Processing and Tensile Testing of 2024 Al Matrix …Processing and Tensile Testing of 2024 Al Matrix...

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–

14th, 2014, IIT Guwahati, Assam, India

481-1

Processing and Tensile Testing of 2024 Al Matrix Composite

Reinforced with Al2O3 Nano-Particles

Kapil Kumar 1*

, Dhirendra Verma 2, Sudhir Kumar

3

1Mechanical Engineering Department, NIET, Greater Noida-201306, UP, India

E-mail: [email protected]* 2Mechanical Engineering Department, NIET, Greater Noida-201306, UP, India

E-Mail: [email protected] 3Mechanical Engineering Department, NIET, Greater Noida-201306, UP, India

E-Mail: [email protected]

Abstract

The fabrication of metal matrix nano composites (MMNCs) using mechanical stir casting process

generally results poor distribution of nano particles having high porosity in the matrix. To overcome

the above problems, mechanical stir casting was combined with electromagnetic stir casting process

and formed Hybrid Casting Process. Al 2024/1 % wt Al2O3 nano composite was fabricated by injecting

Al2O3 particulates into Al alloy with the assistance of argon gas. The wettability of the reinforcement

was enhanced by milling nano-SiC particles with micro Mg powder. The examination of composite

was carried out through Scanning Electron Microscopy (SEM), EDAX and tensile testing. SEM

micrograph revealed that the nano particles are fairly distributed in the matrix and also achieved the

fine grain microstructure. The tensile strength of Al2024/1 % wt Al2O3 nano composite has improved

by 43% as compare to the Al2024 alloy. Keywords: Hybrid stir casting, SEM, Tensile strength.

1. Introduction The primary aim of fabricating metal

matrix composite (MMC) is to combine the

desirable attributes of metals and ceramics.

The incorporation of high strength, high

modulus ceramic particles to a ductile metal

matrix produce a material whose mechanical

properties including specific strength, specific

stiffness, wear resistance, excellent corrosion

resistance and high elastic modulus are

intermediate between the metal matrix alloy

and the ceramic reinforcement [2, 5].

Particulate reinforced MMCs are cost-effective

alternatives and have the advantage of being

machinable and workable using conventional

processing methods [3]. Thus, they have

remarkable scientific, technological and

commercial importance because of their

improved properties. Hence, they are being

used extensively for high performance

applications such as in aircraft and automotive

industries [5]. Wrought aluminium alloys have

long being used as a matrix alloy to produce

metal based composites. The main reason of

this is the low density of aluminium. For

producing aluminium based composites, 2xxx

and 7xxx series alloy are among the best

aluminium alloys because of heat treatable [6].

There are various fabrication techniques

available to produce MMCs. The fabrication

processes may vary considerably as changing

the matrix and reinforcement materials.

Fabrication methods can be divided into two

types: solid state, liquid state processes [7].

The liquid state process has some important

advantages such as better matrix–particle

bonding, easier control of matrix structure,

simplicity, low cost of processing, and nearer

net shape as compared to other processing

techniques. Normally, micron-sized particles

are used to improve the ultimate tensile and

the yield strengths of the metal. However, the

ductility of the MMCs deteriorates

significantly with high ceramic particle

concentration. Nano-scale ceramic particles

are used as reinforcements in the matrix to

maintain good ductility, high temperature

creep resistance and better fatigue. A variety

of methods such as stir casting, ball milling,

nano-sintering are available for fabricating the

metal matrix nano composites (MMNC).

However, it is extremely difficult for the

mechanical stirring method to distribute and

disperse nano-scale particles uniformly in

metal melts due to their large surface-to-

Processing and Tensile Testing of 2024 Al Matrix Composite Reinforced with Al2O3 Nano-Particles

481-2

volume ratio and their low wettability in metal

melts, which easily induce agglomeration and

clustering [8]. In general, the surface of non-

metallic particles is not wetted by the metal

melt. As per the literature survey, the

composites fabricated by liquid metallurgy

exhibit excellent bonding between the ceramic

reinforcement and metal when reactive

elements, such as Mg, Ca, Ti, or Zr are added

to improve wettability [2]. A liquid is said to

wet a solid if the contact angle at the interface

is less than 90º [1].

Although, few studies have been reported on

2024 aluminium alloy reinforced with nano

Al2O3 particulates. In this study, the

electromagnetic stirring coupled with

mechanical stirring, which is called hybrid stir

casting technique, was successfully introduced

to distribute and disperse nano Al2O3 particles

into aluminium 2024 alloy melts. Planetary

ball mill was used for uniform mixing the nano

Al2O3 particles with highly reactive Mg

Powder to improve the wettability. To

characterize the segregation and composition

distribution in the as-cast material, scanning

electron microscopy (SEM) technique and

energy dispersive X-ray spectroscopy (EDS)

analysis were used. A mechanical property of

casted nano composite was successfully

analyzed in terms of tensile strength.

2. Experimental procedure The chemical composition of aluminium 2024

alloy is shown in Table 1. Nano Al2O3 particles

of 40 nm size are used as a reinforcement

material. The wettability of nano Al2O3

particles is very less with aluminium 2024

alloy melt. Hence, to improve the wettability,

Mg metal powder of size 20 µm is added to

nano Al2O3 particles. Planetary ball mill is

used for uniform mixing of both powders. The

ball mill was rotated at a speed of 80 rpm for

12 hours and thus uniform composite powder

was prepared as a reinforcement material.

Fig. 1 shows the schematic of designed set-up

that was used in this study. In the mechanical

stir casting method, a mechanical stirrer is

used to distribute the reinforcement in the

matrix melt. In this method, after the matrix

material is melted, it is stirred vigorously to

form a vortex at the surface of the melt, and

the reinforcement material is then introduced

at the side of the vortex. The stirring is

continued for a few minutes before the slurry

is cast [2]. In electromagnetic stir casting

technique, the vortex on the surface of the melt

is formed by stirring the melt with the help of

electromagnetic field. Stirring of the melt is

carried out for 2 min in the mushy zone under

a temperature range from 620°C to 650°C in a

graphite crucible and then the reinforcement

material is introduced in the vortex. The

current was increased gradually from 0 to 25 A

in orders to increase the strength of the

electromagnetic field [12]. The melt is

solidified under the electromagnetic field.

The melting process of aluminium 2024 alloy

was performed in graphite crucible placed in a

resistance furnace. Initially, 900 gram of Al

2024 alloy was placed in the crucible and

heated up to 750 º C for complete melting of

the alloy. Consequently, graphite crucible was

placed in the designed set-up of hybrid stir

casting. Melt was stirred with the help of

mechanical stirrer as well as electromagnetic

stirrer. Initially, current was gradually

increased up to 25 Amp for stirring the melt

with the help of electromagnetic stirring. The

melt was rotated up to 500 rpm with the help

of hybrid stirring. Heat treatment of

reinforcement particles was done at 1100 ºC

for 20 min in an inert atmosphere. Injection of

the reinforcement particles into the melt is

carried out using a stainless steel injection tube

and inert argon gas. In this part of equipment,

the composite reinforcement powder is placed

in a heated chamber and injected to the melt by

pressure of the inert gas.

Table 1 Chemical composition of Al 2024

alloy

Figure 1 Diagram of Hybrid stir casting

After injection of reinforcements, the

mechanical stirrer was driven regularly for 10

min at 400 rpm, moved up and down to obtain

uniform dispersion of reinforcement in the

melt. The melt solidified under the

electromagnetic field produced by 5 Amp

current because tendency to produce shear

stresses in the final product were negligible

under such a low magnetic field.

Element Si Cu Fe Zn Mg Ti V Mn Al

Wt. % 0.16 4.67 0.43 0.14 1.71 0.05 0.001 0.80 92.09



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2.1 Metallurgical Characterization

The microstructures were taken from the

middle of the cross section of the casted

samples. For analyzing the microstructures, the

specimens were prepared by first grinding

through 240, 500, 800, 1000 and 1200 grit

papers followed by polishing with Al2O3

powder and then etched with Keller’s reagent.

The microstructure was investigated by ZEISS

EVO 50 EP scanning electron microscopy.

2.2 Tensile Testing Tensile testing was done on computerized

Instron Universal testing machine. The gauge

diameter and length of tensile test specimens

were 6 mm 30 mm respectively according to

ASTM: B557M-10 standard. Each value of

tensile strength is an average of three tensile

specimens.

3. Result and discussion

3.1 Microstructure Analysis Figure 2 and Figure 3 show the

microstructures of unreinforced aluminium

2024 alloy and nano composite of aluminium

2024 alloy reinforced with nano Al2O3

particles fabricated by different casting

techniques. In stir casting, mechanical stirrer

comes in direct contact with the melt. When

stirrer is incorporated and expelled from the

melt which causes the formation of porosities

in the cast product as shown in Figure 2 (a)

and Figure 3 (a).The thermal concentration

gradient in the semisolid slurry was almost

removed by stirring. In electromagnetic

stirring (EMS) method, the fragmentation of

dendrite arm is achieved by re-melting of the

dendrite arm due to convection in the stirred

slurry. The stirring of slurry continued during

the solidification of an alloy which produces

equiaxed grains in solidified alloy [12]. The

grain refinement of Al 20204 alloy occurs with

increasing the intensity of current up to 25

Amp. In this casting, there is no direct contact

of stirrer and melt. Therefore, casted product is

greatly refined and homogenous with very less

amount of porosities as shown in Figure 2 (b)

but agglomeration of nano particles and

porosities in nano-composite can be seen in

Figure 3 (b). Agglomeration of nano particles

can be refined by producing appropriate

amount of shear force in the melt. This

problem can be reduced in hybrid stir casting.

In hybrid stir casting, molten alloy was stirred

with mechanical stirrer as well as

electromagnetic stirrer. Mechanical stirrer is

pushed out from the melt when it comes in the

mushy state and the melt is stirred and

solidified only under the electromagnetic

stirring. In such process, the more refined

grains of aluminium 2024 alloy are achieved

and small amount of porosity is observed in

Figure 2 (c) and Figure 3 (c), which could be

attributed to gas entrapment during the stirring.

It is observed that as-received Al2O3 nano-

particles are agglomerated before to their entry

into the melt. When such nano Al2O3 powder is

injected into the melt without any pre-

dispersion treatment, it is extremely difficult to

break the clustering in the melt. Thus, it is

crucial to eliminate the initial clustering of

nano-particles. Increase in porosity is due to

the effect of low wettability and agglomeration

of nano particles and pore nucleation at the

matrix-Al2O3 interface. Hence, nano-Al2O3

particles are pre-dispersed with addition of

micro Mg powder to break the initial

clustering as well as to improve the wettability

of nano-particles with the Al melt. Figure 3

shows the variation of porosity with nano

Al2O3 in nano-composite specimens. It

observes a decreased amount of porosity with

changing the fabrication process. Decreasing

flow of melt associated with nano particles

clusters leads to the formation of porosity [11].

It is observed from SEM pictures that the

porosity percentage in nano-composites

fabricated through Stir casting and

electromagnetic stir casting is more than nano-

composite fabricated through hybrid stir

casting.

3.2 Tensile Strength It is described in [10] that the strength of

composite can be increased by making

changes in strength due to the dislocations,

subgrains and residual stress respectively. It is

observed that uniform mixing of nano-sized

Al2O3 particles with micro Mg powder can

help to disperse nano-Al2O3 particles more

uniformly through the 2024 Al matrix which

leads to higher dislocation density and plastic

constraint in the matrix of nano-composite. It

is anticipated in [8] that due to mismatch in

coefficients of thermal expansion of

reinforcement and matrix, thermal stresses are

developed with in the matrix which results in

increased dislocation density thereby increase

in strength of matrix, and thus to the nano-

composite.


481-4

Element O Mg Al Cu

% wt. 11.58 8.8 78.08 1.55

a Porosity

b Micro porosity

c

Fig. 2 SEM micrograph of unreinforced Al 20204 alloy

casted by different processes (a) Stir casting (b)

Electromagnetic stir casting (c) Hybrid stir casting

120

140

160

180

200

220

240

Ten

sile

Str

en

gth

(M

Pa)

Al 2024 alloy Nano composite

Stir

casting

Electromagnetic

Stir casting Hybrid Stir

casting

Fig. 4 Comparison of ultimate tensile strength of

materials fabricated by different processing methods Fig 5 EDAX profile of Al2024+1 wt. % nano Al2O3

Nano-composite fabricated by hybrid stir casting

Fig. 3 SEM micrograph of Al 20204/ 1wt% nano

Al2O3 nano composite fabricated by different

processes (a) Stir casting (b) Electromagnetic stir

casting (c) Hybrid stir casting

a

Porosity

Macro Al2O3

Nano Al2O3

C

Porosity

Nano Al2O3

b

Porosity

Macro Al2O3

Nano Al2O3

Agglomeration



481-5

It is believed that strong mechanical bonding

made between Al 20204 matrix and Al2O3

particles by using ball mill, helps to disperse

Al2O3 particles more uniformly in liquid. Ravi

Kumar and Dwarakadasa [10] have observed

that the properties of composites strongly

depend on the processing technique and

subsequent thermo-mechanical treatment. The

great enhancement in tensile strength is

observed in the nano-composite fabricated by

hybrid stir casting in comparison to monolithic

2024 Al alloy. It happens due to grain

refinement, lack of agglomeration of

reinforcement and good uniform distribution

of Al2O3, which is confirmed by SEM picture

(Figure 3 C) and low degree of porosity which

leads to effective transfer of applied tensile

load to the uniformly distributed strong Al2O3

particles.

Figure 4 reveals that the ultimate tensile

strength (UTS) of Al 2024 alloy is

continuously increased by changing the

fabrication method and its maximum value of

161 MPa obtained in hybrid stir casting.

Conversely, a least value of ultimate tensile

strength (132 MPa) is obtained in 1 wt. %

nano-Al2O3/2024 Al nano-composite

fabricated through EMS method. It may be

caused by formation of oxide layer on the top

surface of melt due to having great affinity of

aluminium melt with the oxygen. When

reinforcement is injected in the melt through

the steel tube with the assistance of argon gas

pressure, clustering of nano-Al2O3 is formed

because sufficient shear force would not be

available to break these clustering. The

formation of oxide layer on the top surface of

melt acts as a barrier for argon gas which is

entrapped in the melt and formed the porosity

by which flow of melt is greatly reduced and

the adequate amount of shear force is not

produced to break the agglomeration of the

nano-particles. In stir casting, the effect of this

layer is reduced by moving the mechanical

stirrer up and down in the melt.

Aforementioned problems are reduced in the

hybrid casting method. Hence, ultimate tensile

strength and yield strength (YS) of nano-

composite fabricated by hybrid casting are

more as compared to the other mentioned

methods. The EDS elemental intensity graph

for 1 wt. % nano Al2O3/2024 Al nano-

composite fabricated by hybrid stir casting is

shown in Figure 5. It shows no extraneous

element in the nano-composite, as all these

elements are already present in the Al 2024

alloy. The wt. % of Mg element is higher

because Mg powder is mixed externally with

nano-Al2O3 particles. In planetary ball milling,

the continuous milling of Mg powder and

nano-Al2O3 particles, the temperature of

composite powder would increase due to

friction. Mg element is more reactive than Al,

therefore, Mg element forms a thin oxide layer

MgO when it comes into the contact with

oxygen, which increases the wetability of

composite powder with Al alloy melt. The

EDS profile also confirms the presence of

oxygen element in the nano-composite which

is the main cause of formation of porosity.

The ultimate tensile strength and yield strength

of 1 wt. % nano Al2O3/2024 Al nano-

composite fabricated by hybrid stir casting

were enhanced by 43% and 86% respectively,

compared with the alloy matrix. A nano-

composite was also fabricated in [13] with the

same matrix and reinforcement materials by

stir casting associated with ultrasonic vibration

technique. The comparison of results of both

nano-composites is shown in Figure 6.

4. Conclusions Based on the present study, the following

conclusions can be made:

1. Nano-Al2O3/2024 composite was

successfully prepared by hybrid stir casting

technique. This process may be suitable for the

production of other nano-particle reinforced

metal matrix composites.

2. Formation of composite reinforcement

powder (nano-Al2O3+ Mg) in ball mill was

helpful to alleviate the problems associated

with poor wettability and nano-particle

distribution in the melt.

3. In hybrid stir casting, electromagnetic

stirring coupled with mechanical stirring was

beneficial to refine grain microstructure, and

enhance the resulting distribution of nano-

particles in the melt.

4. The injection arrangement of reinforcement

in the melt was helpful to reduce

120130140150160170180190200210220230240

Ten

sil

e S

tre

ng

th (

MP

a)

UTS YS

Stir casting +

Ultrasonic

Treatment

Hybrid Stir

casting

Fig. 6 Comparison of tensile strength of nano-

composites fabricated by hybrid stir casting and

method used in [13]


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agglomeration of nano-particles and porosity

in the composite.

5. The ultimate tensile strength and yield

strength of Nano-Al2O3/2024 composite were

enhanced by 43% and 86% respectively as

compared with the alloy matrix.

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Processing and Tensile Testing of 2024 Al Matrix …Processing and Tensile Testing of 2024 Al Matrix...

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