GB2498361A - Silicon carbide reinforced aluminium alloy turbocharger impeller - Google Patents

Abstract

A turbocharger impeller 1, for connection to a shaft 2, is formed from a metal matrix composite material having an aluminium alloy matrix and silicon carbide particulate reinforcement. The average particle diameter of the particulate reinforcement may be 2 microns or less. The particulate reinforcement may be from 10% to 30% of the composite by volume. The impeller may be fitted with a connector 3. The connector may frictionally engage an inner or outer surface of a shaft-side hub extension H of the impeller. The connector may have a threaded portion 12 which screws onto a corresponding threaded portion 7 of the shaft. The material of the connector may have a coefficient of thermal expansion which is equal to or greater than that of the composite material of the impeller. The material of the connector may have a greater strength than the composite material of the impeller.

Description

I

TURBOCHARGER IMPELLER

Field of the Invention

The present invention relates to an impeller of a turbocharger for connection to a turbocharger shaft.

Backrcund of the Invention Modern trends are to require higher pressure ratios from turbochargers on diesel engines.

To achieve these pressure ratios, generally the turbocharger impeller is required to rotate at an increased speed. This increased speed increases the centrifugal stresses on the impeller material and increases the temperature of the impeller material.

Turbocharger impellers are typically made of aluminium alloys to provide low rotational inertia with reasonable strength at a commercially-acceptable cost. For high speed applications, forged aluminium alloy 261 8A billets can be machined into a tightly toleranced shape using 5-axis milling machines. Compared with alternative processes, such as casting, the forged material has higher fatigue and strength properties. By 5-axis milling, the shapes of the impeller vanes can be accurately controlled and made thinner, which improves efficiencies at the high Mach number flows resulting from higher rotor speeds Alternative materials such as titanium alloys, have been used for high speed and high temperature applications. Unfortunately the density of titanium is twice that of aluminium, which results in turbo-lag and poor engine acceleration capability. The cost of titanium alloy billets is also many times that of aluminium alloy billets, and machining of titanium alloys is more costly both in time and tooling.

With higher pressure ratios, the time taken for the turbocharger to accelerate to full operating conditions is longer. Increased turbo lag makes this worse. "Jet assist" acceleration techniques have thus been developed, in which jets of high pressure air are used to blow onto the impeller vanes during start-up. This air accelerates the turbocharger rotor, but also introduces high levels of vibrating stresses on the impeller vanes. Such stresses can cause failure of the impeller in high cycle fatigue.

With high speed turbochargers, connecting the typically steel shaft to the impeller becomes problematic as a high torque has to be transmitted across the joint. Because of the relative weakness of aluminium alloys and the small diameter of the shaft, one option is to provide the impeller with a steel insert containing a screw-threaded socket which can be threaded on to the shaft. This arrangement can take a higher torque than a connection in which the shaft is directly threaded into the aluminium body.

Typically1 such an insert is fitted into the impeller by shrink fitting; the aluminium body of the impeller is heated to expand the bore which is to receive the steel insert, while the insert is cooled, for example using liquid nitrogen, before being inserted into the bore. The resultant interference connection is restricted by the temperature to which the aluminium can be heated before its material properties are affected, and by the temperature to which the steel can be cooled.

While the arrangement described can perform satisfactorily, a problem can arise during cycling of the turbocharger from rest to full load. As the turbocharger starts to spin, the joint is affected by centrifugal forces, whereby the aluminium grows outwards away from the steel insert. This reduces the interference force between the insert and the impeller, and due to design constraints it has been found that this reduction tends to be greater at one end of the insert than at the other. Consequently, the insert is gripped more firmly at one of its ends than at the other. The turbocharger then starts to heat up, and because of the different thermal coefficients of expansion of the aluminium alloy and the steel, the aluminium grows axially more than the steel, causing the two metals to slide over each other, except at the location where the impeller still grips the insert firmly. On shutdown, the centrifugal stresses are removed, but the thermal stresses remain for some minutes as the turbocharger cools.

In this process, the point of grip of the impeller on the insert changes from one end to the other, and as the turbocharger cools, the insert "walks' along the impeller.

In certain very cyclic conditions (for example fast ferry applications in high ambient temperatures), it has been observed that the insert can move so far along the impeller that turbocharger failure can occur. Although the offect can be mitigated to some degree by increasing the original interference between the components, for the reasons mentioned above this solution is limited, and it is therefore desirable to achieve a design which ensures that the point of grip remains at the same location during the operating cycles, rather than shifting from one end of the insert to the other.

Accordingly, EP1394387 proposes an outer steel constraining ring which reinforces the frictional contact between aluminium impeller and the insert. Since the ring does not expand as much as the impeller body as the turbocharger heats up, the point of grip between the impeller and the insert remains within the axial extent of the ring during the whole operating cycle of the turbocharger, thereby preventing the tendency of the impeller to "walk" along the insert. As a consequence, the operating life of the turbocharger can be considerably extended in comparison with the conventional turbocharger without the constraining ring.

However, the assembly of such a joint is relatively complex. First the insert and impeller bore are manufactured to tight tolerances. Then typically the insert is cooled and the impeller heated, and the insert is placed within the impeller bore at a hub extension of the impeller. As the insert warms up and the impeller cools, a shrink fit joint is formed, but because of the non-axisymmetrical shape of the impeller, some distortion occurs within the impeller. Generally, the outer surface of the impeller hub extension must therefore be reground to be axisymmetric so that it will be suitable for the outer joint with the constraining ring. A further ring may then be shrunk onto a flange portion of the insert to prevent the constraining ring from coming off the impeller.

Summary of the Invention

It would be desirable to provide an impeller which has improved capabilities at high pressure ratios and which can be accelerated rapidly to high speeds. Further, it would be desirable if the impeller was simpler to install on a shaft, while retaining a capability to transmit high torques and preventing or reducing any tendency of the impeller to "walk".

Accordingly, in a first aspect the present invention provides an impeller of a turbocharger for connection to a turbocharger shaft, wherein the impeller is formed from a metal matrix composite having an aluminium alloy matrix and silicon carbide particulate reinforcement.

By forming the impeller from such a metal matrix composite, significant material properties of the impeller can be improved. For example, the impeller can be made stiffer, which allows thinner vanes to be machined into the impeller since the natural frequencies of these vanes will be comparatively high. The thinner vanes are then more suitable for high Mach number flows. Also, the strength of the impeller can be increased, it can be made more resistant to ageing at high temperatures, and it can have improved fatigue resistance. In addition, the coefficient of thermal expansion of the impeller can be decreased, thereby reducing the differential thermal forces which encourage the impeller to "walk". This in turn allows a simpler joint between the impeller and the shaft to be adopted.

A second aspect of the invention provides the impeller of the first aspect fitted with a connector for connecting the impeller to the shaft, the connector being frictionally connected to the impeller and having a threaded portion carrying a thread which screws onto a corresponding threaded portion of the shaft.

A third aspect of the invention provides an impeller according to the first aspect which is connected to a turbocharger shaft.

A fourth aspect of the invention provides an impeller fitted with a connector according to the second aspect, which impeller is connected to a turbocharger shaft having a corresponding threaded section, the thread of the threaded portion of the connector screwing onto the corresponding threaded portion of the shaft.

A fifth aspect of the invention provides a turbocharger having the connected impeller and shaft of the third or fourth aspect.

Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.

The volume average particle diameter of the particulate reinforcement may be 2 microns or less. A small particulate size improves the machinability of the metal matrix composite, e.g. allowing the composite to be machined using techniques similar to those used for non-reinforced aluminium alloys.

The particulate reinforcement can be from 10 to 30% of the metal matrix composite by volume, depending on the material properties required.

The aluminium alloy of the matrix may be, for example, a 2000 series alloy such as AA2124 or a 6000 series alloy such as AA6061. The turbocharger shaft is typically formed of steel (e.g. a high strength steel).

The impeller can be fitted to the shaft in a variety of ways. For example, due to the relatively high strength and low coefficient of thermal expansion of the composite, it may be possible to directly thread the shaft to the impeller at respective threaded portions, e.g. with a helicoil formation fitted to the impeller to protect the thread on the impeller, such that a rotationally fixed connection is provided between the impeller and the shaft.

However, preferably, the impeller is fitted with a connector for connecting the impeller to the shaft, the connector being frictionally connected to the impeller and having a threaded portion carrying a thread which screws onto a corresponding threaded portion of the shaft such that the connector provides a rotationally fixed connection between the impeller and the shaft. In this case, the impeller may have a shaft-side hub extension and a surface of the connector can then be frictionally connected to a surface of the hub extension. The frictional connection between the surface of the connector and the surface of the hub extension may transmit, in use, substantially all of the torque between the shaft and the impeller. The connector may be formed of a material having a greater strength than the metal matrix composite of the impeller. The connector may have a sleeve portion which is frictionally connected on a radially outer surface of the hub extension (the connector, which thus has the threaded portion and the sleeve portion, typically being formed as a unitary body). If the coefficient of thermal expansion of the composite is greater than that of the (e.g. steel) material of the connector, during operation, as the impeller assembly heats up, the hub extension grows to a greater degree than the sleeve portion, whereby the joint between the hub extension and the sleeve advantageously tightens, reducing any tendency of the impeller to "walk" and increasing the torque capacity of the joint. However, another option is for the shaft-side hub extension to have a central recess and the connector to be inserted into the recess to frictionally connect an outwardly facing surface of the connector with a radially inner surface of the hub extension. In such an arrangement, due to the relatively low coefficient of thermal expansion of the composite, which can reduce the differential thermal forces acting across the frictional connection, an outer constraining ring, of the type proposed in EP1394387, may not be needed. Advantageously, the threaded portion of the connector can be within the central recess, which helps to achieve an axially compact arrangement.

More generally, however, even if the frictional connection is on the radially outer surface of the hub extension, the hub extension may still have a central recess, and the threaded portion of the connector may be within the recess, for example as a part of the connector which is inserted into the recess (although a clearance may be provided between the inserted part of the connector and the side surface of the recess).

The central recess may be a blind hole (i.e. with an end surface). Thus, the impeller may not have a through-hole extending from one side to another of the impeller.

The surface of the connector which frictionally connects to the shaft-side hub extension of the impeller may be approximately cylindrically shaped. The corresponding surface of the hub extension may be similarly approximately cylindrical.

The frictional connection between the connector and the hub extension can be achieved by e.g. press fitting or shrink fitting.

To provide the rotationally fixed connection, the threads can be positive-locking, e.g. tapered.

However, another option is for the impeller or the connector, as the case may be, to have an abutment surface which engages a corresponding abutment surface of the shaft when the thread portions are screwed together, thereby tightening the threads to provide the rotationally fixed connection.

The metal matrix composite may have a coefficient of thermal expansion in the range of from 14 to 1 7x1 06/K (compared with about 22.7x1 051K which is usual for aluminium alloy). The typically steel shaft may have a coefficient of thermal expansion of about 11 xl 061K. When the impeller is fitted with a connector, the connector may likewise be formed of steel (e.g. high tensile steel such as EN26, or medium carbon steel such as EN8), and have a similar coefficient of thermal expansion to that of the shaft. However, the coefficient of thermal expansion of the material of the connector may be greater than that of the shaft. Indeed, particularly when the connector is inserted into a central recess to frictionally connect an outwardly facing surface of the connector with a radially inner surface of the hub extension, the coefficient of thermal expansion of the material of the connector may advantageously be equal to or greater than the coefficient of thermal expansion of the metal matrbç composite.

In this way, during operation, as the impeller assembly heats up, the connector grows to the same or a greater degree than the hub extension, reducing any tendency of the impeller to "walk", and, in the case of a greater coefficient for the connector, tightening the joint to increase its torque capacity. Further a relatively high coefficient of thermal expansion of the connector can assist with the production of a shrink filled frictional connection. For example, the connector can be formed of magnesium alloy, bronze, brass or stainless steel. Such alloys should preferably also be resistant to galling with the steel of the shaft.

The connector and!or the impeller may have one or more centring portions having respective engagement surfaces which engage with one or more corresponding centring portions of the shaft, the threaded portion of the impeller or the connector, as the case may be and the centring portions of the connector and/or the impeller being distributed along the impeller axis. The thread surface of the impeller or the connector and the engagement surfaces of the connector andIor the impeller can face radially inwardly, and the respective diameters on the shaft of the thread and the engagement surfaces can then decrease towards the impeller.

Generally the impeller has a casing, and the connector and/or the hub extension can then form a seal with a section of the casing. For example, the seal can include a sealing ring, which may be carried by the casing section and which may be received by a corresponding circumferential recess formed on an outer surface of the connector and/or the hub extension.

The sealing ring may have one or more annular grooves on its radially inner face, and the io recess may have corresponding circumferential ribs which are received in the grooves.

Another option is for the seal to include a labyrinth seal, with formations on facing surfaces of the casing section and the connector andfor the hub extension forming the labyrinth.

The connector may be formed with or may carry a circumferential oil thrower formation at its radially outer surface.

Further optional features of the invention are set out below.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a sectional elevation through a turbocharger impeller joined to a shaft by a connector in accordance with an embodiment of the invention; Figure 2 is a close-up schematic view of a seal between a section of a casing of the impeller of Figure 1 and a hub extension of the impeller; Figure 3 is a close-up schematic view of a seal between a section of a casing of an impeller and a sleeve portion of a further embodiment of the connector; Figure 4 shows schematically a sectional elevation of a further embodiment of the impeller; and Figure 5 is a sectional elevation of a further embodiment of the impeller.

B

Detailed Description and Further Optional Features of the Invention Referring first to Figure 1, an impeller 1 is fitted on to a steel turbocharger shaft 2 by means of a connector 3. The impeller is made from a metal matrix composite having an aluminium alloy matrix and silicon carbide particulate reinforcement. For example, the composite can be an AMC Xfine composite based on an AA2124 orAA6O6l aluminium alloy system reinforced with up to 30% by volume of silicon carbide particulate, and available from Aerospace Metal Composites of Farnborough, UK. The volume average particle diameter of the particulate reinforcement may be 2 microns or less to improve the machinability of the composite.

The alloy of the impeller 1 has a coefficient of thermal expansion of 14 to 1 7x1 06/K, and the steel of the shaft 2 has a coefficient of thermal expansion of about 11 xl 06/K. The material of the connector 3 can be medium carbon steel such as EN8, which also has a coefficient of thermal expansion of about 1 1x10/K. In comparison with an impeller 1 formed of conventional unreinforced aluminium alloy (coefficient of thermal expansion of about 22.7x1O6IK), the differential thermal forces acting between the impeller and the connector are thus substantially reduced, reducing the tendency of the impeller ito "walk" off the connector 3 than in the state of the art and allowing a less tight interference fit to be adopted.

In particular, an outer constraining ring, of the type proposed in EP1 394387, may not be needed. However, alternatively, to assist with shrink fitting of the connector 3 to the impeller 1, and to further improve the "walk" resistance and torque capacity, the connector 3 can be formed of a material having a higher coefficient of thermal expansion, such as magnesium alloy (coefficient of thermal expansion of about 26x1 061K), bronze (coefficient of thermal expansion typically of about 1 8x1 05/K, although as high as 20-21 xl O/K for manganese-bronze), brass (coefficient of thermal expansion of about 1 8.7x1 0/K) or stainless steel (coefficient of thermal expansion of in the range of 16-17.3x10/K). Such alloys can also be resistant to galling with the steel of the shaft 2.

The silicon carbide particles increase the stiffness of the aluminium alloy by around 60%.

This allows thinner vanes to be machined into the impeller 1 since the natural frequencies of the vanes is comparatively high. The thinner vanes are more suitable for high Mach number flows and therefore the impeller 1 tends to have a higher efficiency than impellers formed from conventional unreinforced aluminium alloy. In addition, the strength of the composite material is increased by around 20% by introduction of the particulate reinforcement. This allows the impeller ito be run at higher rotational speeds. Further, the composite is more resistant to ageing at high temperatures than conventional alloy, whereby the life of the impeller 1 can be prolonged. The fatigue life of the impeller can also be increased, which can be particularly beneficial when "jet-assist" acceleration techniques are used.

The connector 3 is of cup-like shape and has an outer surface 14 which connects to the impeller 1, a threaded portion 12 with a threaded bore 11 forming the base of the cup, and a flange portion B around the mouth of the cup.

The shaft 2 is formed at its end with a first shoulder 4 surrounding a cylindrical centring portion 5, and a screw-threaded portion 7 of further reduced diameter extending from the end of the centring portion. The connector 3 is inserted into a blind central recess formed in the hub extension H, with the outer surface 14 of the connector 3 frictionally connected to the radially inner surface of the hub extension H. The flange portion 8 of the connector 3 engages against a shaft-side end face 9 of the hub extension H to determine the relative axial positions of the connector 3 and the hub extension H. The flange portion 8 is engaged on its other side by the shoulder 4 on the shaft 2 The centring portion 5 of the shaft is received in a corresponding centring portion 10 of the connector in a close, but not tight, fit.

The threaded bore 11 engages on the screw-threaded portion 7 of the shaft. The threaded portion 12 has a small clearance from the end of the recess.

The connector 3 is fitted on to the hub extension H by cooling the connector 3 to cause it to shrink and by heating the impeller to cause the hub extension H to expand, and then inserting the connector 3 into the central recess of the hub extension H until the flange portion 8 contacts the end face 9 of the hub extension H. On returning from their thermal excursions, the connector 3 and hub extension H frictionally grip across the outer surface 14 of the connector 3 and the radially inner surface of the hub extension H. The outer surface 14 extends over and thereby frictionally contacts most of the axial length of the hub extension H. The outer diameter of the flange portion 8 is provided with an oil capturelthrower ring R, which in this embodiment of the invention is machined into the flange portion 8. Another option, however, is to form the ring R as a separate component.

As shown better in Figure 2, a section 15 of the impeller casing and the outer surface of the hub extension H are in close proximity to help provide a rotating oil and pressure seal between the impeller 1 and the casing. To improve the seal, the hub extension H has a recess 13 on its outer surface which is bounded at one end by the flange portion 8 of the first component of the connector and which receives a sealing ring 16 carried by the casing section 15. To reduce wear between the sealing ring 16 and the hub extension H, the casing section 15 has a small abutment surface 20 on the shaft side (right hand in Figure 1) of the seal ring 16 and against which the sealing ring 16 rests. To provide enhanced sealing, the sealing ring 16 has annular grooves 18 on its radially inner face, and the recess has corresponding circumferential ribs 17 which are received in the grooves, as described in EP A 1130220. Alternatively, however, the sealing ring can be a plain ring (i.e. without grooves) received in a plain recess (i.e. without ribs). The sealing ring 16 co-operates with the casing section 15 and serves to retain lubricating oil to the shaft side of the assembly and compressed air to the impeller side of the assembly (left hand in Figure 1). The compressed air is contained between the body of the impeller 1, the hub extension H with its sealing ring 16, and the impeller casing, within which the impeller assembly is mounted for rotation on overhung bearings (not shown).

After the connector 3 is fitted on to the hub extension H, the screw-threaded portion 7 of the shaft 2 is screwed onto the threaded portion 12 of the connector 3, the respective centring portions 5, 10 ensuring the shaft aligns with the axis of the impeller. The threads are screwed until opposing surfaces of the flange portion 8 and shoulder 4 come into abutment, which causes the threads to tighten and provides a rotationally fixed connection between the impeller 1 and the shaft 2.

By containing the threaded connection between the connector 3 and the shaft 2 in the central recess of the hub extension H, an axially compact arrangement is achieved. The frictional connection between the connector 3 and the impeller transmits, in use, substantially all of the torque between the shaft 2 and the impeller 1. As there is no need to fit a constraining ring of the type described in EP1 394387 to the hub extension H, regrinding operations can be avoided during fitting of the connector 3.

If there is any tendency for the impeller ito "walk", advantageously this can be monitored by measuring the size of the gap that would open up between the flange portion 8 and the end face 9. For this reason, it is preferred that the flange portion 8 and the end face 9 determine the relative axial positions of the connector 3 and the hub extension H. Alternative pairs of facing features that could be configured to abut each and thereby determine the relative axial positions (such as the threaded portion 12 and the end of the recess) are less amenable to inspection.

Figure 3 is a close-up schematic view of a seal between a section of a casing of an impeller and the flange portion 8 of a further embodiment of the connector 3. In this case, instead of a seal formed by a sealing ring, the hub extension H and flange portion 8 on one side and the casing section 15 on the other side have engaging surfaces 19 carrying respective sets of machined grooves which interlock to form a labyrinth seal.

Figure 4 shows schematically a sectional elevation of a further embodiment of the impeller.

This embodiment is similar to the embodiment of Figure 1 except that the shaft 2 has two centring portions 5a, 5b, and the connector has two corresponding centring portions 1 Oa, 1 Ob. The threaded portions 7, 12 of the shaft 2 and the connector are located axially between the engaging pairs of centring portions such that, on each of the shaft and the connector, the respective diameters of the threaded portions and the centring portions decrease towards the impeller. A further difference relative to the embodiment of Figure 1 is that the threads are tapered, so that merely screwing the threaded portions 7, 12 together results in a rotationally fixed connection between the impeller 1 and the shaft 2.

Figure 5 is a sectional elevation of a further embodiment of the impeller 1. Again, the impeller 1 is fitted on to the steel turbocharger shaft 2 by means of a connector 3, the impeller being made from a metal matrix composite having an aluminium alloy matrix and silicon carbide particulate reinforcement. The connector 3 may be made of a high tensile steel such as EN26, whose composition includes about 2.Swt.% nickel.

The connector 3 has an insertion part 23 of cup-like shape which is inserted into the central recess formed in the hub extension H of the impeller 1, and a cylindrical sleeve portion 24 around the hub extension H. An abutment portion 21 at the impeller side end of the sleeve portion 24 engages against an impeller-side end face 22 of the hub extension H to determine the relative axial positions of the sleeve portion 14 and the hub extension H. A lip portion 8' around the mouth of the insertion part 3 joins the sleeve portion 24 and the insertion part 3.

The lip portionS' has a small clearance from the shaft-side end face 9 of the hub extension H, but is engaged on its other side by the shoulder 4 on the shaft 2.

The sleeve portion 24 extends over and frictionally contacts most of the axial length of the hub extension H, although in other embodiments the sleeve portion 24 can extend over only a portion of the axial length, and/or frictional contact can extend between the sleeve portion 24 and the hub extension H over only a portion of the overlap region between the sleeve portion 24 and the hub extension H. It should be noted that the steel of the connector 3 has a lower coefficient of thermal expansion than the metal matrix composite of the impeller 1 and hence the sleeve portion 24 does not expand as much with rising temperature as the hub extension H. This difference in their respective coefficients of expansion ensures that during operation, as the impeller assembly heats up, the joint between the hub extension H and the sleeve portion 24 tightens, reducing any tendency for relative movement between impeller 1 and connector 3 under the influence of centrifugal and thermal stresses, and increasing the torque capacity of the joint.

The impeller assembly is built up as follows. The connector 3 is warmed, and the sleeve portion 24 is slid on to the cylindrical outer surface of the hub extension H until the abutment portion 21 contacts the end face 22 of the hub extension I-L The insertion part 23 of the connector 3 inserts into the central recess of the hub extension H. When the connector 3 cools, the frictional connection is thus formed between the sleeve portion 24 and the hub extension ft However, the connector 3 is sized such that a clearance C prevents the insertion part 23 from contacting the side of the central recess. The screw-threaded portion 7 of the shaft 2 is then screwed onto the threaded portion 12 of the connector, the respective centring portions 5, 10 ensuring the shaft 2 aligns with the axis of the impeller 1. The threads are screwed until opposing surfaces of the lip portion 8' and shoulder 4 come into abutment, which causes the threads to tighten and provides a rotationally fixed connection between the impeller 1 and the shaft 2.

If there is any tendency for the impeller ito "walk", this can be monitored by measuring the size of the gap that would open up between the abutment portion 21 and the end face 22.

Advantageously, the connector 3 is a unitary body, which requires only one interference fit with the impeller 1. This reduces the number of high tolerance forming operations, and simplifies the joining procedure of the connector 3 to the impeller 1. Similarly to the embodiment of Figure 1, by containing the threaded connection between the connector 3 and the shaft 2 in the central recess of the hub extension H, an axially compact arrangement is achieved. The frictional connection between the sleeve portion 24 and the radially outer surface of the hub extension H transmits, in use, substantially all of the torque between the shaft 2 and the impeller 1.

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. For example, in embodiments such as that of Figure 4, instead of the connector having a centring portion 1 Da, the impeller may have a s centring portion at the base of the recess that engages with the centring portion 5b of the shaft. In another example, when the material of the shaft 1 is stronger than the material of the connector 3, the threads carried by the threaded portion 12 of the connector 3 may be protected by a helicoil formation to prevent damage to the threads of the connector 3. In yet another example, the shaft can be directly threaded to the impeller, i.e. without a connector, Ia but typically with a helicoil formation fitted to the impeller to protect the thread on the impeller. 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 spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

Claims (1)

1. <claim-text>CLAIMS1. An impeller (1) of a turbocharger for connection to a turbocharger shaft (2), wherein the impeller is formed from a metal matrix composite having an aluminium alloy matrix and silicon carbide particulate reinforcement.</claim-text> <claim-text>2. An impeller according to claim 1, wherein the volume average particle diameter of the particulate reinforcement is 2 microns or less.</claim-text> <claim-text>3. An impeller according to claim 1 or 2, wherein the particulate reinforcement is from 10 to 30% of the metal matrix composite by volume.</claim-text> <claim-text>4. An impeller according to any one of the previous claims fitted with a connector (3) for connecting the impeller to the shaft, the connector being frictionally connected to the impeller and having a threaded portion (12) carrying a thread which screws onto a corresponding threaded portion (7) of the shaft such that the connector provides a rotationally fixed connection between the impeller and the shaft.</claim-text> <claim-text>5. An impeller fitted with a connector according to claim 4, wherein the impeller has a shaft-side hub extension (H) and a surface of the connector is frictionally connected to a surface of the hub extension.</claim-text> <claim-text>6. An impeller fitted with a connector according to claim 5, wherein the frictional connection between the surface of the connector and the surface of-the hub extension transmits, in use, substantially all of the torque between the shaft and the impeller.</claim-text> <claim-text>7. An impeller fitted with a connector according to claim 5 or 6, wherein the shaft-side hub extension has a central recess and the connector is inserted into the recess to frictionally connect an outwardly facing surface (14) of the connector with a radially inner surface of the hub extension.</claim-text> <claim-text>8. An impeller fitted with a connector according to claim 7, wherein the central recess is a blind hole.</claim-text> <claim-text>9. An impeller fitted with a connector according to claim 7 or B, wherein the threaded portion of the connector is within the central recess.</claim-text> <claim-text>10. An impeller fitted with a connector according to claim 5 or 6, wherein the connector has a sleeve portion (24) which is frictionally connected on a radially outer surface of the hub extension.</claim-text> <claim-text>11. An impeller fitted with a connector according to any one of claims 4 to 10, wherein the coefficient of thermal expansion of the material of the connector is equal to or greater than the coefficient of thermal expansion of the metal matrix composite of the impeller.</claim-text> <claim-text>12. An impeller fitted with a connector according to any one of claims 4 to 11, wherein the connector is formed of a material having a greater strength than the metal matrix composite of the impeller.</claim-text> <claim-text>13. An impeller according to any one of claims 1 to 3 which is connected to a turbocharger shaft.</claim-text> <claim-text>14. An impeller fitted with a connector according to any one of claims 4 to 12, which impeller is connected to a turbocharger shaft having a corresponding threaded section, the thread of the threaded portion of the connector screwing onto the corresponding threaded portion of the shaft.</claim-text> <claim-text>15. A turbocharger having the connected impeller and shaft of claim 13 or 14.</claim-text>