GB2462275A

GB2462275A - A method of connection a turbine shaft to a rotor

Info

Publication number: GB2462275A
Application number: GB0813921A
Authority: GB
Inventors: Tom J Roberts; Robert L Holroyd; Qiang Zhu; James Alexander Mcewen
Original assignee: Cummins Turbo Technologies Ltd
Current assignee: Cummins Turbo Technologies Ltd
Priority date: 2008-07-31
Filing date: 2008-07-31
Publication date: 2010-02-03
Also published as: GB0813921D0

Abstract

A method of connecting a turbine shaft 24 to a rotor 35 by locating at least a portion of a turbine shaft 24 in a mould shaped to define a turbine or compressor wheel, introducing molten metal or metal powder into said mould and solidifying said molten metal or metal powder introduced into the mould so as to produce said rotor 35 connected to said shaft 24. Investment casting or metal injection moulding may be used to form the rotor 35. The shaft 24 has radial and/or axial formations 26 & 28 for secure connection to the rotor 35 and may be made form a mild or low carbon steel while the rotor 35 is an austenitic nickel-based alloy. The portion of the shaft 24 positioned in the mould may be coated with an anti-oxidising agent that is also present in the rotor 35 (e.g. nickel).

Description

METHOD FOR PRODUCING A TURBINE SHAFT

CONNECTED TO A ROTOR

The present invention relates to a method for producing a turbine shaft connected to a rotor, particularly, but not exclusively, a method for producing a one-piece combined turbine shaft and turbine wheel for use in a variable geometry turbine, such as a variable geometry turbocharger.

Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises a housing in which is provided an exhaust gas driven turbine wheel mounted on a rotatable shaft connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems.

In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passage defined between facing radial walls arranged around the turbine chamber; an inlet arranged around the inlet passage; and an outlet passage extending from the turbine chamber. The passages and chambers communicate such that pressurised exhaust emissions, including gaseous and particulate species, admitted to the inlet chamber flows through the inlet passage to the outlet passage via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passage so as to deflect gas flowing through the inlet passage towards the direction of rotation of the turbine wheel.

Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passage can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suite varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passage. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers.

A turbine shaft and its associated rotor(s), i.e. turbine wheel and, optionally, compressor impellor, are currently formed as separate components which are subsequently connected together in one of a number of different ways. For example, turbine wheels are typically cast from a suitable corrosion resistant metal alloy, such as Inconele 713 or the like, and turbine shafts forged from mild (low carbon) steel. Turbine wheels are usually mass centred after casting with a centre defined in a nose end of the wheel to form the centre which is used for subsequent turning operations. The turbine shaft and turbine wheel are then connected together using, for example, friction welding where heat generated through mechanical friction between the two components fuses them together. Friction welding is generally used since it allows dissimilar materials (e.g. a nickel based alloy and mild steel) to be joined together to provide a strong bond between the two components without the bond adding additional weight to the assembled component. Friction welding also generally offers relatively quick joining times and typically gives rise to relatively small heat affected zones. That being said, it is usually necessary to heat-treat the assembled components to relieve any residual stresses in the joined components and, following heat treatment, the turbine shaft must then be machine finished and a further balance operation carried out, all of which add to the cost and complexity of the shaft/rotor joining process.

It will be appreciated that the extreme working conditions under which turbine rotors operate generally necessitates the use of relatively expensive, high performance materials, such as Inconel� 713, which is undesirable from a cost perspective. The complexity and time expended in relation to the various steps (e.g. welding and heat treatment), which are currently required in order to join turbine shafts to rotors, is also less than desirable.

It is an object of the present invention to obviate or mitigate one or more of the problems set out above.

According to a first aspect of the present invention there is provided a method for producing a turbine shaft connected to a rotor, the method comprised of: providing at least a portion of a turbine shaft in a mould shaped to define a rotor; introducing into said mould material from which the rotor is to be formed; and processing said material introduced into the mould so as to produce said rotor connected to said turbine shaft.

In this way, at least some of the disadvantages associated with prior art methods of connecting turbine shafts to rotors, such as turbine wheels, are avoided. By way of example, the method of the present invention avoids the cost and complexity of prior art methods which employ friction welding and heat treatment to mount a turbine wheel on a turbine shaft. As a result of using the method of the present invention, a portion of the turbine shaft now forms the core of the rotor. Consequently the amount of material required to form the rotor can be reduced, which lowers material costs and therefore improves the economic viability of forming the rotor from high performance, although costly, materials, such as castable or. powdered nickel-based superaHoys using processes, such as investment casting or metal injection moulding respectively.

According to a second aspect of the present invention there is provided a one-piece turbine shaft and rotor assembly comprising a turbine shaft with a rotor cast or metal injection moulded thereon. While the assembly may be produced using any desirable technique, it is preferred that it is produced using the method forming the first aspect of the present invention.

According to a third aspect of the present invention there is provided a turbine comprising a one-piece turbine shaft and rotor assembly according to the second aspect of the present invention.

When material is introduced into the mould, by virtue of at least a portion of the shaft being provided in the mould, some of the material is preferably brought into contact with the portion of the turbine shaft within the mould.

Depending upon the conditions (e.g. temperature) under which said contact occurs, the rotor material may immediately bind to the turbine shaft, or it may be necessary to initiate binding in some way. In a preferred embodiment of the first aspect of the present invention the material from which the rotor is to be formed is introduced into the mould at a first temperature and the step of processing the material set out in the first aspect of the present invention comprises changing the temperature of the material to a second temperature, which may be higher or lower than the first temperature. This change in temperature preferably initiates or supports binding of rotor material to the turbine shaft.

The material introduced into the mould may be in any physical form, but is preferably a fluent material. In this way, the fluent material can be conveniently introduced into the mould, for example by pouring and/or injection, and then subsequently processed to solidify the fluent material into a solid mass assuming the shape of the rotor defined by the mould.

The fluent material may be a liquid. In which case, the liquid may be introduced into the mould at a first temperature and said processing of the material may comprise cooling of the liquid to a second lower temperature to facilitate solidification of the material.

Alternatively, the fluent material may be a powder. The powder may be introduced into the mould at a first temperature and said processing of the material may comprise heating of the powder to a second higher temperature to facilitate binding or fusion of the powder particles. If the powdered fluent material introduced into the mould initially contained a binder or solvent it is preferred that said processing of the material further comprises removal of some or all of the binder or solvent that is present in the material.

It is preferred that processing of the material introduced into the mould may be effected at least partly in a non-oxidising atmosphere, such as an atmosphere comprised primarily of nitrogen or argon. This is desirable since it prevents or at least reduces the possibility of the surface of the shaft within the mould undergoing aerial oxidation, which might otherwise hinder binding of the rotor material to the shaft, particularly in a preferred embodiment in which the shaft is formed from a ferrous alloy, such as steel, that is particularly susceptible to aerial oxidation. Similar advantages may be achieved by providing the portion of the turbine shaft within the mould with an anti-oxidising coating. Such a coating may be used with or without processing the rotor material in a non-oxidising atmosphere. Said anti-oxidising coating may comprise one or more chemical elements which is/are also present in the material from which the rotor is formed. This may be desirable where that element of combination of elements is particularly resistant to aerial oxidation and/or if having that element or elements present improves the strength or ease of forming the join between the rotor and the shaft. In a preferred embodiment, said anti-oxidising coating may comprise nickel metal.

The portion of the turbine shaft provided in the mould is preferably the portion of the shaft upon which it is desired to support the rotor once formed. The portion of the rotor residing in the mould may therefore be any desirable portion of the turbine shaft, or may in fact be the entire length of the shaft if which was desirable in any particular application. In a preferred embodiment, the portion of the shaft provided in the mould is an end or terminal section of the turbine shaft, and which preferably extends partly (e.g. up to -10 %, up to -20 %, or possibly up to -50 %) along the axial length of the shaft.

The portion of the turbine shaft provided in the mould may define at least one radially and/or axially extending formation to contact the material which is to form the rotor. Said formation(s) may provide a mechanical interlock with the rotor when the rotor is formed on the shaft, which could resist torque and/or axial forces tending to separate the rotor from the shaft during use. The formation(s) may also add strength to the combined rotor/shaft part, and/or increase the strength of the chemical bond between the rotor material and the shaft material due to the increased interfacial surface area available for bonding.

The or each formation may comprise a plurality of axially spaced radially extending flanges. One or more of these flanges may define a substantially continuous circumferential edge, and/or a circumferential edge that is stepped or chamfered so as to define a series of straight edges interconnected at their ends, which could, for example, define a polygonal or nut-shaped' radial cross section. Additionally or alternatively, the or each formation may comprise a radially inwardly directed circumferential groove, which might extend partly or fully around the circumference of the shaft. One advantage of providing such a groove or grooves is to increase the mechanical strength of the combined shaft/rotor part.

It will be appreciated that it is generally preferable to form the rotor from one material or combination of materials, such as a high performance superalloy, due to the extreme operating conditions that this part will be required to accommodate, and the turbine shaft (or at least the portion of the shaft which will be imbedded within the rotor) from a different material or combination of materials, such as a relatively cheap steel, to save cost, which is possible because of the lower operational requirements placed on this part.

Preferably the material from which the rotor is to be formed comprises a corrosion resistant alloy. The rotor material may comprise an austenitic alloy, optionally in combination with one or more further materials or alloys. The rotor may be formed from a nickel-based alloy, such as an lnconel� alloy, for example, Inconel� 713 or the like.

As mentioned above, it is preferred that the turbine shaft is formed from a material which is relatively cheap for cost reasons, but the shaft material must still exhibit the appropriate properties to satisfy the operational requirements for that part. In a preferred example, the turbine shaft comprises a ferrous alloy, such as a mild or low carbon steel.

The strength of the connection between the rotor and the turbine shaft may be enhanced by careful selection of the material(s) from which the rotor and the turbine shaft is/are formed. Materials can be chosen which exhibit chemical and/or physical properties which, when combined at the interface between the connected components, provide a connection of increased strength and/or resistance to degradation over the operational lifetime of the turbine.

By way of example, the rotor and at least the portion of the shaft which will be imbedded within the rotor, may be formed from materials exhibiting different coefficients of thermal expansion such that their dimensions, e.g. radial cross sectional areas, change by different amounts in response to a particular change in operating temperature.

The rotor may be formed from a material which exhibits a first coefficient of thermal expansion and the shaft may be formed from a different material which exhibits a second coefficient of thermal expansion which may be larger or smaller than the first coefficient of thermal expansion.

In a preferred embodiment, the first coefficient of thermal expansion is larger than the second coefficient of thermal expansion. For example, the rotor may be formed from a nickel-based alloy and the turbine shaft may be formed from a mild steel which exhibits a lower coefficient of thermal expansion than the nickel-based alloy. In this way, when the shaft and rotor are connected together and subjected to the same temperature change, the cross sectional area of the rotor will increase to a greater extent than the turbine shaft, which will increase the compressive force of the rotor on the shaft and thereby increase the mechanical strength of the connection between the components.

The first aspect of the present invention define above employs moulding to form a rotor directly onto a turbine shaft. This is achieved by carrying out the method such that at least a portion of the shaft resides within the mould when the rotor material is introduced into the mould and then processing the shaft / rotor material / mould assembly to produce the combined part incorporating the rotor connected to the turbine shaft.

It is preferred that the mould is formed by coating a blank shaped to define the rotor with a suitable mould-forming material and then removing the blank from the formed mould. Any suitable blank may be used, for example, the blank may be suitably shaped or sculpted from wax or the like. Moreover, any desirable mould-forming material may be employed, such as a ceramic material.

The blank may be removed from the final mould using any appropriate technique. A suitable technique may be to heat the blank to a temperature above the solidus temperature of the material from which the blank is formed so that the blank material can flow out of the mould or be relatively easily ejected from the mould.

In a preferred embodiment of the first aspect of the present invention, the blank is connected to the portion of the turbine shaft upon which the rotor will eventually be formed. In this way, once the mould is formed and the blank removed, said portion of the turbine shaft is provided within the mould ready for introduction of the rotor material. A further consequence is that said portion of the turbine shaft will be exposed to the surrounding environment. It may therefore be advantageous to remove the blank under a non-oxidising atmosphere, such as a nitrogen or argon atmosphere, to avoid undesirable oxidation of the newly exposed surface(s) of the turbine shaft This may be particularly important where heating is applied to aid removal of the blank from the mould since the heating may also support or encourage oxidation of the exposed section(s) of the turbine shaft. Additionally or alternatively, the exposed portion of the turbine shaft may be provided with an anti-oxidising coating, such as nickel plating. This coating may be applied at any suitable time during the turbine shaft/rotor assembly manufacturing process prior to removal of the formed blank from the turbine shaft, but it is preferred that said anti-oxidising coating is applied prior to applying the blank to the turbine shaft.

In the preferred embodiment where the mould blank is provided on the turbine shaft prior to forming the mould, it may be advantageous to position the blank as accurately as possible with respect to the turbine shaft since this will determine the accuracy with which the rotor is formed on the turbine shaft.

It is envisaged that it may be relatively easy to accurately and reproducibly position the shaft relative to the blank, which will therefore reduce the need for subsequent mass centring operations. That being the case, it is preferred that the method of the present invention further comprises approximately mass centring the blank with respect to the turbine shaft. Any suitable machine may be employed for this operation but it is preferred that mass centring is carried out using a suitably controlled jig.

The above description of the method of the present invention mentions only a single mould / turbine shaft assembly, but it should be appreciated that any desirable number of moulds and dedicated shafts may be connected together to improve the efficiency of the moulding process. In a preferred embodiment the method further comprises connecting a plurality of said moulds together, for example in a conventional tree-arrangement, prior to introducing rotor material into the moulds.

It should be appreciated that while the foregoing discussion refers to the connection of a rotor to a turbine shaft, the rotor may be a turbine wheel or a compressor wheel, and that any desirable number of moulds may be associated with a single turbine shaft. That is, it may be preferably in some applications to employ moulding techniques falling within the scope of the present invention to produce a turbine shaft with two, or possibly even more, rotors connected to the shaft. For example, techniques according to the present invention can be used to form both the turbine wheel and compressor impeller on the turbine shaft of a turbocharger.

Other advantageous and preferred features of the invention will be apparent

from the following description.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an axial cross-section through a known variable geometry turbocharger; Figure 2 is a schematic perspective view of an embodiment of a turbine shaft which can be used in the method according to the first aspect of the present invention; and Figure 3 is a schematic cross-section view of the turbine shaft of Figure 2 connected to a turbine wheel where the combined shaft/rotor component has been produced according to the method according to the first aspect of the present invention.

Referring to figure 1, this illustrates a known variable geometry turbocharger comprising a housing comprised of a variable geometry turbine housing I and a compressor housing 2 interconnected by a central bearing housing 3. A turbocharger shaft 4 extends from the turbine housing I to the compressor housing 2 through the bearing housing 3. A turbine wheel 5 is mounted on one end of the shaft 4 for rotation within the turbine housing 1, and a compressor wheel 6 is mounted on the other end of the shaft 4 for rotation within the compressor housing 2. The shaft 4 rotates about turbocharger axis 4a on bearing assemblies located in the bearing housing 3.

The turbine housing 1 defines an inlet volute 7 to which gas from an internal combustion engine (not shown) is delivered. The exhaust gas flows from the inlet volute 7 to an axial outlet passage 8 via an annular inlet passage 9 and the turbine wheel 5. The inlet passage 9 is defined on one side by a face 10 of a radial wall of a movable annular wall member 11, commonly referred to as a "nozzle ring", and on the opposite side by an annular shroud 12 which forms the wall of the inlet passage 9 facing the nozzle ring 11. The shroud 12 covers the opening of an annular recess 13 in the turbine housing 1.

The nozzle ring 11 supports an array of circumferentially and equally spaced inlet vanes 14 each of which extends across the inlet passage 9. The vanes 14 are orientated to deflect gas flowing through the inlet passage 9 towards the direction of rotation of the turbine wheel 5. When the nozzle ring II is proximate to the annular shroud 12, the vanes 14 project through suitably configured slots in the shroud 12, into the recess 13.

The position of the nozzle ring 11 is controlled by an actuator assembly of the type disclosed in US 5,868,552. An actuator (not shown) is operable to adjust the position of the nozzle ring 11 via an actuator output shaft (not shown), which is linked to a yoke 15. The yoke 15 in turn engages axially extending actuating rods 16 that support the nozzle ring 11. Accordingly, by appropriate control of the actuator (which may for instance be pneumatic or electric), the axial position of the rods 16 and thus of the nozzle ring 11 can be controlled.

The speed of the turbine wheel 5 is dependent upon the velocity of the gas passing through the annular inlet passage 9. For a fixed rate of mass of gas flowing into the inlet passage 9, the gas velocity is a function of the width of the inlet passage 9, the width being adjustable by controlling the axial position of the nozzle ring 11. Figure 1 shows the annular inlet passage 9 fully open.

The inlet passage 9 may be closed to a minimum by moving the face 10 of the nozzle ring 11 towards the shroud 12.

The nozzle ring 11 has axially extending radially inner and outer annular flanges 17 and 18 that extend into an annular cavity 19 provided in the turbine housing 1. Inner and outer sealing rings 20 and 21 are provided to seal the nozzle ring 11 with respect to inner and outer annular surfaces of the annular cavity 19 respectively, whilst allowing the nozzle ring 11 to slide within the annular cavity 19. The inner sealing ring 20 is supported within an annular groove formed in the radially inner annular surface of the cavity 19 and bears against the inner annular flange 17 of the nozzle ring 11. The outer sealing ring 20 is supported within an annular groove formed in the radially outer annular surface of the cavity 19 and bears against the outer annular flange 18 of the nozzle ring 11.

Gas flowing from the inlet volute 7 to the outlet passage 8 passes over the turbine wheel 5 and as a result torque is applied to the shaft 4 to drive the compressor wheel 6. Rotation of the compressor wheel 6 within the compressor housing 2 pressurises ambient air present in an air inlet 22 and delivers the pressurised air to an air outlet volute 23 from which it is fed to an internal combustion engine (not shown).

The above-described turbocharger may be modified for certain applications by the provision of axially extending balancing holes (not shown) in the radial wall of the nozzle ring 11 to balance the pressure within the nozzle ring cavity 19 with the pressure applied to the nozzle ring face 10 by gas flow through the inlet passage. The turbocharger may also include radially extending holes (not shown) in the axially extending flanges 17, 18 of the nozzle ring 11 to provide a bypass path for exhaust gas to flow through the nozzle ring cavity 19, bypassing the inlet passage, as the nozzle ring 11 nears the fully closed position and thereby prevent excessive pressures building up in the engine cylinders and avoid excessive heat generation during engine braking.

Figure 2 shows a first embodiment of a turbine shaft 24 which can be connected to a rotor (not shown in Figure 2) using the method of the present invention. The turbine shaft 24 is similar to shaft 4 described above in relation to Figure 1, but the shaft 24 of Figure 2 is provided at its domed turbine end with a radially outwardly extending flange 26 which is provided with a plurality of flat surfaces 27 so as to provide the flange 26 with polygonal or nut-like radial cross-section. The shaft 24 defines a further radially extending flange 28 of substantially circular radial cross-section which is axially spaced with respect to the other flange 26 so as to define a radially inwardly directed groove 29 between the two flanges 26, 28. The purpose of the flanges 26, 28 and groove 29 will be explained in more detail below in relation to Figure 3.

The remainder of the structure of the shaft 24 is essentially conventional but briefly comprises a region 30 which is located axially adjacent to the circular flange 28 and which defines a circumferential groove 31 for receipt of a split 0-ring (not shown). The shaft 24 further comprises a wasted region 32 which extends into an intermediate region 33 of slightly larger radius, which then extends into a terminal region 34 opposite to the turbine end 25 of the shaft which is of slightly smaller radius than the intermediate region 33. The shaft 24 has been forged out of mild steel using entirely conventional processing techniques. It will however be appreciated that the shaft 24 may be formed from any desirable material to suit a particular application.

Referring now to Figure 3, the shaft 24 is shown connected to a turbine.wheel of generally similar exterior profile to the turbine wheel 5 described above in relation to Figure 1. As can be seen from Figure 3, the turbine wheel 35 has been connected to the turbine shaft 24 such that the entire turbine end 25 of the shaft 24 is embedded within the turbine wheel 35. The pair of flanges 26, 28 extending radially from the turbine shaft 24 are also embedded within the turbine wheel 35 so as to provide a mechanical interlock between the turbine shaft 24 and the material from which the turbine wheel 35 has been formed. The way in which the combined turbine shaft 24 and turbine wheel 35 assembly of Figure 3 has been produced will now be described in more detail.

In a preferred embodiment of the method forming the first aspect of the present invention the shaft 24 is forged using conventional techniques to have the structure shown and described above in relation to Figure 2. The domed turbine end 25 and flanges 26, 28 define the portion of the shaft 24 which is to be received within the structure of the turbine wheel 35. This portion of the shaft 24 is initially coated with a corrosion and/or oxidation resistant coating, such as nickel plating for reasons that will become evident below. The shaft 24 is then positioned on a jig (not shown) and a wax core or blank (not shown) formed on the aforementioned portion of the shaft 24 such that the wax core assumes the shape and position of the eventual turbine wheel 35.

Since the shaft 24 can be accurately positioned on the jig and the wax core centred on the shaft 24, the need for final balancing of the combined shaft 24 and turbine wheel 35 component is reduced.

A ceramic shell (not shown) is then formed around the shaft and the wax core using conventional techniques. A plurality of shells may be combined in a typically fashion to form what is often referred to as a "tree arrangement" whereby a common flow path for the mouldable material is defined which extends through the plurality of moulds and thereby improves the efficiency and consistency of the moulding process. The ceramic shell associated with the or each shaft 24 and wax core is then heated to a sufficient temperature to at least partially liquefy the wax core so that it then flows out of the shell thereby defining a mould around the shaft 24 which mirrors the shape of the eventual turbine wheel 35. The heating process is carried out in an inert, non-oxidising atmosphere to limit the affects of oxygen on the portion of the shaft 24 within the mould. Oxidation and carbonisation of this portion of the shaft 24 is further avoided as a result of the protective coating of nickel plating already applied to the portion of the shaft 24 within the mould, described previously.

An appropriate material, such as a nickel based superalloy is then introduced into the or each mould, for example via a port (not shown) defined by the mould adjacent to the turbine end 25 of the shaft 24.

In a first embodiment of the method according to the present invention, the nickel based superalloy is liquid, castable, Inconel� 713 which, after Introduction into the mould is then allowed to cool and solidify in the usual way so as to assume the shape of the turbine wheel 35 defined by the mould.

As the liquid Inconel� 713 contacts the portion of the shaft 24 within the mould, the nickel alloy forms a chemical bond with the steel of the shaft 24 and, as the Inconel� 713 cools and solidifies, the solid structure of the turbine wheel 35 forms a mechanical interlock with the flanges 26, 28 of the shaft 24.

Once a sufficient period of time has elapsed and the nickel alloy has adequately solidified, the mould is then removed in the usual way and the one-piece shaft 24 and wheel 35 component then subjected to any final balancing operation that may be required to form the finished article.

In a second embodiment, the nickel based superalloy introduced into the mould is powdered lnconel� 713 which is suitable for metal injection moulding ("MIM"). In this embodiment, the powdered Inconel� 713 with its associated binders and/or solvents is first injected into the mould to assume the general shape of the eventual wheel 35. Any binders and/or solvents are then removed in the usual way. The powdered lnconel 713 material within the mould is heated so as to cause the powder particles to fuse and bind together to form a substantially solid mass which is then cooled to form the wheel 35.

Once one or more of such sintering cycles have been completed, the one-piece shaft 24 and wheel 35 component is then released from the mould in the usual way. Additional balancing operations and/or machining may be required in order to form the finished article.

The first and second embodiments of the present invention described above employed casting and metal injection moulding respectively to form the turbine wheel 35 on the turbine shaft 24. Both embodiments offer significant advantages over prior art methods of joining separate turbine shafts and rotors, and both produce final articles (i.e. rotors supported on turbine shafts) which offer performance benefits compared to corresponding articles produced using prior art methods. By way of example, the portion of the steel shaft 24 embedded within the turbine wheel 35 significantly reduces the amount of material required to form the wheel 35. This significantly reduces the raw material costs for each wheel and may enable yet more expensive, higher performance, materials to be considered for use in turbine rotors than would otherwise be the case. It should be noted that as the size of the rotor increases, the relative cost saving is also likely to increase.

Moreover, forming the shaft 24 and wheel 35 in one piece avoids the need for the welding and heat treatment processes previously required to join these two components together, which will provide both cost and time benefits.

Additionally, by performing the mass centring process on the shaft/wax core assembly rather than the rotor itself, overall balancing may be improved and the final balancing operation either may not be needed, or may not need to remove as much rotor material as in prior art methods. Consequently, it may be possible to use less material in the nose of the wheel 35 which would contribute to further raw material cost savings.

In addition to the above advantages, the second preferred embodiment employing metal injection moulding may offer further benefits even though the cost of suitable powdered materials is currently higher than the corresponding castable material. This is because metal injection moulding has a much higher degree of dimensional accuracy than casting techniques, such as investment casting. It is therefore possible to provide a turbine shaft within the rotor mould that is almost machine-finished, which significantly reduces final shaft/rotor balancing requirements. Moreover, since the heating required to bind the powdered alloy is relatively localised and applied over only a relatively short period of time, the exposed surface of the portion of the shaft within the mould is less likely to suffer from aerial oxidation. Satisfactory results may therefore be achieved without applying nickel plating to the turbine shaft and/or carrying out the sintering process in an inert atmosphere.

By using metal injection moulding technology the dimensional accuracy of the blades formed on the rotor can be improved which should therefore provide benefits in terms of balancing. It may also be possible to form thinner blades if this was desirable in a particular application.

Claims

CLAIMS1. A method for producing a turbine shaft connected to a rotor, the method comprised of: providing at least a portion of a turbine shaft in a mould shaped to define a rotor; introducing into said mould material from which the rotor is to be formed; and processing said material introduced into the mould so as to produce said rotor connected to said turbine shaft.
2. A method according to claim 1, wherein said material is introduced into the mould at a first temperature and said processing of the material comprises changing the temperature of the material to a second temperature.
3. A method according to claim 1 or 2, wherein the material introduced into the mould is a fluent material.
4. A method according to claim 3, wherein the fluent material is a liquid.
5. A method according to claim 4, wherein the liquid is introduced into the mould at a first temperature and said processing of the material comprises cooling of the liquid to a second lower temperature to facilitate solidification of the material.
6. A method according to claim 3, wherein the fluent material is a powder.
7. A method according to claim 6, wherein the powder is introduced into the mould at a first temperature and said processing of the material comprises heating of the powder to a second higher temperature to facilitate binding of the powder particles.
8. A method according to claim 7, wherein said processing further comprises removal of any binder present in the material introduced into the mould.
9. A method according to any preceding claim, wherein processing of the material introduced into the mould is effected at least partly in a non-oxidising atmosphere.
10.A method according to any preceding claim, wherein said portion of the turbine shaft is provided with an anti-oxidising coating.
11.A method according to claim 10, wherein said anti-oxidising coating comprises a chemical element which is also present in the material from which the rotor is formed.
12.A method according to claim 10 or 11, wherein said anti-oxidising coating comprises nickel metal.
13.A method according to any preceding claim, wherein the portion of the turbine shaft provided in the mould is an end section of the turbine shaft.
14.A method according to any preceding claim, wherein said portion of the turbine shaft is provided with at least one radially and/or axially extending formation to contact the material which is to form the rotor.
15.A method according to claim 14, wherein said formation comprises a plurality of axially spaced radially extending flanges.
16.A method according to claim 14 or 15, wherein said formation comprises a radially inwardly directed circumferential groove.
17.A method according to any preceding claim, wherein said mould is formed by coating a blank shaped to define the rotor with a suitable mould-forming material and then removing the blank from the formed mould.
18.A method according to claim 17, wherein the blank is removed by heating the blank to a temperature above the solidus temperature of the materiai from which the blank is formed.
19.A method according to claim 17 or 18, wherein the blank is removed under a non-oxidising atmosphere.
20.A method according to claim 17, 18 or 19, wherein said mould-forming material comprises a ceramic material.
21.A method according to any one of claims 17 to 20, wherein said blank is formed from wax or the like.
22.A method according to any one of claims 17 to 21, wherein said blank is connected to said portion of the turbine shaft
23.A method according to claim 22, wherein said blank is approximately mass centred with respect to said turbine shaft.
24.A method according to claim 23, wherein said mass centring is carried out using a jig.
25.A method according to claim 22, 23 or 24, wherein said portion of the turbine shaft is provided with an anti-oxidising coating prior to connecting the shaft to the blank.
26.A method according to any preceding claim, wherein the method further comprises connecting a plurality of said moulds together prior to the introduction into the moulds of the material from which the rotor is to be formed.
27.A method according to any preceding claim, wherein the turbine shaft comprises a ferrous alloy.
28.A method according to claim 27, wherein the ferrous alloy is mild or low carbon steel.
29.A method according to any preceding claim, wherein the rotor is a turbine wheel or a compressor wheel.
30.A method according to any preceding claim, wherein the material from which the rotor is to be formed comprises a corrosion resistant alloy.
31.A method according to claim 30, wherein said alloy is an austenitic alloy.
32.A method according to claim 30 or 31, wherein said alloy is a nickel-based alloy.
33.A one-piece turbine shaft and rotor assembly comprising a turbine shaft with a rotor cast or metal injection moulded thereon.
34.A one-piece assembly according to claim 33, wherein said assembly has been produced using a method according to any one of claims I to 32.
35.A turbine comprising a one-piece turbine shaft and rotor assembly according to claim 33 or 34.
36.A method for connecting a shaft to a rotor substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.
37.A one-piece turbine shaft and rotor assembly substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.