Patent Publication Number: US-2016243609-A1

Title: Quasi self-destructive core for investment casting

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/747,653, filed Jan. 23, 2013, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under N00019-06-C-0081 awarded by the Department of the Navy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Exemplary embodiments of the invention generally relate to investment casting, and more particularly, to a core for forming a passage in an investment casting mold. 
     Investment casting is a commonly used technique for forming metallic components having complex shapes and geometries, especially hollow components such as those used in aerospace applications for example. The production of an investment cast part generally involves producing a ceramic casting mold having an outer ceramic shell with an inside surface corresponding to the shape of the part, and one or more ceramic cores positioned within the outer ceramic shell, corresponding to interior passages to be formed within the part. Molten alloy is introduced into the ceramic casting mold and is then allowed to cool and to harden. The outer ceramic shell and ceramic core(s) are then removed to reveal a cast part having a desired external shape and hollow interior passages in the shape of the ceramic core(s). 
     In comparison to other processes, for example sand casting or permanent mold casting, investment casting provides flexibility while maintaining tight tolerances. In particular, controlled solidification investment casting (CSIC) uses rapid directional cooling to enhance microstructure and mechanical properties. CSIC, therefore, may be useful for an expanded range of applications, particularly in the aerospace industry. However, investment casting is limited by the design of passages within the mold. Unlike a sand core used in a sand casting process, the ceramic cores used in CSIC are difficult to remove or destroy without affecting the molded part. As a result, the process of designing passages severely restricts the use of CSIC for applications requiring complex cored passages. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to yet another embodiment of the invention, a method for manufacturing a composite core for forming a passage in an investment casting mold is provided including arranging a core element adjacent an interior surface of a generally hollow structural element to form a preform. Slurry having particles of varying sizes is layered about the structural element. Heat is then applied to the preform. 
     According to another embodiment, a method of forming a passage in a cast component is provided including arranging a composite core into an interior of a mold. Material of the component is then poured into the mold. The material is cured to form the component. A force is then applied to an exposed portion of the composite core such that the composite core deforms inside the component. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a composite core according to an embodiment of the invention; 
         FIG. 2  is a cross-sectional view of a preform according to an embodiment of the invention; 
         FIG. 3  is side view of a structural element of the preform according to an embodiment of the invention; 
         FIG. 4  is a perspective view of a preform including layers of slurry according to an embodiment of the invention; and 
         FIG. 5  is a cross-sectional view of a component formed from an investment casting mold having a passage formed by a composite core according to an embodiment of the invention. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now to  FIG. 1 , a cross-section of a composite core  20  for forming a passage in an investment casting mold is illustrated. When inserted into a mold (not shown), the composite core  20  includes a generally hollow structural element  40  and a shell element  60  arranged about the exterior  46  of the structural element  40 . The structural element  40  is configured to deform, and therefore break the shell element  60  coupled thereto, when a force is applied to an end  42  ( FIG. 2 ) of the structural element  40 . 
     The composite core  20  is formed using a preform  30 , illustrated in more detail in  FIG. 2 . The preform  30  includes the generally hollow structural element  40  as well as a core element  50  positioned adjacent the interior surface  44  of the structural element. The structural element  40  may be pre-formed and the core element  50  inserted into the hollow center  47  of the structural element  40 , or alternatively, the structural element  40  may be formed around the exterior of the core element  50 . 
     An example of the structural element  40 , shown in  FIG. 3 , is the general size of a passage being formed within an investment casting mold. The material used to form the structural element  40  is selected based on the material of the component being cast. For example, the material of the structural element  40  may be the same alloy as the component being cast. Exemplary metallic materials include, but are not limited to, steel, copper, and nickel for example. In the illustrated embodiment, the structural element  40  is fabricated from a coiled wire  48  such that the structural element  40  behaves in a manner similar to a tensile or compression spring. The specifications of the wire  48  are selected to facilitate contact between the structural element  40  and the core element  50 , as well as the ultimate breakdown of the composite core  20 . As a result, the cross-section of the wire  48  may be any of a variety of shapes, such as circular, square, triangular, or trapezoidal for example, and the coils of the wire  48  need not be evenly spaced as shown. Considerations for the strength and ductility of the structural element  40  include the ability of the structural element  40  to support itself once coupled to the core element  50 , the ability of the structural element  40  to support the composite core  20  once the shell element  60  is formed, and the ability of the structural element  40  to deform when a force is applied thereto. 
     The core element  50  acts as a base to support the outer shell element  60  as it is formed about the structural element  40 . The core element  50  is made from a material configured to melt during the formation of the composite core  20 , prior to the casting process, or during the casting process. In one embodiment, the core element  50  is a wax core, the contour of which is substantially similar to a passage being formed in a mold. In another embodiment, the core element  50  is a metallic mesh or foil, for example made from the same material as the working metal to be poured into the investment casting mold. The metallic mesh or foil  50  is bonded to the interior surface  44  of the structural element  40 , such as through a brazing process for example. The gauge of the foil or mesh  50  is selected to support the shell element  60  as it is formed about the structural element  40 . Once the metallic mesh or foil  50  and the structural element  40  are coupled, the contour of the preform  30  may be altered to a desired shape. 
     After the preform  30  is assembled, the outer shell element  60  is formed, for example through a shelling process. As illustrated in  FIG. 4 , the preform  30  is coated with a slurry  62  having particles of varying sizes. In one embodiment, the material of the slurry  62  used to form the outer shell  60  is substantially identical to the material used to form the investment casting mold, such as ceramic for example. Alternatively, the material of the slurry  62  may be modified to facilitate breakdown of the outer shell  60  when a force is applied to the structural element  40 . The slurry  62  is arranged in a plurality of layers extending outwardly from the surface  52  of the core element  50  to at least the outer surface  46  of the structural element  40  such that the structural element  40  and the shell element  60  are integrally formed. In one embodiment, for example where the core element  50  is a wax core, the surface  52  of the core element  50  may be dipped in the slurry  62  before being inserted into the structural element  40 , to aid in the formation of an inner surface of the shell element  60 . As a result, slurry  62  is positioned about the structural element  40  such that when the composite core  20  is formed, the shell element  60  extends beyond both the inner diameter  44  and the outer diameter  46  of the structural element  40  (see  FIG. 1 ). 
     After layering the slurry  62  about the structural element  40 , the slurry  62  is hardened, such as by firing the preform  30  in an oven or kiln for example. Heat causes the slurry  62  to strengthen and solidify into a cured, rigid, shell element  60 . The core element  50  is designed to melt, or otherwise degrade during the making of the composite core  20 , or during the formation of the finished component. Therefore, application of heat transforms the preform  30  to a composite core  20 , having a generally hollow cross section that allows the structural element  40  and the shell element  60  to be easily removed. When the composite core  20  is formed, the outer surface  64  of the shell element  60  may be substantially uniform, or alternatively, may include slight variations, such as waves or grooves for example. 
     Referring now to  FIG. 5 , a component  80  formed using an investment casting mold and at least one composite core  20  is illustrated. To remove the composite core  20  from a passage  82  of the component  80 , a portion of the shell element  60  is broken to reveal an end  42  of the structural element  40 . A force is then applied to the exposed end  42 , causing the structural element to deform  40 . Because the shell element  60  is formed about the structural element  40 , deformation thereof causes the brittle shell element  60  to shatter and break away from coiled wire  48  of the structural element  40 . The pieces of the shell element  60  and the structural element  40  may then be easily removed from the passage  82  of the component  80 . 
     The composite core  20  may be constructed to create a complex cored passage within an investment casting mold, thereby expanding the range of applications to which controlled solidification investment casting (CSIC) may be applied. Further, by incorporating waves or grooves into the outer surface  64  of the shell element  60 , the passage  82  can have specific patterns such as rifling. The rapid and directional solidification of the investment casting process will result in high quality castings having enhanced microstructures. Because a significant portion of the CSIC process is automated, more stringent quality control measures may be implemented to improve and stabilize the casting process. Forming parts that were previously too complex using a CSIC process will reduce both scrap rates and production cycle time. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.