Fabrication and Studying the Mechanical Properties of A356 Alloy Reinforced with Al2O3-10% Vol. ZrO2 Nano-particles through Stir Casting

doi:10.4236/ampc.2011.12005

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Advances in Materials Physics and Chemistry, 2011, 1, 26-30

doi:10.4236/ampc.2011.12005 Published Online September 2011 (http://www.SciRP.org/journal/ampc)

Fabrication and Studying the Mechanical Properties of

A356 Alloy Reinforced with Al2O3-10% Vol. ZrO2

Nanoparticles through Stir Casting

Mohsen Hajizamani, Hamidreza Baharvandi

Faculty of Materials and Manufacturing Processes, MUT, Tehran, Iran

E-mail: [email protected]

Received May 28, 2011; revised July 6, 2011; accepted July 20, 2011

Abstract

Al2O3-ZrO2 with a high level of hardness and toughness is known as ceramic steel. Due to its unique proper-

ties it can be used as a reinforcement in fabrication of metal matrix composites. In this study, nanoparticles

of Al2O3-10% ZrO2 with an average size of 80 nm were used to fabricate Al matrix composites containing

0.5, 1, 1.5 and 2 wt.% of the reinforcement. The fabrication route was stir casting at 850˚C. There is no re-

port about usage of this reinforcement in fabrication of composites in the literature. The microstructures of

the as-cast composites were studied by scanning electron microscope (SEM). Density measurement, hard-

ness and tensile properties were carried out to identify the mechanical properties of the composites. The re-

sults revealed that with increasing the reinforcement content, density decreased while yield, ultimate tensile

strength and compressive strength increased. Also, hardness increased by increasing the reinforcement con-

tent up to 1 wt.% Al2O3-10% ZrO2 but it decreased in the samples containing higher amounts of reinforce-

ment.

Keywords: Stir Casting, Al Matrix Composite, Al2O3-10% ZrO2 Nanoparticles

1. Introduction

Composites containing discontinuous reinforcements es-

pecially particulate metal matrix composites have found

commercial applications [1-3] because they can be fab-

ricated economically by conventional techniques. Al-

alloy based composites have attracted attentions due to

their processing flexibility, low density, high wear resis-

tance, heat treatment capability and improved elastic

modulus and strength [4]. AMCs are fabricated by inc-

orporating ceramic particles like SiC, B4C, and Al2O3

with particle size of micron or nano-scale into Al-alloy

matrix [5].

Ultra fine particles such as nanoparticles noticeably

reduce interparticle spacing resulting in increased me-

chanical properties. On the other hand, nanoparticles

have a high tendency to form agglomerates. Thus, for

each technique and matrix, it is important to find out the

optimum size, reinforcement content and parameters of

fabrication to minimize agglomeration [6].

Factors such as different particle sizes, density, ge-

ometries, flow or the development of an electrical charge

during mixing may lead to agglomeration [7]. In this

process, mixing of matrix and reinforcement is a critical

step to obtain a homogenous distribution of reinforcing

particles in matrix. Since by reducing ceramic particle

size the stress concentration level on each particle is de-

creased and makes it difficult to be fractured, nanoscale

ceramic particles have attracted attentions in academia

and industry [8,9].

Generally, wettability of the reinforcement ceramic

particles by a liquid metal is very poor. Good wetting

between ceramic particles and liquid metals leads to a

proper bonding between these two during and after cast-

ing.Various techniques like pretreatment of particles [11],

adding elements such as magnesium and lithium into the

matrix as surface active agents [12,13], coating or oxi-

dizing the ceramic particles [14,15], cleaning the particle

surface by ultrasonication and different etching methods

[16,17] have been tried to improve wettability. Among

various techniques to fabricate metal matrix composites

reinforced with ceramic particles, stir casting is one of

acceptable routes for commercial production. However,

this method needs delicate optimization of parameters

M. HAJIZAMANI ET AL.

like casting temperature, stirring velocity, reinforcement

content, etc. [18,19]. In this research, four composites

with different Al2O3-10% ZrO2 content as reinforcement

were fabricated via stir casting. Al2O3-10% ZrO2 nano-

particles were wrapped in aluminum foil to facilitate

addition to the molten aluminum alloy. The casting tem-

perature was fixed at 850˚C and simultaneous stirring of

molten aluminum at constant stirring velocity was car-

ried out. Then specific tests were carried out to identify

the effect of reinforcement content on the mechanical

properties of the as-cast composites.

2. Experimental

Aluminum alloy (A 356) was used the matrix and nano-

sized Al2O3-10% ZrO2 was employed as the reinforce-

ment in fabrication of samples. Chemical composition of

A356 is presented in Table 1.

The samples were prepared using a resistance furnace,

equipped with a stirring system. After smelting of alu-

minum ingots, 3 g Keryolit was added to the molten

metal and stirring was carried out at constant rate of 420

rpm for 14 min. The stirring rate was adapted, according

to the results of literature and previous works [19,20].

Al2O3-10% ZrO2 nanoparticles were wrapped in alumi-

num foils thence added to the molten metal during stir-

ring. The casting was performed at 850˚C. Steel mold

was used for casting of specimens. Finally, the as-cast

composites were prepared for subsequent microstructural

and mechanical analyses. Bulk density measurement was

carried by Archimedes method. Theoretical density was

calculated by using simple rule of mixtures. Porosity of

the composites was estimated using the following rela-

tion:









mcmpp p

Porosity11V V

 

 

where ρmc is the measured density of the composites, ρm

is the theoretical density of the matrix alloy and V is the

volume fraction of Al2O3-10% ZrO2. It should be

metioned that the weight percentages of the reinforce-

ment were converted to volume percentage to be used in

the above relation. Microstructural studies of the as-cast

samples were carried out by scanning electron micro-

scope (SEM-Philips XL 30). The tension tests were car-

ried out in air at room temperature (Instron Universal

Testing Machine-1195 machine).

Also, the compressive strength test was conducted in

air at room temperature (Zwick testing machine). At least

3 specimens were used for each composite sample.

Brinell method was used to measure the hardness of the

samples after grinding and polishing them down to 1 μm.

At least 5 indentations on two polished specimens were

done to obtain data of hardness.

3. Results and Discussion

3.1. Microstructural Studies of As-Cast

Composites

The microstructural examination of the as-cast compos-

ites generally revealed that Al2O3-10%ZrO2 nanoparti-

cles were not distributed uniformly in the matrix and

regional clusters of particles exist (see Figure 1). Since

the wettability of particles by molten matrix is poor a

uniform distribution of particles cannot be observed in

the composites. In addition, other factors like stirring

speed, pouring conditions, solidification rate, etc. have

Table 1. Chemical composition of A356 alloy.



ElementSiMnFe NiTi Zn Sr MgAl

Wt.%7.22 0.01 0.150.016 0.13 0.04 0.01 0.45balance

Figure 1. SEM images of as-cast Al- Al2O3-10% ZrO2 composites containing (a) 0.5 wt.%, (b) 1 wt.%, (c) 1.5 wt.%, (d) 2

t.% Al2O3-10% ZrO2 nanoparticles.

M. HAJIZAMANI ET AL.

noticeable influence on the distribution of particles [20].

3.2. Density and Porosity Measurements

The measured densities of the as-cast composites vs.

reinforcing nanoparticles content are shown in Figure

2(a). It is clear that by increasing the reinforcement con-

tent, density decreased.

The high amount of porosity in the samples can be as-

cribed to air bubbles entering the melt either independ-

ently or as an air envelope to the reinforcing particles

[17]. The results of the measured densities demonstrate

that by increasing reinforcing nanoparticles content, den-

sity decreased because of higher possibility of agglom-

eration at higher percentages of nanoparticles. Agglom-

eration, in turn, leads to porosity formation. In short, by

increasing nanoscaled reinforcements, porosity content

increased. This result is confirmed by porosity content vs.

amount of Al2O3-10% ZrO2 nanoparticles in Figure 2(b).

(a)

(b)

Figure 2. (a) Relative density samples vs. weigh percent of

reinforcement. (b) Volume percent of porosity in samples vs.

weigh percent of reinforcement.

3.3. Tensile Behavior

The results of tensile tests for the samples are presented

in Figure 3(a). it is clear that by increasing Al2O3-10%

ZrO2 nanoparticle content, yield and ultimate tensile

strength (UTS) increased.

Beneficial effect of Al2O3-10% ZrO2 addition on the

strength could be explained by the reduction of mean

free path by increasing Al2O3-10% ZrO2 volume fraction,

and also with the increased density of dislocations gen-

erated as a result of the difference in thermal expansion

coefficients of the matrix and reinforcement [21]. Ther-

mal expansion coefficients of A356, Al2O3 and ZrO2 are

about 23.5 × 10–6, 8.1 × 10–6 and 10.3 × 10–6 1/˚C, re-

spectively. Also, low level of ductility in the as-cast state

may be ascribed to the high porosity content, early void

formation at low strains during tensile elongation and

heterogeneous particle distribution. Therefore, ductility

is expected to decrease by increasing reinforcement con-

tent [22].

3.4. Compressive Behavior

The result of compressive tests is shown in Figure 3(b).

It can be understood from these results that by increasing

the Al2O3-10% ZrO2 content, the compressive strength

increased continuously. Although the porosity content of

the samples increased by increasing Al2O3-10% ZrO2

content (See Figure 2(b)), compressive strength increa-

sed. This demonstrates that porosity content has no dis-

advantageous effect on compressive strength and the

content of reinforcement plays the major role i.e. the

compressive strength increased by increasing Al2O3-

10% ZrO2 content.

The plastic flow of matrix is constrained due to the

presence of these rigid and very strong Al2O3-10% ZrO2

nanoparticles.

The matrix could flow only with the movement of

Al2O3-10% ZrO2 particle or over the particles during pla-

stic deformation. While Al2O3-10% ZrO2 content is sig-

nificantly higher, the matrix gets constrained considera-

bly to the plastic deformation because of smaller in-

ter-particle distance and thus results in higher degree of

improvement in flow stress. It has been understood that

the plastic flow of the composite is due to the plastic

flow of the matrix [23]. The strain hardening of the

composite is primarily due to hardening of the matrix

during its plastic flow. The strain hardening of matrix is

expected to be influenced by the following factors: (i)

dislocation density and dislocation to dislocation interac-

tion, (ii) constraint of plastic flow due to resistance of-

fered by Al2O3-10% ZrO2 nanoparticles [6].

M. HAJIZAMANI ET AL.

(a)

(b)

Figure 3. (a) True stress vs. true strain curves for fabri-

cated composites. (b) The compressive strengths of fabri-

cated composites vs. reinforcement content.

3.5. Hardness Measurements

The hardness of the samples vs. Al2O3-10% ZrO2 content

is presented in Figure 4. It is clear that the hardness of

all

Figure 4. Measuerd values of hardness (BHN) vs. weight

percentage of Al2O3-10% ZrO2 for fabricated compo sites.

composites as shown by figure 4 was higher than that of

the matrix. This is because of the presence of hard

Al2O3-10% ZrO2 nanoparticles. By increasing the rein-

forcement content up to 1 wt.% Al2O3-10% ZrO2 the

hardness increased but the hardness of the sample con-

taining 1.5 and 2 wt.% of Al2O3-10% ZrO2 decreased.

This is because of heterogeneous distribution of nano-

particles and high porosity content. It should be noted

that these results are the average number of at least 5

indentations, thus some indentations were carried out in

the regions containing no or low contents of reinforcing

particles or containing high porosity amounts.

4. Conclusions

Al-alloy based composites reinforced with Al2O3-10%

ZrO2 nanoparticles were fabricated by stir casting at

850˚C. Microstructural and mechanical behaviors were

studied. It was concluded that by increasing the rein-

forcement content, density decreased while yield, ulti-

mate tensile strength and compressive strength increased.

Ductilities of the composites were low because of high

porosity content, early void formation at low strains dur-

ing tensile elongation and heterogeneous particle distri-

bution. Also, by increasing the reinforcement content up

to 1 wt.% Al2O3-10% ZrO2 hardness increased but the

hardness of the sample containing 1.5 and 2 wt.%

Al2O3-10% ZrO2 decreased.

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