Patent Application
20180345357
This application is based upon, and claims the benefit of priority from GB Patent Application No. 1708953.3, filed on 6 Jun. 2017, the entire contents of which are hereby incorporated by reference.
The present disclosure relates to a method and apparatus for positioning a core in a wax pattern die prior to molten wax injection in order to form a wax pattern. The wax pattern is intended to be used for the subsequent formation of a shell mould around the wax pattern, there following a subsequent casting process for production of a metal casting.
The investment casting process is used to create metal components, e.g. turbine blades, by introducing molten metal into a ceramic shell of the desired final shape and subsequently removing the ceramic shell.
The process is an evolution of the lost-wax process whereby a component of the size and shape required in metal is manufactured using a wax pattern die into which molten wax is injected and allowed to solidify. The wax pattern is then dipped in ceramic slurry to create a shell on the wax pattern. The wax is removed and the shell fired. The resulting ceramic shell has an open cavity of the size and shape of the final component. Molten metal is introduced into the shell in order to form the component having near net-shape. The ceramic shell is subsequently removed, either physically and/or chemically.
In order to make a component e.g. an aerofoil blade, with internal cavities e.g. internal cooling channels, a ceramic core is required. This is manufactured separately and is placed inside the wax pattern die prior to wax injection. After casting the metal in the ceramic shell and around the ceramic core, the ceramic core is removed. This can be done by leaching the ceramic core away using alkaline solution, for example, to leave the hollow metal component.
It is important to locate and support the ceramic core in a fixed positional relationship within the ceramic shell in order to accurately control the shape of the hollow metal component after casting.
Ceramic cores may be manufactured via particle injection moulding (PIM). A ceramic material, such as silica, is suspended in an organic binder (vehicle) to create a feedstock. This feedstock is then injected into a die cavity of the required size and shape and allowed to harden to create a “green” component comprising the ceramic and binder components. The binder is subsequently thermally or chemically removed and the ceramic is consolidated by sintering at elevated temperatures; this gives the final ceramic core.
New cooling concepts often require a complex configuration of core passages to give the most efficient level of cooling on the final component. To allow increased complexity of internal cooling passages whilst maintaining manufacturability of the ceramic core, the core can be manufactured in two pieces and assembled together.
The ceramic core can be positioned with respect to the shell by maintaining exposed portions of the ceramic core for contact with the shell. This requires that the wax pattern is not formed at these portions of the core. It is known to locate cores in the wax pattern die by positioning them on “prints”. Prints are surfaces or features on the core that contact corresponding metal location points that are integral to the wax pattern die. The prints allow shell material to make contact with the core during the ceramic slurry dipping process and this contact secures the core and prevents relative movement between the core and the shell during subsequent casting after the wax has been removed.
The prints can be formed in any practical shape. They may make point contact with the locating features in the wax pattern die, being provided for example by shapes with are at least part spherical, cylindrical, rectangular or conical. Alternatively, prints can be full form prints, where the location point is shaped to the profile of the core.
As opposed to being formed as part of the wax pattern die, the prints can be “non-formed surfaces”. Such a surface is provided by a non-integrally formed feature, that is, a feature which is not formed in the initial injection moulded surface of the core. Some examples of non-formed surfaces useful as location features for positioning the core in the wax pattern die are discussed below.
US 2002/0148589 discloses the provision of locator elements on ceramic cores for use in manufacturing turbine aerofoils. The moulded and fired ceramic core is placed in a die specifically to form moulded wax locator elements on the concave and convex sides of the aerofoil shape of the ceramic core. The moulded wax locator elements take the form of hemispherical protrusions which adhere to the surface of the ceramic core. When solidified and cooled, the ceramic core with the moulded wax locator elements is placed into a wax pattern forming die cavity. The moulded wax locator elements assist with the accurate positioning of the core within the die. Molten wax is then injected into the wax pattern forming die cavity in order to form the desired wax pattern for the turbine aerofoil. In effect, the moulded wax locator elements act as wax chaplets for the wax pattern moulding process.
JP 2000-246392 discloses the provision of a location surface for a ceramic core by generating a wax sheet of known dimensions and adhering this to the core, with the aim of preventing unwanted fracture of the core during injection of molten wax to form the wax pattern prior to forming the shell for investment casting.
In another known process, a print may be provided as part of the core that is positioned in the wax pattern die as discussed above but additional material is added to the surface of the print. This approach is known as forming a “slip print”. The additional material is removed before casting the metal into the shell mould. This provides clearance to allow for radial shrinkage. Suitable materials for slip prints include varnishes, paints and tapes.
US 2009/0308564 discloses a method for forming a wax pattern around a ceramic core in a wax pattern die. The approach used in US 2009/0308564 acknowledges that the dimensions and shape of the actual core may deviate from those of an ideal core. The actual core is measured and compared with the ideal core and a best fit spatial relationship of the actual core to the spatial envelope for the ideal core or for the cavity of the die is determined. The actual core is located in the wax pattern die and its position in the die adjusted using movable core positioning members driven by motors according to the best fit spatial relationship. The movable core positing members are rods which extend into the die cavity from outside the die. The movable core positioning members act as core locating features within the wax pattern die and can be used to place the core optimally for wall section control. A similar technology is available known under the trade name Smartwall™ by Camcraft in the United Kingdom (see http://www.camcraft.co.uk/automation.aspx (accessed 4 Jan. 2017)).
It is possible for the same prints to be used for locating the core in the wax pattern die as well as positioning the core within the shell. However, there are examples where the wax pattern die “prints” are different to the “prints” used in the shell process. It is therefore possible to distinguish between location points on the core used for locating the core in the wax pattern die and the “prints” which are the core surfaces that are used to hold the core to the shell.
It is known to add clearance around core print regions in order to locate the core in the wax pattern die by other means. Other means include using chaplets (e.g. EP 0343095 A1 and U.S. Pat. No. 7,246,652B2) or using bumpers (e.g. U.S. Pat. No. 5,296,308A). The wax that flashes over the prints can be cleared subsequently by manual dressing, in order to allow the shell to adhere to the core surface and thereby support the core during casting. This allows the core to be positioned in the wax pattern die in the aerofoil region, without constraining it in the print regions (by having clearance). However, the bumper positions are generally pre-determined, and the chaplet positions come in fixed sizes.
In the case where the core is assembled from multiple components, then not only must the positional relationship between the core and the shell be controlled, but also the positional relationship between the component parts of the core must be controlled.
U.S. Pat. No. 5,295,530 discloses the manufacture of a single cast thin wall structure formed using multiple cores. As shown in FIG. 5 of U.S. Pat. No. 5,295,530 (not reproduced here), a first core component is coated with a pattern wax and a second core is placed on top of the pattern wax coating. Pockets are drilled through the second core component into the first core component and rods used to secure the position of the second core component with respect to the first core component. A further pattern wax coating is formed on the second core component and further rods placed in the second core component and protruding from the further pattern wax coating. The casting shell is formed to cover the further pattern wax coating and the protruding rods. When the wax is removed, there remains the second core component suspended between the first core component and the casting shell by the rods. U.S. Pat. No. 5,394,932 discloses a composite core formed from first and second core components which join together via a tongue and groove arrangement.
U.S. Pat. No. 6,186,217 discloses a multi-piece core assembly for creating multi-wall components. The core components fit together by an arrangement of protrusions and recesses forming joints, the joints having an entry hole permitting the introduction of ceramic adhesive through the entry hole into the joint.
U.S. Pat. No. 6,557,621 discloses the assembly of core components by locating protruding members from one component into pockets of another component and using adhesive to hold the components together.
The inventors have realised that the prior art approaches to the positioning of the core in the wax pattern die require improvement in order to satisfy the requirements for better control over the thickness of the cast component between the core and the shell as the core and cast complexity increases. Thus, there is a need for tighter positional control of the core within the wax pattern die, in particular where there can be expected to be variation of the shape of the core from the ideal, designed shape, and also where there can be expected to be core-to-core variation. More generally, there is a need for tighter positional control of the core with respect to a receiving device (such as the wax pattern die or an apparatus intended to allow assembly of core components in a fixed positional relationship).
Accordingly, there is a need for a method and apparatus for positioning a core within a receiving device that addresses the problems associated with the prior art approaches discussed above.
In a first aspect, the present disclosure provides a method for assembling a core component with respect to a receiving device, the core component being for manufacturing a wax pattern containing the core component for formation of a shell mould for investment casting, the method comprising the steps:
In a second aspect, the present disclosure provides an apparatus for assembling a core component with respect to a receiving device, the core component being for manufacturing a wax pattern containing the core component for formation of a shell mould for investment casting, the apparatus comprising:
In a third aspect, there is provided an investment casting process for manufacturing a cast metal component, the process comprising the steps:
In a fourth aspect, the present disclosure provides a cast component e.g. a turbine blade or guide vane having cavity, channel, arrangement of cavities and/or arrangement of channels formed by the process of the third aspect.
In a fifth aspect, the present disclosure provides a gas turbine engine having a cast component according to the fourth aspect.
Accordingly, the present disclosure allows the position of the core in the receiving device to be precisely controlled based on the actual shape of the core, in the sense that the modification of the precursor location features can be carried out based on the shape of the specific core on which the precursor location features are formed, thereby to form final location features that suit the shape of the specific core.
In the background discussion above, the possibility was noted of there being wax pattern die “prints” which are different to the “prints” used in the shell process, and for there to be clearance around core print regions in order to locate the core in the wax pattern die by other means. However, such approaches do not permit the core to be adaptively or easily positioned using core measurement data.
Optional features of the present disclosure will now be set out. These are applicable singly or in any combination with any aspect of the present disclosure.
In some embodiments, the investment casting process provides a multi-cavity cast component, such as a gas turbine component. There may be provided an arrangement of first and second core components in order to form the required arrangement of cavities and/or channels in the cast component. The cavities may be used for cooling during gas turbine operation, e.g. in a ducted fan turbine engine.
In some embodiments, the receiving device comprises a wax pattern die, the core component being arranged in the wax pattern die by contacting the arrangement of final location features with corresponding positioning features of the wax pattern die. The method may subsequently include 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.
In some embodiments, the receiving device comprises an assembly apparatus. This may hold the core component by any suitable fixing means, such as by clamping. Subsequent to the holding of the core component (the first core component) in the assembly apparatus, a second core component may configured to be assembled relative to the first core component, to provide a core assembly. The second core component may be treated in a similar manner to the first core component, by modification of at least one of an arrangement of precursor location features to form an arrangement of final location features. In this way, the second core component may be held in the same or a different receiving device such as the same or a different assembly apparatus. The second core component may then be positionally fixed with respect to the first core component. This may be achieved, for example, by using a removable joining material, such as a sacrificial material, intended to be removed before or during the investment casting process. The receiving device may be viewed as further comprising the first core component. In that case, the second core component may be positioned with respect to the first core component via said final location features.
The modification of said at least one of the precursor location features to said required shape to provide said arrangement of final location features can be done by machining.
The precursor location features may be formed by one or more of a variety of approaches. For example, the approaches used in the prior art documents mentioned above may be used. Thus, the precursor location features may be formed by moulding a material against the surface of the core component. Suitable moulding materials include wax, plastics, adhesives, cements, formable ceramic materials. For example the precursor location features may be formed using wax injection as disclosed in US 2002/0148589
The precursor location features may be formed via other approaches. For example, the precursor location features may be formed by freeform deposition of material on the surface, such as by 3D printing. Pressure sintering of ceramics may be used, or stamping processes. After the core component is formed, it can be loaded into a fixture and into additive manufacturing apparatus which then deposits a material onto the surface in a desired shape. Examples of such apparatus include extrusion based 3D printing or jetting technology. Typically, a subsequent stage to harden and then fire the material is needed if the location feature is formed from ceramics. If the material is formed from plastics (as chaplets) then this can be used as is—there is a benefit here particularly for complex geometry prints. Pressure sintering is well known, and allows the locator feature to be formed with a more complex shape than could be formed integrally. Stamping, for example, can be used to indent the surface of the core component that surrounds the precursor location feature, thereby forcing material to be raised at the precursor location feature. This raised area can then be machined to a required shape once the core component has been fired and measured.
It is also possible for the precursor location features to be formed integrally with the core component. This may be done at the time of moulding of the core component.
In some embodiments, the machining 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. 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 measured data may be used to determine the difference in shape between the core component, being an actual core component, and a nominal (i.e. ideal, designed) core component. For the actual core component, therefore, the machining of the precursor location feature may then be carried out based on the determination of this difference. In this way, it is possible adaptively to take into account core component distortion.
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 may be located at one or more of the root region, tip region and aerofoil region. In particular, the precursor location features may be located at the root region and/or the tip region.
In embodiments where there is an assembly of core components, the first core component may be positioned first and the second core component may be positioned relative to the first core component with reference to the positioning of an aerofoil region of the second core component. Precursor location features formed at the root region and/or the tip region may then be machined to final location features to take account of the positioning of the aerofoil region of the second core component.
It is within the scope of the present disclosure to implement the techniques of U.S. Pat. No. 6,347,660B and/or U.S. Pat. No. 6,557,621B using the approach of the present disclosure. These prior art approaches use tongue and groove or plug and socket features to locate core components with respect to each other when assembling a complex core. In some embodiments of the present disclosure, corresponding precursor location features, in the form of precursor tongue and groove features and/or precursor plug and socket features, can be machined to a required shape to provide an arrangement of final location features.
More generally, in some embodiments, it is possible to reduce the variation caused by assembling two distorted core components, using the approach of the present disclosure to machine precursor location features on one or more of the core components.
In some embodiments, the measurement is used to determine an optimum assembled position of the first and second core components.
In some embodiments, the measurement is used to determine a best fit the core components to reduce or minimize a characteristic or function of several characteristic of the cast component.
In some embodiments, a population of first and second core components is provided, the measurement being used to determine one or more matching pairs of first and second core components amongst the population.
Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings in which:
With reference to
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.
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.
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
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.
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.
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.
Two ceramic core components 102, 104 are illustrated in
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.
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.
The second core component 104 is then locked in position relative to the first core component 102 using a sacrificial material (not shown).
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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
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.
The machined surfaces to form the final location features are once more designated as M in
As shown in
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.
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.
All references referred to above are hereby incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1708953.3 | Jun 2017 | GB | national |
