Equal Channel Angular Pressing of Al-SiC Composites Fabricated by Stir Casting

doi:10.4236/ojmetal.2013.32A1004

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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

Email: *[email protected]

Received May 9, 2013; revised June 19, 2013; accepted June 29, 2013

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.

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.

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.

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.

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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