Al-2024 alloy can be reinforced with different percentages of Silicon oxide particles using a stir casting method. The solution treatment of the composite sample and the unreinforced alloy was carried out at 550 ᴼC for 2h followed by aging at 150 ᴼC for various aging times between 1h and 5 hrs. The existence of SiO2 particles led to increasing the peak hardness of the alloy. The results revealed that peak hardness of the composite sample took place at shorter times than that of the unreinforced alloy for the samples solution treated for 2 h for both the composite and the unreinforced alloy that led to the fastest aging kinetics and the maximum hardness.

1. Proceedings of the 2nd
International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
15
EFFECT OF SILICON OXIDE (SIO2) REINFORCED PARTICLES ON
AGEING BEHAVIOR OF Al-2024 ALLOY
Gowri Shankar M.C1
, Jayashree P.K2
, Dr. U.AchuthaKini3
, Dr. S.S Sharma4
1, 2, 3, 4
(Mechanical& Manufacturing Engineering Dept, Manipal Institute of Technology, Manipal, Karnataka, India)
ABSTRACT
Al-2024 alloy can be reinforced with different percentages of Silicon oxide particles using a stir casting method.
The solution treatment of the composite sample and the unreinforced alloy was carried out at 550 ᴼC for 2h followed by
aging at 150 ᴼC for various aging times between 1h and 5 hrs. The existence of SiO2 particles led to increasing the peak
hardness of the alloy. The results revealed that peak hardness of the composite sample took place at shorter times than
that of the unreinforced alloy for the samples solution treated for 2 h for both the composite and the unreinforced alloy
that led to the fastest aging kinetics and the maximum hardness. Matrix microhardness, tensile strength and wear test
measurements were carried out after each step of heat treatment.
Keywords: Age-hardening, Al2024, Silicon Oxide (SiO2), Stir casting,
1. INTRODUCTION
In recent years, Aluminium alloys have attracted attention of many researchers, engineers and designers as a
promising structural material in different industries like aerospace and automotive. Special 2xxx series of Al alloys have
been studied extensively because of their high strength to weight ratio, good formability, age hardenability and other
appropriate properties. Among Al alloys, 2024 Al has the highest hardness [1].But some of the mechanical properties
such as low wear resistance; have limited application of these materials. Adding SiC reinforcing particles to these
materials leads to increase the wear resistance [2]. Aging treatment can significantly increase properties of some of Al
alloys and their composites, especially 2xxx and 6xxx series alloys. In investigation of age-hardening kinetics, it has been
shown that the addition of ceramic particles to age-hardenable Al alloys has different effects on precipitation of
composite compared with unreinforced alloy[3–4]. Considering strict importance and widespread applications of these
materials, in this research the effect of heat treatment conditions and also Silicon oxide (SiO2) reinforcing particles on the
artificial aging kinetics and behavior of a 2024 Al- SiO2 composite produced by stir casting technique was studied. Lot of
research work were conducted to synthesize aluminium composites especially with wide variety of reinforcement
materials. These works has been systematically presented in the chronological order of the work progress.
Nieh and Korluk examined the changes in hardness resulting from ageing treatment in a 6061 aluminium alloy
reinforced with 23 vol% B4C particulate. The peak ageing time for the matrix of the composite was about 3 h, whereas
the un-reinforced matrix material exhibited a peak hardness value only after 10 h of ageing at the same ageing
temperature of 450 K. The accelerated ageing phenomenon was hypothesized to be a consequence of the short circuit
path for pipe diffusion of solutes which is induced by the high dislocation densities and interracial stresses resulting from
the reinforcement. [5].
Cottu et al. [6] showed that age-hardening kinetics of Al–Cu–Mg alloy-10 wt.% SiC fiber composite was
enhanced bythe presence of the reinforcement during heat treatment. They explained this by the plastic deformation
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induced during heat treatment due to the difference between coefficients of thermal expansion (CTE) of matrix and
reinforcement.
Considering strict importance and widespread applications of these materials, in this research the effect of heat
treatment conditions and also Silicon oxide (SiO2) reinforcing particles on the artificial aging kinetics and behavior of a
2024 Al- SiO2 composite produced by stir casting technique was studied.
2. EXPERIMENTAL PROCEDURE
The properties of materials adopted and methods followed for the fabrication and testing of MMCs in the
present studies are presented in the following sections.
2.1 Aluminium Alloy Matrix
The matrix for the present studies selected was Al2024 alloy and were procured from Fenfee Metallurgicals,
Bangalore, in the form of ingots. The chemical composition of Al2024 alloy is given in Table 1. It is extensively used in
applications like aircraft structural components, aircraft fittings, hardware, truck wheels and parts for the transportation
industry. This alloy is reinforced by particles of SiO2 (3 and 6 wt.% ) with an average diameter of 150µm.
Table 1: Chemical composition of AA2024
2.2 Preparation of Composites
The Al2024-SiO2 composites were prepared by the vortex method [7]. The SiO2 contents used for the
preparation of the composites were 0%, 3%, and 6%. This is because SiO2 compositions of 9% and above would lead to
rejection from the melt. Addition of SiO2 reinforcements into the molten aluminium alloy melt above its liquidus
temperature of 700 °C was carried out by creating a vortex in the melt using a mechanical stainless steel stirrer coated
with aluminite (to prevent migration of ferrous ions from the stirrer material into the aluminium alloy melt). The melt
was rotated at a speed of 500 rpm in order to create the necessary vortex. The SiO2 particles were preheated to 200 °C to
improve wettability and added to the melt through the vortex. The SiO2 reinforcement particles were added, and the melt
was thoroughly stirred and subsequently degassed by passing hexachloroethane (C2Cl6) solid degasser. The molten metal
was then poured into permanent moulds for casting. The preheated particles were introduced into the vortex and stirred
for eight minutes at a speed of 500 rpm. A pouring temperature of 720 °C was adopted and the molten composite was
poured into prepared moulds. The cast aluminium- SiO2 reinforced composite specimens were obtained after
solidification in air for about 2 hours. The solidified stir cast composite are shown in Fig.1. The microstructure of the
composite after casting was examined to study the effect of reinforcements particles. Photographs were taken for a
polished section of each sample.
Figure. 1: As cast specimens solidified in the crucible

3. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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2.3 Hardness Test
Vickers Hardness test were conducted and average of at least six indentations are taken at room temperature.
Hardness measurements were performed using a Vickers hardness tester (MATZUSAWA MICRO VICKERS
HARDNESS TESTER, MODEL-MMT X 7A) with a load of 100 gmf and dwell time of 15 seconds was given. The
cylindrical specimens of 12mm diameter and length 15mm were used for the test. Before testing the specimen surface
was polished in metcobainpol polishing machine using diamond paste abrasives.
2.4 Tensile Test
The tensile strength tests were performed on Electronic tensometer (model-ER3) as per ASTM-E8M standard
for as-cast, as quenched and peak hardened condition. Circular cross section specimen was prepared with diameter 6 mm
and gauge length 24mm. The load cell value was kept to 20.5 kN and test mode was selected as break. The cross head
speed was kept constant at 12 mm/ min, with length increment value of 0.01 mm.
2.5 Wear Test
The Wear resistance tests were performed on pin-on-disc tribo-meter under dry sliding conditions (WEAR AND
FRICTION MONITOR TR- 201CL). The test were conducted on 8 mm diameter, 25 mm long cylindrical specimens
(ASTM G-99) against a rotating EN-32 steel disc (count face) having hardness 63Rc. Care should be taken to note that
the test sample’s end surfaces were flat and polished metallo-graphically prior to testing. Conventional aluminium alloy
polishing techniques were used to get ready the contact surfaces of the monolithic aluminium specimen for wear test. The
procedure involves grinding of aluminium surfaces manually by 240, 320, 400, and 600 grit silicon carbide papers and
Fine polishing was done in a rotating disc polishing machine with velvet cloth impregnated with diamond paste. The
track diameter was kept constant at 60mm. The test was conducted for three different loads (10 N, 20 N and 30 N) with a
constant speed of 300 rpm. Initial weight of the sample was noted down. For a particular load at a particular speed the
test was run for 1 hr. At an interval of 15 minutes the weight loss was noted by weighing the sample.
2.6 Precipitation hardening/ Age hardening treatment
An aging study was conducted on specimens taken from the matrix alloy and composites in order to determine
the time required to attain peak hardness. The aging process essentially involves a solution treatment of the sample for 2
h at 550 ᴼC, then quenching in water followed by artificial aging at 150ᴼC for various aging times between 1hr to 5 hrs.
3. RESULTS AND DISCUSSION
In the following sections, the Microstructure structural analysis, Hardness, Tensile strength, Wear resistance
strength in both untreated and heat treated behavior of the materials examined are presented.
3.1 Microstructure Structural Features
Fig.2, shows the optical micrographs of Al2024 alloy reinforced with 0 %, 3% and 6 wt.% of SiO2 composites.
Micrographs reveal that there is fairly uniform distribution of SiO2 particulates throughout the matrix alloy and the
porosity is lower. It is reported that higher hardness is always associated with lower porosity of the MMCs [8]. Also, it
can be observed that there is good bonding between the matrix and the reinforcement particulates resulting in better load
transfer from the matrix to reinforcement material.
Figure 2: Microstructure of Al2024 alloy with different percentage of SiO2 (a) 0 wt.%; (b) 3 wt.%; (c) 6 wt.% reinforced
composite (Magnification 100X)

4. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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3.2Vickers hardness test
The resistance to indentation or scratch is termed as hardness. It is a function of the stress required to produce
some specific types of surface deformation. Figure 3 is a graph showing the effect of SiO2 reinforcement on the hardness
of cast Al2024-SiO2 particulate composites. Each value represented is an average of six measurements shown in Table 2.
For the 6% Wt. composition, there was an increase in the hardness value until the peak hardness was achieved after 2
hours. For the 3% Wt. composition the peak hardness was achieved after 2 hrs 30 mins. But the peak hardness of
unreinforced Al2024 alloy achieved after 3 hrs 45 mins. The hardness values decreased after the maximum limit was
achieved. The hardness of the composite sample and the unreinforced alloy that both were solution treated for 2 hrs with
no aging treatment, decreased due to more dissolution of the precipitates and will accelerate aging kinetics.
Table 2: Hardness of composites (Before and after age hardening)
Figure 3: Variation of hardness values with time (peak hardened condition)
One can notice that an increase in the hardness of the composite materials occurred after the aging treatment as
shown in Figure 3. However, it is of interest to note that peak hardness was observed at lower aging times for the
reinforced composites compared to the Aluminium base alloy (3 hrs 45 min for the Al alloy and 2 hrsfor the composite).
These results indicate that the addition of reinforcement to the aluminum matrix accelerates the aging kinetic. This
behavior can be related to the high matrix dislocation density induced by the mismatch between the matrix and the
reinforcement [9-10]. It is well known that high dislocation density in the metal matrix promotes dislocation-assisted
diffusion of the aging elements. On the other hand, the influence of the reinforcement on the aging behavior can be
attributed to the heterogeneous nucleation capability of metastable phases on the SiO2 particles. It can, therefore, be
concluded that both nucleation and growth of the Guinier-Preston zones (GP zones) are influenced by the ceramic
particles.
3.3Tensile test
Figure 4 is a graph showing the effect of reinforcement content on the Ultimate Tensile Strength (UTS) of cast
Al2024, Al2024- SiO2under various conditions. It can be seen that as the SiO2 weight percentage increased from 3% to 6
%, the UTS of the composite material increases monotonically by significant amounts. In fact, as the SiO2 content is
increased from 0% to 6%, the UTS increases by about 20 to 40% as shown in Table 3. This increase in UTS may be due
to the reinforcement particulates acting as barriers to dislocations in the microstructure [10]. Thermal mismatch between
the metal matrix and the reinforcement is a major mechanism for increasing the dislocation density of the matrix and,
therefore, increasing the composite strength.
Type of specimen
VHN (Vickers Hardness Number)
pre-heat
treatment
As quenched (Solution treated) Peak hardened
Al2024+0 % Wt. 136.00 123.05 176.10 (3 hrs. 45 mins)
AA2024+3 % Wt. 140.85 130.43 184.46 (2 hrs 30 mins)
AA2024+6 % Wt. 145.75 134.37 191.2.4 (2 hrs)

5. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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3.4 Tribological Properties
Wear is the progressive loss of material due to relative motion between a surface and the contacting substance or
substances. The wear damage may be in the form of micro-cracks or localized plastic deformation. Wear test was
conducted in a pin-on-disc tribometerunder dry sliding conditions. The results of the wear test conducted in the
laboratory conditions are shown in Table 4 and 5 for composite specimens containing 3 and 6% of SiO2 respectively. Fig.
5 and 6 indicates that the wear factor for Al2024 composites containing SiO2 under an applied load of 10, 20 and 30 N
with a constant speed of 300 rpm respectively. Wear rate increases with increase in applied load in both the composites,
because the applied load affects the wear rate of alloy and composites significantly and is the most dominating factor
controlling the wear behavior. The cumulative volume loss increases with increasing applied normal load and the contact
surface temperature increases as the applied load increases. The peak hardened Al2024-6% wt. SiO2 composite is having
superior wear performance under different conditions.
Figure 4: Comparison of tensile test results at various stages of heat treatment
Table 3: Ultimate tensile test under different conditions
Figure 5: Wear behaviour for 3% SiO2 composite
0
50
100
150
200
250
UltimateTensileStrength
(MPa)
Conditions
0%
3%
6%
0
0.05
0.1
0.15
0.2
0.25
0 20 40
Weightloss(gms)
Normal load (N)
Wear behaviour for 3% SiO2composite
as quenched
pre-heat
treatment
peak
hardened
Ultimate Tensile strength (MPa)
Condition 0% 3% 6%
As-cast 110.6 135.8 154.2
As-quenched 90.1 115.4 139.1
Peak hardened 187.2 197.8 226.4

6. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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Table 4: Wear test results for 3% sio2 composite conducted at 300rpm for 1hr duration in dry conditions
Table 5: Wear test results for 6% SiO2 composite conducted at 300 rpm for 1 hr duration in dry conditions
Figure 6: Wear behaviour for 6% SiO2 composite
0
0.05
0.1
0.15
0.2
0.25
0 10 20 30 40
Weightloss(gms)
Normal load (N)
Wear behaviour for 6% SiO2 composite
as
quenched
pre-heat
treatment
peak
hardened
As quenched
Load (N) 10 20 30
Initial weight (gms) 2.931 2.859 2.701
Final weight (gms) 2.859 2.701 2.498
Weight loss (gms) 0.072 0.158 0.203
Pre-heat treatment
Initial weight (gms) 2.851 2.794 2.703
Final weight (gms) 2.794 2.703 2.552
Weight loss (gms) 0.06 0.091 0.151
Peak hardened
Initial weight (gms) 2.711 2.671 2.571
Final weight (gms) 2.671 2.591 2.451
Weight loss (gms) 0.04 0.06 0.12
As quenched
Load (N) 10 20 30
Initial weight (gms) 2.931 2.865 2.745
Final weight (gms) 2.865 2.745 2.575
Weight loss (gms) 0.066 0.12 0.17
Pre-heat treatment
Initial weight (gms) 2.625 2.569 2.478
Final weight (gms) 2.569 2.478 2.334
Weight loss (gms) 0.056 0.091 0.144
Peak hardened
Initial weight (gms) 2.424 2.393 2.321
Final weight (gms) 2.393 2.321 2.224
Weight loss (gms) 0.031 0.072 0.097

7. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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5. CONCLUSIONS
• Composite containing 3% and 6% by Wt. of SiO2 were successfully produced by stir casting with fairly uniform
distribution of silicon oxide in the aluminium alloy matrix.
• The composite containing 6% by wt of SiO2 showed superior mechanical properties than that containing 3%.
• Composite were successfully heat treated in different conditions like quenching in water (at 25o
C) and peak
hardened state.
• The hardness (VHN), tensile strength and wear characteristics were improved much in the peak hardened state.
• It was found that in comparison to the 2024 aluminum alloy, the precipitation kinetic was accelerated by adding
the reinforcement. This effect reduced the time for obtaining the maximum hardness by the aging heat treatment.
From the studies in overall it can be concluded that Al2024-SiO2 exhibits superior mechanical and tribological
properties.
• Aluminium composites (Al2024-SiO2) can be used in wear resistant applications there by reducing the cost of
components in various applications.
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