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ABSTRACT Aluminium alloy 7075 has been widely used in aircraft structural application over many
years. It is, however, prone to both stress corrosion cracking (SCC) and exfoliation
corrosion. Overaging has been used to resolve these problems. However, a strength
penalty of between 10% and 15% is thus incurred. Therefore, a process known as
retrogression and reaging (RRA) treatment, which capable to restore peak strength of T6
temper while retaining resistance to SCC and exfoliation of T7 temper. This project was
set up to study the influents of retrogression and reaging (RRA) treatment and overaging
heat treatments on the mechanical properties and microstructure evolution of this
aluminium alloy. The mechanical properties of aluminium alloy were determined by
using three mechanical tests, which are tensile test, Rockwell hardness test, and
Charpy impact test. From the review of microstructure evolution, the precipitation in
RRA treated aluminium alloy is extremely fine and distributed homogeneously
inside the grains. It is slightly denser and more stable than that resulting from the T6
temper; whilst the grain boundary precipitation is quite different from that resulting
from T6 treatment, the particles being coarser, and much alike to the precipitation
resulting from T7 temper. The retrogression temperature is the main property
controlling factor. Higher retrogression temperature increases the dissolution degree
and promotes the formation of stable precipitates in reaging stage. Aluminium alloy
7075-T6 and aluminium alloy 7075-RRA possess highest yield strength, ultimate
tensile strength, and Rockwell hardness among those heat treated aluminium alloy
7075. However, Charpy impact strength of aluminium alloy 7075-T6 and aluminium
alloy 7075-RRA are lowest among those heat treated aluminium alloy 7075.
Aluminium alloy 7075 (solution hardened) possess lower yield strength, ultimate
tensile strength, and Rockwell hardness compare with aluminium alloy 7075-T6 and
aluminium alloy 7075-RRA. However, aluminium alloy 7075 (solution hardened)
possess highest Charpy impact strength among all types of aluminium alloy 7075 in
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this project. Aluminium alloy 7075-T73 possesses lowest yield strength, ultimate
tensile strength, and Rockwell hardness among all types of aluminium alloy 7075 in
this project. However, aluminium alloy 7075-T73 possesses higher impact strength
compare to aluminium alloy 7075-T6 and aluminium alloy 7075-RRA.
ii
ABSTRAK Aloi aluminium 7075 telah banyak tahun digunakan secara luasnya dalam sektor
pembinaan struktur kapal terbang. Walaubagaimanapun, bahan ini masih mempunyai
satu kelemahan, iaitu lemah terhadap retakan karatan tegasan (SCC) dan kakisan
pengelupasan. Proses rawatan penuaan lebih dapat menyelesaikan masalah tersebut.
Proses rawatan penuaan lebih ini akan menurunkan tahap kekuatan bahan sebanyak
10% hingga 15%. Oleh sebab itu, satu proses rawatan bernama kemunduran dan
penuaan semula (RRA) telah diperkenalkan. Proses rawatan tersebut bukan sahaja dapat
memulihkan kekuatan tinggi yang terdapat dalam bahan bajaan T6, malah ia juga dapat
mempertahankan rintangan terhadap SCC dan kakisan pengelupasan yang merupakan
ciri-ciri bahan bajaan T7. Sifat-sifat mekanik aloi aluminium dapat diuji kaji dengan
menggunakan kajian tarikan, kajian kekerasan bahan, dan kajian hentaman Charpy.
Pengenapan yang berlaku dalam rawatan RRA adalah sangat halus dan disebarkan
secara serata dalam butiran. Enapan ini adalah lebih stabil banding dengan bajaan T6
tetapi batasan butiran adalah berbeza banding dengan bajaan T6. Selain itu, ciri-ciri
zarah yang kasar adalah hampir sama dengan bajaan T7. Suhu adalah factor yang
sangat penting dalam rawatan RRA. Suhu kemunduran yang tinggi dapat meningkat
darjah pelarutan yang banyak memanfaatkan dalam formasi enapan yang lebih stabil
dalam proses penuaan semula. Aloi aluminium 7075-T6 dan aloi aluminium 7075-
RRA memiliki kekuatan alah, kekuatan tegangan muktamad, dan kekerasan yang
paling tinggi banding dengan kebanyakan jenis aloi aluminiun 7075 yang telah
diproseskan dengan rawatan haba. Walaubagaimanapun, kekuatan hentaman aloi
aluminium 7075-T6 dan aloi aluminium 7075-RRA adalah paling rendah banding
dengan kebanyakan jenis aloi aluminiun 7075 yang telah diproseskan dengan
rawatan haba. Aloi aluminium 7075 (yang telah melalui proses pengerasan larutan)
memiliki kekuatan alah, kekuatan tegangan muktamad, dan kekerasan yang lebih
rendah banding dengan aloi aluminium 7075-T6 dan aloi aluminium 7075-RRA.
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Walaubagaimanapun, kekuatan hentaman aloi aluminium 7075 (yang telah melalui
proses pengerasan larutan) adalah paling tinggi banding dengan semua jenis aloi
aluminium 7075 dalam projek ini. Aloi aluminium 7075-T73 memiliki kekuatan alah,
kekuatan tegangan muktamad, dan kekerasan yang paling rendah banding dengan
semua jenis aloi aluminium 7075 dalam projek ini. Walaubagaimanapun, aloi
aluminium 7075-T73 memiliki kekuatan hentaman yang lebih tinggi banding dengan
aloi aluminium 7075-T6 dan aloi aluminium 7075-RRA.
iv
DEDICATION I would like to dedicate this paper to my family because of all the wonderful things
they do for me and supporting me all the way. Besides, I also like to dedicate this
paper to my supervisors Madam Intan Sharhida Binti Othman, and Dr. Mohd Warikh
bin Abd Rashid because they give me guidance and advices throughout the time I
doing this project.
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ACKNOWLEDGEMENT I am grateful for the help and guidance of Madam Intan Sharhida Othman and Dr.
Mohd Warikh bin Abd Rashid throughout this project. Their ability to remain
unruffled in the face of apparent catastrophe has helped to maintain the project on an
even keel. The encouragement and support from my academic advisor, Mr. Jeefferie Abd.
Razak, has also been of the highest value.
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TABLE OF CONTENT Abstract i
Abstrak iii
Dedication v
Acknowledgement vi
Table of Content vii
List of Tables x
List of Figures xi
List of Abbreviations xiii CHAPTER 1: INTRODUCTION
1.1 Background of Project 1
1.2 Problem Statement 2
1.3 Objective 2
1.4 Scope of Study 3
1.5 Importance of Study 3 CHAPTER 2: LITERATURE REVIEW 5
2.1 Heat Treatment of Aluminium Alloy 5
2.1.1 Temper Designation of Aluminium Alloy 6
2.1.2 Solution Heat Treatment 8
2.1.3 Quenching 10
2.1.4 Artificial Aging 13
2.1.5 Overaging 14
2.1.6 Retrogression and Reaging (RRA) Treatment 15
2.2 Aluminium Alloy 7075 17
2.3 Mechanical Properties Test 20
2.3.1 Tensile Test 20
2.3.2 Rockwell Hardness Test 23
2.3.3 Charpy Impact Test 24
vii
2.3.4 Effect of RRA Treatment on Hardness of Aluminium Alloy 25
7075-T6
2.3.5 Effect of RRA Treatment on Tensile Strength of Aluminium 26
Alloy 7075-T6
2.4 Microstructure Evolution of Aluminium Alloy 7075 27
2.4.1 Microstruture Evolution of Aluminium Alloy 7075-T6 Temper 28
2.4.2 Microstructure Evolution of Aluminium Alloy 7075-T73 28
Temper
2.4.3 Microstruture Evolution of Aluminium Alloy 7075-T6 After 29
RRA Treatment
2.5 Relevant Studies or Researches 32 CHAPTER 3: METHODOLOGY
3.1 Introduction 34
3.1.1 Project Flow Chart 35
3.1.2 Materials Preparation 36
3.1.2.1 Aluminium Alloy 7075 Preparation 36
3.2 Heat Treatment Process 37
3.2.1 Solution Hardening 37
3.2.2 T6 Temper 37
3.2.3 T73 Temper 37
3.2.4 RRA Heat Treatment 38
3.3 Mechanical Testing 39
3.3.1 Tensile Test 39
3.3.2 Rockwell Hardness Test 41
3.3.3 Charpy Impact Test 41 CHAPTER 4: RESULT AND DISCUSSION
4.1 Introduction 43
4.2 Observation of Heat Treatment of Aluminium Alloy 7075 43
4.3 Mechanical Testing of Aluminium Alloy 7075 44
4.3.1 Tensile Test for Aluminium Alloy 7075 44
4.3.1.1 Analysis of Tensile Test Graph of Different Heat 49
Treated Aluminium Alloy 7075
viii
4.3.1.2 Analysis of Tensile Test Graph of All Aluminium 51
Alloy 7075-RRA
4.3.2 Rockwell Hardness Test for Aluminium Alloy 7075 52
4.3.3 Charpy Impact Test for Aluminium Alloy 7075 55 CHAPTER 5: CONCLUSION AND RECOMMENDATION
5.1 Conclusion 58
5.2 Recommendation 59 REFERENCES 63 APPENDICES A
APPENDICES B
APPENDICES C
ix
LIST OF TABLE Table 2.1 Basic temper designation (Kaufman, 2000). 6
Table 2.2 Subdivisions of “T” temper heat treatable alloys (Kaufman, 7
2000).
Table 2.3 Chemical composition of aluminium alloy 7075 (ASTM, 18
B211M-03).
Table 2.4 General properties of aluminium alloy 7075, aluminium alloy 18
7075-T6, and aluminium alloy 7075-T73.
Table 2.5 Standard dimensions of 12.5 mm tensile specimen. 22
Table 2.6 Experimental parameter of solution heat treatment, aging, 32
overaging and RRA treatment of relevant studies or researches.
Table 3.1 Label of specimens at various heat treatment processes. 38
Table 3.2 Standard dimensions of 12.5 mm tensile specimen. 40
Table 3.3 Quantity of specimen needed for each experiment. 44
Table 3.4 Quantity of specimen needed for whole project. 44
Table 4.1 Result of tensile test (ultimate tensile strength) for all specimens 47
of aluminium alloy 7075.
Table 4.2 Result of hardness test (Rockwell hardness scale B) for all 55
specimens of aluminium alloy 7075.
Table 4.3 Result of Charpy impact test for all specimens of aluminium 58
alloy 7075.
x
LIST OF FIGURES Figure 2.1 DTA curves at 20oC/min heating for aluminium alloy 7075 9
(Hatch, 1983).
Figure 2.2 Schematic representation of temperature effects on factors that 12
determine precipitation rate (Hatch, 1983).
Figure 2.3 Tensile strength of eight alloys as a function of average 13
cooling rate during quenching (Hatch, 1983).
Figure 2.4 Instron 5585 universal tensile testing machine. 20
Figure 2.5 Important points in stress-strain curve of ductile material. 21
Figure 2.6 Important material properties prediction based on the shape of 22
stress-strain curve of a material.
Figure 2.7 Standard 12.5 mm round tension test specimen (ASTM, E8M- 22
04).
Figure 2.8 Mitutoyo HR-522 series Rockwell type hardness test machine. 23
Figure 2.9 Gunt W400 Charpy impact tester. 24
Figure 2.10 Precipitation sequence of aluminium alloy 70775-T6 (Reda et 26
al. 2007).
Figure 2.11 TEM microstructure of the aluminium alloy 7075-T6 temper 28
(F. Viana et al. 1999).
Figure 2.12 TEM microstructure of the aluminium alloy 7075-T7 temper 29
(Viana et al. 1999).
Figure 2.13 TEM microstructure after retrogression at 200oC (F. Viana et 30
al. 1999).
Figure 2.14 TEM microstructure after RRA (retrogression at 180oC) 31
(F. Viana et al. 1999).
Figure 3.1 Flow chart of whole project. 35
Figure 3.2 Instron 5585 universal tensile testing machine. 40
Figure 3.3 Standard 12.5mm round tension test specimen (ASTM, E8M- 40
04).
Figure 3.4 Mitutoyo HR-522 series Rockwell type hardness test machine. 42
Figure 3.5 Gunt W400 Charpy impact tester. 43
xi
Figure 4.1 Effect of retrogression period (5 minutes, 10 minutes, and 15 48
minutes) on ultimate tensile strength of aluminium alloy
7075-RRA.
Figure 4.2 Effect of different heat treatment process on ultimate tensile 49
strength of aluminium alloy 7075.
Figure 4.3 Tensile test graphs of aluminium alloy 7075 from different 51
heat treatment process.
Figure 4.4 Tensile test graphs of all aluminium alloy 7075-RRA. 53
Figure 4.5 Effect of retrogression period (5 minutes, 10 minutes, and 15 56
minutes) on hardness of aluminium alloy 7075-RRA.
Figure 4.6 Effect of different heat treatment process on hardness of 57
aluminium alloy 7075.
Figure 4.7 Effect of retrogression period (5 minutes, 10 minutes, and 15 58
minutes) on impact strength of aluminium alloy 7075-RRA.
Figure 4.8 Effect of different heat treatment process on impact strength 59
of aluminium alloy 7075.
xii
LIST OF ABBREVIATIONS AA - Aluminium alloy
GP - Guinier-Preston
RRA - Retrogression and reaging
SCC - Stress corrosion cracking
T6 - Temper assignation for aluminium alloy that has been solution
heat treated and artificially aged to achieve precipitation hardening.
T7 - Temper assignation for aluminium alloy that has been solution
heat treated and aged in a furnace to an over-aged condition.
xiii
CHAPTER 1
INTRODUCTION
1.1 Background of Project Pure aluminium is too soft for most of the structural applications. Therefore, it is
usually alloyed with other elements to improve its mechanical properties. Optimum
strength of aluminium can be achieved by alloying and heat treatments, which
greatly promote the formation of small and hard precipitates that interfere with the
motion of dislocations. Aluminium alloy 7075 is an aluminium alloy with zinc as the major alloying element.
It possesses good mechanical properties with good fatigue strength and average
machinability, but it is not weldable and has less resistance to stress-corrosion
cracking than many other aluminium alloys. It is widely used for aircraft structural
materials because it possesses a high strength with low density (Li, J. F. et al., 2007).
However, this material has a problem where it is highly susceptible to stress
corrosion cracking (SCC), especially when it aged to the maximum strength, T6
temper. Therefore, over-aging treatment such as T73 has been developed. However,
the strength of the aluminium alloy 7075 with these over-aging treatments is
decreased. A heat treatment method called retrogression and reaging (RRA) treatment was
devised some time ago by Cina and Ranish (1973), and Cina (1974). Retrogression
and reaging (RRA) is an intermediate heat treatment that able to increase
dramatically the SCC resistance of the aluminium alloy 7075-T6 without sacrificing
its maximum strength. RRA treatment consists of two main stages, retrogressing the
aluminium alloy 7075-T6 structure at high temperature within the two-phase field for
1
a short period, then, follow by reaging the retrogressed aluminium alloy 7075-T6 at
its original T6 temper condition. In this project, experiment is conducted in order to study the effects of the RRA heat
treatment process on the mechanical properties and microstructure evolution of
aluminium alloy 7075-T6.
1.2 Problem Statement In aerospace application, aluminium alloy 7075 is frequently used due to its high
strength to weight ratio. This aluminium alloy prior to be used for structural such as
aerospace applications is typically aged up to T6 temper. The aluminium alloy 7075
in the T6 temper possesses high strength properties but unfortunately it also known
to be highly susceptible to stress corrosion cracking (SCC). The aluminium alloy has
to be over-aged (T73) to solve the SCC resistance problem, however, this over-aging
process reduces the strength of the aluminium alloy by 10-15% compared to the T6
temper. In order to overcome this material properties problem, a heat treatment
known as retrogression and reaging (RRA) will be use to replace the over-aging
process (T73). RRA is an intermediate heat treatment that able to enhance stress-
corrosion cracking resistance without any sacrifice of yield or tensile strength in
aluminium alloy 7075-T6.
1.3 Objective The objective of this Projek Sarjana Muda, PSM are:
i. To study the influent of retrogression and reaging, RRA treatment on
aluminium alloy 7075.
ii. To study mechanical properties and analysis of microstructure of aluminium
alloy 7075.
2
1.4 Scope of Study The project can be divided into three main stages:
i. Material preparation stage
ii. Material properties testing stage
iii. Material microstructure evolution review stage The first stage of this project will covers the preparation of raw material, aluminium
alloy 7075 to become aluminium alloy 7075-T6 through solution heat treatment,
quenching, and artificial aging by using several experimental parameter. Besides that,
it also covers the retrogression and reaging, RRA treatment with several
experimental parameters of respective aluminium alloy 7075-T6 in order to optimize
its mechanical properties. The second stage of this project will covers material mechanical properties testing
through several testing method. The material mechanical properties testing methods
that are cover in this project are Rockwell hardness test, fracture toughness test, and
tensile test. All of these material mechanical properties testing will be conduct based
on ASTM standard. Lastly, the third stage of this project will cover review of microstructure evolution of
aluminium alloy 7075 from relevant journals.
1.5 Importance of Study Aluminium alloy 7075 is a material with high strength to weight ratio. Hence, it is
commonly used by aerospace industry in aircraft’s structural construction (Li, J. F. et
al., 2007). Therefore, study of microstructure evolution and mechanical properties of
aluminium alloy 7075 is very important for aerospace industry in order to produce
high strength and high stress corrosion cracking resistance properties by determine
the optimum experimental parameter of its heat treatment processes. Moreover, it is
3
believed that application of aluminium alloy 7075 will be further extending to other
industrial sector in the future due to its high mechanical and high stress corrosion
cracking resistance properties.
4
CHAPTER 2
LITERATURE REVIEW
2.1 Heat Treatment of Aluminium Alloy The optimum strength of aluminium is achieved by alloying and heat treatments that
promote the formation of small and hard precipitates, which interfere with the motion
of dislocations. Aluminium alloys that can be heat treated to form these precipitates
are considered heat treatable alloys. Pure aluminium is not heat treatable because no
such particles can form while many heat treatable aluminium alloys are not weldable
because welding would destroy the microstructure produced by careful heat
treatment. The initial strength of heat-treatable aluminium alloys is enhanced by the addition of
alloying elements such as copper, magnesium, zinc, and silicon. Since these elements
singly or in various combinations show increasing solid solubility in aluminium with
increasing temperature, it is possible to subject them to thermal treatments that will
impart pronounced strengthening. Virtually all heat treatable aluminum alloys are strengthened by precipitation
hardening. Precipitation hardening involves raising the temperature of the alloy into
the single phase region so that all of the precipitates dissolve. The alloy is then
rapidly quenched to form a supersaturated solid solution and to trap excess vacancies
and dislocation loops which can later act as nucleation sites for precipitation. The
precipitates can form slowly at room temperature (natural aging) and more quickly at
slightly elevated temperatures, typically 100°C to 200°C (artificial aging). The
degree of hardening obtained depends on the size, number and relative strength of
5
the precipitates. These factors are determined by the composition of the alloy and by
the tempering temperature and tempering time.
2.1.1 Temper Designation of Aluminium Alloy The temper designation system of aluminium alloy is used for all forms of wrought
and cast aluminium and aluminium alloys, with the exception of ingot. The temper
designation system is based on the sequence of basic treatments used to produce
various tempers. The temper designation follows the alloy designation with the two
separated by a hyphen. As shown in Table 2.1 below, basic designations consist of a
letter while subdivisions of those basic tempers. Major subdivisions of basic temper
designation are indicated by one or more digits following those letters.
Table 2.1: Basic temper designation (Kaufman, 2000).
Description
Wrought or cast aluminium product made by some
shaping process or casting where there is no special
Fabricated control over the thermal condition during working
or strain-hardening processes to achieve specific
properties.
Wrought or cast aluminium product that has
undergone some shaping process or casting, and
Annealed which product at some point in the process has been
annealed to maximize subsequent workability or
increase toughness and ductility to a maximum.
Only applies to aluminium alloys that age naturally
and spontaneously after solution heat treating Solution heat-treated
(holding at high temperature followed by quenching
or relatively rapid cooling to room temperature).
Non-heat-treatable wrought aluminium alloys that
Strain-hardened have had their strength increased by strain
hardening at room temperature.
6
Temper Designation
F
O
W
H
Most widely used for heat treated alloys, and
Thermally treated to applies to any product form of any heat treatable
produce tempers alloy that has been given a solution heat treatment
other than F, O or H followed by a suitable quench and either natural or
artificial aging. “T” designation denotes a stable temper other that “F”, “O”, and “H”. The “T”
designations are always followed by one or more digits. Each of the numeral is
indicates a specific sequence of basic treatments. In Table 2.2, numerals 1 to 10
indicate specific sequences of the heat treatment process.
Table 2.2: Subdivisions of “T” temper heat treatable alloys (Kaufman, 2000).
Description
Aluminium alloy that has cooled directly from high temperature hot
working process and then naturally aged to a substantially stable
condition.
Aluminium alloy that has been cooled from high temperature hot working
process and then cold worked before being naturally aged to a
substantially stable condition.
Aluminium alloy that has been given a solution heat treatment following
by hot working, quenching, cold working, and being naturally aged to a
substantially stable condition.
Aluminium alloy that has been given a solution heat treatment and without
any cold work, naturally aged to a substantially stable condition.
Aluminium alloy that has been cooled form a high temperature shaping
process and then artificially aged without any intermediate cold work.
Aluminium alloy that has been solution heat treated and without any
significant cold working, artificially aged to achieve precipitation
hardening.
Aluminium alloy that has been solution heat treated and without any
significant cold working, aged in a furnace to an over-aged condition.
Aluminium alloy that has been solution heat treated, cold worked for
strain hardening, and then artificially aged.
7
T
“T” Temper
T1
T2
T3
T4
T5
T6
T7
T8