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Available online at www.sciencedirect.com

Wear 264 (2008) 638647

Microstructure and the wear mechanism of grain-refinedaluminum during dry sliding against steel disc

A.K. Prasada Rao a,, K. Das b, B.S. Murty c, M. Chakraborty b

a Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Republic of KoreabDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, India

cDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology, Madras, India

Received 31 August 2006; received in revised form 28 March 2007; accepted 30 May 2007

Available online 17 July 2007

Abstract

This article discusses on the influence of grain refinement on the wear mechanism of commercially pure Al. In this work, commercially pure Al,

grain refined using AlTi, AlTiB grain refiner master alloys, prior to casting. These castings after machining have been subjected to dry-sliding

wear against high-chromium hardened steel disc at a constant load of 50 N and speed of 1 m s1. The effect of grain refinement of aluminum on its

wear behavior has been investigated. The sub-surface and the worn surfaces of the specimens were characterized in order to understand the wear

behavior of aluminum against steel disc. Although it hasbeen found that wear mechanismof aluminum is same for both untreated and grain refined,

untreated aluminum exhibits higher wear loss than that of grain-refined aluminum. The results also show that grain refinement has a significant

effect on the transfer of Fe from the steel disc to the worn surface and sub-surface of Al specimens.

2007 Elsevier B.V. All rights reserved.

Keywords: Wear; Grain refinement; Aluminum; Microstructure

1. Introduction

Grain refinement has been a common foundry practice for

Al and its alloys since last few decades [13]. It has been

reported that grain refinement by melt inoculation with AlTiB

or AlTiC type grain refiner results in fine equiaxed grains

[13]. On the other hand several reports have been found on

the mechanical deformation behavior of pure aluminum [4,5].

Much work has been reported in wear of Al alloys especially

AlSi alloys. However, very few reports have been found on

the dry-sliding behavior of pure Al against steel disc. Goto and

Buckley [6] studied the effect of fretting wear behavior of Al

against aluminum under humid conditions. It has been reportedthat the humidity has less influence in altering the coefficient

of friction during fretting wear of aluminum. This perhaps is

due to the stable oxide layer formed near the sliding surfaces.

In another investigation [7] on the dry-sliding wear behavior

of commercial pure Al against a steel disc, it was shown that

Corresponding author. Tel.: +82 54 2792823; fax: +82 54 2795887.

E-mail addresses: [email protected], [email protected]

(A.K. Prasada Rao).

adhesion is the major mode of wear in pure aluminum rubbed

against a steel disc. However, present authors have reported that

the grain refinement has a significant influence in enhancing the

wear resistance of commercial pure Al [8]. Nevertheless, earlier

work[8] did not emphasize on grain shape and detail study of

the Fe-transfer during dry-sliding.

From the survey of the literature it has been understood that

little investigation has been done in understanding the influence

of grain refinement treatment on the microstructure and its sub-

sequent effect on the wear mechanism of commercial pure Al

during dry sliding against a steel disc. Nevertheless, detailed

microstructural features and wear mechanism of grain-refined

aluminum were not investigated in the past work [68].

2. Experimental details

2.1. Grain refinement procedure

One kilogram of aluminum was taken in a zirconia coated

graphite crucible (preheated at 300C) and melted under a cover

flux (50 wt% NaCl + 50 wt% KCl) in a pit type resistance fur-

nace. The melt was brought to a temperature of 720 5 C and

0043-1648/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.wear.2007.05.010
mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.wear.2007.05.010http://dx.doi.org/10.1016/j.wear.2007.05.010mailto:[email protected]:[email protected]

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A.K. Prasada Rao et al. / Wear 264 (2008) 638647 639

Table 1

Details of the aluminum specimens used in the present study

Sample code Grain refiner

alloy

Addition level of

grain refiner (wt%)

Holding time

(min)

HP-1 0

HP-2 Al3Ti 0.33 5

HP-3 Al3Ti 0.33 120

HP-4 Al3Ti 0.50 5HP-5 Al3Ti 0.50 120

HP-6 Al3Ti 0.67 5

HP-7 Al3Ti 0.67 120

HP-8 Al3Ti 1.00 5

HP-9 Al3Ti 1.00 120

HP-10 Al3Ti 1.50 5

HP-11 Al3Ti 1.50 120

HP-12 Al5Ti1B 0.10 5

HP-13 Al5Ti1B 0.10 120

HP-14 Al5Ti1B 0.20 5

HP-15 Al5Ti1B 0.20 120

then degassed (to remove H2) using commercial degasser, hex-achloroethane (C2Cl6). After degassing, the grain refiner master

alloy has been plunged into the melt in the form of chips, duly

packed in an aluminum foil. The melt was stirred for 30 s with

zirconia-coated graphite rod, after which no further stirring was

carried out. Parts of the melt were poured at regular intervals

(0, 5, and 120 minhere after referred to as holding time) into a

cylindricalgraphitemould(25 mm diameter and150 mm height)

with its top open for pouring. Zero minute holding time refers to

the castings obtained from untreated melt (HP-1). Table 1 gives

the details of aluminum specimens obtained after grain refine-

ment treatment. The castings were cut transversely, polished

and etched with Kellers reagent for microstructural characteri-

zation and with Poultons reagent for revealing macrostructure.

Grain size was measured by linear intercept method (by using

Lieca Image Analyzer) at a magnification of 100. The length

and breadth of the grains were obtained as an average of

100 readings vertically and 100 horizontally. The grain size

presented is the square root of the mean product of length

and breadth readings obtained from the vertical and horizontal

intercepts.

The aspect ratio of the grains has been measured by linear

intercept method following the similar procedure used for grain

size measurement. The aspect ratio has been considered as the

ratio of length and breadth of the Al grains.

2.2. Dry-sliding wear studies

Wear characteristics of aluminum were studied by using a

pin-on-disc wear-testing machine (TR-20, DUCOM) equipped

with LVDT sensors for acquiring height loss and friction force

data. Schematic diagram of the pin-on-disc wear testing machine

has been shown in Fig. 1. Steel disc used in the present study

has the Rockwell hardness of (Rc) 64 and surface roughness, Raof 0.15m. Four samples for each condition were tested and the

average of the height loss was obtained. From the height loss,

volume loss and wear rate were calculated. Wear tests were con-

ducted in dry conditions in order to avoid effect of lubricating

Fig. 1. Schematic diagram of the pin-on-disc type wear testing machine.

medium. Wear specimens were obtained by machining the cylin-

drical castings such that the longitudinal axis of the wear sample

coincides with that of the casting. The surface roughness of the

specimen has been measured by using Surtronic 3P machine

(Rank Taylor Hobson Ltd.). The roughness values obtained lie

in the range of 0.4750.614m. Height loss versus sliding dis-

tance plot is obtained from the computer interface connected to

the wear-testing machine, which in turn is plotted as volume lossversus sliding distance. The wear rate is determined as the slope

of thelinear fit of volume lossslidingdistanceplot in thesteady-

state regime (sliding distance of 5001500 m). Wear resistance

reported is the reciprocal of the wear rate. The computer aided

pin-on-disc wear testing machine used in the present study also

gives the force of friction directly as one of the out puts. Thus

coefficient of friction presented is the ratio of the force of fric-

tion and the normal load applied. Apparent area of contact is

assumed to be same as the area of cross-section of the cylin-

drical specimen (pin of diameter 8 mm and 25 mm in length).

Sliding velocity was chosen as 1 m s1 for all the experiments.

The worn surfaces and microstructure of the sub-surfaces were

examined under SEM (JEOL, JSM-5800, Japan)/EDX micro-analyser interfaced with Link ISIS software for EDX, X-ray dot

mapping and Line-scan analysis(ISI 300 Oxford Instruments

Ltd., UK).

3. Results and discussion

A number of experiments have been conducted on the grain

refinement of the commercially pure aluminum using Al3Ti

and Al5Ti1B grain refiner master alloys. The mechanism of

grain refinementis believed to be by heterogeneous nucleation of

Al-grains during solidification of molten aluminum. The nucle-

ating particles being Al3Ti or TiB2, which are added in the formof AlTi or AlTiB type grain refiner master alloys [13].

However, present work is focused on the effect of final as-cast

microstructure of aluminum on its dry-sliding wear behavior

against steel disc.

3.1. Macrostructure and microstructure

Fig. 2 shows a series of macrostructure of commercial pure

aluminum both with and without grain refiner addition. It is

obvious from the figure that Al in untreated condition (HP-1)

shows coarse columnar grain structure. The specimens denoted

as HP-2, HP-4, HP-6, HP-8 and HP-10 represent the aluminum

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640 A.K. Prasada Rao et al. / Wear 264 (2008) 638647

Fig. 2. Photomacrographsof commercially pure aluminum(CPAL) in untreatedcondition (HP-1) and grain refined with Al3Ti (HP-2HP-11), and Al5Ti1B

(HP-12HP-15). Thetop row (HP-2,4, 6, 8, 10, 12 and14) corresponds to 5 min

of holding while the bottom row (HP-3, 5, 7, 9, 11, 13 and 15) corresponds to

120 min of holding.

grain refined with the addition of 0.33, 0.50, 0.67, 1.00 and

1.50 wt% of Al3Ti grain refiner master alloy, respectively for a

holding time of 5 min. Similarly, HP-12 andHP-14 denotethe Al

specimens of grain refined with Al5Ti1B grain refiner master

alloy with addition levels of 0.10 and 0.2 wt% at a holding time

of 5 min.

It has been observed that the increase in the addition levelof Al3Ti grain refiner results in the decrease in the grain size.

In addition to the decrease in the grain size it is also observed

that the grain morphology changes from coarse columnar struc-

ture to fine equiaxed type of structure with the increase in the

addition level of the grain refiner. The addition of 0.33 wt%

of Al3Ti grain refiner results in the microstructure consist-

ing of pre-dominantly columnar grains along with some coarse

equiaxed grains. It can also be seen that the number of fine

equiaxed grains increase while the number of coarse columnar

grains vanish gradually with the increase of the addition level of

the grain refiner. This can be attributed to the increased number

of nucleating particles introduced in the form of the grain refiner

master alloy. Similar behavior is also noticed in the case of Algrain refined with 0.10 and 0.20 wt% of Al5Ti1B grain refiner

for a holding time of 5 min.

The specimens denoted as HP-3, HP-5, HP-7, HP-9 and HP-

11 represent Al grain refined with Al3Ti grain refiner for a

holding time of 120 min, respectively. Although it has been

found that the grains gradually become finer and equiaxed with

the increase in addition level of the grain refiner, it is observed

that grainscoarsen on longerholding time of 120 minwhen com-

pared to those of 5 min holding. This can be explained from the

fading phenomenon of the grain refiner [13]. Similar observa-

tions have also been made in the case of Al grain refined with

Al5Ti1B grain refiner (HP-12HP-15) as seen in Fig. 2.

Fig. 3. Effect of grain refinement on the aspect ratio of the Al grains.

From Fig. 2 it has been understood that the addition of grainrefiners to Al result in the change in the shape of the grains from

coarse columnar to fine equiaxed. It is also evident from Fig. 2

that some of the grain-refined Al castings reveal the co-existence

of both equiaxed and columnar grains in their macrostructure,

while some show completely equiaxed grains. Hence, the aver-

age grain aspect ratio has been measured separately and plotted

against the grain size as shown in Fig. 3.

It is clear from Fig. 3 that the aspect ratio increases with

the increase in the grain size. In other words, grains tend to be

more equiaxed with the grain refinement. It is also found that

the grains larger than 300m have greater aspect ratio, suggest-

ing a columnar equiaxed transition zone, with a co-existence of

columnar and equiaxed grains. Hence, the range of grain sizes

has been classified into three zones designated as columnar,

columnar + equiaxed, equiaxed, as shown in Fig. 3.

The results discussed above have shown that it is possible to

produce a varied microstructure with differentgrainsize by grain

refinement. However, earlier reports [13,8] in this field does not

appear to consider the grain shape as an important parameter,

however present work considers both grain size and grain shape

(aspect ratio) for scaling grain refinement.

3.2. Wear

Dry-sliding wear experiments were conducted using acomputer aided pin-on-disc wear-testing machine at constant

sliding velocity (V= 1 m s1) and constant normal load applied

(N= 50 N). During wear testing, two plots are generated as out

comes they are; height loss (m) versus time (s) curve; andforce

of friction versus time (s). Time is expressed in terms of sliding

distance by multiplying with sliding velocity (V= 1 m s1) and

volume loss (mm3) was calculated by multiplying the height

loss with the area of cross-section, which however keeps the

nature of the curves un-altered. Fig. 4(a)(c) shows the height

loss versus time plots for HP-1 (columnar), HP-5 (colum-

nar + equiaxed), HP-14 (equiaxed) samples for 01800 m sliding

distance (inclusive of both running-in and steady-state regime)

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Fig. 4. (ac) Height loss vs. sliding time plots obtained during the dry sliding

of commercial pure aluminum against steel disc.

(Note: Dry-sliding experiments were conducted for all the speci-

mens from HP-1 to HP-15, however, a few representative results

are presented in this article). Fig. 4(a) shows the height loss of

commercial pure Al in untreated condition (HP-1) as a function

of time of sliding; volume loss was obtained by multiplying

Fig. 5. Effect of grain size on the wear rate of grain-refined aluminum

during steady-state regime, 5001500m (normal load= 50 N, sliding veloc-

i t y = 1 m s1).

height loss with cross-section area. The wear rate (mm3/m)

is calculated from the slope of the volume loss versus sliding

distance curve in the steady-state regime (5001500 m sliding

distance). It hasbeen found that thewear rate (slopes of thelinear

fits) decrease with the decrease in the grain size, which indi-

cates that the wear rate decreases with the decrease in the grain

size of aluminum. This can be attributed to the grain boundary

strengthening of aluminum leading to strain hardening.

The height loss plots exhibit some fluctuations in the curve;

these fluctuations are possibly due to the entrapment and release

of the debris particles in between the sliding surfaces. Another

reasonfor thefluctuationscouldbe dueto thedelamination of the

tribolayers. However, it is difficult to confirm the exact cause for

such fluctuations since the dry-sliding system is quite complex.

Fig. 5 shows the plot, which demonstrates the effect of grain

size on the steady-state wear rate. It can be seen that the wear

rate increases with the increase in the grain size. It is found that

the wear rate increases with the increase of the grain size in a

linear way up to about 500m of grain size. However, there is

a sharp rise in the wear rate at grain size greater than 500m.

Such behavior may be attributed to the change in the grain shape

from equiaxed to columnar (with increase in the grain size) ones.

Fig. 6 shows the effect of grain aspect ratio on the wear rate

of commercial pure aluminum with a range of grain sizes. This

figure shows a sharp increasein the wear rate with the increase inthe aspect ratio from 1 to 1.5, while it remains virtually same up

to three and again increases sharply up to eight. This is due to the

fact that during grain refinement, in addition to the decrease in

the grain size, the grain shape is also transformed from columnar

to equiaxed, which is evident from Figs. 2 and 3. Interestingly

in similar finding it was reported that aluminum with colum-

nar grain structure would exhibit anisotropy in the mechanical

behavior [4,5]. This shows that grain morphology has a signifi-

cant role in improving the wear resistance of as-cast aluminum.

However, it is observed that during grain refinement, the grains

transform from coarse-columnar to fine-equiaxed morphology

resulting in increasing the mechanical isotropy.

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Fig. 6. Effect of grain aspect ratio on the dry sliding wear rate (steadystate) of

cast aluminum (load= 50 N, sliding velocity = 1 m s1).

3.3. Friction

The force/coefficient of friction developed during the dry

sliding of Al pin against the steel disc under normal load 50 N

has been plotted against sliding distance and shown in Fig. 7.

The coefficient of friction shown in Fig. 7 was computed from

Fig. 7. Force of friction (F) developed during dry sliding of aluminum against

hardenedhighchromesteeldiscunder a constantappliedloadof 50N andsliding

velocity of 1 m s1.

force of friction using Coulombs law of friction (= F/N). It is

observed that initially theforce of friction increasesrapidly up to

certain sliding distance of about 300 m, indicating a running-in

wear regime. On further sliding, beyond 300m, thefriction force

Fig. 8. Effect of grain size of aluminum and sliding distance on the force of friction during dry sliding against steel disc (load = 50 N, velocity = 1 m s1

).

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almost remains constant as seen in Fig. 7, suggesting a steady-

state regime. It is also observed that the magnitude of the force

varies from 10 to 35 N (approx.) for various specimens studied.

Interestingly, the specimen corresponding to un-treated Al (HP-

1) exhibits low force of friction; while the same increases from

20 to 35 N for the remaining specimens obtained from grain-

refined Al as enlisted in Fig. 7. The reason for the variation of

the force of friction with the increase in the sliding distance may

be explained as below

Initially surface of the specimen pin mounted on the steel

disc does not have complete contact with the disc surface due

to the asperities formed during the machining of the specimen

pin. In other words, the contact area between pin and disc is less

than that of the area of cross-section of the specimen pin, which

leads to increase in the pressure acting on the pin, particularly at

the running-in wear regime (

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Fig. 12. Schematic representation of the tribolayer in Al castings specimens without grain refinement and with grain refinement indicating the forces acting on the

specimens.

distance. This can be due to the decrease in the grain boundary

area with the increase of grain size. Dry-sliding leads to shearing

phenomenon near the sliding surfaces [9]. This results in defor-

mation of the tribolayer due to shear. However, it is well knownthat the shear strength increases with the decrease in the grain

size of aluminum. The increase in the friction force (Fig. 7) with

the sliding distance can be attributed to the strain hardening of

the tribolayer. Study of the friction plots shown in Fig. 8 reveals

a decreasing trend with the increase of the grain size. Neverthe-

less, it is observed that these plots fluctuate at some grain sizes.

This kind of fluctuation in the force of friction can be explained

as follows

During sliding the tribolayer is work hardened due to plastic

deformation; this leads to the formation of a hard layer due tomechanical mixing which increases the force of friction. The

mechanically mixed layer (MML) containing AlFeO com-

pounds is formed during sliding and mechanical alloying, which

adheres to the sliding surfaces (pin) and increases the wear resis-

tance by preventing further wear of the pin. On further sliding,

this layer gets separated out from the pin surface due to delami-

Fig. 13. SEM photomicrographs of the worn surface of aluminum (a and b) without grain refinement and (c and d) with grain refinement.

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nation leaving behind the fresh pin surface, which is reflected as

the drop in the force of friction at some points in the plots shown

in Fig. 7. However, it should be noted that several phenomena

occur simultaneously during the dry sliding of aluminum, which

make the situation quite complicated and hence it is difficult

to attribute the fluctuations in the friction force to grain size

alone.

3.4. Surface analysis

Worn aluminum test pins were sectioned across their lon-

gitudinal axis in order to study the sub-surface. Fig. 9(a) and

(b) corresponds to the optical photomicrographs revealing the

sub-surface of un-treated aluminum at different magnifications.

The figure clearly reveals coarse columnar grains in the region

away from the worn surface. However, the region just adjacent

to the worn surface shows deformed grains along with fine par-

ticles embedded in the sub-surface. These microstructures also

show that the grains adjacent to the worn surface resemble as

if they are compressed into flat bands, which indicates the plas-tic deformation of the grains (Note: The sliding direction of

the pin is perpendicular to the plain of paper for all specimens

studied).

Similar microscopic study was done on grain-refined alu-

minum(HP-14) wearpins.The optical photomicrographs shown

in Fig. 10(a) and (b) at different magnifications reveal fine

equiaxed grains in the region away from the worn surface. How-

ever, the region adjacent to the worn surface (i.e., sub-surface)

shows fine grains deformed in the form of flat narrow bands

along with fine particles embedded into the worn surface.

A comparative study of sub-surfaces (worn surface is shown

by arrows) of aluminum in un-treated and grain-refined condi-tions shown in Figs. 9(a) and (b) and 10(a) and (b), respectively,

suggests that the extent of deformation is greater in case of

untreated aluminum than that of grain refined aluminum. This

is well in agreement with the theory of plastic deformation

proposed by Ashby [10]. According to this theory, grains

divide the matrix into boxes, which lead to piling up of the

dislocations in the grain boundaries resulting in strain hard-

ening, i.e., the specimens with fine equiaxed grains exhibit

higher strain hardening tendency than Al pins with coarser

grains.

Micro-hardness studies were conducted along the vertical

sectioned surface starting from the worn surface. Fig. 11 sug-

gests that the variation in the micro-hardness of aluminummeasured from the worn surface and away. It can be seen that

grain refined aluminum exhibits higher hardness than that in un-

treated condition. It is evident from Fig. 11 that the hardness of

the specimen decreases with the distance from the worn surface,

which indicates that the sub-surface nearer to the worn surface

was hardened due to strain hardening effect than the region away

from the worn surface. The increase in the hardness can be fur-

ther accounted to the formation of MML (mechanically mixed

layer) by the mutual solubility of the sliding materials or due to

the formation of some AlFe intermetallic compounds. Present

results reconfirm earlier report of Rigney et al. [11] that the

plastic deformation changes the sub-surface microstructure in

ways, which make the material unstable to local shear leading

to delamination.

In order to understand the role of grain size on wear behav-

ior more clearly, a schematic diagram (Fig. 12) representing the

forces acting on the pins during dry sliding against a hard steel

disc may be considered. The specimen pins are subjected to two

forces, they are normal load applied (N) and force of friction (F),

which is developed during rubbing of the pin against the steel

disc. Primarily the deformation occurs due to the shear force,

which is nothing, but the force of friction, Facting on the sliding

surface, normal to the longitudinal axis of the pin as shown in the

schematic diagram (Fig. 12). It can be noticed from Figs. 7 and 8

that the grain-refined Al specimens show higher force of fric-

tion than that of un-treated Al. This observation suggests that

grain size has a significant role in deciding the force of friction.

From the above results it has been found that finer grain size

(higher grain boundary area) results in greater strength. Hence,

it is understood that the force required for shearing the specimen

during dry sliding increases with the grain refinement, which is

clearly noticed from the increasing trend of the friction forcefrom Fig. 7.

Worn surfaces of the aluminum specimenswere studied under

SEM followed by X-ray dot mapping. Fig. 13(a)(d) represents

the SEM photomicrographs of the worn surfaces of aluminum

(without grain refinement) and grain-refined aluminum, respec-

Fig. 14. SEM photomicrographs of the worn debris of aluminum (a) untreated

and (b) grain refined condition (load= 50N, V = 1 m s1

and 1800m).

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tively. Fig. 13(a and b) shows the mechanically mixed layer

(MML) adhered to the aluminum pin surface. A careful study

of Fig. 14(a) and (b) which shows the SEM photomicrographs

of the wear debris indicates that the debris particles generated

from grain-refined Al are of finer size (35m) when com-

pared to that of un-treated aluminum, which lie in the range of

810m in size. This is probably dueto thegeneration of coarser

debrisin case of un-refinedaluminum, unlikein thegrain-refined

aluminum when tribolayer with finer grains is delaminated to

form finer debris during mechanical alloying. These particles

get embedded into the soft Al substrate to form mechanically

mixed layer. During this process some of the particles may be

left un-embedded, which form a part of debris. Loose debris

particles thus formed enter the gap between the pin and the disc

resulting in three-body abrasion. This could be the possible rea-

son for the scatter in the force of friction/coefficient of friction

plot in steady-state regime as evident from Fig. 7. Therefore,

it can be understood that a combination of various wear mech-

anisms like delamination, three-body abrasion, exist in during

the dry sliding of both grain-refined and un-grain-refined Alspecimens.

Figs. 15 and 16 show the X-ray elemental mapping of the

sub-surface of worn specimens of untreated (HP-1) and grain-

refined aluminum (HP-14), respectively. It can be observed from

the elemental distribution shown in Fig. 15, that Fe is transferred

into the aluminum matrix during sliding. It is also seen that the

amount of Fe is more near the worn surface and low in the region

away from the worn surface, indicating the transfer of Fe from

thesteeldisc duringdry-sliding. However,it is interesting to note

that the diffusion of Fe is greater for untreated aluminum (HP-1)

when compared to that of grain-refined aluminum (HP-14). This

again can be attributed to the embedding of greater number of

AlFe particles into softer matrix of untreated aluminum, unlike

in grain-refined case.

3.5. Wear mechanism

Dry sliding of aluminum pins against steel disc results in rub-

bing action, which induces large plastic strain at the sub-surface

of the sliding pin. Such plastic strain leads to the local strain

hardening at the sub-surface of the tribolayer. During which

some iron diffuses into the worn surface of the aluminum due to

mutual solubility of the sliding materials as proposed by Rigney

et al. [11]. This has been confirmed by the decrease in the hard-

ness measured across the sub-surface of the worn aluminum

test pins. It is important to note that the magnitude of hardness

varies with the extent of grain refinement of aluminum. Theplastic deformation further leads to change in the microstruc-

ture of the sub-surface, making the material unstable to local

shear causing delamination. These delaminated asperities get

entrapped between the sliding surfaces resulting in further

plastic strain due to mechanical alloying. Loose debris parti-

cles move in between the sliding surfaces causing three-body

abrasion.

Fig. 15. X-ray mapping of sub-surface of un-treated aluminum wear test specimen (HP-1) (1800 m, 50 N and 1 m s1

) (1000).

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Fig. 16. X-ray mapping of sub-surface of grain-refined aluminum wear test specimen (HP-14) (1800m, 50 N and 1 m s1) (1000).

4. Conclusion

Grain refinement of aluminum by inoculating with AlTi or

AlTiB grain refiners leads to decrease in the size and aspect

ratio of the grains. This in turn increases the grain boundary

area and results in improved strength. It has also been found

that the force of friction generated during dry sliding of alu-

minum pins against steel disc increases with decrease in the

grain size, suggesting the improvement in the shear resistance

of aluminum due to grain boundary strengthening. Hence, it is

understood that the wear resistance of grain-refined aluminum

increases with the decrease in grain size and grain aspect ratio

(equiaxed). A wear mechanism proposed suggests that, Al pins

with and without grain refinement exhibit similar wear mecha-

nism, although the magnitude of the wear rate is lower for the

grain-refined aluminum than that of untreated aluminum.

Acknowledgement

One of the authors (A.K. Prasada Rao) would like to

acknowledge the support by Prof. N.J. Kim, Center for

Advanced Aerospace Materials, POSTECH, Republic of

Korea.

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