GB2401407A

GB2401407A - a hollow component with internal vibration damping

Info

Publication number: GB2401407A
Application number: GB0310283A
Authority: GB
Inventors: David Anthony Williams
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2003-05-03
Filing date: 2003-05-03
Publication date: 2004-11-10
Also published as: GB0310283D0

Abstract

A component such as a blade for use in a gas turbine engine comprises outer panels 2, 4 defining an internal cavity 10 within which a warren girder structure 6 is provided. At least one damping strip 12 is provided in the cavity 10 and secured to one of the panels 2, 4 or to the warren girder structure 6. Each damping strip is secured to the respective surface at one end, for example adjacent the root of the blade, and makes frictional contact with the respective part over the remainder of its length. Vibration induced in the blade causes relative movement between the body of the blade and the damping strip, the resulting frictional resistance causing energy loss which damps the vibration. The components may be secured together by a diffusion bonding process.

Description

2401 407

A HOLLOW COMPONENT WITH INTERNAL DAMPING

This invention relates to a hollow component provided internally with a friction damping element, and to a method of manufacturing such a component. The invention is particularly, although not exclusively, concerned with components for use in gas turbine engines, for example fan blades.

Blades of gas turbine engines are subject to vibration induced by flutter and distortions in the gas flow over the blades. It is known to damp such vibrations by coating the outer surface of the blade with a suitable damping material, for example as disclosed in US 3758233. That document discloses a fan blade coated with a ceramic material, such as magnesium aluminate (MgO.AI2O3). A problem with such coatings is that they impose constraints on the surface finish obtainable on the aerodynamic surfaces of the blade. Furthermore, such coatings tend to be vulnerable both to erosion and foreign object damage (FOD) with the result that the aerodynamic performance of the blades, and their response to vibration, can be degraded.

Conventionally, rotors of gas turbine engines are assembled from a rotor disc and a plurality of blades which are secured to the periphery of the disc. The means of attachment between the blades and the disc, for example a fir-tree root arrangement, frequently provides some frictional damping which reduces the amplitude of any vibrations and so increases the resistance of the components to high cycle fatigue failure. It is becoming more common for blades and discs to be welded together to form unitary bladed discs, or blisks. Blisks have no mechanical joint at the roots of the blades, and so the damping effect achieved at such joints is absent. There is consequently an increased need for alternative damping means to be provided in blisks.

A further development in blade manufacture is disclosed in EP 0568201, and comprises the manufacture of blades, such as fan blades, by a superplastic forming and diffusion bonding technique which results in a hollow blade, ie a blade having at least one - 2 internal cavity. In the technique disclosed in EP 0568201, at least two sheets are laid in face- to-face contact with a predetermined pattern of stop-off material applied to one of the sheets. The sheets are diffusion bonded together, except where this is prevented by the stop-material. Subsequently, internal pressure is created between the sheets, causing them to deform superplastically to form cavities in the regions where diffusion bonding was prevented by the stop-off material. This technique can be used to manufacture hollow fan blades which can be welded to a disc to form a brisk.

GB 2078310 discloses a damping system for a gas turbine rotor blade. The damping system comprises a pin which lies within a passage which extends longitudinally of the blade. Frictional contact between the pin and the passage absorbs energy to damp vibrations of the blade.

According to one aspect of the present invention, there is provided a hollow component comprising a body having an internal cavity, an elongate damping strip being provided within the internal cavity and being secured at one end of the damping strip to the body, a region of the damping strip being in frictional contact with an internal surface of the body.

In the context of the present invention, the expression "strip" means an element having a thickness which is substantially less than the width of the element, for example the ratio of the thickness to the width is less than 0.2 and more preferably less than 0.1.

The result is that the surface area of the damping strip in frictional contact with the internal surface of the body can be large in relation to the mass of the damping strip.

Preferably, substantially the entire surface of one face of the damping strip is in frictional contact with the internal surface of the body.

The body may comprise an outer peripheral wall which defines the external surface of the component. The outer wall may comprise two panels which are bonded together at opposite edges. - 3

The outer wall defines the internal cavity of the component, but the body may also comprise a partition structure disposed within the internal cavity, the partition structure contacting the outer wall at spaced contact regions. The damping strip may be provided on an internal surface of the outer wall at positions between the contact regions, or may be provided on the partition structure. Furthermore, a plurality of the damping strips may be provided, with at least one being provided on the outer wall, and at least one provided on the partition structure.

The damping strip may be made of a softer or less wear resistant material than the region of the body which it contacts, so that the damping strip will wear preferentially over the body. The body may be made of a titanium alloy, in which case the damping strip may also be made of a titanium alloy of a softer or less wear resistant composition.

The wear resistance of the body, at least in the region contacted by the damping strip, may be enhanced by providing the surface of the body with a suitable coating or surface treatment.

The damping strip may be secured to the body by any suitable process, for example a bonding process. For some applications, for example aerospace applications in which the body and the damping strip are made from titanium alloy, a diffusion bonding process may be appropriate.

In a preferred embodiment, the component may be a component of a turbine engine, and more specifically a gas turbine engine. The component may be a rotor blade so that the damping strip will serve to damp vibration of the blade in operation of the engine in which the blade is fitted. The present invention may be applied to a series of blades permanently secured, for example by welding, to a rotor disc to form a brisk. It will be appreciated, however, that the present invention may also be applied to other components, whether or not of gas turbine engines, for which additional damping is required to minimise vibration. - 4

Where the component is a rotor blade, the damping strip is preferably secured to a region of the body at or near the blade root, the damping strip then extending from the region of securement towards the tip of the blade. It is desirable for the damping strip to contact the body at a region at which the maximum amplitude of vibration occurs, or at least close to that region.

It is preferable for at least part of the surface engaged by the damping strip to be inclined to the radial direction, with respect to the axis of rotation of the rotor to which the blade is to be secured, with that surface of the body being oriented towards the axis. With this orientation, the damping strip is pressed, under the action of centrifugal force when the rotor rotates, into firm contact with the surface of the body to increase the frictional force exerted between the damping strip and the body.

The component, especially if it is in the form of a rotor blade, may be made by a diffusion bonding process as disclosed in EP 0568201.

According to another aspect of the present invention, there is provided a method of manufacturing a hollow component comprising a body having an internal cavity, in which method a plurality of panels are joined together in a diffusion bonding process to form the hollow component, a damping strip being disposed between the panels during the diffusion bonding process whereby a diffusion bond is formed at one end region of the damping strip between the damping strip and one of the panels.

After the diffusion bonding process, the panels may be deformed by applying internal pressure between the panels, thereby to create the internal cavity. The panels may be heated to a temperature at which the internal pressure causes the panels to deform superplastically.

The panels may comprise outer panels which, in the finished component, form an outer peripheral wall of the body, and an intermediate membrane disposed between the outer panels, the membrane being bonded in the diffusion bonding process to each of the outer panels at spaced locations, the damping strip being situated outside the spaced - 5 locations, whereby deformation of the panels under the internal pressure causes the membrane to form partitions extending between the outer panels across the internal cavity.

The diffusion bond between the damping strip and the respective panel may be formed between the damping strip and one of the outer panels or, alternatively, between the damping strip and the internal membrane.

In order to avoid the formation of a diffusion bond being created between the damping strip and the body except at the one end region, a stop-off material may be applied to the damping strip and/or to the panels in regions where no diffusion bonding is required.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a transverse cross-section of a rotor blade of a gas turbine engine; and Figure 2 is a diagrammatic longitudinal cross-section of the rotor blade.

The blade shown in the Figures comprises outer panels 2, 4 between which a warren girder structure or line core 6 is disposed. The panels 2, 4 and the warren girder structure 6 are made from a titanium alloy, and constitute the body of the blade. The panels 2 and 4 are diffusion bonded to each other at the leading and trailing edges (not shown) of the blade, and to the warren girder structure 6 at contact regions 8. The warren girder structure 6 thus provides a plurality of partitions extending across the internal cavity 10 of the blade.

A plurality of damping strips 12 are secured to the body of the blade within the internal cavity 10. As shown for the purposes of illustration, two damping strips 12 are provided - 6 on the panels 2, and two on the warren girder structure 6. However, it will be appreciated that different numbers of damping strips 12, including only a single damping strip, may be provided, and that, if more than one, they may all be provided on one or other of the panels 2, 4 or on the warren girder structure 6.

Referring to Figure 2, it will be appreciated that the damping strips 12 extend parallel to the lengthwise direction of the blade (represented by the panel 2). The blade has a root end 14, which may be provided with a blade securing feature such as a fir-tree root (not shown) or which may alternatively be secured to a rotor disc in a permanent manner, for example by welding. The end opposite the root end 14 comprises a tip 16.

The damping strips 12 are secured to the panel 2 at end regions 18 situated near the root 14 of the blade. The damping strips 12 extend from the end regions 18 towards the tip 16, and terminate shortly before reaching the tip 16. In Figure 2, the damping! strips 12 are shown as extending parallel to the lengthwise direction of the blade, but it will be appreciated that rotor blades are often curved and twisted, and the damping strips 12, while conforming to the internal surface against which they lie, need not be precisely parallel to the lengthwise direction of the blade.

Beyond the end regions 18 at which the damping strips 12 are secured to the panel 2, the damping strips 12 are not secured to the panel 2, but instead lie against it in frictional contact. As shown in Figure 1, the panel 2 is provided with shallow recesses within which the damping strips 12 lie, and similar recesses 20 are provided in the appropriate places on the warren girder structure 6. As shown, the depth of each recess 20 is less than the thickness of the respective damping strip 12, so that the damping strips 12 stand slightly proud of the adjacent surface of the panel 2 or the warren girder structure 6.

It will be appreciated that the damping strips 12 secured to the warren girder structure 6 are disposed in a similar manner to that described above with reference to Figure 2, and are similarly secured to the warren girder structure 6 at end regions corresponding to the end regions 18. - 7

Preferably, the damping strips 12 lie against surfaces of the body 2,4, 6 which are inclined to the radial direction in such a way that, in operation, rotation of the rotor to which the blade is secured causes the damping strips 12 to be pressed, by the resulting centrifugal force, against the adjacent surface of the body 2,4,6.

In operation, vibrations induced in the blade will result in relative displacement between the body 2,4, 6 and the damping strips 12. As a result of the frictional contact between the damping strips 12 and the adjacent surface of the body, energy will be lost, for example as heat, and consequently the vibrations will be damped.

The rubbing of the damping strips 12 against the adjacent surface will result in wear. It is desirable for the damping strips 12 to wear preferentially instead of the panels 2,4 or the warren girder structure 6. Consequently, the damping strips 12 may be made from a less wear resistant material than that of the panels 2,4 and the warren girder structure 6. The damping strips 12 may be made from a titanium alloy, but they are preferably made from a softer or less wear resistant alloy than that from which the panels 2,4 and the warren girder structure 6 are made. Furthermore, a wear-resistant coating may be applied to the panels 2,4 and the warren girder structure 6, at least at the surfaces contacted by the damping strips 12.

The blade may be made in a diffusion bonding and superplastic deforming process as disclosed in EP 0568201. In such a process, the precursors of the panels 2 and 4, and an internal membrane constituting a precursor of the warren girder structure 6, are stacked in face-to-face engagement, with the damping strips 12 laid at appropriate positions between them. Regions of the panels 2 and 4 and of the membrane which forms the warren girder structure 6, other than at the contact regions 8, are coated with a stop-off material to prevent diffusion bonding. The stop-off material is also applied to those surfaces of the damping strips 12 which, in the formed blade, will be exposed to the internal cavity 10 and also to the opposite surfaces of the damping strip 20 apart from the regions 18 at which a diffusion bond is required. The resulting stack is then heated and subjected to high pressure so that diffusion bonds are created between - 8 those contacting metal-to-metal regions corresponding to the contact regions 8 in Figure 1 and the end regions 18 of the damping strips 12 in Figure 2.

When bonding has been achieved, the assembly is heated to a temperature at which it can be hot formed into a desired configuration in which, for example, the assembly has an arcuate cross-section with a twist between the ends of the assembly, approximating to a desired blade profile.

Subsequently, the assembly is heated to a temperature at which superplastic deformation of the elements of the assembly can occur and the assembly is internally pressurised. This forces the panels 2 and 4 apart from each other between their leading and trailing edges. Since the membrane which forms the warren girder structure 6 is diffusion bonded at staggered intervals to the panels 2 and 4, but not bonded (or at least not strongly bonded) where the stop-off layer is present, the membrane will superplastically deform into the configuration shown in Figure 1. The resulting structure is consequently that of a hollow component, carrying the damping strips 12 on its internal surfaces. The component therefore exhibits a reduction in the amplitude of vibration when subjected to excitation, for example by flow conditions around the blade. The reduced amplitude of vibration thus reduces the tendency of the blade to fail under high cycle fatigue conditions.

Since the damping strips 12 are secured at the root end 14 of the blade, and otherwise extend freely (although in frictional contact with the panel 2 and the warren girder structure 6) their mass is self-supported, during rotation of the rotor to which the blade is fixed, so that they do not impose any additional centrifugal loading on the panels 2, 4 and the warren girder structure 6. - 9 -

Claims

1 A hollow component comprising a body having an internal cavity, an elongate damping strip being provided within the internal cavity and being secured at one end of the damping strip to the body, a region of the damping strip being in frictional contact with an internal surface of the body.

2 A component as claimed in claim 1, in which the body comprises an outer wall defining an external surface of the component.

3 A component as claimed in claim 2, in which the outer wall comprises two panels which are bonded together at opposite edges.

4 A component as claimed in claim 2 or 3, in which the damping strip is secured to the body adjacent an internal face of the outer wall.

A component as claimed in any one of claims 2 to 4, in which a partition structure is disposed within the internal cavity of the component, the partition structure contacting the outer wall at spaced contact regions.

6 A component as claimed in claim 5, in which the damping strip is secured to a face of the partition structure.

7 A component as claimed in claim 5 or 6, in which the partition structure comprises a warren girder structure, the contact regions comprising parallel, elongate regions.

8 A component as claimed in claim 7 when appendant to claim 4, in which the damping strip is secured to the outer wall at a location between adjacent contact regions. - 10

9 A component as claimed in any one of the preceding claims, in which the body is formed from titanium alloy.

A component as claimed in any one of the preceding claims, in which the damping strip is made from a material having a lower resistance to wear than the material of the body in the region of frictional contact with the damping strip.

11 A component as claimed in any one of the preceding claims, in which the body is provided with a wear-resistant coating in the region of frictional contact with the damping strip.

12 A component as claimed in any one of the preceding claims, in which the damping strip is made from titanium alloy.

13 A component as claimed in any one of the preceding claims, in which the damping strip is secured to the body by a bonding process.

14 A component as claimed in claim 13, in which the bonding process is a diffusion bonding process.

A component as claimed in any one of the preceding claims, in which the damping strip is disposed in a recess in the adjacent internal surface of the body.

16 A component as claimed in any one of the preceding claims, which is the component of a turbine engine.

17 A component as claimed in claim 16, which is a component of a gas turbine engine.

18 A component as claimed in claim 16 or 17, which is a rotor blade. - 11

19 A component as claimed in claim 18, in which the damping strip is secured to the body of the component at a position adjacent the blade root, and extends in a direction towards the blade tip.

A component as claimed in claim 19, in which the damping strip is inclined, over! at least part of its extent, to the radial direction, with respect to the axis of rotation of the blade in operation.

21 A component as claimed in claim 20, in which the damping strip is disposed, over at least part of its extent, so that centrifugal force acting on the damping strip during rotation of the blade in operation acts to press the damping strip against the adjacent surface of the body.

22 A rotor blade for a gas turbine engine substantially as described herein, with reference to, and as shown in, the accompanying drawings.

23 A gas turbine engine, including a component in accordance with any one of the preceding claims.

24 A method of manufacturing a hollow component comprising a body having an internal cavity, in which method a plurality of panels are joined together in a diffusion bonding process to form the hollow component, a damping strip being disposed between the panels during the diffusion bonding process whereby a diffusion bond is formed at one end region of the damping strip between the damping strip and one of the panels.

A method as claimed in claim 24, in which, after the diffusion bonding process, the panels are deformed by applying internal pressure between the panels, thereby to create the internal cavity.

26 A method as claimed in claim 25, in which, during deformation of the panels, the panels are heated to a temperature at which deformation takes place superplastically. it: - 12

27 A method as claimed in any one of claims 24 to 26, in which an intermediate membrane is disposed between the panels, the membrane being bonded in the diffusion bonding process to each of the panels at spaced locations, the damping strip being situated between adjacent spaced locations on one of the panels, whereby deformation of the panels under the internal pressure causes the membrane to form partitions extending between the panels across the internal cavity.

28 A method as claimed in any one of claim 24 to 27, in which a stop-off material is applied to the damping strip, except at the region where the diffusion bond is to be I formed, to prevent or minimise diffusion bonding between the damping strip and the body over a substantial part of the length of the damping strip.

29 A method of manufacturing a hollow component as claimed in claim 24 and substantially as described herein.