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

i

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.

iii

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.

v

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.

vi

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