CHAPTER - I

INTRODUCTION
1.1 FUSION WELDING AND SOLID STATE WELDING

Welding is known as a versatile metal fabrication process. Welding is a

metal joining process that produces a local coalescence of the metals to be joined

by heating them to the welding temperature, with or without the application

of pressure or with or without the use of a filler metal and application of

pressure. All the welding processes can be divided into two major classes

(i) Fusion Welding and

(ii) Solid State Welding

Fusion Welding is a group of welding processes that uses fusion of parent

metal to make the weld. Fusion welds ordinarily do not require the application

of pressure and may be completed with or without adding of filler material.

Solid State Welding is a group of welding processes which produces

coalescence at temperatures essentially below the melting point of the parent

materials being joined without the adding of a filler metal. Most of the solid state

welding processes require pressure to establish joint. Cold Pressure Welding,

Explosion Welding, Friction Welding, Hot Pressure Welding, Diffusion Welding

and Ultrasonic Welding are examples of Solid State Welding process.

Welding is used to join materials into parts and parts into structures and

assemblies. It is also used to fabricate the machines that make those parts or

materials [Ericsson (2012)]. The pipelines used to transport natural gas and oil
from the Bering Sea across Alaska and Canada to the continental United States or

across the Ural Mountains from Russia to Western Europe stands evident to the

versatility of welding process. The giant 100,000-ton super tankers that ply the

oceans moving oil around the world, the numerous off-shore drilling platforms that

tap new reserves of oil and natural gas, liquid gas storage tanks, the numerous

pressure vessels and pipes in steam and power generation plants, or reaction or

storage vessels in the chemical processing industries are a few areas where the

impact of welding becomes obvious.

Welding is a process in which materials of the similar basic type or class

are joined together through the formation of primary (occasionally secondary)

atomic- or molecular-level bonds under the combined action of heat and pressure.

Metals are joined in welding through the formation of metallic bonds. The key to

all welding is atomic-level inter diffusion between the materials being joined. The

diffusion may occur in the liquid, solid, or mixed state. Nothing contributes to

joining better than real interchange of atoms, ions, or molecules. The first is to

apply pressure and the second is to apply heat.

Heating helps welding to occur in several ways. In the solid state, heating

helps by driving off the adsorbed layers of gases or moisture. Then it breaks down

the oxide or other tarnish layers through differential thermal expansion between

them (or occasionally by thermal decomposition) and finally lowering the yield or

flow strength of the parent materials and allowing plastic deformation under

pressure to bring more atoms into intimate contact across the interface.

Alternatively, heating could help by causing melting of the substrate material to

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occur, allowing atoms to restructure by fluid flow and come together to

equilibrium spacing once solidification occurs, or by melting a similar filler rod

material to afford additional atoms of the same or different but compatible types as

the base material.

Pressure aids the process of welding by disrupting the adsorbed layer of

gases and moisture by deformation, then fractures the ceramic tarnish layer or

brittle oxide to expose clean parent material. Finally, it plastically deforms the

asperities to increase the amount of atoms (and the area) in intimate contact.

The relative amount of pressure and heat necessary to produce welds vary

from one extreme to the other. Very high heat and no pressure can produce welds

by relying on the high rate of diffusion in the solid state at elevated temperatures or

in the liquid state produced by melting or fusion. Little or no heat with very high

pressure can produce welds by forcing atoms together by plastic deformation,

relying on diffusion in the solid state to cause subsequent atomic-level intermixing.

The arc welding process consists of thermally emitted electrons and

positive ions from both the welding electrode and the work piece and the

intervening atmosphere. These positive ions and electrons are accelerated by the

potential field (i.e., voltage) between the source (i.e., one electrode) and the work

piece (i.e., the oppositely charged electrode). They produce heat when they convert

their kinetic energy by collision with the oppositely charged element. Arc welding

includes a large and diverse group of process embodiments or processes. The arc in

arc welding is created between an electrode and a work piece at different polarities.

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 The electrode can be projected to be permanent, serving solely as a source

of energy from electrons and positive ions, or consumed, in which case it serves as

both a source of energy for welding and filler to assist in making the weld. If the

electrode is projected to be permanent, then the process is called "non consumable

electrode arc welding process”. If the electrode is projected to be consumed, then

the process is called "consumable electrode arc welding process". Non consumable

electrodes are usually made from tungsten or carbon (in the form of graphite)

because of their very high melting temperatures.

1.2 THERMAL ASPECTS OF WELDING

All fusion welding, need heat to allow joints to be produced through the

structure of atomic-level bonding. Non-fusion welding processes use friction to

produce heat in order to facilitate weld formation or generate heat in the process of

forming a weld. The major differences among these processes from the standpoint

of heat and its effects are as follows: (a) The peak temperatures reached is a

fraction of the parent material's melting point (i.e., the homologous temperature),

which is highest for fusion welding; (b) The rate of heating is generally highest for

fusion welding; (c) The time at peak temperature tends to be shortest for fusion

welding and (d) the rate of cooling once the heating source is removed also tends

to be highest for fusion welding

It is important that for fusion welding processes, not all of the energy

available in the source reach the work piece to cause desired heating and melting to

produce a weld. Losses occur between the source and the work piece.

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 As a result, lower temperature sources are less prone to certain types of

losses (e.g., radiation of light and heat). The flow of heat or distribution in a

welded assembly is governed primarily by the time-dependence of heat, which is

identified by the generalized equation of heat flow (dealt later in the modelling

chapter). In considering the effects of heat on solidification and melting in the

region of melting and on transformations in the surrounding the Heat Affected

Zone (HAZ), it is important to consider how the heat is distributed. Heat

distribution directly influences the efficiency of melting, the extent and nature of

peripheral heating, and the rate of subsequent cooling. The extent of melting, in

turn, directly affects the weld size and shape, the homogeneity through convection,

the degree of weld shrinkage and weld distortion.

The extent of peripheral heating, in turn, affects the following: (a)

development of thermally induced stresses acting on the solidifying zone; (b) The

rate of cooling in the solidifying zone; (c) The level of heating in the heat affected

zone (which can cause degradation of properties); (d) The rate of cooling in the

HAZ (which determines the final structure and properties in this zone) and (e) the

degree and nature of distortion and/or residual stresses in the newly joined

assembly.

Theoretically it's imperative to understand what happens at a point on the

weld as a function of time, from just before the heat source acts on the point to

after the heat source is removed from the point. Key aspects to note are:

(a) The temperature starts out at the ambient temperature of the

environment prior to the arrival of a moving heat source

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 (b) The temperature rises very rapidly once the heat source acts on the

point

(c) The temperature reaches a maximum or "peak" determined by the

balance between the energy being input and all lost

(d) The temperature remains at a maximum only as long as the source

remains on that spot (which, for a moving source, is only an

instant).

(e) The temperature cools back to the ambient level at a rate dependent

on the thermal mass and thermal-physical properties of the material

and any imposed cooling

For the GTAW process, the peak temperature can be much higher than the

liquidus temperature of the base material being welded. This is typically several

hundred degrees Kelvin higher due to the short "dwell time" once melting is

achieved. Superheat is needed to ensure that melting is complete. Cooling of the

newly formed joint and surrounding HAZ is normally quite rapid (i.e., several

hundred degrees Kelvin per second), near the solidification temperature.

1.3 ALUMINIUM AND ITS ALLOYS

1.3.1 Characteristics of Aluminium Alloys

The unique arrangement of properties provided by aluminium and its alloys

make aluminium one of the economical, most versatile and attractive metallic

material for a broad range of uses from soft, highly ductile wrapping foil to the

most challenging engineering application.

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 Aluminium has a density of 2.7 g/cm3, about one third as much as steel

(7.83 g/cm3). Aluminium resists the kind of progressive oxidation that causes steel

to rust away. Aluminium can resist corrosion by salt water and other environmental

factors, and by a wide range of other physical and chemical agents. Aluminium

surface can be highly reflective and hence, electromagnetic waves, radiant heat,

visible light, and radiant energy are efficiently reflected by it. Aluminium typically

displays excellent thermal and electrical conductivity and is about 50% to 60% that

of copper. Aluminium is non-ferromagnetic; a property of importance in the

electrical and electronics industry. It is non-pyrophoric, which is important in

applications involving handling or exposure of inflammable or explosive materials.

Aluminium is non-toxic and is routinely used in containers for beverages

and food. One of the most important characteristics of aluminium is its workability

and machinability. It can be cast by any known method, rolled to any desired

thickness, forged, drawn, spun, hammered, stamped, and extruded to almost any

conceivable shape. Due to the exciting range of properties of aluminium and

aluminium alloys, this group of metals is extensively used for wide range of

industrial applications. Aluminium and its alloys can be ranked next to steel, in

terms of industrial applications.

1.3.2 Classification of Aluminium Alloys

There are international standards based on which aluminium alloys are

designated. These alloys are distinguished by a four-digit number, which is

followed by a temper designation code. The first digit corresponds to the principal

alloying constituent. The second digit corresponds to variations of the initial alloy.

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The third and fourth digits correspond to individual alloy variations. Finally the

temper designation code corresponds to different strengthening techniques.

1xxx—Pure Al (99.00 % or greater)

2xxx—Al-Cu Alloys

3xxx—Al-Mn Alloys

4xxx—Al-Si Alloys

5xxx—Al-Mg Alloys

6xxx—Al-Mg-Si Alloys

7xxx—Al-Zn Alloys

8xxx—Al + Other Elements

9xxx—Unused Series

Temper Designation System

F – as fabricated

O - Annealed

H – strain-hardened

W - Solution heat-treated

T - Thermally treated to produce stable tempers other than F, O, or

Subdivisions of T Temper thermally treated:

The aluminium alloys are classified into two categories: non- heat treatable

and heat treatable.

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Heat-treatable aluminium alloys

The initial strength of aluminium alloy in this group depends upon the alloy

composition, just as the non heat–treatable alloys. Heat treatable aluminium–

copper alloys conforming to Al 2024-T6 are of moderate strength and possess

excellent welding characteristics over the high strength aluminium alloys. Hence,

alloys of this class are extensively employed in marine frames, storage tanks,

pipelines, and aircraft applications.

Non-heat treatable aluminium alloys

The initial strength of the non-heat treatable aluminium alloys depends

mainly upon the hardening effect and effect of alloying element such as silicon,

manganese, iron, and magnesium. These elements increase the strength either by

solid solution strengthening or in dispersed phase. The non-heat treatable alloys are

mainly found in the 1xxx, 3xxx, 4xxx and 5xxx alloy series depending upon the

alloying elements. The strength of all the non-heat treatable alloys may be

improved by strain hardening.

1.3.3 Applications of AA 5059 Aluminium Alloy

In this research work, the non-heat treatable alloy AA 5059 H-136 is taken

for investigation. This alloy is a magnesium (Mg) based non heat treatable alloy

that is strengthened by mechanical strain hardening and is produced at Koblenz,

Germany, by Aleris International, Inc.[T.Anderson et al (2003)]. The strain

hardening process results in the 5000 series alloy receiving the “H” designation

rather than the “T” designation that is typical for heat treatable alloys. Marine

grade tempers of 5059, such as H116 and H321, have been commercially available

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for quite some time on yachts, ferries, and catamarans. The H136 designation

indicates that during the production process, the plate was only stretched and not

cold rolled. This resulted in a lower cost, more ductile version that may provide

some benefit as structural material. Nowadays, aluminium alloys are used in many

applications in which the combination of high strength and low weight is attractive.

Shipbuilding is one area in which the low weight can be of significant value. The

AA 5059 is the most frequently used aluminium alloy in shipbuilding industries as

a hull material due to its high corrosion resistance. These attributes allow ships to

go faster, travel farther, and carry larger payloads given the same amount of fuel

load. Also AA 5059, Alustar aluminium alloy is used in military to make a milled

vehicle door, due to its excellent ballistic and mine blast deflection characteristics.

Due to its lower weight compared to steel plate, this alloy is used as a component

element in products such as brackets, braces and armaments. The main alloying

element in the 5000 series is magnesium. The higher the Mg content, the greater is

the welded strength of the base metal. Al -Mg alloys of the 5000 series are strain

hard enabled, and have moderately high strength and very high toughness even at

cryogenic temperatures to near absolute zero. They are readily welded by a variety

of techniques. As a result, 5000 series of aluminium alloys find wide application in

building and construction, highway structures, including bridges, storage tanks and

pressure vessels, cryogenic tanks and systems for temperature as low as -270°C

and marine applications.

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1.4 CHALLENGES IN WELDING OF ALUMINIUM ALLOYS

Aluminium alloys are lightweight, have relatively high strength, retain

good ductility at subzero temperatures, highly resistant to corrosion, and are non-

toxic. They have a melting range between 4820C and 6600C, depending upon the

alloy. It is impossible in practice to stop the tenacious oxide film formed due to

oxidation at exposed surfaces. Unlike iron, aluminium has only one allotropic form

so there are no phase transformations which can be exploited to control its

microstructure. The heat generated due to welding can severely hinder the process

of deformation and precipitation-hardening of alloys. So, in order to avoid these

difficulties, a new technique called friction stir welding was developed. It can

weld all aluminium alloys, including those that cannot normally be joined by

conventional fusion welding techniques such as aluminium-lithium alloys.

Dissimilar aluminium alloys can also be joined, for e.g. 5000 to 6000 series or

even 2000 to 7000 series.

1.5 WELDING OF ALUMINIUM ALLOYS

There are many different methods available for joining aluminium and its

alloys. To deploy an appropriate method amongst them, the various factors to be

considered are the geometry, strength of the joint and the environmental conditions

such as moisture, temperature, inert atmosphere and corrosion. The dominant

method for aluminium fabrication is welding. Most alloys of aluminium are easily

weld able [Mishra et al.,(2003)]. Thermal conductivity of aluminium is quite high;

therefore heat is easily conducted away from the welding area. It is essential that

the heat source is powerful enough to rapidly reach aluminium's melting point of

565/650ºC. Coefficient of thermal expansion of aluminium is also high as

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compared to steel, so it is prone to distortion and stress inducement if the proper

welding procedure is not followed.

Gas Tungsten Arc welding (GTAW) and Gas Metal Arc welding (GMAW)

processes are commonly used. Though there are a few problems like porosity, lack

of fusion due to oxide layer, incomplete penetration, cracks, inclusion and undercut

associated with these welding processes, the other methods like Resistance

Welding, Friction Welding and Laser Welding can be employed. Welding results

in many physical and chemical changes such as oxide formation, dissolution of

hydrogen in molten aluminium and decolourization upon heating [George et al,

(2003)].

Aluminium, due to its strong affinity towards oxygen, forms oxides of

aluminium, which is common during fusion welding processes. Since, the

oxidation of aluminium elevates the melting point of the metal and its alloys,

complete fusion is possible when fusion welding process is used. Aluminium oxide

is an electrical insulator and if it is thick enough, it is capable of preserving the arc

which starts the welding process. So it demands the use of inert gas welding and

use of fluxes is necessary while using the process of fusion welding.

Liquid aluminium in its liquid state has a tendency to absorb hydrogen due

to its solubility. This paves way for porosity as the hydrogen remain entrapped,

when the metal solidifies. Thus the elimination of sources of hydrogen becomes

imminent and the metal should be shielded properly from hydrogen. Elaborate pre

treatment on the metal and the machine should take care to check the sources of

hydrogen.

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 Another problem faced when welding aluminium is hot cracking that

occurs due to high thermal expansion of aluminiun. The heat treatable aluminium
alumin

alloy has many alloying elements. Hence, weld crack sensitivity is of concern.
concern

Another problem posed during fusion welding of aluminium is weldability

due to thee copper and magnesium content of certain alloys of aluminium making

them susceptible to cracking

1.5.1 Gas Tungsten Arc Welding

Fig.1.1 Illustration of GTAW process [Gary et al (2005)

Gas Tungsten Arc Welding

conventional
onventional fusion welding processes
process for joining aluminium alloys
alloy especially thin

gauge plates. Figure 1.1 illustrates a schematic diagram of GTAW system. GTAW

uses a permanent, non consumable tungsten electrode to generate an arc to the

work piece. This electrode is shielded by an inert gas such as helium or argon to

prevent electrode degradation by oxidation and hence it has the older and common

names like tungsten inert gas (TIG) welding. The quality of GTA welds are

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relatively high than that of any of the arc welding processes. The GTAW process

can be operated as shown in Fig. 1.2. Current from a power supply is passed to the

tungsten electrode in a torch through a contact tube. The tube is usually water-

cooled copper to prevent overheating. The gas-tungsten arc welding process can be

performed with or without filler (i.e., autogenously). When no filler is used, joints

must be thin and tight-fitting square butts. The GTAW process can be operated in

several different current modes, including Direct Current (DC) with the Electrode

Negative (EN) or Positive (EP), or Alternating Current (AC).

Pressure Regulator
Flow Meter

Tungsten Rod
Argon Gas In
Cooling Water In Solenoid
Valve Argon Cylinder

HF Unit &
Welding Cable & Cooling Water Cooling
Ceramic Cup Water In Tube System
Cooling Water Out
Argon Shielding
Arc
High Frequency
+ Connection
Work

Pedal Switch Power Source – +

Fig.1.2 Essential equipment &accessories of GTAW Process

These different current modes result in distinctly different arc and weld

characteristics. When the work piece is connected to the positive terminal of a

direct-current power supply, the operating mode is referred to as "Direct Current

Straight Polarity" (DCSP) or "Direct Current Electrode Negative" (DC-ve or

DCEN). When the work piece is connected to the negative terminal of a direct

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power supply, the operating mode is referred to as "Direct Current Reverse

Polarity" (DCRP) or "Direct Current Electrode Positive" (DC + or DCEP). In

DCSP, electrons are emitted from the tungsten electrode and are accelerated to

very high velocities and kinetic energy while travelling through the arc. These

high-energy electrons collide with the work piece, give up their kinetic energy, and

generate considerable heat in the work. Consequently, DCSP results in deep-

penetrating, narrow welds but with higher work piece heat input.

About two-thirds of the heat available from the arc (after losses from

various sources) enters the work. High heat input to the work piece may or may not

be desirable, depending on factors such as: (a) Required weld penetration

(dependent on joint thickness); (b) Required weld width (dependent on joint fit

up); (c) Work piece mass (dependent on part size and section thickness); (d) Work

piece thermal conductivity (high conductivity needing higher heat input) and (e)

susceptibility to heat-induced defects, and concern for distortion or residual

stresses (with high heat input being problematic in both regards).

In DCRP, on the other hand, the heating effect of the much higher kinetic

energy electrons is on the tungsten electrode rather than on the work piece. Hence,

larger, water-cooled electrode holders are required resulting in shallow welds and

lower heat input. This operating mode is good for welding thin sections or heat-

sensitive metals and alloys. This mode also results in a scrubbing action on the

work piece by the large positive ions that strike its surface, removing oxide and

cleaning the surface. This mode is preferred for welding metals and alloys that

oxidize easily, such as aluminium and magnesium.

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 There is, however, a third mode, using Alternating Current. This mode

tends to give some of the characteristics of both the DC modes during the

corresponding half-cycles, but with some bias toward the straight polarity half-

cycle. During this half-cycle, the current tends to be higher because of the extra

emission of electrons from the smaller, sharper, hotter electrode versus a large,

blunter, cooler work piece. In the AC mode, reasonably good penetration is

obtained, along with some oxide cleaning action.

The electron emission of tungsten electrodes is occasionally enhanced by

adding 1-2% of thorium oxide or cerium oxide to the tungsten. This addition

improves the current carrying capacity of the electrode, results in less chance of

contamination of the weld by expulsion of tungsten because of localized melting of

the electrode and allows easier arc initiation. While both argon and helium are used

for shielding with the GTAW process, argon offers better shielding because it is

heavier and stays on the work. Arc initiation is also easier because the required

ionization potential is lower than that of helium. Thus, GTAW process is good for

welding thin sections because of its inherently low heat input. It offers better

control of weld filler dilution by the substrate than many other processes (again

because of low heat input), and it is a very clean process. Its limitations are its

limited penetration capability (typically about 3-4 mm) and slow deposition rate

(typically less than 1 kg per hour).

1.5.2 Process parameters of TIG welding

The parameters that affect the quality and outcome of the TIG welding

process are given below:

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a) Welding Current

Higher current in TIG welding can lead to splatter and work piece being

damaged. Again lower current setting in TIG welding leads to sticking of the filler

wire. Sometimes larger heat affected area can be found for lower welding current,

as high temperatures need to applied for longer periods of time to deposit the same

amount of filling materials. Fixed current mode will vary the voltage in order to

maintain a constant arc current.

b) Welding Voltage

Welding voltage can be fixed or adjusted depending on the TIG welding

equipment. A high initial voltage allows for easy arc initiation and a greater range

of working tip distance. Too high voltage, can lead to large variation in welding

quality.

c) Welding speed

Welding speed is an important parameter for TIG welding. If the welding

speed is increased, power or heat input per unit length of weld decreases. Therefore

less weld reinforcement results and penetration of welding decreases. Welding

speed or travel speed primarily controls the bead size and penetration of weld. It is

interdependent with current. Excessive high welding speed decreases wetting

action, increases tendency of undercut, porosity and uneven bead shapes while

slower welding speed reduces porosity.

1.5.3 Friction Stir Welding

Friction Stir Welding (FSW) is a relatively new joining process invented at

The Welding Institute (Cambridge, UK) in 1991 [W. M. Thomas et al.,(1991)] and

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developed initially for aluminium alloys, which allows metal joining without

fusion or filler materials. FSW is used widely employed to weld aluminium and its

alloys for critical applications as well. Since FSW is essentially a solid-state,

without melting, high quality weld can generally be fabricated with absence of

solidification cracking, porosity, oxidation, and other defects typical to traditional

fusion welding.

FSW can be used to join many types of similar and dissimilar material

combinations provided that tool can be developed to operate compatibly in the hot

working temperature range of the work pieces. FSW also has potential for bonding

many materials that are difficult or impossible to be joined by more conventional

methods, including alloys that are susceptible to solidification cracking, high-

strength steels, metal-matrix composites, and other advanced alloys. For many

conventionally welded aluminium alloys the fusion zones are typically weaker than

the base metal.

However, FSW offers a significant quality advantage that it is possible to

make welds where the strength of the fusion zone is identical to that of the base

metal alloy. Additionally, because the energy input used for FSW is relatively low

(no melting occurs), the heat-affected zone (HAZ) or thermo mechanically affected

zone (TMAZ) and residual stresses associated with the welds are relatively small.

Lower residual stresses mean that distortion associated with FSW is not a large

concern as in conventional welding.

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1.5.4 Principle of FSW

The two work pieces to be welded, with square mating (faying) edges are

clamped on a rigid backing plate. The clamping prevents the any movement of

work pieces during welding. The shank, shoulder and pin form a welding tool, and

this tool can be rotated to a prescribed speed and may be tilted normal with respect

to the work piece. The tool is slowly plunged into the work piece material at the

butt line, until the shoulder of the tool forcibly contacts the upper surface of the

material and the pin is at a short distance from the back plate. Fig.1.3 shows the

schematic representation of friction stir welding. Either the rotating tool is made to

move, along the butt line, to the end by applying an axial force or the work piece is

moved to the same effect. The pin is withdrawn on reaching the end which leaves a

keyhole as shown in Fig. 1.4.

Fig.1.3 Schematic representation of friction stir welding [Mishra et.al (2003)]

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The pin is forced or plunged into the work piece until the shoulder contacts the

surface of the work piece. As the tool descends further, its shoulder surface

touches the top surface of the work piece and creates heat.

Fig.1.4 Friction Stir Welding Process

1.5.5 Stages of Friction Stir Welding Process

(a) Rotating tool prior to contact with the plate; (b) tool pin contacts plate

creating shear; (c) shoulder of the tool contacts the plate, restricting further

penetration while expanding the hot zone; (d) plate moves relative to rotating tool

creating a fully re-crystallized, fine grain micro structure

The maximum temperature created by FSW process ranges between 70%

and 80% of the melting temperature of the work piece. This reduces welding

defects and large distortion commonly associated with fusion welding are

minimized or avoided. This heat is conducted to both the tool and the work piece.

The amount of heat conducted into the work piece dictates a successful process

which is defined by the quality, shape and microstructure of the processed zone, as

well as the residual stress and the distortion of the work piece.

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 The amount of heat conducted to the tool dictates the life of tool and the

capability of tool to produce a good processed zone. For instance, insufficient heat

from the friction could lead to breakage of the pin of the tool since the material is

not soft enough. Therefore, understanding the heat transfer aspects of the friction

stir welding is extremely important, not only for the science but also for improving

the process.

The process is especially well suited to butt and lap joint in aluminium

since it is difficult to weld by arc processes. In addition, the FSW process produces

an extremely fine grain structure, giving the stir zone unique deformation

characteristics compared with other welding processes, and making it ideally suited

for applications where impact damage is a concern.

FSW has been demonstrated in a variety of metals, such as steel, titanium,

lead, copper, and aluminium. The process is especially advantageous for joining

aluminium and has been exploited commercially around the world in several

industries.

1.6 MODELING OF WELDING

Welding is a complex process involving the interaction of thermal,

mechanical, electrical and metallurgical phenomena. Since it is a complex

process, the analytical models and numerical models are very much useful to

completely understand the mechanism of heating and bonding.

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1.6.1 Need for Finite Element Method

To predict the behaviour of structure three tools adopted such as

analytical, experimental and numerical methods. The analytical method is used

for the regular sections of known geometric entities or primitives where the

component geometry is expressed mathematically. The solution obtained through

analytical method is exact and takes less time. This method cannot be used for

irregular sections and the shapes that require very complex mathematical

equations. On the other hand, the experimental method is used for finding the

unknown parameters of interest. But the experimentation requires testing

equipment and a specimen for each behavior. This, in turn requires a high

initial investment to procure the equipment and to prepare the specimens.

The solution obtained is exact but the time consumed to find the results and

during preparation of specimens is more. There are many numerical schemes

such as Finite Difference Method, Finite Element Method, Boundary Element

and Volume Method, Finite Strip and Volume Method and Boundary Integral

Methods etc., are used to estimate the approximate solutions to acceptable

tolerance.

1.6.2 The Process of Finite Element Method

The Finite Element Method is used to solve physical problems in

e n gineering analysis and design. The p h y s i c a l problems typically involve

an actual structure component subjected to certain loads. The idealization of

the physical problem to a mathematical model requires certain assumptions

that together lead to differential equations governing the mathematical model.

The Finite Element Analysis solves the mathematical model, which describes the

22
physical problem. The Finite Element Method (FEM) is a numerical procedure; it is

necessary to assess the solution accuracy. If the accuracy criteria are not met, the

numerical solution has to be repeated with refined solution parameters until a

sufficient accuracy is reached.

It is clear that the Finite Element solution will solve selected mathematical

model with all the assumptions, which reflects on the predicted response. The

approximate selection of mathematical model will influence the accuracy of the

solution. The mathematical model is solved and checked for the accuracy then

refinement is made if required. Depending upon the level of accuracy, the

optimization of section or shape is performed by linking the optimization

techniques with Finite Element Method.

1.6.3 Field and Boundary Conditions

The field variables such a displacements, strains and stresses must

satisfy the governing conditions, which can be mathematically expressed in the

form of differential equations. For structured mechanic problems, the boundary

conditions may be kinematic which involves displacements, or static, which

involves forces and moments. The specified temperature or heat flow/heat flux

or convections may be specified in thermal analysis.

1.6.4 Steps Involved In Finite Element Modeling

The broad steps involved in the finite element method are as follows:

1. Divide the continuum into a finite number of sub regions (or elements) of

simple geometry such as line segments, triangles, quadrilaterals. (Square

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 and rectangular elements are subsets of quadrilateral), tetrahedrons and

hexahedrons (cubes) etc.

2. Select key points on the elements to serve as nodes where

conditions of equilibrium and compatibility are to be enforced.

3. Assume displacement functions within each element so that the

displacements at each generic point depend on the nodal values.

4. Satisfy strain-displacement and stress-strain relationships within a

typical element.

5. Determine stiffness and equivalent nodal loads for a typical element

using work or energy principles.

6. Develop equilibrium equations for the nodes of the discritized continuum

in terms of the element contributions.

7. Solve the equilibrium for the nodal displacements.

8. Calculate support reactions at restrained nodes if displaced.

9. Determine strains and stresses at selected points within the elements.

1.7 ORGANISATION OF THE THESIS

In this research work, the subsequent chapters describe in detail the

accomplishment of the objectives based on the material used for the work. The

second chapter provides the literature available and earlier works carried out in the

area of the GTAW and FSW and the simulation of the process along with the

motivation for the present work. The third chapter presents the objectives and the

methodology of the present study. The fourth chapter describes the experimental

procedures for GTAW and FSW joints of AA 5059 aluminium alloy. The

procedure to measure temperature and residual stress are elaborated. Specimen

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preparation for the evaluation of the weld characteristics is also described. The

fifth chapter contains the modelling principle adopted for the research work. The

boundary conditions, heat source model and the governing equations that are

incorporated into the FE model are defined. The sixth chapter deals in detail the

optimization of the GTAW process parameters by RSM. The resulting temperature

profile and residual stress for the optimized process parameters are presented.

Consequently characterization of the weldment, based on tensile properties,

fractography, macro and micro structure and micro hardness is dealt in this

chapter. The modeling of GTAW process and its validation with experimental

results are presented in this chapter. The seventh chapter deals in detail the

optimization of the FSW process parameters by RSM. The resulting temperature

profile and residual stress for the optimized process parameters are presented.

Consequently characterization of the weldment, in terms of tensile properties,

fractography, macro and micro structure and micro hardness is dealt in this

chapter. The modeling of FSW process and its validation with experimental results

are presented in this chapter. The eighth chapter deals with the conclusions of the

present research work.

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