Open Journal of Metal, 2013, 3, 26-33
http://dx.doi.org/10.4236/ojmetal.2013.32A1004 Published Online July 2013 (http://www.scirp.org/journal/ojmetal)
Equal Channel Angular Pressing of Al-SiC Composites
Fabricated by Stir Casting
Farouk Shehata1*, Nahed ElMahallawy2, Mohamed Arab1, Mohamed Agwa1
1Department of Mechanical Design and Production Engineering, Faculty of Engineering,
Zagazig University, Zagazig, Egypt
2Faculty of Engineering, Ainshams University, Cairo, Egypt
Received May 9, 2013; revised June 19, 2013; accepted June 29, 2013
Copyright © 2013 Farouk Shehata et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Stir casting method was used to produce conventional metal matrix composites (MMC) with fairly homogenous disper-
sion of reinforcement material. Commercial pure aluminum and silicon carbide particles (50 µm) were selected as ma-
trix and reinforcement materials respectively. The matrix was first completely melt and kept constant at 750˚C. Then
SiC powder preheated to 800˚C was added during stirring action. No wetting agents were used. The melt mixture was
poured into a metallic mold. The composite contents were adjusted to contain 5% and 10% SiC. The as-cast composites
were processed by Equal Channel Angular Pressing (ECAP) route A. The microstructure and mechanical properties
were studied. Results indicated that as cast AlSiC composites can be successfully fabricated via a cheap conventional
stir casting method, giving fairly dispersed SiC particle distribution and having low porosity levels < 3.6%. The me-
chanical properties have improved compared to as cast composites. ECAP technique has greatly reduced SiC particles
from 50 to 3 µm. After the first ECAP pass, yield strength has almost twice its value in the as cast composites. The
maximum yield of 245 MPa obtained after 8 passes is almost four times the corresponding values of the as cast MMC
composites. Hardness has also increased to 1.5 times its value in the as cast composites after one ECAP pass. The
maximum hardness of 71 HRB obtained after 8 passes, which is almost 3.5 times the corresponding values of the as cast
MMC composites.
Keywords: Metal Matrix Composites; (Al-SiC) Composite; Porosity; Stir Casting
1. Introduction
Metal matrix composites (MMC) have received much
attention because of their improved mechanical proper-
ties and relatively low cost, those have made them attrac-
tive for numerous applications in various fields including
aerospace, automotive and sports industries [1,2]. More
specifically, particulate metal matrix composites (PMMCs)
are attractive not only for their high mechanical proper-
ties, but also because of their isotropic properties com-
pared to the continuously reinforced MMCs [3,4]. Alu-
minum silicon carbide (Al-SiC) composites have recently
drawn the attention of many research scientists and tech-
nologists. Several aspects are to be considered with re-
gard to the metallic matrix, namely, composition, re-
sponse to heat treatments, mechanical and corrosion be-
havior. Since aluminum offers flexibility in terms of
these aspects, accordingly, aluminum alloys have been
used in several studies for research and technological
applications. However, few numbers of researches, if any,
have been published using pure aluminum as a matrix,
they mostly use aluminium alloys.
Pure aluminum tends to solidify progressively from
the die surface toward the thermal center of the casting,
pushing SiC particles towards the centre of casting. At
the end of solidification, any shrinkage is confined along
the thermal centerline of the casting. The progressive so-
lidification of pure metal is therefore, difficult in avoid-
ing SiC from sinking in the bottom of the casting.
Aluminum alloys usually have broad freezing range.
They are in a semisolid state throughout most of the so-
lidification process. The semisolid state and wide mushy
zone would be useful in avoiding SiC sink in bottom of
casting. This usually made aluminum alloys preferred
compared to pure metal in avoiding SiC aggregations.
However, pure aluminium serves as a very ductile matrix
and generally has higher corrosion resistance compared
*Corresponding author.
C
opyright © 2013 SciRes. OJMetal

F. SHEHATA ET AL. 27
to aluminum alloys.
The SiC particles as reinforcement materials are harder
than tungsten carbide; this is the reason why SiC is usu-
ally recommended by many researchers as reinforcement
material [5]. The choice of the composite processing
route is dictated by the volume fraction of the SiC rein-
forcement in the composite. For instance, the stir casting
route is more suitable for low volume fractions < 20%,
whilst the infiltration routes are more appropriate for
high volume fraction of the reinforcement > 40% [6,7].
Also infiltration routes often produce agglomerated par-
ticles in the ductile matrix and as a result they exhibit
extremely low ductility [8,9]. Stir casting is therefore
adopted for this work. The stir casting technique is the
most economical of all the available routes for prepara-
tion of MMC [10]. According to Skibo et al. [11], the
cost of preparing composites material using a stir casting
method is about one-third to half that of competitive
methods such as powder metallurgy, it is projected that
the cost will fall to one-tenth. SiC particles aggregations
or clusters and poor wettability are the main processing
problems in as cast composites. Particle clusters act as
crack or decohesion nucleation sites at stresses lower
than the matrix yield strength, causing the MMC to fail at
unpredictable low stress levels [12,13]. Another process-
ing problem is the chemical reaction of aluminum melt
and SiC forming aluminum carbide compound (Al4C3)
that degrades the mechanical properties. One of the suc-
cessful approaches to avoid the attack of SiC by liquid
aluminum and at the same time improve its wettability
with aluminum alloys was the artificial or intentional
oxidation of the SiC reinforcement [14]. Intentional oxi-
dation has been adopted in this work.
As MMCs with small SiC particles often show a very
inhomogeneous particle distribution, which limits the
ductility and formability of these composite materials.
Conventional secondary deformation processing methods
such as rolling or extrusion have been used so far to im-
prove the homogeneity of the particle distribution. But
this is difficult or impossible in the case of fine particles,
since very high strains would be required [15]. The equal
channel angular pressing (ECAP) is therefore very effec-
tive technique to avoid SiC particle aggregations and
structure refinement due excessive very high shear strains
induced. The processing of materials by ECAP has un-
dergone active development in metals and alloys. How-
ever, the development of ECAP on as cast MMC is not
thoroughly investigated.
The aim of this study was to fabricate Al-SiC metal
matrix composite using pure aluminum as a matrix and
SiC as reinforcement. Stir casting technique was used to
disperse the reinforcement material through the matrix
molten metal. Application of ECAP as an additional pro-
cedure to improve the homogeneity of SiC particle dis-
tribution and improve the mechanical properties of MMC
is carried out. The effects of ECAP to different passes on
SiC particle agglomerations and mechanical properties
are investigated.
2. Materials and Experimental
2.1. Materials
The composition of the commercial purity aluminum
used for casting Al-matrix composite is as shown in Ta-
ble 1.
Using method of sieve analysis, the estimated particle
sizes of SiC were found to be ranged from 35 to 65 μm.
Silicon carbide (SiC) has been used as reinforcement. It
has a theoretical density of 3.1 g/cm3. The SiC contents
in the composites were adjusted to be either 5 or 10 wt%.
SiC was originally produced by a high temperature elec-
tro-chemical reaction of sand and carbon.
2.2. Experimental
The schematic drawing of experimental set up for a stir
casting process is shown in Figure 1. The aluminum was
melted into a graphite crucible inside an electric heating
furnace at 750˚C. No wetting agent to bind molten metal
and reinforcement powder was used. The furnace tem-
perature was kept, above melting point of aluminum, at
750˚C, for 10 minutes. Aluminum dross is then removed
from the surface of the molten metal. Steel Stir impeller
was then lowered down into the molten metal and allowed
to rotate at 200 rpm for 10 minutes. When the vortex
appears, the hot powder of SiC, preheated to 800˚C, was
uniformly added to the molten matrix. The angular ve-
locity of stirrer during adding process is then raised to 300
rpm. The powder is added at a rate of 6 g/min. The cru-
cible containing the melt mixture was then carefully taken
out of furnace and poured into a specially designed per-
manent mold. The mold was left to cool and castings were
ejected. These casting samples are now ready for further
ECAP testing and examinations of density, microstructure,
hardness, tensile and compression.
Figure 2 shows the specially designed die used in this
work. It consists of two channels, the angle between the
channels (die angle)

= 90˚. The billets (with 15 mm
Table 1. Chemical composition of commercial purity aluminum (wt%).
Al% Ti% Zn% Ni% Mg% Mn% Cu% Fe% Si%
99.8377 0.0003 0.0019 0.0018 0.0012 0.0021 0.005 0.09 0.06

F. SHEHATA ET AL.
28
Figure 1. Schematic diagram for the set up of stir cast set.
(a)
(b)
(c)
Figure 2. The die used in this work showing billet inside. (a)
Inside corner; (b) Billet exit; (c) Outside corner.
of diameter and 90 mm of length) were processed at a
pressing rate of 20 mm/min. using a ram attached to a
hydraulic press of 50 ton capacity. All the pressings were
conducted using route A where the billet is not rotated
after each successive pass. The billets were coated with
molybdenum disulphide as a lubricant to minimise the
friction between the billets and die walls
Relative densities were calculated as the ratio of the
experimental to the theoretical densities of samples. Ex-
perimental densities were determined by the Archimedes
method and the theoretical densities were calculated from
the simple rule of mixtures, taking the theoretical density
values for aluminum as 2.7 and SiC as 3.1 g/cm3. The
density data were used to determine the porosity levels
according to the following equation:
theo exp
theo
Porosity% 100




. (1)
Microstructure examination specimens were examined
using an optical microscope and scanning electron mi-
croscopy (SEM). Hardness was measured in as cast com-
posites and after ECAP passes using Rockwell hardness
tester. Tests were conducted using a steel ball indenter of
1.588 mm diameter and load of 100 kg force. The values
reported are average of at least five measurements. The
tension and compression tests were carried out using a
LR300 hydraulic testing machine at initial strain rates of
5 × 10−4 s−1. Cylindrical specimens with gauge length of
20 mm and diameter 4 mm were used for tension tests.
The compression specimens, with height of 25 mm and
diameter of 13 mm were used. This is in compliance with
ASTM standards (E9-89a) for measuring the compres-
sive response of the MMC. Special graphite-based grease
was placed between the surfaces of compression speci-
men and the platen of the compression machine to mini-
mize the friction.
3. Results and Discussions
3.1. Microstructure
Figures 3(a) and (b) shows the microstructures of as cast
Al-5% SiC and Al-10% SiC respectively. The reinforce-
ment particles have shown little clusters forming fairly
uniform particle distribution in composite containing 5%
reinforcement. The clusters were more pronounced in the
composite containing 10% SiC. The same trend was
found more clearly in SEM images.
Figure 4 reveals the composites micrographs for as
cast composites containing 5% and 10% SiC using the
SEM. The micrographs with 10% SiC particles showed
greater cluster or agglomerations and porosities com-
pared to ones containing the 5% SiC. Also the distribu-
tion of SiC particles in 10% specimens was worse than
the distribution in 5%. However, at lower magnifications
of optical microscope (Figure 3), the samples seem to
have a more uniform distribution of the SiC reinforce-
ment. But, with an increase in the magnification, the pre-
sence of particle agglomerations is clearly visible as in
Figure 4(b).
Figure 3. Microstructure of as cast composites. (a) AI-5%
SiC; (b) Al-10% SiC (×120).

F. SHEHATA ET AL. 29
Figure 4. SEM micrographs of as cast composites. (a) AI-5%
SiC; (b) Al-10% SiC.
Another observation in SEM images is the breakdown
of SiC aggregates into smaller individual particles. The
shear force applied on the composite mixture by the im-
peller is the main reason to break down most of the SiC
aggregates and overcome its cohesive force. It is notice-
able in the microstructures that some of the SiC particles
are of fine size. These particles are approximately 35 µm
in size, which is still within the range of initial particle
distribution.
The rotation of the stirrer generates a vortex through
which the SiC particles are drawn into the melt. More-
over, the rotation of stirrer can create high and local
shear forces that are exerted on the clusters helping to
break down SiC cluster particles [16]. Rumpf [17] calcu-
lated the tensile strength of a cluster suggesting that T α
(Fc/d2); where Fc is the interparticle cohesive force and d
is the diameter of the individual particle. Under a high
shear and high intensity of turbulence, liquid metal can
penetrate into the clusters of the particles and displace
the individual particles apart.
During the stirring and mixing process, the air bubbles
are sucked into the melt via the vortex created. The SiC
particles tend to become attached to these air-bubbles or
as air bubbles would envelope the reinforced particles,
leading to the formation of particle-porosity clusters [18].
Figures 5 presents the SEM of Al-5% SiC composite
after the first ECAP pass. It can be seen that SiC rein-
forcement particles (35 - 65 μm) were broken to smaller
particulates (~5 μm). Most of the particulates may not
appear in the figure because their sizes are less than 5
μm.
Figure 6 show the SEM micrograph of as cast Al-
10% SiC after the first ECAP pass. The light gray areas
indicate the SiC particles that are embedded in the alu-
minum matrix. The starting coarse SiC particles are bro-
ken down to less than 3 μm and their distributions are
more uniform compared to as cast structure shown in
Figure 4(b). ECAP has achieved a homogeneous distri-
bution of SiC in the matrix, as it is one of the problems
associated with the production of cast MMCs. Moreover,
the great effect of reducing the SiC particle size as num-
ber of ECAP passes is increased. However, the number
of ECAP passes is limited in Al-10% SiC due to the oc-
currence of surface defects and machining difficulty.
Figure 5. SEM micrograph of as cast Al-5% SiC composite
after the first ECAP pass.
Figure 6. SEM micrograph of as cast Al-10% SiC composite
after the first ECAP pass.

F. SHEHATA ET AL.
30
3.2. Porosity Contents
Table 2 presents the comparison of the theoretical and
the experimental densities of Al-5% SiC and Al-10% SiC
compared to monolithic aluminum. It is shown that the
experimental density is always less than theoretical one.
The densities of the composites are higher than that of
the monolithic aluminum.
The experimental densities of composites have in-
creased as reinforcement is increased up to 5% SiC; upon
further increase to 10% SiC the composite experimental
showed little decrease. This is probably due to air and
gases that usually attached to the reinforcement particles.
The amount of gas porosity in casting depends more on
the volume fraction of reinforcement. However, there are
several strategies that have been used in literature to
minimize porosity, such as vacuum casting, inert gas
bubbling through the melt, die casting, extrusion and rol-
ling.
In this work we adopt severe plastic deformation using
equi channel axial pressing (ECAP) technique to con-
solidate the composite and minimize the porosity.
Figure 7 shows the effect of increase numbers of
ECAP passes on porosity percentage. As number of
ECAP passes are increased, the porosity percentages are
greatly decreased. It reached 1.1% after 8 passes in alu-
minum containing 5% SiC.
The gases that dissolved during stirring of molten
metal would lead to formation of porosities on solidifica-
tion. The theoretical and experimental densities were
used to estimate porosity percentage using Equation (1).
Table 2. Comparison of the theoretical and the experimen-
tal densities.
Theo density
g/cc
Exper. density
g/cc Porosity%
Al 0% SiC 2.700 2.640 2.222
Al 5% SiC 2.720 2.658 2.279
Al 10% SiC 2.740 2.648 3.358
Figure 7. The effect of increasing number of ECAP passes
on porosity percentage.
The maximum value of porosity is 3.36%, shown in Fig-
ure 5, represents the as cast composite without any
ECAP process. Many researchers [17,18] have used al-
ternative stirring processes and reported that porosity
levels were within range of 2% to 4%, which were re-
ferred to as an acceptable level of porosity in cast com-
posites. This indicates that in spite of high level of poros-
ity in this work (up to 3.36%) it is still considered as an
acceptable and suitable for preparing the AlSiC compos-
ites. Porosity level and distribution in MMC usually play
an important role in controlling the mechanical properties.
It is thus necessary that porosity levels be kept to a mini-
mum if high performance in service applications is de-
sired. Applying ECAP technique on as cast composites
gave a great reduction in porosity level. Porosity values
of 1.1 and 1.31 were obtained for Al-5% SiC and Al-10%
SiC respectively after 8 ECAP passes. It is noted that the
greatest porosity reduction (>40%) was achieved after
the first pass. Further ECAP passes may lead to extensive
nucleation of voids which will limit the improvement in
porosity percentage [19,20].
3.3. Mechanical Properties
Figure 8 shows the tensile engineering stress-strain
curves for as cast composites Al-5% SiC and Al-10%
SiC compared to pure aluminum matrix material. These
curves are taken from the load-elongation curves ob-
tained from the tensile testing machine.
Figure 9 shows the compression engineering stress-
strain curves for as cast composites; Al-5% SiC and Al-
10% SiC compared to pure aluminum matrix material.
Again these curves are taken from the engineering load-
reduction in height curves obtained from the compression
testing machine. The results of compression test are simi-
lar to tension test results. The curves showed that, in-
creasing the content of SiC reinforcement particles in-
creased the compressive strength of the composite.
In compression tests, at 20% reduction in height, the
compression strength showed a significant increase with
increase of silicon carbide content in the matrix up to 5%
SiC. Further increase in SiC to 10% also showed an in-
crease in compression but with lower rate. The increase
in compression strength is much higher than that the cor-
responding increases in tension strength.
Figure 10 shows a comparison of various mechanical
properties of pure aluminum, Al-5% SiC and Al-10%
SiC composites produced by stir casting before conduct-
ing ECAP process. The yield and ultimate tensile strengths
(UTS) in both composites increased with increasing the
SiC wt% compared to unreinforced pure metal. The
largest increase was found in Al-10% SiC MMC. The
UTS of both composites showed an increase of 97% and
68% over the corresponding value of the as cast pure
aluminum.

F. SHEHATA ET AL. 31
Figure 8. Engineering stress-strain curves from tension tests
of as cast composites.
Figure 9. Engineering stress-strain curves from compres-
sion tests of as cast composites.
Figure 10. Comparison of various mechanical properties for
as cast composites with different percentages of SiC.
The compression strength (at-0.2 stain) showed much
higher increase of 297% and 218% over the correspond-
ing value of as cast pure aluminum. The improvement in
strengths of MMC is resulting from the effective disper-
sion of the SiC particles fabricated by stir casting method.
This can be attributed to closure action of any micro
cracks that might appear. It should be noticed that there
were no fracture in compression specimens due to the
high ductility of the pure aluminum matrix. The same
trend was found in hardness values as show in histogram
of Figure 8. The magnitude of the hardness increase is
about 67%. The increase in strength and hardness of the
composite is accompanied by a little reduction in uni-
form elongations due to embrittlement action of ceramic
(SiC) particles. Figure 11 shows the 0.2% proof stress of
the composites in as cast (0 Pass) and after each ECAP
pass in compression tests. For composites containing 5%
and 10% SiC, the yield strengths are tremendously in-
creased as numbers of ECAP passes are increased. After
the first pass, yield strength has almost twice the value of
the as cast composites. The maximum yield obtained
after 8 passes for both composites showed almost four
times the corresponding values of the as cast MMC com-
posites. It is also noticed that composites containing 10%
SiC showed little higher strengths (~10%) after all passes
compared to ones contain 5% SiC.
Figure 12 shows the effect increasing number of
ECAP passes on average hardness values for composites
containing 5% and 10% SiC particles. The hardness val-
ues show the same trend as yield strength values. For
composites containing 5% and 10% SiC, the hardness
Figure 11. The 0.2% proof stress of the composites in as
cast (0 Pass) and after each ECAP pass against number of
ECAP passes.
Figure 12. The hardness of the composites in as cast (0 Pass)
and after each ECAP pass against number of ECAP passes.

F. SHEHATA ET AL.
32
values are increased as numbers of ECAP passes are in-
creased. After the first pass, the hardness has increased to
1.5 times the as cast composites. The maximum hardness
obtained after 8 passes for both composites showed al-
most 2.9 to 3.5 times higher than the corresponding val-
ues of the as cast MMC composites.
It was found that 0.2% proof stress and hardness of
both the composites after ECAP are impressively higher
than as cast pure aluminium used in this work. The flow
stress of pure aluminium after eight ECAP passes is re-
ported to be 132 MPa [21] compared to 233 MPa in this
work which is considerably higher than the repotted
value. This may indicate that the SiC is very effective in
increasing strength or hardness when ECAP is applied.
As shown Al-10% SiC shows higher strength than Al-5%
SiC composite due to presence of higher amount of SiC
particles. Dislocations are generated due to mismatch in
thermal expansion coefficient between the matrix and the
reinforcement. As a result, the matrix of composites con-
tains higher dislocation. Higher the volume fraction of
reinforcement higher will be the dislocation density. This
leads to higher hardness and strength with increase in
SiC content.
4. Conclusions
1) Commercial purity aluminum matrix with SiC rein-
forcement can be successfully fabricated using conven-
tional low cost method of stir casting.
2) The distribution of silicon carbide particles has
shown an aggregate structure in as cast composites. The
stir cast leads to breaking down most of the SiC aggregates.
3) Composite reinforced with 10% SiC showed greater
agglomerations and porosities compared to 5% SiC in as
cast condition.
4) ECAP techniques resulted in structural refinement
and SiC particles have greatly reduced from 50 µm to 5
µm in Al-5% SiC and 3 µm in Al-10% SiC after the first
ECAP pass.
5) The as cast AlSiC composites indicated porosities up
to 3.6%. After eight ECAP passes, porosity was reduced
1.1 and 1.31% for Al-5% SiC and Al-10% SiC respectively.
6) After the first ECAP pass, yield strength has almost
twice its value in the as cast composites. The maximum
yield of 245 MPa obtained after 8 passes is almost four
times the corresponding values of the as cast MMC com-
posites.
7) After the first ECAP pass, hardness has almost 1.5
times its value in the as cast composites. The maximum
hardness of 71 HRB obtained after 8 passes is almost 3.5
times the corresponding values of the as cast MMC
composites.
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