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CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Aluminium is the most widely used metal in engineering apart from

iron. It has good electrical and thermal conductivities and high reflectivity to

both heat and light. It is highly corrosion resistant under a great many service

conditions and is nontoxic. Aluminium alloys offer a combination of

mechanical and tribological properties and low density that makes them

highly suitable for composite manufacturing.

2.2 MATRIX MATERIAL

Several metals and alloys have been used as matrix materials;

however, most Research and Development has been concentrated on

aluminium and its alloys. Aluminium has a unique combination of properties

among its class, i.e., light metals. It is not only less expensive than titanium

and magnesium but also easier to fabricate. To tailor its properties such as

strength, stiffness, hardness, wear resistance, thermal expansion etc., a

suitable alloy of aluminium can be paired with appropriate reinforcement. In

recent years, aluminium alloys have attracted attention of many researchers,

engineers and designers as a promising structural material in different

industries like aerospace and automotive. Special 2xxx series of Al alloys

have been studied extensively because of their high strength to weight ratio,

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good formability, age hardenablity and other appropriate properties. The

major characteristics of the 2xxx series are:

Heat treatable

High strength, at room and elevated temperatures

Typical ultimate tensile strength range: 190 to 430

Usually joined mechanically, but some alloys are weldable

The 2xxx series of alloys are heat treatable and possess good

combinations of high strength (especially at elevated temperatures),

toughness, and, in specific cases, weldability. Among Al alloys, 2024 Al has

the highest hardness (Hudaa et al 2009). The use of A2024, therefore, has

been growing gradually in industry as a material of aeroplane constructions,

automobiles, and pulling wheels (Malas et al 2004). AA 2024 alloy is the

most widely used aluminium–copper alloys in forging as well as rivets for

aircraft industry.

Cheng et al (2007) developed an effective approach in achieving

both high strength and high ductility in a 2024 Al alloy. The approach

involves solution-treatment to partially dissolve T-phase particles, cryo-

rolling to produce a fine-structure containing a high density of dislocations

and submicrometer subgrains and aging to generate highly dispersed nano-

precipitates. Such a high density of precipitates enabled effective dislocation

pinning and accumulation, leading to simultaneous increases in strength,

work-hardening ability and ductility.

Mazahery et al (2012) investigated the optimal solidification

conditions to manufacture AA 2024 alloy with minimum wear and maximum

strength. Mechanical and wear properties of unreinforced AA 2024 alloy and

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its composites with different vol. % of coated boron carbide particles were

also experimentally investigated. It was seen that the incorporation of hard

particles to 2024 aluminium alloy contributes to the improvement of the

mechanical properties and wear resistance of the base alloy to a great extent.

2.3 FABRICATIONS OF METAL MATRIX COMPOSITES

A key challenge in the processing of composites is to

homogeneously distribute the reinforcement phases in the matrix to achieve a

defect-free microstructure. The reinforcing phases (powders/fibers/whiskers)

in aluminium matrix composites are incorporated into an aluminium alloy

mostly by conventional methods such as stir casting, squeeze casting, and

powder metallurgy. The main problem of hybrid composites is associated

with their fabrication processes. In addition, the mechanical properties of

MMCs are sensitive to the processing technique used to fabricate the

materials. However, powder metallurgy appears to be the preferred process in

view of its ability to give more uniform dispersions. Moreover, PM could be

remarked as a highly effective and economic method compared with other

alternatives.

Mahdavi et al (2011) investigated the applicability of in situ

powder metallurgy (IPM) method for processing the Al6061/SiC/Gr hybrid

composites, effect of SiC content on the tribological behavior of the hybrid

composites. The amount of porosity and hardness are decreased by increasing

of graphite content in the composites. However, for identical graphite

contents the porosity and hardness of Al/30SiC/Gr hybrid composites are

higher than those of Al/Gr composites.

Gui (2001) observed that Plasma spraying is a feasible route to

produce aluminium composite coatings reinforced with SiC particles. A

considerably uniform distribution of SiC particles can be found in the

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composite coatings. Good compatibility and strong bonding between the

sprayed layer and the substrate were obtained. Because of non-wetting nature

of graphite by molten aluminium, non-coated graphite particles exhibited an

inhomogeneous distribution in the coatings and had a certain loss during

plasma spraying. Al/SiC and Al/Gr had clear interfaces, and undesirable

reactions were not found.

Lin et al (2010) investigated the 10%SiCp/Al-Mg composites by

semi-solid mechanical stirring technique. The distribution of SiCp

reinforcement in matrix is improved by the superior wettability between

reinforcement and matrix, with increasing Mg content. The composites

exhibited superior tensile strength compared with Al-Mg alloys. In addition,

the mechanical properties of the composites increased with the addition of

Mg content.

Hassan et al (2008) studied the dry sliding wear behaviour of some

powder metallurgy (PM) Al–Mg–Cu alloys manufactured by powder

metallurgy with different weight percentage of Cu. The wear study of the

metal matrix composites reinforced with 5 or 10 vol. % silicon carbide

particles (SiC) have been carried using pin-on-disk apparatus. From the study,

they observed that both hardness and wear resistance were enhanced by the

addition of Cu and/or SiC to the Al-4 wt% Mg alloy. The formations of

mechanically mixed layer (MML) as a result of material transfer from counter

face disk to the samples and vice versa were observed in all tested specimens.

2.4 STRENGTHENING MECHANISMS IN METAL MATRIX

COMPOSITES

A composite is an artificially made multiphase material where the

constituent phases are chemically dissimilar and are separated by a distinct

interface. It has been described that the flow stresses of the composites are

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substantially higher than those of the unreinforced alloy. Load transfer from

matrix to reinforcement phase due to shear stresses at the fibre-matrix

interface were of greatest importance in the fibre reinforced composite,

whereas in the particle-reinforced composites small grain size plays a

predominant role. The increase in dislocation density as a result of thermal

and geometrical mismatch is also important while considering strengthening

of the matrix.

Karamis et al (2012) made an attempt to improve the strength of Al

6061 Al metal matrix composites by Reciprocating Extrusion (RE). The

billets were extruded under a pressure of 17.5 MPa at 573 K with a 10:1

extrusion ratio. The reciprocating extrusions were carried out by using up to

15 passes. A homogeneous dispersion of SiCp and refined grain structure of

the test materials were obtained by RE.

Kumar et al (2012) fabricated the aluminium hybrid composite by

powder metallurgy (PM) method with a combination of two reinforcements,

namely, Glass and Silicon carbide particles. The addition of higher Glass and

SiC content as reinforcement in the Aluminium matrix increases the strength

property, because of reduction in pore size or increase in relative density. In

general, the strain hardening index value increases with increasing addition of

Glass and SiC because the pore size decreases. This reduces the geometric

work hardening and increases the matrix work hardening. The strength

coefficient value increases with increasing addition of Glass and SiC because

of better densification. The coarser particle size (150 µm) of Glass and SiC

added in hybrid composites shows a higher strain hardening index and higher

strength coefficient values due to better load transfer rate of Glass and SiC to

the Aluminium matrix compared to fine particle size. The formability stress

index increases with increasing addition of Glass and SiC because of better

densification and decrease in pore size. The variation of the strength

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coefficient value (K) with respect to the percentage of SiC content in (Al–

Glass–SiC) hybrid composites are summarized as shown in Figure 2.1. Al–

4% Glass–various percentage of SiC hybrid composite shows better

formability stress index value compared to Al–4% SiC–various percentage of

Glass hybrid composite.

Figure 2.1 The variation of the strength coefficient value with respectto the percentage of SiC content (Al–Glass–SiC) hybridcomposite.(Kumar et al 2012)

2.5 TRIBOLOGICAL BEHAVIOUR OF Al COMPOSITES

The enhancement in tribological properties of AMCs has been

effectively attainable by introducing the ceramic particles. It has been

generally observed that the increasing the SiC or Al2O3 particle content

enhances the wear resistance of the base alloy. The wear resistance of the

composite was found to be considerably higher than that of the matrix alloy

and increased with increasing particle content. The hard particles resist

against destruction action of abrasive and protect the surface. This result is

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consistent with the rule that in general, materials with higher hardness have

better wear and abrasive resistance.

2.5.1 Al -SiC composites

Aluminium metal matrix composites reinforced with SiC

particulates are known for higher modulus, strength and wear resistance

compared to conventional alloys.

Narayan et al (1995) fabricated the AA 2024-15vol. % Al2O3

particulate (average size, 18 pm) composites using the liquid metallurgy

route. The step loading method has been adopted in a pin-on-disc machine to

generate wear data in the range 20-280 N. The AA 2024-15vol.%Al,O,

composite shows better seizure resistance than does the unreinforced alloy in

the peak-aged condition. In the as-extruded condition the wear resistance of

the unreinforced alloy is better than that of the composite.

Iwai et al (1995) studied the wear properties of SiC whisker-

reinforced 2024 aluminium alloys (designated as SiCw-Al) with volume

fraction of whiskers ranging from 0 to 16% manufactured by a PM technique.

The severe to mild wear transition occurred with sliding distance for the

SiCw-Al composites as well as the unreinforced Al 2024. The initial sliding

distance required to achieve mild wear decreased with increasing volume

fraction of whiskers, in severe wear, since whiskers prevented the wear crack

propagation, wear particles became small but their removal rate became high

for the Al-SiC composites. The formation of a hard worn surface due to

plastic flow, the uniform dispersion of the whiskers in the subsurface and the

presence of additional oxidation on the contact surfaces and wear particles

prevent adhesion.

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Rao et al (2011) investigated the effect of applied pressure on the

tribological behaviour of SiCp reinforced aluminium. The overall results

indicated that the dispersion of 10 wt% SiCp to the base alloy made the

seizure resistance to improve by 33%, similarly 25 wt% SiCp, the seizure

resistance enhanced by 50%. The wear rates in all the samples increase

marginally with applied load prior to reaching the transition load (Figure 2.2).

In seizure condition, the wear surface is characterized by the formation of

parallel lips, destruction of MML (wave like material flow) along the sliding

direction. The hard SiC particles may sometimes causes scratching action

over the MML and thus leading to cavities and grooves over the MML in

worn surface of composite.

Figure 2.2 Wear rate as a function of applied pressure ( Rao et al 2011)

Sahin et al (2011) studied the abrasive wear behaviour of SiCp/Al

composites prepared by liquid metallurgy method using pin-on-disc

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configuration. The experimental results demonstrated that the addition of the

20 wt. % SiC particle to the Al alloy led to a dramatic improvement in wear

resistance of the base alloy (Figure2.3). The wear resistance of the MMCs

increases with increase wt. % of SiC particles. Furthermore, SEM

examination indicated that a relatively small amount of wear craters,

combined with re-attachment of debris particles due to occurring the small

amount of fractured particles in MMCs. In general, smoothening mechanism

was observed for both types of materials under higher loads as well.

Moreover adhesion, chipping and abrasion were more obviously observed for

the alloy matrix.

Figure 2.3 Average volumetric wear rate as a function of applied load

for the alloy matrix and its SiCp reinforced composite,

tested against 70 µm size of abrasive( Sahin et al 2011)

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Al-Rubaie et al (1999) observed the effect of three-body abrasion

of aluminium matrix composites reinforced with silicon carbide particles

(SiCp). MMCs were fabricated by a powder metallurgy route involving a

final hot extrusion step. Using a wet monolayer tester, three-body abrasive

wear tests were conducted under a constant load against silicon carbide and

alumina abrasives with four different grits sizes. The reinforcement of

aluminium matrix with SiC improved the abrasion resistance of all

composites tested against all the abrasives used.

Rahimian et al (2011) observed the effect of production parameters

on wear resistance of Al–Al2O3 composites. Alumina powder with a particle

size of 12, 3 and 48 µm and pure aluminium powder with particle size of 30

µm were used. It was found that increasing sintering temperature resulted in

increasing density, hardness and wear resistance and homogenization of the

microstructure. However at certain sintering temperatures and time,

considerable grain growth and reduction of hardness value occurred, leading

to the degradation of wear resistance. However, after raising the particle size

of alumina, relative density initially increases and then drops to lower values.

Increasing weight percent of alumina powder leads to higher hardness and

consequently improves the wear resistance of Al–Al2O3 composite. The use of

fine alumina particles has a similar effect on hardness and the wear resistance.

Figure 2.4 shows the variation of the wear rate of the composites as a function

of (a) Al2O3 particle content and (b) Al2O3 particle size in different sliding

distance.

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Figure 2.4 Variation of the wear rate of the composites as a function of(a) Al2O3 particle content and (b) Al2O3 particle size indifferent sliding distance ( Mehdi Rahimian et al 2011)

Sameezadeh et al (2011) studied the nanocomposites of AA 2024

aluminium alloy matrix reinforced with different volume fractions of

nanometric MoS2 intermetallic particles ranging from 0 to 5% using

mechanical alloying technique. The prepared composite powders were

consolidated by cold and hot pressing and then heat treated to solution and

aged condition. The effects of MoSi2 particle size, volume fraction and also

heat treatment on the hardness and wear properties of the composites were

investigated using Brinell hardness and pin-on-disc wear tests. The results

indicated that although T6 heat treatment increases the hardness of all

samples compared to as hot pressed (HP) condition, the age-hardenability

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(aging induced hardness improvement) decreases after mechanical alloying

and with increasing MoSi2 volume fraction due to the high dislocation density

produced during mechanical alloying. With increasing the volume fraction of

nano-sized MoSi2 particles up to 3–4%, the hardness of the composites

continuously increases and then declines most probably due to the particle

agglomeration. The wear sliding test disclosed that the wear resistance of all

specimens in T6 condition is higher than that of HP condition and increases

with increasing MoSi2 content.

Kwok and Lim (1999) investigated the friction and wear behaviour

of four Al/SiCp composites over a wide range of sliding conditions by the use

of a specially adapted high-speed tester of the pin-on-disk configuration.

Generally, wear rate increased with increasing load, but it varied in a rather

complex manner with speed depending on which regime the sliding condition

fell into. Three regimes of tribological behaviour, demarcated by sliding

speed, were observed for these composites. In Regime I, lower rates of wear

are observed while in Regime II, catastrophic failures occur when a certain

critical load is exceeded, resulting in the rapid adhesion of a large amount of

specimen material to the counterface: it is no longer possible to continue with

the test when this happens. In Regime III, extensive melting of the composites

takes place, and under such a sliding condition, the size of reinforcement

particles appears to have an important influence on the rate of wear of these

composites.

Abarghouie and Reihani (2010) investigated the friction and wear

behaviours of artificially aged 2024 Al and 2024 Al/20 vol.% SiC composite

prepared by powder metallurgy method in the temperature range 20–250 °C.

Dry sliding wear tests were conducted at a constant sliding velocity of 0.5

m/s, an applied load of 20 N, and a sliding distance of 2500m using a pin-on-

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disc apparatus. All specimens showed a transition from mild-to-severe wear

above a critical temperature. In the mild wear regime, the wear rate and the

friction coefficient of the composite specimen were higher than those of the

unreinforced alloy as shown in Figure 2.5. The SiC particles led to an

increase in the critical transition temperature and in the severe wear regime,

they caused a considerable improvement in the wear resistance. Analysis of

worn surfaces and wear debris indicated that the dominant wear mechanisms

of the unreinforced alloy were micro ploughing and slight adhesion in the

mild wear regime, whereas the composite specimen showed microcutting and

oxidation mechanisms in the same regime.

Das et al (2006) have discussed the formation of mechanically

mixed layer (MML) In Al composites consisting of debris and smeared and

fragmented SiC particles. SiC needles in MML and in the subsurface region

were fragmented into finer particles thus demonstrating the occurrence of

subsurface damage during abrasive wear of LM13-SiC composites. The size

and volume fraction of SiC particle reinforcement do not have significant

effect on friction coefficient of A356-SiC composites with volume fraction of

6%, 12% and 34% and its value remained relatively constant at 0.4 under dry

sliding.

Rohatgi et al (1997) have reported that the aluminium matrix

composites reinforced with hard particles like SiC, exhibit higher coefficient

of friction than with soft particles like Gr. AMCs reinforced with soft

reinforcement particles of Gr have been reported to be possessing better wear

characteristics owing to the reduced wear because of the formation of a thin

layer of Gr particles, which prevents metal to metal contact of the sliding

surfaces.

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Figure 2.5 Variations of (a) friction coefficient, and (b) wear rate withtest temperatures for aged 2024 Al/20 vol. % SiC compositeand aged unreinforced 2024 Al, at the applied load of 20 N.(Mousavi Abarghouie and Seyed Reihani 2010)

2.5.2 Al -Gr Composites

Yang et al (2004) studied the effect of Graphite particulates coated

with an electroless copper introduced into an aluminium alloy by

compocasting method to make A356.2 Al/2 wt. %, 4 wt. %, 6 wt. % and 8

wt.% graphite particulate composite. Variations in friction coefficient, wear

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rate, wear debris and electrical contact resistance were studied related to

variations in graphite particulate content under different normal loading and

sliding speeds. Composites containing the 4wt. % or 6 wt.% graphite

particulate exhibited the lowest wear rate and friction coefficient, and these

properties were insensitive to the variation in sliding speed and normal

loading. The wear debris become smaller as the graphite content increased,

this is reflected by the lower electric contact resistance. The rest of the tribo-

layer consisted of the lamellar graphite films structure, which were elongated

over long distances in the direction of sliding, thus, reducing shear stresses

transmitted to the sub-surface regions, the amount of graphite film released on

the worn surface increases with increasing the graphite particulate content.

Hsiao and Jen (2000) investigated the pure graphite particles and

graphite particles coated with an electroless nickel (EN) film introduced into

an aluminium alloy via powder metallurgy to form two kinds of 6061

aluminium alloys. Tribological performances exhibited in unlubricated

frictional contacts and in oil lubricated contacts were compared for both

alloys. Variations in seizure resistance, friction behavior, wear mechanism,

wear particle size, and wear loss were studied related to variations in graphite

content of the aluminium composite material. In dry contacts, the use of the

EN film was significantly beneficial in lowering the wear rate of the upper

specimen, although it did not produce a great reduction in the wear rate of the

lower specimen compared to that for pure graphite. Similarly, friction

coefficients were at relatively lower levels when EN-coated graphite was

used. In dry contacts, the average size of wear debris produced by the

aluminium alloys with pure graphite was relatively fine, irrespective of the

graphite content.

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Riahi and Alpas (2001) studied a systematic investigation of the

role played by the tribo-layers that form on the contact surfaces during the

sliding wear of graphitic cast aluminium matrix composites. The graphitic

composites include A356 Al–10% SiC–4% Gr and A356 Al–5% Al2O3–3%

Gr that are being developed for cylinder liner applications in cast aluminium

engine blocks. Three main wear regimes, namely, ultra-mild, mild and severe

wear were observed. At nearly all sliding speeds and loads in the mild wear

regime a protective tribo-layer was formed. By increasing the speed and load

the tribo-layer covered a larger proportion of the contact surface and became

more compact and smoother. The hardness of the tribo-layers increased with

the applied load and speed and reached values as high as 800 kg/mm2. The

tribo-layers were removed by extrusion process at the onset of severe wear.

The topmost part of the tribo-layer consisted of iron-rich layers. The rest of

the tribo-layer consisted of fractured SiC and Al3Ni particles and thin graphite

films, which were elongated over long distances in the direction of sliding,

thus, reducing shear stresses transmitted to the subsurface regions. It was

shown that because of the thicker and more stable tribo-layers on the contact

surfaces of graphitic composites, than that of non-graphitic composites and

the A356 Al alloy, the graphitic composites displayed a transition from mild-

to-severe wear for all load and sliding speed combinations, which were

considerably higher than those of the A356 aluminium alloy and the non-

graphitic A356 Al–20% SiC composite (Figure3.6).

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Figure 2.6 Comparison of the mild-to-severe wear transitionboundaries of the graphitic A356 Al–10% SiC–4% Gr andA356 Al–5% Al2O3–3% Gr composites with those of thenon-graphitic composite A356 Al–20% SiC and theunreinforced matrix A356 Al alloy. (Riahi and Alpas 2001)

Akhlaghi and Zare-Bidaki (2009) assessed the influence of graphite

content on the dry sliding and oil impregnated sliding wear characteristics of

sintered aluminium 2024 alloy–graphite (Al/Gr) composite materials using a

pin-on disc wear test. The composites with 5–20 wt. % flake graphite

particles were processed by in situ powder metallurgy technique. For

comparison, compacts of the base alloy were made under the same

consolidation processing applied for Al/Gr composites. It was found that an

increase in graphite content reduced the coefficient of friction for both dry

and oil impregnated sliding, but this effect was more pronounced in dry

sliding (Figure 2.8). Hardness and fracture toughness of composites decreased

with increasing graphite content. In dry sliding, a marked transition from mild

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to severe wear was identified for the base alloy and composites. The

transition load increased with graphite content due to the increased amount of

released graphite detected on the wear surfaces. The wear rates for both dry

and oil impregnated sliding were dependent upon graphite content in the

alloy. In both cases, Al/Gr composites containing 5 wt.% graphite exhibited

superior wear properties over the base alloy, whereas at higher graphite

addition levels a complete reversal in the wear behavior was observed. The

wear rate of the oil impregnated Al/Gr composites containing 10 wt. % or

more graphite particles was higher than that of the base alloy (Figure 2.7).

These observations were rationalized in terms of the graphite content in the

Al/Gr composites which resulted in the variations of the mechanical

properties together with formation and retention of the solid lubricating film

on the dry and/or oil impregnated sliding surfaces.

Figure 2.7 The variation in the measured wear rate with the weightpercent of graphite in the composites for both dry slidingand oil impregnated sliding. (Akhlaghi and Zare-Bidaki2009)

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Figure 2.8 The variation in the measured coefficient of friction with theweight percent of graphite in the composites for both drysliding and oil impregnated sliding. (Akhlaghi and Zare-Bidaki 2009)

Hassan et al (2008) have reported decrease in hardness with

increase in % reinforcement of Gr due to increased porosity. The implication

of these observations is that the % reinforcement of Gr in Al–Gr composites

is bounded by certain limit beyond which it is not beneficial to add Gr as

reinforcement. Hard ceramic particulates of SiC when added as a second

reinforcement is a panacea towards the difficulties encountered with high %

reinforcement of Gr in Al–Gr composites.

2.5.3 Hybrid Composites

Aluminium matrix composites with multiple reinforcements

(hybrid AMCs) are finding increased applications because of improved

mechanical and tribological properties. Hence hybrid composites are better

substitutes for single reinforced composites. Al–Gr composites containing

SiC are referred as Al–SiC–Gr hybrid composites. The salient observations of

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some of the studies on Al–SiC–Gr hybrid composites are highlighted in the

following few lines.

Ted et al (2000) studied the tribological behavior of self-

lubricated aluminium/SiC/graphite hybrid composites with various amount of

graphite addition synthesized by the semi-solid powder densification (SSPD)

method. As the amount of graphite increases, the hardness and coefficients of

thermal expansion of the composites decreases. Fracture toughness decreases

monotonically as the graphite content increases. It was found that the seizure

phenomenon which occurred with a monolithic aluminium alloy did not occur

with the hybrid composites. The amount of graphite released on the wear

surface increases as the graphite content increases, which reduces the friction

coefficient (Figure 2.10). Graphite released from the composites bonded onto

the wear surfaces of the counter faces. However, the amount bonded is small,

and X-ray mappings showed no significant difference in the amounts bonded

for different graphite additions. Wear becomes more stable, and wear debris

particles become smaller as the graphite content increases, which is reflected

by the lower electric contact resistance. More fracturing is shown on the wear

surface of the composite with high graphite addition as a consequence of poor

fracture toughness by comparison with composites with low graphite. There

seems to be less abrasive wear on the composites than on the counter faces.

The wear rate of the composite increases as the amount of graphite content

increases up to 5% then falls to a lower value for an 8% addition (Figure 2.9).

However, the wear rate of the counter face increases as the amount of

graphite in the composites increases up to 8%.

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Figure 2.9 Weight loss of the composites and the counterparts in 5 minof wear process for various graphite additions. (Ted Guoet al 2000)

Figure 2.10 Variations of friction coefficient with the percentage ofgraphite addition (Ted Guo et al 2000)

Jinfeng et al (2008) investigated the 40%SiC/5%Gr/Al composites

with various-sizes graphite particle addition by squeeze casting technology,

and their friction and wear properties. Results showed that after the addition

of graphite the friction coefficient of composites decreased and the wear

resistance increased by 170 to 340 times. In addition, wear resistance was

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improved with increasing of graphite particle size, which is attributed to the

enhancement of integrity of lubrication tribo-layer composed of a complex

mixture of iron oxides, graphite as well as fractured SiC particles and some

fine particles containing aluminium. Moreover, the wear loss of counter face

steel is decreased by a factor of about 2/3. Figure.2.11 shows the wear loss of

the composites and counter faces of wear process for SiC/Al and SiC/Gr/Al

Figure 2.11 Wear loss of the composites and counterfaces of wearprocess for SiC/Al and SiC/Gr/Al (Leng Jinfeng et al 2009)

Mahdavi and Akhlaghi (2011) studied the effect of size of silicon

carbide particles on the dry sliding wear properties of composites. In this

study, wear behaviour of Al6061/10 vol % SiC and Al6061/10 vol % SiC/ 5

vol % graphite composites processed by in situ powder metallurgy technique

has been investigated using a pin-on-disk wear tester. The debris and wear

surfaces of samples were identified using SEM. The increased SiC particle

size reduced the porosity, hardness, volume loss, and coefficient of friction of

both types of composites. Moreover, the hybrid composites exhibited lower

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coefficient of friction (Figure 2.12) and wear rates (Figure 2.13). The wear

mechanisms are changed from mostly adhesive and micro-cutting in the

Al/10SiC composite containing fine SiC particles to the prominently abrasive

and delamination wear by increasing of SiC particle size. While the main

wear mechanism for the unreinforced alloy was adhesive wear, all the hybrid

composites were worn mainly by abrasion and delamination mechanisms.

Figure 2.12 Variation of the coefficient of friction in different composites with SiC particle size. (Mahdavi and Akhlaghi 2011).

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Figure 2.13 Variation of the wear rates of Al/SiC and Al/SiC/Grcomposites containing different sized SiC particles at thesliding distances of 500 and 1000 m (Mahdavi and Akhlaghi2011).

Basavarajappa et al (2006) investigated the dry sliding wear

behavior of Al 2219 alloy and Al 2219/SiCp/Gr hybrid composites under

similar conditions. The composites are fabricated using the liquid metallurgy

technique. The dry sliding wear test is carried out for sliding speeds up to 6

m/s and normal loads up to 60 N using a pin on disc apparatus. It was found

that the addition of SiCp and graphite reinforcements increases the wear

resistance of the composites. The results of Variation of wear rate with

applied load at a sliding speed of 3 m/s for a sliding distance of 5000 m are

summarized in Figure 2.14. Abrasion is the principle wear mechanism for the

composites at low sliding speeds and loads. At higher loads, the wear

mechanism changes to delamination.

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Figure 2.14 Variation of wear rate with applied load at a sliding speed of3 m/s for a sliding distance of 5000 m( Basavarajappa et al.2006)

2.6 STATISTICAL ANALYSIS OF Al COMPOSITES

The experimental results are analyzed with the help of statistical

analysis software, which is widely used in many fields of engineering

research. Kumar et al (2010) developed a new mathematical model to predict

the abrasive wear rate of AA7075 aluminium alloy matrix composites

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

rotatable design matrix was used to optimize the required number of

experiments. The model was developed by response surface method. Analysis

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

Composites with larger reinforcement size and high volume fraction showed

improved abrasive wear resistance as compared to other combinations. It is

inferred that the size of abrasive exerted the greatest effect on abrasive wear,

such that the higher the abrasive size, the higher the wear rate. When the size

47

of reinforcement is smaller than the abrasive size, the fracture and micro-

cutting of the reinforcement are more dominant.

Sahin (2010) investigated the aluminium alloy matrix reinforced

with 15 wt% SiC particles by powder metallurgy (PM) 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, but the percentage of 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 size and hardness was about

30.90 while interaction of other factors was pooled.

Devaraju et al (2013) studied the effect of reinforcement particles

such as Silicon carbide (SiC), Graphite (Gr) and rotational speed on wear and

mechanical properties of Aluminium alloy surface hybrid composites

fabricated via Friction stir processing (FSP). Taguchi method was employed

to optimize the rotational speed and volume percentage of reinforcement

particles for improving the wear and mechanical properties of the surface

hybrid composites. The fabricated surface hybrid composites have been

examined by optical microscope for dispersion of reinforcement particles and

revealed that the reinforcement particles (i.e. SiC and Gr) are uniformly

dispersed in the nugget zone. It is also observed that the microhardness at

optimum condition is increased due to the presence and pining effect of hard

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SiC particles. The wear resistance of the surface hybrid composite is

increased due to the mechanically mixed layer generated between the

composite pin and steel disk surfaces which contained fractured SiC and Gr.

Regression models were developed to predict the quality characteristics

(microhardness, UTS, YS, %EL and wear rate) within the selected range of

process parameters (SiC, Gr and rotational speed).

Basavarajappa et al (2007) investigated the Aluminium metal

matrix composites reinforced with SiC and graphite (Gr) particles by liquid

metallurgy route. Dry sliding wear behaviour of the composite was tested and

compared with Al/SiCp composite. A plan of experiments based on Taguchi

technique was used to acquire the data in a controlled way. The incorporation

of graphite particles in the aluminium matrix as a secondary reinforcement

increases the wear resistance of the material. The smearing of the graphite

and formation of protecting layer between the pin and the counter face

enables in reducing the wear volume loss. Sliding distance is the wear factor

that has the highest physical as well as statistical influence on the wear of

both composites. SiCp composite present a contribution of sliding distance

(57.57%), load (24.34%), and sliding speed (6.8%). SiCp–Gr reinforced

composites present a contribution of sliding distance (57.24%), the load

(22.58%) and sliding speed (9.66%). The interactions between the wear

parameters have statistical significance but do not have any physical

significance.

Suresha and Sridhara (2010) analyzed the influence of addition of

graphite (Gr) particulates as a second reinforcement on the tribological

behaviour of aluminium matrix composites reinforced with silicon carbide

(SiC) particulates under dry sliding condition. Experiments are also

conducted on composites with % reinforcement of SiC similar to hybrid

composites for the sake of comparison. Parametric studies based on design of

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experiments (DOE) techniques indicate that the wear of hybrid composites

decreases from 0.0234 g to 0.0221 g as the % reinforcement increases from

3% to 7.5%. But the wear has a tendency to increase beyond % reinforcement

of 7.5% as its value is 0.0225 g at.% reinforcement of 10%. This trend is

absent in case of composites reinforced with SiC alone. Load and sliding

distance show a positive influence on wear implying increase of wear with

increase of either load or sliding distance or both.

2.7 Al NANO COMPOSITES

Kollo et al (2009) studied the effect of High-energy planetary

milling in aluminium powders with 1 vol.% of silicon carbide (SiC)

nanoparticles. A number of milling parameters were modified for constituting

the relationship between the energy input from the balls and the hardness of

the bulk nanocomposite materials. It was shown that mixing characteristics

and reaction kinetics with stearic acid as process control agent can be

estimated by normalised input energy from the milling bodies. For this, the

additional parameter characterizing the vial filling was determined

experimentally. Depending on the ball size, a local minimum in filling

parameter was found, laying at 25 or 42% filling of the vial volume for the

balls with diameter of 10 and 20mm, respectively. These regions should be

avoided to achieve the highest milling efficiency. After a hot compaction,

difference of hardness for different milling conditions was detected.

Therewith the hardness of the Al–1 vol.% nanoSiC composite could be

increased from 47HV0.5 of pure aluminium to 163HV0.5 when milling at the

highest input energy levels.

Gajewska et al (2012) studied the effect of an AlN reinforcement

size on properties of aluminium alloy matrix composites produced via

mechanical alloying/uniaxial hot pressing. Composite samples were prepared

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using pre-alloyed AA7475 powder with 10 wt.% of AlN additions of differing

average particle sizes: <40 m, ~1 m or <1 m. Powders were milled in a

high energy ball mill for up to 40 hours and then hot pressed in vacuum at

380°C/600 MPa. Milled powder analysis revealed that addition of microsized

hard reinforcing phase allows to achieve a higher level of crystallite size

refinement as compared with submicron reinforcement, under the same

milling conditions. The SEM microstructures of compacted samples

confirmed uniform dispersion of the ceramic phases independent of the size.

All of the composites were characterized by fine matrix grain size (<200 nm)

and a high density of even finer intermetallic, Zn, Cu, Mg or Fe rich

precipitates. The obtained hardness results were highest for ~1 m AlN

addition - near 320 HV which is a 30% improvement over the matrix itself.

The best properties of composites reinforced with intermediate particles

suggest that the achievement of a compromise between the structure

refinement during MA and grain growth restraint during hot pressing is of a

particular importance in composite materials produced in this way.

Nemati et al (2011) investigated the wear behavior of aluminium

alloy matrix composites produced using powder metallurgy technique of ball

milled mixing in a high energy attritor and using a blend–press–sinter

methodology. Matrix of pre-mechanical alloyed Al–4.5 wt.% Cu was used to

which different fractions of nano and micron size TiC reinforcing particles

(ranging from 0 to 10 wt.%) were added. The powders were mixed using a

planetary ball mill. Consolidation was conducted by uniaxial pressing at 650

MPa. Sintering procedure was done at 400°C for 90 min. The results

indicated that as TiC particle size is reduced to nanometre scale and the TiC

content is increased up to optimum levels, the hardness and wear resistance of

the composite increase significantly, whereas relative density, grain size and

distribution homogeneity decrease. Using micron size reinforcing particulates

from 5% to 10 wt.%, results in a significant hardness reduction of the

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composite from 174 to 98 HVN. Microstructural characterization of the as-

pressed samples revealed reasonably uniform distribution of TiC reinforcing

particulates and presence of minimal porosity. The wear test disclosed that the

wear resistance of all specimens increases with the addition of nano and

micron size TiC particles (up to 5 wt.%).

2.8 LIMITATIONS OF EXISTING LITRATURE

Based on the above discussion, it can be found that the preform-

based powder metallurgy is an effective process to fabricate the metal matrix

composites. It offers advantages compared with stir-casting, and squeeze

casting because of its low manufacturing temperature, which avoids strong

interfacial reactions, minimising undesired reactions between the matrix and

the reinforcement. An additional advantage of powder metallurgy is the

uniformity in the reinforcement distribution. This uniformity improves not

only the structural properties but also the mechanical strength as well as

imparts high wear resistance. In general, PM aluminium matrix composites

exhibit good levels of mechanical properties compared with those from other

alternative manufacturing processes.

Reasonably fewer studies are found on the self lubricated wear

characteristics of lightweight aluminium-based composites. However, studies

on the self lubricated Al hybrid composites are scanty in the literature.

Moreover, no systematic attempt has been made to study the influence of the

hybridisation of SiC on the tribological properties of aluminium-based

composites prepared by conventional powder metallurgy route. Furthermore,

it is evident from these studies that the majority of the alloys chosen as

matrices have been the A356, 6xxx and 7xxx series alloys. Although some

studies have been reported on the 2xxx series alloys reinforced with both

silicon carbide and graphite particulates, much less attention has been given

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to the AA 2024 alloy matrix composites, which, has the highest hardness

among all Al alloys. Therefore, in the present study, aluminium hybrid

composites on the dry sliding friction and wear properties are investigated.

Some of the limitations identified based on the literature survey of

the topics are listed as below.

From the literature study, it is observed that silicon carbide/Gr

aluminium composites are synthesized using different routes,

such as stir- casting, squeeze casting, etc. However, the

powder metallurgy route is rarely found in the literature.

From the literature study, it was observed that the majority of

the alloys chosen as matrices have been the A356, 6xxx and

7xxx series alloys. Although some studies have been reported

on the 2xxx series alloys reinforced with both silicon carbide

and graphite particulates, much less attention has been given

to the AA 2024 alloy matrix composites, which, has the

highest hardness among all Al alloys. Therefore the present

investigation, both nano-sized and micro sized SiC and Gr

particles were used as reinforcement for a nano and micro -

aluminium alloy matrix (AA 2024).

Limited study has been made on aluminium hybrid

composites involving both hard and soft reinforcements even

though it is very interesting and provides scope to overcome

some of the challenges posed by use of single.

Most of the reported research focuses on the effect of either

one or two factors on the dry sliding wear behaviour of hybrid

composites. But no systematic attempt has been made to study

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the influence of the hybridisation of both SiC and Gr on the

tribological properties of aluminium-based composites

Factorial technique and Taquchi method has not been revealed

so far about mechanical and tribological behaviour of the

hybrid Al composites. Analysis of variance (ANOVA) was

used to investigate the influence of the parameters on both the

wear loss and the coefficient of friction.