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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
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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
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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.
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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.
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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,
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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.
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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
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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
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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,
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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
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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
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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.
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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.
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