Journal of Minerals & Mateials Characterization & Engineering, Vol. 10, No.6, pp.493-505, 2011
jmmce.org Printed in the USA. All rights reserved
493
Sand Slurry Erosive Wear Behavior of Hot Extruded Al6061-SiC Composites
C.S. Ramesh*, R. Keshavamurthy
Department of Mechanical Engineering, PES Institute of Technology, 100ft Ring Road, BSK III
Stage, Bangalore-560 085.
Corresponding Author: csr_gce@yahoo.co.in
ABSTRACT
Aluminum based composites are very popular in automotive and aerospace segments. In
particular aluminum alloy-SiC composite systems are widely studied and seriously explored for
various other applications in defense and space. However they have not been looked at as
potential materials in naval and chemical applications where synergistic effects of both wear
and corrosion need to be addressed to. This needs the assessment of wear behavior of
composites. From the survey, it is evident that major focus is on mechanical and corrosion
properties of cast composites. However, meager information is available as regards the slurry
erosive wear behavior of cast and extruded metal matrix composites.
In the light of the above, the present investigation is aimed at studying in detail the slurry erosive
wear behavior of cast and hot extruded Al6061 and Al6061-SiC composites in sand slurry.
Al6061-SiC composites have been prepared primarily by vortex method. Hot extrusion of these
composites and the matrix alloy has been carried out at 5500C using a 500T hydraulic press.
Both as cast and hot extruded composites have been subjected to microstructure studies,
microhardness, and slurry erosive wear tests. The hot extruded composites exhibit higher
hardness, and slurry erosive wear resistance when compared with as cast alloy and its
composites.
Keywords: Slurry erosive wear, Hot extrusion, Composites, Silicon carbide.
1. INTRODUCTION
Aluminum alloys have excellent mechanical properties coupled with good corrosion resistance.
However, they possess poor wear resistance. To improve the above said property, researchers
494 C.S. Ramesh, R. Keshavamurthy Vol.10, No.6
have successfully dispersed various hard and soft reinforcements such as SiC, Al2O3, glass,
graphite, mica, and coconut shell char in aluminum alloys by different processing routes [1-3]. In
recent years, considerable interest has been shown in extending the use of metal matrix
composites in the marine environment [4]. This demands evaluation of corrosion as well as
erosion-corrosion characteristics of the composite materials under simulated marine
environment.
A few studies have been reported by different investigators on erosive –corrosive wear behavior
of Al alloy and its composites [5]. Erosion–corrosion of MMCs takes place usually by three
different mechanisms (i) corrosion of the matrix alloy (ii) abrasion /erosion of the matrix as
reinforcements by the impact of suspended erodent. The corrosion of the matrix takes place by
an oxidation reaction [6] when Al3+ is released from the matrix to the slurry. This results in
exposure of large fresh area of the specimen surface to the slurry which causes further removal
of matrix alloy leading to higher material loss in the initial stages.
As time progresses, the slurry adjacent to the specimen surface gets saturated with Al3+, resulting
in a reduction in pH of slurry. As a result, the dissolution rate of Al3+ in the slurry is reduced and
the excess Al3+ is deposited over the specimen surface which combines with OH- to form
Al(OH)3 compound over the specimen surface. This passive layer so long as it is not broken,
protects the matrix from direct contact with the slurry and slows down the rate of weight loss.
Additionally [7-8] the gas evolved during corrosion is entrapped into the crater micro sites
(formed due to coming out of particulates or removal of matrix) and protects these sites from the
slurry which further slows down the rate of weight loss.
It is reported that characteristics of aluminum metal matrix composites have been improved by
subjecting the cast composites to hot extrusion as studied and reported by Alkpas et.al [2],
Straffelini et.al [9], Joshi[10], and Ganesh et.al [11]. Hot Extrusion has considerably improved
the microstructure and the mechanical of stir cast Al-Si-Pb alloys and greatly decreased the
porosity. At room temperature the hot extruded Al-Si-Pb alloys have demonstrated better wear
resistance than base alloys as reported by An et.al [12].
Contrary to these findings, some other investigators [13-14] have reported reduced wear rates of
the composites where the matrix alloy contained hard particles such as SiC, Al2O3, SiO2, Si3N4
and glass. Of all the techniques available for preparing the composite, the liquid metallurgy route
is the most popular one as reported by Ramesh et.al [15]. However, the composites produced by
this route will have inherent casting defects thereby limiting the practical applications of
composites. To overcome this, it is very much essential to adopt the secondary forming process
on cast composites in particular hot extrusion. From the literature survey it is quite evident that
most of the researchers have focused their attention on friction and wear behavior of primarily
processed composites. However meager literature is available as regards the tribological
Vol.10, No.6 Sand Slurry Erosive Wear Behavior 495
properties of secondary processed metal matrix composites In the light of the above, this paper
discusses the effect of hot extrusion of Al6061-SiC on its hardness and erosive wear properties.
2. EXPERIMENTAL DETAILS
2.1 Composite Preparation and Hot Extrusion
A batch of 3.5kgs of Aluminum 6061 alloy was melted using a 6KW electric furnace. The melt
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 SiC (preheated to 700oC for 2 hrs) were added slowly in to the vortex
while continuing the stirring process. The stirring duration was 10 min. The composites melt
maintained at a temperature of 710ºC was then poured in to preheated metallic moulds. The
stirrer blades used were made of stainless steel and were coated with ceramic material to
minimize the iron pickup by the molten metal. The amount of SiC was varied from 4 to 8 wt%
in steps of 2. The cast matrix alloy and its composites were hot extruded at a temperature of
5500C with an extrusion ratio of 9.0 using a 500T hydraulic press at National Physical
Laboratory, New Delhi, India. Microstructural studies, hardness, and erosive wear tests were
conducted on both the cast and extruded matrix alloy and composites. Hardness of all the studied
materials were eva l u a ted at 50g for a duration of 10 seconds using micro hardness tester.
2.2 Fabrication Details of Erosion Test Set Up
Erosion test set up fabricated is shown in Fig.1. This machine consists of a motor with a speed
control unit having the specification of a mixer grinder with a maximum speed of 8000 rpm. The
slurry mixture is filled in the stainless steel jar. The jar has a provision for holding the specimen
along with it. The specimen holder assembly details are shown in Fig 2. The specimen is
cylindrical having 8 mm diameter and 15 mm height. The specimen is held in the specimen
holder at an inclination of 0o as per the ASME standards.
3. RESULTS AND DISCUSSIONS
3.1 Microstructure
The microstructure of cast and extruded matrix alloy Al6061 and its composites are shown in
Fig.3. The microstructures clearly indicate the homogeneity in the distribution of reinforcements
in the matrix alloy. Further, it is evident that the particles have got damaged during the extrusion
process in addition to be i ng o ri e n ted along the extrusion direction.
496 C.S. Ramesh, R. Keshavamurthy Vol.10, No.6
Fig. 1. Erosion testing set up
Fig. 2. Specimen holder drawing
Vol.10, No.6 Sand Slurry Erosive Wear Behavior 497
25µm25µm25µm
25µm25µm25µm
(a) Cast Al6061 alloy (b) Extruded Al6061 alloy
25µm25µm25µm
25µm25µm25µm
(c)Cast Al6061-8wt%SiC composite (d) Extruded Al6061-8wt%SiC composite
Fig. 3. Microphotographs of cast and extruded Al6061 and its composites
3.2 Microhardness
The variation of hardness of composites with increased contents of reinforcement is shown in
Fig.4. It is observed that increased content of the reinforcement results in enhanced hardness of
the composites. This can be attributed to the fact that SiC is a hard ceramic. Addition of hard
phase to a soft ductile matrix leads to improved hardness as reported by Ramesh and Seshadri
[16]. However; the extruded composites exhibit a higher hardness when compared with cast
composites. This can be attributed to healing up of casting defects during extrusion. For the
matrix alloy, an increase in hardness of 12% is observed and for cast Al6061-8wt% SiC an
increase in hardness of 36% is noticed on hot extrusion.
3.3 Slurry 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
498 C.S. Ramesh, R. Keshavamurthy Vol.10, No.6
higher hardness of the composite as discussed earlier higher the hardness better is the erosive
wear resistance of the materials.
0
10
20
30
40
50
60
70
80
90
100
Al6061 alloyAl6061-4wt% Si CAl6061-6wt% SiCAl6061-8wt% SiC
Percentage of Reinforcement
Microhardness (VH N )
As Cast
Hot Extruded
Fig. 4. Variation of microhardness of cast and extruded Al6061 alloy and Al6061-SiC
composites.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Al6061 Al6061-
4%SiC Al6061-
6%SiC Al6061-
8%SiC
Percentage weight of reinforcemen t
S lurry erosisive wear rate in
mm3/m2-Hr
As Cast
Extruded
S lurry erosive w ear resistance
In mm
3
/m
2
-hr
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Al6061 Al6061-
4%SiC Al6061-
6%SiC Al6061-
8%SiC
Percentage weight of reinforcemen t
S lurry erosisive wear rate in
mm3/m2-Hr
As Cast
Extruded
S lurry erosive w ear resistance
In mm
3
/m
2
-hr
Fig. 5. Variation of Slurry erosive wear rate of cast and hot extruded Al6061 alloy and
Al6061-SiC composites.
Vol.10, No.6 Sand Slurry Erosive Wear Behavior 499
3.3.2 Effect of slurry concentration
The slurry erosive wear rates of base Al6061 alloy and Al6061-SiC 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.
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 SiC particles in the matrix alloy. It is also clear that extrusion have a
significant effect on slurry erosive wear resistance of both the matrix alloy and all the composite
systems studied.
Speed of Rotation:6000rpm,
Ti me duration:3 0mins
3700
4200
4700
5200
5700
6200
6700
7200
6090 120
Slurry Concentration in grms/ltr
Slurry erosive w ear resistance
in mm3/m2 - hr
Ca st Al6061
Extruded Al6061
Ca st Al6061-4wt%Si C
Extruded Al6061-4wt% SiC
Ca st Al6061-6wt%Si C
Extruded Al6061-6wt%SiC
Ca st Al6061-8wt%Si C
Extruded Al6061-8wt%SiC
Slurry erosive wear resistance
In mm
3
/m
2
-hr
Speed of Rotation:6000rpm,
Ti me duration:3 0mins
3700
4200
4700
5200
5700
6200
6700
7200
6090 120
Slurry Concentration in grms/ltr
Slurry erosive w ear resistance
in mm3/m2 - hr
Ca st Al6061
Extruded Al6061
Ca st Al6061-4wt%Si C
Extruded Al6061-4wt% SiC
Ca st Al6061-6wt%Si C
Extruded Al6061-6wt%SiC
Ca st Al6061-8wt%Si C
Extruded Al6061-8wt%SiC
Slurry erosive wear resistance
In mm
3
/m
2
-hr
Fig. 6. Variation of Slurry erosive wear rate of cast & hot extruded Al6061 alloy and
Al6061-SiC composites for different slurry concentrations.
500 C.S. Ramesh, R. Keshavamurthy Vol.10, No.6
As Cast Hot Extruded
60 gms/ltr
90 gms/ltr
120 gms/ltr
Fig.7.SEM of worn slurry erosive wear test specimens of as cast and
hot extruded Al6061-SiC composites at different slurry
concentrations.
3.3.3 Effect of speed of slurry rotation
The slurry erosive wear rate of Al6061 alloy and Al6061-SiC 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
cast and extruded base alloy and its composites studied. At very high speed of 8000 rpm
Vol.10, No.6 Sand Slurry Erosive Wear Behavior 501
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. However, increased content
of reinforcement in the matrix alloy reduces the slurry erosive wear rate for all the speeds of
slurry rotations studied in both cast and extruded conditions. This can be attributed to the higher
hardness of composites with increased content of SiC particles in the matrix alloy. It is also clear
that extrusion have profound effect on slurry erosive wear resistance of both the matrix alloy and
all the composite systems studied. This can be attributed to the fact that extruded specimens
possess higher hardness when compared to the cast ones as discussed earlier.
Time : 30mins,
Slurry concentra tion : 9 0g rms/ltr
4000
4500
5000
5500
6000
6500
7000
7500
4000 6000 8000
Speed of slu rry rotation in rpm
Sl urry e rosiv e wea r ra te in
mm3/m2-hr
Cast Al6061
Extruded Al6061
Cast Al6061-4wt%SiC
Extruded Al6061-4wt%
SiC
Cast Al6061-6wt%SiC
Extruded Al6061-
6w t%SiC
Cast Al6061-8wt%SiC
Extruded Al6061-
8w t%SiC
Slurry erosive w ear resistance
In mm
3
/m
2
-h r
Time : 30mins,
Slurry concentra tion : 9 0g rms/ltr
4000
4500
5000
5500
6000
6500
7000
7500
4000 6000 8000
Speed of slu rry rotation in rpm
Sl urry e rosiv e wea r ra te in
mm3/m2-hr
Cast Al6061
Extruded Al6061
Cast Al6061-4wt%SiC
Extruded Al6061-4wt%
SiC
Cast Al6061-6wt%SiC
Extruded Al6061-
6w t%SiC
Cast Al6061-8wt%SiC
Extruded Al6061-
8w t%SiC
Slurry erosive w ear resistance
In mm
3
/m
2
-h r
Fig.8. Variation of Slurry erosive wear rate of cast and extruded Al6061 and Al6061-SiC
composites for different speed of slurry rotation.
3.3.4 Effect of time duration
The slurry erosive wear rates of cast & extruded Al 6061-SiC 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
502 C.S. Ramesh, R. Keshavamurthy Vol.10, No.6
formation of passive layer over the exposed surface of the specimens which retards the slurry
erosive wear rate by acting as a protective layer [17]. However, increased content of SiC in cast
and extruded matrix alloy reduces the wear rate for all time du r a t ion. This can be attributed to the
higher hardness of composites with increased content of SiC in matrix alloy. Probable deposits
are observed in the present study as shown in SEM photographs in Fig. 11. SiC in matrix alloy.
Probable deposits are observed in the present study as shown in SEM photographs.
As Cast Hot Extruded
4000 rpm
6000 rpm
8000 rpm
Fig.9.SEM photographs of worn slurry erosive wear test specimens
of as cast and hot extruded Al6061-SiC composites at different speed
of slurry rotation.
Vol.10, No.6 Sand Slurry Erosive Wear Behavior 503
Speed = 4000 r pm, Sl urr y Concent r at i on = 90 grm s/ltr
2800
3300
3800
4300
4800
5300
30 45 60
Time duration in minutes
Slurry erosive wear rate
in mm3 /m2 -h r
Cast Al6061
Extruded Al6061
Cast Al6061-4wt%Si
C
Extruded Al6061-
4wt% SiC
Cast Al6061-6wt%Si
C
Extruded Al6061-
6w t%SiC
Cast Al6061-8wt%Si
C
Extruded Al6061-
8w t%SiC
Slurry erosive wear resistance
In mm
3
/m
2
-hr
Speed = 4000 r pm, Sl urr y Concent r at i on = 90 grm s/ltr
2800
3300
3800
4300
4800
5300
30 45 60
Time duration in minutes
Slurry erosive wear rate
in mm3 /m2 -h r
Cast Al6061
Extruded Al6061
Cast Al6061-4wt%Si
C
Extruded Al6061-
4wt% SiC
Cast Al6061-6wt%Si
C
Extruded Al6061-
6w t%SiC
Cast Al6061-8wt%Si
C
Extruded Al6061-
8w t%SiC
Slurry erosive wear resistance
In mm
3
/m
2
-hr
Fig.10. Variation of Slurry erosive wear rate of cast & extruded composites for different
test durations.
As Cast Hot Extruded
30 min
45 min
504 C.S. Ramesh, R. Keshavamurthy Vol.10, No.6
60 min
Fig.11. SEM of worn slurry erosive wear test specimens for different
test durations.
4. CONCLUSIONS
Al6061-SiC composites have been successfully prepared by liquid metallurgy route. Up to
8wt%SiC has been successfully dispersed in the matrix alloy. Cast Al6061 and Al6061-SiC have
been successfully hot extruded.
There is a significant increase in slurry erosive wear rate of cast & extruded base alloy and its
composites with an increase in speed of slurry rotation. However, extruded Al6061-SiC
composites exhibited better slurry erosive wear resistance when compared with Al6061 alloy and
cast Al6061-SiC composites under identical test conditions.
With an increase in test duration, there is a reduction of slurry erosive wear rates of both cast and
extruded base alloy and its composites. However, extruded Al6061-SiC composite possesses
higher slurry erosive wear resistance when compared with the base alloy and cast Al6061-SiC
composites.
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