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PROJECT REPORT ON
STUDY OF MECHANICAL PROPERTIES
OF Al 7075 ALLOY AFTER HEAT
TREATMENT
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR
AWARD OF DEGREE OF
MASTER OF TECHNOLOGY
Production Engineering.
Submitted by
Amandeep Singh Bhui (1539018)
Gurpreet Singh (1539020)
Under the guidance of
Dr. Sarabjeet Singh Sidhu
(Assistant Professor)
DEPARTMENT OF MECHANICAL ENGINEERING BEANT COLLEGE OF ENGINEERING AND TECHNOLOGY, GURDASPUR
November, 2017
ii
BEANT COLLEGE OF ENGINEERING AND TECHNOLOGY, GURDASPUR
CANDIDATE'S DECLARATION
I hereby certify that the work which is being presented in the project report entitled “Study of
Mechanical Properties of Al 7075 alloy after Heat treatment” by “Amandeep Singh Bhui
(1539018) and Gurpreet Singh (1539020)” in partial fulfillment of requirements for the award
of degree of M.Tech. (Production Engineering) submitted in the Department of Mechanical
Engineering at BEANT COLLEGE OF ENGINEERING AND TECHNOLOGY,
GURDASPUR under PUNJAB TECHNICAL UNIVERSITY, JALANDHAR is an authentic
record of my own work carried out during a period from August 2017 to Nov. 2017 under the
supervision of Dr. Sarabjeet Singh Sidhu. The matter presented in this project report has not
been submitted by me in any other University / Institute for the award of M.Tech Degree.
Gurpreet Singh Amandeep S. Bhui
Student (s)
This is to certify that the above statement made by the candidate is correct to the best of my/our
knowledge.
Dr. Sarabjeet Singh Sidhu
SUPERVISOR
iii
ABSTRACT
Aluminum alloys are used in a large number of applications including automobiles,
transmission of electricity, aerospace and defense industries due to the concepts of high
strength to low weight ratio. But aluminium-zinc alloy as in 7075 Al alloy is susceptible to
embrittlement because of micro-segregation of MgZn2 precipitates which may lead to
catastrophic failure of components produced from it. Even if Al 7075 has higher hardness,
higher strength, excellent wear resistance, and high-temperature corrosion protection, it is in
need of further enhancement of properties for increasing its applicability.
In this study, Aluminium alloy 7075 was selected as specimen for analyzing the various testing
results on its mechanical properties after heat treatment. The purpose of heat treating is to
analyze the mechanical properties of the Al 7075 alloy, i.e. hardness, Yield strength, tensile
strength and impact resistance. In the present study, selected samples were heat-treated by
elevating the temperature to 480°C for 2 hours and then quenched in different mediums in
order to investigate the effect on the mechanical properties of the Aluminium 7075 alloy. The
changes in mechanical behavior as compared to untreated samples were investigated in terms
of changes in tensile strength, hardness and impact strength. Results showed that the
mechanical properties of Aluminium 7075 alloy can be improved by customized heat treatment
for a specific application.
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ACKNOWLEDGEMENT
“A DREAM DOESN’T BECOME REALITY THROUGH MAGIC,
IT TAKES DETERMINATION AND HARDWORK!!”
In order to complete any project or mission, it is ensured that the person associated with it is
patient, sincere and humble. With the absence of any one of these attributes, errors can be
committed. At the outset, we would like to propose a word of thanks for the people who gave
us unending support and help. Many people have been the constant source of inspiration,
encouragement and assistance in several ways.
Firstly, we want to thanks and express my sincere gratitude to our research based project
supervisor, Dr. Sarabjeet Singh Sidhu, for his guidance, support, encourage and helpful
advices throughout this project. We had various interesting discussions regarding the present
project that significantly contributed to removing all the technical road blocks and helped in
shaping the path of achieving the goals of the present research activity. Without his encourage
and support, this research work could not be completed and presented to all.
We would like to extend our sincere thanks to Dr. Ranjit Singh (HOD), Department of
Mechanical Engineering, Beant College of Engineering and Technology for giving us this
wonderful opportunity to work in desired area of interest.
We extend our sincere thanks to all teaching staff of Mechanical Engineering
Department, those who helped us in completing this project successfully.
Lastly we also thanks the people who directly or indirectly gave encouragement and
support throughout the project.
Amandeep Singh Bhui (1539018)
Gurpreet Singh (1539020)
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LIST OF FIGURES
Page No.
Figure 1: Preparing samples for tensile test 23
Figure 2: Preparing samples for impact test 24
Figure 3: Preparing samples on lathe machine 24
Figure 4: Prepared samples for heat treatment 24
Figure 5: Muffle furnace used for experimentation 25
Figure 6: Samples inside furnace 25
Figure 7: Broken samples after tensile testing 25
Figure 8: Broken samples after impact testing 26
Figure 9: Impact testing machine 27
Figure 10: Specimen for charpy impact test 27
Figure 11: Samples after testing 27
Figure 12: Variation of tensile strength 29
Figure 13: Schematic illustration of the solid diffusion processes 30
Figure 14: Variation of percentage elongation during tensile test 31
Figure 15: Variation of impact strength 31
Figure 16: Variation of brinell hardness 32
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LIST OF TABLES
Page No.
Table 1: Wrought alloys designation system 14
Table 2: Chemical composition of Al 7075 alloy 18
Table 3: Percentage variation of results compared to standard 28
Table 4: Percentage variation of elongation during tensile test 30
vii
CONTENTS
Page No.
Candidate's Declaration ii
Abstract iii
Acknowledgement iv
List of Figures v
List of Tables vi
Chapter 1: INTRODUCTION 9
1.1 Influence of alloying elements 9
1.2 Strengthening & Softening techniques 12
1.3 Wrought Alloys 14
1.4 Requirement of alloy to be treated 15
1.5 Sequences of heat treatment 16
1.6 Ageing 17
1.7 Al 7075 alloy 18
Chapter 2: LITERATURE REVIEW 19
Chapter 3: PROBLEM FORMULATION 21
Chapter 4: RESEARCH OBJECTIVE 22
Chapter 5: METHODOLOGY 23
5.1 Material Preparation 23
5.2 Heat treatment of Al 7075 alloy 24
5.3 Tensile testing 25
5.4 Hardness test 26
5.5 Impact testing 26
viii
Chapter 6: RESULTS AND DISCUSSION 28
6.1 Effect of heat treatment on mechanical properties 28
6.2 Tensile strength 29
6.3 Impact strength 31
6.4 Brinell hardness 32
Chapter 7: CONCLUSION 33
Chapter 8: REFERENCES 34
9
CHAPTER – 1
INTRODUCTION
Alloys are materials having metallic properties and are composed of two or more elements, at
least one of which is a metal. In the case of aluminum alloys, most of them contain 90 to 96%
aluminum. Aluminum alloys are categorized under two headings i.e. wrought alloys and cast
alloys. Aluminum casting alloys are used in a large number of applications including
automobiles, trucks, transmission of electricity, development of transportation infrastructures,
and in the aerospace and defense industries. The fast growth of aluminum alloys in industrial
applications is related to their high strength-to-weight ratio which improves the mechanical
properties and performance of the products. Among different foundry alloys, aluminum casting
alloys are very popular, as they have the highest cast-ability ratings, possess good fluidity and
comparably low melting points. [4] Their light weight and high strength-to-weight ratio are the
main reasons why cast iron and steel components are being increa singly replaced by aluminum
alloys, particularly in the automotive industry. Choosing one casting alloy over another tends to
be determined by the relative ability of the alloy to meet one or more of the characteristics
required for a specific application.
In 1825 aluminium was made in laboratory, because of its high manufacturing cost, it gain
popularity in late of 20th century. Aluminium alloy have long been of interest in various
industries due to its increased performance in comparison with ferrous alloys. Aluminium
alloys plays a significant role in the applications where high strengt h to weight ratio is
necessary. Aluminium alloys are the prime candidates in the aerospace community due to their
modest specific strength, ease of manufacture and low cost. Uses of Titanium alloys and
composites have opened the doors for the applications in aerospace industry [8, 10]. But dew to
their high cost and unease of manufacture and restrictions in material handling, the extent of
application of these are quite restricted. Aluminum alloys are commonly used in aerospace
applications such as fuselage, wing, and support structures due to low density and good
mechanical properties. Aluminum is also used in land and marine vehicles as well as food
containers and other structural applications [11].
1.1 Influence of alloying elements on the Aluminium cast alloys.
There are many ways of changing properties of aluminium cast alloys. One of them is of course
change in composition of the alloy. Although the influence of elements that are noticeable in
the alloy is mainly considered, elements which are known as impurities cannot be omitted, and
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its effect are not always negative. Influence of all alloying elements as well as impurities which
can be found in aluminium alloys are as following. [3]
Silicon
Addition of Si to the aluminium alloys has a great number of benefit s. It is one of the elements
which do not increase the weight of the alloys and in the same time improves it properties. The
casting ability of Al-Si alloys are on extremely high level which lowers costs of producing Al-
Si castings. Mechanical properties of aluminium alloys depends more on the distribution of
added silicon then on the amounts of it. In these alloys where the Si particles are uniformly
distributed represent increase in ductility, while alloys in which these particles are acicular,
show small increase in strength [12]. While adding silicon to the Al alloy corrosion resistance
is only slightly affected. Generally it stays on the same level or is slightly better than in case of
pure aluminium. With increase content of Si decrease of the fluidity and the freezing range is
observed. Moreover silicon expands during solidification, so it compensates the shrinkage of
the aluminium. When the content of Si in the Al-Si alloys is as high as 25% volume shrinkage
of these alloys reaches zero level.
Copper
Changes in mechanical properties of alloy while adding copper can be observed in its strength
and ductility. Copper has the biggest influence on high temperature strength. These changes,
like in Al-Si alloys, do not depend on the amount of added copper but rather on the way how it
is distributed in solid solution. Alloys in which Copper can be found in the form of evenly
distributed sphereodised particles show biggest increase in strength without negative effects on
ductility, while alloys with Copper present as continuous network at grain boundaries appear to
be less ductile without noticeable increase in strength. Addition of copper will also reduce
corrosion resistance of the alloy.
It happens because Copper disperses the oxide film which appears on the metal surface and it
this way it prevents alloy to be electrically neutral. It leads to the fact that Al -Cu alloys can
corrode not only by contacting another materials but also another Al-Cu alloy.
Magnesium
Magnesium is material which is lighter then aluminium and shows the same strength
properties. It is the main alloying element in some Al alloys, but in the majority of them is
rather considered as impurity. The role of magnesium in aluminium-silicon alloys is to
precipitate _‖ phase (Mg2Si). [7] Al-Mg alloys are characterized with high strength with good
ductility. Moreover magnesium can, as one of the few elements, increase modulus of elasticity
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of Al alloys. Proper amount of magnesium in alloy will also give extremely high response to
heat treatment. Another property which is very good in Al-Mg alloys is corrosion resistance. It
is better in salt water and in mild alkalis then in pure aluminium. The worst thing when it
comes to Al-Mg alloys is it poor castability when the content of magnesium is small (2-4%). It
appears to be better with higher amounts of magnesium (up to 12%)
Iron
Iron added to aluminium alloys negatively influence its corrosion resistance. As far as
mechanical properties are concerned, Fe improves strength of the alloy and in the same time
reduces its ductility. Iron improves also resistance to hot tearing during solidification.
Formation of beta iron needles has detrimental effect on mechanical properties of aluminum
alloy. It happens because needle-shape like iron phases act as stress risers and crack
propagation can start in these points.
Manganese
In wrought alloys manganese is added to obtain better results during work hardening. In Al-Si
alloys Mn improves properties in high temperatures and similarly to silicon reduces shrinkage
formation during solidification. Nevertheless the most important feature of adding manganese
to the alloy is the fact that such addition result in change of iron phases in the alloy. Iron is
changed from the needle like shape to more spherical one which results in worse crack
propagation in the alloy.
Nickel
Nickel slightly improves both strength and ductility of the alloys at both room and elevated
temperatures. What is more, when adding nickel together with iron, corrosion resistance against
hot water is improved.
Chromium
Additions of chromium to the Al-Si alloys will effect in little increase in strength of these
alloys. It will also cause slightly worst tensile properties.
Zinc
Zinc in Al-Si alloys improves its machinability but decrease high temperatures strength. It also
increase tendency to hot tearing.
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Tin and lead
Similarly to Zn, addition of Sn and Pb improves machinability and decrease high temperature
strength of the alloys.
1.2 Strengthening and Softening techniques
Besides modifying Al alloys by changing its content, there are several other methods of
modifying these alloys. That is why aluminium alloys are almost always subjected to
strengthening or softening processes. Aluminium alloys can be divided into two groups:
Strengthening by means of work hardening and annealing.
Modified by heat treatment.
1.2.1 Work hardening and annealing
Work hardening is process in which metal is strengthening by for example rolling or forging. It
is main technique of improving properties of alloys which cannot be heat treated. During these
treatments number of dislocations in material increasing, and the strength of metal improves.
This is caused because grain shape changes and microstructure becomes more inhomogeneous
which do not allow fracture propagation in easy way. Work hardening is method of improving
properties of these alloys which cannot be heat treated. Nevertheless changes in structure of
alloys also affect reaction of these alloys to annealing and hot working.
Annealing is the process during which previously work hardening material is placed in furnace
in elevated temperatures. The great amount of commercially used Al alloys is generally
annealed in temperature range between 300°C and 420°C. When annealing time is too long,
recrystallization can be followed by grains growth and the softening can occurs. [4]
1.2.2 Heat treatment
Although precipitation or ageing hardening phenomenon of aluminium alloys was discovered,
by German metallurgist Alfred Wilm, over 100 years ago it is still not fully known and further
experiments are still being held all over the world. There is no exactly predicated influence of
ageing temperatures, ageing times and exact alloys composition on the mechanical properties
on these alloys. Latest research revels for example that however tensile strength in some alloys
decrease during first hours of ageing it starts to increase later and reach values better than as-
cast alloys [5].
13
Heat treatment is defined as an operation or combination of operations involving heating and
cooling of a metal or alloy for this case involving the mild steel in the solid state in such ways
as to produce certain microstructure and desired mechanical properties (hardness, toughness,
yield strength, ultimate tensile strength, Young‘s modulus, percentage elongation and
percentage reduction). Annealing, normalizing, hardening and tempering are the most
important heat treatments often used to modify the mechanical properties of engineering
materials particularly steels. Annealing is defined as a heat treatment that consists of heating to
and holding at a suitable temperature followed by cooling at an appropriate rate, most
frequently applied in order to soften iron or steel materials and refines its grains due to ferrite-
pearlite microstructure; it is used where elongations and appreciable level of tensile strength are
required in engineering materials. Hardening is the heat treatment processes in which increases
the hardness of a steel piece by heating it to a certain temperature and then cooling it rapidly to
room temperature. Tempering is the process of imparting toughness at the cost of its hardness
to an already hardened piece of steel by reheating it to a certain temperatur e and then cooling it
rapidly. The temperature of heating depends on the toughness to be imparted and hardness to
be reduced. In normalizing, the material is heated to the austenitic temperature range and this is
followed by air cooling. This treatment is usually carried out to obtain a mainly pearlite matrix,
which results into strength and hardness higher than in as received condition. It is also used to
remove undesirable free carbide present in the as-received sample [1]. The main objective of
heat treatment is to make the material system structurally and physically fit for engineering
application.
1.2.3 Quenching
Quenching is the process of rapid cooling of material systems to room temperature to preserve
the solute in solution. The cooling rate needs to be fast enough to prevent solid -state diffusion
and precipitation of the phase. The rapid quenching creates a saturated solution and allows for
increased hardness and mechanical properties of the material system. In addition to this studies
have shown that the highest degree of corrosion resistance have been obtained through the
maximum rates of quenching. To achieve supersaturated solid solution (desired optimum
condition for precipitation hardening) process called quenching is used. Rapid cooling of the
alloy to the room temperature evades forming type of precipitation which has negative
influence on mechanical properties and corrosion resistance [10]. The solid solution formed at
solution heat treating temperature is preserved.
14
Heat is dissipated from the object by movement of the quenching medium by conduction
currents. The difference in temperature between the boiling point of the medium and actual
temperature of the medium is the major factor influencing the rate of heat transfer in liquid.
Very fast cooling is more effective and thus strength is higher. Properties like Yield Strength,
Ultimate Tensile Strength and % elongation varies with quenching mediums i.e. quenchants.
1.3 Wrought Alloys
Cast aluminium alloys are grouped into different series of alloys, namely, 1xx.x series, 2xx.x
series, 3xx.x series, and so on. The principal alloying element or elements in each series
characterizes that series, as shown in Table 1.1
Alloy Series Principal Alloying Element
1xx.x Aluminium (99% minimum)
2xx.x Copper
3xx.x Manganese
4xx.x Silicon
5xx.x Magnesium
6xx.x Magnesium and Silicon
7xx.x Zinc
8xx.x Other elements
Table-1: Wrought alloys designation system
The International Alloy Designation System is the most widely accepted naming scheme for
wrought alloys. Each alloy is given a four-digit number, where the first digit indicates the
major alloying elements, the second — if different from 0 — indicates a variation of the alloy,
and the third and fourth digits identify the specific alloy in the series. For exa mple, in alloy
3105, the number 3 indicates the alloy is in the manganese series, 1 indicates the first
modification of alloy 3005, and finally 05 identifies it in the 3000 series.
15
1000 series are essentially pure aluminium with a minimum 99% aluminium content by
weight and can be work hardened.
2000 series are alloyed with copper, can be precipitation hardened to strengths comparable
to steel. Formerly referred to as duralumin, they were once the most common aerospace
alloys, but were susceptible to stress corrosion cracking and are increasingly replaced by
7000 series in new designs.
3000 series are alloyed with manganese, and can be work hardened.
4000 series are alloyed with silicon. Variations of Aluminum-silicon alloys intended for
casting (and therefore not included in 4000 series) are also known as silumin.
5000 series are alloyed with magnesium, and offer superb corrosion resistance, making
them suitable for marine applications. Also, 5083 alloy has the highest strength of not heat-
treated alloys.
6000 series are alloyed with magnesium and silicon. They are easy to machine,
are weldable, and can be precipitation hardened, but not to the high strengths that 2000 and
7000 can reach. 6061 alloy is one of the most commonly used general-purpose aluminium
alloys.
7000 series are alloyed with zinc, and can be precipitation hardened to the highest strengths
of any aluminium alloy (ultimate tensile strength up to 700 MPa for the 7068 alloy).
8000 series are alloyed with other elements which are not covered by other
series. Aluminium-lithium alloys are an example.
1.4 Requirements of the alloy to be heat treated
Not all aluminium alloys can be heat treated. Heat treatable alloys are only these from groups
2XX.X, 3XX.X and 7XX.X (these in which main alloying element is copper, magnesium and
zinc respectively). Properties of rest of aluminium alloys can be improved only by cold
working because process of precipitation hardening do not occurs in them.
The main requirement for an alloy system to respond to heat treatment is a significant decrease
in solid solubility of one or more of the alloying elements with decreasing temperature [4].
What is more, alloys which are cast using high-pressure die-casting method are not suitable for
heat treatment. During high pressure die casting gas bubbles are trapped inside the casting and
create so called porosity. During heat treatment process these gas pores expand and distort the
casting which makes component unusable.
16
1.5 Sequences of heat treatment
There are many different sequences of heat treatment which improves properties of aluminium
alloys. Its designation consists of one letter T means ‗heat treated‘ and a number showing how
the sequence looks like. The most common sequences are as following:
T1 heat treatment
cooled after casting or hot working
naturally aged
T2 heat treatment
cooled after casting or hot working
cold worked
annealing
T3 heat treatment
solution heat treated
cold worked
T4 heat treatment
solution heat treated
naturally aged
T5 heat treatment
cooled after casting or hot working
artificially aged
T6 heat treatment
solution heat treated
quenched
naturally aged
artificially aged
T7 heat treatment
solution heat treated
quenched
naturally aged
overaged
T8 heat treatment
solution heat treated
cold worked
17
artificially aged
T9 heat treatment
solution heat treated
artificially aged
cold worked
Most common sequences of heat treatment of aluminium alloys are T5 and T6. To specify heat
treatment more precisely one or more digits could be added to T1 -T9. For example T351 means
that the parts were solution heat treated, then stress were relieved by controlled amount of
stretching. Aluminium receives no further straightening after stretching. This process can be
applied to plate, rolled or cold finished rods and bars.
1.6 Ageing
Ageing is divided for natural ageing, when keeping samples in room temperature and artificial
ageing conducted in elevated temperatures. Most of the properties like hardness and electrical
conductivity are influence by ageing, depending on time and temperature [11].
While ageing, supersaturated solution is decomposing to disperse precipitates. Ageing is a key
factor in all heat treatment processes because it allows to obtain mechanical properties even
two times better than in as-cast state.
Natural ageing
Hardening process starts at room temperature after quenching (quenching keeps alloying
elements in supersaturated solid solution). During natural ageing clusters are formed. This
effect can negatively affect properties of the alloy because when clusters are not stable and
critical radius is not attained; fewer coarse precipitates are formed [10]. Nevertheless natural
ageing cannot be omitted during production process and that is why it is generally taken into
consideration while performing research in the field of heat treatment of aluminium alloys.
Artificial ageing
Artificial ageing causes precipitation hardening which is a process where properties of material
can be changed by mean of heat treatment. During precipitation hardening allotrope
transformation does not occur [9]. Artificial ageing process is based on phase separation which
happens in supersaturated solid solution in room temperature or in slightly elevated
temperature.
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Heat-treatable alloys like 7xxx series (primary alloying elements: zinc, magnesium, and
copper) are particularly desirable for aerospace applications as they develop high specific
strength. These aluminum alloys generally undergo thermo mechanical processing in their
production.
In this study, the hardness, tensile properties, impact strength and microstructure of Al 7075
alloy was investigated. This alloy was selected for study based on its promising potential for
use in aerospace, marine and automotive applications.
1.7 Alloy 7075
Aluminum alloys fall into two general categories: heat-treatable and non-heat treatable. Series
7xxx alloys are considered the high strength aircraft alloy family, are heat treatable by solution
and aging. Al alloy 7075 was introduced in 1943 by Alcoa and is primarily an aircraft and
aerospace alloy. 7075 is typically used in applications requiring a combination of high strength
and moderate toughness and corrosion resistance, including aircraft structures, gears and sha fts,
missile parts and various defense equipment. The chemical composition of 7075 is given in
Table 1.2
Concentration [wt%]
Cr Cu Fe Mg Mn Si Ti Zn Al
0.18-0.28 1.2-2.0 0.50 2.1-2.9 0.30 0.40 0.20 5.1-6.1 Balance
Table-2: Chemical Composition of Al 7075 Alloy
7075 Al alloy due to its attractive comprehensive properties such as low density, high strength,
ductility, toughness and resistance to fatigue. It has been extensively utilized in aircraft
structural parts and other highly stressed structural applications.
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CHAPTER – 2
LITERATURE REVIEW
Patricia Mariane Kavalco et. al. (2009) overviewed the inter-granular corrosion (IGC)
process and contrasted it to pitting corrosion. It was shown that the propensity for IGC of heat
treatable aluminum alloys was cooling rate dependent. If an aluminum alloy was slowly cooled
from an elevated temperature, alloying elements precipitate and diffuse from solid solution to
concentrate at the grain boundaries, small voids, on un-dissolved particles, at dislocations, and
other imperfections in the aluminum lattice. For optimal properties, it was desirable to retard
this diffusion process and maintain the alloying elements in solid solution. This was done by
quenching from the solution temperature [1].
Adeyemi Dayo Isadare et. al. (2013) investigated the effects of annealing and age hardening
heat treatments on the micro-structural morphology and mechanical properties of 7075 Al
alloy. It was also found out that age hardening and annealing heat treatment operation
eliminated micro-segregations and improved mechanical properties of 7075 Al alloy. It was
concluded that micro-segregation can be eliminated by rapid solidification and appropriate heat
treatment process [2].
Jasim M. Salman et. al. (2013) studied to improve properties of 7075-T6 such as impact
toughness, thermal age hardening behaviour and corrosion resistance in 3.5% NaCl solution by
using quenching in 30% polyethylene glycol and addition alloying elements, i.e. boron (B) to
this alloy. Results showed improved impact toughness by (30%) when quenching in water, and
by (50 %) when quenching in 30% PAG corresponding to the base alloy at aging temperature
150ºC [3].
Paul A. Rometsch et. al. (2014) described the effects of homogenisation, solution treatment,
quenching and ageing treatments on the evolution of the microstructure and properties of 7xxx
alloys. it was demonstrated how improvements in the microstructure and properties can be
obtained by, 1) dissolving unwanted coarse constituent particles such as the S-phase through
well-controlled high temperature treatments, and 2) controlling the quench rate and factors that
influence the quench sensitivity [4].
Tolga Dursun et. al. (2014) reviewed important recent advances in aluminium aircraft alloys
that can effectively compete with modern composite materials. Recent developments in high
strength Al–Zn and Al–Li alloys, damage tolerant Al–Cu and Al–Li alloys, has been successful
20
in improving the static strength, fracture toughness, fatigue and corrosion resistance through
the design and control of chemical composition, and/or through the development of more
effective heat treatments [5].
Guo Lianggang et. al. (2015) developed true stress–strain curves at temperatures of 300 –
500°C and strain rates of 0.01–10 s -1
by the isothermal compression tests, providing a data
basis for the establishment of hot processing map of as-cast 7075 aluminum alloy. The stable
area with homogeneous grain resulted from dynamic recovery, namely the temperatures at
425–465 °C and the strain rates at 0.01–1 s -1
, was suggested to be the suitable processing
window for the as-cast 7075 aluminum alloy [6].
Ankitkumar K. Shriwas et. al. (2016) reviewed the microstructure of aluminium alloys series
and their emphasis on the mechanical properties were discussed. It was observed that 7075 -T6
aluminium alloy provides good yield strength, ductility and fracture toughness. It was observed
that in 7075 -T6 aluminium alloys energy required for crack propagation is larger than crack
formation. There is large scope for research in the microstructure analysis for 7075 -T6
aluminium alloy [7].
P. Rambabu et. al. (2017) overview of the historical development of aerospace aluminium
alloys, followed by a listing of a range of current alloys with a descriptio n of the alloy
classification system and the wide range of tempers in which Al alloys are used. A description
was given of the alloying and precipitation hardening behaviour, which is the principal
strengthening mechanism for Al alloys. Various aerospace applications of Al alloys were
enlisted. The Indian scenario with respect to production of primary aluminium and some
aerospace alloys, and the type certification process of Al alloys for aerospace applications were
described. Finally there was a critical review of some of the gaps in existing aerospace Al alloy
technologies [9].
21
CHAPTER – 3
PROBLEM FORMULATION
From the literature it is observed that heat treatable aluminum alloys are widely used in aircraft
structural, marine, automotive parts applications and are susceptible to localized corrosion in
chloride environments, such as pitting, crevice corrosion, intergranular corrosion and stress
corrosion cracking (SCC). Pitting is the most common corrosion process encountered with
aluminum alloys, and is a major cause in variations in the grain structure between adjacent
areas on the metal surfaces in contact with a corrosive environment. Intergranular
(intercrystalline) corrosion occurs most commonly in the following aluminum alloys: -
Al-Cu-Mg (2xxx); Al-Mg (5xxx), which is similar to the Al-Cu-Mg alloys; Al-Mg-Si (6xxx);
and Al-Zn-Mg-Cu (7xxx) [10].
During normal operation aircraft and marine ships are subjected to natural corrosive
environments due to humidity, rain, temperature, oil, hydraulic fluids and salt water [11].
High strength aluminium alloys such as the 7075-T6 are widely used in aircraft and marine
structures due to their high strength-to-weight ratio, machinability and relatively low cost.
However, due to their compositions, these alloys are susceptible to corrosion. Fatigue life and
strength properties are always important design data for such structures; therefore an approach
is made for enhancing the strength and corrosive properties of Al 7075 with heat treatment
method using different quench mediums.
Titanium powder and CNTs were chosen as quenchants in addition with oil because of their
high corrosion resistance, optimal mechanical properties in load bearing applications and weak
response to magnetism [12].
22
CHAPTER – 4
RESEARCH OBJECTIVES AND OUTLINE
The goal of this research is to examine the effects of modified solution heat treatment on the
microstructure, mechanical properties, and SCC resistance of Aluminium alloy 7075. This is
done through an experimental program which included heat treating specimens of the alloy to
various metallurgical conditions, a two part testing program with one phase designed to
evaluate the resulting mechanical and physical properties and the other phase to assess the
microstructure and SCC susceptibility under various conditions.
This was accomplished by the following objectives: -
To determine the tensile properties in relation to the effects of different quenching mediums.
To determine the change in impact strength using various quenchants.
An examination of the Stress Corrosion Cracking (SCC) during the heat treatment of
Aluminium alloy 7075.
Analyse the data obtained in terms of the effects of heat treatment parameters on the
Ultimate Tensile Strength, Yield Strength and %Elongation values, employing mathematical
analysis.
Determine the change in hardness during various quenching mediums.
23
CHAPTER – 5
METHODOLGY
5.1 Material Preparation
The present investigation was carried out on Al 7075 alloy. The material was purchased from
Bharat Aerospace Metals, Mumbai in extruded rod shape having diameter 20 mm and length
915 mm (2 lengths). From the raw material, six samples were prepared for tensile test
according to ASTM E8M standard (dia. 12 mm and gauge length 60mm). Lathe machine was
used to make the dumble shape for the tensile test. Similarly, six samples were prepared for
impact test according to ASTM standard E23. Firstly, square shape of 14 mm was made on
lathe machine and then square of 14 mm reduced to 10 mm using grinder according to ASTM
standard followed by v – notch for the final test.
Fig.-1: Preparing samples for Tensile test on lathe machine
24
Fig.-2: Converting rod bar to square for Fig.-3: Preparing samples on Lathe machine.
impact sample on lathe machine.
Fig.-4: Prepared Samples for Heat Treatment
5.2 Heat Treatment of Al 7075 alloy
The prepared samples of aluminium 7075 alloy were subjected to solutionizing treatment at a
temperature of 480°
C for a period of 2 hours using muffle furnace, followed by quenching in
five different quenchants viz; Water, Brine Solution, Oil, Oil + TiO2 , and Oil + CNTs and one
sample was furnace cooled.
25
The Heat treatment was conducted according to the ASTM standard B918 – 01 i.e. the standard
for the heat treatment of aluminium alloys.
Fig.-5: Muffle Furnace used for Experimentation. Fig.-6: Samples placed inside furnace for heating.
5.3 Tensile testing
Tensile testing of all these specimens was conducted per ASTM standards. Six samples were
tested; five were quenched and one was annealed i.e. furnace cooled. The tests were carried out
at room temperature using FIE make Universal Testing Machine, UTE – 20 electromechanical
testing machine. Load – displacement plots were obtained and ultimate tensile strength and
percentage elongation values were calculated from this load – displacement diagrams.
Fig.-7: Broken Samples after Tensile Testing.
26
5.4 Hardness test
The control and the heat treated samples were subjected to the Brinell hardness test. The
specimens were polished to 600 microns and mounted on the machine using a dwell time of 15
seconds. The diameter of the impression left by the ball was then measured using the Brinell
calibrated hand lens and the corresponding Brinell hardness number was determined.
5.5 Impact testing
Impact testing of all these specimens was conducted per ASTM Standard E 23. The tests were
carried out using Charpy impact test method on En Kay Enterprises impact-testing machine.
The amount of impact energy absorbed by the specimen before yielding was read off on the
calibrated scale attached to the machine as a measure of impact strength in Joules.
Fig.-8: Broken Samples after Impact Testing.
27
Fig.-9: Impact Testing Machine. Fig.-10: Specimen for Charpy Impact Test.
Fig.-11: Samples after Testing.
28
CHAPTER – 6
RESULTS AND DISCUSSION
Mechanical properties (hardness, tensile strength, yield strength, elongation and percentage of
elongation) of the treated and untreated samples are determined using standard methods. For
hardness testing, oxide layers formed during heat treatment were removed by stage-wise
grinding and then polished. Average Brinell Hardness Number (BHN) readings were
determined by taking two hardness readings at different positions on the samples and tensile
test using universal testing machine. Impact energy was recorded using the Charpy impact
tester.
6.1 Effect of heat treatment on mechanical properties
The effect of heat treatment (annealing, quenching) on the mechanical properties (ultimate
tensile strength, hardness, percentage elongation, and impact strength) of the treated and
untreated samples is shown in Table 3. The function of annealing is to restore ductility and also
removes internal stresses but its Brinell Hardness Number is less than hardening because here
carbon get more time to react with oxygen in the atmosphere for slow cooling rate. The
function of quenching is to increase the hardness of the specimen and so its Brinell hardness
number is larger than annealing because here carbon cannot get more time to react with oxygen
(for quick cooling rate), so carbon is trapped with the specimen and formed martensite.
Sample Type
Tensile
Strength
(MPa)
%age
change
Brinell
Hardness
(BHN)
%age
change
Impact
Strength
(Joule)
%age
change
Standard 510 --- 150 --- 27 ---
Water
Quenched 515 1% 161 7% 68 152%
Brine solution
Quenched 541 6% 172 15% 91 237%
Oil Quenched 543 6% 165 10% 83 207%
(Oil + TiO2)
Quenched 535 5% 156 4% 52 93%
(Oil + CNT)
Quenched 471 -8% 145 -3% 80 196%
Annealed 220 -57% 71 -53% 8 -70%
Table-3: Percentage Variation of Tensile strength, Brinell hardness, and Impact strength as compared
to the standard Al-7075-T6 alloy.
29
Comparing the mechanical properties of annealed sample with the untreated sample, annealed
sample showed that lower tensile strength (220 MPa), impact strength (8 Joule) and hardness
(71 BHN). The decrease in tensile strength and hardness can be associated with the formation
of soft ferrite matrix in the microstructure of the annealed sample by cooling.
The mechanical properties of the brine quenched sample revealed that it had the highest value
of hardness (172 BHN), impact strength (91 Joule) and tensile strength of 541 MPa were
obtained. The specimen was in muffle furnace at 480°C for 2 hours and then brine quenched.
This treatment increased the tensile strength and hardness but there was massive reduction in
elongation and reduction in area respectively.
6.2 Tensile strength
Fig.-12: Variation of tensile strength of standard specimen compared to the different heat treatments.
From Table-3 and Fig.-1 it can be inferred that, there is only 1% increase in tensile strength
with water quenching, and the maximum value of tensile strength (543 MPa) was achieved by
oil quenching. Drastic decrease in tensile strength (-57%) can be seen in the case of annealing,
this is due to inter-granular corrosion in which upon slow cooling from the solutionizing
temperature, alloying elements precipitate and diffuse from solid solution to concentrate at the
grain boundaries, small voids, on un-dissolved particles, at dislocations, and other
30
imperfections in the aluminium lattice occur as shown in Fig. 2 [1]. Hence annealing is not the
preferred heat treatment process for aluminium alloys.
Fig.-13: Schematic illustration of the solid diffusion processes that may occur during solution heat treatment of aluminium alloy.
Elongation during tensile testing
From the load vs displacement graphs provided by the testing lab, the percentage elongation of
each specimen was calculated. The length of dumble prepared for testing was 220 mm.
Sr. No. Sample type Elongation in length
(mm) %age Elongation
1 Water Quenched 9.6 4.36%
2 Brine solution
Quenched 12.3 5.61%
3 Oil Quenched 14.75 6.70%
4 (Oil + TiO2)
Quenched 14.3 6.5%
5 (Oil + CNT)
Quenched 12.84 5.83%
6 Annealed 18.9 8.6%
Table-4: Percentage Variation of Elongation during Tensile test of Al-7075-T6 alloy.
31
Fig.-14: Variation of elongation during tensile test in the different heat treatments.
From the table-4 and fig.-3, it was observed that the minimum elongation was in the case of
water quenched sample (4.36%), and the maximum elongation was in the case of annealed
sample (8.6%). In the case of oil quenched sample the percentage elongation was highest
among the quenched samples with 6.7% elongation. It is obvious from the results that the
percentage elongation is not that high because quenching makes the material brittle and for
brittle materials the necking phase is diminished.
6.3 Impact strength
Fig.-15: Variation of impact strength of standard specimen compared to the different heat treatments.
32
From table-3 and fig.-4 it was observed that maximum increase of 237% in impact strength in
the case of quenching with brine water, and minimum of 52% increase in impact strength in the
case of quenching with a solution of oil+TiO2. Again a drastic decrease in impact strength was
observed when the sample was annealed.
6.4 Brinell Hardness
Fig.-16: Variation of brinell hardness of standard specimen compared to the different heat treatments.
From table-1 and fig.-4 it was observed that maximum increase in brinell hardness of 15% was
in the case of quenching in brine solution and minimum ( -8%) in the case of quenching in
oil+CNT solution. Annealing of aluminium alloy again results in drastic decrease in hardness
as annealing makes the alloy soft and moreover as discussed earlier that, alloying elements
precipitate and diffuse from solid solution to concentrate at the grain boundaries and small
voids.
33
CHAPTER – 7
CONCLUSION
From the results, it was concluded that mechanical properties of aluminium 7075 alloy can be
customize according to different applications. In our work, we had used five samples for
different quenching mediums i.e. water, brine solution, oil, oil mixed TiO2, oil mixed CNTs
and one sample was annealed. Following points are concluded on the behalf of results:
Tensile strength of the specimen enhances during quenching and decrease drastic to 220
MPa during annealed due to slow cooling.
Quenching is best suited for increasing the hardness of the specimen.
Brine solution had the maximum impact strength of 91 Joule with the hardness of 172
BHN.
Results show that the percentage elongation is not that high because quenching makes
the material brittle and for brittle materials the necking phase is diminished.
34
CHAPTER – 8
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