Abstract:
A core, for use in a casting mould to form a cavity in a cast component such as a blade or vane of a gas turbine engine. The core has a relatively fragile thin-walled region. A bead is formed along a lateral edge of the thin-walled portion in order to reduce cracking or other damage in the thin-walled portion.

Description:
BACKGROUND 
     This invention relates to a core for use in a casting mould, and is particularly, although not exclusively, concerned with a ceramic core for use in a mould for casting aerofoil components such as turbine blades and stator vanes of a gas turbine engine. 
     Stator vanes and blades in turbine stages of a gas turbine engine are commonly provided with internal cavities and passages to allow the flow of cooling air within the component. The blades and vanes may be made by casting, and the cavities and passages may be formed at least partially by positioning a ceramic core within the casting mould. More specifically, such components may be made by a form of investment casting known as the “lost-wax” process. In the lost-wax process, a wax pattern of the component to be cast is formed by injection moulding, around the ceramic core. The wax pattern, including the core, is then dipped into a ceramic slurry, which is then dried. The dipping process is repeated until an adequate thickness of ceramic has been built up, after which the ceramic mould is heated to melt the wax, which is removed from the mould interior. Molten alloy is poured into the mould. When the alloy has solidified, the mould is broken and the ceramic core is removed by leaching to leave the finished cast component. 
     SUMMARY 
     Some aerofoil components include a cavity having a narrow region which is formed by a core having a correspondingly thin-walled portion. The thin-walled portion may be perforated, so that, in the casting process, pedestals are formed within the narrow cavity region to support the walls of the component. 
     The thin-walled portion of the core is very fragile, and consequently the core is prone to breakage in the manufacturing process, either through mishandling or through stresses induced during the moulding of the wax pattern, owing to wax pressures or stresses imparted by the die, or during the casting process itself, owing to molten metal momentum (where it is a metallic material being cast) or to induced strains during casting material cooling. 
     According to the present invention there is provided a core for use in a casting mould, to form a cavity in a component cast in the mould, the core including a thin-walled portion extending from a thicker portion of the core towards a terminal edge of the core, characterised in that a lateral edge of the thin-walled portion terminates at a bead which is thicker than the thin-walled portion, the bead defining a lateral edge of the core. 
     The bead serves to reinforce the lateral edge of the thin-walled portion, thus resisting damage to the lateral edge and cracking within the thin-walled portion. 
     The bead may be one of two beads disposed at opposite lateral edges of the thin-walled portion, both beads defining lateral edges of the core. The lateral edges may be substantially parallel to each other. Alternatively the lateral edges may be at an angle to one another. 
     The terminal edge of the core may be defined by a rib which is thicker than the thin-walled portion, and which, when two beads are provided at opposite lateral edges, may extend between respective ends of the beads. 
     The thin-walled portion may be perforated, in which case the perforations may comprise holes which lie on at least one line-extending transversely of the or each lateral edge. 
     The component to be cast in the mould may include an aerofoil portion including a cavity portion formed by the thin-walled portion. 
     Another aspect of the present invention provides a cast component having a cavity formed by a core as defined above. 
     The component may have an external surface which extends generally parallel to an internal surface of a cavity region formed by the thin-walled portion, and to a surface portion of the bead adjacent to the thin-walled portion. 
     The component may have an aerofoil portion and a shroud portion, the cavity region formed by the bead being situated at the transition from the aerofoil portion to the shroud portion. 
     The component may be a blade or vane for a gas turbine engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: — 
         FIG. 1  shows a turbine stator vane; 
         FIG. 2  shows a ceramic core in accordance with the prior art, for use in the manufacture of the vane of  FIG. 1 ; 
         FIG. 3  is a partial sectional view of the core of  FIG. 2  taken on the line A-A in  FIG. 2 , and of the vane cast using the core; 
         FIG. 4  corresponds to  FIG. 3  but shows a core and vane in accordance with the present invention; and 
         FIG. 5  corresponds to  FIG. 4 , but shows an alternative form of core and vane. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The vane shown in  FIG. 1  comprises an aerofoil portion  2  and inner and outer shroud portions  4 ,  6 . The vane has an internal cavity  8  which opens to the exterior at a passage  10  in the shroud portion  6  and a corresponding passage (not visible) in the shroud portion  4 . The cavity  8  also communicates with the exterior through a slot  12  at the trailing edge of the vane. The vane is made from a high temperature aerospace alloy by a lost-wax casting process. 
     The cavity  8  and the passages  10  are formed in the vane during the casting process by a core  14  shown in  FIG. 2 . The core has a main body  16  which forms the cavity  8 , and extensions  18  which form the passages  10 . The body  16  is of generally aerofoil shape, and has a thicker portion  20 , which tapers down to a thin-walled portion  22 , that is to say a portion having a thin cross-section. The thin-walled portion  22  terminates, at a location corresponding to the trailing edge of the vane of  FIG. 1 , in a rib  24  which is thicker than the thin-walled portion. The rib  24  serves to form the end of the slot  12  in the cast vane. 
     The body  20  has lateral edges  26 , which also constitute the lateral edges of the thin-walled portion  22 . The thin-walled portion  22  is perforated by holes  28 . In the cast vane as shown in  FIG. 1 , the holes  28  form pedestals  30  which extend between walls  32 ,  34  of the aerofoil portion  2  defining the cavity  8 . The holes  28 , in the embodiment shown in  FIG. 2 , are disposed in an array constituted by rows of holes lying on lines extending perpendicularly between the lateral edges  26 . As illustrated, one such line is represented by the section line A-A. 
       FIG. 3  shows, on the left side, a partial section view of the thin-walled portion  22  taken on the section line A-A. 
     It will be appreciated that the thin-walled portion  22  is fragile, by comparison with the thicker portion  20  of the body  16  and the rib  24 . Furthermore, the perforation by the holes  28  contributes to the weakness of the thin-walled portion  22 . In practice, damage to the core  14  is often initiated by failure at one of the edges  26  of the thin-walled portion  22 , and the crack may propagate into the thin-walled portion  22 , frequently between individual holes  28 , for example along a line of holes extending between the lateral edges  26 . 
     Cracking of this kind creates a potential path for metal ingress (where a metallic material is being cast) and hence result in casting flash in the cast component. For example, as represented in  FIG. 1 , casting flash  36  may form between individual pedestals  30  in the cast vane, these gaps corresponding to cracked regions between adjacent holes  28  in the core  14 . 
     This flash  36  restricts air flow within the cavity  8 , and can lead to cooling air starvation at the trailing edge of the vane, resulting in local overheating. If detected during inspection of the casting, it may be possible to carry out salvage work to remove accessible flash, but frequently this cannot be performed economically and the component must be rejected. If not detected and remedied there may be premature deterioration of the trailing edge of the aerofoil portion  2  in service. 
       FIG. 4  shows a modification of the core  14  to avoid damage to the core. A bead  38  is provided along the lateral edge  26  of at least the thinnest part of the thin-walled portion  22 . Being thicker than the thin-walled portion  22 , the bead  38  resists damage, and in particular the initiation of cracks at the lateral edge  26 , and so substantially reduces damage within the thin-walled portion  22 . This minimises the occurrence of regions of flash  36  in the cast component. Consequently, the economic consequences of component rejection and salvage work can be avoided. 
     The right side of  FIG. 4  shows the region of the vane of  FIG. 1  corresponding to the core shown on the left side of  FIG. 4 . The aerofoil portion  2  merges into the outer shroud portion  6  at a curved transition surface  40  on each side. A bead cavity region  42 , corresponding to the bead  38 , is formed at this transition between the aerofoil portion  2  and the shroud portion  6 , this bead cavity region  42  having a bulbous or “mushroom” shape including diverging surface regions  44 . The corresponding surface regions  46  on the bead  38  are shaped so that the surface regions  44  of the bead cavity region  42  generally follow the curvature of the transition surfaces  40  and preferably are approximately parallel to them. The result is that the rate of change of the wall thickness of the vane at the lateral edges of the cavity is minimised. Preferably, the wall thickness remains generally constant over the inner and outer (or “pressure and suction”) walls  32  and  34 , past the bead cavity region  42  and into the shroud portion  6 . This has advantages in that residual stresses are reduced in the finished component, and stress concentrations during engine operation can be avoided. 
     An alternative configuration for the bead  38  and the resulting bead cavity region  42  is shown in  FIG. 5 . In this embodiment, the bead shape is modified so that the surface regions  44  follow an alternative profile for the transition surface  40 , being more in the form of a truncated teardrop. 
     Because the bead is situated within the transition between the aerofoil portion  2  and the inner and outer shroud portions  4 ,  6 , it does not affect the trailing edge of the aerofoil portion  2 , so that the airflow regime over the vane is not disrupted. Also, the bead  38  is small by comparison with the total flow cross-section over the slot formed by the thin-walled portion  22  of the core  14 . Consequently, the cooling airflow distribution through the slot is substantially unaffected by the bead cavity region  42 .