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CHAPTER 1
INTRODUCTION
Aluminium is the most abundant metal in nature. It is ductile and can be
readily cast and machined. Several properties set aluminium apart from other
metals. First, it is lighter than all other engineering metals except magnesium
and beryllium. It has a density of about 0.1lb/in3
(2990 kg/m3
). A second
important property of aluminium is its thermal and electrical conductivity. The
third property that is responsible for the wide use of aluminium alloys is their
corrosion resistance. Aluminium is not widely used for chemical resistance, but
for applications involving atmospheric corrosion resistance it is probably the
most widely used metallic material. There are hundreds of commercially
available aluminium alloys. It can be seen that aluminium alloys can be cast by
all the common casting techniques.
The usual alloying additions to aluminium in order to improve physical
properties include Cu, Si, Mg, Zn, and Mn. Aluminium alloy 2024 is an
aluminium alloy, with copper as the primary alloying element. It is used in
applications requiring high strength to weight ratio, as well as good fatigue
resistance. Aluminium alloy 7075 is an aluminium alloy, with zinc as the
primary alloying element. 7000 series alloys such as 7075 are often used in
transport applications, including marine, automotive and aviation, due to their
high strength-to-density ratio.
The difficulty of making high-strength, fatigue and fracture resistant
welds in aerospace aluminium alloys, such as highly alloyed 2XXX and 7XXX
series, has long inhibited the wide use of welding for joining aerospace
structures. These aluminium alloys are generally classified as non-weldable
because of the poor solidification microstructure and porosity in the fusion
1

zone. Also, the loss in mechanical properties as compared to the base material is
very significant. These factors make the joining of these alloys by conventional
welding processes unattractive. Some aluminium alloys can be resistance
welded, but the surface preparation is expensive, with surface oxide being a
major problem.
Friction stir welding (FSW) was invented at The Welding Institute (TWI)
of UK in 1991 as a solid-state joining technique, and it was initially applied to
aluminium alloys. The basic concept of FSW is remarkably simple. A non-
consumable rotating tool with a specially designed pin and shoulder is inserted
into the abutting edges of sheets or plates to be joined and traversed along the
line of joint. FSW is considered to be the most significant development in metal
joining in a decade and is a ‘‘green’’ technology due to its energy efficiency,
environment friendliness, and versatility. As compared to the conventional
welding methods, FSW consumes considerably less energy. No cover gas or
flux is used, thereby making the process environmentally friendly. The joining
does not involve any use of filler metal and therefore any aluminium alloy can
be joined without concern for the compatibility of composition, which is an
issue in fusion welding. When desirable, dissimilar aluminium alloys and
composites can be joined with equal ease.
Aluminium is a lightweight metal (density = 2.71 g/cm3
) having good
corrosion resistance to the atmosphere and many aqueous media, combined with
good electrical and thermal conductivity. The observed corrosion behaviour of
aluminium is sensitive to small amounts of impurities in the metal; all these
impurities, with the exception of magnesium, tend to be cathodic to aluminium.
In general, the high-purity metal is much more corrosion-resistant than
commercially pure aluminium, which, in turn, is usually more resistant than
aluminium alloys.
2

Corrosion is the destructive attack of a metal by chemical or
electrochemical reaction with its environment. The three main reasons for the
importance of corrosion are: economics, safety and conservation. To reduce the
economic impact of corrosion, corrosion engineers, with the support of
corrosion scientists, aim to reduce material losses, as well as the accompanying
economic losses, that result from the corrosion of piping, tanks, metal
components of machines, ships, bridges, marine structures, and so on. Corrosion
can compromise the safety of operating equipment by causing failure (with
catastrophic consequences) of, for example, pressure vessels, boilers, metallic
containers for toxic chemicals, turbine blades and rotors, bridges, airplane
components, and automotive steering mechanisms. Safety is a critical
consideration in the design of equipment for nuclear power plants and for
disposal of nuclear wastes. Loss of metal by corrosion is a waste not only of the
metal, but also of the energy, the water, and the human effort that was used to
produce and fabricate the metal structures in the first place. In addition,
rebuilding corroded equipment requires further investment of all these resources
— metal, energy, water, and human.
3

CHAPTER 2
LITERATURE SURVEY
Friction stir welding (FSW) was invented at The Welding Institute (TWI)
of UK in 1991 as a solid-state joining technique by W. M. Thomas et al.
Frankel and Xia were first to investigate pitting and stress corrosion cracking
behaviours of FSW 5454Al and compare 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. Frankel and Xia 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 lowest ductility of FSW5454Al-H34 in both air and solution
was attributed to a defect associated with some remnant of original interface.
The breakup of the original interface depends on the process parameters as well
as tool design. It is important to completely breakup and distribute the oxide
surface layer to avoid crack nucleation sites.
4

The experimental observations that the pitting and SCC resistances of
FSW welds were superior or comparable to those of the base material were also
recently reported by Corral et al., Zucchi et al. and Meletis et al.
Corral et al. investigated the effect of FSW on the corrosion behaviour
of a very common heat-treatable aircraft aluminium alloy (2024Al-T4) and a so-
called third-generation Al–Li alloy (2195Al). Anodic polarization curves
showed that the diffusion-limiting current densities and corrosion potentials of
both 2024Al and 2195Al FSW welds were nearly identical to those of the base
alloys for a 0.6 M NaCl solution. Furthermore, static immersion tests for 20 h
and 25 days showed an even amount of by-product build-up both on the FSW
zones and base metal sections.
Similarly, Zucchi et al. reported that the 5083Al FSW weld exhibited a
higher corrosion resistance in EXCO solution (4 M NaCl–0.5 M KNO3–0.1 M
HNO3) and a lower pitting tendency than the base alloy. Further, a higher
pitting potential and a lower cathodic current were observed in the FSW weld
than in the base alloy. Additionally, SSRT showed that FSW joint was not
susceptible to SCC in both EXCO and 3.5% NaCl + 0.3 g/l H2O2 solutions. In
comparison, MIG joints were susceptible to SCC in both solutions.
More recently, Meletis et al. investigated SCC behaviour of FSW
7075Al-T6, 2219Al-T87 and 2195Al-T87 by two types of experiments: (a)
four-point bending at different loading levels under alternate immersion (AI)
conditions in 3.5% NaCl solution for 90 days, and (b) slow strain rate tension of
specimens pre-exposed (PE) under AI in 3.5% NaCl solution. Four-point
bending results revealed that no stress corrosion cracks were present in these
samples, indicating no SCC susceptibility for any of the FSW alloys for the
given exposure period and loading levels. The SSRT results show that under
5

more severe SSRT experiments, FSW 2219Al and 2195Al still showed no SCC
susceptibility, whereas FSW 7075Al showed a reduced ductility with increasing
PE time. Meletis et al. suggested that the observed environmental susceptibility
in FSW 7075Al was due to hydrogen embrittlement.
The investigations by Lumsden et al., Hannour et al. and Paglia et al.
demonstrated that FSW welds of 7075Al, 7010Al, 2024Al, and 7050Al were
more susceptible to intergranular attack than the base alloy. They showed a
typical example of corrosion attack of 7075Al-T651 following extended
exposure to a modified EXCO solution (4 M NaCl–0.5 M KNO3–0.1 M HNO3
diluted to 10%). It is evident that after 24 h exposure to the modified EXCO
solution, the corrosion was very localized in the HAZ, including the outer edges
of the TMAZ, and neither the base alloy nor the weld nugget showed evidence
of corrosive attack. For extended exposure times, the intergranular attack
became more severe in the initial attack region and attack region spread to
whole TMAZ previously unattacked. Finally, the intergranular attack was also
developed in the nugget zone. However, no intergranular corrosion was detected
in the parent metal. Similar results were also reported by other investigators in
FSW 7075Al-T651, 2024Al-T351, 7010Al-T7651 [163,164], namely
intergranular attack occurred preferentially in the HAZ adjacent to the TMAZ.
Paglia et al. further verified that the HAZ in the retreating side exhibited
higher susceptibility than that in the advancing side. However, Biallas et al. and
Paglia et al. reported that preferential corrosion attack occurred in the TMAZ
for FSW 2024Al-T3 and in the TMAZ-nugget interface for FSW 7050Al-
T7451. Clearly, the pitting potential of corrosion zone was not only
significantly lower than that of the base alloy, but also lower than that of the
nugget zone for all FSW aluminium welds. These studies indicated that the
hottest regions within the HAZ were the most susceptible to intergranular
6

corrosion and had the lowest pitting potential followed by the nugget.
Microstructural examinations on the hottest regions of the HAZ revealed
significant Cu depletion at grain boundaries.
Based on the experimental observations, Lumsden et al. attributed the
mechanism of intergranular corrosion to a Cu depletion model linking
intergranular corrosion with pitting corrosion. This is consistent with previous
studies that the pitting potential decreases with a decrease of Cu. Furthermore,
widened PFZs, coarse grain boundary phases and coarse intragranular
precipitates in the HAZ were also considered responsible for the preferential
corrosion in the HAZ. It should be pointed out that in addition to alloy
chemistry, both residual microstructure in FSW welds and corrosion medium
exert a significant effect on the corrosion behaviour of FSW aluminium alloys.
This is why contradictory trends were reported for 2024Al. This requires further
research to establish the dominating factors influencing corrosion properties of
FSW welds.
7

CHAPTER 3
SCOPE AND OBJECTIVE
Aluminium is a soft, durable, lightweight, ductile and malleable metal. It
is non-magnetic. It has one-third the density and stiffness of steel. It is easily
machined, cast, drawn and extruded. It has good mechanical properties along
with good thermal and electrical properties. It shows high corrosion resistance.
Extraction of aluminium is also easy. So it is widely used in many commercial
and industrial applications. It finds application in transportation, construction,
packaging, electrical transmission lines, household utensils, aviation, aerospace,
marine, etc.
Friction stir welding is a solid-state welding technique. It is highly useful
for welding high strength aluminium alloys. It is used when good mechanical
properties are required in the as welded condition. It provides improved safety
due to the absence of toxic fumes and spatter of molten material. These welded
metals are used in highly corrosive environments as in the case of marine
application, e.g., ships.
electrochemical reaction with its environment. The three main reasons for the
importance of corrosion are economics, safety and conservation. The corrosion
of weld depends on the materials, weld parameters and environment. This study
aims at finding the optimum parameters of the weld for high corrosion-resistant
weld.
8

CHAPTER4
STUDY OF ALUMINIUM
Aluminium was first produced in the laboratory in 1825 by reducing
Aluminium chloride. However, wide acceptance of aluminium as an
engineering material did not occur until World War II. Aluminium is the most
abundant metal in nature. Some 8% by weight of the earth’s crust is aluminium.
Many rocks and minerals contain a significant amount of aluminium.
Unfortunately, aluminium does not occur in nature in metallic form. In rocks,
aluminium is present in the form of silicates and other complex compounds.
The ore from which most aluminium is presently extracted, bauxite, is a
hydrated aluminium oxide. From 1825 when aluminium was discovered, to
about 1890, aluminium was produced on a small scale by complex and
expensive mechanical reductions of aluminium compounds.
4.1 General Characteristics
Aluminium is a good electrical conductor. It is ductile and can be readily
cast and machined. It has a face-centered cubic structure as do other “metallic”
metals, such as copper, silver, nickel and gold. Several properties set aluminium
apart from other metals. First, it is lighter than all other engineering metals
except magnesium and beryllium. It has a density of about 0.1 lb/in3
(2990
kg/m3
).A comparison with other metals is shown in figure.
`
9

Fig1. Density of aluminium compared to other metals
A second important property of aluminium is its thermal and electrical
conductivity. It has about 60% of the conductivity of pure copper (IACS).
Because of its lower density, aluminium has a higher conductivity than copper
per unit mass. For example, a 10 mm diameter aluminium wire will have the
same resistivity as a 6mm diameter copper wire and still be about 13% lighter
than the copper wire. This is an important consideration in long power
transmission cables.
The third property that is responsible for the wide use of aluminium
alloys is their corrosion resistance. Aluminium is not widely used for chemical
resistance, but for applications involving atmospheric corrosion resistance it is
probably the most widely used metallic material. Architectural applications of
aluminium are everywhere – railings, windows, frames, doors, fishing and so
on.
10

4.2 Alloy Designation
4.2.1 Wrought
The most commonly used alloy designation system in the United States is
that of the Aluminium Association. For wrought alloys, it is based on four digits
corresponding to the principal alloying elements.
Table1. Principal alloying elements
Corresponding pure aluminium (99%min) 1000
Copper(major alloying element) 2000
Manganese 3000
Silicon 4000
Magnesium 5000
Magnesium and silicon 6000
Zinc 7000
Other elements 8000
Unused series 9000
The second digit in this system designates milli control or lack of same
on specific elements. The last two digits have no significance except in the 1xxx
series they coincide with aluminium content above 99% in hundredths. A third
digit can be used to indicate a variation of the two-digit temper designation
where properties are slightly different from those of the two digit temper. The
meanings of the numbers following the T temper designations are as follows.
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Table2. Temper designations
xxxx-F As fabricated, no special controls
xxxx-W Solution heat treated (used only on alloys that naturally age harden)
xxxx-O Annealed(wrought alloys only)
xxxx-H Strain hardened
xxxx-T Thermally treated to produce effects other than F, O or H
Table3. Meanings of numbers in T temper designations
xxxx-T1 Cooled from a hot working temperature and naturally aged
xxxx-T2 Cooled from an elevated temperature, cold worked and naturally
aged
xxxx-T3 Solution heat treated and cold worked
xxxx-T4 Solution heat treated and naturally aged
xxxx-T5 Cooled from a hot work temperature and furnace aged
xxxx-T6 Solution heat treated and furnace aged
xxxx-T7 Solution heat treated and stabilized
xxxx-T8 Solution heat treated, furnace aged and cold worked
xxxx-T9 Solution heat treated, furnace aged and cold worked
xxxx-T10 Cooled from an elevated temperature, cold worked and furnace aged
12

4.2.2 Cast Alloys
In U.S cast alloys have been identified by a four digit identification
number with the last digit separated by a decimal. A letter prefix is occasionally
used to signify alloy or impurity limits. The first digit indicates the alloy group.
The second and third digits identify an alloy within a group, and last digit
indicates product form. A last digit of 0 indicates a casting; a digit of 1 indicates
an ingot form. The designations for groups of cast alloys are shown below.
Table4. Cast aluminium alloy designations
Case alloy designation Major alloying elements
1-99(old system) Aluminium + silicon
1xx.x 99.5 min. aluminium
2xx.x Copper
3xx.x Silicon + copper or magnesium
4xx.x Silicon
5xx.x Magnesium
6xx.x Unused series
7xx.x Zinc
8xx.x Tin
9xx.x Other element
13

4.3 Aluminium Products
There are hundreds of commercially available aluminium alloys. It can be
seen that aluminium alloys can be cast by all the common casting techniques.
Investment castings are made from alloys in the permanent mold category.
Castings are used for everything from engine blocks to camera parts. Wrought
aluminium products include foil, sheet, bar, rod, wire, tubing, powder metals
and structural shapes such as I’s, channels and angles. Open and closed die
forgings are used for many aerospace and aircraft applications. Extrusions
account for between 10% and 20% of all aluminium products. They are widely
used for special shapes for everything from pencils to sailboat masts. They can
be extremely useful in machine design for clutches, mouldings and part nests.
Wrought aluminium products are commercially available with a wide
range of special finishes. These include mechanical finishes, chemical finishes
and coatings. Mechanical finishes include cold finished, buffed and textured.
Chemical finishes include such things as etched, bright dipped and chemical
conversion coatings. Anodizing, painting, plating and vitrified coatings are
included in the coating category. There are number designation systems for
these surface finishes, but there are so many of these finishes that the best way
to learn about them is to request a finish manual from an aluminium supplier.
These manuals are usually free and contain actual samples of aluminium with
these finishes. If a proposed application could benefit from a pretreated surface,
mill finishes should certainly be investigated.
14

4.4 Advantages
The following are some of the noteworthy advantages of using
aluminium.
● One third of the weight of steel
● Good thermal and electrical conductivity
● High strength to weight ratio
● Can be a hard surface by anodizing and hard coating
● Most alloys are weldable
● Will not rust
● High reflectivity
● Can be die cast
● Easily machined
● Good formability
● Non magnetic
● Non toxic
15

CHAPTER 5
STUDY OF CORROSION
electrochemical reaction with its environment. The corrosion scientist studies
corrosion mechanisms to improve (a) the understanding of the causes of
corrosion and (b) the ways to prevent or at least minimize damage caused by
corrosion. The corrosion engineer, on the other hand, applies scientific
knowledge to control corrosion. Both the scientific and engineering viewpoints
supplement each other in the diagnosis of corrosion damage and in the
prescription of remedies.
5.1 Importance of Corrosion
The three main reasons for the importance of corrosion are:
1. Economics
2. Safety
3. Conservation
5.1.1 Economics: To reduce the economic impact of corrosion, corrosion
engineers, with the support of corrosion scientists, aim to reduce material losses,
as well as the accompanying economic losses, that result from the corrosion of
piping, tanks, metal components of machines, ships, bridges, marine structures,
and so on.
5.1.2 Safety: Corrosion can compromise the safety of operating equipment by
causing failure (with catastrophic consequences).
5.1.3 Conservation: Loss of metal by corrosion is a waste not only of the
metal, but also of the energy, the water and the human effort that were used to
produce and fabricate the metal structures in the first place.
16

5.2 Economic Losses
Economic losses are divided into (1) direct losses and (2) indirect losses.
Direct losses include the costs of replacing corroded structures and machinery
or their components. They also include the extra cost of using corrosion-
resistant metals and alloys instead of carbon steel where the latter has adequate
mechanical properties but not sufficient corrosion resistance. There are also the
costs of galvanizing or nickel-plating of steel, adding corrosion inhibitors to
water and dehumidifying storage rooms for metal equipment. Indirect losses are
more difficult to assess, but a brief survey of typical losses of this kind compels
the conclusion that they add several billion dollars to the direct losses already
outlined.
Examples of indirect losses are as follows:
1. Shutdown
2. Loss of Product
3. Loss of Efficiency
4. Contamination of Product
5. Over design
5.3 Causes of Corrosion
Any metal surface is a composite of electrodes electrically short-circuited
through the body of the metal itself. So long as the metal remains dry, local-
action current and corrosion are not observed. However, on exposure of the
metal to water or aqueous solutions, local-action cells are able to function and
are accompanied by chemical conversion of the metal to corrosion products.
Local-action current, in other words, may account for the corrosion of metals
exposed to water, salt solutions, acids, or alkalies.
17

5.4 Definition of Anode and Cathode
A combination of two electrical conductors (electrodes) immersed in an
electrolyte is called a galvanic cell in honour of Luigi Galvani, a physician in
Bologna, Italy, who published his studies of electrochemical action in 1791. A
galvanic cell converts chemical energy into electrical energy. On short -
circuiting such a cell (attaching a low-resistance wire to connect the two
electrodes), positive current flows through the metallic path from positive
electrode to negative electrode.
The electrode at which chemical reduction occurs (or + current enters the
electrode from the electrolyte) is called the cathode. The electrode at which
chemical oxidation occurs (or + electricity leaves the electrode and enters the
electrolyte) is called the anode.
Corrosion of metals usually occurs at the anode. Nevertheless, alkaline
reaction products forming at the cathode can sometimes cause secondary
corrosion of amphoteric metals such as Al, Zn, Pb and Sn, which corrode
rapidly on exposure to either acids or alkalies.
5.5 Types of Corrosion Damage
5.5.1 General Corrosion or Uniform Attack
This type of corrosion includes the commonly recognized rusting of iron
or tarnishing of silver. ‘Fogging’ of nickel and high-temperature oxidation of
metals are also examples of this type. Generally, for uniform attack, the initial
corrosion rate is greater than subsequent rates. Duration of exposure should
always be given when corrosion rates are reported because it is often not
reliable to extrapolate a reported rate to times of exposure far exceeding the test
period. Rates of uniform attack are reported in various units, with accepted
terminologies being millimetres penetration per year (mm/y) and grams per
18

square meter per day (gmd). For handling chemical media whenever attack is
uniform, metals are classified into three groups according to their corrosion
rates and intended application. These classifications are as follows:
A. < 0.15 mm/y (< 0.005 ipy) — Metals in this category have good
corrosion resistance to the extent that they are suitable for critical parts,
for example, valve seats, pump shafts and impellors, springs.
B. 0.15 to 1.5 mm/y (0.005 to 0.05 ipy) — Metals in this group are
satisfactory if a higher rate of corrosion can be tolerated, for example, for
tanks, piping, valve bodies, and bolt heads.
C. > 1.5 mm/y (> 0.05 ipy) — Usually not satisfactory.
5.5.2 Pitting
This is a localized type of attack, with the rate of corrosion being greater
at some areas than at others. If appreciable attack is confined to a relatively
small, fixed area of metal, acting as anode, the resultant pits are described as
deep. If the area of attack is relatively larger and not so deep, the pits are called
shallow. Depth of pitting is sometimes expressed by the pitting factor, the ratio
of deepest metal penetration to average metal penetration as determined by the
weight loss of the specimen. A pitting factor of unity represents uniform attack.
Many metals, when subjected to high-velocity liquids, undergo a pitting type of
corrosion called impingement attack, or sometimes corrosion-erosion. Fretting
corrosion, which results from slight relative motion (as in vibration) of two
substances in contact, one or both being metals, usually leads to a series of pits
at the metal interface. Metal-oxide debris usually fills the pits so that only after
the corrosion products are removed do the pits become visible.
5.5.3 Dealloying, Dezincification and Parting
Dealloying is the selective removal of an element from an alloy by
corrosion. Dezincification is a type of attack occurring with zinc an alloy (e.g.,
yellow brass) in which zinc corrodes preferentially, leaving a porous residue of
19

copper and corrosion products. The alloy so corroded often retains its original
shape, and may appear undamaged except for surface tarnish, but its tensile
strength and ductility are seriously reduced. Dezincified brass pipe may retain
sufficient strength to resist internal water pressures until an attempt is made to
uncouple the pipe, or a water hammer occurs, causing the pipe to split open.
Parting is similar to dezincification in that one or more reactive components of
the alloy corrode preferentially, leaving a porous residue that may retain the
original shape of the alloy. Parting is usually restricted to such noble metal
alloys as gold-copper or gold-silver. It is used in gold refining. Copper-base
alloys that contain aluminium are subject to a form of corrosion resembling
dezincification, with aluminium corroding preferentially.
5.5.4 Intergranular Corrosion
`This is a localized type of attack at the grain boundaries of a metal,
resulting in loss of strength and ductility. Grain-boundary material of limited
area, acting as anode, is in contact with large areas of grain acting as cathode.
The attack is often rapid, penetrating deeply into the metal and sometimes
causing catastrophic failures. At elevated temperatures, intergranular corrosion
can occur because, under some conditions, phases of low melting point form
and penetrate along grain boundaries; for example, when nickel-base alloys are
exposed to sulphur-bearing gaseous environments, nickel-sulphide can form and
cause catastrophic failures. This type of attack is usually called sulphidation.
5.5.5 Cracking
If a metal cracks, when subjected to repeated or alternate tensile stresses
in a corrosive environment, it is said to fail by corrosion fatigue. In the absence
of a corrosive environment, the metal stressed similarly, but at values below a
critical stress, called the fatigue limit or endurance limit, will not fail by fatigue
even after a very large, or infinite, number of cycles. A true endurance limit
20

does not commonly exist in a corrosive environment. The metal fails after a
prescribed number of stress cycles no matter how low the stress. The types of
environment causing corrosion fatigue are many and are not specific. If a metal,
subject to a constant tensile stress and exposed simultaneously to a specific
corrosive environment, cracks immediately or after a given time, the failure is
called stress-corrosion cracking. The stress may be residual in the metal, as
from cold working or heat treatment, or it may be externally applied. The
observed cracks are intergranular or trans-granular, depending on the metal and
the damaging environment. Failures of this kind differ from intergranular
corrosion, which proceeds without regard to whether the metal is stressed.
Almost all structural metals (e.g., carbon and low alloy steels, brass, stainless
steels, Duralumin, magnesium alloys, titanium alloys, nickel alloys and many
others) are subject to stress-corrosion cracking in some environments.
Fortunately, either the damaging environments are often restricted to a few
chemical species, or the necessary stresses are sufficiently high to limit failures
of this kind in engineering practice. As knowledge accumulates regarding the
specific media that cause cracking and regarding the limiting stresses necessary
to avoid failure within a given time period, it will be possible to design metal
structures without incidence of stress-corrosion cracking. Highly stressed metal
structures must be designed with adequate assurance that stress-corrosion
cracking will not occur.
5.6 Corrosion in Aluminium
Al3+
+ 3e−
→Al φ° = −1.7 V
Aluminium is a lightweight metal (density= 2.71 g/cm3
) having good
corrosion resistance to the atmosphere and many aqueous media, combined with
good electrical and thermal conductivity. It is very active in the Emf Series, but
21

becomes passive on exposure to water. Although oxygen dissolved in water
improves the corrosion resistance of aluminium, its presence is not necessary to
achieve passivity. It is usually assumed that the passive film is composed of
aluminium-oxide, which, for air-exposed aluminium, is estimated at about 2–10
nm (20–100 Å) in thickness. The observed corrosion behaviour of aluminium is
sensitive to small amounts of impurities in the metal; all these impurities, with
the exception of magnesium, tend to be cathodic to aluminium. In general, the
high purity metal is much more corrosion resistant than commercially pure
aluminium, which, in turn, is usually more resistant than aluminium alloys.
5.6.1 Corrosion in Water and Steam
Aluminium tends to pit in waters containing Cl−
, particularly at crevices
or at stagnant areas where passivity breaks down through the action of
differential aeration cells. Traces of Cu 2+
(as little as 0.1 ppm) or Fe3+
in water
reacts with aluminium, depositing metallic copper or iron at local sites. The
copper or iron, being efficient cathodes, shifts the corrosion potential in the
noble direction to the critical potential, thereby both initiating pitting, and by
galvanic action, stimulating pit growth.
5.6.2 Effect of pH
Aluminium corrodes more rapidly both in acids and in alkalies compared
to distilled water, with the rates in acids depending on the nature of the anion.
At room temperature, the minimum rate occurs in the pH range approximating
4–8.5. Corrosion rates of aluminium in the alkaline region greatly increase with
pH, unlike iron and steel, which remain corrosion-resistant. The reason for this
difference is that Al3+
is readily complexed by OH−
, forming AlO2
−
.
Al + NaOH + H2O → NaAlO2 + 3/2 H2
22

This reaction proceeds rapidly at room temperature, whereas for iron a
similar reaction forming NaFeO2 and Na2FeO2 requires concentrated alkali and
high temperatures.
5.6.3 Corrosion Characteristics
Aluminium is characterized by sensitivity to corrosion by alkalies and
pronounced attack by traces of copper ions in aqueous media. In addition,
aluminium is subject to rapid attack by mercury metal and mercury ions and
anhydrous chlorinated solvents (e.g., CCl4, ethylene dichloride, and propylene
dichloride). The rate of attack can be appreciable in either dilute or concentrated
alkalies. For this reason, when aluminium is cathodically protected,
overprotection must be avoided in order to ensure against damage to the metal
by accumulation of alkalies at the cathode surface. Lime, Ca(OH)2, and some of
the strongly alkaline organic amines (but not NH4OH) are corrosive. Fresh
Portland cement contains lime and is corrosive; hence, aluminium surfaces in
contact with wet concrete may evolve hydrogen visibly. The corrosion rate is
reduced when the cement sets, but continues if the concrete is kept moist or
contains deliquescent salts (e.g., CaCl2). A drop of mercury in contact with an
aluminium surface rapidly breaks down passivity accompanied by
amalgamation (i.e., formation of an aluminium amalgam). In the presence of
moisture, the amalgamated metal quickly converts to aluminium-oxide, causing
perforation of piping or sheet. Mercury ions present in solution in only trace
amounts similarly accelerate corrosion, producing intolerably high rates of
attack.
In summary, aluminium is resistant to the following:
1. Hot or cold NH4OH.
2. Hot or cold acetic acid. Aluminium is resistant to citric, tartaric, and
malic acids.
3. Fatty acids. Aluminium equipment is used for distillation of fatty acids.
23

4. Nitric acid, > 80% up to about 50 °C (120 °F).
5. Distilled water.
6. Atmospheric exposure. Excellent resistance to rural, urban, and
industrial atmospheres; lesser resistance to marine atmospheres.
7. Sulphur, sulphur atmospheres, and H2S.
8. Fluorinated refrigerant gases, such as Freon.
Aluminium is not resistant to the following:
1. Strong acids, such as HCl and HBr (dilute or concentrated), H2SO4 HF,
HClO4, H3PO4 and formic, oxalic and trichloroacetic acids.
2. Alkalies. Lime and fresh concrete are corrosive, as well as strong
alkalies. Corrosion by soap solutions can be inhibited by adding a few
tenths percent of sodium silicate (not effective for strong alkalies).
3. Mercury and mercury salts.
4. Seawater. Pitting occurs at crevices and surface deposits, especially
when trace amounts of heavy metal ions are present.
5. Waters containing heavy metal ions (e.g., mine waters or waters
previously passing through copper, brass, or ferrous piping).
6. Chlorinated solvents.
7. Anhydrous ethyl, propyl, or butyl alcohols at elevated temperatures.
8. Contact with wet woods, in particular beech wood. Any wood
impregnated with copper preservatives is especially damaging.
5.6.4 Corrosion in Aluminium Alloys
The usual alloying additions to aluminium in order to improve physical
properties include Cu, Si, Mg, Zn and Mn. Of these, manganese may actually
improve the corrosion resistance of wrought and cast alloys. One reason is that
the compound MnAl6 forms and takes iron into solid solution. The compound
(MnFe) Al6 settles to the bottom of the melt, in this way reducing the harmful
24

influence on corrosion of small quantities of alloyed iron present as an impurity.
No such incorporation occurs in the case of cobalt, copper, and nickel, so that
manganese additions would not be expected to counteract the harmful effects of
these elements on corrosion behaviour. The Duralumin alloys (e.g., types 2017
and 2024) contain several percent copper, deriving their improved strength from
the precipitation of CuAl2 along slip planes and grain boundaries.
5.6.5 Exfoliation
Exfoliation is a type of anodic path corrosion in which attack of rolled or
extruded aluminium alloy results in surface blisters followed by separation of
elongated slivers or lamina of metal. It occurs in various types of aluminium
alloys in addition to the copper-bearing series. Proper heat treatment may
alleviate such attack. Exfoliation is commonly experienced on exposure of
susceptible aluminium alloys to marine atmospheres.
5.6.6 Stress-Corrosion Cracking
Pure aluminium is immune to stress-corrosion cracking (S.C.C.). Should
a Duralumin alloy, on the other hand, be stressed in tension in the presence of
moisture, it may crack along the grain boundaries. Hence, in heat treatment
procedures, it is better practice to aim at a slightly over-aged rather than an
under-aged alloy. High concentrations of zinc in aluminium (4–20%) also
induce susceptibility to cracking of the stressed alloys in the presence of
moisture. Many high-strength aluminium alloys are available; specific
composition ranges and heat treatments for these alloys are usually chosen with
the intent of minimizing susceptibility to S.C.C. Solution heat treatment
temperature affects stress-corrosion susceptibility by altering the grain boundary
composition as well as the alloy metallurgical microstructure. As mentioned
earlier, cladding of alloys can serve to cathodically protect them from either
intergranular corrosion or S.C.C.
25

CHAPTER 6
FRICTION STIR WELDING
The difficulty of making high-strength, fatigue and fracture resistant
welds in aerospace aluminium alloys, such as highly alloyed 2XXX and 7XXX
series, has long inhibited the wide use of welding for joining aerospace
structures. These aluminium alloys are generally classified as non-weldable
because of the poor solidification microstructure and porosity in the fusion
zone. Also, the loss in mechanical properties as compared to the base material
is very significant. These factors make the joining of these alloys by
conventional welding processes unattractive. Some aluminium alloys can be
resistance welded, but the surface preparation is expensive, with surface oxide
being a major problem. Friction stir welding (FSW) was invented at The
Welding Institute of UK in 1991.
6.1 Process
It is a solid-state joining technique, and it was initially applied to
aluminium alloys. The basic concept of FSW is remarkably simple. A non-
consumable rotating tool with a specially designed pin and shoulder is inserted
into the abutting edges of sheets or plates to be joined and traversed along the
line of joint. The tool serves two primary functions: (a) heating of workpiece,
and (b) movement of material to produce the joint. The heating is
accomplished by friction between the tool and the workpiece and plastic
deformation of workpiece. The localized heating softens the material around
the pin and combination of tool rotation and translation leads to movement of
material from the front of the pin to the back of the pin. As a result of this
process a joint is produced in ‘solid state’. Because of various geometrical
features of the tool, the material movement around the pin can be quite
complex. During FSW process, the material undergoes intense plastic
26

deformation at elevated temperature, resulting in generation of fine and
equiaxial recrystallized grains. The fine microstructure in friction stir welds
produces good mechanical properties. FSW is considered to be the most
significant development in metal joining in a decade and is a ‘‘green’’
technology due to its energy efficiency, environment friendliness, and
versatility. As compared to the conventional welding methods, FSW
consumes considerably less energy. No cover gas or flux is used, thereby
making the process environmentally friendly. The joining does not involve any
use of filler metal and therefore any aluminium alloy can be joined without
concern for the compatibility of composition, which is an issue in fusion
welding. When desirable, dissimilar aluminium alloys and composites can be
joined with equal ease. In contrast to the traditional friction welding, which is
usually performed on small axisymmetric parts that can be rotated and pushed
against each other to form a joint, friction stir welding can be applied to
various types of joints like butt joints, lap joints, T butt joints, and fillet joints.
Fig2. Schematic diagram of friction stir welding process
27

6.2 Process Parameters
FSW/FSP involves complex material movement and plastic deformation.
Welding parameters, tool geometry, and joint design exert significant effect on
the material flow pattern and temperature distribution, thereby influencing the
microstructural evolution of material. In this section, a few major factors
affecting FSW/FSP process, such as tool geometry, welding parameters, joint
design are addressed.
6.2.1 Tool Geometry
Tool geometry is the most influential aspect of process development. The
tool geometry plays a critical role in material flow and in turn governs the
traverse rate at which FSW can be conducted. An FSW tool consists of a
shoulder and a pin as shown schematically in Figure. As mentioned earlier, the
tool has two primary functions: (a) localized heating, and (b) material flow. In
the initial stage of tool plunge, the heating results primarily from the friction
between pin and workpiece. Some additional heating results from deformation
of material. The tool is plunged till the shoulder touches the workpiece. The
friction between the shoulder and workpiece results in the biggest component of
heating. From the heating aspect, the relative size of pin and shoulder is
important, and the other design features are not critical. The shoulder also
provides confinement for the heated volume of material. The second function of
the tool is to ‘stir’ and ‘move’ the material. The uniformity of microstructure
and properties as well as process loads is governed by the tool design. Generally
a concave shoulder and threaded cylindrical pins are used.
With increasing experience and some improvement in understanding of
material flow, the tool geometry has evolved significantly. Complex features
have been added to alter material flow, mixing and reduce process loads.
28

Fig3. Schematic diagram of the FSW tool
6.2.2 Welding Parameters
For FSW, two parameters are very important: tool rotation rate (v, rpm)
in clockwise or counter clockwise direction and tool traverse speed (n, mm/min)
along the line of joint. The rotation of tool results in stirring and mixing of
material around the rotating pin and the translation of tool moves the stirred
material from the front to the back of the pin and finishes welding process.
Higher tool rotation rates generate higher temperature because of higher friction
heating and result in more intense stirring and mixing of material as will be
discussed later. However, it should be noted that frictional coupling of tool
surface with workpiece is going to govern the heating. So, a monotonic increase
in heating with increasing tool rotation rate is not expected as the coefficient of
friction at interface will change with increasing tool rotation rate.
In addition to the tool rotation rate and traverse speed, another important
process parameter is the angle of spindle or tool tilt with respect to the
workpiece surface. A suitable tilt of the spindle towards trailing direction
ensures that the shoulder of the tool holds the stirred material by threaded pin
and move material efficiently from the front to the back of the pin. Further, the
insertion depth of pin into the workpiece (also called target depth) is important
for producing sound welds with smooth tool shoulders. The insertion depth of
29

pin is associated with the pin height. When the insertion depth is too shallow,
the shoulder of tool does not contact the original workpiece surface. Thus,
rotating shoulder cannot move the stirred material efficiently from the front to
the back of the pin, resulting in generation of welds with inner channel or
surface groove. When the insertion depth is too deep, the shoulder of tool
plunges into the workpiece creating excessive flash. In this case, a significantly
concave weld is produced, leading to local thinning of the welded plates. It
should be noted that the recent development of ‘scrolled’ tool shoulder allows
FSW with 08 tool tilt. Such tools are particularly preferred for curved joints.
Preheating or cooling can also be important for some specific FSW
processes. For materials with high melting point such as steel and titanium or
high conductivity such as copper, the heat produced by friction and stirring may
be not sufficient to soften and plasticize the material around the rotating tool.
Thus, it is difficult to produce continuous defect-free weld. In these cases,
preheating or additional external heating source can help the material flow and
increase the process window. On the other hand, materials with lower melting
point such as aluminium and magnesium, cooling can be used to reduce
extensive growth of recrystallized grains and dissolution of strengthening
precipitates in and around the stirred zone.
6.2.3 Joint Design
The most convenient joint configurations for FSW are butt and lap joints. A
simple square butt joint is shown in Figure. Two plates or sheets with same
thickness are placed on a backing plate and clamped firmly to prevent the
abutting joint faces from being forced apart. During the initial plunge of the
tool, the forces are fairly large and extra care is required to ensure that plates in
butt configuration do not separate. A rotating tool is plunged into the joint line
and traversed along this line when the shoulder of the tool is in intimate contact
30

with the surface of the plates, producing a weld along abutting line. On the other
hand, for a simple lap joint, two lapped plates or sheets are clamped on a
backing plate. A rotating tool is vertically plunged through the upper plate and
into the lower plate and traversed along desired direction, joining the two plates.
Many other configurations can be produced by combination of butt and lap
joints. Apart from butt and lap joint configurations, other types of joint designs,
such as fillet joints, are also possible as needed for some engineering
applications.
It is important to note that no special preparation is needed for FSW of
butt and lap joints. Two clean metal plates can be easily joined together in the
form of butt or lap joints without any major concern about the surface
conditions of the plates.
Fig4. Joint configurations for friction stir welding: (a) square butt (b) edge
butt (c) T butt joint (d) lap joint (e) multiple lap joint (f) T lap joint and (g)
fillet joint.
6.3 Application
6.3.1 Aerospace
It is well known that high-strength aluminium alloys such as 2XXX and
7XXX series are widely used for aerospace structures such as fuselage, fins,
wings, etc. Unfortunately, such high-strength aluminium alloys are difficult to
31

join by conventional fusion welding due to the occurrence of hot cracking
during welding. Therefore, conventionally, a great amount of joining in the
aerospace structures is achieved by means of riveting. This results in increased
manufacturing complexity and cost. The emergence of friction stir welding
provides an opportunity to alter traditional approach for producing lightweight
assemblies for pervasive cost savings at the system level.
Eclipse Aviation is revolutionizing aircraft manufacturing by adopting FSW
for joining skins components and structure in Eclipse 500 aircraft. Other
remarkable successes include adoption of FSW by Boeing for its Delta rocket
tanks and C17 internal structures. The combined efforts of aerospace industries
have produced miles of FSW welds in commercial set-up without defects.
6.3.2 Armour
High-strength aluminium alloys have been used as armour due to a
combination of high ballistic performance and static strength. Such an armour
alloy was conventionally welded by MIG using Al–Mg filler. However, the
major problems associated with the MIG welds are: (a) stress corrosion
initiating at the weld toe, (b) exfoliation occurring in the solution treated and
naturally aged part of the HAZ, and (c) liquation due to the formation of low
melting point grain boundary films. With the emergence of new solid-state FSW
process, a defence research agency in the UK started a program to evaluate
FSW for aluminium armour in 1995. Preliminary investigations on exfoliation
corrosion and stress corrosion cracking tests verified the advantages of FSW
over MIG in terms of weld quality. Further research is focused on the
development of real joint designs for property verification and the application of
techniques to increase the speed of welding and the thickness of plate that can
be joined. However, GMAW and GTAW produce low ductility in butt welds in
2519Al alloy, with the result that the welds do not pass the ballistic shock test
32

required for combat vehicle applications. This prevents many simple butt weld
designs from being used in the vehicle structure. Although other joint types in
areas where plates must be joined have been resorted, this results in greater
complexity and concomitant higher manufacturing costs.
FSW, being a solid-state process, has been shown to produce superior as-
welded mechanical properties when compared to typical arc welding processes
in other aluminium alloys such as 5083Al, 6061Al, and 2219Al. Therefore, in
the past few years, attempts were made in General Dynamics Land Systems
(GDLS) and Concurrent Technologies Corporation (CTC) to friction stir weld
2519Al-T87. It was shown that sound-quality one inch thick flat-butt weld and
1–2-in. thick 908 corner welds can be successfully made by friction stir
welding. FSW 2519Al-T87 exhibited an ultimate tensile strength of 389 MPa
while maintaining a ductility of nearly 14%, representing an increase of 124
MPa in tensile strength and 300% increase in ductility over GMAW minimum
properties. Further, Colligan demonstrated that both flat and 908 corner weld
panels passed the ballistic shock test with less than 12 in. of cracking, even
though the impacting velocities were about 30% over the specification
requirement. Currently, mine-blast testing of FSW article is under progress to
further evaluate the suitability of FSW for joining armour aluminium alloys.
33

CHAPTER 7
EXPERIMENT
7.1 Welding
Two aluminium alloys were selected. One is from the AA2XXX series–
AA2024. The other is from the AA7XXX series– AA7075. Three plates each of
5mm thickness of these alloys were taken. The dimensions of the plates are
100mm x100mm. The Friction Stir Welding of these plates was carried on these
plates using three different weld parameters listed below. Thus, three different
samples were prepared.
Table5. Weld Parameters of the three samples
SAMPLE A B C
LOAD (kN) 10
12
16
ROTATIONAL
SPEED (rpm)
400 600 1200
WELD SPEED
(mm/min)
30 40 40
These samples were left as such for six months. During this period the
defects in the welded region, if present, would have been attacked by
atmospheric corrosive agents. The aged plate is then taken for further analysis.
34

Fig5. Welded sample A cut into pieces
7.2 Corrosion test
To examine the effect of corrosion on the weld it was decided to immerse
the welded region in strong alkaline solution for specific time periods. Then
NaOH solution of pH 8 was prepared. The welded portion of each sample was
cut into five pieces of 10 mm width.. These were separately immersed in 100 ml
of the NaOH solution prepared. They were immersed for different time periods.
They were removed after one hour, two hours, three hours, four hours and five
hours. left undisturbed for five hours. After removing the samples from the
solution, they were washed in distilled water .Then they were washed with
acetone to prevent further corrosion of the samples. These samples were
concealed in airtight covers and labelled. A few photographs of the samples
tested are shown in the figure below.
35

a) Sample A b) Sample B
c) Sample C
Fig6. Samples dipped in NaOH
7.3 Microscopic examination
Each specimen was examined under metallurgical microscope. The
effects of corrosion were hard to find under it. So the samples were examined
with a Scanning Electron Microscope (SEM). The images were taken at the
portion where the welded region met with the parent metal and at the centre of
the welded region. The Energy Dispersive X-Ray Analysis (EDAX) was also
carried out for the welded and corroded region. The SEM images are shown
below.
36

7.3.1 Sample A
The SEM images of five hour specimen of sample A are shown in the
figure below.
a) Left side b) Right side
c) Centre
Fig7. SEM images of sample A
Fig7. shows the scanning electron microscopic images of sample A. It has
three parts: (a) showing the left side of the weld zone, (b) showing the right side
of the weld zone and (c) showing the centre of the weld zone.
The sample A shows severe attack of the alkaline solution on the surface
of the welded plate. The corrosion of the metal is found to have occurred in the
welded zone. The oxides of metal are formed on the surface. Pitting corrosion is
found to take place in the welded zone.
37

7.3.2 Sample B
The SEM images of five hour specimen of sample B are shown in the
figure below.
c) Centre
Fig8. SEM images of sample B
Fig 8. shows the scanning electron microscopic images of sample B. It
has three parts: (a) showing the left side of the weld zone, (b) showing the right
side of the weld zone and (c) showing the centre of the weld zone.
The alkaline solution, in which the welded plate was immersed, is found
to have caused some effect on the surface. There are no severe traces of
corrosion in sample B. The sample B shows considerable corrosion resistance.
38

7.3.3 Sample C
The SEM images of five hour specimen of sample C are shown in the
figure.
c) Centre
Fig9. SEM images of sample C
Fig 9. shows the scanning electron microscopic images of sample C. It
has three parts: (a) showing the left side of the weld zone, (b) showing the right
side of the weld zone and (c) showing the centre of the weld zone.
The welded surface is found to be least attacked by the alkaline solution
in sample C. There are traces of oxides present on the surface. It is not as severe
in sample A.
39

CHAPTER 8
RESULTS AND DISCUSSION
The Energy Dispersive X-ray Analyses of the three samples are:
8.1 Sample A
Fig10. EDAX images of sample A
The EDAX images of sample A are shown in the fig10. This shows the
presence of oxides of aluminium alone. The spectrum shows that 23.56% of O
and remaining Al are present. Thus, the welded zone is severely corroded. The
pitting corrosion has occurred on the surface due to the effect of the alkaline
solution.
40

8.2 Sample B
Fig11. EDAX images of sample B
The EDAX image of the sample B shows the presence of 28.34% of O,
18.75% of C, 5.20% of Cu, 1.29% of Mg, 1.21% of Si, 1.12% of Na, 0.86% of
Fe, 0.71% of Mn, 0.50% of Cl, 0.42% of Ca and remaining Al by weight. This
shows that the percentage composition by weight of sample B shows small
deviation from that before corrosion.
41

8.3 Sample C
Fig12. EDAX images of sample C
The EDAX of sample A shows the presence of 28.92% of O, 16.52% of
C, 3.51% of C, 0.96% of Fe, 0.82% of Si, 0.74% of Mg, 0.42% of Ca and
remaining Al by weight. This shows that the composition percentage by weight
of the corroded region shows slight variation from parent metal composition.
42

Thus upon experimental analysis, followed by imaging of the specimen
with Scanning Electron Microscope, to study the microstructure, and the Energy
Dispersive X-ray Analysis of the specimen, to study the composition, showed
that two out of three specimen were much resistant to corrosion than the third
specimen.
The specimen B with weld parameters 12 kN, 600 rpm and 40 mm/min
and the specimen C with weld parameters 16 kN, 1200 rpm and 40 mm/min are
suitable for application. The specimen A with weld parameters 10 kN, 400 rpm
and 30 mm/min is susceptible to corrosion. So it is not suitable for application
in highly corrosive environments such as seawater.
43

CHAPTER 9
CONCLUSION
The aluminium metal and its alloys have a wide range of application such
household utensils, const ruction equipment, packaging, vessels used in
industries, pipes, aircrafts, ships, marine equipments, weapons, etc. They are
mainly used for their corrosion resistance property. High strength alloys of
aluminium are used in aircrafts and ships. They can be welded easily only by
using Friction Stir Welding technique. So care has to be taken that there is no
probability of corrosion in the welded region. Our project work reveals that the
so called non-corrosive alloys of aluminium are also affected by the universal
process of corrosion. But it can be reduced by using the optimum parameters of
the weld. Welding can take place at any set of parameters, but a safe set of
parameters to weld ,which will prevent the welded zone from corrosion should
be chosen.
In our project we conclude that welded region is susceptible for corrosion
when the axial load and the rotational speed are kept low. As we increase the
value of these parameters the welding is done more and more perfectly. Out of
the three sets of parameters we have chosen, the third set, i.e., the welded
sample C shows more corrosion resistance than the other two sets of
parameters. So we conclude that welding the aluminium alloy plates of AA2024
and AA7075 at 16 kN axial load, 1600 rpm rotational speed and 40 mm/min
weld speed is most suitable.
44

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47

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