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Mechanical characterization of precipitation
hardened Al7075-grey cast iron powder
reinforced metal matrix composites
Jamaluddin Hindi 1*, Kini Achuta1 and Murthy Amar1
1Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology,
Manipal Academy of Higher Education, 576104, India
Abstract. Al7075 alloy is the most commonly used by the aerospace
industry. Al7075 alloy is characterized by its improved properties such as
higher toughness, specific strength and hardness. The current work focuses
on the preparation and characterization of age hardened Al7075-Grey cast
iron composites. Two stage stir casting technique is used for the
preparation of the composite. Age hardening treatment is imparted to
enhance the mechanical characteristics. The variation of hardness and
tensile strength with respect to aging temperature and percentage of
reinforcement is analyzed. The composites exhibit higher hardness and
tensile strength as the reinforcement percentage is increased at an aging
temperature of 1000C.
1 Introduction
Composite material is a combination of dissimilar materials which result in
property enhancement than those of the individual elements. There will not be
any change in the chemical, physical, and mechanical properties of individual
elements i.e. matrix and reinforcement [1-4]. Composite materials are
characterized by high strength to weight ratio, higher stiffness with low density
resulting in considerable reduction in the weight of the component.
Enhancement in the desirable properties is mainly due to the reinforcement
which is generally harder, refractory, and stiffer than the base material [5]. The
reinforcement phase can be continuous or discontinuous fiber or particulate.
Metal matrix composites can be defined as a combination of two or more
dissimilar materials, one of which is a metal, in which enhancement of
desirable properties are achieved by systematic combinations of different
constituents [6-8]. Very high specific strength and specific modulus can be
achieved in MMCs consisting of continuous or discontinuous fibers, whiskers,
or particles [9].
* Corresponding author: jamalhindi@gmail.com
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons
Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).
MATEC Web of Conferences 144, 02007 (2018) https://doi.org/10.1051/matecconf/201814402007
RiMES 2017
Aluminum alloy composites are hardened by reinforcement of ceramic refractory
particulates into the base matrix material. Due to their innate nature and creation of
nucleation sites in the matrix material, reinforcements can enhance tensile related properties
of the matrix material [10-12]. Particulate reinforced composites are characterized by
economic manufacturing methods with finer grains. Selection of the type of reinforcement
is based on the property enhancement and monetary investment [13,14]. Generally,
continuous fiber reinforced MMCs have unidirectional properties in the direction of the
fiber, but are costly. Chopped fibers can produce significant property enhancement in the
two dimensionally in the direction of their orientation, at moderate cost. MMCs provide
moderate but isotropic properties [15-19].
Grey cast iron is one of the types of cast irons which have unique microstructure
containing carbon in free and combined form. Carbon is present as cementite and graphite.
The weight percentage of these two constituents can be altered by the addition of graphite
formers like Si. The fractured surface of the grey cast iron specimen is greyish in colour
due to the presence of free carbon as graphite and combined carbon as cementite. The
graphite also provides gray cast iron with an additional damping capacity for the alloy or
matrix since it absorbs the energy [21-23].
2 Materials and methods
2.1 Al 7075 alloy
Al7075 alloy is the most commonly used by the aerospace industry. Al7075 alloy is
characterized by its improved properties such as higher toughness, specific strength and
hardness. The Al-Zn-Mg alloys [Al7075] are used in as cast and age hardened condition.
As cast alloys are generally homogenized. Table 1 shows the chemical composition of
Al7075 alloy [20].
Table 1. Chemical composition of Al7075 alloy.
Al Cr Cu Fe Mg Zn Si Mn Ti Others
89.79
0.08
1.35
0.3
2.21
5.67
0.4
0.08
0.06
0.06
2.2. Grey cast iron
The chemical composition of grey cast iron is shown in table 2.
Table 2. Chemical composition of the grey and cast iron (wt.%).
Element in
wt.% C S P Si Mn Fe
GCI 3.61 0.024 ≤0.022 1.30 0.41 Bal
2
MATEC Web of Conferences 144, 02007 (2018) https://doi.org/10.1051/matecconf/201814402007
RiMES 2017
Aluminum alloy composites are hardened by reinforcement of ceramic refractory
particulates into the base matrix material. Due to their innate nature and creation of
nucleation sites in the matrix material, reinforcements can enhance tensile related properties
of the matrix material [10-12]. Particulate reinforced composites are characterized by
economic manufacturing methods with finer grains. Selection of the type of reinforcement
is based on the property enhancement and monetary investment [13,14]. Generally,
continuous fiber reinforced MMCs have unidirectional properties in the direction of the
fiber, but are costly. Chopped fibers can produce significant property enhancement in the
two dimensionally in the direction of their orientation, at moderate cost. MMCs provide
moderate but isotropic properties [15-19].
Grey cast iron is one of the types of cast irons which have unique microstructure
containing carbon in free and combined form. Carbon is present as cementite and graphite.
The weight percentage of these two constituents can be altered by the addition of graphite
formers like Si. The fractured surface of the grey cast iron specimen is greyish in colour
due to the presence of free carbon as graphite and combined carbon as cementite. The
graphite also provides gray cast iron with an additional damping capacity for the alloy or
matrix since it absorbs the energy [21-23].
2 Materials and methods
2.1 Al 7075 alloy
Al7075 alloy is the most commonly used by the aerospace industry. Al7075 alloy is
characterized by its improved properties such as higher toughness, specific strength and
hardness. The Al-Zn-Mg alloys [Al7075] are used in as cast and age hardened condition.
As cast alloys are generally homogenized. Table 1 shows the chemical composition of
Al7075 alloy [20].
Table 1. Chemical composition of Al7075 alloy.
Al
Cr
Cu
Fe
Mg
Zn
Si
Mn
Ti
Others
89.79
0.08
1.35
0.3
2.21
5.67
0.4
0.08
0.06
0.06
2.2. Grey cast iron
The chemical composition of grey cast iron is shown in table 2.
Table 2. Chemical composition of the grey and cast iron (wt.%).
Element in
wt.%
C
S
P
Si
Mn
Fe
GCI
3.61
0.024
≤0.022
1.30
0.41
Bal
Grey cast iron has good potential as a reinforcement material due to following reasons:
It is also an industrial waste which comes in the form of chips during machining
operation. These chips can be re casted to be used as reinforcement.
Grey cast iron is hard due to the presence of cementite but also has high self-
lubricating property and machinability due to the presence of free graphite in the form
of flakes.
It serves as a hybrid reinforcement because microscopically there are three phases in
GCI at room temperature i.e. cementite, ferrite and graphite.
It will not react with Al7075 which will be used as a base matrix material.
2.3. Preparation of the reinforcement phases
Grey cast iron (GCI) rod is cast using chill casting technique with secondary operation i.e.
turning operation to machine the casting to remove foreign material inclusions near the
mould wall and slag at the top of the casting. Hardness values at three sample zones of
casting is measured using Rockwell B scale to analyse the homogeneity of hardness values
of the sample. To achieve homogenous chemical composition and hardness, the specimen is
annealed at 8000C for 10 hours [24-26] The casting is turned in lathe to collect grey cast
iron dust (debris), washed and preheated to 3000C for 1 hour to remove volatile substances
present. Debris is ground in planetary ball mill as shown in figure 2 to reduce the size of
particles upto 50 μm and sieving is performed to obtain uniform grade of particles. Sieving
and grinding cycles is repeated till the required quantity of uniform grade of particles are
obtained. The particles were seived through ASTM-140 sieve to obtain uniform grade of
particles of average size 50 μm [18].
2.4 Stir casting of Al7075–GCI composites
Al7075-T6 rods of 1 inch diameter and 7 inch length were procured from the supplier. It
was cut to 1 inch length and required number of pieces were placed in the crucible and
melted in a furnace at 7500C. After complete melting hexa chloroethane was added as a
degasification agent. Alkaline powder was added to remove the slag. Permanent mold
cavities for cylindrical and flat billets are cleaned using a wire brush and are coated with
graphite powder and water emulsion. Assembled molds were preheated to a temperature of
4500C for an hour in a separate furnace. Reinforcement prior to the addition to the molten
metal was separately pre heated to 4500C to remove volatile substances [9]. Three
compositions of the composite were prepared by varying the weight percentage of the
reinforcement. Stirring of the molten metal was carried using a mechanical stirrer placed on
top of the Induction furnace. Required amount of reinforcement was added to the melt
through a funnel placed in position. Vortex was created by rotating the stirrer at suitable
speed, two step stirring was carried out to overcome heat loss and drop in furnace
temperature. After the completion of stirring the molten metal with reinforcements was
poured to a pre heated permanent mold and allowed to solidify at room temperature.
3 Results and discussion
3.1 Measurement of Peak hardness
Peak hardness values are noted for all the compositions. Age hardening curves were plotted
for each specimen at half an hour interval. From the analysis of age hardening curves it is
evident that lower the temperature of aging, higher will be the peak hardness value. As the
3
MATEC Web of Conferences 144, 02007 (2018) https://doi.org/10.1051/matecconf/201814402007
RiMES 2017
aging temperature is increased peak hardness can be obtained at a faster rate but sacrificing
on the value of peak hardness. At both lower and higher aging temperature, material
exhibits higher peak hardness as the weight percentage of the reinforcement is increased .
Figure 1 and 2 shows Age hardening curve at lower and higher aging temperature at 100
and 2000C. Figure 3 shows variation in peak hardness with respect to wt.% of GCI for as-
cast and age hardened specimens.
Fig. 1. Age hardening curve at lower aging temperature 1000C.
Fig. 2. Age hardening curve at higher aging temperature 2000C.
Fig. 3. Peak hardness vs wt.% of GCI for as-cast and age hardened specimens.
4
MATEC Web of Conferences 144, 02007 (2018) https://doi.org/10.1051/matecconf/201814402007
RiMES 2017
aging temperature is increased peak hardness can be obtained at a faster rate but sacrificing
on the value of peak hardness. At both lower and higher aging temperature, material
exhibits higher peak hardness as the weight percentage of the reinforcement is increased .
Figure 1 and 2 shows Age hardening curve at lower and higher aging temperature at 100
and 2000C. Figure 3 shows variation in peak hardness with respect to wt.% of GCI for as-
cast and age hardened specimens.
Fig. 1. Age hardening curve at lower aging temperature 1000C.
Fig. 2. Age hardening curve at higher aging temperature 2000C.
Fig. 3. Peak hardness vs wt.% of GCI for as-cast and age hardened specimens.
3.2 Measurement of tensile strength
From the analysis of the graph shown in figure, it is evident that lower the temperature of
aging, higher will be the ultimate tensile strength value. At both lower and higher aging
temperature, material exhibits higher ultimate tensile strength as the weight percentage of
the reinforcement is increased. Figure 4 shows the variation in ultimate tensile strength with
respect to wt.% of GCI for as-cast and age hardened specimens. The ductility decreases as
the percentage of reinforcement is increased irrespective of the aging temperature as shown
in figure 5.
Fig. 4. Ultimate tensile strength vs wt.% of GCI for as-cast and age hardened specimens.
Fig. 5. Ductility vs wt.% of GCI for as-cast and age hardened specimens.
3.3 Microstructure analysis
The specimens are systematically polished with a series of silicon carbide embedded emery
papers starting from coarser 100 microns to finer 600 microns in the steps of 100 microns.
At every stage of polishing specimens are water washed and dried with acetone. Super
finishing operation known as buffing is performed on disc polisher with wet diamond paste
of 50 microns. Finally, the mirror like polished specimens is etched with etchant (Keller’s
reagent) [6]. Microstructures of all the samples are recorded in metallurgical microscope at
300X magnification. Figure 6 shows the microstructure of heat treated composites at 300X
as recorded by ImageAnalyzer. Homogenised specimens show better dispersion of
reinforcement without clustering.
5
MATEC Web of Conferences 144, 02007 (2018) https://doi.org/10.1051/matecconf/201814402007
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Fig. 6. Microstructure of homogenized (a) AA7075 composite with 1% Grey cast iron powder (b)
AA7075composite with 3% Grey cast iron powder (c) AA7075 composite with 5% Grey cast iron
powder.
4 Conclusions
Al 7075 alloy can be successfully reinforced with grey cast iron powder using two step stir
casting technique. Age hardening treatment significantly improves the mechanical
properties of the composite material. Lower the aging temperature better is the peak
hardness values in age hardening. Substantial improvement in the bulk hardness is observed
in the composite with increase in the weight percentage of reinforcement. Substantial
improvement in the ultimate tensile strength is observed in the composite with increase in
the weight percentage of reinforcement. Peak hardness and maximum tensile strength was
observed in Al 7075-5% GCI aged at 1000C. The ductility decreases as the percentage of
reinforcement is increased. The microstructure study reveals uniformity in the
reinforcement distribution without any agglomeration.
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MATEC Web of Conferences 144, 02007 (2018) https://doi.org/10.1051/matecconf/201814402007
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Al 7075 alloy can be successfully reinforced with grey cast iron powder using two step stir
casting technique. Age hardening treatment significantly improves the mechanical
properties of the composite material. Lower the aging temperature better is the peak
hardness values in age hardening. Substantial improvement in the bulk hardness is observed
in the composite with increase in the weight percentage of reinforcement. Substantial
improvement in the ultimate tensile strength is observed in the composite with increase in
the weight percentage of reinforcement. Peak hardness and maximum tensile strength was
observed in Al 7075-5% GCI aged at 1000C. The ductility decreases as the percentage of
reinforcement is increased. The microstructure study reveals uniformity in the
reinforcement distribution without any agglomeration.
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