WO1996041068A1

WO1996041068A1 - Anti-fretting barrier

Info

Publication number: WO1996041068A1
Application number: PCT/CA1996/000353
Authority: WO
Inventors: Lijue Xue; Ashok K. Koul; William Wallace
Original assignee: National Research Council Of Canada
Priority date: 1995-06-07
Filing date: 1996-06-04
Publication date: 1996-12-19

Abstract

A cold-worked low-friction metallic shim or a low-friction nonmetallic composite shim is bonded onto the surface of a component with a layer of adhesive to form an anti-fretting barrier. This barrier is incorporated between the tightly fit components which are subject to high cyclic stresses, e.g. a blade and a disc slot in a gas turbine engine. In one form, the barrier is bonded to the surface of only one component. Bonding the shim significantly delays fretting damage on the surface of the component. The low friction nature of the shim significantly reduces the fretting damage on the contact surface of the other component. In another form, the barrier is bonded to the contact surfaces of both components, which eliminates or reduces fretting damage on the component surfaces. Reduced fretting action is confined to the consumable shim surfaces. Any fretting crack initiated in the barrier will not propagate directly into the component. Bonding the shim onto a component will significantly increase the fretting fatigue life of the component when compared to the fretting fatigue life of the component assembly obtained by using a freely inserted and unbonded shim.

Description

ANTI-FREΓΠNG BARRIER

FIELD OF THE INVENTION

This invention relates to a method for increasing the fretting fatigue and fretting wear resistance of titanium, nickel and iron base alloys, or any other alloys which are prone to fretting damage accumulation, in tightly fit components, particularly those subject to high cyclic stresses.

BACKGROUND OF THE INVENTION

Fretting is a type of surface wear damage caused by small amplitude oscillating slip between two contacting surfaces under a normal pressure. Fretting fatigue occurs when the fretting is produced on the surface of a component that suffers cyclic stressing, and it can dramatically reduce the fatigue life of the component.

Fretting fatigue and fretting wear problems are widespread. Any mechanically fastened joint or any surfaces in contact under small relative movement may experience fretting damage if one or both contacting surfaces are subjected to cyclic loads or vibration. Severe fretting fatigue problems are frequently observed in the disc-blade fixing regions of fan, compressor and turbine components in gas turbine engines. The fretting action between the blade dovetail or root and the corresponding disc slot surface induces early crack initiation either in the dovetail region of the blade or in the rim slot surface of the disc, which drastically reduces the fatigue life of the blade or the disc. In extreme situations, this phenomenon may cause premature failure of the blade or the disc.

U.S. Patent 5,106,243 describes the use of a metallic reinforced shim or a multi-layer shim for improving fretting fatigue resistance of turbine engine blades and rotors. U.S. Patent 5,240,375 (General Electric Co.) presents the use of a multilayer clad shim for the same purpose. However, these two methods deal with a freely inserted shim. Relative movement between the shim and the component contact surfaces still exists, which eventually causes fretting damage accumulation. U.S. Patent 5,026,122 discloses the use of an intermediate disc in a wheel of a commercial vehicle to avoid fretting corrosion in the support region of the wheel disc. The intermediate disc can be solidly attached to the wheel disc by way of adhesive, by friction setting or by casting. The intermediate disc serves as an intermediate layer between the wheel and the hub flange (or a brake drum) which are bolted i.e. fastened together. As a result, there is a "permanent fit" between the components and no high cyclic stresses exist.

The latter invention does not address the problem of fretting fatigue and corrosion in tight-fit or close-fit (not fixed) components which are subjected to cyclic stresses, such as blades/disc slots in turbine engines etc.

Various anti-fretting solutions are proposed in U.S. patents Nos. 5,236,788; 5,312,696; and 5,292,596. Among these, certain coatings are applied onto the surface of a component subject to fretting corrosion. Such coatings, applied e.g. by vapor deposition, plasma spraying etc. affect the surface of the component.

SUMMARY OF THE INVENTION

In turbine engines, fans, compressors etc, it is customary to install blades as separable, detachable components to be installed slidably with a tight fit, or close fit, in disc slots. Total immobilization of blades is possible, but difficult and often not economical.

Inevitably, the resulting assembly, in operation, is subject to high cyclic stresses resulting in fretting wear and corrosion. It has been determined that a significant improvement in the durability and fretting fatigue life of such detachable components and their assembly can be achieved if the contact surface of at least one of the components, e.g. the root of the blade or the corresponding surface of the disc slot, is provided with an anti-fretting barrier of the present invention, the barrier comprising a shim shaped to fit at least part of the contact surface of the component subject to fretting wear or fatigue, and a layer of an adhesive, or generally speaking a bonding agent, for bonding said shim to the surface. A cold worked low friction metallic shim (such as brass, bronze, Co-base, Ni- base or Fe-base materials) or a low friction nonmetallic composite shim (such as graphite epoxy) is bonded onto the working, or contact surface (meaning the surface subjected to fretting wear or fatigue) of a component with a layer of an adhesive (such as epoxy glue, epoxy film or ceramic adhesive) to form an anti-fretting barrier. This barrier is incorporated between the tightly fit components to reduce fretting- induced damage (fretting wear or fretting fatigue). The barrier, as mentioned above, is applied to at least one of the said contact surfaces of the components.

In one form, the said barrier is only applied to the contact surface of one component which suffers high cyclic stressing. Bonding the shim prevents fretting damage accumulation on the surface of the component. The shim is selected to have a low friction surface co-operating with the other component, and the low-friction nature of the shim significantly reduces the fretting damage on the contact surface of the other component.

In another form, the said barrier is applied on the contact surfaces of both components, which eliminates fretting damage on the component surfaces. The reduced fretting action is confined to the consumable shim surfaces. For both forms, any fretting crack initiated in the barrier will not propagate directly into the component, because of the nature of the adhesive and its (l°^w-^temP^erature) application, not affecting the physical structure nor the microstructure of the material of the underlying component.

For the purpose of this specification, "tightly fit" may be defined as a mechanical configuration of two components in which, under a normal pressure, a small amplitude oscillating slip (between 10 and 90 μm) exists between the components.

According to one aspect of the invention, a method of applying the said barrier to a component comprises applying a layer of an adhesive (bonding agent) to a clean surface of the component, placing a layer of shim on the adhesive and pressing the shim tightly against the component surface. The pressure can be introduced manually or with the help of an automated or a semi-automated facility (such as clamp, vacuum press or any other suitable methods). For certain types of adhesives, moderate heating which does not alter the microstructure of the component, may be needed to cure the adhesive to achieve the maximum shear strength. Alternatively, the bonding agent may be applied to the shim first and then the shim pressed onto the surface of the component.

The dimensions of the applied barrier should cover at least a large part of the contact (working) region of the component surface.

The component can be made of titanium, nickel and iron base alloys, or any other alloys or materials which are sensitive to fretting damage accumulation.

To achieve best improvement of fretting resistance, selection of the combination of shim/adhesive materials for an anti-fretting barrier will depend on the component materials, loading conditions and service temperatures. For example, a brass shim plus epoxy adhesive may be suitable for use in low-to-moderate temperature applications, while a Stellite 6B (Co-base alloy) shim plus metallic adhesive may be used for elevated temperature applications (say, typically in the range about 500°C - 850°C).

The invention prevents the working surface of a component from accumulating the fretting damage by applying a non-detachable anti-fretting barrier (bonded shim) onto it. Compared to a freely inserted shim, bonding a shim onto the component surface ensures that the shim remains in place and eliminates any relative movement between the shim and the component surface. As a result, little fretting damage is produced on the said component surface. The fretting damage is restricted to the consumable shim only. Furthermore, if a crack is initiated and propagates in the shim material, it may not propagate into the component due to the discontinuity introduced by the adhesive layer; the anti-fretting barrier, due to the use of an adhesive, or a bonding agent which does not affect the microstructure of the underlying surface, also has a crack-arresting effect. This invention also permits the use of other conventional surface modification techniques for improving the fatigue resistance of said components. The fatigue life of a component may be further improved by applying surface hardening (e.g. shot peening), soft coating (e.g. CuNiln coating), solid lubricant selected from the group consisting of dichalcogenides of molybdenum (e.g. MoS_∑), dichalcogenides of tungsten (e.g. WS^, graphite, and fluorides of alkali metals (e.g. CaFj) or other state-of-the art techniques (such as ion implantation or laser shock hardening), to the component or shim surface, in combination with the anti-fretting barrier.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.1 is a graph showing the effects of an anti-fretting barrier according to the invention on the fretting fatigue life of a base and shot peened Ti-6A1-4V component;

Fig.2 is a schematic drawing of an anti-fretting barrier of the invention bonded to the blade dovetail surface of a blade/disc dovetail configuration assembly;

Fig.3 is a schematic drawing of the anti-fretting barrier bonded to the disc rim dovetail surface of a blade/disc dovetail configuration assembly;

Fig.4 is a schematic drawing of the anti-fretting barrier bonded to both the blade dovetail surface and the disc rim dovetail surface of a blade/disc assembly;

Fig.5 is a schematic drawing of the anti-fretting barrier bonded to the blade serration surface of a blade/disc serration configuration assembly;

Fig.6 is a schematic drawing of an anti-fretting barrier bonded to the disc rim serration surface of a blade disc serration configuration assembly; and

Fig.7 is a schematic drawing of an anti-fretting barrier bonded to both the blade serration surface and disc rim serration surface of a blade/disc assembly. DETAILED DESCRIPTION OF THE INVENTION

The anti-fretting barrier of the present invention is useful for reducing fretting damage for instance in gas turbines (fan, compressor and turbine blade and disc dovetail or serration surfaces). In addition, the barrier could find a wide range of applications for improving the fretting fatigue and fretting wear resistance of components in other industries, such as automobile, power generation and any machinery or mechanical equipment where fretting fatigue and fretting wear contribute to deterioration and failure of parts.

Figure 1 shows the effects of an anti-fretting barrier of the invention (termed "bonded shim") on the fretting fatigue life of base and shot peened Ti-6A1-4V material (6% Al, 4% V, balance titanium, all by weight). Bonding a low friction brass shim onto a T.-6A1-4V specimen surface with epoxy glue eliminates the fretting action between the shim and the specimen surface, which results in a dramatic increase in the fretting fatigue life of the specimen when compared to the fretting fatigue life of the specimen with a freely inserted brass shim (General Electric Co.), or to base component (no shim), with or without shot peening. The fretting fatigue life of the base T.-6A1-4V material with the said barrier (brass shim bonded with epoxy glue) is considerably longer than the T.-6A1-4V specimens subjected to a shot peening treatment (a process commonly used for improving the fretting fatigue resistance). When applying the said barrier on the shot peened T.-6A1-4V, the fretting fatigue life is further improved.

The shim may be made of conventional metals, a low friction nonmetallic composite material consisting of graphite, boron or glass fibers, and epoxy adhesives; or from Ag-base alloys. In the embodiment of Fig. 1, the shim was a cold-rolled alpha brass shim (70 wt. % Cu and 30 wt. % Zn) supplied by Paxam Limited. The thickness of the shim was about 0.004" (0.10 mm).

The bonding agent may be selected from a group of metallic adhesives consisting of aluminum, nickel and stainless steel composite, or from a group of ceramic adhesives based on aluminium oxide, zirconium oxide, silicon oxide, or magnesium oxide, said adhesives optionally including other metalolic or non-metallic agents, e.g. graphite. In the embodiment of Fig. 1, the adhesive was a 5 MINUTE EPOXY made by LEPAGE LIMITED. The glue consists of epoxy resin and hardener. The two parts were mixed together at a ratio of 1: 1 before use. The layer was cured at room temperature. The thickness of the adhesive layer was about 0.002"-0.003" (0.051-0.076 mm).

Exemplary embodiments of the invention are described and illustrated as follows:

Example 1.

As shown in Fig. 2, a blade/disc dovetail configuration assembly for a turbine engine has a disc 10 having a dovetail slot 12 in its rim, a blade 14 having a dovetail sidewall root 16 sized to fit the disc dovetail surface, and an anti-fretting barrier bonded to the blade dovetail sidewall surface 16, the anti-fretting barrier including a shim 18 shaped to fit the blade dovetail sidewall surface 16 and a layer 20 of an epoxy adhesive to bond the shim 18 onto the blade dovetail sidewall surface 16.

Example 2.

As shown in Fig. 3, a blade/disc dovetail configuration assembly for a turbine engine has a disc 10 having a dovetail slot 12 in its rim, a blade 14 having a dovetail sidewall root 16 sized to fit the disc dovetail surface, and an anti-fretting barrier bonded to the disc rim dovetail surface 12, the anti-fretting barrier including a shim 18 shaped to fit the disc rim dovetail surface, and a layer 20 of an adhesive to bond the said shim onto the disc rim dovetail surface.

Example 3.

As shown in Fig. 4, a blade/disc dovetail configuration assembly for a turbine engine has a disc 10 having a dovetail slot 12 in its rim, a blade 14 having a dovetail sidewall root 16 sized to fit the disc dovetail surface 12; and an anti-fretting barrier bonded to both the blade dovetail sidewall surface and the disc rim dovetail surface, the anti-fretting barrier including shims 18, 19 shaped to fit the blade dovetail sidewall surface and the disc rim dovetail surface respectively, and layers 20, 21 of adhesive to bond the shims 18, 19 to the blade dovetail sidewall surface and the disc rim dovetail surface respectively.

Example 4.

As shown in Fig. 5, a blade/disc serration configuration assembly for a turbine engine has a disc 10 having a serration slot 22 in its rim; a blade 24 having a serration sidewall root 26 sized to fit the disc serration slot 22; and an anti-fretting barrier bonded to the blade serration sidewall surface of the root 26, the anti-fretting barrier including a shim 18 shaped to fit the blade serration sidewall surface, and a layer 20 of adhesive to bond the shim 18 onto the blade serration sidewall surface.

Example 5.

As shown in Fig. 6, a blade/disc serration configuration assembly for a turbine engine has a disc 10 having a serration slot 22 in its rim, a blade 24 having a serration sidewall root 26 sized to fit the disc serration slot 22, and an anti-fretting barrier bonded to the disc rim serration surface of the slot 22, the anti-fretting barrier including a shim 18 shaped to fit the disc rim serration surface, and a layer 20 of adhesive to bond the shim 18 to the disc rim serration surface.

Example 6.

As shown in Fig. 7, a blade/disc serration configuration assembly for a turbine engine has a disc 10 having a serration slot 22 in its rim, a blade 24 having a serration sidewall root 26 sized to fit the disc serration slot 22; and an anti-fretting barrier bonded to both the blade serration sidewall surface of the root 26 and the disc rim serration surface of the slot 22 respectively, the anti-fretting barrier including shims 18, 19 shaped to fit the blade serration sidewall surface and the disc rim serration surface respectively, and layers 20, 21 of adhesive to bond the shims 18, 19 to the blade serration sidewall surface and the disc rim serration surface respectively.

Table 1 corresponds to Fig. 1 and lists numerically the fretting fatigue data of the base and shot peened T.-6A1-4V materials with and without the anti-fretting barrier of the invention (termed "bonded shim"), in comparable conditions. It will be seen also that the use of the anti-fretting barrier of the invention improves fretting fatigue life in a dramatic manner compared to the prior art freely-inserted shims e.g. of the General Electric Co. patent referenced herein.

Although the invention has been described with reference to the above specific example embodiments it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims:

Table 1. Effect of bonded shim on the fretting fatigue life of T1-6A1-4V.

Conditions Specimen No. Fretting Fatigue Life Average Fretting Fatigue Life (cycles to failure) (cycles to failure)

BB1 66,416

BB2 65,083

Base Material BB3 57,749 67,100

BB4 79,232

SH2 190,886

Shim Only SH17 103,443

SH18 98,297 131,800

SH21 134,480

SH3 1,374,220

Bonded Shim SH19 5,881,242 4,113,000 (Invention)

SH20 5,083,438

SP11 1,817,760

Shot Peening Only SP14 1,993.280

SP15 1,416,030 2,272,800

SP17 3,864,240

Shot Peening Plus SH4 >5,5B5,280 Bonded Shim >5,263,900

SH7 >4,942,526

Claims

What we claim is:

1. A method of reducing fretting or sliding wear between contacting surfaces of components subject to cyclic stresses, the method comprising the following steps:

(a) cleaning at least one of said contacting surfaces to remove any rust and dirt, if necessary,

(b) applying a low-friction shim onto said at least one contacting surface, and

(c) bonding said shim to said contacting surface by means of a bonding agent, said bonding agent being selected such as not to impair the structure of said surface, in a manner to form a non-detachable anti-fretting barrier resistant to working conditions of said component.

2. The method of claim 1, wherein said cleaning step comprises degreasing said surface.

3. The method of claim 1, wherein said bonding agent is selected from a group of metallic adhesives consisting of aluminum, nickel and stainless steel composites.

4. The method of claim 1, wherein said bonding agent is selected from a group of ceramic adhesives based on aluminium oxide, zirconium oxide, silicon oxide, or magnesium oxide, said adhesives optionally including other metallic or non-metallic agents.

5. The method of claim 1, wherein said shim is made of low friction nonmetallic composite material consisting of graphite, boron or glass fibers, and an epoxy adhesive.

6. The method of claim 1, wherein said shim is made of an Ag-base alloy.

7. The method of claim 1 wherein said contacting surface of said component is shot peened before step b).

8. A structural component for use in an assembly comprising two slidably contacting components which are subject to fretting or sliding wear, said component comprising an anti-fretting barrier which comprises

(a) a shim shaped to fit a contact surface of said component, and

(b) a layer of a bonding agent between said shim and said contact surface for non-detachably bonding said shim to said contact surface.

9. The component of claim 8 wherein the outer surface of said shim is coated with a soft CoNiln coating.

10. The component of claim 8 wherein the outer surface of said shim is coated with a solid lubricant selected from the group consisting of dichalcogenides of molybdenum, dichalcogenides of tungsten, graphite, and fluorides of alkali metals.

11. An assembly comprising two detachable components subject to fretting or sliding wear due to cyclic stresses, wherein at least one of said components of said assembly comprises an anti-fretting barrier comprising

(a) a shim shaped to fit a contact surface of said component, and

12. The assembly according to claim 11 wherein both detachable components comprise said anti-fretting barrier.

13. The assembly of claim 11, wherein said assembly is a blade/disc assembly wherein said contact surface is a blade dovetail surface or a disc rim dovetail surface respectively.

14. The assembly of claim 11, wherein said anti-fretting barrier is bonded to both the blade dovetail surface and the disc rim dovetail surface respectively.

15. The assembly of claim 11, wherein said assembly is a blade-disc assembly wherein said contact surface is a blade serration surface or a disc rim serration surface respectively.

16. The assembly of claim 15, wherein said anti-fretting barrier is bonded to the disc rim serration surface.

17. The assembly of claim 15, wherein the said anti-fretting barrier is bonded to both the blade serration surface and the disc rim serration surface respectively.