processing and tensile testing of 2024 al matrix composite
TRANSCRIPT
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
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–
14th, 2014, IIT Guwahati, Assam, India
481-3
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.
Processing and Tensile Testing of 2024 Al Matrix Composite Reinforced with Al2O3 Nano-Particles
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
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–
14th, 2014, IIT Guwahati, Assam, India
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]
Processing and Tensile Testing of 2024 Al Matrix Composite Reinforced with Al2O3 Nano-Particles
481-6
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.
References
1. Pai B.C, Ray Subrat, Prabhakar K.V.,
Rastogi P.K. (1976), Fabrication of
Aluminium-Alumina (Magnesia)
Particulate Composite in Foundries using
Magnesium Additions to the Metals,
Material Science and Engineering, vol.24,
pp 31-44.
2. Hashim J., Looney L., Hasmi M.S.J.
(1999), Metal Matrix Composite:
Production by the stir casting method,
Journal of Material Processing
Technology, vol. 92-93, pp. 1-7.
3. Hong Soon-Jik, Kim Hong-Moule, Huh
Dae, Suryanarayana C., Chun Byong Sun
(2003), Effect of clustering on mechanical
properties of SiC particulate-reinforced
aluminium alloy 2024 metal matrix
composites, Materials Science &
Engineering A, vol. 347, pp. 198-204.
4. Hassan S.F., Gupta M. (2004),
Development of high performance
magnesium nano-composites using nano-
Al2O3 as reinforcement, Materials Science
& Engineering A, vol. 392, pp. 163-168.
5. Kok M. (2005), Production and
mechanical properties of Al2O3 particle-
reinforced 2024 aluminium alloy
composites, Journal of Materials
Processing Technology, vol. 161, pp. 381-
387.
6. Abdollahi Alireza, Alizadeh Ali,
Baharvandi Hamid Reza (2014), Dry
sliding tribological behavior and
mechanical properties of Al2024–5
wt.%B4C nanocomposite produced by
mechanical milling and hot extrusion.
Materials and Design, vol. 55 pp. 471–
481.
7. Tahamtan S., Halvaee A., Emamy M.,
Zabihi M.S. (2013), Fabrication of
Al/A206–Al2O3 nano/micro composite
by combining ball milling and stir casting
technology. Materials and Design, vol. 49,
pp. 347–359.
8. Mazahery A., Abdizadeh H., Baharvandi
H.R. (2009), Development of high-
performance A356/nano-Al2O3
composites. Materials Science and
Engineering A, vol. 518, pp. 61–64.
9. Rahimian Mehdi, Parvin Nader, Ehsani
Naser (2010), Investigation of particle
size and amount of alumina on
microstructure and mechanical properties
of Al matrix composite made by powder
metallurgy. Materials Science and
Engineering A, vol. 527, pp. 1031–1038.
10. Kumar N.V. Ravi, Dwarakadasa E.S.
(2000), Effect of matrix strength on the
mechanical properties of Al–Zn–Mg/SiCP
composites. Composites: Part A, vol. 31,
pp. 1139–1145.
11. Sajjadi S.A., Torabi Parizi M., Ezatpour
H.R., Sedghi A. (2012), Fabrication of
A356 composite reinforced with micro
and nano Al2O3 particles by a developed
compocasting method and study of its
properties, Journal of Alloys and
Compounds vol. 511, pp. 226– 231.
12. Chung Il-Gab, Bolouri Amir, Kang
Chung-gil (2012), A study on semisolid
processing of A 356 aluminium alloy
through vacuum-assisted electromagnetic
stirring, International journal of advanced
manufacturing Technology, vol. 58, pp.
237-245.
13. Su Hai, Gao Wenli, Feng Zhaohui, Lu
Zheng (2012), Processing, microstructure
and tensile properties of nano-sized
Al2O3 particle reinforced aluminum
matrix composites, Materials and Design,
vol. 36, pp. 590–596.