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CHAPTER 1

INTRODUCTION

1.1 MATERIALS

"Engineers make things out of materials," says the quote,

emphasizing the importance of materials and their performance in the field of
engineering. Materials have great impact on everyone’s life as it dominates
human history. Bricks (made of clay) and straw were used as composites in
the past. Traditional monolithic materials have certain limitations while
meeting today's advanced technological demands. Material science evolution
has resulted in the development of composite materials. In today’s world,
composite is an essential component, because of its high strength-to-weight
quotient and stiffness-to-weight ratio. The composites used today are, at the
cutting edge of materials technology, with performance and cost-effectiveness
suitable for high-demand applications such as spacecraft, automotive, and so
on. These materials combine with the best characteristics of dissimilar
materials to improve their physical and mechanical properties. According to
Manocha et al. (1980), the significance of composites as engineering
materials is that, 200 or more out of over 1600 materials are existing
composites. During earlier times, research on composites relied on continuous
carbon or boron fibers, which doesn’t yield high-quality composites due to an
adverse chemical reaction between the reinforcements and the base matrix.
New fibers such as Aluminum oxide, boron carbide, and silicon carbide have
accelerated research in metal and ceramic matrix composites.
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Metal and ceramic matrix composites works on finding

commercial applications in automobiles, aircraft, construction, electronic
equipment, and other fields, and it is all because of the evolution of many
low-cost fibers and modern manufacturing techniques.

Nonferrous metals include aluminum. It is silvery-white in color

and has a soft and ductile texture. Aluminum has an atomic number of 13.
Aluminum is regarded as a low-density material with the distinct
characteristics of lower weight density and improved corrosion resistance.
Aluminum is known to be an excellent thermal and electrical conductor,
having around 60% of copper, and only 30% of copper density.

(Source: Ashby et al. n.d.)

Figure 1.1 Classification of Engineering Materials
These materials are essential in the aircraft, marine and automotive
industries. Besides, aluminum is used as a structural material in civil
constructions, power transmission systems and machinery parts. Aluminum
constitutes 60% of thermal and electrical conductivity of copper and only
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30% of the density. This present research work was to study the
microstructural behavior of an aluminium metal matrix with different
percentages of agro-waste reinforcements and to improve the mechanical
properties and wear behavior of the composite materials (Al-Al2O3-BSA).
Mechanical properties like density, hardness, impact strength, and elongation
of the hybrid composites were studied. Wear behavior, like the wear rate of
the composite, was studied, and it also improved compared with pure
aluminium alloy. The corrosion rate of the composite was improved; this was
studied using a diluted HCl solution. The microstructure of the composite
specimens was analyzed using an optical microscope and SEM analysis. This
hybrid composite material was better suited for use in all types of automobile
and marine industries.

1.2 ALUMINUM ALLOYS

The materials of aluminum alloy are considered to be the best

alternatives for replacing existing conventional materials to achieve the
desired properties. They are commonly used in automobile, industrial,
marine, defense, and aerospace applications, because of their superior
properties such as low density, high strength, and good dimensional stability.
Most automotive, marine, and aerospace industries now expect more reliable,
low-cost materials to increase the efficiency of engine by significantly
reducing wear and friction. The global quest for sustainable development has
compelled many researchers to focus on developing new materials with
superior mechanical and tribological properties. Composite materials are
created by combining two or more dissimilar materials that result in superior
desired properties. Metal Matrix Composites (MMCs) are ideal for emerging
materials due to their superior properties. Many researchers have attempted
to improve the behavior of Aluminum Metal matrix Composites (AMCs)
with various reinforcements. Aluminum alloys provide enough strength to be
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used in the design and construction of light and strong structures. Casting is
the primary method of producing aluminum and aluminum alloys. After the
casting process, they are extruded and heat treated to increase their strength
even further. Age hardening is used to increase the yield strength of heat-
treatable aluminum alloys. The maximum strength of the material is
determined by the time of age hardening. It has been observed that proper age
hardening can increase yield strength up to 350 MPa.

There are several subclasses of aluminum alloy, such as 1000,

2000 to 8000. Their chemical composition determines their classification.
AA6061 is a member of the subclass 6000 series that is known for its
workability, weldability (typically TIG or MIG) and corrosion resistance. It is
an aluminum alloy that can be heat treated. The chemical composition and
mechanical properties of AA6061 are shown in table 1.1. & 1. 2 (ASME
Handbook volume 2, 1990) respectively.

Table 1.1 Chemical composition of AA6061 (ASME Handbook volume 2,

1990)
Si Fe Cu Mn Mg Cr Zn Ti Al
0.4-0.8 0.7 0.15 – 0.15 0.8 - 0.04 – 0.25 0.15 Balan
(max) 0.4 (max) 1.2 0.35 (max) (max) ce
Table 1.2 Mechanical Properties of AA6061 (ASME Handbook volume
2, 1990)
Condition Tensile Yield Elastic Hardness Shear Elongation
Strength Strength Modulus (HB) Strength (diameter 13 mm
(MPa) (MPa) (GPa) (MPa) specimen) (%)
0 124 55 69 30 83 30
T4 241 145 69 65 165 25
T6 310 276 69 95 207 17
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The special characteristics of AA6061 allow it to be used in wide

range of applications. AA6061 is commonly used in the following
applications.

 Home built Aircraft structures (i.e. fuselage, wings)

 Automotive parts (i.e. body sheet, chassis)

 Packing the foods and beverages

 Bicycle frames

 Constructions

Materials are crucial in determining the dependability and

efficiency of any system or process. Structures draw special attention in this
regard because they are subject to mechanical and thermal loading conditions
in the working environment. Metallic materials like steel account for a large
portion of the utility of material in structures. Aluminum alloys are used as
structural materials in automotive and aircraft structures to reduce weight and
improve fuel efficiency.

1.3 COMPOSITE MATERIALS

The composite material is made up of two or more components,

one in a microscopic level, and other in different chemical phases. The terms
"matrix" and "reinforcement" refers to these phases. In this kind of
composite, the physical and chemical characteristics of both phases are
preserved. They generate combined properties which can’t be found with
either of the constituent working alone. The reinforcement is maintained in
the desired place and orientation by the matrix. While the matrix is the
substance that supports the reinforcement, the reinforcement in the composite
serves as the main load-bearing part. The matrix also distributes the load
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among the reinforcements and guards against environmental hazards brought

on by high humidity and temperature. Before creating the composites, the
reinforcements may occasionally be functionalized with hydroxyl groups of
materials to enhance the bonding between the matrix and the reinforcement.
The geometry and quantity of distributed phases affect the composites
characteristics.

Embedded reinforcements (fibers, particles, flakes, and/or fillers)

in a matrix material makes the multiphase material known as composite
(metals, polymers, or ceramics). These components are either not soluble in
one another or only joinable at the macroscopic level. To achieve the
appropriate form and transfer the load to the fibers, the reinforcements are
held in place by the base matrix. The insertion of reinforcements increases
the matrix's mechanical characteristics (2003) Surap.

According to Chawla (2007), a material can be named as

composite if it satisfies the below said conditions:

 It is manufactured (naturally occurring composites are

excluded)

 It consists of two or more physically and/or chemically

distinct, suitably arranged, or distributed phases with an
interface separating them.

 It has characteristics that are not represented by any of the

components in isolation.
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1.4 CLASSIFICATION OF COMPOSITES

Composite materials are classified into three categories based on

the matrix material namely Polymer Matrix Composites (PMC), Metal
Matrix Composites (MMC) and Ceramic Matrix Composites (CMC).

Metal matrix Aluminum,Magnesium,

composites(MMC) Copper,Titanium etc.,

Polymer Matrix Epoxy polyester,vinyl Ester

Matrix based Composites(PMC) etc.,

Silicon Carbide,
Classification of composites

Ceramic Matrix Zirconium Carbide ,

Composites(CMC) Boron Carbide,
Alumina etc.,

Natural Fiber
Fiber Reinforced
Composites(FRC)
Synthetic Fiber

Metalic
Reinforcement Particle Reinforced
based composites(PRC)
Ceramic

Mono Layered
Laminated
Composites(LAC)
Multi Layered

Figure 1.2 Classifications of Composites

1.4.1 Polymer Matrix Composites (PMC)

Polymer Matrix Composites (PMC) are created,by incorporating

high-strength, high -modulus fibers like glass, carbon, and kevlar into the
polymer matrix,. The diameter of fiber could be in the range of 10 m. Carbon
Nanotubes (CNT) and other nanoscale fibers, which are 1 to 100 nm in size,
are employed as efficient PMC reinforcements. Jute, straw, kenaf, sisal, and
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cotton are among the natural fibers that are employed as reinforcement.
Natural fibres such as jute, straw, kenaf, sisal, and cotton are also used as
reinforcement. The availability, low cost, low density, high modulus, and
biodegradability of natural fibres make them suitable reinforcements, even
though they lack durability compared to synthetic fibers. Thermoset or
thermoplastic polymers are used as matrix materials. Polyester, Epoxy,
Phenolic, Polyethylene, Polystyrene and Nylons are the most commonly used
matrix materials. Maximum strength, service temperature and humidity,
smoke emission and cost are crucial factors that decide the type of polymer
matrix to be selected for composite. These polymer matrix composites are
manufactured by using the following techniques

 Lay-up process

 Filament winding process

 Resin Transfer Moulding process

 Pultrusion process

 Compression Moulding process

 Vacuum Impregnation process

Lower maximum working temperature, sensitivity to moisture and

radiation are major disadvantages of Polymer Matrix Composites.

1.4.2 Ceramic Matrix Composites (CMC)

In most cases, the Ceramic Matrix Composite (CMC) is created to

improve the fracture toughness of the base ceramic materials. The
reinforcement is added as secondary phase to the ceramic matrix which acts
as a barrier for cracking propagation and increasing the material's fracture
toughness. This fracture requires more energy than the ceramic fracture. It
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also pulls out the fiber or whisker and provides resistance to crack opening.
This process is known as crack bridging. Reinforcements in the shapes of
fiber and whisker shapes are commonly used in ceramic matrix composites.
The whiskers are similar to fibers .They have a low length-to- diameter ratio.
Silicon Nitride (Si3N4), Alumina (Al2O3), Boron Carbide (B4C), Boron
Nitride (BN), Aluminum Nitride (AlN), Titanium Boride (TiB2), and
Titanium Carbide (TiC) are extensively studied as matrix materials. Carbon,
Silicon Carbide (SiC), and Oxide fibers, as well as SiC whiskers, are
commonly used as CMC reinforcements.

The ceramic matrix composites are fabricated in the following ways

 Solid state hot pressing

 Sol-gel route

 Chemical vapour infiltration

 Liquid metal infiltration

 Gas metal reaction

The formation of oxides in the matrix is caused by the high-

temperature heat treatment of CMCs. This interfacial oxide layer provides a
grip between the matrix and reinforcement surfaces. It causes fiber fracture
because of the high-stress accumulation at the interface. The need for high-
temperature treatments slow down the growth of CMCs more than other
groups of composites.

1.4.3 Metal Matrix Composites (MMC)

The research activity for developing the technology for Metal

Matrix Composites sped up in the early 1980’s. The initial momentum for the
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development of these materials has come from the aerospace and military
industries, which required materials of high strength-to-weight ratio, high
modulus-to-weight ratio, and a material to withstand severe temperature and
corrosive atmosphere. The low strength metallic alloys are commonly used as
matrix material. The reinforcements are classified based on their shape and
size such as fibers, particles, and whiskers.

The main advantage of MMC’s over Polymer Matrix Composites

is that, it is suitable for applications that require long-term load-bearing
capacity and resistance for critical environments. This is because the metals
possess comparatively high elasticity modulus and yield strength over
polymers. In addition, the metals are plastically deformable and they can be
strengthened by several heat treatment processes. The properties like
insensitivity to moisture, higher thermal and electrical conductivity, and
chemical inertness, fatigue and wear resistance make MMC as superior
composite to polymer matrix composites.

The first attempt at Metal Matrix Composites was made on steel

wire- reinforced copper as continuous fiber-reinforced composites. MMC’s
initial level of development focused only on the effort and performance of the
material. Later, the boron filament with high modulus and graphite fibers was
tried in Aluminum matrix. The property degradation and poor wettability of
fibers occurs experienced with molten Aluminum alloy. To improve the
wettability of fibers with the alloy of Aluminum and magnesium,the fibers are
surface treated. The discontinuous reinforcement of fiber in Aluminum and
titanium plays a vital role in structures. Discontinuous reinforcement is
comparatively easier than continuous reinforcement in metallic materials.
They also offer excellent specific strength, stiffness, thermal conductivity,
affordability, and high structural efficiency. The Metal Matrix Composites are
capable of retaining mechanical properties at high temperatures. For example,
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Boron reinforced Aluminum maintains its properties at 510°C. The thermal

conductivity of graphite-reinforced Al, Cu and Mg is superior to the
monolithic metal.

The engineering viability of using MMC’s as industrial application

is well- proven. The lack of property data, price of synthetic ceramic
reinforcements, difficulties in manufacturing and shortage of design
guidelines restricts their applications. The advancement in understanding the
manufacturing processes of MMCs is required to further develop the process
for high productivity and controlled internal defects of the material. It is
necessary to gain a better understanding of the relationship that exists
between the microstructure, size, and spatial distribution of reinforcement,
and the mechanical properties of the composite.

The most studied metal matrices are Al, Cu, Mg, Ti, Ni, and
their alloys. The common reinforcements used in metal matrices are ceramics.
Soft metals usually lack in strength and thermal resistive properties. These are
fulfilled to an extent by reinforcing hard ceramic particles into the matrix. The
reinforcements are used in different shapes like fiber, whisker and particles.
They are also reinforced in a range of sizes from microns to nanoscale.

1.5 ALUMINUM METAL MATRIX COMPOSITES

(AL-MMCS)

The matrix metal in Aluminum Metal Matrix Composites is either

a pure Aluminum or an Aluminum alloy. The most widely experimented
aluminum alloys are from the series 2xxx, 5xxx, 6xxx, and 7xxx. This series
of aluminum alloys are found to be intense applications of automotive parts
and aircraft structures. The ceramic reinforcements are commonly tried with
base alloys namely SiC, Al2O3, TiC, TiB2, B4C, BN, Zircon, etc. The
reinforcements in the matrices are used in different size to improve the base
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properties of alloys. Researchers identified that there is a significant

improvement in the structural, thermal and tribological properties of alloys
that are reinforced with ceramic particles. As a result of that, Al MMCs are
now being used in industrial applications. The industries and applications of
Al MMCs are as follows (Mavhungu et al. 2017).

Table 1.3 Industrial applications of Al MMCs

(Source: Mavhungu et al. 2017).
Manufacturer Al MMC Component
Duralcan, Martin Al/SiCp Pistons
Marietta and Lanxide
Duralcan, Lanxide Al/SiCp Brake rotors, Calipers,
liners
GKN, Duralcan Al/SiCp Propeller shaft
Toyota Al/Al2O3 Piston rings
DuPont, Chrysler Al/Al2O3 Connecting rod
Martin Marietta Al/TiCp Piston, Connecting rods
Honda Al/Al2O3/Cf Engine Blocks
Lotus Elisse, Al/SiCp Brake rotors
Volkswagen
Chrysler Al/SiCp Brake rotors
GM Al/SiCp Rear brake drum, drive
shaft, engine cradle
Dia-Compe, Manitou Al/SiCp Bicycle fork brace

1.6 TYPES OF REINFORCEMENT IN COMPOSITES

Reinforcement improves the stiffness, strength and temperature

resistance of composites with decrease in density. To achieve these properties,
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the type of reinforcement, production methodologies and chemical

compatibility with the base matrix alloy should be considered. Figure 1.1
depicts the various types of reinforcements used in composites.

Figure 1.3 Classification of reinforcements

1.6.1 Fiber Reinforcement

Fiber-reinforced composites comprise elevated strength and

modulus fibers bonded to a matrix with specific interfaces. These Fibres
promote the strength and stiffness ratio of the composites. These composites
are classified based on the length of the fiber. Long fibers with a high aspect
ratio are known as continuous fibres. Short fibers with low aspect ratios are
known as discontinuous fibres. In discontinuous fibers reinforcement, the
isotropic behavior may be achieved by orienting the short fiber randomly, but
the optimal mechanical properties cannot be attained. In continuous fiber
reinforcement, the fibers are aligned in one direction thus enhancing the
mechanical properties. The commonly used fibres are E- glass fibers, natural
fibers, carbon fibers, aramid fibers, boron fibers, etc.,
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1.6.2 Laminates

Laminar Composites comprise minimum of two layers of bonded

dissimilar materials. The orientation of reinforced fiber may be single or
bi-directional based on the end use of composite. A hybrid laminate is
manufactured by embedding dissimilar materials or of the same material with
a distinctive reinforcement pattern. Generally, manmade fibres are favorite
due to their good combination of physical, mechanical and thermal behavior.

1.6.3 Particulate Reinforcement

Particulate Reinforcement is a particle having the same dimension

in all directions. Particulates are the most familiar and economical
reinforcement materials. They generate the isotropic property of MMCs
which confirms a potential application in structural fields. The Particulate
reinforced composite comprises a homogenous matrix embedded with coarse
spherical particles. Their formation will improve the stiffness but will not
strengthen the composites. The sharing of load by the particles with the
matrix will be less than that of a fibre-reinforced one.

1.7 HYBRID ALUMINUM METAL MATRIX COMPOSITES

The hybrid of Aluminum Metal Matrix Composites is

manufactured by adding two or more reinforcements in the base alloy. These
are made to further improvise the properties of Al-MMCs. For example, the
hardness of the composites increases when ceramic reinforcements are added.
It reduces the machinability of the composite. This kind of problem may be
answered by hybrid composites with a proper selection of primary and
secondary reinforcements (Singh and Chauhan, 2015). The particular
combination of reinforcements and their composition raises the reliability and
flexibility of the composites for different applications. Al/SiC/B4C,
Al/SiC/Fly ash, Al/SiC/Bamboo leaf ash, Al/Al2O3/Rice Husk Ash and
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Al/SiC/mica are a few hybrids of Al MMCs that are investigated for

mechanical behavior. There are lots of research works going on in this field.
Researchers are using synthetic ceramics and agricultural or industrial wastes
as secondary reinforcements to manufacture a hybrid of Al-MMCs. Recent
researches in HAMCs focuses on developing the existing composite material
based on the selective application requirement. HAMCs are majorly used as a
lightweight constructional material for automotive, aerospace, marine and
sporting utilities (Abdizadeh et al. 2014).

1.8 MANUFACTURING PROCESS OF AL-MMCS

The fabrication process available for aluminum metal matrix

composites are of two types. They are (a) The Solid phase fabrication method
(b) The Liquid phase fabrication method (c) Other Methods. Powder
Metallurgy (PM) and diffusion bonding are popularly known manufacturing
processes in solid phase methods. In liquid phase, stir casting, squeeze
casting, compo casting and liquid-metal infiltration is often used to fabricate
Al-MMCs (Huda et al. 1993).

1.8.1 Solid-State Processing Techniques

In this technique, reinforcement materials are used in the form of

particles or whiskers. This process requires the use of a die and a punch of
the desired shape. The matrix and reinforcement particles are combined and
placed inside the required shape die. The powder blend is compacted by
applying pressure (cold pressing). The compacted preforms are heated till it
reaches below the melting point of the base metal. This heat treatment
process is called as sintering and it is done to enhance the bonding between
the particles in the compacted preform. In the case of hot pressing, the
mixture of powders is pressed and heated together. This process is
inexpensive when comparing to any other liquid processing technique as it
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does not require the melting and casting of the base metal. The composites
with reinforcements in the form of whiskers or fibers are easily manufactured
in the Powder Metallurgical route. Low processing temperature and control
of dissolution of reinforcement in the matrix are other attractive features of
this technique. The following block diagram represents the Powder
Metallurgy process (Huda et al. 1993).

Metal Powders
(Alloys)
Finished
Mixing Compacting Sintering
Component
Additives
(Binding)

Figure 1.4 Powder Metallurgy Process

(Source : Hashim et al. 1999)

Figure 1.5 Comparison between different manufacturing methods
adopted in Metal Matrix Composites

The main disadvantage of this procedure is that it is difficult to

achieve a homogenous distribution of particulates in the matrix. The blending
process requires a highly cleaned atmosphere. It is ensured when the
inclusions present in the material decreases the properties like fatigue life and
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fracture toughness. Diffusion bonding is another solid-state fabrication

method of MMCs (Abouelmagd et al. 2004). In this process, the matrix
material is taken in the form of foil or sheet. The reinforcements and the
matrix surfaces are chemically treated before the process to have proper
chemical bonding between them. The reinforcements are then located in the
desired orientation and pressure is applied to them to bind them together.

1.8.2 Liquid Processing Techniques

The researchers largely adopt this method due to its simplicity,

flexibility, easiness and inexpensiveness. The matrix alloy is melted in a
graphite or ceramic furnace in a controlled atmosphere. The reinforcing
particles are mixed into the melt. A motorized stirrer made up of steel is used
to agitate the melt and disperse the reinforcement uniformly throughout the
molten metal (Acharya et al. 2017). The flow of molten metal inside the
crucible depends on the stirrer profile, speed of the stirrer, duration of the
stirring process, amount of molten metal in the crucible, and the gap between
the stirrer and the wall of the crucible.

(Source: Suresh et al. 2019)

Figure 1.6 Schematic of Stir Casting Setup
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The molten mixture is then casted into the required shape of the
mold. In the case of squeeze casting, this molten mixture is poured into a die
and a plunger is used to squeeze the mixture in a required pressure to turn the
composite into the required shape. The die and plunger remain closed until
the solidification of the mixture gets completed. Squeeze casting is a process
that combines casting and forging. The main advantage of squeeze casting is
that it controls the orientation of the grains during solidification, which
reduces porosity and improves mechanical properties. The schematic stir-
casting process is depicted.

The stir casting process is better than squeeze casting process as it

has the ability to manufacture larger size castings, inexpensiveness and the
tendency to produce less damage to reinforcement. The maximum limit of
reinforcement to have proper bonding with the matrix is 30% of the volume.
Ultrasonic-assisted casting is another technique that is used for processing
nano-sized reinforcements (Akbari et al. 2013). In this process, an ultrasonic
probe is dipped into the molten metal instead of the stirring process in stir
casting method. The propagation of ultrasonic waves in molten metal makes
the reinforcement to disperse uniformly throughout the melt. Spray casting,
vacuum infiltration and pressure-less infiltration are a few other liquid
processing techniques for manufacturing Aluminum Metal Matrix
Composites.

1.8.3 Infiltration Method

The infiltration method is the process through which the infusion

of molten metal is performed. This method can be accomplished by pressure
or pressure less infiltration. In the melt infiltration method of manufacturing
technique the reinforcement particles are placed as a preliminary step into the
mold die cavity and the molten melt is poured in and solidified without any
external source of pressure. External pressure is applied to the inert gas,
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vacuum pressure, and squeeze infiltration techniques in the pressure

infiltration technique (Akbari et al. 2013). Figure 1.7 depicts the schematics
Squeeze infiltration Setup

(Source: Pulkit Garg et al. 2019)

Figure 1.7 Schematic of Squeeze infiltrations Setup

1.8.4 Spray Deposition Method

Spray deposition is a matrix material atomization process in which

heated reinforcement particles are injected into the die with a fine diffused
stream of droplets via pressurized inert gas. This method is less expensive
than other liquid metallurgy processes. The schematic diagram (Figure 1.8)
shows the spray atomization and deposition processing of particulate-
reinforced metal matrix composites. Spray atomization and deposition
technology is a solid-liquid state metallurgical process in which discontinuous
reinforcements and matrix alloys are mixed. The unique characteristics of the
process will differ from solid, liquid and two-stage processing methods. The
merit of the process is that the mixing and solidification are performed in a
single operation. The technique is beneficial for the composite informing of
near net shape from the geometrical shapes of billets, ingots, plates, tubes
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Etc., In comparison to liquid Rheo casting, the contact time between the
matrix and the reinforcement is significantly shorter.

Source: (Ashish Agrawal et.al. 2018)

Figure 1.8 Schematic of spray atomization and deposition Setup

1.9 MECHANICAL BEHAVIOR OF ALUMINUM METAL

MATRIX COMPOSITES

Once the manufacturing process gets over, the Metal Matrix

Composites are taken for several tests to study the behavior. The mechanical
behavior of the Al MMCs is investigated by conducting numerous tests like
tensile, compression, bending, impact, hardness and Fractrography as per
ASTM standards. The mechanical properties of Metal Matrix Composites are
determined through the microstructural changes in the base alloy which is
caused due to the addition of particulates to the matrix. Sometimes, the
reinforcements in the stir casting process are reactive and forms harmful
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compounds at the interfaces of matrix and reinforcement. These compounds

are undesirable and deteriorate the tensile properties by making the material
more brittle in nature. For example, when the SiC fraction is increased in the
Al matrix, it forms the compound Al4C3.

To improve the mechanical properties of composites, the

interfacial bonding between matrix and reinforcement should be improved.
This assists in transferring the applied load to the matrix into the
reinforcement. Another important factor in developing mechanical properties,
particularly tensile properties, is porosity. As it is a well-known fact, these
pores act as a stress concentration zones during tensile loading. They generate
micro-cracks in the matrix and these micro-cracks are coalesce together to
form macro cracks. These cracks propagate and lead to the fracture in the
material. The maximum control in the porosity decreases the possibility of
premature failure of the material. The increase in volume fraction of the
reinforcement particles leads to decrease in the effective load transfer from
the matrix to reinforcement decreases. The dispersion of particulates in the
matrix should be improved for the composites to perform better.

1.10 WEAR BEHAVIOR OF ALUMINUM METAL MATRIX

COMPOSITES

Tribology is the study of friction, wear and lubricants. The wear

resistance of the material is yet another important property of the mechanical
components as there is friction between the mating parts. When the
components are worn out, it leads to the problems like misalignment,
inefficiency, vibration and premature failure due to dynamic stress. It is
essential to maintain the wear rate of the material within the specified limit.
Aluminum alloys (i.e. Al-Si) are used in piston rings, pistons and cylinder
blocks which are expected to have high wear resistance as they undergo
continuous sliding action. Al-MMCs are manufactured also to increase the
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wear resistance of aluminum alloys. The wear resistance of the materials is

tested in different configurations like Pin-on-Disc, Pin-on-Roller, Ball on
Disc, Reciprocating friction, etc. as per ASTM standards. Before and after the
wear process the wear rate of the material is calculated by mass loss .Worn
surface microstructures are also studied to explain the nature of wear (i.e.
abrasive, adhesive, corrosion and oxidation).

The wear resistance of the Metal Matrix Composites is

predominantly controlled by the bonding between the matrix and
reinforcement. The poor bonding pulls put the particle which stimulates a
high wear rate of the material. The hardness of the Metal Matrix Composite
directly influences the wear rate of the material.

Wear mechanisms are generally classified as abrasive wear,

adhesive wear, corrosive wear, oxidative wear, fretting wear, fatigue wear,
scoring, impact wear, catastrophic wear and erosive wear. Among these,
abrasive wear, adhesive wear, and scoring are the most frequently observed
wear mechanisms in dry sliding cases. Abrasive wear occurs due to the
presence of hard particles like SiC and Al2O3 in the contacting surfaces. The
relative motion between the contacts surfaces make these hard particles to cut
either of the surfaces. Third body wear occurs when these hard particles roll
in between the contact surfaces. In the case of adhesive wear, a bonding
between the contact surfaces lead to the transfer of material between both
surfaces. The localized cold weld also happens due to adhesive wear. It is
most commonly observed in the high-temperature wear process. Scoring is
defined by extensive grooves formed on the worn-out surface in the direction
of sliding.
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1.11 CORROSION BEHAVIOR OF ALUMINUM METAL

MATRIX COMPOSITES

1.11.1 Corrosion

Corrosion is the slow or progressive degradation of the material

properties caused by the chemical or electrochemical reaction when exposed
to atmospheric environmental circumstances.

1.11.2 Types of Corrosion

Corrosion is generally unpredictable. Many factors such as metal

composition and manufacturing processes may affect the material by
degrading its properties due to its usage in different environmental conditions.
The formation of such corrosion is classified as follows.

 Uniform corrosion

 Pitting corrosion

 Galvanic or two metal corrosion

 Crevice corrosion

 Intergranular corrosion

1.11.2.1 Uniform corrosion

Uniform corrosion is the most common form of corrosion. This

corrosion occurs due to the electrochemical reaction, which prolongs
uniformly over a long period and it spreads over a large area. The corroded
metals lead to failure more thinly and uniformly. Uniform corrosion occurs
due to the degradation of the bulk material on a tonnage basis. These
corrosion forms are not identified extensively at the material design stage
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because the component's durability is examined appropriately by carrying out

simple tests. The uniform corrosion's resistivity can be controlled and reduced
by the proper selection of materials, surface coatings, and cathodic protection.

1.11.2.2 Pitting corrosion

Pitting is one of the most severe types of corrosion. Such corrosion

occurs when there is a low level of oxygen or a higher level of anions, which
causes the rate of corrosion to be faster in specific areas rather than uniform
corrosion. During severe damage, the majority of the surface remains
protected. Pitting corrosion causes component materials to fail when
perforation occurs at minimum weight percentages comparing to the overall
material removal rate of the structure. Pits are more difficult to identify,
because of their small size and they are mostly covered by corrosion
fragmented particles. Pits may occur in the same condition, because of the
non-uniformity in depths measuring and carrying out the amount of corrosion
and conducting a comparative study on pitting is also time-consuming. it is
also difficult to predict the occurrence of pitting corrosion, by performing
laboratory-level experiments. The components subjected to pitting takes a
longer time to appear in most engineering applications.

1.11.2.3 Galvanic or two metal corrosion

Galvanic corrosion is also referred to as bimetallic corrosion. This

process is an electrochemical discharge in which one metal gets corroded in
preference to another metal when they are in contact with an electrolyte. The
process occurs when two different metals submerge in the conductive liquid
and are electrically associated. In this corrosion, one cathode metal is
protected, whereas the other anodic metal gets corroded. The corrosion rate is
accelerated in anodic metal as comparing to the uncoupled metal.
 25

1.11.2.4 Crevice corrosion

The deep localized corrosion occurs regularly in a crevice and

other protected areas on the surface of the metals that are subjected to
corrosion.

This is commonly associated with minor volumes of stagnant

solution caused by holes, gasket surfaces, lap joints, surface deposits, and
crevices beneath bolt and rivet heads.

1.11.2.5 Inter granular corrosion

The impact of grain boundaries is not considered more significant

in most engineering applications while using metals. In metals, grain
boundaries are slightly affected more due to corrosion than that of the matrix.
However, under some conditions interface of the grain is more reactive which
results in inter granular corrosion. Localized damage may occur on grain
boundaries due to relatively minor inter granular corrosion. Impurities at the
grain boundaries cause corrosion, as do the enhancement of one of the
alloying elements and the reduction of one of these elements at the grain
boundaries. Metals such as iron are less soluble in the Aluminum matrix. As a
result, they segregate at the grain boundaries causing inter granular corrosion.

Aluminum (Al) and aluminum oxide (Al2O3) are commonly used in

industries such as aerospace, automotive, construction, electronics, packaging,
and transportation due to their lightweight, corrosion resistance, and electrical
conductivity properties. These industries might potentially be interested in
exploring novel hybrid materials for various applications.

Bamboo stem ash, on the other hand, is not typically used as a

structural or functional material in industrial sectors. However, bamboo has
been gaining attention as a sustainable and renewable resource due to its rapid
 26

growth and low environmental impact. The utilization of bamboo stem ash as
a filler or reinforcement in composite materials could be explored to enhance
specific properties or reduce costs.

Regarding the economic viability of the Al-Al2O3-Bamboo stem

ash hybrid compared to currently used materials, it would require a detailed
cost analysis, considering factors such as material availability, production
processes, performance characteristics, and market demand. Without specific
data on this hybrid material, it is difficult to make a conclusive statement
about its economic feasibility compared to existing alternatives.

1.12 ENGINEERING APPLICATIONS

Construction Material: Used as a structural material for civil

constructions under static loads (trusses, roofing sheets and columns).

 Food packing containers

 Carriages in Ships

Automotive applications: Bicycle frames, truck cabins, Bearings,

Bushes, camshafts, engine cylinders and pistons, connecting rods, engine
casings, etc.

 Marine industries

 Aerospace industries

 Sports equipment

Electrical appliances: capacitors, motor, transformer and

generator casings
 27

Machinery: Textile industries, coal mining and food

processing Industries

Miscellaneous applications: Turbine blades, steam, and thermal

power plants, Pipelines in water and fuel transportation, household
appliances, and furniture.

The Al-Al2O3-Bamboo stem ash hybrid is not a widely recognized

or established material in industrial applications. Therefore, there is limited
information available on its specific relevance and economic viability
compared to currently used materials. However, some general insights on
potential technical and economic benefits:

Technical Benefits:

Lightweight: Aluminum and bamboo stem ash are both lightweight

materials, which can be advantageous in industries that prioritize weight
reduction, such as aerospace and automotive.

Corrosion Resistance: Aluminum and aluminum oxide offer good corrosion

resistance, making the hybrid potentially suitable for applications in
environments where corrosion is a concern, such as marine or offshore
structures.

Strength and Stiffness: The inclusion of bamboo stem ash may enhance the
strength and stiffness properties of the hybrid, depending on the specific
composition and processing techniques used.

Thermal and Electrical Conductivity: Aluminum and aluminum oxide

exhibit good thermal and electrical conductivity, which can be beneficial in
industries that require efficient heat transfer or electrical conductivity.
 28

Economic Benefits:

Cost Reduction: Bamboo is generally considered a renewable and low-cost

resource compared to some traditional materials. If the inclusions of bamboo
stem ash in the hybrid results in cost savings, it could potentially offer
economic benefits.

Sustainability: The use of bamboo stem ash aligns with sustainability goals
due to the renewable nature of bamboo and its potential to reduce the reliance
on non-renewable resources.

To determine the specific industries where the Al-Al2O3-Bamboo

stem ash hybrid may be most relevant and whether it is more economical than
currently used materials, detailed research, testing, and economic analysis
would be required. It is recommended to consult with experts in materials
science and engineering, conduct comprehensive cost evaluations, and assess
performance characteristics in specific applications to obtain accurate and up-
to-date information.

1.13 OBJECTIVES AND SCOPE

Metal Matrix Composites are currently finding applications in the

field of automotive and aircraft industries due to their outstanding specific
strength. The novel MMCs are also formulated and studied for their improved
properties. The versatile processing route of MMCs made them a suitable
material in such fields. The stir casting technique is identified as a low-cost
and simple method to fabricate MMCs, particularly aluminum MMCs.

AA6061/Al2O3 Metal Matrix Composites are investigated by many

researchers and that showed improved mechanical and tribological properties.
Porosity and lack of dispersion are a few problems mentioned by the
 29

researchers in these composites. These problems are identified as factors that

deteriorate the properties of the composite material.

Hybrid Metal Matrix Composites are manufactured to overcome

the drawbacks of conventional Metal Matrix Composites. The Hybrid MMCs
are fabricated by reinforcing two or more materials in the matrix. In the case
of Aluminum Metal Matrix Composites, studies on mechanical and
tribological properties are extensively available. The development in the field
of nanomaterial encourages the use of nano-sized particles, fiber and whiskers
as novel reinforcement in aluminum matrix. These nano-composite materials
show outstanding property variation over conventional composite materials.
The structural materials under friction are expected to have self-lubricating
properties. This may be possible by adding a solid lubricant like graphite,
MoS2 and h-BN as one of the reinforcements in the composite. These solid
lubricants are capable of serving two functions. One is to improve the
tribological properties of the material and then to act as a lubricant where the
liquid lubricant can’t serve the purpose. These Hybrid Metal Matrix
Composites can be a suitable replacement in structures of automotive and
aircraft industries as they can offer superior properties than straight metal
matrix composites. The designer would strongly demand a composite material
that offers improved properties and possesses self-lubricating capacity. In the
perspective meeting, the above-said demand, the current research work was
initiated.

To meet the above requirements the following points are set as

objectives:

 To make Aluminum Hybrid Composite specimens reinforced

with alumina and boron carbide in various volume fractions.
 30

 To examine the distribution of reinforcements in the cast

specimen using optical micrographs, scanning electron
micrographs and XRD images.

 To investigate how reinforcements affect mechanical

properties such as density, hardness, tensile strength and
impact strength.

 To investigate the factors influencing the frictional and wear

behavior of Aluminum Hybrid Composites at various
reinforcement percentages, sliding loads, sliding speeds, and
siding distances using a pin-on-disc apparatus.

 To investigate the mechanism of wear on the worn surfaces

of composite specimens by scanning an electron microscope.

 To investigate the corrosion behavior of composite materials

using immersion tests.

1.14 OVERVIEW OF THE RESEARCH

This Research contains seven chapters. The first chapter of the

research introduces the field of research. It explains the evolution of
materials, the need for composite materials and the types of composite
materials. It also elucidates Aluminum Metal Matrix Composites, their
applications and the manufacturing processes. The mechanical and wear
properties of Aluminum Metal Matrix Composites are also concisely
expressed.

The second chapter constitutes the literature survey. It highlights

the published research work in the selected field during last two decades. The
chapter explains various works carried out in the selected field by the
researchers and their findings. It also validates the novelty of the selected area
 31

accomplished in this Research work. It describes the objective of the research

work.

The third chapter elaborates research methodology. It describes

the composites manufacturing process and material characterization before
and after manufacturing using an Optical Microscope (OM), a Scanning
Electron Microscope (SEM), an Energy Dispersive Spectroscope (EDS), and
an X-ray Diffraction technique (XRD). The chapter describes the standard
testing procedures used to characterize the materials.

The fourth chapter discusses the mechanical properties of

composites. It explains how reinforcement and porosity affect composite
tensile and compression behavior. The chapter also analyses Scanning
Electron Microscope (SEM) images of fractured surfaces of composites and
explains the mechanism of failure.

The fifth chapter discusses the wear behavior of composites. The

impact of dry sliding process parameters on material wear behavior is
discussed, as well as the effect of friction. SEM images of the worn surface
and subsurface are displayed and discussed. This chapter describes the wear
mechanism and the effect of subsurface changes on the wear rate of the
composite.

The sixth chapter examines the corrosion behavior of various

composite combinations as well as their respective corrosion rates.

The seventh chapter explains the summary and results of all

investigations on the experimental research. Furthermore, it discusses the
scope for future research.
 32

1.15 SUMMARY

The chapter elaborates the relevant background theory of selected

area of the research work. It describes the materials used, manufacturing
techniques and mechanical and wear properties of Al-Metal Matrix
Composites. The chapter explains the scope of the research work and overall
perspective.