An investigation of the fretting wear of two aluminium alloys

An investigation of the fretting wear of two aluminium alloys

Tribology ELSEVI :R SCIENCS’ 0301-679X(95)00118-2 is,;mational Vol. 30, No. 1, pp. l-7. 1997 Copyright 0 1996 Elsevier Science Ltd Printed in Grea...

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Tribology

ELSEVI :R SCIENCS’

0301-679X(95)00118-2

is,;mational

Vol. 30, No. 1, pp. l-7. 1997 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0301-679X/96/$15.00 +O.OO

investigation of t e fretting ar of two aluminium alloys 2. R. Zhou*t,

S. R. Gut

and L. Vincent*

Frett ng fatigue is one of the most detrimental loadings for crack nucleation. Aeronautical aluminium alloys have been analysed during the first cycles of fretting so to determine the evolution of the maximum tangential force and the coefficient of friction. Several stages have been identified as velocity accommodation takes place in the superficial oxide layers, in the two first bodies or in the third body (debris bed). The effects of the test frequency or of the surface roughness were shown to depend on these stages The effect of the brittleness of the first body has been analysed by means of the comparison between AI-Li and AI-Si alloys. The latter brittle alloy better resisted crack nucleation despite its lower fatigue strength. This could be related to the early detachment of debris in the fretting contact. Copyright 0 1996 Elsevier Science Ltd Keywords:

fretting,

aluminium

alloys,

coefficient

of friction,

Introduction Fretting is used by engineers to describe the action of two contacting surfaces which are pressed together under a normal clamping force and which undergo a cyclic tangential shearing causing a relative movement on the two surfaces. This movement is of a low order of magnitude and fretting is manifest in many contact components. In fretting wear, this relative displacement is always imposed while in fretting fatigue it is essent-ially a deformation induced by the oscillating external load’,“.

cracks

associated with three kinds of material response: (no degradation (ND); cracking (C); particle detachment (PD) (Fig lb)). C rat k’m g. m d uced by fretting is clearly the most dangerous degradation mechanism, especially in the case of the existence of external fatigue. The ratios between the maximum tangential force F,,,, and the normal force Fn are higher in the mixed regime than in the partial slip regime and in the slip regime4. Cracking first appears in the mixed regime but the stress level can remain high enough to nucleate cracks in the gross slip regime even after debris has been formed.

Previous analyses of fretting wear have resulted in introducing two kinds of fretting maps3,4: Running Condrtion Fretting Maps (RCFM) which have been associated with three fretting regimes (partial slip, mixec fretting, and slip (Fig la)), and Material Response Fretting Maps (MRFM) which have been

* Tribc!ogy Laboratory, Southwest Jiaotong University, 6IOO31, Chengdu, Peoples’s Republic of China i Department of Materials Science and Engineering, Tsinghua Univemify, 100084, Beijing, Peoples’s Republic of China $ D&urtement de Matkriaux-MCcanique Physique, URA CNRS 447, Ecole Centrale de Lyon, 69131, Ecuky ctd>x,‘France Received 10 January 1994: revised 12 September 1995; accepted II Decemoer 1995

DC+/-pm)

D(+/hm)

Fig 1 Two kinds of fretting (a) RCFM; (b) MRFM

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maps on the Al-Li

30 Number

1 1997

alloy:

1

Fretting

wear

of aluminium

alloys:

Z. R. Zhou

et al. Table 2 Chemical Material

Li

AI-Li AI-Si

1.9

Si

analysis Cu

Fe Mn

Na

Mg

Zn

0.032.1 0.05 0.03 1.6 0.03 21.4 2.08 0.87 0.55 0.05

The experimental following ranges:

parameters

were

controlled

imposed displacement amplitude D: from to 100 pm; l normal load F,: from 100 N to 1000 N; l frequency f: 1, 5 and 12.5 Hz; 0 number of cycles N: from 1 to 5 X 104. l

Fig 2 The fretting

rig schema

This paper deals with the analysis of the variation in the maximum tangential force depending on experimental parameters at the different fretting stages under the gross slip regime of two aluminium alloys.

In the case of the gross slip regime the fretting was divided into four stages:

A sphere on plane configuration was employed in the fretting tests with a sphere radius of about 1 m. Two values of the plane asperity were tested. One set of surfaces were polished with diamond paste (6 and 3 pm), finished with alumina powder (0.1 pm) and then ultrasonically cleaned in pure alcohol (maximum roughness R, = 0.4 pm). The second batch was only hand-lapped to 400# sand paper and cleaned with alcohol before testing (maximum roughness R, = 4 pm). The two contacting surfaces were made of the same alloy (homogeneous contact) and the sphere roughness was constant (R, = 0.4 km).

first fretting stroke; transition leading to particle detachment i.e. transition from two to three body contact; o particle detachment; o steady state or third body accommodation. l

First

stroke

This friction stage concerns the very first stroke. Classically, the calculation of the coefficient of friction for any peculiar pair of metallic surfaces depends on the surface roughness, on deformation properties and on interfacial shear strengthsx9. The influences of mechanical and material parameters on the coefficient of friction are discussed below.

Ff IN1

Two aluminium alloys were used. Their main mechanical properties and chemical compositions are listed in Tables 1 and 2.

P 0.4

500 t

t

properties 50

UTS (MPa)

AI-Li AI-Si 2

520 490 Tribology

process

l

The fretting rig is a tension compression hydraulic machine. Reciprocating movement of a given shape and amplitude is applied to the first specimen (2) (curvature R = 1 m) which is connected to the piston. The second {I} is fixed to the framework via a movable trolley enabling the application of the normal load F, (Fig 2).

Material

*15 pm

Results

procedure

Table 1 Mechanical

in the

During the fretting test, the tangential force F, variation versus the instantaneous displacement was recorded for every cycle. The results will be discussed using an analysis of the maximum tangential force F,,, and of the ratio p = Ftmax /F,,. This ratio is equal to the coefficient of friction in the case of the slip regime. The fretting scars on the surfaces were examined with optical or scanning electron microscopes.

Contact stresses and skin tangential traction stress determine crack nucleation with respect to the nucleation resistance of the upper layers4. They strongly depend upon the coefficient of frictions-‘. Therefore, knowledge of the coefficient of friction is required to predict the critical fretting condition leading to the crack nucleation.

Experimental

Ti

A%

Hv

14 0.8

200 160

-

(rk, 425 410

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loo

50

+/-

3Epm

-

-

+/-

100

lE0pm

Fig 3 Variation of the tangential force F, and the friction coefficient p with the displacement amplitude in the case of the AI-Li alloy, F, = 500 N, f = 1 Hz, R, = 0.4 pm 1 1997

Fretting Sphere contact

(R, = 0.4 pm)-plane

(R, = 0.4 pm)

In ttis study, the effect of parameters such as displacement amplitude, normal load, frequency and aluminium alloy type were not obvious. Examples of the dynamic variation of the tangential force and of the friction coefficient versus the imposed displacement for tEe first pass and two fretting amplitudes are shown in the Fig 3. It was generally observed that the coefficient of friction depends slightly upon these parameters. This result corresponded with statistical calculation4. Values were found in the range of 0.2 and 0.35 for several alumillium alloys. Sphere

(R, = 0.4pm)-plane

(R, = 4pm)

wear

of aluminium

alloys:

Z. R. Zhou

et al.

mean pressure value of 75 MPa), the coefficient of friction rose markedly as shown in Fig 4. On the other hand, identical friction coefficients were obtained for normal loads of more than 250 N. Plastic deformation can play an important role as discussed in the next part. A great effect of frequency was also noted in the case of the low normal load values. The coefficient decreased with the frequency. Figure 5 respectively shows p values of about 0.7 and 0.15 for frequencies of 1 Hz and of 12.5 Hz. The relative sliding speed depends on the displacement amplitude and on the frequency of the oscillatory

contact

The direction of fretting tests was parallel to the polishing direction of the plane. The influence of the contact load here appeared strong. When the normal force was less than 150 N (which corresponded to a

Fig 7 F,-D curves for the 10 first F, = 500 D, D = k25 pm: (a) Al-S; 125

25

12.5

fretting cycles, (b) Al-Li

25

Fig 4 [email protected] of roughness on the maximum tangential and the friction coefficient. AI-Li alloy, force F, = 1.50 N, D = &25 pm

AI-Li

I.

N (cycle) 2

4

6

8

10

Fig 8 Variation in friction coejj‘icient vs the number of fretting cycles. F, = 500 N, D = +25 pm 125

25

12.5

2s

Fig 5 influence of the frequency on the maximum tangential force and on the friction coefJicient. Al-Li alloy, IF, = 1.50 N, D = &25 pm, R, = 4 pm

~‘5 La) Fn=lSLl

2i (b)

N

Fn=SOO

N

Fig 6 Il’nfluence of the normal load (a, b) on the F,--D curves during the first cycle. Al-Li alloy, f = 1 Hz, D = +25 pm, R, = 0.4 Frn

Fig 9 Wear scar on the contact surface after 10 cycles. F, = 500 N, D = k25 p, Al-Li Tribology

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Z. R. Zhou

et al. movement. However, for the same sliding speed, the friction coefficient in the case of low amplitude (e.g. 10 pm) with a high frequency (e.g. 5 Hz) differed from that of high amplitude (e.g. 50 pm) with a low frequency (e.g. 1 Hz). Velocity accommodation appeared to be different with the modification of the displacement amplitude and of the frequency. Transition

2 mm

I

The friction mechanism in this stage appeared complex because of new surface geometries and material strain hardening phenomena. The maximum tangential force increased from the first stroke due to local asperities or to more or less global (sub-surface) cyclic plastic deformation. By comparison, this increase was more obvious under the higher normal load or for smoother surfaces. Figure 6 indicates that the difference d(F,) at the end of the first cycle was greater in the fretting test for a normal force of 500 N than that for a normal force of 150 N.

I

Fig 10 Wear scars on the contact surface after 10 cycles, F, = 1000 N, D = 5100 pm, Al-Li

-25

Fig 11 Variation in the tangential stage for the Al-Si alloy

Whatever the normal load or the surface roughness, the cyclic plastic deformation effect is much less sensitive for the aluminium silicon alloy due to its high brittleness (elongation to failure A% = 0.8%). The F,-D curve thus appeared to join at the end of the first cycle (dF, = 0), which was not obtained in the case of the aluminium-lithium alloy (Fig 7). For fretting cycles between 1 and 10, the tangential force rapidly increased for the Al-Li alloy due to cyclic work hardening, while it slowly increased for the AlSi alloy (Fig 7) due to the influence of the surface geometry (increase in wear scar dimensions) only. The first parallelepipedic F,-D curves are not closed until after a number of cycles. This depends on experimental parameters (normal load, etc.), especially for the aluminium lithium alloy.

25

D (pm1

force

in the wear

2mm

I

I

Fig 12 Surface observation 4

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after 1000 cycles. F, = 500 N, D = k50 pm Al-Li Volume

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Fretting

When considering friction coefficient number of cycles coefficient of the for tke Al-S1 alloy

wear

of aluminium

alloys:

2. R. Zhou

et al.

Surface observations of the aluminium lithium samples have revealed two different wear scars after 10 cycles. Low normal force and low displacement amplitude induced small multi-wear scars (Fig 9), but with high normal force and high displacement, wear scars were very large and deep (Fig 10). These scars were related to the local cyclic plastic deformation.

the tangential force at D = 0, the evolution as a function of the is shown in the Fig 8. The friction Al-L1 increased more rapidly than from identical initial values.

Wear

stage

The so-called wear stage is related to the particle detachment process. Particles can be detached by several mechanisms. Tangential force was suddenly released during sliding when a particle formed. For the ductile Al-Li alloy, the tangential force may decrease considerably’ due to large particle detachment (resulting from the intense cyclic plastic deformation zone) near the maximum or minimum displacement point of one cycle. On the other hand, for the brittle aluminium silicon alloy, the tangential force decreased but was repeated many times during the sliding part

N= 5~10~ Fi fN) N=103

Fig 1.’ F,-D czlrves in the steady stage for the AI-Si alloy. F, = 1000 N, D = ‘3.5 pm

+/-I

00 urn

750

750

+/-SO pm Ft (N1

Ft IN1

-750

-750

-loo

D (pm)

loo

-2.5

(al Fig 14 F,-D

0 (pm)



(b)

curves after 5 x 10” cycles for both alloys: (a) effect of displacement; (b) effect of frequency

Mounting

\ Black debris

Fig 15 Crack formation under the slip regime for Al-Li

alloy. F, = 1000 N, N = 106 cycles

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of each cycle (Fig 11). This means that small particles detached and this often repeated itself all along the sliding part. During this stage, new cyclic plastic deformation zones nucleated and particles could detach from these zones. The coefficient of friction strongly varied as a function of the displacement for the Al-S1 alloy and of the fretting cycles for both alloys. Figure 12 is an example of the surface degradation after 1000 cycles for the Al-L1 2091 alloy. The contact surface was completely degraded, and ejected particles (i.e. wear particles) could be observed at the contact edge. Steady

stage

In this stage, the first two bodies were totally separated by a third body (powder bed). The contact was described by the kinetics of the debris formation, the ejection rate and the debris morphologyiO,il. As compared with the cycles of the wear stage, a low

750 N T

N 10’

10‘2

10‘3

-

Fig 17 Variation in the ratio F., D = 22.5 Frn, F, = 500 N, f = 1 Hz: (a) for Al-Li alloy; (b) for AlSi alloy increase in the tangential force at the end of each cycle was obtained for the aluminium lithium alloy and discontinuities in the F,-D curves disappeared for the aluminium silicon alloy (Fig 13). Cross-section analyses often revealed a thick layer of black debris due to the oxidation of the crushed metallic detached particles3. Effects of experimental parameters were tested in the slip regime after 5 x lo4 cycles. The effect of the displacement and of the frequency on the coefficient of friction appeared much lesssensitive (Fig 14). Small discontinuities were still registered in the case of the lower frequency.

-750 N

Synthesis

(a)

For the Al-Li alloy, crack nucleation was noted beneath the worn zone of the contact surface for the highest number of fretting cycles (Fig 15). On the other hand, the same experimental condition did not nucleate cracks in the aluminium silicon alloy despite its lower fatigue resistance.

750 N

Friction logs showed that the tangential force always remained low in the case of the Al-Si alloy (Fig 16). The ratio p of the maximum tangential force over the normal load reached smaller values in the case of the Al-Si alloy (Fig 17).

-750 N

(b)

Fig 16 Examples of friction logs, D = +25 pm, F, = 500 N, f = 1 Hz: (a) for the Al-Li alloy; (b) for Al-Si alloy 6

All the results were obtained in the case of the gross slip regime. In fact, two other fretting regimes were identified in the running conditions fretting maps3. The regime moved from partial slip, to mixed and slip with the increase in displacement for a given normal load or with the decrease in normal load for a given amplitude. The two other fretting regime cases are not detailed here.

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The Al-S1 alloy only suffered a small amount of cyclic plastic deformation before an easy and rapid detachment of particles. The displacement was then accommodated in the debris bed (third body) which diminished the coefficient of friction, reduced the stress .field (although a theoretical calculation is impossible due to insufficient knowledge of rheological properties and the accommodation site in the third body), and prevented any crack formation. Conse-

Volume 30 Number 1 1997

Fretting quenly, from the effect of the third bodies, a brittle mate:-ial appears to be more suitable to prevent cracking under fretting wear owing to the rapid contact surface degradation which thus protects the volume and prevents the p ratio on which the local stress applied at the contact edges depends in order to reach high values.

(1) (2)

(3)

of aluminium

1. Waterhouse

R.B.

Fretting

Corrosion,

2. Waterhouse 1981

R.B.

Fretting

Fatigue,

Z. R. Zhou

et a/.

Pergamon, Applied

Oxford,

Science,

1972 London,

3. Zhou Z.R., Fayeulle S. and Vincent L. Cracking behaviour various aluminium alloys during fretting wear. Wear 1992, 317-330 4. Zhou Z.R. Fissuration cation au cas d’alliages ECL de Lyon, France,

the slip regime of alloys, the following

The coefficient of friction mainly depends on initial surface roughness and contact pressure. The ductility of the metal is an important factor under high friction. The coefficient of friction rapidly increases with increasing work-hardening due to the cyclic plastic deformation induced in The upper layers. (Compared to the aluminium lithium, the aluminium silicon resists better against the fretting cracking owing to a lower coefficient of friction at all the fretting stages.

alloys:

References

Conclusion From experimental results in fretting wear for two aluminium conclusions were established:

wear

5. Mindlin A.S.M.E.,

R.D. Compliance .I. App. Mech.,

6. Hamilton a circular 7. Johnson 1985 8. Kayaha growth 9. Tahor years.

induite par d’aluminium 1992

Tribology

petits debattements: appliaeronautiques. T&W de

of elastic bodies 1949, 16 259-268

in contact.

Trans.

G.M. and Goodman L.E. The stress field created sliding contact. .I. Appl. Me&. 1966, 371-376 K.L. T. with

Contact

Mechanics,

and Kato a junction

Cambridge

Y..MCcanisme 1988

International

University

by

Press,

K. Experimental analysis of junction model. Wear, 1978, 51, 105-116

D. Friction and wear-developments IMechE 1987, C245187, 157-172

10. Godet M. The third body Wear 1984, 100, 437-452 11. Berthier France,

of 155,

approach: et tribologie.

Volume

over

a mechanical Thbe

30 Number

the last view

de INSA

fifty

of wear. de Lyon,

1 1997

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