H.N.Reddappa et al. / International Journal of Engineering Science and Technology (IJEST)
EFFECT OF COLD QUENCHING ON
WEAR RATE OF AL6061- BERYL
COMPOSITES
H.N.Reddappa1*, K.R.Suresh2, H.B.Niranjan3 and K.G.Satyanarayana4
1
Research scholar, Dept. of IE&M, Bangalore Institute of Technology, Bangalore, India
2
Princiapal & professor, Bangalore Institute of Technology, Bangalore, India
3
Princiapal & professor, G.S.S.I.T, Bangalore, India
4
Consulting Chief scientist & Head, BMS R&D Center, Bangalore, India
*Corresponding author: Ph: +919448203812, Email: [email protected]
Abstract:
In recent years, aluminum alloy based metal matrix composites (MMC) are gaining importance in several
aerospace and automobile applications. Al6061 has been used as matrix material owing to its excellent
mechanical properties coupled with good formability and its wide applications in industrial sector. From
literature survey it has been observed that hardness and tensile strength of Aluminum alloy-beryl composites
increased significantly with increasing beryl content. The cast matrix alloy and its composites have been
subjected to solutionizing treatment at a temperature of 5300C for 18hr. followed by quenching in ice. Heat
treatment has profound effect on both hardness and tensile strength of Al6061-beryl composites, among them
composite with 10% beryl shows maximum. Accordingly, the aims of the present study are (i) study the effect
quenching media and (ii) sliding distance on the wear properties of these composites in both ‘as cast’ and ‘heat
treated’ conditions. It was observed that the quenching has significant effect on tensile, hardness and wear
behavior, exhibiting enhancement as compared to ‘as cast’ composites.
Keywords: Al6061, beryl, solutionizing, Wear
1. Introduction
Applications of aluminum based Metal matrix composites (MMCs) continue to expand in space, automobile,
and structural industries, due to their high strength, high stiffness, and better wear resistance, particularly when
component weight reduction is a key objective (Howell, G.J et al 1995; Rawal, S. 2001 et al). In recent years,
among all the Al alloys, Al6061 is gaining much popularity as a matrix material to prepare MMCs owing to its
excellent mechanical properties and good corrosion resistance (Ramesh et al 2005). In addition, Al6061 alloy is
heat treatable and as a result further increase in strength can be expected (Appendino et al 1991; Salvo and
Surey 1994; Gupta and Surappa 1995; Anwar Khan et al 2002). However, the major focus is on processing and
characterization of Al based composites (Appendino et al 1991; Pramila Bai et al 1992; Doel and Bowen 1996;
Gui et al 2000). Although the synergetic effect of heat treatment and the type of reinforcement plays a dominant
role in dictating the final mechanical properties of composites, meager information is available, pertaining to the
heat treatment of Al based composites that too with beryl as reinforcement.
It is has also been reported (K. R. Suresh et al 2003) that Al356 alloy–beryl composite has been processed
both through gravity die casting and squeeze casting techniques followed by hot forging using closed die.
Amongst these, hot forged composites showed improved hardness and tensile strength with decreasing wear rate
(K. R. Suresh et al 2002). In fact, improvement in both ultimate tensile strength (UTS) and hardness over the
matrix by 33% and 66% respectively were observed for the hot forged composites. All these properties have
been attributed to refined microstructure due to the presence of beryl particulates, their uniform distribution,
effects of pressure both during squeeze casting and after forging of the composites (K. R. Suresh et al 2002).
The literature review reveal that there is little report (H.N.Reddappa et al 2010) available on the effect of
quenching media and heat treatment on the mechanical and wear properties of Al-beryl composites. Hence, this
paper reports the results of studies on the (i) effect of quenching media (ii) sliding distance on (a) ‘as cast’ (b)
‘solutionized’ (c) ‘solutionized and ice quenched’ conditions.
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2. Experimental
2.1. Materials
Beryl, which is naturally occurring and having the formula [Be3Al2 (SiO3)6] was used as the reinforcing
material, while Al6061 alloy has been used as the matrix. The beryl particles used were of 45-60μm size. They
have a density of 2.6-2.8g/mm3 which is almost on par with that of Al6061 having hardness of 7.5 to 8.5 on
Mho’s scale, together with hexagonal structure (K.R.Suresh et al 2002). The chemical composition of the matrix
alloy (Al6061) and the reinforcement (beryl particles) are presented in the Table 1(a & b) respectively.
Table 1(a) Chemical composition of Al6061.
Element
Mg
Si
Fe
Cu
Ti
Cr
Zn
Mn
Al
Wt. %
0.92
0.76
0.28
0.22
0.1
0.07
0.06
0.04
Bal
Table 1(b) Chemical composition of beryl.
Element
SiO2
Al2O3
BeO
Fe2O3
CaO
MgO
Na2O
K2O
MnO
Wt. %
65.4
17.92
12.25
0.8
1.34
0.48
0.55
0.004
0.05
2.2. Methods
For the preparation of the composite, liquid metallurgy route was adopted as described in earlier works (K.G.
Satyanarayana et al 2002; T. V. Clyne et al 1993; S. Suresh et al 1993; W.F.Smith 1993; A.R. Anwar Khan et al
2002). Briefly, Al6061 alloy was melted in graphite coated crucible, degassed and vortex was created using a
ceramic-coated steel impeller for about 10 minutes at stirring speed of 400rpm. Then, preheated 10% wt. of
beryl particles of size 45-60μm were introduced into this vortex, which was maintained at 720°C. Then, molten
composite slurry at 710°C was poured into coated steel molds.
After casting the Al6061-10% beryl composites, wear test specimens were prepared by machining the
cylindrical bar castings. Each specimen was Φ8mm×25mm length in its dimensions. The specimen surfaces
were polished with 1µm diamond paste. Each result presented is an average of three trials of each composite
tested under identical test conditions. The cast composites and the base alloy specimens were subjected to
solutionizing for 18hr. at a temperature of 5300C and quenched in water. All the ‘as-cast’, ‘solutionised’ and ‘ice
quenched’ samples were prepared suitably for determining wear properties. Matrix alloy treated with same
procedure has been taken as datum for comparison.
2.3. Wear tests
Wear tests were carried out using a pin-on-disc apparatus (MODEL:TR20-LE, WEAR AND FRICTION
MONITOR, DUCOM MAKE, BANGALORE, INDIA) as per ASTM-G99-95 standard. The schematic diagram
of the wear testing machine used is depicted in fig.1. The tests were carried out by rubbing a composite
specimen against rotating hardened high carbon high chromium steel of hardness Rc60, which was used as a
counter disc under dry conditions. The test environment was kept at a room temperature and a relative humidity
of 78%. The wear test was conducted with varying sliding distances (600m, 1200m, 1800m, 2400m) with
constant speed of 1.66m/s. Load was varied from 9.81 to 29.43N. The wear losses of sample pins were
measured as height loss in microns which was recorded using an LVDT transducer of accuracy of 1μm. The
measurement of wear loss of the pin was used to evaluate the volumetric loss, which in turn was used to
compute the specific wear rate of the composites. Lastly, wear surface were studied with SEM to determine the
wear mechanism undergone by the material.
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Fig.1 Schematic diagram of pin-on-disc test setup.
3. Results and discussions
3.1. Morphology
The microstructure of Al6061-10% wt. beryl composite is shown in fig.2(a-c). The microstructure clearly
indicates fairly uniform distribution of beryl particles in the matrix along with evidence of minimal porosity in
the composite. Further, an excellent bonding between the matrix and the reinforcement particles is observed.
(a)
(b)
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(c)
Fig. 2 Microstructure of Al6061-10% beryl composite (a) as cast (b) solutionised (c) ice quenched
3.2. Hardness
The hardness of the aluminum alloy has been observed to be enhanced with the dispersion of beryl particles
(H.N.Reddappa et al 2010). The effect of quenching media on hardness is presented in fig. 3. It can be observed
that for solutionized and quenched Al6061-10% beryl components exhibit higher hardness as compared to as
cast samples. The ice quenched samples show an increase of 12.66% increase in hardness when compare to as
cast Al6061 alloy.
As cast
Solutioniz e d
Ice que nche d
100
Hardness, BHN
80
60
40
20
0
0
2
4
6
8
Wt. % of be ryl
10
12
Fig. 3 Hardness of Al6061-beryl composites (as cast, solutionized and ice quenched)
The solutionizing treatment results in the formation of intermetallic phase of Mg & Si which is observed to be
harder than aluminum leading to higher hardness. Of the heat treated composites, the ice quenched composites
exhibit higher hardness than solutionised composites which may be due to combined effect of enhanced bonding
between the beryl particles and matrix due to lower temperature and the formation and stabilization of Mg2Si
intermetallic phase with matrix (N.R. Prabhu Swamy et al 2007; M.R.Rosenberger et al 2005; B.N. Pramila Bai
et al 1992; H.N.Reddappa et al 2010)..
3.3. Adhesive wear
3.3.1 Effect of ice quenching
Al6061-10% beryl composites have been taken up for the wear studies as this composition exhibited reasonably
good strength and hardness (H.N.Reddappa et al 2010). Heat treatment of these components influences the wear
behavior as can be seen in the following fig.4
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As cast
Solutioniz e d
2.60
Ice que nche d
2.50
Specific wear rate (mm
3
/N-m)*10 -3
H.N.Reddappa et al. / International Journal of Engineering Science and Technology (IJEST)
2.40
2.30
2.20
2.10
2.00
600
Sliding distance ,m
Fig. 4 Specific wear rate of Al6061-10% beryl composite (Sliding distance:600m, Load: 19.62N)
The solutionised composite exhibit lower wear rate of 1.9%, further ice quenching reduces wear rate by 11.9%.
The solutionised components show higher hardness when compared to the ‘as cast’ one. This may be attributed
to the formation of harder Mg2Si intermatallic phase and ice quenching adds further hardness to the components
due to the stabilization of intermatallic phase (N.R. Prabhu Swamy et al 2007; M.R.Rosenberger et al 2005;
B.N. Pramila Bai et al 1992; H.N.Reddappa et al 2010).
3.3.2 Effect of sliding distance
When the sliding distance is increased from 600 to 1200m the wear rate decreases due to the presence of hard
ceramic phase. Increase in sliding distance to 1800-2400m increases the wear rate, may be marginally as can be
observed from the fig.5.
Solutioniz e d
Ice que nche d
3.00
2.50
Specific wear rate (mm
3
/N-m)*10 -3
As cast
2.00
1.50
1.00
0.50
0.00
600
1200
1800
2400
Sliding distance ,m
Fig. 5 Specific wear rate of ‘as cast’, ‘solutionised’ and ‘ice quenched’ Al6061-10% beryl composites
3.3.3 Worn surfaces
Micrographs of the worn surface of the (a) Solutionised and (b) Ice quenched composites specimens (at a load
of 40N and sliding speed of 1.66m/s for a sliding distance of 2400m) are shown in fig.6. The studies carried out
through SEM of the wear tracks have made it possible to deduce that the path was not homogenous, but, a nonuniform one, showing wear outstanding zones and areas with grooves along the sliding direction and plastic
deformation after a sliding distance of 2400m. Grooves have been formed by the reinforcing materials. On these
surfaces, areas with a fractured appearance can be observed. It can be seen that the layer of material has been
removed as debris from the surface and that the debris is in the form of thin sheets. In reference to SEM studies
of worn-out surfaces, it could be observed that wear damage was caused by plastic flow of the matrix with an
accumulation of material within valleys as reported by Mahagundappa M. Benal et. al. who had also observed
similar results.
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(a)
(b)
Fig. 6 Scanning electron microstructure photographs of the worn surface of (a) solutionised (b) Ice quenched Al6061-10% beryl
composites at 14.71N load, 1.66m/s sliding speed for a sliding distance of 2400m.
The possibility of debonding of the particle due to the continuous sliding which makes the particle get loosened
from the matrix and get struck between the sliding surfaces whereby it would act as abrader leading to short
duration of abrasive wear. This results in the enhanced wear rate.
Conclusions
The main conclusions of the dry sliding wear experiments performed with ‘as cast’, ‘solutionised’ and ‘ice
quenched’ Al6061-10% beryl composites in a pin-on-disc apparatus, under different sliding distances of 600m,
1200m, 1800m and 2400m are:
•
Studies carried out to determine the influence of quenching media on the wear resistance of the Al606110% beryl composites revealed that the ice quenching was the one that provided the matrix greater hardness
and therefore these showed the high wear resistance.
•
The effect of solutionizing enhances the hardness by 2.53 % and further ice quenching increases this to
12.66%.
Wear rate of the ice quenched composite is lower than either ‘solutionised’ or ‘as cast’ composites.
Increase in the sliding distance from 600m to 1200m decreases the wear rate by 36.49% in the ice quenched
condition.
Further increase in the sliding distance behind 1200m increases the wear rate very marginally by 12%.
•
•
•
Acknowledgements
One of the authors (Mr. H.N.Reddappa) acknowledges Vokkaligara Sangha, Bangalore for financial support.
Also, the authors sincerely thank the Management of Bangalore Institute of Technology, G.S.S. Institute of
Technology and BMS College of Engineering with which the authors are associated presently for their support
and encouragement for this work.
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