Mechanical properties of Al 6063 metal matrix composites ...lejpt.academicdirect.org/A33/089_104.pdf · The mechanical behavior of Al 6063 metal matrix composites reinforced with

Leonardo Electronic Journal of Practices and Technologies

ISSN 1583-1078

Issue 33, July-December 2018

p. 89-104

89

Engineering, Enviroment

Mechanical properties of Al 6063 metal matrix composites reinforced

with agro-wastes silica particles

Oluyemi Ojo DARAMOLA 1,6*, Olusegun Adebayo OGUNSANYA 2, Olajide Samson

AKINTAYO 3, Isiaka Oluwole OLADELE 4, Benjamin Omotayo ADEWUYI 5 and

Emmanuel Rotimi SADIKU 6

1-5 Department of Metallurgical and Materials Engineering, Federal University of

Technology, Akure, 340001, Nigeria 6Institute of NanoEngineering Research (INER), Department of Chemical, Metallurgical and

Materials Engineering (Polymer Technology Division) and the Tooling Centre Soshanguve

Campus, Tshwane University of Technology, Pretoria 117, South Africa

E-mail(s): 1,[email protected]; 1,[email protected]; 2 [email protected]; [email protected];

[email protected]; [email protected]; [email protected]

* Corresponding author phone: +2348166814002, +276403311197

Received: August 09, 2018 / Accepted: October 29, 2018 / Published: December 30, 2018

Abstract

The mechanical behavior of Al 6063 metal matrix composites reinforced with agro-

waste silica particles was investigated. The composites were prepared by double-stir

casting with 2, 4, 6 and 8 vol. % of agro-waste silica particles as reinforcement in the

Al 6063 metal matrix. Mechanical properties (hardness, tensile, fracture toughness and

impact strength) and density measurements were used to characterize the cast

specimens. Test results showed that the silica reinforcement resulted in density

reduction and improved mechanical properties compared to the unreinforced Al 6063

metal matrix. However, percentage elongation and fracture toughness reduced gradually

with increase in silica volume fraction with the latter above the minimum permissible

limit (6-8 vol. %) allowed for aluminum matrix composites. This shows that agro-waste

silica particles is a promising reinforcement material for production of low cost

aluminum matrix composites.

mailto:[email protected]

Mechanical properties of Al 6063 metal matrix composites reinforced with agro-wastes Silica particles

Oluyemi Ojo DARAMOLA, Olusegun Adebayo OGUNSANYA, Olajide Samson AKINTAYO, Isiaka Oluwole

OLADELE, Benjamin Omotayo ADEWUYI and Emmanuel Rotimi SADIKU

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Keywords

Aluminum; Agro-waste silica particles; Metal matrix composites; Double-stir casting;

Density; Percentage elongation; Fracture toughness, Impact strength

Introduction

Aluminum matrix composites (AMCs) are progressively used to produce several

components in aerospace, automotive, marine and nuclear industries. The advancement of

AMCs is one of the momentous marks in the history of engineering materials. They are

established to improve the performance of orthodox aluminum alloys which cannot meet the

constraint of modern engineering products [1,2]. Topical engineering applications require

materials that have high level of functionality, lighter, durable, resilient, and less expensive [3].

These features are not realizable with lightweight monolithic aluminum, titanium and

magnesium alloys [4,5].

Metal matrix composite (MMC) is a genuine combination of metal matrix and

reinforcing materials, tailored towards some properties such as high strength, lesser density,

high stiffness, hardness, and fatigue resistance. However, the development of low cost MMCs

reinforced green materials has been one of the major innovations in the field of materials to

curtail environmental pollutions. Agro waste particles such as rice husk ash, coconut shell ash,

fly ash, groundnut shell ash particles are highly hazardous to the environment and to human

health and it’s a great menace to the environment as it can act as a basis for land contamination,

air pollution, react and infiltrate into water bodies in areas where they are discarded. The

necessity to protect the environment is also an apprehension of this study to make certain the

effective utilization of the agricultural waste [6]. Several researchers have reflected the

development of less expensive composites by using industrial and agro waste derivatives as

reinforcing materials in MMCs and polymer matrix composites (PMCs) [4,5,7,8].

AMCs reinforced particles usually enhanced its mechanical properties in comparison to

matrix materials. Amongst various reinforcements used such as SiC, Al2O3, TiB2, graphite,

silica sand and others [9,10]. Agro waste particles are one of the most economical and low

density reinforcement obtainable in enormous quantities as solid waste by-products [4]. Silica

particles (with density 2.32 g/cm3) extracted from agro waste (rice husk ash) is less dense

compared to synthetic particulate reinforcement materials such as Alumina with density of 3.9


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91

g/cm3 and silicon carbide with density of 3.2 g/cm3 which are frequently used as reinforcements

[11] and consequently make the composite produced to be denser which limits the application

of such materials subject to the weight percent of the reinforcing phase. Presently, metallurgical

grade silica, with 98-99% silica purity is obtained from quartz rocks by carbiothermic reduction

using electric arc furnace and this make synthetic silica reinforcements sold at high cost [7].

Sol-gel process which is a low temperature alkali extraction and simple chemical method as

well as low energy consumption method can be used to produce pure amorphous silica particles

which contains high percentage of silica and makes it a desirable reinforcement for composites

[7].

The specific objective of the current research is to study the mechanical behaviour of Al

6063 metal matrix composites reinforced with agro-waste silica particles.

Materials and methods

Silica particles of average particle size of 500 nm produced by Daramola et al. [7] was

used as reinforcements in this research. Al 6063 alloy which was used as the matrix was

purchased at NIGALEX PLC., Lagos, Nigeria. The chemical compositions of silica particles

and Al-Mg-Si alloy are shown in Tables 1 and 2 respectively.

Table 1. Chemical Composition of Silica Particles (wt.%)

Element Si O Na Cl K

% Composition 63.35 31.93 1.75 0.79 0.16

Table 2. Chemical Composition of Al 6063 Matrix Alloy (wt.%)

Element Al Si Fe Cu Mn Mg Ni Zn V

% Composition 98.76 0.47 0.23 0.22 0.012 0.39 0.001 0.01 0.01

Composite Production

The composites were fabricated using a double stir casting process according to

Alaneme and Aluko [12]. The process started with the utilization of charge calculations in

determination of the quantities of the alloy billets and agro-waste silica particles required to

produce composites containing 2, 4, 6 and 8 vol. % silica particles reinforcement. The silica

particles were initially preheated separately at a temperature of 250 °C to eliminate surface




92

moisture which helps reduce clotting of silica particles, improves wettability and ensure

homogenous dispersion of the particles within the molten Al 6063 alloy. The Al 6063 alloy was

heated to a temperature of 750 °C ± 30 °C (which above the liquids temperature of the Al 6063

alloy) to ensure high fluidity and complete melting of the alloy. The liquid alloy was then cooled

in the crucible furnace to a semi-solid temperature of about 600 °C. The preheated silica

particles were then charged into the semi-solid melt at 600 °C and the slurry was manually

stirred for 5 minutes. The composite slurry was then superheated to 800 °C ± 50 °C and a second

stirring performed using a mechanical stirrer. The stirring operation was performed at a speed

of 100 rpm for 10 min before casting into rods in prepared sand moulds fitted with metallic

chills. The composites produced were designated

Sample Designation

The composites produced were given designations based on the weight percent of the

reinforcing phase. 100% Al 6063 Alloy, 2 wt.% SiO2 + 98 wt.% Al 6063 Alloy, 4 wt.% SiO2 +

96 wt.% Al 6063 Alloy, 6 wt.% SiO2 + 94 wt.% and 8 wt.% + 92 wt.% Al 6063 were designated

as shown in Table 3.

Table 3. Samples Designation

Sample Composition Designation

100 vol. % Al 6063 Alloy

2 vol.% SiO2 + 98 vol.% Al 6063Alloy

4 vol.% SiO2 + 96 vol.% Al 6063 Alloy

6 vol.% SiO2 + 94 vol.% Al 6063 Alloy

8 vol.% SiO2 /Al 6063 Alloy

A

B

C

D

E

Density Measurement

The experimental density of each grade of composite produced was determined by

dividing the measured weight of a test sample by its measured volume using a digital

weighing balance with tolerance of ±0.0001; while the theoretical density was evaluated by

using Eq. (1).

(1)


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Where: is the density of Al 6063 alloy reinforced with silica particles,

ρAl 6063 is the density of the Al 6063 alloy, vol.Al 6063 is the volume fraction of the Al 6063 alloy,

is the density of the silica particles and is the volume fraction of the silica

particles

The experimental densities were compared with the theoretical densities for each

composition of the composites produced; and it served as basis for evaluation of the percent

porosity of the composites using the relations in Eq. (2) according to (Daramola et al) [4].

(2)

Where: is the theoretical density (g/cm3) and ρEX is the experimental density (g/cm3).

Mechanical Testing

Test samples were machined from the as-cast composite rods using lathe machine for

tensile, hardness, impact and fracture toughness tests specimens following standard procedures.

Tensile Test

The tensile properties (ultimate tensile strength, percentage elongation and fracture

toughness) of the composites produced were evaluated by using an Instron universal tensile

testing machine in accordance to ASTM E8M-91 standard [13]. Specimens for the test were

machined from the as-cast composite cylindrical rods of 150 mm length into round specimens

of 15 mm diameter, 30 mm gauge length. Results are average of three individual tests sample.

Hardness Test

The hardness test according to ASTM E18-07 standard [14] was conducted on the

prepared samples using the INDENTEC hardness testing machine 2007 model with an applied

load of 60 kg. Prior to hardness testing, the test samples were machined and polished to obtain

a smooth plane parallel surface. A Rockwell hardness “A” scale was utilised for the hardness

measurements. Multiple hardness tests (five measurements) were performed on each sample

and the average value was taken as a measure of the hardness of the specimen.




94

Impact Strength

Specimens were subjected to impact test on an Izod V-notch impact testing machine in

accordance to ASTM E23-05 standard [15]. The pendulum of the machine is allowed to swing

freely through a known angle; some energy was used to break the specimen while the energy

was recorded directly on the scale attached to the machine.

Fracture Toughness

The fracture toughness (material’s resistance to crack propagation) of the composites

was determined by using an approach based on the uniaxial tensile experiment [16]. The as-

cast composites were machined for the CNT testing using gauge length (l), specimen diameter

(D), notch diameter (d), and notch angle of 30 mm, 4.5 mm, 3.6 mm, and 60 0 respectively. The

specimens were then subjected to tensile loading to fracture using an instron universal tensile

testing machine. The fracture load was determined from the load-extension plots obtained and

the fracture toughness (K1C) evaluated using the relations in Eq. (3) according to Dieter [17].

(3)

Where: Pf is the fracture load, D and d, are the diameter of the specimen and the diameter of

the notched section respectively. The assessment of the material’s resistance to crack

propagation was also confirmed by using the relation in Eq. (4), according to Nath and Das

[18].

(4)

Where: σy is the yield stress.

Results and discussion

Composite Density and Percentage Porosity


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A summary of the composite densities and percentage porosity is presented in Table 4.

It can be observed that the density of the produced composites decreases with increasing volume

percent of the reinforcement phase. This due to the fact that silica particles possess lower

density (2320 kg/m3) compared to aluminum (2700 kg/m3) thereby giving the composites an

overall decrease in density. This is in agreement with the results obtained by Prasad and Krishna

[19], Daramola et al. [4] and Zuhailawati et al. [9] who had developed particulate reinforced

aluminum composites and experienced linear reduction in composite densities with increasing

reinforcement content. Generally, the composites experienced higher percent porosity as

compared to the unreinforced alloy. The presence of porosity can be attributed to gas

entrapment during mixing, hydrogen evolution and shrinkage during solidification, and air

bubbles entering the slurry either independently or as an air envelope to the reinforcement

particles. However, the percentage porosity of the composites was below the maximum

allowable limit of 4% which is encouraging.

Table 4. Composites density and estimated percentage porosity.

Sample Designation Theoretical Density

(g/cm3)

Experimental

Density (g/cm3)

Porosity

(%)

A

B

C

D

E

2.701

2.6924

2.6848

2.6772

2.6696

2.6890

2.6430

2.6121

2.6246

2.6313

0.44

1.83

2.71

1.96

1.43

Mechanical Properties

Ultimate Tensile Strength (UTS)

The plot showing the variation of the tensile strength of the unreinforced Al 6063 alloy

and the Al 6063/SiO2 composites is shown in Figure 1. The UTS increased linearly with

increasing silica content. However, the highest tensile strength was experienced in sample E

which shows a 72 % increment over the unreinforced alloy. This demonstrates that by

increasing the silica content, the ultimate tensile strength increases as the silica reinforcement

helps increase the load bearing capacity of the composites. As the silica content increases, there

is little or insignificant site of contacts between silica particles.




96

0

50

100

150

200

250

A B C D E

Ult

imat

e te

nsile

str

engt

h (M

Pa)

Samples

Figure 1. Variation of the ultimate tensile strength of Al 6063 alloy and its composites

The matrix has efficaciously isolated the silica particles individually so that they can act

as distinct entities. Consequently, cracks will be arrested by the ductile matrix and its

propagation among the silica particulates tends to be challenging since they function as barriers

to dislocations during the application of stress. The enhancement of the tensile strength of the

composite is accredited to the fact that the silica particles have greater modulus, thus offering

greater opposition to dislocation motion [20].

Hall-Petch relationship shows that the tensile strength is inversely proportional to the

square root of the diameter of the particle. On account of the size of the silica particles (within

the submicron range), the surface area is increased thus providing more resistance to the applied

load.

The dislocation motion is retarded due to frequent change in orientation of dislocations

at grain boundaries. The good bonding and clear interface delay the detachment of silica

particles from the aluminum matrix during tensile loading. The applied tensile load is

effectively transferred to the silica particle. Orowan strengthening mechanism is prevalent due

to the dispersion of silica particles all over the matrix since the interface and the particle are

both strong. Hence, dislocations bow around the particle and they are immobilized [21].

Sequentially, dislocation immobilization, dislocation pile-up due to more reinforcing particles

added, dislocation intersection, dislocation entanglement and dislocation multiplication occurs

which consequently increases the strength of the composites.


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Percentage Elongation

The ductility of the composite decreases with increase in the volume fraction of the

silica particles. Silica is a hard phase particle which when added, increases dislocation density

in the composite leading to a drop in the ductility of the material. The plot showing the variation

of the percentage elongation of the samples developed is presented in Figure 2.

0

2

4

6

8

10

12

14

16

18

20

A B C D E

Pe

rce

nta

ge e

lon

gati

on

(%

)

Samples

Figure 2. Variation of percentage elongation of Al 6063 alloy and its composites

It can also be observed that an inverse relationship exists between the tensile strength

and the percentage elongation of the composites. As the tensile strength of the composites

increases with increasing silica content, the elongation reduces.

Young Modulus of Elasticity

The plot showing the young’s Modulus of Elasticity for Al 6063 and Al 6063/SiO2

composites is illustrated in Figure 3.




98

0

10

20

30

40

50

60

A B C D E

Yo

un

g m

od

ulu

s o

f e

last

icit

y (G

Pa)

Samples

Figure 3. Variation of Young modulus of elasticity of Al 6063 alloy and its composites

The Young’s Modulus of Elasticity of the composites increases with increase in the

volume fraction of the silica particles with an optimum value of 46.644 GPa at 8 vol.%; The

increase in Young’s modulus is governed by the fact that the filler gives good reinforcement

with the matrix. Furthermore, the particle size of the filler is small (0.50 μm) which led to high

aspect ratio [7].

Hardness

The variation of the hardness properties of Al 6063 alloy and its composites is shown in

Figure 4. It was observed that the hardness of the composite increased in a linear trend with the

increase in volume fraction of the silica particles. This occurs due to increase in surface area of

the matrix and thus the grain sizes are reduced. The presence of such hard surface area offers

more resistance to plastic deformation which leads to enhanced hardness.


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0

10

20

30

40

50

60

70

80

A B C D E

Har

dn

ess

(H

RA

)

Samples

Figure 4. Variation of Rockwell hardness of Al 6063 alloy and its composites

From Figure 4, it can be seen that the hardness of the composites increases linearly from

61.2 HRA for sample A to 71.4 HRA for sample E. This shows that silica addition improves

the hardness of the unreinforced Al 6063 matrix.

Impact Strength

The impact strength of a filled composite depends on the degree of matrix/filler

adhesion, but in a more complex manner than the tensile strength [22]. The variation of the

impact strength of Al 6063 alloy and its composites in terms of energy lost per unit cross-

sectional area at the notch is presented in Figure 5.




100

0

10

20

30

40

50

60

70

A B C D E

Imp

act

stre

ngt

h (

KJ/

m2)

Samples

Figure 5. Variation of impact strength of Al 6063 alloy and its composites

It can be observed that a marginal increment is experienced as the silica content

increases. This implies that the composite developed possessed good interfacial adhesion. The

highest impact strength was experienced in sample E(8 vol.% SiO2) with a value of 63.33 KJ/m2

which represents a 76 % improvement over the unreinforced alloy. Generally, agro-waste silica

addition improves the impact strength of Al 6063 matrix.

Fracture Toughness

The variation of fracture toughness of Al 6063 alloy and its composites produced are

shown in Figure 6.


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0

2

4

6

8

10

12

14

A B C D E

Frac

ture

tou

ghne

ss (

MPa

m1/

2 )

Samples

Figure 6. Variation of fracture toughness of Al 6063 alloy and its composites

The fracture toughness values were reported as valid plane strain fracture toughness

since it met the plain strain conditions stated by Nath and Das [18] which is generally

acceptable. It is observed that fracture toughness of the composites developed is lower than that

of the Al 6063 alloy. This is attributed to the hard, rigid and brittle silica particulates which tend

to be more susceptible to rapid crack propagation [23].

Conclusions

The mechanical properties of Al 6063 matrix composites containing 2, 4, 6 and 8 vol.%

agro silica particles as reinforcement was investigated. The results showed that the ultimate

tensile strength, hardness values and impact strength of the developed composites increased

with increase in the silica volume fractions while the percentage elongation and fracture

toughness decreased with increase in silica volume fractions.

The overall densities of the composites reduced with increasing concentration of silica

particles making the produced composites lighter and stronger. It has also been established that

low porosity levels between intervals 1.43% - 2.71% were observed in Al 6063/SiO2

composites developed.




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Acknowledgements

The financial assistance of Technology Innovation Agency (TIA), South Africa in

conjunction with the Department of Science and Technology (DST) towards this research is

hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the authors

and are not necessarily to be attributed to TIA

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Mechanical properties of Al 6063 metal matrix composites ...lejpt.academicdirect.org/A33/089_104.pdf · The mechanical behavior of Al 6063 metal matrix composites reinforced with