Abstract:
A method of forming a cast product ( 30 ) by providing a core ( 52 ) having a plurality of sections ( 54 ) and one or more gaps ( 55 ) there-between. The core further includes an insert member ( 60 ) spanning the gap ( 55 ) between adjacent sections ( 54 ). The core ( 52 ) is located within a mold ( 68 ) and a liquid phase material is introduced into gap ( 55 ) between the core sections. The liquid phase material is solidified in the gap so as to form a cast feature of a resulting solid product and the core sections ( 54 ) are removed from the solid product ( 30 ) such that the insert member ( 60 ) remains securely held within the feature ( 74 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to cast products having internal features and more particularly, although not exclusively, a casting process for producing products having cooling passages therein. 
     There are a number of machine components for which it is necessary to provide internal features such as cavities or passages. The complexity of such internal features provides a technical challenge when the intended component is manufactured by casting. 
     The provision of cooling passages for components which operate in use within high temperature environments is one example in which such complex internal passages are required. Cooling of components is of particular importance for high temperature gas turbine engines in order to ensure that components within the engine are maintained at a suitable operational temperature without deterioration to performance. It is widely acknowledged that the use of internal cooling channels can allow components to operate effectively in hot environments which exceed the melting temperature of the component material. 
     It is known to provide cooling arrangements in which coolant flow cascades between a plurality of cooling chambers in order to maximise the cooling efficiency and effect. The cascading of cooling flow is used to ensure successive impingement of the coolant flow onto surfaces to be cooled. This technique may be suitable for a number of different types of components and is applied to rotor rims for turbines in a gas turbine engine. Cooling in this manner typically requires a plurality of successive cooling chambers to be defined by internal wall formations in the component. Flow between those chambers is permitted by the provision of openings in the walls such that flow entering a first chamber passes into a second chamber via said openings and then into a further chamber from the second chamber by virtue of further openings. The openings are arranged such that the flow impinges on the surfaces to be cooled in the relevant chambers prior to passing into another chamber. 
     Whilst such cooling passages are preferable from an operational point of view, the formation of such chambers and openings by way of casting or moulding is a complex process. In an investment or ‘lost wax’ casting process, a core is required which defines the shape of the interior of the component. The core is removed to leave the negative internal space within the component once formed. However a problem exists in that exit apertures must be provided in the component in order to allow removal of the core. 
     Additional problems arise due to the intricate nature of the core used to define the internal features of the component. The shape of a core which is suited to providing cooling chambers separated by relatively thin walls typically results in a delicate structure which may not be capable of supporting its own weight. A support in the form of a spine is often required to hold the core bodies in a fixed relative position and to maintain tolerances relative to the cast. 
     The spine is a manufacturing feature and, once removed, leaves unwanted apertures in the final component. 
     Exit apertures due to removal of a spine and/or the core itself are undesirable in the final component and can cause short circuits or otherwise prevent correct operation of the internal cooling network. Accordingly these passages need to be closed in the final component. Conventional methods of closing the exit apertures involve brazing or welding of closures, which methods are time consuming and can cause detrimental thermal stresses in the final component. Repeated thermal loading of the component can lead to problems on account of thermal stresses, such as cracking or component failure. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide a method of casting products which can provide internal features in a product in an improved manner. It is a further aim of the present invention to provide a cast product and/or articles for use in the casting of a product which mitigate the problems described above. 
     According to one aspect of the present invention there is provided a method of forming a cast product comprising: providing a core having a plurality of sections and one or more gaps there-between, wherein the core comprises an insert member spanning the gap between adjacent sections of the core; locating the core within a mould; introducing a liquid phase material into the gap between the core bodies in the mould; allowing the liquid phase material to solidify in the gap so as to form a feature of a resulting solid product; and removing the core sections from the solid product such that the insert member remains securely held within the feature. 
     According to one embodiment, the feature comprises an internal feature in the resulting product. In one embodiment, the cast features are internal walls within the resulting product. The feature may comprise a wall, which may be provided between internal cavities or chambers of the product. 
     The insert member may comprise a material which is different to the material of the remainder of the core. The insert member may be formed of a first material and the core sections are formed of a second material, wherein the first and second materials are different. The insert member may comprise or consist of a metal or ceramic material. The core sections may be formed of a ceramic material. 
     In one embodiment the core sections define internal cavities within the resulting product. The plurality of sections may comprise a plurality of first sections and the core may comprises a plurality of further sections. The further sections may depend from the first sections and may be connected thereto by one or more pedestals. The first sections, the further sections and the pedestals may be formed of the same material. The further sections may be of smaller volume than the first sections. 
     The core may define a network of internal cooling cavities in the resulting product. 
     The insert member may comprise opposing retaining features, shaped to retain the insert member in the feature of the solid product once cast. The insert member may comprise a neck region and opposing retaining formations depending therefrom. The insert member may comprise a tapered portion and may comprise a pair of opposingly tapered portions. 
     In one embodiment, the core comprises one or more retaining formations for positioning the core within the mould. The retaining formations may comprise arm members depending outwardly there-from and the arm members may be received within corresponding locating formations in the mould. The retaining members may be arranged so as to suspend the core within the mould. 
     According to one embodiment, the resulting product is a gas turbine engine component. 
     According to a second aspect of the invention, there is provided a mould core for use in an investment casting process, the core comprising: a plurality of sections spaced by a gap there-between and an insert member having a first portion located in a first section and an opposing portion located in a second core section so as to span the gap there-between; wherein the insert member is formed of a material which is different to the material of the core sections. 
     According to a third aspect of the invention, there is provided a cast product comprising a plurality of internal cavities and one or more internal walls there-between, said cavities in combination defining an internal cooling passage within the product, the one or more internal walls comprising an aperture having an insert member therein, said insert member comprising a neck portion seated within the aperture and opposing retaining portions depending outwardly from said neck portion so as to retain the insert member within the wall. 
     The insert member may be formed of a single solid body. 
     According to a fourth aspect of the invention, there is provided a gas turbine engine comprising a product according to the third aspect. 
     According to a fifth aspect of the present invention, there is provided an insert member for use in the creation of cast product according to the first aspect. 
     The terms ‘cast’ or ‘casting’ as used herein should be construed as relating to the forming of a product whereby liquid phase material is allowed to solidify within a cast, mould, shell, die or similar formation so as to define the shape of the solidified material therein. 
     Any of the optional features described herein in relation to any one aspect or embodiment of the invention is applicable to all further aspects or embodiments wherever practicable. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       One or more working embodiments of the present invention are described in further detail below by way of example with reference to the accompanying drawings, of which: 
         FIG. 1  shows a half longitudinal section of a gas turbine engine to which the invention may be applied; 
         FIG. 2A  shows a three-dimensional view of a turbine seal segment according to the present invention; 
         FIG. 2B  shows a cut-away three-dimensional view of the seal segment of  FIG. 2A ; 
         FIG. 3  shows a three-dimensional view of a body and core for creation of a turbine seal segment according to the present invention; 
         FIG. 4  shows a three-dimensional view of a core for creation of internal formation within the seal segment of  FIG. 2 ; 
         FIG. 5  shows a cross section of a core according to a further embodiment of the present invention; and, 
         FIG. 6  shows a sectional view of a cast product with cast members in place. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , and intermediate pressure turbine  18 , a low-pressure turbine  19  and a core engine exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines the intake  12 , a bypass duct  22  and a bypass exhaust nozzle  23 . 
     The gas turbine engine  10  works in a conventional manner so that air entering the intake  12  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  17 ,  18 ,  19  respectively drive the high and intermediate pressure compressors  15 ,  14  and the fan  13  by suitable interconnecting shafts. 
     Alternative gas turbine engine arrangements may comprise a two, as opposed to three, shaft arrangement and/or may provide for different bypass ratios. Other configurations known to the skilled person include open rotor designs, such as turboprop engines, or else turbojets, in which the bypass duct is removed such that all air flow passes through the core engine. The various available gas turbine engine configurations are typically adapted to suit an intended operation which may include aerospace, marine, power generation amongst other propulsion or industrial pumping applications. 
     The present invention is particularly suited to components which may be manufactured using investment casting techniques, which may be otherwise referred to a ‘lost wax’ castings. Such components may be mounted in the vicinity of the turbines  17  to  19 —particularly the high pressure turbine  17 —and may comprise seal segments which form a closely-fitting rim or ring about the turbine or else vanes, such as nozzle guide vanes immediately downstream of the turbine. 
       FIG. 2  shows an example of a component which may be formed according to the present invention in the form of a turbine seal segment  30 . The component  30  has a cast body  32  in which are defined a plurality of internal features or structures in the form of walls  34  and  35 . A first set of internal walls  34  depend inwardly from outer wall  36  so as to define a series of larger internal cavities or chambers  38 A,  38 B,  38 C. A second set of internal walls  35  depend inwardly from external wall  40  so as to define a second series of relatively smaller internal chambers  42 A,  42 B,  42 C. The first  38 A,  38 B,  38 C and second  42 A,  42 B,  42 C sets of internal chambers are separated by internal wall  44 . 
     Internal wall  44  extends generally laterally across the component  30  between opposing side walls, whereas the internal walls  34  and  35  are generally perpendicular thereto, so as to define generally right-angled internal chambers  38 A,  38 B,  38 C and  42 A,  42 B,  42 C. Additional formations in the form of turbulators are cast into the walls of the smaller internal chambers  42 A,  42 B,  42 C to promote heat transfer between the chamber walls and a coolant flowing there-through. 
     A plurality of apertures  46 A,  46 B,  46 C and  48 A,  48 B,  48 C are provided in the internal wall  44 . The apertures  46 A,  46 B,  46 C provide inlets into the second chambers  42 A,  42 B,  42 C from the relevant first chamber  38 A,  38 B,  38 C, whereas the apertures  48 A,  48 B;  48 C provide an outlet from the second chambers  42 A,  42 B,  42 C to the relevant first chamber  38 A,  38 B,  38 C. With reference to  FIG. 2B , coolant can thus flow from the left-most chamber  38 A,  38 B,  38 CA via apertures  46 A,  46 B,  46 CA into the chamber  42 A,  42 B,  42 CA there-beneath. The coolant exits chamber  42 A,  42 B,  42 CA into the central chamber  38 A,  38 B,  38 CB via apertures  48 A,  48 B,  48 CB. Coolant enters chamber  42 A,  42 B,  42 CB from central chamber  38 A,  38 B,  38 CB via apertures  46 A,  46 B,  46 CB and passes there-along prior to exiting into chamber  38 A,  38 B,  38 CC via apertures  48 A,  48 B,  48 CC. From chamber  38 A,  38 B,  38 CC, coolant can enter chamber  42 A,  42 B,  42 CC via apertures  46 A,  46 B,  46 CC. 
     Internal cooling of component in this manner by passage of coolant into and from successive chambers may be referred to herein as cascade cooling or cascade impingement cooling. Using this technique coolant undergoes multiple passes to and from a surface to be cooled (in this case external wall  40 ) prior to exiting the component. This has a beneficial impact on cooling efficiency. 
     Turning now to  FIGS. 3 and 4 , investment casting is used to form body  30  within a mould (not shown). The material  50  from which the body  30  is formed is cast about a core member  52  as shown in  FIG. 3 . In this embodiment, the core member  52  is substantially formed of a ceramic material although other known core materials may be used. The core member  52  is removed from the material  50  once cast using conventional techniques as would be known to the person skilled in the art. The remaining material  50  is then machined and/or otherwise processed and/or treated in order to result in the component  30 . 
     The core member  52  is shown in isolation in  FIG. 4 . The core member  52  comprises a plurality of sections which form the corresponding internal cavities in the final component. In this example, the sections  54 A,  54 B and  54 C respectively form the chambers  46 A,  46 B,  46 CA,  46 A,  46 B,  46 CB and  46 A,  46 B,  46 CC in the final component. The sections  54 A,  54 B and  54 C are spaced by gaps  55  which form walls  34  in the final component. In order to provide the cascade cooling effect described above, it is preferable that the gaps  55  are continuous such that walls  34  have no apertures therein, which would serve to short-circuit the cascade cooling gas path in the final component. 
     The series of sections  56 A,  56 B and  56 C respectively form the individual cooling passageways  42 A,  42 B,  42 CA,  42 A,  42 B,  42 CB and  42 A,  42 B,  42 CC as shown in  FIG. 2B . The sections  56 A,  56 B,  56 C are suspended from sections  54 A,  54 B,  54 C by ties or pedestals  58  formed of the same core material, which, when removed, form the apertures  46 A,  46 B,  46 C and  48 A,  48 B,  48 C in the final component. 
     The intricate and delicate nature of the core  52  results in a need to support the core sections throughout at least some stages of the component manufacturing process. 
     This is achieved using one or more core insert members  60  as shown in  FIGS. 5 and 6 , which span the gaps between core sections and serve to hold the core sections in a fixed relative position. 
     An exemplary cross section of a core  62  which comprises two adjacent core sections  64 , separated by a gap  66 , is shown in  FIG. 5 . This embodiment would produce a component having two main internal chambers, rather than the three chambers  38 A,  38 B,  38 C shown in  FIG. 2 . The invention may be applied to a core having two or more core sections and a corresponding component produced thereby to have two or more internal chambers. 
     The core  62  is shown held within a mould, which is depicted schematically at  68 . The core  62  has support features in the form of arms  70  and  72  depending outwardly there-from and which are received in corresponding location formations in the mould  68 . The core insert members  60  also help to maintain tolerances to the cast surface in conjunction with the arms  70 ,  72  which project out of the casting. 
     However, unlike the provision of a continuous spine through the core  62 , the core insert member  60  is formed of a different material to the core  62  and associated arms  70 ,  72 . In this embodiment, the insert member  60  is formed of a Zirconia or Alumina material although any material which is capable of withstanding the casting process/melt temperatures may be used provided it meets the functional requirements of the component in which it is to be inserted. 
     The core insert member  60  is doubly tapered in shape so as to form a neck region  61  at its centre which is smaller in dimension than its opposing sides. The insert member is generally circular in plan such that its shape may be likened to a unison of two opposing frusto-conical halves. The insert member may otherwise be described as being generally hourglass shaped. 
     With the core  62  and insert member  60  therein held within the mould  68 , molten material can be allowed to enter the mould  68  to thereby form the component body about the core  62 . It will be appreciated that various optional methods for casting are available which may include casting within a vacuum, single crystal casting or directionally solidified castings, any of which may be use din conjunction with the present invention. 
     Once cast, the component is removed from the mould  68  and the core removed there-from using conventional techniques. However the core insert member  60 , being formed of a different material to that of the core, is maintained within the internal wall of the core body. The cast component can be machined and otherwise treated as required for use. The insert member  60  remains in the core throughout the casting process and is then ultimately retained by the metal cast around it. 
     An example of such a component is shown in section in  FIG. 6 , in which the core insert members  60  are held fast within the cast internal walls  74 . The shape of the members  60  ensure that they cannot slide out from the walls in which they are cast. Furthermore, the dual taper of the members ensure that the insert members are resilient to operational fluid pressures which may be applied to the component in use. 
     In the event that the component may undergo heating during operation, the thermal expansion properties of the members  60  are typically closely matched to that of the component material. Any slight discrepancy therein may be accommodated for by the dual taper of the member, such that the member cannot come loose. Whilst it is acknowledged that a portion of the member  60  will protrude form the wall  74  into the internal cavity, such protrusion is not considered to cause undue detriment to the efficiency of the cascade cooling circuit. 
     The taper and dimensions of the core insert member may be tailored to suit the operational requirements for the end component. For example the taper may be increased for components which will undergo relatively high coolant pressure loading in use. 
     The insert member described above provides a solution to the problems associated with removable/soluble core investment casting, which is effective in terms of cost and function. Further specific advantages of the invention are considered to include: 
     formation of a strong link between the core bodies; 
     retention of insert member within the internal wall is favoured by shrinkage of metal during casting process; 
     time and cost penalties of high tolerance machining operations are avoided; 
     potential scrap caused by high tolerance machining operations is avoided; 
     inspection requirements are reduced; 
     a consistently air-tight barrier between core bodies is provided; and, 
     the location of the insert member in the wall is not critical since it is a free, cast-in feature. 
     In addition to turbine seal segments, the invention may be applied to nozzle guide vanes or other cast components for which internal features require the use of delicate and/or complex cores.