CORE POSITIONING IN WAX PATTERN DIE, AND ASSOCIATED METHOD AND APPARATUS

Abstract

For forming a shell mould for investment casting, first and second core components are provided. These are formed of ceramic material. The core components are assembled together for manufacturing a wax pattern containing the core component for formation of a shell mould for investment casting. The core components have an arrangement of precursor location features. The method includes machining at least one of the precursor location features to a required shape to provide an arrangement of final location features. The core components are then positioned with respect to each other and/or with respect to a wax pattern die by contacting the arrangement of final location features with corresponding positioning features of a receiving device and/or the wax pattern die.

Claims

1. A method for assembling a core component with respect to a receiving device, the method comprising the steps: forming a core component of ceramic material; providing an arrangement of precursor location features at the surface of the core component; machining at least one of the precursor location features to a required shape to provide an arrangement of final location features; positioning the core component with respect to the receiving device by contacting the arrangement of final location features with corresponding positioning features of the receiving device.

2. A method according to claim 1 wherein the receiving device comprises a wax pattern die.

3. A method according to claim 2 wherein the core component is arranged in the wax pattern die by contacting the arrangement of final location features with corresponding positioning features of the wax pattern die.

4. A method according to claim 2 subsequently including the steps of introducing molten wax around the core component in the wax pattern die and allowing the molten wax to solidify to form the wax pattern containing a core component.

5. A method according to claim 1 wherein there is provided an arrangement of first and second core components, the first and second core components being positioned relative to each other.

6. A method according to claim 5 wherein the receiving device comprises an assembly apparatus for holding the first core component, the method subsequently including the step of configuring the second core component to be assembled relative to the first core component, to provide a core assembly.

7. A method according to claim 6 wherein the receiving device further comprises the first core component, the second core component being positioned with respect to the first core component via said final location features.

8. A method according to claim 1 wherein the precursor location features are formed by moulding or depositing a material on the surface of the core component.

9. A method according to claim 1 wherein the precursor location features are formed integrally with the core component.

10. A method according to claim 1 wherein the modification of the precursor location feature is carried out in order to take account of core distortion based on measured data representative of the shape of at least part of the core component.

11. A method according to claim 10 wherein the measured data is used to determine the difference in shape between the core component, being an actual core component, and a nominal core component.

12. A method according to claim 11 wherein the modification of the precursor location feature is carried out based on the determination of the difference in shape between the actual core component and the nominal core component.

13. A method according to claim 11 wherein the measured data is used to determine an optimum assembled position of the first and second core components.

14. A method according to claim 11 wherein the measured data is used to determine a best fit of the core components to reduce or minimize a characteristic or function of several characteristics of the cast component.

15. A method according to claim 11 wherein a population of first and second core components is provided, the measured data being used to determine one or more matching pairs of first and second core components amongst the population.

16. A method according to claim 1 wherein, where the cast component is intended to be an aerofoil component so that the core component has a root region, a tip region and an aerofoil region, the precursor location features are located at one or more of the root region, tip region and aerofoil region.

17. A method according to claim 16 wherein the precursor location features are located at the root region and/or the tip region.

18. An apparatus for assembling a core component with respect to a receiving device, the apparatus comprising: a core component of ceramic material; and a receiving device, wherein the core component has at its surface an arrangement of final location features formed by machining at least one of an arrangement of precursor location features at the surface of the core component to a required shape, the arrangement of final location features being capable of positioning the core component in the receiving device by contacting the arrangement of final location features with corresponding positioning features of the receiving device.

19. An investment casting process for manufacturing a cast metal component, the process comprising the steps: forming a core component of ceramic material; providing an arrangement of precursor location features at the surface of the core component; machining at least one of the precursor location features to a required shape to provide an arrangement of final location features; positioning the core component in a wax pattern die by contacting the arrangement of final location features with corresponding positioning features of the wax pattern die; introducing molten wax around the core component in the wax pattern die and allowing the molten wax to solidify to form the wax pattern containing a core component; forming a shell mould around the wax pattern; removing the wax pattern; introducing a molten metal into the shell mould to fill space between the shell mould and the core component; allowing the molten metal to solidify; and removing the shell mould and the core component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings in which:

[0065] FIG. 1 shows a longitudinal cross-section through a ducted fan gas turbine engine.

[0066] FIG. 2 shows a schematic view of a prior approach to the assembly of first and second core components relative to each other for wax pattern injection.

[0067] FIG. 3 shows a first core component, designated the reference core component, with 6 datum points, corresponding to the datum points used on the first core component in FIG. 2.

[0068] FIG. 4 shows a second core component, designated the slave core component, with 6 datum points at locations differing to the datum points used for the second core component in FIG. 2.

[0069] FIG. 5 shows a schematic view of the root regions of the reference core component and slave core component, viewed end-on from the root, with the adjustment in shape of the root of the slave core component due to machining.

[0070] FIG. 6 indicates the surfaces of the slave core component that are machined relative to the slave datum points.

[0071] FIG. 7 shows the reference and slave core components and indicates measurement of surfaces of the core components and the formation of machined surfaces.

[0072] FIG. 8 shows the use of the machines surfaces indicated in FIG. 7 being used for location of the core components during assembly for wax pattern moulding.

[0073] FIG. 9 shows a flow chart of a method according to an embodiment of the present disclosure.

[0074] FIG. 10 shows a flow chart of another method according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0075] With reference to FIG. 1, a ducted fan gas turbine engine incorporating an embodiment of the present disclosure is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

[0076] During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate-pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate-pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high-pressure compressor 14 where further compression takes place.

[0077] The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 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 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

[0078] The embodiments of the present disclosure relate to the manufacture of cast metal components with complex internal geometries, for example to turbine blades at the high, intermediate and/or low-pressure turbines 16, 17, 18 in FIG. 1, the turbine blades having interconnected internal cavities to assist with cooling of the blades in use of the engine.

[0079] A suitable cast component can be formed according to an embodiment of the present disclosure via investment casting. A ceramic core, or an assembly of ceramic core components, is prepared, this being held in a ceramic shell mould. The shell mould is formed around a wax pattern which in turn is formed around the core. The wax pattern is removed after the shell mould is formed. Molten metal is introduced into the shell mould to fill space between the shell mould and the core. The molten metal is allowed to solidify in a known manner to form a desired grain structure for the component (e.g. single crystal or columnar grain structure). The shell mould and the core are then removed. This can be carried out in a known manner, for example by leaching away the ceramic of the shell mould and core using a suitable alkaline solution.

[0080] There now follows a more detailed explanation of the manner in which the core is processed in order to form the wax pattern around the core.

[0081] First, an approach to the assembly of core components is described, which is not necessarily prior art, but which is of use in understanding the contribution provided by the embodiments of the present disclosure.

[0082] Two ceramic core components 102, 104 are illustrated in FIG. 2, assembled in position relative to each other as required for the investment casting process (the shell mould is not illustrate). The assembly of the core components defines a root region 106, and aerofoil region 108 and a tip region 110, these corresponding to parts of the intended cast component.

[0083] For the process of assembling the core components relative to each other, the first core component 102 is placed in a 6 point location fixture. Fixed datum points 112 are illustrated by black dots. As can be seen, each of these fixed datum points is located at the root region or tip region. The effect of this is that if there is any dimensional distortion present in the core, due to the manufacturing process, for the actual core compared with the nominal (i.e. ideal, designed) shape of the core, this distortion is in the central portion of the core i.e. the aerofoil region.

[0084] The second core component 104 is then itself placed in a 6 point location fixture. Again, the datum points 112 are located at the root region and tip region, with the same effect that any dimensional distortion is located in the aerofoil region. Note that, additionally, the second core component may be located partially or fully on location features of the first core component, and not only on the fixture location positions. Suitable location features here include raised ceramic conical locators (bumpers), for example. Alternatively, there may be male and female locators as described in U.S. Pat. No. 6,347,660 B1, used for locating the two core components together.

[0085] The second core component 104 is then locked in position relative to the first core component 102 using a sacrificial material (not shown).

[0086] The effect of this assembly process is that the root and tip region of the core assembly have very good dimensional repeatability, but this is at the expense of the effect that any distortion present has been transferred to the aerofoil region of the core assembly. Upon subsequent casting, this results in a dimensionally consistent core exit hole. This is useful, because it is required for location within the wax pattern die, blade machining and engine tolerances. However, this can have an adverse effect on ceramic core dimensional variability (on the aerofoil region) and ultimately an adverse effect on the yield of satisfactory cast components from the process.

[0087] The datum points 112 are hard location points (e.g. semi-spherical button features) that are used for locating the core during assembly. Clamps (not shown) are used to hold the core components in position to ensure contact with the location points.

[0088] However, with general improvements in the design of cast components, there is a basic problem that the core and cast product complexity is increasing. This means that the component part designs are requiring better control over wall thickness, in particular requiring that the wall thickness is smaller and with less variation in the wall thickness and from a nominal designed shape. Similar issues apply to the positioning of complex features.

[0089] The prior art approaches of adding non-formed features to the core allows manipulation of the location of the core. However, this is carried out in a similar manner for each core, and does not allow optimization of the position of each core, taking into account variations in shape of the cores from core to core. The movable core positioning members disclosed in the prior art within a wax pattern die are complex to operate and typically add unwanted features into the wax pattern that have to be subsequently finished out of the casting.

[0090] It is known that the ceramic cores can be machined to achieve an overall core length or to achieve a specific feature length such as the trailing edge passage. Furthermore, it is known to machine a notch into the root region of a core at a known radial position, in order to position a feature in the correct radial location.

[0091] The embodiments of the present disclosure include machining of a precursor location feature to form a final location feature. The machining can therefore take account of the specific characteristics of the core on which the precursor location feature is formed. This allows the shape and dimensions of the final location feature to be specific to that core, allowing optimization of the final location feature and therefore careful control of the positioning of the core in the receiving device (such as the wax pattern die or the assembly apparatus).

[0092] In an embodiment of the present disclosure, a core component is intended to be located in a six point fixture. Precursor location features are formed on the core component, for example using wax injection as disclosed in US 2002/0148589. These precursor location features may be formed at positions where it is considered that optimal feature control is required. The precursor location features on the core component are then machined to a final shape, this final shape being determined based on an assessment of any required adjustment of the positioning of the core component in the six point fixture. As will be understood, to take account of the machining process, the precursor features typically are oversized to allow a suitable adjustment of the final location features to be adjusted. In this embodiment, it may not be necessary to carry out measurement of the shape of each individual core component before machining the precursor location features. Instead, the machining may be carried out based on prior knowledge of the difference between the shape of the precursor location features and the required shape for the final location features for the core component for suitable positioning of the core component in the receiving device.

[0093] For example, measurement of the shape of the core components may have been carried out historically, or on a sample basis. The machining may be carried out by taking an average or some other mathematical manipulation of the historical or sample data.

[0094] Note that the measurement of the assembled core may be used to feedback adjustments for further components, where machining takes place based on a fixed process, and such machining may not be adapted based on the dimensions of the particular component being machined.

[0095] In another embodiment of the present disclosure, the machining of the precursor location features is carried out in order to take account of core distortion based on measured data representative of the shape of at least part of the core component. Measurement of the shape of the core component can be carried out by any suitable process, for example by using linear displacement sensors, touch trigger probes or other devices. Alternatively the data may be obtained by performing a scan (e.g. using structured light, laser scanning, CT scanning, etc.). The purpose of the measurement is to determine the difference in shape between an actual core component and the shape of the nominal (i.e. ideal, designed) core component. For the actual core component, therefore, the machining of the precursor location features is then carried out based on the determination between the difference in shape between the actual core component and the shape of the nominal core component. In this way, it is possible adaptively to take into account core component distortion.

[0096] In another embodiment, the present disclosure allows for the assembly of multiple core components, e.g. for forming cast components with dual wall designs. This permits account to be taken of distortion in an assembled core. The precursor location feature can be formed in an oversizing manner on one or more of the core components and then machining these to the desired shape to account for the distortion. This applies to the assembly of the core components together and additional or alternatively applies to the positioning of the assembled core in the wax pattern die.

[0097] FIG. 3 shows a first core component 202 having 6 datum points 212. These are located in the root 206 and tip 210 regions, as in FIG. 2. The first core component 202 is also referred to here as the reference core component. The first core component is held in a six point fixture (not shown) as for FIG. 2. A second core component 204, shown in FIG. 4, is provided. The second core component 204 is also referred to here as the slave core component. Datum points 214 are formed on the aerofoil region 208 of the second core component 204. The second core component is held in a six point fixture (not shown) at the datum points 214, therefore allowing careful control of the positioning of the aerofoil region. Where there has been distortion of the aerofoil region, given that the aerofoil region is being positioned more carefully than the root region, the distortion of the second core component may be manifested by a distortion of the root (or tip) region of the second core component relative to the first core component. This is illustrated in FIG. 5, where the outline of the root region 206a of the first core component is shown and the outline of the root region 206b of the second core component is shown prior to machining. As can be seen, the root region 206b is positionally distorted relative to root region 206a. However, the root region 206b can be considered to be a precursor location feature of the core component. It is deliberately oversized relative to the root region 206a of the first core component and is then machined to become root region 206c of the second core component. The degree of oversize of the root region 206b allows for machining of the root region 206c to become the final (machined) location feature.

[0098] Machining of the root region 206b can be carried out by any suitable machining method, such as 5 axis machining, fixtured single axis machining, etc.

[0099] In addition to machining of the root region 206b, location features at the tip region of the second core component 204 can also be machined. In FIG. 6, the second core component 204 is shown in which root region 206c is shown after machining and also machined location feature 210c is shown at the tip region 210 of the second core component. Machined surfaces are indicated by the reference M.

[0100] FIGS. 7 and 8 show a further example, again involving first core component 302 and second core component 304. Before assembly of the core components together, the shape of the core components is measured, as indicated by measurement points 320. This measurement provides a determination of the difference in shape between each actual core component and the shape of the nominal (i.e. ideal, designed) core component. By analysis, using a suitable algorithm, it is then possible to determine an optimum assembled position of the two core components. The algorithm can, for example, operate to best fit the core components to reduce or minimize the overall error in terms of concave aerofoil metal thickness, convex aerofoil metal thickness and/or the metal web thickness formed by the space between the two ceramic core components. Alternatively, the algorithm can operate to best fit the cores to minimise the theoretical component stress during service, or optimize some other computed characteristic or function of several characteristics that are considered to be important within the component lifecycle. Alternatively, several core components of each type can be measured and the algorithm can select core components for assembly by matching. The core component matching can be carried out in addition to determining the optimum assembled position of each core component pair.

[0101] Based on the determination explained above, for each actual core component, the machining of the precursor location features is then carried out in order to achieve the required positional relationship between the core components and/or between the assembled core components and the wax pattern die.

[0102] The machined surfaces to form the final location features are once more designated as M in FIG. 7.

[0103] As shown in FIG. 8, the machined surfaces (here indicated using dots) are then used as datum points for assembly of the first core component 302 with the second core component 304. In this way the consequences of ceramic core component dimensional variation are reduced.

[0104] Accordingly, the embodiments of the present disclosure allow account to be taken of core component distortion when locating one or more core components together or within a wax pattern die, by machining of precursor location features formed on the core component to form final location features. In this way, better dimensional control of complex cores and assembled cores is enabled, including the possibility of optimal positioning of key design features within the cat component (e.g. turbine component). This enables more complex designs and tighter dimensional tolerance of these designs.

[0105] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the scope of the invention.

[0106] All references referred to above are hereby incorporated by reference.