study on slurry erosive wear behaviour of al 6061 based ...fig. 2 slurry erosion testing set up...
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Corresponing author: Abrar Ahamed E-mail address: abrar_gce@rediffmail.com
Doi: http://dx.doi.org/10.11127/ ijammc.2016.04.14 Copyright@GRIET Publications. All rights reserved.
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Advanced Materials Manufacturing & Characterization Vol 6 Issue 2 (2016)
Advanced Materials Manufacturing & Characterization
journal home page: www.ijammc-griet.com
Study on Slurry Erosive Wear Behaviour of Al 6061 based Composites
Abrar Ahamed1*, Anwar Khan2
1 Department of Mechanical Engineering, Birla Institute of Technology, off shore campus, UAE 2 Department of Mechanical Engineering, Ghousia College of Engineering, Ramanagaram, Karnataka, India
Abstract In the present investigation, Titanium di oxide (TiO2) reinforced Al
6061 composites were fabricated using liquid metallurgy route. The
performance of Al 6061 based Titanium di oxide composites in slurry
erosive environment was examined. The slurry consists of equiaxed
sand particles of size approximately 600 µm in 3.5%NaCl solution. The
slurry erosive wear studies on Al 6061 alloy and Al 6061-TiO2
composites were carried out at different sand concentration, varying
rotational speed and at different time duration of exposure. It was
observed that slurry erosive weight loss decreased with increase in
TiO2 reinforcement. However, there was increase in slurry erosive
weight loss with increase in slurry concentration, rotational speed and
time duration of exposure.
Key words: Aluminium 6061 composites, TiO2, slurry erosive wear.
1. INTRODUCTION
Aluminum alloys are preferred engineering material for
automobile, aerospace and mineral processing industries for
various high performing components that are being used for
varieties of applications owing to their lower weight and
excellent thermal conductivity properties. Among several series
of aluminum alloys, heat treatable Al6061 and Al7075 are much
explored, among them Al6061 alloy are highly corrosion
resistant and are of excellent extricable in nature and exhibits
moderate strength and finds many applications in the fields of
construction (building and high way), automotive and marine
applications [1]. The composites formed out of aluminum alloys
are of wide interest owing to their high strength, fracture
toughness, wear resistance and stiffness. Further these
composites are superior in nature for elevated temperature
application when reinforced with ceramic particle [2]. In recent
years, the use of fly ash as a reinforcement material in Al alloys
has been reported to be desirable from both environmental and
economic points of view due to its availability as a low cost
waste material [3].
Slurry erosion can be defined as a type of wear or loss of
material experienced by a component, when exposed to high
velocity stream of slurry mixture of solid particles in a liquid,
usually water [1]. When the components are entrained in such
environments, the design life of the component is greatly
reduced, resulting in huge economic losses. The areas in which
components suffering from this problem include, mining
machinery components, hydraulic transport of solids in
pipelines, marine, oil gas and power generation industries [2–
5]. Erosive wear is a complex phenomenon due to presence of
too many variables such as
1. Target parameters: includes, composition, microstructure, mechanical properties [6-8].
2. Process parameters: viz particle size, shape, velocity and particle concentration [6, 8-10].
3. Environmental parameters: temperature, humidity, etc. [11].
Many peoples have studied and reported on slurry erosive
wear behavior of metal matrix composites. Caron et al. [12]
75
studied the slurry erosive wear behavior of 5083-Al2O3
composites. They have noticed that, slurry erosive wear of
composites increased with increase in Al2O3 content in the
matrix material. Ramachandra et a.l [13] have reported on
slurry erosive wear behavior of Al/SiC composites, slurry
erosive wear resistance increases with increasing if SiC content.
The formation of passive layers on the surface of the slurry
erosive specimens decreased wear loss by forming protective
layers against the impact of slurry. Ramachandra and
Radhakrishna [14] have reported on slurry erosive wear
behavior of Al-12wt%Si alloy reinforced with fly ash
composites. They have reported that use of flyash has enhanced
the slurry erosion wear resistance of the developed composites
which has been attributed to the formation of protective
passive layer on the worn surfaces. Li et al. [15] have proposed
the effect of time duration on slurry erosive wear of aluminum
alloy and have found that wear rate increases with an increase
in test time duration. Aso et al. [16] have investigated the effect
of impact velocity and sand concentration on erosive wear and
have reported that, with increase in sand concentration and
impact velocity increases the wear rate. Girish et al. [17] have
studied slurry erosion of ductile materials under normal impact
condition and reported that velocity and particle size has
strong dependence on erosion wear but solid concentration has
relatively weak dependence. Acharya et al. [18] have reported
increasing the hardness of target material decreases the wear
rate.
This paper reports on the slurry erosive wear behaviour of cast
Al6061 and Al6061-TiO2 composites. Cast Al6061 and its
composites have been produced by liquid metallurgy route
which is a very popular technique owing to its economy and
versatility coupled with large-scale production. The extent of
incorporation of TiO2 in the matrix alloy has been tried out
from 4wt% to 10wt% in steps of 2wt%. Microstructure studies,
micro hardness and slurry erosive wear tests using the
fabricated erosive test rig have been conducted on the base
matrix Al6061 and Al6061-TiO2 composites.
2. EXPERIMENTAL PROCEDURE
Aluminum 6061 was used as matrix material owing to its
excellent mechanical properties coupled with good formability
and its wide applications in industrial sector. The material was
procured from M/s Plast-Met-Chem in the form of plates. The
chemical composition of the material is given in table. 1
Table 4.1 Chemical composition of Al6061
Titanium dioxide was chosen as reinforcement owing to its
high hardness and low co-efficient of thermal expansion. TiO2 is
highly wear resistant and also has good mechanical properties,
including high temperature strength and thermal shock
resistance [19]. The properties of Titanium dioxide are listed in
Table. 2.
Table. 2 Properties of Titanium dioxide
Density 4.2g/cc
Tensile Strength 300-350 Mpa
Vickers Hardness 980 kgf/mm2
Compressive Strength 800-1000 MPa
Modulus 240 GPa
A batch of 3.5kgs of Aluminum 6061 alloy was melted using a
6KW electric furnace as shown in Fig.1. The metal was
degassed using commercially available chlorine based tablets
(Hexachloroethane). The molten metal was agitated by use of
mechanical stirrer rotating at a speed of 300 rpm to create a
fine vortex. Preheated TiO2 powders (preheated to 700C for 2
hrs) were added slowly into the vortex while continuing the
stirring process. The stirring duration was 10 min. The
composites melt maintained at a temperature of 710C was
then poured in to preheated metallic moulds. The stirrer
blades were made of stainless steel and coated with ceramic
material to minimize the iron pick up by the molten metal. The
amount of TiO2 was varied from 4 to 10 wt% in steps of 2 wt%.
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Fig.1. Photograph of Electric Resistance Furnace.
The samples were prepared for microstructure, hardness and
slurry erosive wear tests.
The micro hardness test was performed by applying 100 grams
of load for a period of 5secs on the polished surfaces of both the
matrix alloy and composites. The hardness was noted by taking
the diagonal length of indentation produced. The test was
carried out at five different locations in order to negate the
possible effect of indenter resting on the harder particles. The
average of all the five readings was taken as hardness of
sample.
Slurry Erosion test set up was fabricated using a commercially
available domestic mixer grinder. Fig.2 shows the details of the
fabricated test set up. This machine consists of a motor with a
speed control unit having the specification of a mixer grinder
with a maximum speed of 10,500 rpm.
Fig. 2 Slurry Erosion testing set up
Slurry erosion test were carried on Al6061 alloy and Al6061-
TiO2 composites. The slurry erosive wear test samples of 8 mm
diameter and 15 mm height were machined from the castings
and prepared by polished as per standard metallographic
procedure. The samples were cleaned in acetone and weighed
using an electronic microbalance before and after the wear
tests. The polished samples were fixed on spindles with the
help of a screw. The samples were dipped into slurry pot made
of stainless steel. The slurry was prepared by mixing 3.5% of
NaCl and silica sand with distilled water. The studies were
carried out at different sand concentration, varying rotational
speed and at different time durations while keeping the
impinging particle size to be constant as an average of 300
microns. After the test, specimens were dried and cleaned
before measuring weight loss.
3. RESULTS AND DISCUSSIONS
3.1 Microstructure
The optical microphotographs of the cast Al6061 and Al6061-
TiO2 composites are shown in Fig. 3. The micrographs clearly
indicate the evidence of minimal porosity in both the base alloy
and the composites. The distribution of TiO2 particles in a
matrix alloy is fairly uniform. Further these microphotographs
reveal an excellent bond between the matrix alloy and the
reinforcement particles.
Al 6061 alloy Al 6061-4wt%TiO2
composite
Al 6061-6wt%TiO2
composite
Al 6061-10wt%TiO2
composite
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Fig. 3. Microphotographs of as cast Al 6061 alloy and its
composites
3.2 Microhardness:
The variation of micro hardness with increased contents of TiO2
particles in the matrix Al6061 alloy is shown in Fig. 4. It is
observed that with increased content of TiO2 particles in the
matrix alloy, there is a significant improvement in the micro
hardness of the composites. An improvement of around 43% is
observed in Al6061-10wt%TiO2 composites when compared
with the unreinforced Al6061 matrix alloy. This trend is similar
with the result of other researchers [20].
The improvement in hardness of composites can be attributed
to the following factors.
1. TiO2 is a hard reinforcement. Hard reinforcement in a soft and ductile matrix always enhances the hardness of the matrix alloy in general.
2. Probable increased density of dislocations due to thermal mismatch between the matrix alloy and TiO2 particles due to the large differences in coefficient of thermal expansion. The increased level of dislocations thereby increases the resistance of the materials to plastic deformation leading to improved hardness.
Fig. 4 Variation of micro hardness with increased content
of TiO2
3.3 Erosive Wear results:
3.3.1. Effect of Percentage weight of reinforcement:
There is a significant reduction in the slurry erosive wear rate
of the composites with an increase in the percentage weight of
the reinforcement as shown in the Fig. 5. This can be attributed
to the higher hardness of the composite as discussed earlier.
Higher the hardness better is the erosive wear resistance of the
materials.
Fig. 5 Variation of Slurry erosive wear rate of cast Al6061
and cast Al6061-TiO2 composites
The improved performance of Al6061-TiO2 composites when
compared with Al6061 alloy can be attributed to following
reasons:
1. Aluminum alloys when exposed to slurry media / NaCl solution, it reacts with water to form a stable passive oxide layer on the surface [21-22]. Formation of such oxide layer protects the surface of alloys from erosive and corrosive action of slurry.
2. Improved hardness of Al6061-TiO2 composites can also be attributed to improvement in slurry erosive wear resistance when compared with Al6061 alloy. Higher the surface hardness of the material lowers the material removal rate by mechanical action of solid particles in the slurry.
3. Presence of TiO2 particles in the material reduces the effective metallic area of the composite exposed to slurry and reduces the formation and growth of corrosion pits in the material which may be another reason for improved wear resistance of the composites [23]. From the microphotographs it is observed that there exist minimum micro porosities in the composites. Composites with lower the porosities exhibits better wear and corrosion resistance as reported by several researchers [24-25].
4. Good bond that between matrix and reinforcement also resist the corrosion attack and slurry erosive wear [26-27].
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3.3.2. Effect of Slurry Concentration:
Fig. 6 Variation of Slurry erosive wear rate of cast Al6061
and cast Al6061-TiO2 composites for different slurry
concentration
The slurry erosive wear rates of base Al6061 alloy and Al6061-
TiO2 composites with different concentration of silica slurry at
a given slurry rotation and time duration is as shown in Fig .6. It
is observed that increased slurry concentration results in
higher slurry erosive wear rate of both the base alloy and its
composites studied. Increased slurry erosive wear rates at
higher slurry concentrations can be attributed to the fact that
increased abrasive particle concentration in the slurry will
enhance the probability of larger impingement surface in the
slurry. This in turn will result in increased deterioration of the
material from its surface. Also, as the concentration of solid
mass increases in the slurry the sand particles can interact with
target material more strongly leading to increased mass loss
[28]. This phenomenon is clearly observed on subjecting the
worn surfaces to SEM studies. Increased slurry concentration
has resulted in higher density of cracking and also certain
deposit formations over the exposed surface as shown in Fig .7.
However, increased content of reinforcement in the matrix
alloy reduces the slurry erosive wear rate for all the slurry
concentrations studied. This can be attributed to the higher
hardness of composites with increased content of TiO2 particles
in the matrix alloy.
60 gms/ltr 90 gms/ltr
120 gms/ltr
Fig.7 SEM photographs of worn slurry erosive wear test
specimens for different slurry concentrations.
3.3.3. Effect of speed of slurry rotation:
Fig.8: Variation of Slurry erosive wear rate of cast Al6061
and cast Al6061-TiO2 composites for different speeds of
slurry rotations
The slurry erosive wear rate of Al6061 alloy and Al6061-TiO2
composites with different speed of silica slurry rotation at
constant time duration and slurry concentration is as shown in
Fig.8. It is observed that increased speed of slurry rotation
results in higher slurry erosive wear rate of both the base alloy
and its composites studied. At very high speed of 10,500 rpm
maximum slurry erosive wear rate is observed. The increased
speed of the slurry rotation will tend to increase the velocity of
impingement of the abrasive grains present in the slurry.
Increased impingement velocity will lead to higher rates of
material removal from the surfaces resulting in higher slurry
erosive wear rate. The larger extent of impingement at higher
speed of slurry rotation is demonstrated by SEM photographs
as shown in Fig.9. These SEM photographs clearly indicate the
presence of several craters on the worn surfaces. Higher the
speed of slurry rotation larger is the extent of crater formation
noticed on the worn surfaces as evident from Fig.9. However,
79
increased content of reinforcement in the matrix alloy reduces
the slurry erosive wear rate for all the speeds of slurry
rotations studied. This can be attributed to the higher hardness
of composites with increased content of TiO2 particles in the
matrix alloy.
8000rpm 9000rpm
10500rpm
Fig.9 SEM photographs of worn slurry erosive wear test
specimens for different speeds of slurry rotations.
3.3.3. Effect of time duration:
The slurry erosive wear rates of Al 6061-TiO2 composites with
different time duration at a given slurry rotation and speed is
as shown in Fig.10. It is observed that increased time duration
results in reduction of slurry erosive wear rate for both the
base alloy and the composites. This can be attributed to the fact
that the surface of the specimen gets strain hardened with as
the abrasive particles frequently impinge over its surface. This
phenomenon will lower the material loss from the surfaces.
Further decrease in weight loss can also be due to formation of
passive layer over the exposed surface of the specimens which
retards the slurry erosive wear rate by acting as a protective
layer [29]. However, increased content of TiO2 in matrix alloy
reduces the wear rate for all time duration. This can be
attributed to the higher hardness of composites with increased
content of TiO2 in matrix alloy. Probable deposits are observed
in the present study as shown in SEM photographs in Fig.11.
Fig.10 Variation of Slurry erosive wear rate of cast Al6061
and cast Al6061-TiO2 composites for different time
durations of exposure.
30 min 45 min
60 min
Fig.11 SEM photographs of worn slurry erosive wear test
specimens for different time durations.
4. CONCLUSIONS
Al 6061-TiO2 composites have been cast successfully by liquid metallurgy route.
Micro structure clearly confirms minimum porosity in composites developed.
Significant increase of 43% is noticed in hardness for composites having TiO2 as reinforcement.
Al 6061- TiO2 composites possess superior slurry erosion resistance when compared with Al 6061 alloy.
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With increase in TiO2 reinforcement there is increase is slurry erosive wear resistance.
With increase in speed, slurry concentration and test duration there is increase in slurry erosive wear rate observed.
However, under all test conditions studied Al 6061-TiO2 composites have exhibited lower slurry erosive wear rate when compared with Al 6061 matrix alloy.
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