Ceramic Core for an Investment Casting Process

Abstract

Described is a ceramic core for producing cast component for a gas turbine engine, the core comprising: a first cavity forming member; a second member adjacent to or opposite the first cavity forming member; and a removable web which joins the first and second members.

Claims

1. A ceramic core for producing cast component for a gas turbine engine, the core comprising: a first cavity forming member; a second member adjacent to or opposite the first cavity forming member; and a removable web which joins the first and second members, wherein the removable web is plate-like member having a thickness in the range of approximately 0.1 mm to approximately 2 mm.

2. A ceramic core as claimed in claim 1, wherein the second member is a cavity forming member.

3. A ceramic core as claimed in claim 1, wherein either or both of the first and second members are for providing a cooling passage member in a cast fluid cooled component.

4. A ceramic core as claimed in claim 1, wherein the removable web extends between three or more members.

5. A ceramic core as claimed in claim 1, wherein at least one of the members is a stock for holding the core within a mould.

6. A ceramic core as claimed in claim 3, wherein cooling passage member of the first or second member is a multi-pass cooling passage.

7. A ceramic core as claimed in claim 1, wherein the removable web extends across a corner region which is formed by a junction of the first and second members.

8. A ceramic core as claimed in claim 1, wherein at least one of the members is a strut which extends between two other members, the strut having a smaller transverse section than the other of the members.

9. A ceramic core as claimed in claim 1, wherein the removable web extends substantially perpendicularly from a surface of one or more of the members.

10. A ceramic core as claimed in claim 1, wherein the removable web is polygonal when viewed in the direction normal to a surface of the plate-like member.

11. A ceramic core as claimed in claim 1, wherein the first member provides an inlet passage in a hub region of a gas turbine blade.

12. A ceramic core as claimed in claim 1, wherein a plurality of removable webs extend from a common member.

13. A ceramic core as claimed in claim 12, wherein the plurality of removable webs are diametrically opposed about the common member.

14. A gas turbine component made using the ceramic core of claim 1.

15. A core production facility having a first plurality of ceramic cores according to claim 1, and a second plurality of cores which is the same as the first plurality of cores but with the removable web removed.

16. A method of forming a ceramic core for an investment casting process comprising: providing a ceramic core comprising: a first cavity forming member; a second member adjacent to or opposite the first cavity forming member; and a removable web which joins the first and second members, wherein the removable web is plate-like member having a thickness in the range of approximately 0.1 mm to approximately 2 mm; and removing the removable web prior to the ceramic core being used in a casting process for a gas turbine component.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0044] Embodiments of the present disclosure will now be described with the aid of the following drawings of which:

[0045] FIG. 1 shows a longitudinal section of a conventional gas turbine engine.

[0046] FIG. 2 shows a partial perspective view of a turbine stage of a conventional gas turbine engine.

[0047] FIG. 3 shows the steps of a conventional investment casting process.

[0048] FIGS. 4a and 4b show perspective views of a conventional gas turbine blade, FIG. 4a being a cut away to reveal some exemplary cooling flow passages.

[0049] FIG. 5 shows a conventional generic ceramic core typically used to cast a turbine blade.

[0050] FIG. 6 shows a ceramic core according to the present disclosure.

[0051] FIG. 7 shows planform section A-A of the strut and web region of the ceramic core shown in FIG. 6.

[0052] FIG. 8 shows a flow diagram indicating the steps for manufacturing a core of the present disclosure.

[0053] FIG. 9 shows a production facility in which the removable core may be removed.

DETAILED DESCRIPTION OF DRAWINGS

[0054] FIG. 4 shows a known turbine blade 410 notionally similar to the one depicted in FIG. 2. The blade 410 includes an aerofoil portion 412 having leading 414 and trailing 416 edges with pressure 418 and suction (obscured from view) surface walls extending therebetween.

[0055] The aerofoil portion 412 extends from a hub 420 which includes a platform 422 and attachment fixture in the form of a so-called fir tree root 424. The aerofoil 412 extends in span between the hub platform 422 and a tip 426 which includes a shroud 428. The platform 422 and shroud 428 extend laterally from the aerofoil to having leading and trailing edges and lateral or circumferential edges which face corresponding faces of adjacent components in the rotor array to provide radially inner and radially outer segmented annuli. The radially inner platform and radially outer shroud define the main gas path of the turbine blade.

[0056] The partial cutaway shown in FIG. 4a reveals the internal cooling passages which extend from an inlet located in the root 424 of the blade 410. The passages include so-called multi-pass 432 or serpentine type which include multiple serially connected spanwise passages, and single spanwise passages 434. The position and size of the cooling passages are determined by the required cooling duty and will be part specific. There may be any combination of either or both single or multi-pass cooling passages which may extend spanwise or chordwise. There may be multiple passages across the thickness of the aerofoils such that the suction and pressure surfaces have different cooling passage distributions. The multi-pass passages may meander aft rather than fore towards the leading edge. It will be appreciated that other arrangements will be possible.

[0057] The cooling passages are exhausted at various locations, some or all of which providing external cooling to the surface of the component. In the example shown, the cooling passages include distributions of film cooling holes 436 on the flanks of the aerofoil including spanwise arrays along the leading edge, the pressure surface mid-chord and local to the trailing edge. Suction surfaces tend to have a reduced number of film cooling holes due to the reduced thermal loading. The tip of the blade also includes cooling holes which are provided at the terminal end of the multi-pass and single pass cooling passages.

[0058] As described above, the cooling passages are formed within the body of the blade when the component is cast using a ceramic core.

[0059] A multi-pass ceramic core is shown in FIG. 5. The core does not correspond to the cooling passages of the blade shown in FIG. 4a, but is notionally similar to the core which would have been used.

[0060] The core 510 includes a tip 512, a root 514, a leading edge portion 516 and a trailing edge portion 518 which relate to the orientation of core as it would be presented in the cast component. The dimension between the tip 512 and root 514 is referred to as the span of the core 510, with the chord denoting the dimension between the leading 516 and trailing 518 edge portions.

[0061] The core 510 includes a multi-pass core passage member 520 which includes a plurality, i.e. three in the present case, of serially connected spanwise longitudinal members which are connected by u-bends to provide a meandering or serpentine multi-pass cooling passage member. There is also a single spanwise longitudinal core passage member 522 which extends between the tip 512 and root 514. The single core passage member is located at the leading edge portion 516 of the core 510, with the multi-pass passage 520 member being aft thereof and extending meandrously towards the trailing edge 518 from a mid-chord position. The passage members are connected to a spar 524 which is located at the root 514 of the core 510. In the example shown, there are two connections to the spar, each attributed to one of the core passage members. The core passage members are sized according to the required flow for the resultant passage within the cast component. The root of the core provides the inlet holes for the core passages in the cast component.

[0062] The tip of the core is provided with a stock 528 (commonly referred to as a print or tip print) which is used: to hold the core 510 within the mould used to apply the sacrificial moulding as described above; to hold the composite core within the shell mould; and, to provide support for the individual cooling passages. The stock is typically outside of the cast part but it may form a cavity or hollow in the tip of an aerofoil in some instances. A plurality of struts 530 extend from the stock 528 from a first end to a second end which is integrally connected to the core passage members. The struts 530 are elongate members which are separate from each other and provide a through-passage in the tip wall of the cast product. Thus, in the example of a turbine blade such as that shown in FIG. 4a, the struts 530 provide openings may be sealed and bored to provide cooling holes 530 of the appropriate size in the tip shroud. As such, the stock does not form part of the cast product in this example. However, there may be instances where the stock or an equivalent feature at the tip of the core does form part of the cast product.

[0063] A difficulty with the strutted core design shown in FIG. 5 is the permitted thickness required of the struts 530. The struts 530 must be manufactured to have a thickness sufficient to allow the core forming processes to be successfully carried out. Thin struts are difficult to reliably fill with ceramic slurry during the injection process, particularly with higher viscosity ceramics. Even when the strut volumes are fully occupied, weaknesses can occur where two slurry flows meet and fail to knit properly due to localised temperature fluctuations. Additionally, corner portions of the multi-pass core passage members tend to resist core shrinkage and the resultant tensile stresses can lead to a mechanical weakening and failure of the cores post firing. Other causes of failure in the tip region may occur.

[0064] Providing a larger sectioned strut can overcome these difficulties, however, too large a strut is also problematic as the holes left by the struts may need to be reduced or entirely closed and the closing process, such as welding which is typically used to do this, may result in distortion of the component which needs to be compensated for in the thickness of the component walls.

[0065] The issues with struts limit the geometry of a ceramic core so as to have fewer passages and/or fewer thick to thin transitions or generally simpler designs without, for example, some of the desirable surface features such as turbulator strips. The issues can also affect the material type and strength which can be used for injecting the ceramic cores.

[0066] FIG. 6 shows a core 610 to according to an embodiment of the present disclosure. The core 610 is shown from a front facing perspective view which corresponds to the pressure surface of the aerofoil. The core 610 is similar to that described in FIG. 5 and thus includes a tip 612, a root 614, a leading edge portion 616 and a trailing edge portion 618 which relate to the orientation of core as it would be presented in the cast component. The dimension between the tip 612 and root 614 is referred to as the span of the core 610, with the chord denoting the dimension between the leading 616 and trailing 618 edge portions.

[0067] The core 610 includes a plurality of structural members in the form of cooling passage members 620, a stock 628, a spar 624 and struts 630 which are reduced section members which bridge between two of the other structural members. The spar 624 is a structural member which connects two of the cooling passage members directly and which may itself be a cooling passage member in the form of an inlet.

[0068] The core 610 may include one or more multi-pass core passage members 620 which include a plurality, i.e. three in the present case, of serially connected spanwise longitudinal members which are connected by u-bends to provide a meandering or serpentine multi-pass cooling passage member. There is also a single spanwise longitudinal core passage member 622 which extends between the tip 612 and root 614. The single core passage member is located at the leading edge portion 616 of the core 610, with the multi-pass passage 620 member being aft thereof and extending meandrously towards the trailing edge 618 from a mid-chord position. The core passage members have a thickness in the dimension which extends between the suction and pressure walls and an axial chord length which extends between the leading and trailing edges.

[0069] The passage members connected to a spar 624 which is located at the root 614 of the core 610. In the example shown, there are two connections to the spar, each attributed to one of the core passage members. The core passage members are sized according to the required flow for the resultant passage within the cast component. The root of the core provides the inlet holes for the core passages in the cast component.

[0070] The tip of the core is provided with a stock 628 which is used to hold the core 610 within the mould used to apply the sacrificial moulding as described above. A plurality of struts 630 extend between the stock 628 and the core passage members.

[0071] The struts 630 are elongate members which may be straight and may have a substantially constant cross section along their length. There are three struts shown in FIG. 6, each extending from tip end of one of the core passage members 620, 622. There is a trailing edge strut 630T, a leading edge strut 630L and a mid-chord strut 630M. It will be appreciated that there may be greater or fewer struts than is shown in FIG. 6. The struts have a first end and a second end which are each connected, either directly or via a transition portion, to a face of the stock and the core passage respectively. Each core passage return or terminal end in the tip region includes a strut. The struts may be provided to provide structural rigidity to the ceramic core so that it can withstand the subsequent process steps such as the injection of the sacrificial material. Thus, any free end or cantilevered end of a member or members may be attached to a strut to tie it to an another structural element to provide rigidity and additional strength.

[0072] The struts 630 are notable as having a considerably smaller sectional area than the core passages in the example shown and may be defined by a sharp reduction in the sectional area of the core passage or other structural member to provide the thinner section. The transition between the core passage member 620 and 622 and strut 630 may be an abrupt one in which the strut 630 abuts a face of the core passage as per the mid chord strut, or may be tapered as per the leading edge strut 630L where the sectional area of the cross passage member decreases gradually as it morphs into the strut.

[0073] As shown in FIG. 7, the struts 630 may be positioned along the camber line of the core which generally corresponds to the camber line of the aerofoil. The struts may be substantially polygonal in planform section with heavily filleted, i.e. rounded, longitudinal edges. The struts 630 may be longitudinally straight or curved.

[0074] The core of FIGS. 6 and 7 includes a plurality of removable webs 640 which span between respective struts 630. The web 640 is a plate-like member of ceramic material which extends in span and chord between the struts 630 and has a thickness which extends between the pressure and suction surface sides of the core 610. The thickness of webs 640 is significantly less than that of the struts 630, cooling passage members 620, 622, stock 628 or other structural members to which it may attach. The removable webs 640 may have a thickness in the range between 0.1 mm to 2 mm but will typically be in the region of around 0.8 to 1.5 mm. The removable webs may be specified as a ratio of the associated strut thickness so, for example, may be approximately between 0.1 and 1 of the strut thickness but will typically be somewhere between 0.3 and 0.5. It will be appreciated that the thicknesses of the individual struts and removable webs may vary in themselves and also relative to each other. The webs 640 are removable in that they do not form part of the core which is used in the subsequent investment casting, but are provided to the benefit of producing the core.

[0075] The webs 640 are formed with the ceramic core during the injection process (or alternative core forming method). Thus, the webs 640 are made from the same material as the rest of the core 610, are integrally formed therewith and undergo the same manufacturing process until they are removed, typically after firing. The inclusive processing steps may therefore include moulding, solidification and firing of the core. It will be appreciated that other processes may also be shared and the removable web may be removed prior to firing the core. Further, the core may be made using an additive layer procedure.

[0076] The web 640 may extend along the length of the struts 630 or other structural member. The webs 640 may extend between the structural members along a curved path. In the present case, this provides the web 640 with a curved profile in the planform section. The joint between the structural member and web 640 may be at the approximate lateral mid-portion of the structural member in section as shown in the planform section of FIG. 7.

[0077] The web 640 may extend perpendicularly from the adjoining face. Providing a perpendicular transition between the web and adjoining face of the strut or other portion or member of the core may help reduce stresses in the joint. The web may begin to curve after the perpendicular transition.

[0078] The web 640 may extend fully between the struts 630 in chord and may be completely continuous so as not to include any breaks, notches or apertures. The webs 640 may also extend in span from the tip face of the core passage members and the radial inner edge or face of the stock, thus providing a closed web which is attached, at least partially, on all sides.

[0079] In the alternative, the webs 640 may include geometric features such as local thickening or reducing features such as notches or apertures or the like where the design permits. The removable webs may be partial and may not extend full width between the first and second members. The plate-like shape of the webs may be, for example, hour glass or bow tie shaped. The webs may be take the form of a strip which extends along the struts. The strip may extend around multiple members to provide a peripheral support with a central aperture. The web may be attached on two or more sides. The web may be attached on three sides or four sides. The majority of the perimeter of the removable web may be joined with a structural member of the ceramic core. One or more of the struts may have a web extending from opposing sides thereof. The webs may be diametrically opposing.

[0080] The provision of a web increases the flow section for the ceramic slurry upon injection or pouring, increases the structural strength and rigidity of the area and features local to the web.

[0081] It will be appreciated that the webs are extraneous features of the core with a functionality limited to the formation of the core. Hence, once the core has been prepared, the webs are removed using a suitable technique. Such a technique may include manual removal and dressing of the adjoining portions by hand, or may include machining of the ceramic where possible. Such machining may include CNC machining.

[0082] A further application of the webs 642 is shown at the hub end of the core 610. In this example, the web 642 is provided between a cooling passage member 622 and the spar 624. It will be appreciated that the removable webs 640 are not restricted to the tip or hubs and may be employed anywhere on the core 610.

[0083] The cooling passage member 622 may extend from a face of the spar at an angle. The angle may be approximately ninety degrees as shown, or any which can benefit from the advantages provided by a webbed support. The web 642 spans between the spar surface and cooling passage member to brace the corner region where the two components meet. The corner web may be triangular or some other three sided shape. For example, the hypotenuse of the web 642 may be curved or include multiple facets.

[0084] In the example shown, the there are two webs which are on opposing sides of an elongate member which extends at an angle from a cross piece.

[0085] FIG. 8 shows a method for manufacturing the ceramic core 810 according to the present disclosure. The first step is to provide a mould 812 for receiving a ceramic slurry for producing the core. The ceramic slurry will typically comprise ceramic particulates and a binder material as is well known in the art. The mould is shaped to provide the core required for an investment cast component such as the aerofoil described above. Hence, the core includes at least a first member, a second member and a removable web which spans between the first and second member.

[0086] The ceramic is introduced into the mould by injection 814 or pouring before being solidified and fired 816. Once fired the web can be removed 818 using a suitable process. The removal process may be via a machine such as a CNC milling machine which uses a rotating tool to cut the removable web out. Alternatively, or additionally, the web may be removed by hand using appropriate tools.

[0087] FIG. 9 shows a schematic representation of a production facility 910 which may be used to remove the removable web. The production facility may be any suitable facility which is capable of removing the webs. Thus, the cores may be manufactured in a facility having a process line in which there are a plurality of first cores 912 and plurality of second cores 914 which are differentiated at least by the presence or absence of the removable web. Thus, the first cores may include the removable web and the second cores may be the same as the first cores but with the web removed. The production facility may or may not produce the ceramic cores and may or may not perform the remaining steps of the core forming or investment casting process. For example, the cores may be made and fired at a first location before being imported into the facility for the removal of the web and inclusion in a composite core or other part of an investment casting process. The facility will also include some form of web removing capability 916 which may be mechanised or manual.

[0088] Although the above described embodiment relates to a blade for a gas turbine engine, it will be appreciated that a similar core could be used for any hollow cast member. In the case of a gas turbine, this may include a nozzle guide vane for a turbine or a compressor for example. It is contemplated that other components may be cast using the above described web. Thus, generally, the removable web may be deployed between any two structural members in any ceramic core. Thus, there may be a first member and a second member having a removable web extending therebetween. The first and second members may be adjacent or opposite one another in the sense that they may directly connect with one another so as to be adjacent, or be separate from or connected indirectly via a third member so as to be opposite one another. In this context, opposite may or may not include the first and second members facing one another.

[0089] The first and second members, and third where the case may be, will generally be thicker than the web which will be plate-like in most instances.

[0090] The components described above generally relate to air cooled components. It will be appreciated that the cooling may be achieved by other fluids such as steam.

[0091] It will be understood that the invention is not limited to the described examples and embodiments and various modifications and improvements can be made without departing from the concepts described herein and the scope of the claims. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features in the disclosure extends to and includes all combinations and sub-combinations of one or more described features.