19

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

The literature survey is carried out as a part of the thesis work to have an overview of the production processes, properties, wear behavior, and machinability of metal matrix composites. Composite structures have shown universally a savings of at least 20% over metal counterparts and a lower operational and maintenance cost. As the data on the service life of composite structures is becoming available, it can be identified that they are durable, maintain dimensional integrity, resist fatigue loading and are easily maintainable and repairable. The various factors that influence the machinability and wear characteristics of AMCs are illustrated in the cause and effect diagram as shown in the Figure 2.1.

Figure 2.1 Cause and Effect diagram of various factors that influence machinability and wear characteristics of AMC

20

This chapter provides a review of the published literature on

processing, properties, wear performance, machinability and optimization of

process parameters of metal matrix composites to place the research problem

in perspective.

2.2 MATERIAL SELECTION

2.2.1 Matrix Material

As it is much more than dispersing glue in MMC, the matrix alloy

should be chosen only after giving careful consideration to its chemical

compatibility with the reinforcement, its ability to wet the reinforcement, and

its own characteristics properties and processing behaviour. As a rule of

alloying element addition, the added element should not form intermetallic

compounds with the matrix elements and should not form highly stable

compounds with the reinforcements. The best properties can be obtained in a

composite system when the reinforcement whiskers or particulates and matrix

are physically and chemically compatible as possible.

The matrix can be selected on the basis of oxidation and corrosion

resistance or other properties. Generally Al, Ti, Mg, Ni, Cu, Pb, Fe, Ag, Zn,

Sn and Si are used as the matrix material, but Al, Ti, Mg are used widely.

Recently, researchers all over the world are focusing mainly on

aluminium because of its unique combination of good corrosion resistance,

low density and excellent mechanical properties. The unique thermal

properties of aluminium composites such as metallic conductivity with

coefficient of expansion that can be tailored down to zero, add to their

prospects in aerospace and avionics. The choice of Silicon Carbide as the

reinforcement in aluminium composite is primarily meant to use the

composite in missile guidance system replacing certain beryllium components

21

because structural performance is better without special handling in

fabrication demanded by latte

In addition, literature also reveals that most of the published work

has considered aluminium-based composites with their attractions of low

density, wide alloy range, heat treatment capability and processing flexibility.

2.2.2 Reinforcement

Reinforcement increases the strength, stiffness and the temperature

resistance capacity and lowers the density of MMC. In order to achieve these

properties the selection depends on the type of reinforcement, its method of

production and chemical compatibility with the matrix and the following

aspects must be considered while selecting the reinforcement material.

Size diameter and aspect ratio

Shape Chopped fiber, whisker, spherical or irregular

particulate, flake, etc

Surface morphology smooth or corrugated and rough

Poly or single crystal

Structural defects voids, occluded material, second phases

Surface chemistry e.g. SiO2 or C on SiC or other residual

films

Inherent properties strength, modulus and density

Even when a specific type has been selected, reinforcement

inconsistency persists because many of the aspect cited above in addition to

contamination from processing equipment and feedstock may vary greatly.

22

Since most ceramics are available as particles, there is a wide range of

potential reinforcements for particle-reinforced composites.

The use of graphite reinforcement in a metal matrix has a potential

to create a material with a high thermal conductivity, excellent mechanical

properties and attractive damping behaviour at elevated temperatures.

However, lack of wettability between aluminium and the reinforcement, and

oxidation of the graphite lead to manufacturing difficulties and cavitations of

the material at high temperatures.

It is proven that the ceramic particles are effective reinforcement

materials in aluminium alloy to enhance the mechanical and other properties.

The reinforcement in MMCs is usually of ceramic materials; these

reinforcements can be divided into two major groups, continuous and

discontinuous. The MMCs produced by them are called continuously (fibre)

reinforced composites and discontinuously reinforced composites. However,

they can be subdivided broadly into five major categories: continuous fibres,

short fibres (chopped fibres, not necessarily the same length), whiskers,

particulate and wire (only for metal). With the exception of wires,

reinforcements are generally ceramics, typically these ceramics being oxides,

carbides and nitrides. These materials are used because of their combinations

of high strength and stiffness at both room and elevated temperatures.

Common reinforcement elements are SiC, A12O3, TiB2, boron and graphite.

Particulates are the most common and cheapest reinforcement

materials. These produce the isotropic property of MMCs, which shows a

promising application in structural fields. The SiC-particulate-reinforced

aluminium matrix composites have a good potential for use as wear resistant

materials. Actually, particulates lead to a favorable effect on properties such

as hardness, wear resistance and compressive strength.

23

Application: If the composite is to be used in a structural

application, the modulus, strength, and density of the

composites are important, which requires a high modulus, low

density reinforcement. Particle shape may also be important,

since angular particles act as local stress raisers, reducing

ductility. When the composite is used in thermal management

applications, the coefficient of thermal expansion and thermal

conductivity are important. If the composite is to be used in

wear resistant applications, hardness is important.

Method of composite manufacture: There are two generic

methods for composite manufacture, powder metallurgy (P/M)

and methods involving molten metal. Reaction of the

reinforcement can severely degrade the properties of the

composites, and hence the reinforcement has to be chosen after

considering the matrix alloy, and the processing time and

temperature.

Cost: A major concern for using particulates is to reduce the

cost of the composites. Therefore, the reinforcement of

reproducible grade has to be readily available in quantities, size

and shape required at low cost.

2.3 PROCESSING AND PROPERTIES

The attractive physical and mechanical properties that can be

obtained with metal matrix composites such as high specific modulus,

strength and thermal stability have been documented extensively. The various

factors controlling the properties of particulate MMCs and the influence of

the manufacturing route on the MMC properties has also been reviewed by

several investigators (Uematsu et al 2008, Ceschini et al 2009, Zhu et al 2012,

24

Abdizadeh et al 2011, Cocen et al 1997, Liu et al 2008a, Samuel et al 1993,

Ganesh & Ferreira 2009, Maiti & Chakraborthy 2008, Liu et al 2008b,

Vishista & Gnanam 2009, Wang et al 2009, Tekmen et al 2003, Logsdon &

Liaw 1986 and Ko & Yoo 1999).

Muratoglu et al (2006b) investigated the joining characteristic of

SiC particulate reinforced aluminum metal matrix composites (MMCs) with

pure aluminum by diffusion bonding process. The joining quality of the

Al/SiCp MMCs was studied to determine the influences of SiCp particulates

with homogenization and age hardening on bonding properties. The

experimental results indicate that the application of aging before and after

diffusion bonding decreases SiC particulate accumulation, and increases other

elemental concentration at interface. Especially, the application of aging

treatment before the diffusion bonding of Al/SiCp MMCs to pure Al,

increased Cu% concentration at interface which treats as the insert alloy.

Umasankar et al (2014) have made an attempt to understand the

influence of processing parameters on the mechanical properties like sintered

density and micro hardness. It has been observed that the compacting pressure

and reinforcement percentage have higher impact than the sintering

temperature on microhardness and density. Further to evaluate the fracture

resistance strength, sintered composites have been subjected to static ball

indentation. It has been observed that higher reinforcement and compacting

pressure enhance the failure load while sintering temperature and time exert

constrained influence.

Karamis et al (2012) had manufactured different MMCs to

determine their tribological properties. AA2124 matrix material, reinforced by

SiC, B4C or Al2O3 (of different particle sizes), was used for manufacturing by

powder metallurgy. The reinforcing particles were included at 10%, 20% and

30% volume fraction (vf). The matrix and reinforcement powders were

25

compressed at 600 MPa and 615°C for 30 min in an argon atmosphere. For

comparison with the metal matrix composites (MMC), unreinforced AA2124

samples and GGG40 cam material samples (both induction-hardened and

untreated) were also prepared for tribological tests. Tribological tests were

conducted at 50 N loading with 900 rpm revolution for 30 min under dry

conditions.

Uthayakumar et al (2013) had emphasized on the dry sliding wear

behavior of aluminum reinforced with 5% SiC and 5% B4C hybrid composite

using a pin on disc tribometer. Wear performance of the hybrid composites

were evaluated over a load ranges of 20 100 N, at the sliding velocities from

1 to 5 m/s. Detailed metallurgical examination and energy dispersive analysis

were carried out to assess the effect of SiC and B4C particles on the wear

mechanisms. The Focused Ion Beam (FIB) technique is used to characterize

the tribo layers that have been formed at the worn surfaces of composites. The

experimental results show that the hybrid composites retain the wear

resistance properties up to 60 N load and sliding speed ranges 1 4 m/s. The

enhancement of wear resistance with small amount of SiC and B4C is

achieved by the cooperating effect of reinforcement particles.

Colin et al (1993) investigated the influence of some processing

parameters on the extent of interfacial reaction in squeeze cast aluminium

matrix composites reinforced with 12 pm diamerer, continuous stainless steel

fibres. The average thickness of the reaction layer at fibre/matrix interfaces

was measured by image analysis. When casting was made in a die at room

temperature, the thickness of the reaction layer was affected on a distance of

several mm from the lateral surface or from the bottom of the preform. The

results indicate that the major part of the reaction occurs before solidification

of the liquid metal. The control of the extent of interfacial reaction can be

26

achieved through optimization of both infiltration parameters and features of

the preform such as the volume fraction of the fibres.

Rajmohan et al (2014) had synthesised and characterized the hybrid

aluminium matrix reinforced with micro SiC particles, and nanocopper oxide

(CuO) particles prepared by sintering process. First the powder mixtures

containing fixed weight (wt)% of SiC and different wt% of nanocopper oxide

as reinforcement constituents that are uniaxially cold pressed. Afterwards the

green compacts are sintered in an electric muffle furnace. Microstructure and

mechanical properties such as tensile strength, microhardness and density of

the composites are examined. Microstructure of the samples has been

investigated by using scanning electron microscope (SEM), X-ray diffraction

(XRD) and Atomic Force Microscope (AFM). The results indicated that the

increase in weight% of nano Cuo particles improves the mechanical

properties.

The effects of Sc and Zr on the aging behavior, precipitation

strengthening and precipitate coarsening of the composites were investigated

by Lai (2013). The addition of Sc resulted in considerable precipitation

hardening of the composite matrices for all aging temperatures applied.

Alloying both Sc and Zr in the Al B4C composites produced a remarkable

synergistic effect, which provided not only an increase in strength at peak

aging but also improved the stability of the mechanical properties. The

precipitate volume fraction, the average radius and the size distribution of

nanoscale Al3Sc and Al3(Sc,Zr) precipitates formed during aging were

measured. The Al3(Sc,Zr) precipitates generally exhibited better coarsening

resistance than Al3Sc precipitates.

Scanning electron microscope (SEM), X-ray diffraction (XRD)

techniques were used to characterize the sintered composites by Meijer et al

(2000). The effect of temperature on the density, hardness, strength, and

27

microstructure of composites was investigated. Detailed failure behavior was

analyzed. It was found that the segregation of SiC appeared at higher

temperature. The highest micro-hardness of 80MPa occurred at 700°C. The

strength tended to increase with the increasing temperature due to the

formation of Al2Cu. Both ductile and brittle fracture features were observed.

Mukherjee et al (1997) had developed a fracture mechanics

approach to examine the interfacial debonding process in MMCs and ceramic-

matrix composites (CMCs) during a fiber push-out test. The equivalent

domain integral (EDI) method is implemented in a finite element code and is

used to compute the strain-energy release rates for the interface crack. The

cooling process from the composite consolidation temperature, specimen

preparation for the push-out test and the actual testing are included in the

finite element simulation. A strain-energy-based debonding criterion is used

to predict the interfacial behavior. The experimentally observed phenomenon

of bottom debonding in MMCs is explained from the energy release rate

variation for the loading and support end cracks. It is shown that processing-

induced residual stresses significantly affect the initiation and propagation of

interface cracks. The advantage of the EDI method over conventional

methods for modeling interface crack propagation, by eliminating the need for

singular elements and thus remeshing with crack advance is demonstrated

through the simulation of the push-out test.

Ekici et al (2010) had investigated the effects of particle size,

volume fraction, random dispersion and local concentration underneath a

spherical indenter on the indentation response of particle reinforced metal

matrix Al 1080/SiC composites. The ceramic particles in certain sizes and

volume fractions were randomly distributed through the composite structure

in order to achieve a similar structure to an actual microstructure as possible.

The particle size and volume fraction affected considerably indentation depths

28

and deformed indentation surface profiles. The indentation depth increases

with increasing particle size, but decreases with increasing particle volume

fraction. The experimental indentation depths were in agreement with

numerical indentation depths in case the local particle concentration effect is

considered. The local particle concentration plays an important role on the

peak indentation depth. For small particle sizes and large volume fractions the

random particle distribution affects the deformed surface profiles as well as

the indentation depths. However, its effect is minor on residual stress and

strain distributions rather than levels in the indentation region.

Narayana Murthy et al (2003) involved in the development of

processing maps, a simple instability condition for assessing the extent of

plastic deformation in a workpiece prior to the formation of defects and

potential of the instability condition, the published flow stress data of 6061

Al-10 vol. % Al2O3 particulate reinforced metal matrix composite is

considered. Instability maps at different strain levels were superimposed

while delineating the unstable regions in the processing maps. This takes into

account the dependence of strain rate sensitivity and strain hardening

coefficient of the material on the plastic instability during hot deformation.

The stable and unstable regions in the map are verified with the

microstructural observations of the deformed compression specimens as well

as the industrial forging trials. To examine its validity, a comparative study is

made with the flow localization concept for titanium alloys and titanium

aluminides. The standard flow localization concept shows inconsistency in

predicting the unstable regions in the processing maps, whereas, the present

instability criterion is found to be consistent. Further studies were made on

the hot deformation of 6061 Al with varying volume fractions and sizes of

SiC particulate reinforcements.

29

Corrochano et al (2009) reported on six Al Mg Si composites

reinforced with 15 vol. % of MoSi2 intermetallic particles, together with three

unreinforced monolith Al Mg Si (AA6061) alloys have been processed by

powder metallurgy to quantify the roles of alloy matrix grain size and

reinforcement particle on their solutionized hardness and ageing response. In

the range studied, hardness of solutionized composites follows a Hall Petch

mechanism. Moreover, it can be rationalised as the sum of the hardness of the

alloy matrix with the same matrix grain size (d) and a term HR, that accounts

for 17 27% of total hardness, is roughly constant and independent of

reinforcing size and distribution. Matrix grain size is responsible for 50 65%

of hardness, whereas the contributions of solid solution and dtrengthenings

account for 17 26%.

Maiti & Chakraborty (2008) investigated on aluminium matrix

composites reinforced with molybdenum aluminide nanoparticles synthesized

by ball milling and reactive sintering of the mixture of aluminium and 10 wt%

hydrated molybdenum oxide powders. Sintering the as milled powder in air

below 750°C produced MoAl12 intermetallic compound nanoparticles, at

750°C produced a mixture of MoAl5 and MoAl4 nanoparticles and at 800°C

under Argon atmosphere produced predominantly MoAl4 intermetallic nano-

particles in the Al matrix. The powder compacts sintered in air below 750°C

produced MoAl12 whereas at 750°C or above formed the Al matrix

composite reinforced with the MoAl5 nanoparticles. These nanoparticles

become agglomerated to take up some irregular shaped flakes in the metal

matrix. The reaction between Al and hydrated Mo oxide powders was found

to be a favorable way to produce predominantly a particular Mo Al

intermetallic compound at a particular temperature. The Al2O3 particles

formed as another reaction product, in all the above reactions, remain

distributed in these composites. The composites thus formed were

characterized by SEM, XRD and TEM analysis.

30

Liu et al (2008b) found that only when the concentration of sodium

dodecyl sulfate was much higher than its critical micelle concentration, could

PMMA/Al2O3 composite particles with high percentage of grafting be

prepared. The same results were obtained between the experimental and

stoichiometric amounts of tris (dodecylbenzenesulfonate) isopropoxide

(NDZ), indicating that single-molecule-layer adsorption had taken place

between NDZ and Al2O3. Analysis using FTIR, TEM and XPS showed that

PMMA/Al2O3 composite particles with core shell structure had been

successfully synthesized by in situ emulsion polymerization. Compared to

Al2O3, thermal stability and dispersibility of the composite particles showed

marked improvement.

Near fully dense Fe3Al (10Ti)/TiC composites were synthesized by

Jia Li et al (2008) using mechanical alloying technique and hot-pressing

sintering methods. Based on the Orowan strengthening effect offered by the

nano-TiC particles, higher three-point bending strength and hardness values,

1310 MPa and 90 HRA, were achieved in Fe3Al(10Ti)/40 vol.% TiC

composite. The addition of Ti favored the improvement of hardness and room

temperature bending strength of composites by the ordering strengthening

effect and solid solution hardening effect. The microstructures of as-

synthesized composites were investigated by XRD, SEM and TEM. Fe3Al

particles had equiaxed morphology. TiC particles with grain size ranging

from 50 to 200nm were homogeneously dispersed in Fe3Al matrix. The larger

TiC particles with sub-micrometer were mainly located at grain boundaries

and the smaller ones were within matrix grains.

Ibrahim et al (1991) analysed the physical and mechanical

properties of particulate reinforced MMCs. Reinforcement materials include

carbides, nitrides and oxides. In an effort to optimize the structure and

properties of particulate reinforced MMCs various processing techniques have

31

evolved over the last 20 years. The processing methods utilized to

manufacture particulate reinforced MMCs can be grouped depending on the

temperature of the metallic matrix during processing. Accordingly, the

processes can be classified into three categories such as liquid phase

processes, solid state processes, and two phase (solid-liquid) processes.

Regarding physical properties, strengthening in metal matrix composites has

been related to dislocations of a very high density in the matrix originating

from differential thermal contraction, geometrical constraints and plastic

deformation during processing.

Nano-sized ceramic particle reinforced aluminum matrix

composites fabricated by Su et al (2012) by using conventional stir casting

technique usually present poor distribution of nanoparticles within the matrix

and high porosity. In this study, nano-Al2O3/2024 composites were prepared

by solid liquid mixed casting combined with ultrasonic treatment. The

obtained composite exhibited fine grain microstructure, reasonable Al2O3

nanoparticles distribution in the matrix, and low porosity. Solid liquid mixed

casting technique was effective in inhibiting the agglomeration of

nanoparticles in the matrix. The application of ultrasonic vibration on the

composite melt during the solidification not only refined the grain

microstructure of the matrix, but also improved the distribution of nano-sized

reinforcement. Compared with the matrix, the ultimate tensile strength and

yield strength of 1 wt.% nano-Al2O3/2024 composite were enhanced by 37%

and 81%, respectively. The better tensile properties were attributed to the

uniform distribution of reinforcement and grain refinement of aluminum

matrix.

Liu et al (1994) fabricated metal matrix-particulate composites

(MMPCs) using powder metallurgy (PM) as the fabricated composites

possess a higher dislocation density, a small sub-grain size and limited

32

segregation of particles, which, when combined, result in superior mechanical

properties. The various PM-related processes currently in use in the

fabrication of MMPCs are reviewed, outlining the common problems

encountered in each of these fabrication processes. The more recently

developed PM techniques to fabricate MMPCs are also discussed.

Kalkanli & Yilmaz (2008) studied about 7075 matrix alloy and SiC

at the liquid state before vertical pressure casting. Four different additions of

SiCp were made and the weight fractions used are 10%, 15%, 20% and 30%.

Composites were processed by vertical pressure/squeeze casting machine

developed at Middle East Technical University (METU) under 80 MPa

pressure. The mold is specially designed to produce specimens for tensile and

three point bend tests. Both as-cast and heat treated aluminum composites

were examined and the T6 heat treatment was applied. Three point bend tests

were performed to reveal the fracture strength of the aluminum composites.

The 10 wt% SiCp aluminum matrix composites showed the maximum

flexural strength both for the as-cast (450 MPa) and heat treated conditions

(588 MPa). The maximum flexural strength increased by about 40 MPa (10%)

for the as-cast and 180 MPa (44%) for heat treated composites. Hardness tests

were performed to determine the maximum value. For the as-cast specimens

the hardness values increased from 133 to 188 Vickers (10 kg) with an

increase in SiCp content from 0 to 30 wt% and for the heat treated specimens

the hardnes values increased from 171 to 221 Vickers (10 kg). The peak

hardness values were obtained for the 24 h precipitation heat treatment.

Nam et al (2000) observed that the dynamic behavior of metal-

matrix composites (MMCs) varies with impact velocity. MMCs with 15 vol%

of fibers were fabricated by the squeeze-casting method. AC8A aluminum

alloy was used as the matrix, and alumina and carbon fibers were used as

33

reinforcements. Tensile and vibration tests were conducted to obtain the

tensile failure stress and elastic modulus of the MMCs. A low-pass filter and

instrumented impact test machine were adopted to study the dynamic

behavior of the MMCs at various impact velocities. Stable impact signals

were obtained by using the low pass filter. As the impact velocity increases,

the impact energy absorbed by each material increases but its dynamic

fracture toughness does not change much. To show the relationship between

crack initiation energy and dynamic fracture toughness, a simple model is

proposed which uses the strain energy and stress distribution at a notch. The

crack initiation energy is proportional to the square of dynamic fracture

toughness and inversely proportional to elastic modulus.

Osso et al (1993) synthesized nanometer-sized -A12O3-metal

composites performed by room temperature ball-milling of mixtures of metal-

oxides and aluminium as shown by Matteazzi and Le Caer. The average

crystallite size of the alumina-metal composite so obtained is in general about

10nm. Such composites may also be prepared by direct grinding of a mixture

of -A12O3 and of a metal or an alloy.

Kennedy & Wyatt (2000) discussed the microstructure and

mechanical properties of aluminium/TiC MMCs made by powder processing

(PM). Particle clustering is more prevalent in cast than in PM composites, but

the grain-refining nature of TiC particles significantly reduces the degree of

clustering commonly observed in cast MMCs. Melting PM material enables

oxide films to `trawl' the particles into large clusters. The stiffness and

ductility are similar for cast and the PM composites but melting the PM

material results in significant reductions in strength and ductility. In all cases,

composite ductility is enhanced by extrusion through the removal of porosity

and the break-up of particle clusters. Modulus measurements as a function of

plastic strain indicate that rates of damage accumulation are lowest, and hence

34

interfacial bonding is strong in cast composites as a result of the attainment of

intimate contact and strong chemical bonding between the two phases.

Srivatsan & Hajri (2002) reported on the cyclic stress-amplitude-

controlled fatigue response and fracture behavior of aluminum alloy 7034

discontinuously reinforced with silicon carbide particulates (SiCp). In view of

the limited ambient temperature ductility, test specimens of the 7034/SiCp

composite, in both the under-aged and peak-aged conditions were cyclically

deformed under stress-amplitude control at an elevated temperature

corresponding to the aging temperature of the alloy. The cyclic fatigue tests

were conducted at two different load ratios with the objective of documenting

the conjoint influences of intrinsic composite microstructural effects, nature

of loading, and magnitude of cyclic stress amplitude on cyclic fatigue life and

fracture characteristics. The final fracture behavior of the composite is

discussed in light of the concurrent and mutually interactive influences of

composite microstructural effects, deformation characteristics of the

composite constituents, nature of loading, and resultant fatigue life.

The production methods and properties of metal matrix composite

materials reinforced with dispersion particles, platelets, non continuous

(short) and continuous (long) fibres are discussed by Kaczmar et al (2000).

The most widely applied methods for the production of composite materials

and composite parts are based on casting techniques such as the squeeze

casting of porous ceramic preforms with liquid metal alloys and powder

metallurgy methods. On account of the excellent physical, mechanical and

development properties of composite materials, they are applied widely in

aircraft technology and electronic engineering, and recently in passenger-car

technology also.

Investigations of composite materials based on EN AW-2124

aluminum alloy reinforced with the BN or Al2O3 particles with various weight

35

ratios of 5, 10 and 15% are presented by Dobrzanski et al (2006). Powders of

the starting materials were mixed in the laboratory vibratory ball mill to

acquire the uniform distribution of reinforcement particles in the matrix

material. The components were initially compacted at cold state in a die with

the diameter of 26mm in the laboratory vertical unidirectional press with a

capacity of 350 kN. The obtained P/M compacts were heated to a temperature

of 480/500°C and finally extruded with the extrusion pressure of 500 kN.

Bars with a diameter of 8mm were obtained as the end product. Based on the

microstructural examinations of the obtained composite materials, the

uniform distribution of the reinforcing particles in the aluminum matrix was

revealed. Hardness tests, static tensile tests and the compressive tests made it

possible to demonstrate that all these properties change along with the

reinforcing particles concentration change. As an example, hardness increased

from 89 HV1 for the material without the reinforcing phase to 123 HV1 for

15% of BN, and the ultimate compressive strength decreases along with the

increase of the reinforcing phase fraction from 667MPa for 5% to 472MPa for

15% of BN.

Li et al (1999) computationally constructed the samples by

assembling digitally acquired micrographs obtained by serial sectioning. The

material samples considered vary in volume fraction and in particle size.

Furthermore, equivalent microstructures with actual particles replaced by

ellipses (in 2-D) or ellipsoids (in 3-D) are computationally simulated for

efficiency. The equivalent microstructures are tessellated by a particle surface

based algorithm into a mesh of Voronoi cells. Various 3-D characterization

functions are developed to identify particle size, shape, orientation and spatial

distribution in the actual materials and to compare with 2-D micrographs.

Through this analysis, differences between 2-D and 3-D characterization are

established. Results indicate that it may not be sufficient to use 2-D section

information for characterizing detailed microstructural features like particle

36

shapes, orientations and near-neighbor distances. The second part of this

sequence of papers will describe the important relationship of these features

to damage evolution in these same materials. This sequence of papers is

perhaps one of the first on 3-D physical characterization of the phase and

damage structure for this class of materials.

Metal matrix composites based on aluminium alloys were produced

by Al-Rashed et al (1993) using powder metallurgy route, involved

unidirectionally hot pressing under 500 MPa for 15 minutes at temperature

about 0.95 Ts [Solidus Temperature]. Metal matrix contains different weight

percents of SiC, WC and SiN with different particle size. Wear and

mechanical tests have been carried out on composites, and it was found that

about 90% of wear reduction occured in composite with 30% Sic compared

with pressed matrix.

Sahoo & Pradhan (2013) investigated the influence of process

parameters like cutting speed, feed and depth of cut on flank wear and surface

roughness (Ra) in turning Al/SiCp metal matrix composites using uncoated

tungsten carbide insert under dry environment. The experiments have been

conducted based on Taguch 9 orthogonal array. Abrasion and adhesion are

observed to be the principal wear mechanism from images of tool tip. No

premature tool failure by chipping and fracturing was observed and

machining was steady using carbide insert. Built-up-edge formation is noticed

at low and higher cutting speed and at high feed combination and

consequently surface quality affected adversely. The optimal parametric

combination for flank wear and surface roughness are found to be v1 f1 d3

and v3 f1 d3 respectively and is greatly improved through Taguchi approach.

Mathematical models for flank wear and surface roughness are found to be

statistically significant.

37

Gopalakannan & Senthilvelan (2013) fabricated metal matrix nano-

composite (MMNC) of Al 7075 reinforced with 1.5 wt% SiC nano-particles

by a novel ultrasonic cavitation method. The high resolution scanning

electron micrograph (SEM) and field emission scanning electron micrograph

(FESEM) shows uniform distribution and good dispersion of the SiC

nanoparticles within the aluminum metal matrix. Electrical discharge

machining (EDM) was employed to machine MMNC with copper electrode

by adopting face centered central composite design of response surface

methodology. Analysis of variance was applied to investigate the influence of

process parameters and their interactions. Further a mathematical model has

been formulated in order to estimate the machining characteristics. It has been

observed that pulse current was found to be the most important factor

affecting all the three output parameters such as material removal rate (MRR),

electrode wear rate (EWR) and surface roughness (SR). The optimum

parameter of combination setting has been identified for the MMNC are

voltage 50.00 V, pulse current 8.00 A, Pulse on time 8.00 pulse off

time 9.00

minimizing EWR and SR using desirability function approach.

Alidokht et al (2013) studied a new processing technique, friction

stir processing (FSP) and used to incorporate SiC and MoS2 particles into the

matrix of an A356 Al alloy to form surface hybrid composite. The tool

rotation rate was changed from 630 to 1600 rpm and a tool tilt angle of 3° was

applied. Higher tool rotation rate was found to cause a more uniform

dispersion of reinforcing particles and thus decreases particles clustering. Dry

sliding wear tests were conducted using a pin-on-disc machine. The

subsurface deformation was assessed as a measure of variation in

microhardness along the depth normal to the cross-section of the worn

surface. It was found that the wear resistance of the processed samples

improved significantly as compared to that of the as-cast alloy.

38

Microstructural analysis showed that a MoS2 rich layer on the top of worn

surface helped to decrease the plastic deformation in subsurface region and

alleviate severe wear. The improvement in wear resistance of surface hybrid

composite compared to that of the as-cast alloy was found to be more

pronounced under higher applied loads.

Umanath et al (2013) had investigated the wear behaviour of

Al6061-T6 discontinuously reinforced with silicon carbide (SiC) and

aluminium oxide (Al2O3) composite. The test specimens are prepared and

tested as per ASTM standard. The experiments are conducted by using a pin

on disc wear tester. Empirical relation is established to estimate the wear

using statistical regression analysis and analysis of variance (ANOVA). The

results indicated that the wear resistance of the 15% hybrid composite is

better than that of the 5% composite. The fracture surface of composites

shows the ductile tear ridges and cracked SiC and Al2O3 particles indicating

both ductile and brittle fracture mechanism.

2.4 MACHINING

Over the last few decades, Metal Matrix Composites (MMCs) have

emerged as a material system offering tremendous potential for future

applications. The primary advantages offered by these materials are their

improved mechanical properties, particularly in the areas of wear, strength

and stiffness. Of the MMCs, Aluminum matrix composites have grown in

prominence due to their low density, low melting point and low cost.

However, machining these materials remains a challenging task mainly due to

the high abrasiveness of the reinforcing phases. Conventional machining

processes such as turning, milling or drilling are adopted for machining

MMCs. The existing and ongoing developments in machining MMCs - tool

life, tool wear, machinability and understanding chip formation mechanism

have been studied. Most of the studies discussed in this review focuses on

39

Aluminum matrix composites (Lin et al 1995, El-Gallab & Sklad 1998, El-

Gallab & Sklad 2004, Andrewes 2000, Zhang et al 2001, Palanikumar 2007,

Muller & Monaghan 2000, Palanikumar et al 2006, Lloyd 1994, Engelstad &

Reddy 1994, Zitoune et al 2010, Kilickap et al 2005, Dabade et al 2007,

Coelho et al 1995, Tosun & Muratoglu 2004a, Tosun & Muratoglu 2004b,

Quan & Ye 2003, Yanming & Zehua 2000, Pramanik et al 2007, Ramkumar

et al 2004, Munda & Bhattacharyya 2008, Hocheng & Tsao 2006).

Kadiver et al (2014), had discussed the effects of ultrasonic

vibration on burr size reduction, drilling force and surface roughness with two

different vibration systems. To this end, two vibration structures were built,

one to excite the workpiece (the workpiece vibration system) and the other to

vibrate the tool (the tool vibration system). Besides, the effects of amplitude,

feed rate, cutting speed, and SiC particle content on the drilling process of

Al/SiCp metal matrix composites are studied. In all tests TiN-coated HSS drill

tools with a diameter of 5 mm were utilized for drilling. Based on the attained

results, it was demonstrated that suitable ultrasonic vibration reduced burr

height, drilling force and surface roughness more so than conventional

drilling. Meanwhile, in the workpiece vibration system, enhanced surface

roughness and higher drilling force were obtained as compared to the tool

vibration system.

Sikder & Kishawy (2012) had presented an analytical force model

to predict the forces generated during machining of metal matrix composites

and investigates the effect of particle size on machining forces. Several

aspects of the cutting mechanics such as shear force, ploughing force, and

particle fracture force are considered to estimate the generated cutting forces.

Chip formation force is obtained using the Johnson Cook constitutive model.

The ploughing force is formulated using the slip-line field theory, while the

fracture force is calculated using Griffith Theory. The predicted results are

40

compared to experimentally measured data under different conditions. The

results show acceptable agreement between the theoretically predicted and

experimentally measured cutting forces.

Davim (2002) studied the machinability of the metal matrix

composites (MMCs) A356/SiC/20p with brazed polycrystalline diamond

(PCD) tools and chemical vapour deposition (CVD) diamond coated tools.

The experimental procedure consisted of turning operations, during which

cutting force, cutting tools flank wear and surface roughness obtained in

composite workpiece were measured. The obtained results showed that PCD

tools are important in cutting this composite type of reduced machinability.

Actually, CVD diamond coated tools show short life, as tools wear evolution

becomes very fast after coating rupture.

Ramulu et al (2002) conducted drilling studies on Al2O3 aluminum-

based metal matrix composites by using different drills (high-speed steel,

carbide-tipped, and polycrystalline diamond (PCD) drills to produce holes in

10 and 20 vol. % (Al2O3)p/6061. The drilling forces were recorded using

dynamometer. Surface finish was evaluated by a surface profilometer, and

tool wear and geometry were inspected using optical and scanning electron

microscopes (SEM). The drilling characteristics were evaluated in terms of

drilling forces, tool wear, chip formation, and drilled-hole quality. It was

found that PCD drills outperformed all other drills in terms of drilled-hole

quality and minimum drilling forces induced. Better results achieved with the

PCD drills are in line with other similar studies with other processes.

Surface roughness variations on the drilled surface and extension of

surface and subsurface deformation due to drilling was investigated by

Basavarajappa et al (2007b). The influence of different tools and cutting

conditions on Al2219/15%SiCp and Al2219/15%SiCp-3%Graphite (hybrid)

composites is investigated experimentally. The composites are fabricated by

41

liquid metallurgy method. The drilling tests are conducted with carbide and

coated carbide tools. The surface roughness decreases with the increase in

cutting speed and increases with the increase in feed rate. The surface is

analyzed using scanning electron microscope (SEM). Microhardness profiles

indicate that the subsurface deformation extends up to a maximum of 120 mm

below the machined surface for Al2219/15SiCp-3Gr composite when

compared to 150 mm in Al2219/15SiCp composite.

Morin et al (1995) conducted experiments on drilling of 6061 T-6

aluminum alloy as well as in a particle reinforced metal matrix composite

(MMCp) consisting of 20 vol.% SiC particles, 12 pm in diameter, in a 6061

aluminum matrix (6061/SiC/20%, or Duralcan@ F3S20S). HSS (high speed

steel) drills of 10 mm diameter were used, and measurements were made of

thrust (normal force), torque and flank wear for several feed rates and drill

speeds. It was found that while drilling Duralcan C0 with unworn drills, both

torque and thrust varied with feed rate raised to the power 0.81, as for

classical materials. When flank wear V, became significant, torque varied

linearly with V, but no empirical relation with physically meaningful

parameters was found to fit the thrust data. Speed had no significant effect on

wear or on drilling forces. Flank wear proceeded linearly with depth of

material drilled, or with the total distance passed by the lip or cutting edge of

the drill. A linear relation between both thrust and torque against flank wear

was observed, so that either thrust or torque may be measured to give an

indication of wear of the drill. The linear relation between torque and wear of

the drill implies a linear variation of specific cutting energy with flank wear.

Drilling forces are controlled by the matrix material and not by the particles.

Singh et al (2004) investigated the effect of current (C), Pulse on-

time (P) and flushing pressure (F) on metal removal rate (MRR), tool wear

rate (TWR), taper (T), radial overcut (ROC), and surface roughness (SR) on

42

machining as-cast Al-MMC with 10% SiCP reinforcement. ELEKTRAPULS

spark erosion machine was used for the purpose and jet flushing of the

dielectric fluid, kerosene, was employed. Brass tool of 2.7mm was chosen

to drill the specimens. An L27 orthogonal array (OA), for the three machining

parameters at three levels each, was opted to conduct the experiments. The

experiments were performed in a random order with three successive trials.

Analysis of variance (ANOVA) was performed and the optimal levels for

maximizing the responses were established. Scanning electron microscope

(SEM) analysis was done to study the surface characteristics.

The dynamics of drilling of high volume fraction glass fibre

reinforced composite was investigated by Velayudham et al (2005). This type

of composite is currently used in ballistic applications. At high fibre volume,

fibres do not show much relaxation and normal hole shrinkage associated

with polymeric composites is not observed during drilling. Peak drilling

thrust, dimension of holes drilled and vibration induced during drilling are

observed to correlate with each other. Vibrations study has been attempted

through wavelet packet transform and the results demonstrated its capability

in signal characterization.

Tsao & Hocheng (2008) presents the prediction and evaluation of

thrust force and surface roughness in drilling of composite material using

candle stick drill. The approach is based on Taguchi method and the artificial

neural network. The experimental results indicate that the feed rate and the

drill diameter are the most significant factors affecting the thrust force, while

the feed rate and spindle speed contribute the most to the surface roughness.

In this study, they established a correlation between the feed rate, spindle

speed and drill diameter with the induced thrust force and surface roughness

in drilling composite laminate. The correlations were obtained by multi-

variable regression analysis and radial basis function network (RBFN) and

43

compared with the experimental results. The results indicate the RBFN is

more effective than multi-variable regression analysis.

Ciftci et al (2004b) carried out the machining tests for SiCp-

reinforced metal matrix composites (MMCs) containing two levels of SiC

particles (8 and 16 wt%) of different mean particle sizes 30, 45 and 110 lm

prepared using a melt stirring squeeze casting route. Uncoated and triple-

layer coated carbide cutting tools at various cutting speeds under a constant

feed rate and depth of cut were used for the study. The effects of cutting speed

and coating of tool on tool wear were investigated. Furthermore, surface

roughness measurements were carried out on the machined surfaces. The

reinforcement particle size and its weight fraction together with the cutting

speed were found to be the major factors affecting the tool wear. Coated

carbide cutting tools performed better than uncoated carbide cutting tools for

all the materials machined in terms of tool wear. However, uncoated cutting

tools produced better surface finish in terms of mean Ra values, particularly at

lower cutting speeds. Although the tool wear mechanism remained one of

abrasion, detailed examination of the cutting edges under scanning electron

microscope (SEM) showed that higher cutting speeds led to edge chipping.

The investigation done by Suresh Kumar Reddy et al (2008) is to

enhance the knowledge about the machinability of Al alloy reinforced with

SiC using TiAlN coated carbide end mill cutters. Investigations on surface

quality and the extent of sub-surface damage of machined Al/SiC PMMC and

Al alloy were carried out at different levels of cutting conditions. The

comparison of Al/SiC PMMC and Al alloy on the basis of surface integrity

(surface roughness, residual stress, microstructure and microhardness) was

tried out in order to know the machinability of two materials. The results

show that the presence of the reinforcement enhances the machinability in

44

terms of both surface roughness and lower tendency to clog the cutting tool,

when compared to a non-reinforced Al alloy.

Palanikumar et al (2008) presented the results of experimental

investigation on mechanical and machinability properties of SiCp reinforced

aluminium metal matrix composite. The influence of reinforced ratios of 5, 10

and 15 wt. % of SiCp on mechanical properties was examined. The effect of

machining parameters, such as cutting speed, feed rate and depth of cut on

tool wear and surface roughness was studied. It was observed that increase of

reinforcement element addition produced better mechanical properties such as

impact toughness and hardness, but tensile strength showed different trend;

increased upto 10 wt.% of SiCp reinforced and then decreased when 15 wt.%

of SiCp reinforcement addition. Machinability properties of the selected

material were studied and higher SiCp reinforcement produced a higher tool

wear. It is also identified that the surface roughness was generally affected by

feed rate and cutting speed.

Lin et al (1998) characterized the mechanism of chip formation,

cutting forces and surface roughness. XRD, SEM and mechanical properties

of the block samples produced by powder metallurgy technique were

investigated. It was found that increasing the duration of mechanical alloying

resulted in the formation higher amounts of Al3C4 particles which therefore

raised the hardness, but on the contrary decreased the transverse rupture

strength of the samples. During the machining of MMCs, elemental and arc

chips formation were observed. High volume fraction of Al4C3 in the matrix

decreased formation of built-up edge (BUE) and surface roughness at high

cutting speeds. Furthermore, it was concluded that the effect of Al4C3 on the

crack formation in shear plane, reduced the cutting force, shortened the chip

contact length and the chip segment thickness.

45

Pramanik et al (2008) investigated the machining forces, chip

formation, surface integrity and shear and friction angles to understand the

machinability of metal matrix composites (MMCs). However, because of the

complexity of the reinforcement mechanisms of the ceramic particles, a fair

assessment of the machinability of MMCs is still a difficult issue. The major

findings are that the surface residual stresses on the machined MMC are

compressive, the surface roughness is controlled by feed, particle pull-out

influences the roughness when feed is low, particles facilitate chip breaking

and affect the generation of residual stresses and the shear and friction angles

depend significantly on feed but are almost independent of speed.

Sahin et al (2002) investigated the machinability of 2024

aluminium alloy reinforced with Al2O3 particles using varying size and

weight fraction of particles up to 30 wt. % by a vortex method for different

cutting conditions. Experiments were carried out with TiN (K10) coated

carbide tools and TP30 coated carbide tools at various cutting speeds. Tool

wear and surface roughness in the turning of Al2O3 particle-reinforced

aluminium alloy composite was investigated with special attention paid to the

effects of material structures. The experimental results showed that tool life

increased with increasing the cutting speed for both cutting tools and the tool

life of TiN (K10) tool was significantly longer than that of TP30 tool. It is

observed that the major wear form of the tools are the combination of

rounding of nose and flank wear in addition to removal of coated layer from

the substrate for the TiN (K10) tool but edge chipping and rounding of nose

was evident for the TP30 tool. Moreover, the optimum surface roughness was

obtained at a speed of 160 m/min while the maximum surface roughness

value was found in the machining of the 10% Al2O3 composites with particle

size of 16 mm. The surface roughness also increased with the increasing

weight percentage of the particles. Furthermore, physical appearance of chips

produced by TiN (K10) cutting tools were discontinuous and smaller sizes

46

while the appearance of chips produced by TP30 cutting tools were

continuous type and larger size.

El-Gallab & Sklad (2000) developed a robust 3-D finite element

model of a cutting tool, taking into consideration the thermal and mechanical

interactions at the tool/chip and tool/workpiece interfaces. Temperature-

dependant material properties are incorporated in the model. The model was

applied to the special case of the turning of Al/SiC particulate metal matrix

composites, where high tool wear rates represent a challenge to the industry.

The temperatures and stresses predicted by the model are in agreement with

experimental measurements and tool wear observations. The wear patterns

predicted by the model include crater wear, tool pitting and tool chipping.

Thus, the model presented could be utilized in the selection of the tool

material, geometry and cutting parameters that would result in the least tool

wear, and hence helps to reduce machining costs and tool change down-time.

Controlling tool wear is expected to enhance the surface integrity of the

workpiece.

2.5 WEAR PERFORMANCE

Wear of metals is probably the most important to understand the

aspects of tribology. It is certainly the youngest of the tri of topics, friction,

lubrication and wear, to attract scientific attention, although its practical

significance has been recognizes throughout the ages.

One third of our global energy consumption is consumed wastefully

in friction. In addition to this primary saving of energy, very significant

additional economics can be made by the reduction of the cost involved in the

manufacture and replacement of prematurely worn out components. The

dissipation of energy by wear impairs strongly the national economy and the

47

life style of most of people. So, the effective decrease and control of wear of

metals are always desired.

Wear causes an enormous annual expenditure by industry and

consumers. Most of this is replacing or repairing equipment that has worn to

the extent that it no longer performs a useful function. For many machine

components a very small percentage of the total volume has been worn away.

Wear is not an intrinsic material property but characteristics of the

engineering system which depend on load, speed, temperature, hardness,

presence of foreign material and the environmental condition. Widely varied

wearing conditions causes wear of materials. It may be due to surface damage

or removal of material from one or both of two solid surfaces in a sliding,

rolling or impact motion relative to one another. In most cases wear occurs

through surface interactions at asperities. During relative motion, material on

contacting surface may be removed from a surface, may result in the transfer

to the mating surface, or may break loose as a wear particle. The wear

resistance of materials is related to its microstructure and may take place

during the wear process and hence, it seems that in wear research emphasis is

placed on microstructure. Wear of metals depends on many variables, so wear

research programs must be planned systematically.

were conducted by mechanical engineers and metallurgists to generate data

for the construction of motor drive, trains, brakes, bearings, bushings and

other types of moving mechanical assemblies.

It became apparent during the survey that wear of metals was a

prominent topic in a large number of the responses regarding some future

priorities for research in tribology (Abouelmagd 2004, Unlu 2008, Wang et al

2007, Dhokey & Paretkar 2008, Mahapatra & Patnaik 2009, Attia 2001,

48

Izciler & Muratoglu 2003, Lim et al 2003, Choi et al 2010, Bonollo et al

1993, Amirthan & Balasubramanian 2011, Chang et al 2013, Wang et al

2010).

Amro & Qutub (2009) investigated experimentally the effect of

heat treatment on the hardness, wear behavior, and friction properties of 6061

Al composite reinforced with sub-micron Al2O3 (10% vol.) produced by

powder metallurgy. Heat treatment of the as-received composite starts by the

solution treatment at a temperature of 550°C for a period of two hours

followed by quenching in chilled water and then age hardening at 175°C for

different periods. It is illustrated that heat treatment has relatively small effect

on the hardness of the composite. This can be attributed to the large interface

areas between the matrix and the sub-micron alumina in the composite, which

reduces the whole concentration of vacancies in the matrix. The results shows

reduction in efficiency of age hardening. For this reason, wear and friction

tests were limited to the heat treated composite with four hours aging only.

Ahlatci et al (2004) had studied the effect of Si addition up to 8%

Si on the abrasive wear and mechanical properties of Al Si/60 vol.% SiC

composites produced by pressure infiltration technique. Optimum properties

in terms of improved strength and abrasion resistance without significant loss

in toughness were obtained when the matrix alloy contained 1% Si. At Si

contents higher than 1%, dramatic decrease of toughness was accompanied a

reduction in strength and abrasion resistance.

Muratoglu & Aksoy (2006) studied the influence of temperature (in

the range 20 200°C) on the abrasive wear behaviour of a 2124 Al/SiC

composite produced by powder metallurgy techniques. Abrasive wear tests

were conducted at constant distance and 10N load using pin-on-disk

apparatus. Some specimen taken from the composite materials were

49

artificially aged (T6) to determine ageing effects. Moreover, worn surfaces

and the sub surfaces were examined by using SEM, EDS and optical

microscopy. Wear test results obtained at all test temperatures showed that the

weight loss of the aged specimens was less than that of the non-aged

specimens. It was also observed that the better wear resistance was seen for

specimens worn at RT both the aged and non-aged specimens. There was no

(or little) net change in wear rate for the test temperature above 50°C both the

aged and non-aged specimens. Due to contact between SiC particles in the

composite material and abrasive paper, broken or loosened hard SiC

particulates embedded to the soft layer under worn surface which observed at

50 200°C, caused an increase in strength of surface of the composite

specimens and so resulted little change in wear rate.

Wear behaviour of the composite was investigated by Sahin (2010)

to find out effects of operating variables and hardness in terms of the Taguchi

approach, on a pin-on-disc machine and compared with the previous work on

the composite produced by liquid metallurgy method. Analysis of variance

(ANOVA) was also employed to investigate which design parameters

significantly affected the wear behaviour of the composite. The results

showed that abrasive grain size exerted the greatest effect on the abrasive

wear, followed by the hardness, which supported the previous work, but the

percentage contribution was very different. The percentage contributions of

the grain size and hardness were about 81.57 and 11.09, respectively. This

might be because of production method of PM, particle size, model used by

not considering the interaction effects, and testing condition. Moreover, larger

particle sizes of the composites showed more wear resistance than those of

others. As for the case of earlier work the percentage contributions of the

grain size and type of material (hardness) were about 29.90, 17.90,

respectively. However, the percentage contribution of interaction of abrasive

50

size and hardness was about 30.90 while interaction of other factors was

pooled.

Dry sliding wear behaviour of the composite was tested and

compared with Al/SiCp composite by Basavarajappa et al (2005). A plan of

experiments based on Taguchi technique was used to acquire the data in a

controlled way. An orthogonal array and analysis of variance was employed

to investigate the influence of wear parameters like as normal load, sliding

speed and sliding distance on dry sliding wear of the composites. The

objective was to investigate which design parameter significantly affects the

dry sliding wear. It shows that graphite particles are effective agents in

increasing dry sliding wear resistance of Al/SiCp composite.

Zhang & Wang (2007) have investigated the friction and wear

behavior of the same brake materials dry sliding against two different brake

drums made of aluminum matrix composite reinforced with different sizes

p, respectively, in place of the conventional

cast iron brake drum for a Chase Machine. It has been found that the brake

material against the latter showed better friction performances and wear

resistance than those against the former associated with small-size SiCp

pullout and thin tribo film formation. In both cases, the friction coefficient

decreased with the increase of load and speed, and converged gradually at two

temperatures of 177 and 316°C. Friction fade took place at high temperatures,

followed by excellent recovery upon cooling. Also, the specific wear rate is

observed with the increase of load and speed, but shows an increase with

temperature.

Ghosh & Saha (2011) in this investigation, studied the crack

density and wear performance of SiC particulate (SiCp) reinforced Al-based

metal matrix composite (Al-MMC) fabricated by direct metal laser sintering

51

(DMLS) process have been studied. Mainly, size and volume fraction of SiCp

have been varied to analyze the crack and wear behavior of the composite.

The study has suggested that crack density increases significantly after 15

volume percentage (vol.%) of SiCp. The paper has also suggested that when

size (mesh) of reinforcement increases, wear resistance of the composite

drops. Three hundred mesh of SiCp offers better wear resistance and above

300 mesh the specific wear rate increases significantly. Similarly, there has

been no improvement of wear resistance after 20 vol.% of reinforcement. The

scanning electron micrographs of the worn surfaces have revealed that during

the wear test SiCp fragments into small pieces which act as abrasives to result

in abrasive wear in the specimen.

Kumar & Balasubramanian (2008) reported that the dry sliding

wear behaviour of AA7075 aluminium/SiCp composites fabricated by powder

metallurgy technique. Five factors, five levels, central composite, rotable

design matrix is used to optimize the required number of experiments. The

wear test has been conducted in a pin-on-roller wear testing machine, under

constant sliding distance of 1 km. An attempt has been made to develop a

mathematical model by response surface method (RSM). Analysis of variance

(ANOVA) technique is applied to check the validity of the developed model.

t-test is utilised to find out the significance of factors. The effects of

volume percentage of reinforcement, particle size of reinforcement, applied

load, sliding speed and hardness of counter part materials on dry sliding wear

behaviour of AA7075 aluminium/SiCp have been analysed in detail.

Pin-on-disk dry sliding wear tests at sliding speeds ranging from

0.6 to 1.25 m/s and under loads ranging from 3.98 to 6.37MPa (50 80 N)

were conducted by Tang et al (2008) for pin specimens of composites with

Al-5083 matrices reinforced with 5 and 10 wt.% B4C particles. The wear rate

of the composite with 10 wt.% B4C was approximately 40% lower than that

52

of the composite with 5 wt.% B4C under the same test condition. Two stages

were observed in the reduction of pin length/sliding distance curves in several

specimens, with the length reduction rate in the first stage being one to two

orders of magnitude lower than that in the second stage. The low length

reduction rate in the first stage corresponded with a flat stage with a low

coefficient of friction (COF) in the COF/sliding distance curve.

Vieira et al (2009) studied the influence of centrifugal casting

processing parameters on the wear of Al alloy/SiCp functionally graded

composites. A non-commercial Al alloy (Al 10Si 4.5Cu 2Mg) was selected

to be the matrix of the composites and the reinforcing particles were SiCp with

SiCp functionally graded metal matrix

different mould rotating speeds (1500 and 2000 rpm). By centrifugal casting a

gradient in the distribution of SiC particles across the thickness of the cast

ring was obtained. The sliding wear behavior was studied using a ball-on-ring

configuration (sliding wear parameters: 3N, 0.5 m/s, 1800m, room

temperature) with a high-carbon chromium steel ball (AISI 52100) as counter

body. A good correlation was evidenced between the dry sliding behaviour of

functionally graded aluminium matrix composites and the distribution of SiC

reinforcing particles. The dominant wear mechanisms were identified and

correlated with the microstructure of the functionally graded composites.

Effect of matrix alloy and influence of SiC particle on the sliding

wear characteristics of high strength aluminium alloys AA7010, AA7009 and

AA2024, composites was examined by Rao & Das (2010) for varying applied

pressure and a fixed sliding speed of 3.35 m/s. The results revealed that the

wear resistance of the composite was noted to be significantly higher than that

of the alloy and is suppressed further due to addition of SiC particles. The

overall observation among the matrix alloys, AA7010 alloy shows maximum

53

wear resistance than that of the other, and can withstand the seizure pressure

up to 2.6 MPa. The wear mechanism was studied through worn surfaces and

microscopic examination of the developed wear tracks. The wear mechanism

strongly dictated by the formation and stability of oxide layer, mechanically

mixed layer (MML) and subsurface deformation and cracking. The overall

results indicate that the high strength aluminium alloys and composite could

be considered as an excellent material where high strength and wear

resistance components are prime importance especially designing for

structural applications in aerospace and general engineering sectors.

Tribological behavior of aluminium matrix composite

(AMC)/brake pad tribo-couple under dry sliding conditions was studied by

Uyyuru et al (2006) using Pin-on-Disc machine. Brake pad material was used

as pins while the AMC formed the rotating disc. Series of experiments were

performed to characterize the tribological nature of the tribo-couple. Load and

sliding speeds were varied over a range to represent actual braking conditions

in passenger cars. Effect of volume fraction and size distribution of

reinforcement on wear and friction coefficient has been studied. It was

observed that a heterogeneous tribo-layer was formed over the worn surfaces

during the wear tests. Presence of tribo-layer was believed to cause two

effects: acting as a lubricant layer and acting as a source of wear debris.

Morphology and topography of worn surfaces and debris were studied using

scanning electron microscope (SEM), electron probe micro analyzer (EPMA),

and X-ray diffraction (XRD) techniques. When the reinforcement in the

matrix has wide size distribution, wear rate and friction coefficients are found

to be higher compared to composite containing mono-size reinforcement.

Wan et al (2007) investigated the non-lubricated, sliding friction

and wear behavior of Ti3Si (Al)C2 and SiC-reinforced Ti3Si(Al)C2 composites

against AISI 52100 bearing steel ball using a ball-on-flat, reciprocating

54

tribometer at room temperature. The contact load was varied from 5 to 20N.

For monolithic Ti3Si (Al) C2, high friction coefficients between 0.61 and 0.90

and wear rates between 1.79×10 3 and 2.68×10 3 mm3/Nm were measured.

With increasing SiC content in the composites, both the friction coefficients

and the wear rates were significantly decreased. The friction coefficients

reduced to a value between 0.38 and 0.50, and the wear rates between

2.64×10 4 and 1.93×10 5 mm3/Nm when the SiC content ranged from 10 to 30

vol.%. The enhanced wear resistance of Ti3Si(Al)C2 is mainly attributed to the

facts that the hard SiC particles inhibit the plastic deformation and fracture of

the soft matrix, the oxide debris lubricate the counter pair, and the wear mode

converts from adhesive wear to abrasive wear during dry sliding.

Kumar & Balasubramaniuan (2010) developed a new mathematical

model to predict the abrasive wear rate of AA7075 aluminum alloy matrix

composites reinforced with SiC particles. Five factors, five levels, central

composite, rotable design matrix were used to optimise the required number

of experiments. The model was developed using response surface method.

Analysis of variance technique was applied to check the validity of the model.

-test was utilised to find out the significant factors. The effect of

volume percentage of reinforcement, reinforcement size, applied load, sliding

speed and abrasive size on abrasive wear behaviour were analysed in detail.

Zhiqiang et al (2005) reported a study on the wear property of

powder metallurgy aluminum matrix composites 9Si/Al Cu Mg. A ring on

rock wear-testing machine is used to evaluate the wear property of the

composites, in which a GCrl5 steel ring is used as the counter face material.

The wear behavior of the composites under different conditions is studied.

The optical microscope and scanning electron microscope are used to analyze

the worn surfaces and the subsurface of the composites in order to research

55

the wear mechanism of the composites. Results indicate that the weight loss

of the composite were lower than that of the matrix alloy.

Extruded AlSi7 Mg alloy based SiCp reinforced (AlSi7 Mg/SiCp)

composites and the matrix alloy were wear tested on a pin on disk type tester

by Bayhan & Onel (2010). The work was planned so that some response

surface (RS) models can be used to examine the wear behaviour of composite

samples. The effects of friction load, sliding distance and reinforcement

content on the wear rate and weight loss of AlSi7 Mg/SiCp composites were

evaluated by using RS optimization procedure. In the applications of RS

models to engineering problems, the estimated RS models usually have a

maximum or a minimum point. Through this article the RS optimization

procedure was employed to optimize the reinforcement content and sliding

distance for the minimization of wear rate and weight loss of tested

composites. During the tests, the values of reinforcement content, friction

load and sliding distance were changed on the intervals (0%, 20%), (49 N,

169 N), (100 m, 1000 m), respectively. It was shown that there exists some

optimum values of reinforcement content and some optimum values of sliding

distance which minimize the wear rates. In this concern, the average value of

optimum reinforcement contents and the average value of optimum sliding

distances of AlSi7 Mg/SiCp composites minimizing the wear rate were found

as 13% and 595 m, respectively. Also the average value of optimum

reinforcement content minimizing the weight loss was found as 13%.

Yang (2007) discussed about the earlier work with turning and pin-

on-disc tests and found that the wear coefficient values of tungsten carbide

obtained could vary significantly due possibly to the different nominal contact

areas used in the various testing processes. They reported the results obtained

from research work carried out to determine the effect of nominal specimen

contact area on the wear coefficient of A6061 aluminium matrix composite

56

(MMC) containing 20% of alumina particles. The pin-on-disc testing method

was used to conduct the wear tests with speeds of 200 and 275 m/min and

loads of 74 and 98 N. Square pins with a length of 25mm and with two

different nominal contact areas of 6×6mm2 and 10×10mm2 were used. The

discs were made of AISI-01 cold work tool steel with a hardness of about 62

HRC. Wear tests were carried out at distances from 250 to 12,000 m. It was

found that the wear coefficient values obtained from the pins with a smaller

nominal contact area were indeed lower by an average of about 12%, due to

the availability of smaller asperity wear volumes. The results also agree with

the adhesive wear theory. Hence one should exercise extreme care in the

interpretation of wear coefficient data obtained from different testing

methods, or from using different nominal specimen contact areas.

Gurcan & Baker (1995) investigated the wear resistance of four

AA6061 MMCs together with the monolithic AA6061 alloy, all in the T6

condition, using a pin-on-disc test. In addition to the widely studied 20 vol.%

Saffil MMCs, the present investigation considered a hybrid of 11% Saffil +

20% SiC, and a high volume fraction SiC, MMC. AA6061 + 60% SiC. The

wear behaviour against P400 SiC grit adhesive bonded paper and against

BS817M40 (EN24) steel were explored under an applied load of 9.8 N with a

nominal contact pressure of 0.5 MPa. It was found that after testing against

Sic grit, AA6061 + Saffil showed little advantage over the monolithic alloy,

but the other three composites had a significant improvement in wear

resistance. The hybrid and the AA6061 + 60% SiC showed the best

performance. Only small improvements were noted for AA6061 + Saffil and

AA6061 + 20% SiC over the monolithic alloy, when tested against steel.

The wear behaviour of AE42 magnesium alloy and AE42+20%

saffil short fibre composite is investigated by Kumar et al (2007) under dry

sliding condition using a pin-on-disc set-up in the load range of 5 40N with

57

sliding speeds of 0.838, 1.676 and 2.513 m/s for a constant sliding distance of

2.5 km. Both alloy and the composite wear rate increases with increasing

loads and the wear rate of the composite is lower at lower loads. At all sliding

speeds, a crossover in wear rate is observed with the increase in load, i.e.,

above a certain load the wear rate of the composite becomes greater than that

of the alloy, and the crossover shifts to lower loads with increase in the

sliding speed. Severe sub-surface plastic deformation and fibre breakage are

found to be the dominant mechanism for the unreinforced alloy and the

composite, respectively.

A new formulation of the wear coefficient was developed and

tested experimentally by Yang (2003). Two different types of pin-on-disc

wear tests were conducted using three commercial, A6061 aluminum-based

metal matrix composites (MMCs). One type of test resulted in a spiral track

and the other a circular track. Hardened tool steel discs were used as the

sliding counterface for MMC pins having 10, 15 and 20% alumina

reinforcements. A new wear equation was derived and shown to be a better

predictor of steady-state wear coefficients.

Hassan et al (2009) studied about the friction and wear behavior of

Al Mg Cu alloys and Al Mg Cu based composites containing SiC particles

at room conditions with a pressure of 3.18 MPa and a sliding speed of 0.393

m/s using a pin-on-disk wear testing machine. This study is an attempt to

investigate the effects of adding copper as alloying element and silicon

carbide as reinforcement particles to Al 4 wt% Mg metal matrix. The wear

loss of the copper containing alloys was less than that for the copper free

alloys. It was observed that the volume losses in wear test of Al Mg Cu alloy

decrease continuously up to 5%. It was also found that the silicon carbide

particles play a significant role in improving wear resistance of the Al Mg

Cu alloying system. The formation of mechanically mixed layer (MML) due

58

to the transfer of Fe from counterface disk to the pin was observed in both

Al Mg Cu alloys and Al Mg Cu/SiC composites.

Deuis et al (1997) found that for adhesive wear, the influence of

applied load, sliding speed, wearing surface hardness, reinforcement fracture

toughness and morphology are critical parameters in relation to the wear

regime encountered by the material. In this review contemporary wear

theories, issues related to counterface wear, and wear mechanisms are

discussed. Other areas of research relevant to adhesive wear of Al-5 alloys

and aluminium composites containing discontinuous reinforcement phases,

such as the role of the reinforcement phase, are also presented.

The effects of applied load, sliding velocity and SiC volume

fraction on the transitional behavior between mild and severe wear in SiC

particulate reinforced copper matrix composites were studied Zhan et al

(2004) under dry sliding wear condition. Increasing SiC fraction or decreasing

sliding velocity delays the occurrence of severe wear up to higher transition

load. Mechanically mixed layer (MML), which is markedly harder than that

of the bulk material, is absent in the post-transition regime. The coverage rate

of MML is affected by applied load and sliding velocity. SiC particulates act

as load-bearing components and lessen the frictional deformation extent in the

subsurface region. In the pre-transition regime, microcrack propagation

induced detachment of MML and subsurface material are the primary wear

mechanism. In the severe wear process, thermally activated subsurface

deformation plays a significant role in the tear of surface layer from the

substrate material.

In the study of Kundu et al (2013), the experimental investigation

was done for hybrid metal matrix composites with SiC, Al2O3 and graphite

reinforced aluminium alloy (Al 6061T6) composites samples, processed by

stir casting route. The aluminium alloy was reinforced with 10 wt. % (SiC,

59

Al2O3) and 5 wt. % of graphite to mixture the hybrid composite. Dry Sliding

Wear of the hybrid composite were tested and it was found that when the

wear resistance of the hybrid composites can be increased when compared to

Al6061 T6 alloy. The parameters such as load, sliding speed and sliding

distance were identified to affect wear rate. The design of experiments (DOE)

approach using taguchi method was employed to analyze the wear behaviour

of hybrid composites. Signal-to-noise ratio and analysis of variance

(ANOVA) were used to investigate the influence of parameters on the wear

rate.

Prasad & Ramachandra (2013) studied the influence of the process

parameters on wear resistance in squeeze casting of LM6 Al-flyash composite

using Taguchi method. The parameters studied include percentage wt. of

flyash, squeeze pressure, and squeeze time. In Taguchi method, a four level

orthogonal array has been used to determine the S/N ratio. In this

investigation, composites have been produced by incorporating fly ash as a

reinforcement material and eutectic Al Si alloy as a matrix. Stir casting route

has been adopted to disperse fly ash (from 5% to 12.5%wt.) in the Al Si alloy

matrix which is followed by applying the squeeze pressure of 30, 60, 90 and

120 bar for a varying squeeze time. The Pin-on-disc test was conducted on the

specimen prepared out of these castings to determine the sliding wear

behavior of the composite. The results of experimental investigation on wear

resistance of flyash reinforced aluminium metal matrix composite shows that

the inclusion of the flyash by weight percentage and the squeeze pressure are

the recognized parameters to cause appreciable improvement in the wear

resistance of the squeeze cast components.

Israa (2013) studied the effect of addition of different weight

percent from SiCp (2, 4, 6, 8 ) to Al 4 Cu alloy which have been fabricated

by liquid metallurgy method on the dry sliding wear behavior and mechanical

60

properties. Wear characteristics of Al SiC composites have been investigated

under dry sliding conditions and compared with base alloy. Dry sliding wear

tests have been carried out using pin-on-disk wear test under normal applied

loads 5, 10, 15 and 20 N and at different sliding velocity of (2.7, 3.7, 4.7)

m/sec. It was also observed that the wear rate varies linearly with increasing

normal applied load. The wear mechanism appears to be oxidative for both Al

Cu alloy and composites under the given conditions of load and sliding

velocity as indicated by optical microscopic of the worn surfaces. Further, it

was found from the experimentation that the wear rate decreases linearly with

increasing weight percent of silicon carbide. The best results have been

obtained at 8 % wt SiC. It is also observed that the yield strength, tensile

strength increases with increasing wt% of SiC, but the ductility decreases.

2.6 OPTIMIZATION OF PROCESS PARAMETERS

A detailed survey was done for different optimization techniques to

optimize the process parameters in order to yield optimum performances (Ilo

et al 2012, Muthukrishnan & Davim 2009, Balasubramanian et al 2008,

Petropoulus et al 2008, Enemuoh &Gizawy 2003).

Bhushan (2013) , had presented the findings of experimental

investigations into the effects of cutting speed, feed rate, depth of cut and

nose radius in CNC turning of 7075 Al alloy 15 wt% SiC (particle size 20

40

methodology (RSM) has been used to accomplish the objective of the

experimental study. The machining parameters such as cutting speed, feed

rate, depth of cut and nose radius are optimized by multi-response

considerations namely power consumption and tool life. A composite

desirability value is obtained for the multi-responses using individual

desirability values from the desirability function analysis. Based on composite

desirability value, the optimum levels of parameters have been identified, and

61

significant contribution of parameters is determined by analysis of variance.

Confirmation test is also conducted to validate the test result. It is clearly

shown that the multi-responses in the machining process are improved

through this approach. Thus, the application of desirability function analysis

in response surface methodology proves to be an effective tool for optimizing

the machining parameters of 7075 Al alloy 15 wt% SiC (20 40

composite. Result of this research work show that when turning is be carried

out at values of machining parameters obtained by multi response

optimization through desirability analysis route this will reduce power

consumption by13.55% and increase tool life by 22.12%.

Taskesen & Kutukde (2014) evaluated the machining parameters

and optimized with grey relational analysis in drilling B4C reinforced metal

matrix composites (MMCs) produced by powder metallurgy. HSS, TiAlN

coated and uncoated cementide carbide drills were used under dry cutting

conditions. The drilling parameters such as feed rate, spindle speed, drill

material and wt.% of B4C particles were optimized based on multiple

performance characteristics including thrust force, torque and surface

roughness. The experimental study showed that increasing the weight fraction

of the B4C resulted in a considerable increase in the thrust force. Furthermore,

average surface roughness of drilled hole decreased with increasing particle

content for carbide tools and increased for HSS tools. Among the tools used,

TiAlN coated carbide drills showed the better performance with regard to the

surface roughness. Moreover, ANOVA analysis indicated that the most

effective factor on grey grade was found to be weight fraction and followed

by drill material, feed rate and spindle speed, respectively.

Davim & Conceicao (2001b) had investigated on an hybrid

technique based on an evolutionary search over a design space obtained by

experimental way is considered. The machining forces, the surface finish and

62

the tool wear are experimentally measured considering the feed and the

cutting velocity as predefined parameters. Using genetic algorithms for the

optimal search of cutting conditions, the chromosomes represent cutting

conditions defined according to a temporal scale and are composed by

random keys.

Ramesh & Suresha (2014), had focused on identifying the factors

such as filler type, filler loading, grit size of SiC paper, normal applied load

and sliding distance on two-body abrasive wear behaviour of the hybrid

composites. Abrasive wear tests were carried on carbon fabric reinforced

epoxy composite (C-E) filled with filler alumina (Al2O3) and molybdenum

disulphide (MoS2) separately in different proportions, using pin-on-disc

apparatus. The experiments were planned according to Taguchi L18

orthogonal array by considering five factors, one at two levels and the

remaining at three levels, affecting the abrasion process. Grey relational

analysis (GRA) was employed to optimize the tribological parameters having

multiple-response. Analysis of variance (ANOVA) was employed to

determine the significance of factors influencing wear. Also, the comparative

specific wear rates of all the composites under dry sliding and two-body

abrasive wear were discussed. The analysis showed that the filler loading, grit

size and filler type are the most significant factors in controlling the specific

wear rate of the C-E composite. Optimal combination of the process

parameters for multi performance characteristics of the composite under study

is the set with filler type as MoS2, filler loading of 10 wt.%, grit size 320, load

of 15 N and sliding distance of 30 m. Further, the optimal parameter setting

for minimum specific wear rate, coefficient of friction and maximum

hardness were corroborated with the help of scanning electron micrographs.

Palanikumar (2011) presented an effective approach for the

optimisation of drilling parameters with multiple performance characteristics

63

16,

4-level orthogonal array has been used for the experimentation. The drilling

parameters such as spindle speed and feed rate are optimised with

consideration of multiple performance characteristics, such as thrust force,

workpiece surface roughness and delamination factor. Response table and

response graph are used for the analysis. The analysis of grey relational grade

indicates that feed rate is the more influential parameter than spindle speed.

The results indicate that the performance of drilling process can be improved

effectively through this approach.

2.7 SUMMARY OF LITERATURE REVIEW

The following observations can be drawn from the literature review

as the bases of this research. Aluminium alloy reinforced with SiC particles

has distinct properties and the literature review has established clear strategy

for achieving high wear resistant and better machinability characteristics. In

addition, to extend the wide applications of these composites in various

engineering sectors especially in structural members, it is desirable to have

better mechanical properties, wear, and machinability performance. This can

be achieved by proper selection of processing parameters, testing factors,

type, size and quantity of reinforcement.

The literature survey clearly indicated the need for selection of

reinforcement, its quantity, size and suitable process parameters for achieving

desired results. The results showed that matrix specific conditions are to be

selected for desired results. Various research works have been carried out

with different types of reinforcement, various particle sizes of reinforcement,

different fabrication techniques and also various testing conditions. However,

in aluminium metal matrix composites reinforced with SiCp fabricated

through powder metallurgy process, not much work has been reported so far.

64

From the available literature, it can be seen that the optimization

and modeling of wear performance and machinability of the composites has

not been investigated in detailed way yet. Therefore, there is an urgent need

for performance analysis of Al 2219-SiCp composites during wear and

mahinability testing.