On the Electrical and Thermal Conductivities of Cast A356/Al<sub>2</sub>O<sub>3</sub> Metal Matrix Nanocomposites

doi:10.4236/msa.2011.29159

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Materials Sciences and Applicatio ns, 2011, 2, 1180-1187

doi:10.4236/msa.2011.29159 Published Online September 2011 (http://www.SciRP.org/journal/msa)

On the Electrical and Thermal Conductivities of

Cast A356/Al2O3 Metal Matrix Nanocomposites

El-Sayed Youssef El-Kady, Tamer Samir Mahmoud, Ali Abdel-Aziz Ali

Mechanical Engineering Department, Faculty of Engineering, King Khalid University (KKU), Abha, Kingdom of Saudi Arabia.

Email: [email protected]

Received May 13th, 2011; revised June 2nd, 2011; accepted June 7th, 2011.

ABSTRACT

To assess the effect of the dispersion of Al2O3 nanoparticles into A356 Al alloy on both the electrical and thermal con-

ductivities, A356/Al2O3 metal matrix nanocomposites (MMNCs) were fabricated using a combination of rheocasting

and squeeze casting techniqu es. Two different sizes of Al2O3 nanoparticles were dispersed into the A356 Al alloy, typi-

cally, 60 and 200 nm with volume fractions up to 5 vol%. The effect of the nanoparticles size and volume fra ction on the

electrical and thermal conductivities was evaluated. The results revealed that the A356 monolithic alloy exhibited better

electrical and thermal conductivities than the MMNCs. Increasing the nanoparticles size and/or the volume fraction

reduces both the thermal and electrical conductivities of the MMNCs. The maximum reduction percent in the thermal

and electrical conductivities, according to the A356 monolithic alloy, were about 47% and 38%, respectively. Such

percentages were exhib ited by A356/Al2O3 MMNCs containing 5 vol% of nanoparticles having 60 and 200 nm, respec-

tively.

Keywords: Nanocomposites, Thermal Conductivity, Electrical Conductivity, Aluminum Alloys

1. Introduction

Metal matrix nanocomposites (MMNCs) reinforced by

nanoscale particulates is one type of nanocomposite.

MMNCs reinforced with nanoparticles are widely used in

industry. Several applications of Al and Mg MMNCs are

found in automotive and aerospace industry [1,2].

Conventional particulate reinforced metal matrix com-

posites (MMCs) usually have a metallic phase (usually

Al or Mg) and a ceramic reinforcement phase (most

commonly SiC, Al2O3 and graphite particulates) com-

posed of micron scale particles. Ingot metallurgy (I/M)

(or casting techniques) and powder metallurgy (P/M) are

common methods to produce conventional MMCs [2].

MMCs have higher yield strength, stiffness and lower

coefficient of thermal expansion than those of the mono-

lithic matrix alloy. Currently, there are several fabrica-

tion methods of MMNCs, including mechanical alloying,

powder metallurgy, casting techniques, electrochemical

deposition, friction stir processing, laser and sol-gel

technology [1]. Each of these methods has its own ad-

vantages and disadvantages. Mechanical alloying, pow-

ered metallurgy and casting technique are the most

commonly used techniques for producing bulk MMNCs

[3]. With electrochemical deposition, laser technology

and sol-gel fabrication methods, only thin films or layer

of nanostructured MMNCs are formed.

The rheocasting (compocasting), as a semi-solid phase

process, can produce good quality MMCs [4,5]. The

technique has several advantages: It can be performed at

temperatures lower than those conventionally employed

in foundry practice during pouring; resulting in reduced

thermochemical degradation of the reinforced surface,

the material exhibits thixotropic behaviour typical of stir-

cast alloys, and production can be carried out using con-

ventional foundry methods. The preparation procedure

for rheocast composites consists of the incorporation of

the ceramic particles within very vigorously agitated

semi-solid alloy slurry which can achieve more homo-

genous particles distribution as compared with a fully

molten alloy. This is because of the presence of the solid

phase in the viscous slurry that prevents the ceramic par-

ticles from settling and agglomerating. However, the

composites produced by rheocasting suffer from the high

porosity content, which has a deleterious effect on the

mechanical properties. It has been reported that the po-

rosity can be reduced by means of mechanical working

such as extrusion or rolling on the solidiﬁed composites

On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites1181

2 3

or by applying a pressure to the composite slurry during

solidiﬁcation [6,7].

However the mechanical and tribological characteris-

tics of MMNCs were extensively studied [8,9], only limi-

ted data on the thermal and electrical behavior of such

nanocomposites are available [10,11]. Until now the ther-

mal and electrical behaviour of MMNCs were not suffi-

ciently determined and published. Accordingly, the aim

of the present work is to study the effect of the Al2O3

nanoparticles additives on both the electrical and the

thermal conductivities of A356 Al alloy. The effect of

the Al2O3 nanoparticles size and volume fraction of the

aforementioned conductivities was evaluated. The A356/

Al2O3 MMNCs were fabricated a combination of rheo-

casting and squeeze casting techniques.

2. Experimental Procedures

The A356 Al-Si-Mg cast alloy was used as a matrix. The

chemical composition of the A356 Al alloy is listed in

Table 1. Nano-Al2O3 particulates were used as reinforc-

ing agents. The Al2O3 nanoparticles have two different

average sizes, typically, 200 and 60 nm. Several A356/

Al2O3 MMNCs were fabricated with different volume

factions of nanoparticles up to 5 vol%.

The A356/Al2O3 MMNCs were prepared using a com-

bination of rheocasting and squeeze casting techniques.

Preparation of the composite alloy was carried out ac-

cording to the following procedures: About 1 kg of the

A356 Al alloy was melted at 680˚C in a graphite crucible

in an electrical resistance furnace. After complete melt-

ing and degassing by argon gas of the alloy, the alloy

was allowed to cool to the semisolid temperature of

602˚C. At such temperature the liquid/solid fraction was

about 0.7. The liquid/solid ratio was determined using

primary differential scanning calorimeter (DSC) experi-

ments performed on the A356 alloy. A simple mechani-

cal stirrer with three blades made from stainless steel

coated with bentonite clay was introduced into the melt

and stirring was started at approximately 1000 rpm. Be-

fore stirring the nano-particles reinforcements after heat-

ing to 400˚C for two hours were added inside the vortex

formed due to stirring. After that, preheated Al2O3

nanoparticles were introduced into the matrix during the

agitation. After completing the addition of Al2O3 nano-

particles, the agitation was stopped and the molten mix-

ture was poured into preheated tool steel mould and im-

mediately squeezed during solidiﬁcation using a hydrau-

lic press of 50 ton capacity for 5 minutes.

Table 1. The chemical composition (wt%) of the A356 alloy.

Si Fe Cu Mn Mg Zn Al

6.6 0.25 0.11 0.002 0.14 0.026Bal.

The MMNCs were heat treated at T6 before testing.

The MMNCs were solution treated at 540˚C ± 1˚C for

three hours and then quenched in cold water. After cool-

ing specimens were artificially aged at 160˚C ± 1˚C for

12 hours.

Samples from the fabricated composites were cut from

the cast ingot for microstructural examinations. Speci-

mens were ground under water on a rotating disc using

SiC abrasive discs of increasing finesse up to 1200 grit.

Then they were polished using 10 µm alumina paste and

3 µm diamond paste. Microstructural examinations were

conducted using both optical and scanning electron mi-

croscopes (SEM). Microstructural examination was per-

formed in the unetched condition. The porosity of the

MMNCs was measured using the typical Archimedes

(water displacement) method.

The electrical measurements of the A356 monolithic

alloy and the A356/Al2O3 MMNCs were measured using

SIGMASCOPE SMP10 apparatus made by Helmut

Fischer GmbH + Co.KG, Germany. The apparatus can

measure the electrical conductivity of all non-magnetic

metals and even stainless steel, etc. Samples from the

MMNCs and the monolithic alloy having a cylindrical

shape of 20 mm diameter and 10 mm length were cut

from the cast and polished from the two end faces before

electrical conductivity measurements. The electrical con-

ductivity measurements were performed according to

ASTM E-1004. The apparatus measures the electrical

conductivity using the eddy current method.

The thermal conductivity of the MMNCs was measu-

red using the apparatus shown in Figure 1. The appara-

tus has been specially designed for the determination of

thermal conductivity for both good conductors and thin

specimens of insulants. The apparatus consists of a self-

clamping specimen stack assembly with electrically

heated source, calorimeter base, Dewar vessel enclosure

to ensure negligible loss of heat, and constant head cool-

ing water supply tank. A multipoint thermocouple switch

is mounted on the MMNCs specimen and two mercury

and glass thermometers are provided for water inlet and

outlet temperature readings as shown in Figure 2.

Three NiCr/NiAl thermocouples are fitted and connec-

tions are provided for a suitable potentiometer instrument

to give accurate temperature readings. The specimens

have cylindrical shape of about 10 mm diameter and 20

mm length. The end faces of these specimens are very

carefully prepared by grinding and lapping operations.

Two small holes were drilled in each specimen for inser-

tion of the thermocouples. Heat was applied to the top of

the stack of specimens by contact with the thermostati-

cally protected heater block, and is collected at the base

of the stack by contact with a water cooled calorimeter.

The heat transmitted through the specimens can thus be

On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites

1182 2 3

Figure 1. A photograph of the apparatus used to measure

the thermal conductivity of the nanocomposites.

Figure 2. A schematic drawing shows the specimen, the

positions of the thermocouples and the heating and cooling

methods of the specimen.

calculated from a measurement of water flow and tem-

perature rise of the water. Heat losses by radiation and

conduction from the heater block other than from contact

with the specimen were neglected, since measurement is

made of heat collected from the samples rather than heat

delivered to them. A steady flow of cooling water is

maintained at the end away from the heating block and it

leaves at the end nearer to it. Thermometers Twi and Two

are provided to measure the temperatures of the inlet and

outlet water, respectively. Two holes of 1.5 mm diameter

are drilled in the specimen rod and thermocouples are

inserted in these holes to measure the temperatures T1

and T2 of the rod at these places. The temperatures of the

two thermometers and the two thermocouples rise ini-

tially and ultimately become constant when the steady

state is reached. The water coming out of the apparatus is

collected in a beaker for a fixed time measured by a

stopwatch and the flow rate of water is determined by

timing the collection of 100 cm3 per second sample of

water. The cross-section area of the rod is calculated by

measuring its diameter and the distance between the

holes in the rod was measured.

It is assumed that the cross-sectional area normal to

the flow of heat transfer is constant and the periphery is

insulated due to the use of Dewar vessel enclosure to

ensure negligible loss of heat. The heat transfer through

the specimen rod is governed by Fourier’s Law:

QkA

 (1)

where Q is the rate of heat transfer in the x-direction, k is

the thermal conductivity of the specimen material, A is

the cross-sectional area of the specimen normal to the

x-direction, and dT/dx is the temperature gradient in the

x-direction. At steady state condition the temperature of

the specimen does not change with time at any point and

the heat conducted along the specimen must go into the

flowing water. The heat taken by the water was calcu-

lated using the following equation:





wo wi

QmcT T (2)

where m is the mass flow rate of cooling water, Cp is the

specific heat of water, and Two, Twi are the cooling water

outlet and inlet temperatures, respectively. By applying a

thermal balancing hence, the same rate of heat transfer Q

is used in Equation (1) and (2). Thus,







pwowi

mc TTx

kAT T



 (3)

The thermal conductivity measurement depends on the

measurement of the heat flux Q and temperature differ-

ence (T2 − T1). The axial flow method has been long es-

tablished and has produced some of the most consistent

and highest accuracy results. The present measurement

focused mainly on reduction of radial heat losses in the

axial heat flow developed through the specimen from the

electrical heater mounted at one end.

3. Results and Discussion

3.1. Microstructural Observations

Figure 3 shows typical micrographs for the microstruc-

ture of the monolithic A356 alloy as well as the fabric-

cated A356/Al2O3 MMNCs after heat treatment. It is

clear from Figure 3(a) that the structure of the mono-

lithic A356 Al alloy consists of primary α phase (white

regions) and Al-Si eutectic structure (darker regions).

Fine needle-like primary Si particulates were distributed

along the boundaries of the α-Al dendrites. Figures 3(b)

and 3(c) show micrographs of A356/Al2O3 MMNCs con-

On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites1183

2 3

40 μm

(a)

40 μm

(b)

40 μm

(c)

40 μm

(d)

10 μm

(e)

Figure 3. Optical micrographs for (a) A356 monolithic alu-

minum alloy; (b) A356/3 vol% Al2O3 (60 nm) nanocompo-

sites; (c) A356/3 vol% Al2O3 (200 nm) nanocomposites; (d)

A356/5 vol% Al2O3 (200 nm); (e) A356/5 vol% Al2O3 (200

nm) nanocomposites.

taining 3 vol% of Al2O3 nanoparticles having 60 and 200

nm, respectively. Clusters of Al2O3 nanoparticles in the

microstructure of the A356/Al2O3 MMNCs were ob-

served. Figure 3(d) shows high magnification optical

micrograph of Al2O3/5 vol% (200 nm) MMNCs. It is

clear that clusters of nanoparticles clusters are located

inside the -grains as well as near the eutectic structure.

Figure 4(a) shows high magnification SEM micrograph

of the 5 vol% Al2O3 nanoparticles (200 nm) showing that

nanoparticles are agglomerating near the Si particles of

the eutectic structure. The XRD analysis for the nanopar-

ticles is shown in Figure 4(b). Increasing the volume

fraction of the Al2O3 nanoparticles dispersed inside the

A356 alloy increases the agglomeration percent. The

(a)

(b)

Figure 4. (a) High magnification SEM micrograph of the 5

vol% Al2O3 nanoparticulates (200 nm) showing that na-

noparticulates are agglomerating near the Si particles of the

eutectic structure; (b) XRD analysis for the particles shown

in (a).

On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites

1184 2 3

MMNCs containing 5 vol% of Al2O3 nanoparticles ex-

hibited the highest agglomeration percent when com-

pared with those containing 1 and 3 vol%.

Porosity measurements indicted that the MMNCs have

porosity content lower than 2 vol%. Such low porosity

content is attributed to the squeezing process that was

carried out during the solidification of the MMNCs. In

cast MMCs, there are several sources of gases. The oc-

currence of porosity can be attributed variously to the

amount of hydrogen gas present in the melt, the oxide

film on the surface of the melt that can be drawn into it at

any stage of stirring and the gas being draw into the melt

by certain stirring methods. Vigorously stirred melt or

vortex tends to entrap gas and draw it into the melt. In-

creasing the stirring time allows more gases to be entered

into the melt and hence reduce the mechanical properties.

The amount of liquid inside the semi-solid slurry in-

creases with increasing the temperature which on the

other hand reduces the viscosity of the solid/ liquid slurry.

Nanoparticle distribution in the A356 Al matrix alloy

during the squeezing process depends greatly on the vis-

cosity of the slurry and also on the characteristics of the

reinforcement particles themselves, which inﬂuence the

effectiveness of squeezing in to break up agglomerates

and distribute particles. When the amount of liquid inside

the slurry is large enough, the particles can be rolled or

slid over each other and thus breaking up agglomerations

and helping the redistribution of nanoparticles and im-

proving the microstructure.

3.2. The Thermal Conductivity of MMNCs

Figure 5 shows the variation of the thermal conductivity

against the time for different MMNCs specimens having

different volume fractions and sizes of Al2O3 nanoparti-

cles. The figure shows that there is a significant variation

of the thermal conductivity during the first few minutes

and it approaches to a relatively constant value after 120

minutes or two hours (Steady state condition). The A356/5

vol% contains Al2O3 nanoparticles of 200 nm and 60 nm

exhibited the maximum and the minimum values of

thermal conductivity of 213 and 105 W/m.K, respectively.

The present experimental measurements show a signify-

cant difference in the value of the average thermal con-

ductivity for different A356/Al2O3 MMNCs. The steady

state technique and the axial flow method are satisfactory

methods for measuring the thermal conductivity of

MMNCs.

The variation of the thermal conductivity of the A356/

Al2O3 MMNCs with the volume fraction of the nanopar-

ticles at several nanoparticles sizes is shown in Figure 6.

The thermal conductivities of the A356/Al2O3 MMNCs

showed disturbing results. It is suggested that the clus-

tering of the Al2O3 nanoparticles is the reason of such

(a)

(b)

(c)

Figure 5. Variation of average thermal conductivity with

the time for (a) the A356 monolithic alloys; (b) A356/Al2O3

60 nm) MMNCs and (c) A356/Al2O3 (200 nm) MMNCs. (

On the Electrical and Thermal Conductivities of Cast A356/Al2O3 Metal Matrix Nanocomposites

1185

Figure 6. The average thermal conductivity of the A356/Al2O3 MMNCs.

results. Generally, the Al2O3 nanoparticles addition de-

creased the thermal conductivity of the MMNCs when

compared with the A356 monolithic alloy. The MMNCs

containing up to 3% of 60 nm Al2O3 nanoparticles showed

better thermal conductivities than those containing 200

nm nanoparticles. Only MMNCs containing 3%-60 nm

and 5%-200 nm of Al2O3 nanoparticles showed practi-

cally the same thermal conductivity of A356. The reduce-

tion of the thermal conductivity of the MMNCs due to the

addition of the Al2O3 nanoparticles may attribute to the

insulating nature of the Al2O3 nanoparticles itself and the

increase of the porosity content with increasing the vo-

lume fraction of the nanoparticles. Figure 6 shows also

the percent of reduction/increase of the thermal conduc-

tivity of A356 due to the dispersion of Al2O3 nanoparti-

cles having several sizes and volume fractions. The ma-

ximum percentage of reduction of thermal conductivity

was about 47% for MMNCs containing 5 vol% of 60 nm

particulates.

The reduction of the thermal conductivity due to the

addition of ceramic nanoparticles was also reported by

Ke Chu et al. [11]. They reported that the addition of

carbon nanotubes (CNTs) showed no enhancement in

overall thermal conductivity of the Cu composites due to

the interface thermal resistance associated with the low

phase contrast of CNT to copper and the random tube

orientation. Besides, the composite containing 15 vol%

CNTs led to a rather low thermal conductivity due possi-

bly to the combined effect of unfavourable factors in-

duced by the presence of CNT clusters, i.e. large porosity

and lower effective conductivity of CNT clusters them-

selves.

The thermal conductivity of the conventional alumi-

nium based MMCs was investigated by many workers

[12,13]. It has been found that the thermal conductivity

of MMCs is mainly governed by the conductivity of the

individual phases, their volume fraction and shape, and

also by the size of the inclusion phase due to a ﬁnite

metal/ceramic interface thermal resistance. Beside these

factors, the results obtained from the current work show-

ed that the agglomeration % of the particulates has a sig-

nificant influence on the thermal conductivity of the

MMCs. Increasing the volume fraction of the particulates

increases the amount of agglomeration % of the particles

which can lead to disturbing results of the average ther-

mal conductivity of the MMCs. Particles free zones have

higher thermal conductivity than the particles clustered

zones. Another important factor that plays an important

role on the thermal conductivity of the MMCs is the fab-

rication technique and its processing conditions. The

casting techniques have several advantages than other

fabrication techniques such as powder metallurgy. These

advantages include lower cost and the capability of pro-

duction of large components. However, MMCs cast com-

ponents suffer from several defects such as particles seg-

regation and high porosity content. The choice of the

On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites

1186 2 3

proper process parameters of the casting technique has a

significant effect in reduction, but not elimination, of the

particles segregation and porosity content. Squeezing the

MMNCs during solidification helped in reduction of the

porosity content of the MMNCs, however, it did not re-

duce the agglomeration % of the particles. The reduction

of the agglomeration % of the particles can be achieved

by the proper selection of the rheocasting process pa-

rameters which was the previous stage before the squee-

zeing process. The rheocasting process parameters in-

clude; the stirring speed; the shape, position and size of

the stirrer; the stirring temperature; the stirring duration

and pouring technique.

3.3. Electrical Conductivity of MMNCs

Figure 7 shows the variation of the electrical conductivity

of the A356/Al2O3 MMNCs, in MS/m, with the volume

fraction of the Al2O3 nanoparticles. The results revealed

that the MMNCs showed lower electrical conductivities

when compared with the A356 base alloy. Increasing the

volume fraction reduces the electrical conductivity of the

MMNCs. Moreover, the increasing the size of the nano-

particles from 60 to 200 nm reduced significantly the

electrical conductivity of the MMNCs. The maximum

reduction percent in the conductivity, with respect to the

A356 monolithic alloy, was about 38% and exhibited by

A356/Al2O3 MMNCs containing 5 vol% of nanoparticles

having 200 nm.

The reduction of the electrical conductivity of the

A356 due to the addition of the Al2O3 nanoparticles may

be attributed to the clustering of the nanoparticles which

are in fact insulations sites that reduce the electrical

conductivity. Moreover, it has been observed that the

nanoparticles are positioned and clustered at the grain

boundaries of Al grains and decrease the electrical con-

Figure 7. Variation of the electrical conductivity of the

A356/Al2O3 MMNCs with the volume fraction of the Al2O3

nanoparticles.

ductivity. The reduction of the electrical conductivity due

to the addition of ceramic nanoparticles noticed in the

current work has been observed also by Sheikh et al. [10].

In their work, carbon nanotubes were (CNT) added to

pure copper and MMNCs were fabricated using powder

metallurgy. The results revealed that increasing CNT

contents in the Cu composites decreases the electrical

conductivity. In contrast, El-Mahalla wi et al. [14] re-

ported that introducing Al2O3 nanoparticles to A356 al-

loy within the range 1 vol% tends to yield the alloy of

highest electrical conductivity of the MMNCs. The mor-

phology, and size of the eutectic silicon is believed to

have an effect on the electrical conductivity also and any

improvement on the eutectic morphology is believed to

induce a significant effect on the electrical conductivity

of the A356 alloy [14]. The significant effect induced on

cast alloys by adding Al2O3 particulates has been dis-

cussed and reported recently by some researchers. Ge-

nerally, the electrical conductivity of the MMCs is

mainly decided by the conductivity of the matrix alloy as

well as the shape, size and volume fraction of hard ce-

ramic alumina particles [15].

4. Conclusions

Based on the results obtained from the current work, the

following conclusions are drawn:

1) The A356 monolithic alloy exhibited better thermal

conductivity than the MMNCs. The A356/Al2O3 MMNCs

containing up to 3% of 60 nm Al2O3 nanoparticles

showed better thermal conductivities than the MMNCs

containing 200 nm nanoparticles.

2) The A356/Al2O3 MMNCs showed lower electrical

conductivities when compared with the A356 base alloy.

Increasing the volume fraction of the Al2O3 nanoparticles

reduces the electrical conductivity of the MMNCs.

Moreover, increasing the size of the Al2O3 nanoparticles

from 60 to 200 nm reduced significantly the electrical

conductivity of the MMNCs.

3) The maximum reduction percent in the thermal and

electrical conductivities, according to the A356 monoli-

thic alloy, were about 47% and 38%, respectively. These

percentages were exhibited by A356/Al2O3 MMNCs

containing 5 vol% of nanoparticles having 60 and 200

nm, respectively.

5. Acknowledgements

This work is supported by the King Abdel-Aziz City of

Science and Technology (KACST) through the Science

and Technology Center at King Khalid University

(KKU), Fund (NAN 08-172-7). The authors thank both

KACST and KKU for their financial support. Special

Thanks to Prof. Dr. Saeed Saber, Vice President of KKU,

Dr. Ahmed Taher, Dean of the Scientific Research at

On the Electrical and Thermal Conductivities of Cast A356/Al2O3 Metal Matrix Nanocomposites

1187

KKU, and Dr. Khaled Al-Zailaie, Dean of the faculty of

engineering at KKU, for their support.

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