18

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

The literature collected from various sources for the present

research work is presented briefly under the following headings:

1. Metal Matrix Composites

2. Al Matrix material

3. Reinforcement

4. Fabrication methods of MMCs

5. Stir Casting

6. Welding of Metal Matrix Composites

7. Gas Tungsten Arc Welding

8. Friction Stir Welding

9. Design of Experiments

10. Microstructural studies

11. Corrosion analysis

12. Wear analysis

2.2 METAL MATRIX COMPOSITES

Composite materials appear everywhere in life, both man-made

(such as fiberglass) and biologically produced (like mammalian bones). The

purpose of a composite man-made material is to alter and improve the

properties of the matrix material, by the addition of some second material

19

with very different chemical and structural properties. An example of an

important area of composite research is metal-matrix composites, in which a

metallic host material is modified through the addition of a ceramic. The

ceramic may be in the form of particles or fibers. The goal is to supplement

the desirable properties of the metal, such as ductility, by the addition of a

ceramic which can improve the performance of the material in its final

application. Common goals in the creation of new metal-matrix composites

are to achieve increased strength, reduce friction and prevent corrosion.

MMC materials have a combination of different, superior properties

to an unreinforced matrix which are; increased strength, higher elastic

modulus, higher service temperature, improved wear resistance, high

electrical and thermal conductivity, low coefficient of thermal expansion and

high vacuum environmental resistance. These properties can be attained with

the proper choice of matrix and reinforcement Composite materials consist of

matrix and reinforcement. Its main function is to transfer and distribute the

load to the reinforcement or fibers. This transfer of load depends on the

bonding which depends on the type of matrix and reinforcement and the

fabrication technique. The matrix can be selected on the basis of oxidation

and corrosion resistance or other properties (Taya and Arsenault 1989).

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.

Now a days, 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

20

composite in missile guidance system replacing certain beryllium components

because structural performance is better without special handling in

fabrication demanded by latter’s toxicity (Richards Demeis 1989) Recently

aluminium-lithium alloy has been attracting the attention of researches due to

its good wettability characteristics (Huda et al 1993)

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.

Many of these features are also exhibited by magnesium-based systems and

with its lower elastic modulus. Many of the composite fabrication processes

are common to both Al and Mg based systems (Doychak 1992)

The factors influencing the type and form of reinforcement used are

the material properties desired, ease of processing, and part fabrication. In

early stages of developments, only a limited range of reinforcements could be

used. The stability between the components and the differences in their

thermal properties such as coefficient of thermal expansion and coefficient of

thermal conductivity are the limiting factors in the compatibility of the two

materials used to make the composite. A good bond only can be formed by

proper and adequate interaction between the reinforcement and the matrix.

Inadequate interaction results in lack of proper bonding, whereas excessive

interaction leads to the loss of the properties desired and inferior performance

of the MMC.

In recent years, the demand for advanced materials for structural

applications is increasing owing to the conventional metallic alloys not

meeting the needs of industrial sectors. Metal–matrix composites (MMCs)

have become increasingly used for advanced structural applications because

of their high specific stiffness, high specific strength and superior thermal

stability compared to their monolithic alloy counterparts (Tjong et al 2005).

21

MMC types are commonly subdivided according to whether the

reinforcement is in the form of (a) particles, which are at least approximately

equiaxed, (b) short fibers (with or without a degree of alignment), or (c) long

aligned fiber matrix and reinforcement. Specifications of the way in which the

composite material is to be synthesized, and the manner in which a stock item

or component is to be fabricated from this material, are key interwoven issues

for technologists interested in product development. (Zhu and Kishawy 2005)

There are many interdependent variables to consider in designing

an effective MMC material. Since the upper bound on MMC properties is

established by the properties of the matrix and reinforcement material, careful

selection of these components is necessary.

The recent focus is on particulate reinforcements MMCs due to

their low cost of fabrication. The major reinforcements used in aluminum

MMCs are silicon carbide, boron, graphite, and alumina. Most of the current

research work is focused on SiC and Al2O3 reinforced aluminum MMCs, the

main reason being low cost and high availability.

The SiC/Al interface reaction proceeds according to the equation (2.1):

3SiC + 4Al Al4C3 + 3Si (2.1)

The thermal conditions for this reaction depend on the composition

of the MMC and its processing method. As the reaction progresses, the

activity of silicon in liquid aluminum increases and the reaction tends to

saturate. The presence of free silicon in an aluminum alloy has been shown to

inhibit the formation of Al4C3. Temperature control is extremely important

during the fabrication process. If the melt temperature of SiC/Al composite

materials rises above a critical value, Al4C3 is formed, increasing the viscosity

of the molten material, which can result in severe loss of corrosion resistance

22

and degradation of mechanical properties in the cast composite; excessive

formation of Al4C3 renders the melt unsuitable for casting (Lucas and Clarke

1993).

2.3 Al MATRIX MATERIAL

Because 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, to its ability to wet the reinforcement,

and to its own characteristics properties and processing behavior (Mehrabian

et al 1974).

One very crucial issue to consider in selection of the matrix alloy

composition involves the natural dichotomy between wettability of the

reinforcement and excessive reactivity with it (McKimpson and Scott 1989).

Good load transfer from the matrix to the reinforcement depends on the

existence of a strongly adherent interface (Rack 1990, Urquhart 1991). In

turn, a strong interface requires adequate wetting of the reinforcement by the

matrix. However, the attainments of wetting and aggressive reactivity are both

favored by strong chemical bonding between the matrix and reinforcement.

Adjusting the chemical composition to accomplish this delicate compromise

is difficult as many subtleties are involved. To illustrate the complexity,

several examples concerning alloying additions to aluminium matrix metal

relative to Silicon carbide whiskers, Boron reinforced and Graphite reinforced

aluminium composites and the effect of insidious impurities from various

origins have been documented by numerous investigators (Zedias et al 1991,

Clyne et al 1987, Gupta et al 1991, Lloyd 1989, Roos et al 1990).

The best properties can be obtained in a composite system when the

reinforcement whiskers or particulates and matrix are as physically and

chemically compatible as possible. Special matrix alloy compositions, in

23

conjunction with unique whisker coatings, have been devised to optimize the

performance of certain metallic composites (Herbert Dietrich 1991, Chester

1991, Alok and Mishra 2002)

2.4 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:

– Si, Na and Ca in sapphire reinforcement;

Inherent properties – strength, modulus and density.

Even when a specific type has been selected, reinforcement

inconsistency will persist because many of the aspect cited above in addition

to contamination from processing equipment and feedstock may vary greatly

(Eustathopoulos et al 1991, Arsenault 1984, Adeqoyin et al 1991). Since most

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

24

reinforcements for particle reinforced composites (Tiwari et al 1990, Liaw

and Gungor 1990).

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 behavior at elevated temperatures (Zhang

et al 1992) However, lack of wettability between aluminium and the

reinforcement, and oxidation of the graphite (Chester 1991, Thomas and

Cawley 1992) lead to manufacturing difficulties and cavitations of the

material at high temperatures.

Alumina and other oxide particles like TiO2 etc. have been used as

the reinforcing particles in Al-matrix. Alumina has received attention as

reinforcing phase as it is found to increase the hardness, tensile strength and

wear resistance (Ravichandran et al 1992) of aluminium metal matrix

composites. Deonath and Rohatgi (1980) have studied mica, alumina, silicon

carbide, clay, zircon, and graphite as reinforcements in the production of

composites. Numerous oxides, nitrides, borides and carbides were studied by

Zedalis et al (1991) as reinforcements for reinforcing high temperature

discontinuously reinforced aluminium (HTDRA). It has been inferred from

their studies that HTDRA containing TiC, TiB2, B4C, Al2O3, SiC and Si3N4

exhibit the highest values of specific stiffness.

It is proven that the ceramic particles are effective reinforcement

materials in aluminium alloy to enhance the mechanical and other properties

(Taya and Arsenault 1989). 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

25

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 are used because of their

combinations of high strength and stiffness at both room and elevated

temperatures. Common reinforcement elements are SiC, A1203, TiB2, boron

and graphite.

Tamer Ozben et al (2008) examined the influence of reinforced

ratios of 5, 10 and 15 wt.% of SiCp on mechanical properties. The effect of

machining parameters, e.g. 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.

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. The choice of reinforcement is not as arbitrary as this

list of composites might suggest, but is dictated by several factors (Lloyd

1990)

The application: If the composite is to be used in a structural application, the

modulus, strength, and density of the composite will be important, which

requires a high modulus and low density reinforcement. Particle shape may be

important, since angular particles can act as local stress raisers, reducing

ductility. If the composite is to be 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.

26

The method of composite manufacture: There are two generic methods for

composite manufacture, powder metallurgy (P/M) and methods involving

molten metal. For composites processed in the molten state, there are different

considerations such as, compatibility. Alumina is stable in most Mg free Al

alloys, but unstable in Mg alloys, reacting to form Al2MgO4. Reaction of the

reinforcement can severely degrade the properties of the composites, so the

reinforcement has to be chosen after considering the matrix alloy, and the

processing time and temperature.

It was predicted by Friend (1987) that there exists a critical

reinforcement volume fraction above which the composite strength can be

improved relative to that of the unreinforced material and below which the

composite strength decreases, owing to the ineffective load transfer from

matrix to reinforcement in MMCs. For low volume fraction of reinforcement,

the composite strength was observed to be governed by the residual matrix

strength, which decreases with increase in reinforcement volume fraction.

2.4.1 Effect of Particle Size

The deformation and fracture behavior of the composite revealed

the importance of particle size (Friend 1987). A reduction in particle size is

observed (Lewandowski et al 1991) to increase the proportional limit, yield

stress and the ultimate tensile stress. It is well established that large particles

are detrimental to fracture toughness due to their tendency towards fracture. It

would be highly desirable to have a composite system where the reinforcing

particles are relatively fine so as to get the stiffness benefits of a composite

without significantly lowering fracture toughness.

27

2.4.2 Effect of Reinforcement Distribution

Apart from the reinforcement level, the reinforcement distribution

also influences the ductility and fracture toughness of the MMC and hence

indirectly the strength (Lewandowski et al 1991). A uniform reinforcement

distribution is essential for effective utilization of the load carrying capacity

of the reinforcement. Non-uniform distributions of reinforcement in the early

stages of processing was observed to persist to the final product in the forms

of streaks or clusters of uninfiltrated reinforcement with their attendant

porosity, all of which lowered ductility, strength and toughness of the material

(McKimpson and Scott 1989). Important MMC applications in the ground

transportation (auto and rail), thermal management, aerospace, industrial,

recreational and infrastructure industries have been enabled by functional

properties that include high structural efficiency, excellent wear resistance,

and attractive thermal and electrical characteristics.

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.

2.5 FABRICATION METHODS OF MMCs

It has been reported that the energy consumed when aluminium is

recycled is only about 5% of that required in the primary production of

aluminium. There are, however, certain disadvantages associated with the

recycling of aluminium such as the presence of impurities, which to a large

extent impair the mechanical properties of the recycled material. This

problem can be overcome by a careful selection of the constituents and also

the fabrication technique, as they can lead to the formation and piling up of

intermediate phases that are detrimental (Eliasson and Sandstorm 1995,

Allison and Cole 1993, Lloyd 1990).

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In recent years the potential of metal-matrix composite (MMC)

materials for significant improvement in performance over conventional

alloys has been recognized widely. However, their manufacturing costs are

still relatively high. There are several fabrication techniques available to

manufacture the MMC materials: there is no unique route in this respect. Due

to the choice of material and reinforcement and of the types of reinforcement,

the fabrication techniques can vary considerably. The different fabricating

techniques are Powder Blending, Diffusion bonding of foils, stir casting,

squeeze casting, spray deposition, Rheo casting, Physical Vapour Deposition.

Since the research work deals with stir casting technique, the literature survey

for stir casting alone is reported in detail.

2.5.1 STIR CASTING

Normally the liquid-phase fabrication method is more efficient than

the solid-phase fabrication method because solid-phase processing requires a

longer time.

Stir casting involves the addition of particulate reinforcement into

semisolid metal (SSM) by means of agitation. The advantage of stir casting

lies in a lower processing temperature leading to a longer die life and high

production cycle time (Kenney and Courtois 1998). Reduced fluidity can be

achieved in SSM by means of shearing. The greater resultant fluidity of the

SSM also reduces solidification shrinkage, making the fabrication of

structural components with tight tolerance possible (Witulski et al 1996). The

production can be carried out by conventional foundry methods (Gupta et al

1997). Disadvantages that may occur if process parameters are not adequately

controlled include the fact that non-homogeneous particle distribution results

in sedimentation and segregation (Surappa and Rohatgi 1981).

29

Although stir casting is generally accepted as a commercial route

for the production of MMCs (Lin et al 1995) there is however technical

challenges associated with producing a homogeneous, high density

composite. Effectiveness with which mechanical stirring can incorporate and

distribute the particles throughout the melt depends on the constituent

materials, the stirrer geometry and position, the speed of stirring, and the

mixture temperature. Research has been conducted in an effort to optimize the

mechanical properties of MMCs. Little of this work however is concerned

with investigation of time required for particulate distribution. Unfortunately,

in normal practice the effect of the stirring action on the flow patterns cannot

be observed as they take place in a non-transparent molten metal within a

furnace. As such, and because of the fact that direct measurements of metal

flow characteristics can be expensive, time consuming and dangerous, the

current research focuses on methods of simulating fluid and particle flow

during stirring.

2.6 WELDING OF METAL MATRIX COMPOSITES

2.6.1 Classification

Since aluminium based MMCs form the dominant class of these

composites, they are also the application area for which many of the

published joining methods have been developed. It is appropriate to put these

methods into the following groups (Composite Materials Handbook 1999):

Fusion processes

Solid state processes

Other processes.

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2.6.2 Fusion Processes (Mainly Particulate Reinforced MMCs)

There are a number of difficulties associated with fusion welding of

MMCs :

high viscosity of the melt above the melting point

segregation effects when the melt resolidifies

interactions between reinforcement and matrix

gas evolution

High viscosity tends to make the mixing of filler and molten parent

metal difficult. The problem can be alleviated by using Si-rich aluminium

filler wires or, if possible, by employing a matrix alloy with high Si content

(Ellis 1996).

Segregation may take place during fusion welding of SiC-

reinforced Al MMC, since the ceramic particles are rejected by the

solidification front, thus causing formation of particulate-free (unreinforced)

regions. In the case of Al2O3 particle reinforcement, the use of high Mg

containing wires will help trying to lower the melt viscosity by increasing its

temperature and tends to worsen potential problems with reinforcement-

matrix interaction (Ellis 1996). In the Al-SiC case, Al4C3 platelets and silicon

blocks may be formed. The resulting microstructure is extremely brittle and,

in the presence of water, very prone to corrosion. The reaction has often been

reported to take place during electron/laser beam welding, which tends to

create hot weld pools. To avoid problems with formation of Al4C3 platelets, it

is essential to choose the welding parameters carefully. Also the matrix

composition has been shown to be critical (Gittos et al 1994).

31

Gas evolution during fusion welding can be a problem if the MMC

material has been produced by a powder metallurgy route. If the occluded gas

content is too high, gas (especially hydrogen) evolution will occur, leading to

extensive cracking in the heat affected zone (HAZ) and/or weld porosity. This

problem may be solved by application of proper degassing techniques to the

powder (Ellis 1996).

Among fusion welding processes, the following ones are of interest

for joining Al base MMCs:

Gas-Tungsten Arc Welding (GTAW)

Gas Metal Arc Welding (GMAW)

Laser Beam Welding (LBW)

Electron Beam Welding (EBW)

Capacitor Discharge (CD) welding

2.6.2.1 Gas-Tungsten Arc Welding

In GTAW and GMAW, an electric arc is struck between the

workpiece and an electrode, nonconsumable tungsten or consumable metal,

respectively. The hot/molten metal is protected by inert gas flowing around

the electrode. In the GTA case, filler metal may be preplaced in the joint or

fed into the arc from an external source.

The GTA method has been extensively used for welding Al MMCs

based on 6XXX (Al-Mg-Si) alloys. Low heat input and high silicon filler

wires are recommended. In the case of MMCs with Al2O3 particle

reinforcement, Mg-rich wires should be used to prevent the particles from

dewetting and clumping in the weld pool (Ellis 1996).

32

When continuously reinforced Al(6061)-B MMCs was GTA

welded without filler wire, the boron filaments were overheated, leading to

fragmentation and dissolution. The problem was solved by using Si-enriched

(ER4043) filler wire (Kennedy and Courtois 1973).

Welding can be described as the joining of two pieces by a

coalescence of the areas in contact with each other. This can be achieved by

different means. Welding processes could be autogenous welding, involving

only the fusion of the base metals.

Gas tungsten arc welding GTAW uses a non-consumable tungsten

electrode and an inert gas to protect the weld pool, electrode, and arc column.

One of the advantages of GTAW is that the arc remains stable even at very

low welding currents.

The bulk of the heat is produced at the positive terminal. If the

tungsten electrode is connected to the positive pole using DC current i.e.

DCEP (Direct current electrode positive), then it melts because of

overheating. Further, cleaning action due to the breaking of the oxide film on

the specimen occurs during DCEP. GTAW with DCEN (Direct current

electrode negative) leads to efficient penetration during welding. Manual

GTAW of aluminum is performed with AC. In AC type current, the oxide

film removal takes place on the electrode positive half cycle and electrode

cooling and weld bead penetration occurs on the electrode negative half cycle.

After every half cycle, the arc is extinguished and reignited (Kaiser

Aluminum 1978).

Typically, the gas for AC-GTAW welding of aluminum is argon;

however, helium and argon-helium mixtures may also be used. Argon gives a

shallow penetration weld bead, but will leave the weld bright with a silvery

appearance. Argon facilitates easy arc ignition with higher stability. Helium,

33

because of its higher ionization potential, increases arc voltage, and has the

effect of constricting the arc and increasing arc stability. Adding argon to

helium significantly enhances the arc stability (Mathers 2002). Travel speeds

and penetration will be less than with pure helium but greater than with pure

argon. Normally, 25% helium with argon is preferred (Mandal 2002).

The electrode used should not protrude from the nozzle by more

than about 6 mm; although it may be extended to 10 mm if a gas lens is fitted

to the torch. The hemispherical shape of the electrode tip should be

maintained to achieve a stable arc.

Porosity arises from the gas dissolved in the molten weld metal,

which becomes trapped during solidification, forming bubbles in the

solidified weld metal. Hydrogen has low solubility in the solid but high

solubility in molten aluminum, which is a major problem and results in the

above defect. Increasing the heat input increases the weld pool temperature

and enhances the rate of absorption of hydrogen in the molten weld metal;

however, higher heat input can reduce porosity since in that case the rate of

gas evolution from the weld exceeds the rate of absorption, slowing the rate at

which the weld freezes and allows the hydrogen to escape out of the weld.

Among conventional fusion methods, GTAW has lower levels of porosity

than GMAW due to less hydrogen contamination of the filler wire (Mathers

2002).

Oxide film removal is needed to reduce the risk of porosity. It is

also necessary to avoid welding defects such as lack of fusion and oxide film

entrapment. Aluminum oxide forms very rapidly, and has a higher melting

temperature (2060°C) as compared to the melting temperature of pure

aluminum metal (660°C). Increasing the temperature of aluminum above its

melting temperature will result in a layer of oxide, surrounding the molten

aluminum pool. This oxide layer needs to be removed to prevent high risks of

34

early service failures. Proper inert gas is also required for adequate shielding

of the weld pool and the arc column (Mathers 2002).

Another important aspect one needs to focus on is weld preparation

and its design. The convenience with which a weld can be made depends on

the joint design and there are some crucial factors for weld design one should

keep in mind. Some of the important and relevant factors discussed below are

welding speed, welding current and welding position.

a. Welding speed

Welding speed is defined as the linear rate at which the welding arc

moves along the weld joint. It is a very important parameter because it

controls the actual welding time and the total heat input in the specimen. A

proper estimate of the welding speed is required to attain high weld quality.

Excessive welding speeds may cause porosity, undercut, and arc blow. With

slow welding speeds, the penetration decreases with the weld bead getting

wider; however, with increasing plate thickness, the welding speed should be

reduced to facilitate good welds if the current is kept constant.

b. Welding current

Welding current is one of the most influential parameters in

welding. The melting rate is directly proportional to the amount of heat

energy supplied. Increased welding current leads to weld induced distortions,

while low currents lead to lack of fusion and penetration. Further, increasing

heat input leads to wider HAZ. Also, with increase in heat input, the hardness

in the HAZ decreases due to slow cooling rates leading to grain growth. One

therefore, should keep the current within recommended limits; however, it is

found that the arc current needs to be increased for metals with higher thermal

conductivity, such as for aluminum (Luijendijk 2000). This is simply due to

35

the fact that these metals lose heat faster than other metals with low thermal

conductivity, and requires relatively higher heat inputs for better joint fusion.

c. Welding position

Generally, flat or downhand position is preferred for all welding

processes. Flat position welding generally ensures good quality weldment

with sufficient metal deposition rates. The weld pool is larger in this position

with slow cooling rates, allowing the gases to evolve out of the pool and

reducing porosity. For fillet welds in the horizontal-vertical position, the

electrodes are inclined at 50°- 80° in the direction of travel and 40°- 50° to the

flat plate. In that case, the force of gravity tends the weld pool to sag, making

it difficult to obtain the desirable results (Mandal 2002); however, some

specific components and processes require positions other than flat position

during welding.

Electrodes used are typically tungsten or tungsten alloyed with

thoria (ThO2) or zirconia (ZrO2). These compounds improve the arc stability

characteristics and higher service life. Recently, rare earth elements such as

cesium, cerium, or lanthanum have claimed to improve the electrode life and

have reduced the risks arising from radiation during the grinding of thoria

containing electrodes (Mathers 2002).

2.6.2.2 Gas-metal arc welding

GMAW, which is often automated with high welding speed, has

been found to be more adaptable to MMC welding than that of GTAW. As an

example, it has been shown that GMAW produced the best results when both

processes were tried for joining 6061 Al matrix composite reinforced with

B4C particles, using filler metal addition. The GMA method is considered a

viable joining technique for MMCs (Composite Materials Handbook 1999).

36

2.6.2.3 Laser beam welding

In this process, a beam of laser light is focussed using optical lenses

onto the solid material, which is heated above its melting point. LB welding

involves very high power density of the order of 106 W/cm

2. A high power

density is necessary to produce the required interaction with the material,

"beam coupling". This beam coupling is about four times greater for MMCs

than for monolithic aluminium alloys. The result is that the LB method can be

used to produce deep, narrow welds with narrow heat affected zones.

Unfortunately, the high temperature and the laser beam’s interaction with SiC

particles tend to produce a deleterious weld zone microstructure containing

Al4C3, primary silicon and Al-Si eutectic (Ellis 1996). It is possible, though,

to limit the extent of this reaction by controlling the amount and mode of

energy input (Wang et al 2000). Another means of dealing with this problem

has been found to be the addition of a strong carbide-forming element like

titanium, either by using Ti filler wire (Meinert et al 1992) or by placing a Ti

foil between the two MMC blocks to be joined by a butt joint (Wang et al

2000).

2.6.2.4 Electron beam welding

In EB welding, a beam of electrons is accelerated through an

electric field and focussed by a magnetic lens onto the joint zone. Since the

electrons would be scattered by gas molecules, the process must take place in

vacuum. Heat is generated when the beam hits the weld zone. The very high

power density, of the order of 106 W/cm

2, produces a deep, narrow weld (Ellis

1996). Compared to the LB method, EB welding has been found to cause less

of the unwanted Al/SiC reaction (Lienert et al 1993).

37

2.6.2.5 Capacitor discharge welding

Capacitor discharge welding is a special kind of resistance welding,

in which the energy comes from the rapid discharge of electrical capacitors

while force is applied over the joint interface. Because the discharge pulse is

short, of the order of 5 - 25 milliseconds, the CD process may produce less

unwanted reactions and provide somewhat better weld properties than

conventional spot welding. This has been borne out by experiments on several

types of Al/SiC MMCs (Composite Materials Handbook 1999).

2.6.2.6 Diffusion bonding

When joining two solid pieces of material by diffusion bonding, the

two parts are brought together and held under pressure at an elevated

temperature for a sufficient length of time to allow a metallurgical bond to be

formed by diffusion. For Al-base materials, the temperature range is 325 -

520°C; the time needed depends on the temperature and the material to be

bonded. The surfaces must be prepared to a good finish, better than Ra 0.4

µm, and they must be clean. Vacuum or protective atmosphere is also needed

during the process. Al alloys are not especially well suited for diffusion

bonding, since a tenacious and stable surface oxide is naturally formed. The

presence of reinforcement particles places further restrictions on process

variables. By careful process control, a suitable amount of mass transport can

be achieved in order to avoid the formation of either particulate-rich or

particulate-free zones, both of which would cause bond strength impairment

(Ellis 1996).

Diffusion bonding is a preferred joining method for heat transfer

applications such as heat pipes, radiators and heat exchangers (Composite

Materials Handbook 1999).

38

Muratoglu et al (2006) investigated the characteristics of SiC

particulate reinforced pure aluminium metal matrix composites joined by

diffusion bonding process. The experimental results indicated that the

application of aging before and after diffusion bonding decreased SiC

particulate accumulation, and increased other elemental concentration at

interface. The application of aging treatment before the diffusion bonding of

Al/SiCp MMCs to pure Al, increased Cu% concentration at interface which

was treated as the insert alloy.

2.6.3 Solid State Processes

2.6.3.1 Inertia Friction Welding

In friction welding, the heat needed is produced by friction between

the two parts to be joined. A subgroup of friction welding, inertia friction

welding, is used in cases where at least one of the parts is rotationally

symmetric. This part, fixed to a rapidly rotating flywheel, is brought into

contact with the (stationary) mating part under pressure. Under the influence

of the heat evolved, a soft layer is formed at the interface. Normally, this

bonding layer is allowed to cool under pressure. The formation of the bonding

layer involves upset forging and extrusion of material from the interface. With

MMCs, a higher axial force must be applied when joining monolithic

materials, since the flow stress is increased by the reinforcing particles

(Composite Materials Handbook 1999).

Zhou et al (1997) examined the optimum joining parameters for the

friction joining of aluminium-based metal–matrix composite materials. The

properties of MMC/MMC, MMC/alloy 6061 and alloy 6061/alloy 6061 joints

were derived from detailed factorial experimentation. The mechanical

properties of the joints were evaluated using a combination of notch tensile

testing and also conventional tensile and fatigue testing. The frictional

39

pressure had a statistically-significant effect on the notch tensile strength of

joints produced in all base material combinations. The upset pressure had a

statistically-significant influence on the notch tensile strength properties of

alloy 6061/alloy 6061 joints. The notch tensile strengths of MMC/alloy 6061

joints were significantly lower than MMC/MMC and alloy 6061/alloy 6061

joints for all joining parameter settings. It was found that fatigue strength of

MMC/MMC joints and alloy 6061/6061 joints were also poorer than the as-

received base materials.

2.6.3.2 Friction Stir Welding

Being a relatively new method for joining monolithic materials,

friction stir welding appears to be a promising technique for joining of

MMCs. As distinguished from conventional friction welding, the parts to be

joined are not moved relative to each other. Rather, they are firmly clamped

to a backing plate in order to prevent the faces to be forced apart. A

cylindrical, rotating tool is moved along the joint line to produce a plastisized

material zone around the tool through frictional heating. The plastisized

material is forced to move from the front to the back of the tool, thus forming

the weld on consolidation. This solid state process enables the retention of

chemistry and uniform distribution of reinforcing particles in the matrix. The

risk of reinforcement matrix chemical reaction is minimized by the low

welding temperature (Composite Materials Handbook 1999).

So far, the usefulness of FSW has been demonstrated for butt

welding of flat plates. In the case of SiC-reinforced MMCs, it has been found

that it can be used with reinforcement levels up to 25 volume percent. A

problem is that the SiC particles tend to cause a high rate of wear of the tool

(Lee et al 1999). The Friction Stir Welding (FSW) process was

developed by The Welding Institute (TWI) of Cambridge, England in 1991

(Thomas et al 1991). FSW is a mechanical, solid-state joining process which

40

has been proven as a viable joining method for many different joining

configurations including lap joints, T joints, fillet joints, and butt joints. FSW

is currently employed in the railway, aerospace, and maritime industries.

Friction Stir Welding has several distinct advantages over traditional arc

welding.

FSW generates no fumes, results in reduced distortion and

improved weld quality for the proper parameters, is adaptable to all positions,

and is relatively quiet. The major variables of interest in any FSW process are

rotation (spindle) speed, travel speed (feed rate), tool orientation/position (tilt

angle), plunge depth, tool material, tool geometry, and workpiece material

(Mishra and Ma 2005).

An emerging area of FSW research is the welding of Metal Matrix

Composites (MMCs). The microstructural characterization evidenced, in the

FS weld zone of an aluminium matrix (AA7005) composite reinforced with

10 vol.% of Al2O3 particles (W7A10A), a substantial grain refinement of the

aluminium alloy matrix (due to dynamic recrystallization induced by the

plastic deformation and frictional heating during welding) and a significant

reduction of the particles size of the reinforcement (due to the abrasive action

of the tool). Tensile tests showed a high efficiency of the FSW joints (about

80% of the ultimate tensile strength) (Ceschini et al 2007).

Tool wear for threaded steel pin tools declines with decreasing

rotation speed and increasing traverse or welding speeds for the FSW of Al

359+20% SiC MMC. Less than 10% tool wear occurs when the threaded tool

erodes to a self-optimized shape resembling a pseudo-hour glass at weld

traverse distances in excess of 3m. There is only a 7% reduction in the SiC

mean particle size in the weld zone for self-optimized pin tools with no

threads as compared with a 25% variation for threaded tools wearing

significantly at the start of welding. The weld zone becomes more

41

homogeneous for efficient welding with self-optimized tools, and there is a

reduction in the weld zone grain size due to dynamic recrystallization, which

facilitates the solid-state flow. The weld zone is observed to harden by as

much as 30%, in contrast to the base material, as a consequence of the

recrystallized grain size reduction and the SiC particles distributed therein.

(Fernandez and Murr 2004)

Shaowen and Xiaomin (2008) studied the texture patterns on

transverse, longitudinal and horizontal cross-sections in friction stir welds

(FSW) experimentally and their variations with welding parameters were

analyzed. Numerical simulations of the FSW process have been carried out to

understand the texture patterns. Results of this study suggest that the texture

patterns are complex but a dominant theme is the appearance of bands, which

occur in the advancing-side material. The banded pattern on the transverse

cross-section is often in the form of onion rings. The spacing between the

bands on the longitudinal and horizontal cross-sections equals the distance

traveled by the welding tool in one revolution. The texture patterns are found

to correlate well with equivalent plastic strain contours from simulations of

the corresponding FSW process, suggesting that the texture patterns may be

formed because periodically spaced material regions experience very different

levels of plastic deformation during the FSW process.

The joint characteristics of friction-stir-welded A356 alloys,

especially concerning the improvement of mechanical properties at the weld

zone, were observed with various FSW (friction stir welding) speeds. The

microstructures of the weld zone are composed of SZ (stir zone), TMAZ

(thermo-mechanically affected zone) and BM (base metal). The BM shows

the hypoeutectic Al_/Si dendrite structure. The microstructure of the SZ is

very different from that of the BM. The eutectic Si particles are dispersed

homogeneously in primary Al solid solution. TMAZ, where the original

42

microstructure was greatly deformed, is characterized by dispersed eutectic Si

particles aligned along the rotational direction of the welding tool. These

regions are mainly formed at the advancing side and the upper region of the

retreating side. The mechanical properties of the weld zone are greatly

improved in comparison to that of the BM. The hardness of the weld zone

shows more uniform distribution than that of the BM because some defects

are remarkably reduced and the eutectic Si particles are dispersed over the SZ

(Storjohann et al 2005).

The motivation behind investigating friction stir welding as a

method for joining MMCs is explicated by Storjohann, et al. provides a full

assessment of the problems inherent in welding MMCs using traditional

fusion techniques. Storjohann et al. utilize three different fusion methods to

weld Aluminum composites reinforced with SiC whiskers: gas tungsten arc

(GTA), electron beam (EB), and Nd-YAG continuous wave laser beam (LB).

The authors compare these welds with those produced using FSW to

determine what effect, if any, the solid state method has on weld quality. The

problem with fusion methods lies in the formation of an Al4C3 phase as a

result of the interaction between the SiC reinforcement and molten aluminum.

The authors postulate that the amount of Al4C3 present in a resultant weld is

closely related to the peak weld temperature (i.e. a higher temperature

produces a greater abundance of Al4C3). Since FSW is a lower temperature

process that does not melt the workpiece material, it is hypothesized that the

FSW welds will have a smaller concentration of the deleterious Aluminum

Carbide phase (Storjohann 2005).

FSW results in intense plastic deformation around rotating tool and

friction between tool and workpieces. Both these factors contribute to the

temperature increase within and around the stirred zone. Since the

temperature distribution within and around the stirred zone directly influences

43

the microstructure of the welds, such as grain size, grain boundary character,

coarsening and dissolution of precipitates, and resultant mechanical properties

of the welds, it is important to obtain information about temperature

distribution during FSW. However, temperature measurements within the

stirred zone are very difficult due to the intense plastic deformation produced

by the rotation and translation of tool. Therefore, the maximum temperatures

within the stirred zone during FSW have been either estimated from the

microstructure of the weld (Rhodes et al 1997, Liu et al 1997) or recorded by

embedding thermocouple in the regions adjacent to the rotating pin (Mahoney

et al 1998).

The contribution of intense plastic deformation and high-

temperature exposure within the stirred zone during FSW/FSP (Friction Stir

Processing) results in recrystallization and development of texture within the

stirred zone (Benavides et al 1999, Murr et al 1998, Ma et al 2003) and

precipitate dissolution and coarsening within and around the stirred zone

(Tang et al 1998, Kwon et al 2002).

Liu et al (2003) presented the investigations on self-made FSW butt

joints of the aluminum alloys 2024-T3 and 6013-T4. By tensile, fatigue

endurance (SM) and fatigue crack propagation tests it was demonstrated, that

especially the FSW-joints of 2024-T3 sustain high mechanical loadings.

Investigations on the corrosion properties revealed a certain sensitivity of the

2024-T3 joints to intergranular and exfoliation corrosion.

The Figure 2.1 shows schematic illustration of friction stir welding

process. Schematic cross section of a typical FSW weld showing four distinct

zones (Nandan et al 2008) is presented in Figure 2.2

44

Material Specific Issues

Recently, several investigations were conducted on the feasibility

of FSW of aluminum matrix composites such as 6092Al–SiC (Mahoney et al

1998), 6061Al–B4C (Nelson et al 2000), A339–SiC (Murr et al 2000),

6061Al–Al2O3 (Prado et al 2001, Nakata et al 2003), and 7093Al–SiC

(Sharma et al 2001)

No evidence of any chemical reaction between reinforcements and

matrix alloy was detected. However, compared to unreinforced aluminum

alloys, the optimum FSW parameter for producing sound welds was limited to

lower tool traverse speed (Nakata et al 2003). Second, the ceramic particle

distribution in the FSW welds was uniform. However, while it was reported

by several investigators (Nelson et al 2000, Murr 2000) that the particles size

distribution in the FSW composites was essentially identical to that in the

base composites, other investigations revealed significant breakdown of

reinforcement particles in the weld nugget compared to the base composite.

The number of SiC particles in the FSW 7093Al–SiC composite is more than

twice compared to the base composite, though basically same particle volume

fractions were observed in both conditions. This indicated the occurrence of

particle breakage during FSW. It was suggested that the particle damage

occurred mainly by knocking of corners and sharp edges off large particles,

rather than shattering of large particles (Mahoney et al 1998).

45

Figure 2.1 (a) Schematic illustration of the friction stir welding process,

(b) An FSW weld between aluminium sheets, (c) An actual

tool with threaded pin

Figure 2.2 Schematic cross section of a typical FSW weld showing four

distinct zones: (A) Base Metal (B) Heat Affected

(C) Thermomechanically affected and (D) Stirred (nugget) zone

46

Third, the composite welds made by friction stir welding exhibited improved

mechanical properties over that made by the GTAW. The tensile properties of

the FSW composite are considerably superior to those of GTA welded

composite. The yield strength of the FSW composite is even higher than that

of the base material. This indicates that FSW is an effective welding

technique for joining metal matrix composites.

2.7 DESIGN OF EXPERIMENTS

Design of experiments is a scientific approach of planning and

conducting experiments to generate, analyze and interpretation of the data so

that valid conclusions can be drawn efficiently and economically. It has

proved to be very effective for improving the process yield, process

performance and process variability (Jiju Antony 2001).

The general quantitative design of experiments approach deals with

the procedure of selecting the number of trails and conditions for running

them, essential and sufficient for solving the problem that had been set with

the required precision. Design of experiments is more logical and suitable

among the various research techniques selected for designing the experiments.

One of the important statistical techniques that had been recommended for the

design of experiments in engineering investigations is the factorial technique

(Box and Hunter 1957, Cochran and Cox 1963).

In general quantitative approach, statistically designed experiments

and compatible analytical techniques have been implemented to establish

relationships between welding input and output variables, and results have

been derived by employing similar statistical analyses. On the basis of results

from a factorially designed experiment, regression equations are established

using least squares method. To establish the goodness of fit of these

equations, the correlation coefficient and Fisher’s F-ratio are considered.

47

Using student t-test, the significance of each coefficient of different

parameters is tested and final regression models are developed using only

those significant coefficients without sacrificing the accuracy of the models.

There are many advantages in using designed experiments based on full

factorial techniques to produce predictive equations but the best results can be

achieved only if the experimental design and analytical techniques are

compatible (Larry Juffus 1999).

It is essential to design the experiments on a sound basis rather than

on the commonly employed trial and error method in conjunction with a small

number of repeat experiments for confirmation of the result. However, for

quality work and future predictions, trial and error methods are often little

better than the guess work (Harris and Smith 1983).

Statistical theory of experimental designs is well suited to

engineering investigations. One such important design is central composite

rotatable type favored for the exploration of quadratic response surfaces

(Davies 1978, Box and Hunter 1978, Cochran and Cox 1963).

Factorial design is a standard statistical tool to conduct the

experiment in an optimum way to investigate the effect of factors on the

response or output parameter. The most important advantage of design is that

the number of parameters is simultaneously studied for a deeper insight into

the combined effect of the parameters on the response. In addition to that the

interaction between two or more parameters can also be evaluated which is

not possible with the conventional experimental approach since in that

approach all parameters, other that the one investigated, are held constant

(Arya and Parmar 1986)

48

Response surface methodology is a technique to determine and

represent the cause and effect relationship between true mean responses and

input control variables as a two or three-dimensional hyper surface (Gunaraj

and Murugan 1999). Response surface designs are also employed in the

empirical study of relationships between one or more measured response

variables and a number of independent or controllable variables of a process

(Box and Hunter 1978).

Jayabal and Natarajan (2011) reported influence of parameters on

drilling characteristics of natural fibre reinforced composites (NFRC) by Box-

Behnken design, analysis of variance (ANOVA) and response surface

methodology (RSM) techniques. A mathematical model for correlating the

interactions of drilling parameters and the optimum values of responses by

RSM were presented.

Balasubramnian et al (2008) investigated the laser beam welding

trials on austenitic stainless steel for different beam power, welding speed and

beam angle. The experimental trials were conducted based on three level Box-

Behnken design with replications resulting in 15 trials. Finally the results of

the simulation models and experimental results were compared.

2.8 MICROSTRUCTURAL STUDIES

The aluminium alloy (AA6061) matrix composite reinforced with

20 vol.% fraction of Al2O3 particles (W6A20A), welded using the friction stir

welding process was compared with that of the base material and the results

were discussed in the light of microstructural modifications induced by the

FSW process on the aluminium alloy matrix and on the ceramic

reinforcement. The FSW reduced the size of both particles reinforcement and

aluminium grains and also led to overaging of the matrix alloys due to the

frictional heating during welding. The FSW specimens, tested without any

49

post-weld heat treatment or surface modification showed lower tensile

strength and higher elongation to failure respect to the base material (Ceschini

et al 2007).

Fonda et al (2008) studied the microstructural evolution around the

tool during friction stir-welding of aluminum. The rotating tool induces a

gradual rotation of the crystal lattice with a concomitant development of

elongated subgrains. Additional small subgrains form within this structure by

continuous dynamic recrystallization, gradually increasing in misorientation,

eventually developing high-angle (>15°) grain boundaries.

Huseyin Uzun et al (2007) studied the microstructure,

microhardness, EDX analysis and electrical conductivity measurements to

evaluate the weld zone characteristics of friction stir welded AA2124/SiC/25p

composites. The EDX analysis and the microstructure investigations of

AA2124/SiC/25p composite demonstrate the presence of both fine and coarse

SiC particle reinforced AA2124 matrix alloys. The weld nugget exhibits the

relatively homogeneous SiC particle distributions but has fine particle density

bands. In addition, the nugget contains some porosity around the coarse SiC

particles and cracking of some coarse SiC particles. The thermo-mechanically

affected zone (TMAZ), which is adjacent to the weld nugget, has been

plastically deformed and thermally affected. TMAZ exhibits the elongated

grains of Al alloy matrix and the SiC particle-free regions of the composite.

The heat affected zone (HAZ) between TMAZ and unaffected base composite

regions exhibits a similar microstructure both at the retreating and advancing

sides as the base composite.

Microstructural and mechanical characteristics of fusion welds

(GTA) and solid-state welds (FSW) of Al–4.5 Mg–0.26 heat-treatable

aluminium alloy were compared by Cabello Munoz et al (2008).

Microstructures of base metal and welded zones are analyzed by optical

50

Microscopy (OM) and Transmission Electron Microscopy (TEM). Particular

emphasis is laid on the evolution of hardening precipitates in welded areas.

The results suggest that hardening precipitates are comparatively more

affected by the TIG than by the FSW process. This results in a substantial

reduction of mechanical properties of TIG welds that can be partially

recovered through a post-weld heat treatment.

2.9 CORROSION ANALYSIS

Xia and Frankel (1999) were first to investigate pitting and stress

corrosion cracking behaviors of FSW 5454Al and compared them with those

of base alloy and GTAW samples. Their study revealed following important

observations. First, the pits in FSW samples formed in the HAZ, whereas in

GTAW samples the pits formed in the large dendritic region just inside the

fusion zone. Second, FSW welds showed a pitting resistance higher than

those of base alloy and GTAW welds. Xia and Frankel pointed out that

although the differences in pitting potential were not very large, the trend of

higher pitting potential for FSW samples was observed consistently. Third, in

stress corrosion cracking (SCC) tests using U-bent specimens, base alloy and

FSW welds did not show SCC susceptibility in 20 days tests in 0.5 M NaCl

solution, even if polarized at +60 mV in respect to corrosion potential.

However, GTAW U-bent specimens cracked at the same conditions. Fourth,

slow strain rate tests (SSRT) revealed that both base metals and FSW and

GTAW welds, anodically polarized, exhibited a reduction in ductility,

indicating a certain SCC susceptibility. However, the reduction in ductility for

FSW welds was lower than that for GTAW welds.

The experimental observations that the pitting and SCC resistances

of FSW welds were superior or comparable to those of the base material were

also reported by Corral et al 2000, Zucchi et al 2001 and Meletis et al 2003.

51

Aluminum alloys are highly resistant to corrosion and stress

corrosion owing to the formation of a protective oxide layer on their surface.

Corrosion in aluminum alloys takes one of two forms: pitting or intergranular

corrosion. Intergranular corrosion results in selective corrosion at the grain

boundaries, or any precipitate-free zones that might be formed adjacent to

them, with the remainder of the matrix undergoing very little corrosion. Al

6XXX alloys are generally more resistant to intergranular corrosion because

the Mg, Si precipitates which form at grain boundaries during heat treatment

has a similar electrode potential to the matrix. (Lucas and Clarke 1993)

Research on the mechanical and corrosion properties of aluminium

matrix composites is still at the development stage, but the outlook is very

promising. In recent years the aerospace, military and automotive industries

have been promoting the technological development of composite materials to

achieve good mechanical strength/density and stiffness/density ratios

(Velhinho 2003)

Micro arc oxidation (MAO) coatings were applied on friction stir

welds of aluminum alloys to improve their corrosion resistance. Uncoated and

MAO coated welds were subjected to salt spray corrosion. Studies revealed

severe corrosion in uncoated welds, particularly in their thermomechanically

affected and heat-affected zones. MAO-coated welds showed no signs of

corrosion. The results suggest that the corrosion resistance of friction stir

welded aluminum alloys can be substantially improved by applying MAO

coatings (Prasad Rao et al 2008).

The effect of welding parameters (rotation speed and travel speed)

on the corrosion behavior of friction stir welds in the high strength aluminium

alloy AA2024–T351 was investigated. It was found that rotation speed plays a

major role in controlling the location of corrosion attack. Localized

intergranular attack was observed in the nugget region for low rotation speed

52

welds, whereas for higher rotation speed welds, attack occurred

predominantly in the heat-affected zone. The increase in anodic reactivity in

the weld zone was due to the sensitisation of the grain boundaries leading to

intergranular attack. Enhancement of cathodic reactivity was also found in the

nugget as a result of the precipitation of S-phase. The results were compared

with samples of AA2024–T351 that had been heat treated to simulate the

thermal cycle associated with welding, and with samples that had been

exposed to high temperatures for extended periods to cause significant over-

ageing (Jariyaboon et al 2007).

One of the main obstacles to the use of composite materials is the

influence of the presence of the reinforcement on the corrosion resistance.

This is particularly important in Al alloy based composites, where a protective

oxide film imparts corrosion resistance. The addition of a reinforcing phase

could lead to further discontinuities in the film, increasing the number of sites

where corrosion can be initiated and rendering the composite liable to severe

attack (Pardo et al 2005). In the case of pure metals, pitting resistance is

dependant on the electrochemical stability of the passive film. However in the

case of Al alloy based composites, pitting is influenced by the distribution of

particles. Commonly such particles will exhibit electrochemical characteristics

that differ from the behavior of the matrix, rendering the alloy susceptible to

localized forms of corrosion (Pardo et al 2005, Birbilis et al 2005, Aballe et al

2003).

Pitting attack is reported to be the major form of corrosion in

Al/SiC MMCs. Studies on aluminium matrix composites have shown that a

larger amount of pits are formed on composites than on unreinforced alloys.

Investigations have focused on the effect of reinforcement and intermetallic

cathodic phases on the pitting behavior (Barbucci et al 2000). Preferential

attack occurs at the reinforcement/matrix interface. Furthermore, pores,

53

matrix second phases and interfacial reaction products can all influence

corrosion behavior in a significant way (Pardo et al 2005).

In Al alloy based composites, the pit morphologies are

circumferential and appear as a ring of attack around a more or less intact

particle or particle colony. The attack appears to be mainly in the matrix

phase. This type of morphology has been ascribed to localized galvanic attack

of the more active matrix by the nobler particle (Pardo et al 2005, Birbilis

et al 2005, Aballe et al 2003).

Pardo et al (2005) studied the influence of the volume fraction of

SiC reinforcement in aluminium matrix composites (A360/SiC/10p,

A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) immersed in 3.5 mass % NaCl

solution by potentiodynamic polarization. The experimental results showed

that in both cases the corrosion of the composites intensifies as the

concentration of SiC particles increases.

Electrochemical impedance spectroscopy (EIS) has been used

earlier to characterize anodized Al alloys of various process histories. This

approach, which gives accurate, reproducible data, has been used to study the

corrosion resistance of a SiC/Al alloy that had been anodized for increased

corrosion protection. The anodization treatment used here is not as effective

as it is for corresponding wrought alloys. The properties of the anodized layer

must be quite different from those produced with the same treatment. The

presence of SiC particles on the surface produces these changes (Mansfeld

and Jeanjaquet 1986)

2.10 WEAR ANALYSIS

In the last two decades numerous studies on wear properties of

Aluminium based Metal Matrix Composites with different type of

54

reinforcements have been carried out. The wear of aluminum based metal

matrix composites (MMCs) depends on several factors such as volume

fraction, morphology, and size of reinforcing phase as well as the strength of

the interface. Work published in the literature is mainly concerned with SiC,

Al2O3 particles. There are also relatively few discussions on the wear behavior

of aluminum MMCs reinforced with alumna fibers and also with natural

minerals.

Constantin et al (1999) investigated the sliding wear behavior of

Aluminum Silicon Carbide metal matrix composites reinforced with different

volume fraction of particulate against a stainless steel slider. Their results

show that addition of reinforced particles increases the resistance of the

composites to sliding wear under dry conditions, even for small volume

fraction of particles.

Rohatgi et al (1997) in their work report test examination of

abrasive wear resistance of Aluminum alloy (A356) containing fly-ash

particles. Their results show that the wear resistance of specimen containing

fly ash was comparable to that of alumina fiber-reinforced alloy and superior

to that of base A356 alloy.

Miyajima and Iwai (2003) studied the effect of reinforcements on

sliding wear behavior of aluminum matrix composites. Their results show that

the degree of improvement of wear resistance of metal matrix composites

(MMC) is strongly dependent on the kind of reinforcement as well as its

volume fraction. Aluminum metal matrix composites are emerging as

promising friction materials.

Shorowordi et al (2003) studied the effect of velocity on the wear,

friction and tribochemistry of aluminum MMC sliding against phenolic brake

pad. Their results show that higher sliding velocity leads to lower wear rate

55

and lower friction coefficient for Al-B4C and Al-SiC metal matrix

composites. Hutching (1994) studied the ‘Tribological properties of Metal

Matrix Composites’ and opined that under certain conditions MMCs show

high wear resistance but this is not the case always and is some time depended

on the wear mechanism.

Axen et al (1994) studied the friction and wear behavior of an Al-

Si, Mg-Mn aluminum alloy reinforced with 10%, 15%, and 30% volume of

alumina fibers. Their results shows that fiber reinforcement increases the wear

resistance in milder abrasion situations and the coefficient of friction

decreases with increasing fiber content and matrix hardness of composites.

Zongyi et al (1991) studied the abrasive wear of discontinuous SiC

reinforced aluminum alloy composites. Their result shows that the composites

exhibits excellent abrasive resistance compared with the unreinforced matrix

alloy. Alahelisten et al (1993) studied the effect of fiber reinforcement of

aluminum magnesium and Mg-9 Al-1 Zn on the wear properties. Their results

shows that tribological behavior of MMCs depends much on type of MMC

and the type of contact situation i.e. tribosystem. Cao et al (1990) studied the

wear behavior of a SiC whisker reinforced aluminum composite. Their results

show that the SiC whisker–Al composite exhibits a fairly good wear

resistance especially for higher sliding velocities and / or higher loads. Liang

et al (1995) studied the effect of particle size on the wear behavior of SiC

particulate reinforced 2024 Al composites investigated using three tests,

sliding wear test, impact abrasion test, and erosion test. Their results show

that the wear behavior of particulate reinforced aluminum composite is

significantly affected by particle size.

Composites contain large particles exhibited excellent wear under

sliding wear conditions with steady applied load. Wang and Rack (1991)

studied on the comparative assessment of the effect of different types of

56

reinforcement. Their results show that in the case of 20% vol. SiC particles

Vs 20% Volume SiC-whisker (perpendicular or parallel), the steady state

wear rates of the composites were generally independent of the reinforcement

geometry (Particulate or whisker) and orientation (perpendicular Vs Parallel).

Ravikiran et al (1997) studied the effect of sliding speed on wear behavior of

Al-30 wt % SiCp MMC, concluded that the wear rate of pin material (MMC)

decreased with increasing speed, and also the wear rate of the composite

decreased with increasing area fraction of SiC particles. Manish Narayan et al

(1995) have done an experimental study on dry sliding wear behavior of Al

alloy 2024 Al2O3 particle metal matrix composites and have shown that the Al

2014, 15 vol% Al2O3 composite shows better seizure resistance than does the

unreinforced alloy in the peak aged condition and also in the as-extruded

condition the wear resistance of the unreinforced alloy is better than that of

composite.

Tjong et al (1996) studied the wear behavior of aluminum silicon

alloy reinforced with low volume fraction of SiC particles prepared by

Compocasting process. The wear behavior of unreinforced Al-12 % SiC alloy

and metal matrix composites was investigated by them using a block–on-ring

test at room temperature under dry conditions. Their result shows that the

addition of low volume fraction of SiC particles (2 to 8%) is a very effective

way of increasing wear resistance of composite. Yoshiro et al (1995) studied

the wear properties of SiC whisker reinforced 2024 Al alloy with volume

fraction of whiskers ranging from 0 to 16% produced by Powder Metallurgy

technique. Their results show that SiC whisker reinforcement can improve the

wear resistance of aluminum alloy for both severe and mild wear. Huda et al

(1993) reported that a particular fabrication technique depends on the type of

the proper matrix and reinforcement materials to form the MMC.

57

Sannino and Rack (1995) however showed that the effect of the

shape of reinforcement depended on the sliding velocity. It is difficult to

deduce the effects of reinforcement from the literature because in the reported

studies experimental conditions such as contact load and sliding velocity

spread over very wide ranges and these studies employ different kinds of test

apparatus. The effects of sliding velocity on the frictional and wear behavior

of aluminum MMC sliding against ferrous counter body have been studied by

a number of researchers. Their studies reveled that the frictional and wear

characteristics of aluminum MMC depend on the sliding speed in a

complicated way.

Depending upon the sliding velocity range, both increase and

decrease in wear rate with sliding velocity were reported. It is clear from the

above discussions that the wear properties are improved remarkable by

introducing a hard inter metallic compound into the aluminum matrix. It has

also been demonstrated that because the bonding strength between

intermetallic and matrix is very strong, pulling out is prevented even at high

loads.