GB2106016A - Gas turbine rotor assembly - Google Patents

Abstract

A metal ring (11) having aerofoil blades (11a) equally spaced apart around its periphery is mounted on a disc (14), sealed in place under vacuum and subsequently hot isostatically pressed to achieve diffusion bonding across the interface between the ring (11) and the disc (14). Each of the ring (11) and the disc (14) is provided with axially extending flanges (12, 13, 15 and 16) at their axial extents. The flanges (12, 13, 15, 16) are so dimensioned that when the ring (11) is mounted on the disc (14) the flanges (12, 13, 15, 16) engage each other in close fitting relationship in such a way that the ring (11) and the disc (14) have an enclosed annular chamber (20) radially spacing them apart. The joint lines (21, 22) between the flanges (12, 13, 15, 16) are sealed by vacuum brazing so that during hot isostatic pressing, the enclosed annular chamber (20) collapses and diffusion bonding occurs between the ring (11) and the disc (14) as well as between the flanges (12, 13, 15, 16). The diffusion bonds between the flanges (12, 13, 15, 16) provide a ready indication of the degree of diffusion bonding between the ring (11) and the disc (14). <IMAGE>

Description

SPECIFICATION Gas turbine engine rotor assembly This invention relates to a gas turbine engine rotor assembly and in particular to a method of manufacturing a gas turbine engine rotor assembly.

Rotor assemblies which are intended for use in gas turbine engines usually comprise a disc having an annular array of aerofoil blades equally spaced apart around its radially outer periphery. The aerofoil blades are usually attached to the disc by some form of mechanical connection. Thus for instance it is common to provide each aerofoil blade with a fir-tree configuration root which locates in a correspondingly shaped cut-out portion in the disc periphery. Whilst such methods of attachment are satisfactory when employed in medium to large size gas turbine engines, they are not so successful when employed in small gas turbine engines.

Because of the physical limitations in manufacturing small scale components, rotor blade aerofoil cross-sections are disproportionely large in comparison with the rotor disc to which they are required to be fixed. They therefore in turn require relatively large fir tree type root fringes to adequately retain them on the disc at high rotational speeds.

As speeds and temperature are increased in order to meet the demands of impaired performance, then the proportions of fir tree type fixings need to be increased still further. Thus a mechanical limitation is reached with overcrowding of the blade root fixings on the rotor disc periphery. It is therefore desirable to provide an alternative method of attaching rotor blades to discs in which the use of machanical root fixings is eliminated, thereby alleviating the problem of overcrowding on the disc periphery.

In U.K. patent No. 1586331 there is described a method of manufacturing a rotor assembly in which the disc is formed with a plane periphery and the aerofoil blades constitute part of a ring which is adapted to fit on to that periphery. The joint lines between the ring and the disc are brazed under vacuum after which the assembly is hot isostatically pressed to achieve diffusion bonding across the interface between the ring and disc. Thus the method avoids the overcrowding and consequent mechanical limitation of conventional root fixing on a small diameter disc since there is no mechanical connection between the aerofoil blades and the disc.

One disadvantage of producing rotor assemblies by this method is that of achieving satisfactory location of the ring on the disc. Thus since it is difficult to achieve abutting surfaces of the ring and disc which match exactly, there is a possibility that some areas will abut whilst others will be spaced apart by varying distances. This in turn can lead to problems in achieving satisfactory sealing of the joint lines between the ring and disc and also variability in the quality of the diffusion bond across the interface between the ring and disc.

It is an object of the present invention to provide a method of manufacturing a rotor assembly for a gas turbine engine bythe diffusion bonding of a bladed ring to a disc wherein improved location of the bladed ring on the disc prior to diffusion bonding is achieved.

According to the present invention, a method of manufacturing a rotor assembly for a gas turbine engine comprises the steps of fabricating a metal bladed ring comprising a ring member having an annular array of radially extending aerofoil blades mounted around its radially outer periphery and two axially extending annularflanges, one positioned on each axial extent thereof, preforming a metal disc having two axially extending flanges, one on each axial extent thereof, mounting said bladed ring on the radially outer periphery of said disc so that said axially extending flanges on said bladed ring engage said axially extending flanges on said disc in close fitting relationship, said flanges being so diminished that at least a major portion of the radially inner periphery of said bladed ring and the radially outer periphery of said disc are maintained in radially spaced apart relationship so that together with said flanges, they define an enclosed chamber, providing a seal between said axially extending flanges under vacuum conditions with a high temperature melting point sealant so as to fully enclose and seal said enclosed chamber and the interface between said axially extending flanges and subsequently hot isostatically pressing the resultant assembly to collapse said enclosed chamber and achieve diffusion bonding between said axially extending flanges and said bladed ring and disc.

Preferably said axially extending flanges are machined off said resultant rotor assembly after said hot isostatic pressing.

At least one of said flange of each pair of engaging flanges at each axial extent of said rotor assembly is preferably provided with at least one annular groove which is so positioned as to be adjacent its engaging flange to prevent the ingress of said high temperature melting point sealant into said enclosed chamber between said ring member and said disc, and so dimensioned as to collapse during said hot isostatic pressing.

Each of said flanges may be provided with at least one of said annular grooves, said grooves being so positioned that grooves in engaging flanges correspond to define a single enclosed chamber when said flanges engage each other in close fitting relationship.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side view of a ring provided with an array of radially extending aerofoil blades.

Figure 2 is a side view of a disc.

Figure 3 is a side view of the bladed ring shown in Figure 1 mounted on the disc shown in Figure 2.

Figure 4 is a cross-sectional view of a portion of the bladed ring/disc assembly shown in Figure 3 showing the arrangement of the axially extending flanges thereon.

With reference to Figure 1, a bladed ring generally indicated at 10 comprises a ring 11 which is provided with an annular array of equally spaced apart radially extending aerofoil blades 11a mounted around its radially outer periphery. The bladed ring 10 may be fabricated by any convenient method.

Thus, for instance, it may be fabricated by assembling an annular array of aerofoil blades having platforms so that the platforms of adjacent blades abut, and then bonding the abutting blades together by, for instance, electron beam welding. Alternatively, it may be fabricated by forming a ring having a plurality of aerofoil cross-section slots in its periphery, inserting aerofoil blades of corresponding cross-sectional shape in the slots and subsequently fixing the blades in the slots by electron beam welding. It will be appreciated however that other methods of fabricating the bladed ring could be employed if desired.

The ring 11 is provided with two axially extending flanges 12 and 13 as can be seen in Figure4which are positioned on each of its axial extents so as to be in axially spaced apart relationship. After fabrication of the bladed ring 10, the radially inner periphery of the flanges 12 and 13 are machined to the same internal diameter. The radially inner periphery of the ring 11 is however machined to an internal diameter which is slightly greater than that of the flanges 12 and 13.

The bladed ring 10 may be fabricated from any convenient alloy but we prefer to fabricate it from a nickel base alloy and in particular the nickel base alloy knows as MAR MOO2.

The disc 14 shown in Figure 2 is preformed from particles of an alloy which have been compacted under suitable conditions of temperature and pressure so as to consolidate them. The alloy is also a nickel base alloy and is preferably that which is known as "Astralloy". The disc 14 is so configured as to be provided with two axially extending flanges 15 and 16 which are positioned on each axial extent thereof so as to be in axially spaced apart relationship, the axial spacing being the same as that which separates the flanges 12 and 13 on the ring 11. The disc 14 is so formed that the radially outer periphery of each flange 15 and 16 is slightly greater than the inside diameter of each of the flanges 12 and 13.The flanges 15 and 16 are, however machined after consolidation until they each have an outside diameter which is equal to the inside diameter of each of the flanges 12 and 13. The radially outer periphery of the disc 14 is also machined after consolidation but it is so machined that its outside diameter is slightly less than that of the flanges 15 and 16.

All of the flanges 12,13,15 and 16 are additionally machined so that each is provided with an annular groove 17. More specifically, the annular grooves 17 on the flanges 12 and 13 are on their radially inner peripheries and the annular grooves 17 on the flanges 15 and 16 are on their radially outer peripher ies.Theannulargroovesl7intheflangesl2andl3 are adjacent the axially spaced apart by the same distance as that by which the annular grooves 17 in the flanges 15 and 16 are axially spaced apart.

The bladed ring 10 is then heated up until it thermally expands to such an extent that it may be mounted on the radially outer periphery of the disc 14 in manner shown in Figures 3 and 4. The bladed ring 10 is so mounted on the disc 14 that the annular grooves 17 in the flanges 12 and 15 and those in the flanges 13 and 16 cooperate to define the enclosed chambers 18 and 19 respectively. Moreover since the radially inner periphery of the ring 11 and the radially outer periphery of the disc 14 have diameters different to those of the flanges 12,13,15 and 16, then they, together with the flanges 12,13,15 and 16 cooperate to define a further enclosed chamber 20. The whole assembly of the bladed ring 10 and the disc 14 is then allowed to cool so that the ring 11 contracts on to the disc 14, thereby providing a close fit between the flanges 12,13,15 and 16.Thus since the disc 14 and the ring 11 are spaced apart by the annular enclosed chamber 20, the flanges 12,13,15 and 16 provide the only means of location of the bladed ring 10 on the disc 14.

The flanges 12 and 15 together define an exposed annular joint line 21 and the flanges 13 and 16 likewise define an exposed annular joint line 22.

These annular joint lines 21 and 22 are sealed by vacuum brazing so that the interfaces between the flanges 12,13,15 and 16, and the annular enclosed chambers 18,19 and 20 are both enclosed and sealed. The annular enclosed chambers 18 and 19, which may for instance be up to 0.1" diameter, act as braze traps so as to prevent the flow of moltern braze into the interfaces between the flanges 12,13,15 and 16 as well as onto the annular enclosed chamber 20.

It will be appreciated that other high melting point sealants could be employed to seal the joint lines 21 and 22. Thus, for instance, electron beam welding could be employed in the sealing operation.

After the joint lines 21 and 22 have been sealed under vacuum, the whole assembly is subjected to hot isostatic pressing. Thus the assembly is subjected to a pressure of 15,000 pounds per square inch at a temperature of 1 200"C for four hours. This serves to achieve diffusion bonding between the flanges 12,13,15 and 16. Moreover it causes the enclosed chambers 18, 19 and 20 to collapse so that a continous diffusion bond is created between the ring 11, the disc 14 and the flanges 12,13,15 and 16.

It will be seen therefore that prior to the hot isostatic pressing step, the flanges 12,13,15 and 16 provide the sole means of location of the bladed ring 10 on the disc 14. Accurate location of the bladed ring 10 on the disc 14 is thereby ensured.

After hot isostatic pressing has been discontinued, the resultant rotor assembly is inspected by ultrasonic examination. Now the diffusion bond between the ring 11 and the disc 14 is so positioned that it cannot be satisfactorily inspected ultrasonically. This is because effective ultrasonic examination requires the ultrasonic probe to be positioned generally normal to the surface to be inspected and the configuration of the disc 14 and the location of the aerofoil blades 1 lea effectively preclude this. However, the flanges 12,13,15 and 16 are so configured that the diffusion bonds between them can be readily inspected ultrasonically. Thus we have found that if the diffusion bonds between the flanges 12 and 15 and 13 and 16 are satisfactory, then so too is the diffusion bond between the ring 11 and the disc 14.

This is especially so in the case of the enclosed chambers 18 and 19 since if the hot isostatic pressing is sufficient to collapse them and provide diffusion bonding across the resultant interface, then the enclosed chamber 20 will also have collapsed and diffusion bonding will have occurred between the ring 11 and the disc 14. If a satisfactory degree of diffusion bonding is deleted the flanges 12,13,15 and 16 are removed by machining them off. If it is desired to make visual inspection of the diffusion bonds between the flanges 12 and 14, and 13 and 16, the machining operation could be carried out in such a way as to reveal sectional views of the bonds.

In order to ensure that ultrasonic surface effects are avoided, it is usually necessary to ensure that the flange thicknesses are appropriate for the type of ultrasonic equipment which is to be used. Thus the flanges 12,13,15 and 16 each preferably have a thickness greater than 0.3 inches.

Claims (6)

1. A method of manufacturing a rotor assembly for a gas turbine engine comprising the steps of fabricating a metal bladed ring comprising a ring member having an annular array of radially extending aerofoil blades mounted around its radially outer periphery and two axially extending annular flanges, one positioned on each axial extent thereof, preforming a metal disc having two axially extending annular flanges, one positioned on each axial extent thereof, mounting said bladed ring on the radially outer periphery of said disc so that said axially extending flanges on said bladed ring engage said axially extending flanges on said disc in close fitting relationship, said flanges being so dimensioned that at least a major portion of the radially inner periphery of said bladed ring and the radially outer periphery of said disc are maintained in radially spaced apart relationship so that together with said flanges, they define an enclosed chamber, providing a seal between said axially extending flanges under vacuum conditions with a high temperature melting point sealant so as to fully enclose and seal said enclosed chamber and the interfaces between said axially extending flanges and subsequently hot isostatically pressing the resultant assembly to collapse said enclosed chamber and achieve diffusion bonding between said axially extending flanges and said bladed ring and disc.

2. A method of manufacturing a rotor assembly as claimed in claim 1 wherein said axially extending flanges are machined off said resultant resultant rotor assembly after said hot isostatic pressing.

3. A method of manufacturing a rotor assembly as claimed in claim 1 or claim 2 wherein at least one of said flanges of each pair of engaging flanges at each axial extent of said rotor assembly is provided with at least one annular groove which is so positioned as to be adjacent its engaging flange to prevent the ingress of said high temperature melting point sealant into said enclosed chamber between said ring member and said disc, and so dimensioned as to collapse during said hot isostatic pressing.

4. A method of manufacturing a rotor assembly as claimed in claim 3 wherein each of said flanges is provided with at least one of said grooves, said grooves being so positioned that grooves in engaging flanges correspond to define a single enclosed chamber when said flanges engage each other in close fitting relationship.

5. A method of manufacturing a rotor assembly as claimed in any one preceding claim wherein said high temperature melting point sealant is a brazing alloy.

6. A method of manufacturing a rotor assembly substantially as hereinbefore described with reference to and as shown in the accompanying drawings.