1989-microstructure and wear of cast (al-si alloy)-graphite composites

7/21/2019 1989-Microstructure and Wear of Cast (Al-Si Alloy)-Graphite Composites

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Wear, I33 (1989) 173 - 187

173

MICROSTRUCTURE AND WEAR OF CAST Al-Si ALLOY)-GRAPHITE

COMPOSITES*

S. DAS and S. V. PRASAD

Regional Research Laborator y Council of Scienti fi c and Industri al Research), Bhopal

462026 M .P.) I ndi a)

T. R. RAMACHANDRAN

Indi an Insti tut e of Technology, Kanpur 208016 U.P.) Indi a)

Summary

Graphite-particle-dispersed Al-Si alloys have potential for a variety of

antifriction applications. In the present investigation, two Al-Si alloys

LM13 of near eutectic and LM30 of hypereutectic composition) were

chosen as matrix alloys and composites were prepared by casting. Compos-

ites and matrix alloys were heat treated to produce different morphologies

of silicon ranging from plate-like in die-cast alloys to near spherical in heat-

treated alloys.

Wear tests were conducted, under both dry and partially lubricated

conditions, with SAE30 oil on a pin-on-disc wear test apparatus, against a

rotating steel EN25) counterface. In partially lubricated wear tests, the

sliding velocity V was varied from 1.4 to 4.6 m s- and the applied pressure

P from 1 .O to 5 .O MPa. P-V limits of all matrix alloys and composites with

different microstructures were evaluated. Heat-treated composites were

found to possess superior wear properties wear rate, seizure resistance and

P-V

limits) as compared with those of die-cast composites and matrix

alloys. Worn surfaces of heat-treated composites showed the presence of a

graphite film while those of die-cast alloys and composites showed surface

fracture. The role of graphite particle dispersion and the morphology of

silicon on the sliding we::r behaviour is discussed.

1. Introduction

Al-S1 alloys are extensively used in tribological applications such as

pistons and in some cases as cylinder liners) in internal combustion engines.

Although Al-Si alloys meet many of the requirements for such applications,

their poor resistance to seizure makes them vulnerable under poor lubri-

cating conditions. To overcome this problem, several investigators have

*Paper presented at the International Conference on Wear of Materials, Denver, CO,

U.S.A., April 8 - 14, 1989.

0043-1648/89/ 3.50

@ Elsevier Sequoia/Printed in The Netherlands


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dispersed solid lubricant particles such as graphite in Al-Si alloy matrices

[l - 41. Techniques to produce Al-Si alloy graphite composites by various

casting routes have also been developed in recent years [ 51.

There have been a number of reports describing the sliding wear behav-

iour of aluminium alloy-graphite particle composites under dry [ 3, 6, 7 1 as

well as lubricated [ 1, 2, 4, 8, 91 conditions. Studies have shown that the

spreadability of lubricating oil such as SAE30 on Al-S1 alloys increases with

the increasing percentage of graphite dispersion [lo]. The coefficient of

friction of these composites against rotating steel discs and the temperature

rise of the test pin during the pin-ondisc wear test were reported to decrease

with increase in graphite content [3]. However, there have been some

conflicting reports on the dry sliding wear behaviour of aluminium alloy -~

graphite composites. Biswas and Pramila Bai [6] reported that Lhe dry

sliding wear of Al-Si alloy composites containing 2.7 and 5.7 wt.% graphite

particles was found to be higher than that of the matrix alloy. Similar

findings were reported by Gibson

et al.

[3] for composites with a higher

(greater than 8 wt.%) graphite content. The loss in composite tensile strength

and ductility associated with graphite particle addition was believed to be

the cause for such anomalous behaviour. One way to solve this problem is

through proper control of the matrix microstructure.

(Al-Si alloy)-graphite is essentially a three-phase composite consisting

of silicon (primary and/or eutectic), a-aluminium and graphite. The micro-

structure of the matrix (i.e. the size and morphology of the silicon phase)

controls the mechanical properties as well as the wear behaviour of the

composite material. This study was therefore aimed at understanding the

role of matrix microstructure on the wear behaviour of two Al-Si alloys

containing dispersed graphite particles. One material, LM13, is a standard

piston alloy whereas the second, LM30, has a potential for use as cylinder

liners in internal combustion engines in place of much heavier cast iron.

Wear studies in dry and partially lubricated conditions were conducted

under a wide range of loads and sliding speeds.

2. Experimental details

2.1. Material preparation

The two matrix alloys chosen for the present investigation are (i) a

near-eutectic Al-Si alloy (B.S.LM13) and (ii) a hypereutectic Al-Si alloy

(B.S.LM30). The chemical compositions of the alloys are shown in Table 1.

Composites containing 3 wt.% graphite particles (63 - 120 pm) were pre-

pared by the vortex technique which is described fully elsewhere [ll].

Briefly, the various steps involved are melting the alloy, creating a vortex

by mechanical stirring and casting the composite melt into a metallic mould.

To increase the wettability of graphite particles by the Al-Si alloy, magne-

sium (1 wt.%) was added to the melt prior to graphite particle dispersion.


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176

Pressure on the specimen was increased in steps of 0.5 MPa until the speci-

men seized before a sliding distance of 500 m was reached. The onset of

seizure was signalled by a sudden increase in test pin temperatures followed

by vibration and noise from the disc-pin assembly.

2.3.2. ubricated sliding wear

Partially lubricated we.3 tests were performed on the same apparatus

but using a modified procedure. In this case, the steel disc (EN25) was first

dipped in SAE30 lubricating oil. The excess oil was spun off the disc by

.otating it for 5 s at 640 rev mm-l before the commencement of the actual

test.

All lubricated wear tests were carried out at sliding distances up to

2500 m Disc rotation was first fixed at 330 rev min (corresponding sliding

velocity, 1.38 m s-i). A pressure of 1 .O MPa was applied to the specimen and

the test was run for 2500 m. If the sample did not seize at this pressure the

disc was removed, cleaned, reimmersed in SAE30 oil, excess oil removed and

the applied pressure was increased in steps of 0.5 MPa. By this means a

critical applied pressure was determined where seizure occurred within a

sliding distance of 2500 m.

2.4. Microscopy

Samples for microscopic examination were prepared by standard

metallographic procedures, etched with Kellers reagent and examined by

both optical and scanning electron microscopy, the latter equipped with a

wavel~n~d~persive X-ray spectrometer capable of detecting carbon.

Both worn surfaces and wear debris were examined in the scanning electron

microscope. Debris was gold coated prior to examination.

3. Results

3.1.

Microstructure

The microstructures of diecast

(Al-Si

(LM13) alloy)-graphite particle

composites are shown in Fig. 2. Figure 2(a) shows the distribution of gra-

phite particles in the Al-Si alloy matrix while Fig. 2(b) shows the matrix

mi~rost~cture. The microst~cture immedia~ly su~ounding the dispersed

graphite particles (Fig. 2(c)) shows that the graphite particle was pushed into

the last freezing eutectic liquid. The microstructure of the Al-Si (LM13)

alloy matrix in the heat-treated condition is shown in Fig. 3. Clearly, the

heat treatment altered the morphology of the eutectic silicon from plate-like

to nearly spherical. The microstructure of the LM30-graphite particle

composite in the die-cast condition is shown in Fig. 4(a). Graphite particles,

primary silicon and eutectic silicon can be clearly seen. A typical matrix

microstructure of the heat-treated LM30 alloy (Fig, 4(b)) shows similar

changes in the morphology of eutectic silicon as in the case of LM13 alloy.


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(c)

Fig. 2. Microstructure of AI-S1 LM13) alloy-graphite particle composites in the die-

cast condition showing a) the distribution of graphite particles, b) a magnified view of

the matrix microstructure and c) matrix microstructure in the vicinity of the graphite

particle.

Fig. 3. Microstructure of Al-Si LM13) alloy matrix in the heat-treated condition.

3.2. Dry sliding wear

Dry sliding wear rates of die-cast alloys (LM13 and LM30) as a function

of applied pressure are shown in Fig. 5. It can be seen that the wear rates of

both the alloys increased with applied pressure although the wear rate is not


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(a)

(h)

Fig. 4. (a) Microstructure of LM30-~graphite composite in the die-cast condition showing

both primary and eutectic silicon, graphite particles and primary aluminium (P, primary

silicon; E, eutectic silicon; G, graphite). (b) Microstructure of LM30 alloy matrix in the

heat-treated condition. The change in the shape of eutectic silicon from needle-like to

nearly spherical should be noted.

ALMl3

OLM30

+ Seizure

Pressure.

Po

Pressure, MPo

Fig. 5. Effect of applied pressure on the dry sliding wear of Al-& alloys.

Fig. 6. Effect of applied pressure on the dry sliding wear rates of LM13 alloy and LMl3

graphite composites in the die-cast and heat-treated conditi.ons.

directly proportional to the applied pressure. For instance, the wear rate of

LM13 alloy was increased from 1.0 X lo--l2 to 2.5 x lo-l2 m3 m-l when the

applied pressure was increased from 1.0 to 1.5 MPa. Beyond this applied

pressure i.e. at 2.0 MPa), a drastic increase

in wear rate from 2.5

X lo-l2 to

17 X lo-l2 m3 m- was observed. The specimens were also seized at this

pressure before a sliding distance of 500 m was reached. By constrast, there

was no such drastic increase in the wear of LM30 alloy with increase in


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179

applied pressure. Secondly, the LM30 alloy seized at a much higher applied

pressure of 5.0 MPa.

Figure 6 shows the effect of graphite particle dispersion and heat

treatment on the wear rate of LM13 alloy at various applied pressures. Of

the four types of samples tested, the heat-treated LM13 graphite composite

showed lowest wear rates at all applied pressures. Secondly, the seizure

resistance (i.e. tie minimum pressure at which the sample seized) is the

highest for the heat-treated LM13 graphite composite. The seizure resistance

of LM30 alloy was not influenced by heat treatment and/or graphite particle

dispersion. However, heat-treated LM30-graphite composites showed the

lowest wear rates at all applied pressures (Fig. 7).

_I

I 2 3

4 5

c 20 60

Pressure MPa

T me, seconds

Fig. 7. Effect of applied pressure on the dry sliding wear of LM30 alloy and composites

in the die-cast and heat-treated conditions.

4MPa

3MPa

2 M30

Fig. 8. Temperature of the test pin as a function of time for LM30 alloy.

The temperature of the LM30 wear pin as a function of time at various

applied pressures is shown in Fig. 8. It can be seen that the temperature of

the specimen increased rapidly during the first 20 s of the experiment and

thereafter it increased at a much reduced rate. The maximum temperature

of the test pin increased with the applied pressure. At an applied pressure

of 5 MPa there was a sudden increase in the temperature after a sliding

distance of 250 m and this sudden increase in temperature was taken as the

onset of seizure. The temperature rise for all other test pins at various

applied pressures until seizure is shown in Table 2. There appears to be a

direct correlation between time-temperature plots (Table 2) and the applied

pressure wear rate plots (Figs. 6 and 7). In both cases, heat-treated com-

posites showed generally superior properties.

The coefficient of friction was computed by dividing the frictional

force by the normal load (Table 3). It can be seen from Table 3 that there

was a marginal decrease in the coefficient of friction on the LM13 alloy due

to heat treatment or graphite particle dispersion, whereas the combined

action of heat treatment and graphite particle dispersion reduced the coef-


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TABLE 2

Maximum temperature of test pins during sliding wear

_

--___

-_._..

Applied

Temperature C)

pressure

_____.

_._.__

WPa)

LM13 LM13 LM13

LM 13 LM 30 LM 30 LM 30

LM30

die

HT)

graphi te graphi te die

WT)

graphite graphite

cast

HT)

cast

HTj

1.0 44

- -

36 39 47 3s

1.5 50

86 44 60

_

2.0 154a

98 72 90

80 82 80

7-I

2.5

150a 98 106

._ -.

3.0 -

_ 144

118

108 100 116

1 O

3.5

- 150a 130

_

_.

4.0

- 1 5oa

136 130 162

I18

5.0 -

21Ba 158a 206a

160

aSeizure.

TABLE 3

Coefficients of friction

Alloy

Coeff icient of fr iction

LM13 die cast)

0.125

LM13 HT)

0.119

LM13-graphite

0.103

LM13-graphite

HT) 0.059

LM30 die cast)

0.172

LM30 HT)

0.143

LM30-graphite

0.140

LMSO-graphite

HT) 0.071

ficient of friction of LM13 alloy by half. Similarly, the coefficient of friction

of LM30 alloy was reduced from 0.170 to 0.071 because of graphite particle

dispersion and heat treatment.

A scanning electron micrograph of a typical worn surface of die-cast

LM13 alloy is shown in Fig. 9(a). The wear surface is characterized by fairly

long grooves and surface cracks. The tendency for fracture during wear

appears to be less in the case of heat-treated LM13 alloy (Fig. 9(b)). The

worn surface of diecast LM13graphite composite also showed fairly long

grooves (Fig. 10(a)) and no graphite film was detected on this surface.

Figure 10(b) shows a scanning electron micrograph of the worn surface of

heat-treated LM13graphite composite. Fracture was not evident in this case

and formation of patches of graphite film can be seen (Fig. 10(b)). Scanning

electron micrographs of LM30 alloy and LM30graphite composite in the

die-cast condition are shown in Figs. 11(a) and 11(b). There is little visible


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(a)

(b)

Fig. 9. Scanning electron micrographs of worn surfaces of LM13 alloy: a) die-cast condi-

tion and b) heat-treated condition.

(a)

(b)

Fig. 10. Scanning electron micrographs of worn surfaces of LM13 alloy-graphite compo-

site: a) die-cast condition and b) heat-treated condition.

(a)

(b)

Fig. 11. Scanning electron micrographs of worn surfaces of a) LM30 alloy and b)

LMSO-graphite composites in die-cast condition.


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difference between the two wear surfaces. In both cases, there were long

grooves and patches of severely damaged regions. In contrast, wear surfaces

of heat-treated L~30-~phite eomposites (Fig. 12(a)) showed no evidence

of severely damaged regions; the number of grooves was much less ana the

surface showed the presence of graphite film. Figure 12(b) is a carbon X-ray

Fig. 12. a) Scanning dectron micrograph of worn surface of heat-treated L~30-~aphite

composite. b) X-ray dot map of carbon corresponding to a).

A scanning electron micrograph of typical die-cast LM13 alloy debris

at low applied pressure (1.0 MPa) is shown in Fig. 13. Most of the debris

particles are seen to be small and equiaxed. In contrast, debris from die-cast

LM13 alloy at pressures close to seizure (2.0 MPa) were found to be flaky

in nature (Figs. 14(a) and 14(b)). The magnified view of typical flake-type

debris, showing cracks, is shown in Fig, 14(b). Debris from die-cast LM13-

graphite composite were also observed to be similar to those of die-cast

LM13 alloy. However, debris from heat-treated LM13graphite composite

were found to be much smaller in size, e.g. Fig. 15(a). Some of the debris

are flaky and occasionally a few machining chips were also o served

(Fig. 15(b)). In the case of die-cast LM30 alloy and die-cast LM30graphite

Fig. 13. Scanning electron micrograph of LM13 debris obtained at low 1.0 MPa) applied

pressure.


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184

composites, the debris also consisted of large faceted silicon (size, 70 pm)

particles in addition to flake-type debris. One such silicon debris particle is

shown in Fig. 16(a). Figure 16(b) is a silicon dot map corresponding to

Fig. 16(a).

3.3.

Parti all y l ubri cated w ear

P-V

limits of LM13 alloy and LM13 alloy-graphite composites in

die-cast and heat-treated conditions are shown in Fig. 17 while the corres-

ponding curves for LM30 alloy and composites are shown in Fig. 18. Each

point in any one curve represents the minimum applied pressure at which the

specimen begins to seize at a particular sliding velocity (speed). At lower

sliding velocities, the specimens were able to withstand higher applied pres-

sures. With an increase in sliding velocity, however, there was a progressive

decrease in the limiting value of the applied pressure. It can be seen from

Fig. 17 that the maximum

P-V

limits are obtained for the LM13graphite

composites in the heat-treated condition. Similarly, the

P-V

limits of heat-

treated LM30graphite composites were found to be superior to those of the

other LM30-based alloys. Therefore the results of partially lubricated sliding

wear studies appear to be in good agreement with the results of the dry

wear tests. In both cases, the heat-treated composites showed optimum

properties.

12r-

12

1

,

- Ml3CtiT

A LM

13-Grophlte

2 _ 0 LM 13-Graphfte (HT)

[II LM 30-Graphrte (HT)

I

i

Sltdmg velocity, m/s

J I

/

I I 1

I

5 c

I

2

3

4

:

Shdmg velocity, m/s

Fig. 17. P-V limits for LM13 alloy and composites.

Fig. 18. P V limits for LM30 alloy and composites.

4.

Discussion

In the Al-Si alloy system, the eutectic forms at 12.6 wt.% Si [14]. The

microstructure of LM13 alloy (containing 11.0 wt.% Si), solidified in a

metallic mould, consists of primary aluminium dendrites with an average

dendrite arm spacing (i.e. centre-tocentre distance between neighbouring


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185

TABLE 4

Tensile strengths of aluminium alloys and composites

A l loy

Ultimate tensile

strength MPa)

LM13 die cast)

180

LM13 HT) 280

LMl3-~aphite 130

LM13-graphite HT) 190

LM30 die cast)

140

LM30

(HT) 220

LMSO-graphite 98

LMSO-graphite (HT) 160

dendrites) of the order of 18 pm and eutectic silicon in the interdendritic

region and around the dendrites (Fig. 2(b)). The eutectic silicon is plate

like in appearance and some of these plates are interconnected. The heat

treatment of Al-Si alloy resulted in a significant change in the morphology

of eutectic silicon from plate like (Fig. Z(b)) to nearly spherical {Fig. 3). This

change in morphology of silicon due to heat treatment also results in an

increase in the tensile strength of LM13 alloy from 180 to 280 MPa (Table

4). In hypereutectic Al- alloy, the first phase to solidify is primary silicon

as large cuboids and the remaining liquid is solidified as primary aluminium

and Al-Si eutectic phase. In this case also, heat treatment resulted in a

change in morpholo~ of eutectic silicon.

Previous investigators have reported that the wear of Al-Si alloy is not

a linear function of applied pressure [12]. A transition from mild to severe

wear was observed as the load was increased. This transition load was also

reported to depend upon the silicon content [ 121. Our results (Fig. 5) also

confirm a change in wear behaviour from mild to severe with increase in

applied pressure. It is also interesting to note that the wear debris collected

at low applied pressure (1.0 MPa) are small and equiaxed (size, less than

4 pm) compared with large flake-type debris (length, 450 pm; breadth,

250 pm) found at higher applied pressure. The presence of a large amount

of flake-type debris suggests that delamination is the predominant wear

mechanism at higher applied pressure [13 J. Del~ination is based on the

hypothesis that subsurface cracks fpre-existing or nucleated due to the

normal and tangential stresses) propagate during the course of wear, When

such subsurface cracks join the wear surface, flake-type wear particles are

generated. In addition to delamination, the presence of significant numbers

of large faceted silicon wear debris particles in hypereutectic alloys suggests

that the large primary silicon has a tendency to fracture during sliding wear.

When delamination is the operating wear mechanism, the tensile strength

of the material controls the crack propagation and the overall wear behav-

iour. Graphite particle dispersion reduces the tensile strength of the resultant

composites. The tensile strength of LM13 alloy in the die-cast condition was


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186

reduced from 180 to 130 MPa, while that of LM30 alloy was reduced from

140 to 98 MPa from the 3.0 wt.% dispersion of graphite partiefes (Table 4).

This fess in strength offsets the positive effect of dispersed solid lubricant

in reducing friction and shear stresses. In addition to graphite, the sharp-

edged silicon phase also acts as a stress riser. The stress concentration can

be lowered by changing the morphology of silicon from plate shaped to

spherical.

The results of the present study show that, under both dry and Iubri-

cated sliding wear conditions,

superior wear properties were generalfy

observed in the case of heat-treated composites, It is interesting to note

that the worn surfaces of heat-treated composites showed graphite film on

the sliding surfaces whereas no such graphite film was detected on the worn

surfaces of die-cast composites. The combined effect of the increase in

tensile strength and the reduction in metal-to-metal contact (i.e.

between

AI-Si alloy and steel) due to the presence of graphite film on the mating

surface might have resulted in the observed improvements in the friction and

wear properties of the heat-treated composites.

5, Conclusions

(I) The presence of dispersed graphite particles and the morphology

of the silicon phase were found to influence the friction and wear behaviour

of the (Al-% ploys-~phite composites.

(2) For the LM13 alloys and composites, the heat-treated composites

showed least wear and maximum resistance to seizure. Similarly, the heat-

treated LM30 alloy-graphite composites showed optimum wear properties.

(3) The worn surface of the heat-treated composites showed the pre-

sence of a graphite film whereas those of the die-cast alloys and composites

showed a considerable amount of surface cracks.

(4) The coefficients of friction of the LMl3 and LM30 alloys were

reduced by more than 50% because of graphite particle dispersion and heat

treatment.

(5) The P-V limits of the heat-treated composites under partially

lubricated conditions were found to be higher than those of the other

materials.

Two of the authors (S.D. and S.V.P.) are grateful to Dr. R. Kumar,

Director RRL, Bhopal, for his encouragement and permission to publish

this paper.


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1989-microstructure and wear of cast (al-si alloy)-graphite composites

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