FIG. 4  is a schematic view of a prior art equiax grain structure. 
         FIG. 5  is a schematic view of an example casting. 
         FIG. 6  is perspective view of an example airfoil including a columnar grain structure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , an example casting process utilizes a casting furnace assembly  10  having an induction coil  12  for heating a ceramic mold to a temperature in excess of the melting point of the alloy to be cast. The example casting assembly further includes a hot zone  22  and a susceptor  16  which translates heat from the induction coil to the ceramic casting mold  20 . A lid  14  is placed on top of the hot zone to prevent the escape of heat. A ceramic mold  20  is supported on a cold chillplate  18 . The cold chillplate  18  is movable out of the hot zone  22  in a direction indicated by arrow  32 . A baffle  21  separates the hot zone which resides above the baffle  21  from the colder portion of the furnace into which the cast mold is withdrawn. It is within the contemplation of this disclosure that other casting furnace assembly  10  configurations could be utilized. 
     The mold  20  defines the geometric features of a molded part, such as for example an airfoil. A seed  28  is received within a seed cavity  34  of the mold  20 . The seed  28  includes a plurality of preferentially orientated columns  30 . The molten material adopts the orientation of the columns  30  defined within the seed  28  during solidification to form a completed molded article having a desired columnar grain structure. 
     The molding method proceeds by forming the mold  20  to define the desired geometric shape. The mold  20  is defined with the seed cavity  34  for receiving the seed  28 . The seed  28  is a pre-molded block having the desired columnar grain structure. The seed  28  comprises similar material content and mixture as is utilized to cast the article. The material comprising the seed  28  can be identical to that of the molten material for the cast article. Further, the seed  28  could be fabricated from a metallic material or composite material similar, but different than the material utilized for the cast article. As appreciated, any material or composite utilized for the seed  28  is selected to provide the desired results and compatibility in view of the material utilized for the cast article. 
     The solid seed  28  is inserted into the seed cavity  34  prior to the introduction of molten metal and prior to insertion of the mold  20  within the furnace assembly  10 . With the seed  28  disposed within the mold  20 , the mold  20  is placed within the hot zone  22 . Molten material is then poured into the mold  20 . The molten material  20  remains in a molten state within the hot zone  22 . The seed  28  comes into contact with the molten material and partially melts at the interface with the molten material. 
     Once the mold  20  is filled, it is removed from the hot zone  22  into a cooler environment that provides for solidification of the molten material  24 . The mold  20  is supported on the cold plate  18 . The example cold plate  18  is supported for movement out of the hot zone  22  in a controlled manner to control the solidification of the molten material. A solidification front  26  forms as the mold  20  is lowered from the hot zone  22 . The solidified material adopts the columnar orientations and structure of the seed  28 . The columnar orientations are defined by those created and defined in the seed  28 . The orientation of the grains within the completed molded article is thereby defined by the columnar structure fabricated into the seed  28 . 
     Referring to  FIG. 2  with continued reference to  FIG. 1 , a section  36  is shown that illustrates the interface between the seed  28  and the molded article. The seed  28  is shown with defined columns  30 . Upon exposure to the molten material, a portion of the seed  28  melts back and forms a mixed zone  38 . In the mixed zone  38  the added molten material  24  ( FIG. 1 ) mixes with a portion of the seed  28 . Upon solidification, the material from the original seed  28  freezes in a mixture with the molten material. It is in this mixed zone  38  that the columnar grain structure and orientation is adopted by the solidifying molten material. 
     The molded article  40  includes the columnar structure  30  that was defined in the original seed  28 . This columnar structure  30  includes fine dendritic structures to start the growth of columnar structures within the molten material. The columnar structures are numbered and orientated to provide the desired strength, durability or other mechanical properties desired in the completed molded article. Additional mechanical properties that can be defined and provided by the example process includes durability, fatigue strength, dampening and also the preferential orientation of stresses on the completed part. 
     Referring to  FIG. 3 , several columns  30  of grained structures is schematically depicted as blocks  44 . Each of the grained structures  44  are vertically orientated along a vertical axis  46 . Further, the grained structures  44  are commonly orientated in a rotational direction indicated by arrow  48  about the vertical axis  46 . The orientations vertically along the axis  46  and rotationally is provided by the orientations defined in the columnar grained seed  28 . Each block  44  defines the orientation of one column  30 . 
     Referring to  FIG. 4 , a prior art equiax grain structure  50  is schematically illustrated and includes a grain structure where the grains possess different primary and secondary orientations. Each adjacent column represented by the block is facing a different rotational direction as indicated by arrows  52 . 
     Referring to  FIG. 5 , an example mold  54  for the fabrication of an airfoil such as a turbine vane includes several cavities  56 . Each cavity  56  defines the geometry and shape of a completed turbine blade. All the cavities  56  are in communication such that molten material flows to each cavity from a common inlet. Each of the cavities  56  includes a seed cavity portion  58 . Once of the example seed cavities  58  are partially sectioned to show an example seed  60  received therein. The seed  60  is cast, solidified and heat-treated prior to insertion into the seed cavity  58 . The casting process of the seed  60  provides the desired columnar grain orientations that are to be adopted by the completed turbine blade. 
     The seed  60  includes a size that corresponds with each individual cavity  56 . The seed  60  is as wide or wider than the corresponding cavity  56  to provide for columnar growth in a vertical manner without disruptions. Further, the height of the seed  60  is such that upon encountering the molten material, at least some portion of the seed  60  will remain in a solid form. The height of the seed  60  is therefore provided such that sufficient original solidified material with the defined grain structure survives the molding and solidification process. 
     Referring to  FIG. 6 , an example turbine blade assembly  62  includes a columnar grain structure with columns  64  throughout. The columns  64  within the blade assembly  62  are uniform and generated by columns  66  defined by the seed  60 . After the molding process is complete and the blade assembly  62  has solidified, the seed  60  is removed by a machining process as is known. 
     The example method and device provides for the preferential orientation of columnar grain structures within a completed cast article. The example columnar grain structures are defined within a seed that is received within each mold cavity prior to receiving molten metal material. The preferential defined columnar grain structure provides for tailoring of grain structure to obtain desired mechanical characteristics and properties in the completed molded article. 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.