GB2106425A - Gas turbine engine rotor assembly - Google Patents

Abstract

A metal ring (11) having aerofoil blades (12) 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, 14 and 15) which constitute extensions of the interface between the ring (11) and the disc (14). The joint lines (20, 21) between the flanges (12, 13, 14 and 15) are sealed by vacuum brazing so that during hot isostatic pressing, diffusion bonding occurs between the flanges (12, 13, 14 and 15) as well as between the ring (11) and the disc (14). The diffusion bonds between the flanges (12, 13, 14, and 15) 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 which facilitates its inspection.

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 outer periphery. The aerofoil blades are usually attached to the disc by some form of mechanical connection. Thus 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 disproportionately large in comparison with the rotor disc to which they are required to be filed.

They therefore in turn require relatively large fir tree type root fixings to adequately retain them on the disc at high rotational speeds.

As speeds and temperatures are increased in order to meet the demands of improved 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 mechanical root fixings is eliminated, thereby alleviating the problem of overcrowding on the disc periphery.

In U.K. Patent No. 1 586 331 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 that periphery. The joint lines between the ring and disc are brazed under vacuum after which the assembly is hot isostatically pressed to achieve diffusion bonding across the interface between the ring and the disc.

Thus the method avoids the overcrowding and mechanical limitations of conventional root fixings on a small diameter disc since there is no mechanical connection between the aerofoil blades and the disc.

One problem associated with constructing rotor assemblies by this method concerns the inspection of the diffusion bond between the bladed ring and the disc. Thus diffusion bonds of this type are conventionally inspected using ultrasonic techniques. Such techniques are most effective if the ultrasonic waves from the inspection apparatus are directed onto the diffusion bonded joint under investigation in a direction which is substantially normal to the joint.

However in the case of rotor assemblies, it is not possible to do this in view of the location of the diffusion bonded joint and the typical geometry of rotor assemblies. Generally it is only possible to direct ultrasonic waves on to the joint in a direction which is other than normal to the joint.

This leads to poor attenuation of the reflected ultrasonic waves and a correspondingly poor indication of the integrity of the diffusion bonded joint.

It is an object of the present invention to provide a method of manufacturing a rotor assembly for a gas turbine engine by the diffusion bonding of a bladed ring to a disc wherein improved inspection of the diffusion bond is facilitated.

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 at least one axially extending annular flange preforming a metal disc having at least one axially extending annular flange, mounting said bladed ring on the radially outer periphery of said disc so that both they and said axially extending flanges engage each other in close fitting relationship, sealing the joint lines between said axially extending flanges and additionally any other exposed joint lines between said bladed ring and said disc under vacuum conditions with a high melting point sealant so as to fully enclose and seal the interface between said axially extending flanges and said bladed ring and said disc, and subsequently hot isostatically pressing the resultant assembly to achieve diffusion bonding between said axially extending flanges, and said bladed ring and said disc.

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

At least one of said flanges is preferably provided with at least one groove which is so positioned as to be adjacent its engaging flange to prevent the ingress of said high melting point sealant into said interface between said bladed ring 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 grooves, said grooves being so positioned that adjacent grooves correspond to define a single enclosed chamber when said axially extending flanges engage each other in close fitting relationship.

Each of said ring member and said disc is preferably provided with two of said axially extending flanges, one positioned on each axial extent thereof, in axially spaced apart relationship.

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

Fig 2 is a side view of a disc.

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

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

With reference Fig 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 12 mounted on its radially outer periphery. The bladed ring 10 may be fabricated by any convenient method. Thus, for instance, it may be fabricated by assembling a plurality of aerofoil blades having plafforms in an annular array 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 10 could be employed if desired.

The ring 11 is provided with two axially extending annular flanges 12 and 13 as can be seen in Fig 4 which are positioned on each axial extent thereof, in axially spaced apart relationship.

The radially inner surfaces of the flanges 1 2 and 13 are contiguous with the radially inner periphery of the ring 1. After fabrication of the bladed ring 10, the internal periphery of the ring 1 together with that of the flanges 12 and 1 3, is machined to a given inside diameter.

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 known as MAR M002.

The disc 14 shown in Fig 2 is preformed from particles of an alloy which have been compacted under suitable conditions of heat 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 1 4 is so configured as to be provided with two axially extending flanges 1 5 and 1 6 which are positioned on each side thereof in axially spaced apart relationship, the axial spacing being the same as that which separates the flanges 12 and 13 on the ring 1 1. The disc 14 is so formed that the radially outer surfaces of the flanges 1 5 and 1 6 are the radially outer periphery of the disc 14 are contiguous and of the same outside diameter.This outside diameter is slightly greater than the inside diameter of the ring 11 and its associated flanges 12 and 13. The disc 14 and the flanges 1 5 and 1 6 are, however, machined after consolidation until their outside diameter is equal to the inside diameter of the bladed ring 10 and the flanges 12 and 13.

All of the flanges 12, 13, 14and 15 are additionally machined so that each is provided with an annular groove 1 7. More specifically the annular grooves 17 on the flanges 12 and 13 are on their radially inner peripheries and the annular grooves 1 7 on the flanges 15 and 16 are on their radially outer peripheries. The annular grooves 1 7 on the flanges 12 and 13 are adjacent the axial extents of those flanges and are axially spaced apart by the same distance as that by which the annular grooves 17 in the flanges 1 5 and 16 are axially spaced apart.

The bladed ring 10 is then heated up so that it thermally expands to such an extent that it may be mounted on the outer periphery of the disc 14 in the manner shown in Figs 3 and 4. The bladed ring 10 is so mounted on the disc 14 that the annular grooves 17 in the flanges 1 2 and 1 5 and those in the flanges 1 3 and 1 6 cooperate to define two enclosed chambers 1 8 and 1 9 respectively although it will be understood that such groove cooperation is not in fact essential. 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 so as to provide a close fit between them and additionally a close fit between the flanges 12, 13, 14 and 1 5.

The flanges 1 2 and 1 5 together define an annular joint line 20 and the flanges 13 and 16 likewise define an annular joint line 21. These annular joint lines 20 and 21 are sealed by vacuum brazing so that the interfaces between the ring 11 and the disc 14, the flanges 12 and 1 5 and the flanges 13 and 16 are both enclosed and sealed. The annular grooves 17, which may be up to 0.1 " diameter, act as braze traps so as to prevent the flow of molten braze into the interfaces between the flanges 12 and 15, and 13 and 16.

It will be appreciated that other high melting point sealants could be used to seal the joint lines 20 and 21. Thus, for instrance, electron beam welding could be employed in the sealing operation.

After the joint lines 20 and 21 have been sealed under vacuum, the whole assembly is subjected to hot isostatic pressing. Thus the assembly is subject to a pressure of 1 5,000 pounds per square inch at a temperature of 1 2200C for four hours. This serves to achieve diffusion bonding between the ring 11 and the disc 14 as well as between the flanges 12 and 15, and 1 3 and 1 6. Moreover it causes the enclosed chambers 18 and 19 to collapse so that a continuous diffusion bond is created between the ring 1 1, the disc 14 and the flanges 12, 13, 14 and 1 5.

After hot isostatic pressing has been discontinued, the resultant rotor assembly is inspected by ultrasonic examination. Now since the diffusion bond between the ring 11 and the disc 1 4 is so positioned that it cannot be satisfactorily inspected ultrasonically, ultrasonic inspection is made of the diffusion bonds between the flanges 12, and 15, and 13 and 16. Thus the flanges 12, 13, 14 and 15 are so located that an ultrasonic probe may be positioned substantially normally to the diffusion bonds between them, thereby facilitating effective examination of these bonds. Thus we have found that if the diffusion bonds between the flanges 12 and 15, and 13 and 1 6 are satisfactory then so will the diffusion bond between the ring 11 and the disc 14.This is especially so in the case of the enclosed chambers 1 8 and 1 9 since if the hot isostatic pressing is sufficient to collapse them, then diffusion bonding will also have occurred between the ring 11 and the disc 1 4. If a satisfactory degree of diffusion bonding is detected, the flanges, 12, 13, 14 and 15 are removed by machining them off. If it is desired to make a visual inspection of the diffusion bonds between the flanges 12 and 15, and 13 and 16, the maching operations could be carried out in such a way as reveal sectioned views of the bonds.

In order to ensure that ultrasonic surface effects are avoided, it is 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, 14 and 1 5 are preferably greater than 0.3 inches thick.

It will be appreciated that although the present invention has been described with reference to a ring 11 and a disc 14 which are provided with two axially extending flanges, other configurations are possible. Thus for instance the ring 11 and the disc 14 may only be provided with one flange each. The flanges would cooperate in a similar manner to the flanges 12 and 1 5, and 13 and 1 6, and thereby provide evidence of effective diffusion bonding between the ring 11 and the disc 14.

In such an arrangement, the exposed joint line between the ring 11 and the disc 14 on the side thereof without flanges would, of course, have to be sealed with a suitable high temperature sealant in order to ensure the necessary vacuum sealing of the interface between the ring 11 and the disc 14.

Claims (7)

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 at least one axially extending annular flange, performing a metal disc having at least one axially extending annular flange, mounting said bladed ring on the radially outer periphery of said disc so that both they and said axially extending annular flanges engage each other in close fitting relationship, sealing the joint lines between said axially extending flanges and additionally any other exposed joint lines between said bladed ring and said disc member vacuum conditions with a high melting point sealant so as to fully enclose and seal the interface between said axially extending flanges and said bladed ring and said disc, and subsequently hot isostatically pressing the resultant assembly to 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 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 is provided with at least one groove which is so positioned as to be adjacent its engaging flange to prevent the ingress of said high melting point sealant into said interface between said bladed ring 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 adjacent grooves correspond to define a single enclosed chamber when said axially extending flanges engage each other in close fitting relationship.

5. A method of manufacturing a rotor assembly as claimed in any one preceding claim wherein each of said ring member and said disc is provided with two of said axially extending flanges, one positioned on each side thereof, in axial spaced apart relationship.

6. A method of manufacturing a rotor assembly as claimed in any one preceding claim wherein said high melting point sealant is a braze.

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