Centrifugal casting

Abstract

A centrifugal casting apparatus comprising an upper portion into which molten material is poured, the upper portion having a central rotational axis about which the apparatus is rotated; at least one block runner is connected to the upper portion at the proximal end of the block runner and to at least one mould at the distal end of the block runner, the block runner being mounted substantially perpendicular to the axis of rotation; and wherein the moulds are oriented substantially parallel to the axis of rotation of the centrifugal casting apparatus.

Claims

1. A method of casting a component comprising: attaching a mould to a centrifugal casting apparatus, such that the mould is oriented substantially parallel to an axis of rotation of the centrifugal casting apparatus, and applying a molten feedstock through (i) an upper portion of the centrifugal casting apparatus, a central rotational axis of the upper portion coinciding with the axis of rotation of the centrifugal casting apparatus, and (ii) a block runner mounted substantially perpendicular to the axis of rotation to control and contain a turbulence within the molten feedstock, wherein the molten feedstock is a titanium aluminide alloy material, a thinnest part of the mould is oriented in a radial direction away from the axis of rotation, the thinnest part of the mould is a radial outermost part of the mould, and the component is a blade.

2. The method of casting as claimed in claim 1, wherein the mould is pre-heated prior to the casting process.

3. The method of casting as claimed in claim 2, wherein the mould is heated to a temperature between 400 and 900° C.

4. The method of casting as claimed in claim 1, wherein the mould is an investment shell.

5. The method of casting as claimed in claim 4, wherein the investment shell is backed with a ceramic grit.

6. The method of casting as claimed in claim 1, wherein the centrifugal casting apparatus is rotated at 200 to 400 rpm.

7. The method of casting as claimed in claim 1, wherein the mould is mounted perpendicular to the block runner to prevent the turbulence.

8. The method of casting as claimed in claim 1, wherein the step of applying the molten feedstock includes quiescent filling the molten feedstock into the mould via an angle formed between the block runner and the mould.

9. The method of casting as claimed in claim 1, wherein the blade is blade for a gas turbine engine.

10. A centrifugal casting apparatus for casting a component, the apparatus comprising: an upper portion into which molten material is poured, the upper portion having a central rotational axis about which the apparatus is rotated; and at least one block runner connected to the upper portion at a proximal end of the at least one block runner and to at least one mould at a distal end of the at least one block runner, the at least one block runner being mounted substantially perpendicular to the axis of rotation, wherein the at least one mould is oriented substantially parallel to the axis of rotation of the centrifugal casting apparatus, the molten feedstock is a titanium aluminide alloy material, a thinnest part of the mould is oriented in a radial direction away from the axis of rotation, the thinnest part of the mould is a radial outermost part of the mould, and the component is a blade.

11. The centrifugal casting apparatus of claim 10, wherein the at least one mould is removeable.

Description

BRIEF DISCUSSION OF THE FIGURES

(1) Embodiments will now be described by way of example only, with reference to the Figures, in which:

(2) FIG. 1 is a sectional side view of a gas turbine engine;

(3) FIG. 2 is a close-up sectional side view of an upstream portion of a gas turbine engine;

(4) FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

(5) FIG. 4 is a schematic of the casting apparatus of the present disclosure.

(6) FIG. 5 is a flow chart of an embodiment of the casting process of the disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

(7) FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

(8) In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

(9) An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to process around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

(10) Note that the terms “low pressure turbine” and “low pressure compressor” as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the “low pressure turbine” and “low pressure compressor” referred to herein may alternatively be known as the “intermediate pressure turbine” and “intermediate pressure compressor”. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

(11) The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

(12) The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

(13) It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

(14) Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

(15) Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

(16) Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

(17) The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

(18) FIG. 4 shows a schematic representation of the casting apparatus 40. The casting apparatus comprises an upper portion 41, which receives the molten material. The molten material flows along the path of the arrow. The upper portion forms the centre of the casting apparatus through which runs a central axis, it is about this axis that when in use, the casting apparatus is rotated to generate the required centrifugal force. Thus, in use the central axis becomes the axis of rotation. Connected to the upper portion are a number of block runners 42. The block runners are connected to the upper portion at their proximal ends, whilst at their distal ends they are connected to the moulds 43. Each mould may contain a single or multiple cavities defining the component or components to be cast as well as those features commonly required in shape casting processes to achieve the appropriate material integrity. In particular, elements commonly referred to as feeders by those familiar with the art, or alternatively risers, may be attached to the component. Campbell has set out the purpose of a feeder as being to compensate for the volumetric contraction of the molten material as it solidifies, maintain pressure in the melt and to act as a flow-off for the first metal through the filling system. The runner may also act as a feeder and for any particular component geometry feeders may or may not be required..sup.1 The moulds are mounted on the block feeder so that they are oriented approximately parallel with the axis of rotation. The moulds may be mounted above or below the block feeders. For example, the moulds may be mounted within 10 degrees of parallel. The moulds may also be mounted within 5 degrees of parallel. The moulds can be removable from the horizontal block feeders. There can be any suitable number of horizontal block feeders. This for example could be 2, 3, 4, 5, 6, 7, 8, 9 or greater. Any suitable number of moulds may be connected to the block feeder. For example, this may be 1, 2, 3, 4, 5 or greater. The moulds may be aligned so that the thinnest part of the mould is oriented in a radial direction away from the axis of rotation. .sup.1 Campbell J., Complete Casting, pub. Butterworth Heinemann, ISBN-13: 978-1-85617-809-9, pp. 659-696.

(19) In use the molten feed stock is applied into the upper portion 41 of the casting apparatus. For example, feedstock may be melted using induction heating in an appropriate container material or skull melting whereby a resolidified layer forms a solid barrier between a cold crucible and molten charge. Heat for skull melting of the charge can be provided using induction or and electric arc. The feedstock can be formed of any suitable material. This may be pure metals or alloys. Such alloys may include titanium aluminide. The centrifugal casting apparatus is rotated about the axis of rotation. The speed of rotation required to fill the mould will depend on the dimensions of the component and the radial length of the feeder perpendicular to the axis of rotation. A centrifugal force derived from the centrifugal acceleration (g) falling in to the range 150≤g≤6000 ms.sup.−2. For example, the centrifugal casting apparatus may be rotated at 50 to 500 rpm. The centrifugal force acting in this apparatus results in the high-quality components produced by this method. This is because the centrifugal motion moves the metal into the mould as quickly as possible to prevent it from solidifying to such a degree as to stop flow (freezing off) before the mould is full. The mould 43 may be pre-heated so as to increase the time before the metal freezes off; thus, allowing the metal to fill the mould, including any thinner sections. For example, the mould may be heated to a temperature of not less than 400° C. and not more than 1200° C. at any location. To prevent the mould from breaking during the casting process, the shell of the mould may be placed within a container and the external form of the mould supported by filling the container (backing) with ceramic grit and then heated. Using such a method enables the mould to reach a uniform or desired non-uniform temperature during any pre-heating process prior to casting. The moulds may be the same as those used for investment casting.

(20) As the casting apparatus on which the assembled mould and any container is rotated the feedstock is passed down a block feeder 42 through centrifugal motion. The block runner may have a rectangular cross-section. However, the shape of the block runner has been found to not be as important as the angle of the runner. This has been found to work best at a range of not more than 10° inclination to the normal to the axis of rotation. In doing so, the molten feedstock flows from the upper portion to the end of the horizontal block feeder through the centrifugal force on the feedstock as the casting apparatus is rotated. Employing this method allows some of the turbulence to be moved to the inner section of the block feeder. Turbulent flow is still present in the material entering the mould resulting from the rotational movement of the feeder and the centrifugal motion. There are a number of small gas bubbles (air under partial pressure or a controlled atmosphere, for example an inert gas at partial pressure) entrained as a consequence of free surface turbulent flow in the runner are transported in to the component. Under the action of the radial pressure gradient in the liquid they are rapidly separated from the liquid alloy. As such, there is also little turbulent entrainment in the actual component. It has been found that through the use of this method that the turbulence remains within the runner/feedblock 42 and the angle between the runner and the mould acts as a quieting feature. This has been found to work best for angle of 90 degrees, However, the mould may also be mounted at an angle between 75-105 degrees. The moulds 43 of the components are mounted approximately parallel to the axis of rotation of the casting apparatus. In this embodiment the casting moulds are presented as being mounted in a vertically downward direction relative to the horizontal block. However, they may equally be positioned in a vertically upward direction relative to the block feeder. This allows the molten feedstock that has been forced to the end of the horizontal block feeder to enter and to fill the mould.

(21) The mould may be oriented in any orientation. However, it has been found that by orienting the thinnest part of the mould of the component in a radial direction opposite to the rotational axis of the mould improves the quality of the cast component. This is because in this way the orientation of the mould ensures that the maximum centrifugal pressure acts at the thinnest part of the component. This has been found to work for thin edges, even for objects such as the trailing edge of blades for use in gas turbine engines.

(22) The moulds may be the same as those used as for static moulds for investment casting. Alternatively, mould can be designed to allow a quiescent fill process to be employed. The use of a quiescent fill process ensures maximum component quality. This casting method may be used to manufacture any suitable component. This could for example be parts for a gas turbine engine such as blades. Alternatively, it could be used to manufacture turbocharger wheels, engine frame connectors and gimbals. The method may be used for any appropriate material to be used in the casting process. In particular it is suitable to materials that can be used for a number of different casting objects and for a number of different applications.

(23) FIG. 5 presents an example flow chart of the process disclosed. The moulds are attached to the casting apparatus in step 51. The attached moulds are oriented so that they are parallel to the axis of the rotation of the centrifugal casting apparatus. For example, these may be attached so that they extend below the casting apparatus. The moulds may also be connected so that they are oriented so that the thinnest part of the mould is oriented in a radial direction away from the axis of rotation. The metal compound is melted in step 52 to form a molten feedstock; this may be carried out using a suitable process such as those discussed above. The molten metal is added into the upper portion of the casting apparatus in step 53. Prior to this step the mould may be preheated to an appropriate temperature. The apparatus is rotated at any suitable speed so that the resulting centrifugal force generated through the rotation is enough to move the molten metal to the end of the block feeder of the casting apparatus. The metal fills the mould in step 54 and is allowed to cool and solidify. The moulds are then removed from the casting apparatus in step 55. The moulds can then be separated from the cast components in step 56.

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