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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
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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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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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