An Investigation Of The Effect Of Various Heat Treatment Processes On Microstructure And Stress Corrosion Cracking (SCC) Of Aluminium Alloy 7075.

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

AN INVESTIGATION OF THE EFFECT OF VARIOUS HEAT

TREATMENT PROCESSES ON MICROSTRUCTURE AND

STRESS CORROSION CRACKING (SCC) OF ALUMINIUM

ALLOY 7075

This report submitted in accordance with requirement of the Universiti Teknikal Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Engineering Materials)

LENG SIONG CHENG B050710018

FACULTY OF MANUFACTURING ENGINEERING 2011

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA

TAJUK: An Investigation of the Effect of Various Heat Treatment Processes on Microstructure and Stress Corrosion Cracking (SCC) of Aluminium Alloy 7075

SESI PENGAJIAN: 2010/11 Semester 2 Saya LENG SIONG CHENG

mengaku membenarkan Laporan PSM ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut: 1. Laporan PSM adalah hak milik Universiti Teknikal Malaysia Melaka dan penulis. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan

untuk tujuan pengajian sahaja dengan izin penulis.

3. Perpustakaan dibenarkan membuat salinan laporan PSM ini sebagai bahan pertukaran antara institusi pengajian tinggi.

4. **Sila tandakan (√)

SULIT

TERHAD

TIDAK TERHAD

(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia yang termaktub di dalam AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)

Alamat Tetap:

PT 292, Jln Kotaville Indah 9, Tmn Kotaville Indah, Wakaf Bharu, 16250 Tumpat, Kelantan.

Tarikh: _________________________

Disahkan oleh:

PENYELIA PSM

Tarikh: _______________________ ** Jika Laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh laporan PSM ini perlu dikelaskan sebagai SULIT atau TERHAD.

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DECLARATION

I hereby, declared this report entitled “An Investigation of the Effect of Various Heat Treatment Processes on Microstructure and Stress Corrosion Cracking (SCC)

of Aluminium Alloy 7075” is the results of my own research except as cited in references.

Signature : ……….

Author’s Name : LENG SIONG CHENG

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APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment of the requirements for the Degree in Bachelor of Manufacturing Engineering (Engineering Materials). The member of the supervisory committee is as follow:

……… Supervisor

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ABSTRAK

Kajian menyeluruh telah dilakukan terhadap aloi aluminium 7075 kerana keunggulannya dalam sifat-sifat mekanikal hasil daripada process penguatan penuaan serta kegunaannya yang meluas dalam struktur kapal terbang. Walaupun dengan kaedah T6, aloi tersebut mampu mencapai kekuatan bahannya yang tinggi, tetapi ketahanannya terhadap SCC adalah rendah. Dengan kaedah T73, ketahanan AA-7075 terhadap SCC dalam aloi ini mampu ditingkatkan , tetapi kekuatan bahannya tidak mampu diperlihara seperti dalam kaedah T6. Kaedah retrogression

dan re-aging (RRA) pula dikatakan mampu meningkatkan ketahanan bahan terhadap

SCC seperti dalam kaedah T73, tetapi dalam masa sama memelihara kekuatan bahan tersebut seperti dalam kaedah T6. Kajian dimulakan dengan pemeriksaan bahan spesimen menggunakan analisa Arc Spark. Spesimen-spesimen ini kemudiannya melalui pelbagai kaedah rawatan haba, iaitu T6, T73 dan RRA, sebelum diuji dalam ujian DTSCC. Ujian DTSCC dijalankan berdasarkan ASTM G49-85 dan ASTM G139-05, dengan pendedahan bahan ke dalam persekitaran mengakis, iaitu dalam 3.5% larutan natrium klorida selama 5 hari bagi spesimen yang dirawat dengan kaedah rawatan haba yang berlainan. Ujian terikan dijalankan menggunakan mesin ujian universal (UTM) sebelum mikrostrukturnya diperhati di bawah mikroskop imbasan elektron (SEM). Kajian menunjukkan spesimen T6 mengandungi kerentanan SCC yang tinggi; manakala T73 mampu memberi ketahanan pemerakahan kakisan stress yang tinggi tetapi tidak mampu memberi kekuatan bahan seperti T6. RRA mampu menghasilkan mikrostruktur campuran T6 dan T73, yang mana ia mampu memberi sifat-sifat gabungan daripada T6 dan T73.

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ABSTRACT

Aluminium alloy 7075 has been widely studied, due to its excellent mechanical properties developed by age hardening and their extensive uses in the aircraft structure. Although T6 tempered alloys are known to possess high mechanical strength, however it has poor resistance to SCC. The method of T73 tempering is applied to overcome the SCC problem, but the strength of the material is sacrificed due to over-aging. On the other hand, retrogression and re-aging is done as the method is claimed able to improve SCC resistance of the alloy but retaining the high mechanical strength as of T6 tempered alloy. Material specimens are checked and determined before heat treatments by the means of arc spark analysis. Specimens which are heat treated with the three different methods, namely T6, T73 and RRA, are then subjected to DTSCC test according to ASTM G49-85 for test preparation and ASTM G139-05 as the test model. The SCC test parameter includes the exposure of material specimens to 3.5% sodium chloride solution as the corrosive environment, with duration of 5days after the each heat treatment process. Tensile test is done by the means of Universal Tensile Machine (UTM) according to ASTM E8-04, follows by microstructure observation under scanning electron microscope (SEM). The study shows that T6 specimen possesses of high susceptibility to SCC, whereas T73 tempering is able to lower the SCC susceptibility but its strength is lowered in the same time. RRA is capable to produce the microstructure of mixture of both T6 and T73 specimens, where provides it the combined properties of both T6 and T73.

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ACKNOWLEDGEMENT

I would like to express my utmost gratitude to Dr. Mohd Warikh Abd Rashid for his all time guidance and supervision upon the execution of my project as project supervisor. I would also like to thank Mr. Ng Guan Yao (Alans) and Mr. Miron Gakim for their greatest support and guidance as senior and mentor, my team mates: Nor Nadiah Abd Hamid, Noorfazidatul Fariha Mustaffa, Nur Fawwaz Asri and Haris Fahaza Ghazali, and the helpful laboratory technicians Mr. Azhar Shah and Mr. Shafarizat. Credits also go to my graduated seniors Mr. Luei Hong Keat, and Ms Tan Kae Shin for providing me some useful references for my project.

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DEDICATION

To my beloved father Mr.

Leng Kok Oar

and mother Mdm.

Teoh Ling Siau

, my beloved eldest sister Pn.

Nur Atiqah Shuhaily Leng

and family,

my beloved second sister Mdm

Leng Siek Ping

, brother-in-law Mr.

Low Ah Kian

, my lovely niece

Celine Low

, and the new-born nephew

Leonard Low

, my beloved brother Mr.

Leng Siong F att

and family, my

respected Dr

Mohd Warikh Abd Rashid

, Mr.

Ng Guan Yao

and Mr.

Kwan Wai Loon

, and my beloved girl-friend Ms

Chee Chew Yen

…

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PAGE

Abstrak i

Abstract ii

Acknowledgement iii

Dedication iv

Table of Contents v

List of Tables viii

List of Figures ix

List of Abbreviations xii

1. INTRODUCTION 1

1.1 Project Background 1

1.2 Problem Statements 2

1.3 Objectives and Aims 5

1.4 Scope of Project 5

2. LITERATURE REVIEW 6

2.1 Aluminium Alloy 7075 6

2.1.1 Aluminium Alloys Designation for Wrought Alloys 8 2.1.2 Temper Designation of Wrought Aluminium Alloys 9 2.1.3 T- designation for Aluminium Alloys 7075 11 2.2 Heat Treatment of Aluminium Alloy 7075 15

2.2.1 Solution Heat Treatment 16

2.2.2 Quenching 16

2.2.3 Precipitation Hardening 17

2.2.4 Heat Treatment to Overcome the SCC Problems in AA-7075

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2.2.6 Retrogression and Re-aging 21

2.27 Microstructure of Aluminium Alloy 7075 22

2.3 Stress Corrosion Cracking (SCC) 26

2.3.1 Mechanism of SCC 27

2.3.2 SCC Occurrence and Environments 29

2.3.3 Environment Causing SCC 30

2.3.4 The Effect of Electrode Potential 31

2.3.5 Alloy Dependence 32

2.3.6 Stress Effect 33

2.3.7 Stress Corrosion Cracking Problem in Aluminium Alloy 7075

2.3.8 Stress Corrosion Testing 35

3. METHODOLOGY 37

3.1 Introduction 37

3.2 Raw Material Specification 39

3.3 Raw Material Characterisation 40

3.4 Heat Treatment Processes 40

3.4.1 T6 Tempering 41

3.4.2 T73 Tempering 43

3.4.3 Retrogression and Re-Aging (RRA) 46

3.5 Direct Tensile Stress Corrosion Cracking Test (DTSCC) 48

3.6 Tensile Test 50

3.7 Hardness Test 50

3.8 Microstructure Observation 51

4. RESULTS AND DISCUSSIONS 53

4.1 Material Characterisation 53

4.2 Microstructure Observation 54

4.3 Tensile Test 58

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vii

5. CONCLUSION AND RECOMMENDATIONS 64

5.1 Conclusions 64

5.2 Recommendations 65

REFERENCE 66

APPENDIX A 68

APPENDIX B 69

APPENDIX C 70

APPENDIX D 73

APPENDIX E 74

APPENDIX F 75

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viii

LIST OF TABLES

PAGE

Table 2.1 Aluminium Alloys Code Designation 9

Table 2.2 Temper Designations 10

Table 2.3 List of T-temper Designations 12

Table 2.4 Results of Mechanical Tests Done on AA -7075 which are Heat Treated via Tempering Methods T6, T73 and RRA

Table 2.5 Examples of SCC Environments 29

Table 3.1 Specification of the Test Specimen (ASTM E8-04) 39

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LIST OF FIGURES

PAGE

Figure 2.1 Example of microstructure of 7075-T6 Aluminium Alloy 15

Figure 2.2 Phase diagram of aluminium-copper binary system 23

Figure 2.3 TEM bright field micrograph of T6 tempered aluminium alloy 7075

Figure 2.4 TEM bright field micrograph of T7 tempered aluminium alloy 7075

Figure 2.5 TEM bright field micrograph of RRA aluminium alloy 7075

Figure 3.1 Flow chart of project execution 38

Figure 3.2 The Geometry of the Specimen (ASTM E8-04) 39

Figure 3.3 Image of the furnace used 40

Figure 3.4 Temperature- time graph for the solution heat treatment process

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Figure 3.6 Temperature-time graph for T6 tempering 42

Figure 3.7 Temperature-time graph for T73 over-aging (Stage 1) 43

Figure 3.8 Temperature-time graph for over-aging (Stage 2) 44

Figure 3.9 Temperature-time graph for T73 tempering 45

Figure 3.10 Temperature-time graph for retrogression 46

Figure 3.11 Temperature-time graph for RRA 47

Figure 3.12 Universal Testing Machine Used 48

Figure 3.13(a) DTSCC test setup 49

Figure 3.13(b) DTSCC test setup, where the NaCl solution is contained in the green rubber tube, with the specimen soaking in the solution

Figure 3.14 Image of hardness testing machine used 50

Figure 3.15 Image scanning electron microscope used 51

Figure 4.1 Microstructure of heat treated of aluminium alloy 7075 57

Figure 4.2 Tensile strength of non corroded heat treated aluminium

alloys 7075

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Figure 4.3 Tensile strength of corroded heat treated aluminium alloy

7075. 60

Figure 4.4 Comparison of tensile strength between the corroded and

non corroded specimens. 61

Figure 4.5 Reduction of tensile strength of heat treated aluminium

alloy 7075 62

Figure 4.6 Hardness results for heat treated specimens of aluminium

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LIST OF ABREVIATION

Al - Aluminium

AA - Aluminium alloy

AH - Age hardening

ASM - American Society of Materials

ASTM - American Standard for Testing of Materials BCC - Body-centre cubic

FCC - Face-centre cubic

HT - Heat treatment

Nd-YAG - Neodymium-doped Yttrium Aluminium Garnet PH - Precipitation hardening

PHT - Precipitation heat treatment RRA - Retrogression and re-aging SCC - Stress Corrosion Cracking SEM - Scanning Electron Microscope SHT - Solution heat treatment

SSRT - Slow Strain Rate Test UTM - Universal Testing Machine XRD - X-Ray Diffractometer

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CHAPTER 1 INTRODUCTION

1.1 Project Background

Aluminum alloy 7075 is a well known type of aluminum alloy used as structural materials of aerospace, transportation, and sports, where the need of lightweight and high strength are needed. Aluminum alloy 7075 is one of the 7000-series aluminum alloys, which captured its reputation in the aeronautical industries due to their attractive comprehensive properties, such as low density, high strength, ductility, toughness and resistance to fatigue (Li et. al, 2007). However, like other 7000-series aluminium alloy, aluminium alloy 7075 is sensitive to localized corrosion such as intergranular corrosion, exfoliation corrosion and stress corrosion. Further enhancement is necessary in order to extend its further applications.

Li et. al (2007) and Reda et al. (2008) stated that the corrosion resistance is

modifiable via the means of heat treatment. Most of the researchers are favorable to study the effect on corrosion resistance of the aluminum alloy 7075 by comparing the other heat treatment processes to the T6 tempering of aluminum alloy 7075. Aluminum alloy 7075-T6 possesses high strength, but it is highly subjected to localized corrosion. Some heat treatment processes such as T73, T74, and T76 are developed to increase their corrosion resistance, especially T73 tempering, which is develop to enhance the resistance of the aluminum alloy 7075 against stress

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corrosion. Besides of T73, another process named retrogression and re-aging or RRA is another heat treatment process which is well known to enhance the corrosion resistance of aluminum alloy.

This project is done to study the effect of various heat treatment processes on the microstructure and stress corrosion cracking of aluminum alloy 7075. Heat treatment is applied on the material specimens to enhance their corrosion resistance against the stress corrosion cracking. Two different types of heat treatment processes, namely T73 tempering and RRA are applied on the aluminum alloy 7075, with T6 tempering as the reference. Direct tensile stress corrosion cracking test is done on the specimens, where later tensile test is done to determine the residual stress of the material after it undergone DTSCC. Besides, the tensile strength of the materials after DTSCC is then compared to the tensile strength of the materials which are not corroded under DTSCC, at which the difference of strength, indicates the occurrence of SCC within the materials. This cause of such properties is explained by relating with the microstructure appearance of specimens.

1.2 Problem Statement

In modern day transportations, as well as aerospace applications, the demand for lighter materials that possess good mechanical properties have become the direction for researches and developments, with the hope to find the best possible material for the structural components that suits the all requirements. Besides the aspect of light weight and mechanical properties, one other important concern in the structural components for these applications is their susceptibility to corrosion, especially when the materials are alloys.

The problem of corrosion is not a fresh issue in metals and alloys, and many solutions had been proposed to overcome corrosion problem. Among all corrosion failures, stress corrosion cracking can be said as one of the important failure cause. Its occurrences are mostly undetectable or not apparent at the first place. Stress

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corrosions cracks are mostly microcracks, where the cracks normally occur at the grain level. This phenomenon happens in all alloys, depending on the environment and stress level, at which the effect and consequence can be catastrophic compared to mechanical cracking. The corrosion takes place on the surfaces of the material, forming surface discontinuities that eventually becomes the stress raiser or notch to crack propagation at microstructural level, but the mechanism of crack propagation in such phenomenon is not merely caused by the atomic dislocation due to the stress, but it is also caused by the chemical attack on the crack tip, causing inter-granular cracks along the grain boundaries of the material. As the cracking happens within the grain boundaries, it is invisible to naked eyes. These inter-granular cracks will then become a mechanical crack when the crack growth achieves a certain crack size that is quite visible to naked eyes. SCC can happen in also all type of environment, but the corrosion rate maybe faster is the material is subjected to its susceptible environment.

7000 series Al alloys have been widely used as structural materials in aeronautical and transportation purposes due to their attractive comprehensive properties, such as low density, high strength, ductility, toughness and resistance to fatigue. Perhaps due to its lower price and cost of manufacturing, 7000 series AA gains higher popularity as compared to a better but more expensive material known to be titanium alloys. 7075 Al alloy is one type of the 7000 series AA. Not only it possesses the same principle alloying element as other 7000 series AA, i.e. zinc, it is also sensitive to localized corrosion, such as inter-granular corrosion, exfoliation corrosion and stress corrosion cracking. (Li et al., 2007)

For about 40 years as since aluminium alloys begin to gain their wide acceptance in the modern day applications, researches and developments in improving the mechanical properties and the resistance to corrosion had been done vigorously. In the studies done by Li et al. (2007) and Reda et al. (2008), the stress corrosion resistance, in the 7000 series aluminium alloys, including aluminium alloy7075 can be modified by heat-treatment. In the heat treatment done for such purposes, the most common heat treatment methods for aluminium alloy 7075 would be over-aging, as in T7 tempering. Although the aluminium alloy 7075 with T6 tempered, possesses high strength, however their localized corrosion resistance is poor. As enhancement

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upon this problem in T6, over-aging treatments such as T73, T76 and T74 have been developed. However, the strength of these over-aging treated aluminium alloy 7075-T7 is relatively poorer than T6. Retrogression and re-aging, RRA is then developed as it produces a balance of properties for both strength and corrosion resistance for the 7000 series Al alloys. (Reda et. al., 2008) However, the RRA treatment cannot be used for large-section Al alloys due to its very short retrogression time.

In this investigation, by applying the three methods of T6, T73 and RRA tempering, the microstructure of the heat treated aluminium alloy 7075 is altered. T73 tempering and RRA are done to produce the microstructure that would resist the stress corrosion cracking, while the T6 tempering is done to produce materials with properties referable by both T73 and RRA. It is not sufficient to compare only in stress corrosion resistance of both heat treatment processes to T6, as in the same time, other mechanical properties are taken into account as well. In this study, it is crucial to identify the effect on the tensile strength and hardness of the material, besides of looking in the performance of the material to resist stress corrosion cracking. Then, both heat treatment processes is compared to identify which one will actually be the best process of giving the material good stress corrosion cracking resistance and mechanical properties. The identification of the best process is done by looking at the formation of microstructure of both heat treatment processes compared to T6, as well as the reduction rate of the tensile strength when subject to stress corrosion. The reduction rate of the tensile strength will eventually provide the indication of the occurrence of stress corrosion cracking. By identifying these measuring keys, it is possible to answer the problem of which is the best process that would provide good stress corrosion cracking resistance, but in the same time having good mechanical properties.

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1.2 Objectives and Aims

The objectives of the investigation include:

i. To relate the microstructure of the heat treated aluminium alloy 7075 with material’s stress corrosion behaviour.

ii. To study the effect of various heat treatment on the mechanical properties of aluminium alloy 7075.

1.4 Scope of Project

The study in this project focuses on the microstructure formed within the material after undergoing heat treatment processes, and how those microstructures would actually affecting the stress corrosion cracking resistance of the material. In this project as well, the DTSCC is done to simulate the stress corrosion condition on the heat treated material, and the effect is shown in the tensile test results. The determination of the stress corrosion cracking occurrence is determine by comparing the tensile strength of both corroded and non corroded specimens. This study will not include the cost of the processes and the effect of heat treatment on the size and geometry of material.

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CHAPTER 2 LITERATURE REVIEW

2.1 Aluminium Alloy 7075

According to Budinski et al. (2005), aluminium and its alloys are the second only to steel in importance in our modern world. With steels, it is possible to make large structures and tools, but aluminium alloys made large structures possible with a lighter weight.

Aluminium alloys are essential engineering materials. The curious aspect of this material is that it is relatively new to our world. This “commodity metal” was made common for about 60 years. Pure aluminium was first produced in the laboratory in 1825, by the reduction of aluminium chloride, and its wide acceptance did not occur until World War II. Nowadays, aluminium alloys are mostly used in aerospace, marine, and transportation applications as the main material for the structural components.

Budinski et al. (2005) stated that aluminium is a good electrical conductor; it is ductile and can be readily cast or machined. It has a face centred cubic structure, as do other „metallic‟ metals, such as copper, silver, nickel and gold. It is lighter than all other engineering metals, except for magnesium and beryllium. It has the density of about 2990kg/ . Aluminium has the conductivity of about 60% IACS. However due to its lower density, aluminium has a higher conductivity than copper per unit

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mass. For example, a 10-mm-diameter aluminium wire will have the same resistivity as a 6-mm-diameter copper wire. The resistivity of the material is just the inverse of the conductivity of the material. However, the aluminium wire is still 13% lighter than the copper wire. This is a vital consideration for long distance power transmission cables.

Aluminium alloys, in some extent, are regarded as corrosion-resist material. However, the corrosion termed in the case of aluminium is regarded as atmospheric corrosion, rather than chemical corrosion. Aluminium is still subjected to electrochemical corrosion. (Budinski et al., 2005)

In overall, aluminium alloys are best described of having some noteworthy advantages like

i. One-third the weight of steel.

ii. Good thermal and electrical conductivity. iii. High strength-to-weight ratio.

iv. Can be given a hard surface by anodizing and hardcoating. v. Most alloys weldable.

vi. Will not rust. vii. High reflectivity. viii. Can be die cast.

ix. Easily machined. x. Good formability. xi. Nonmagnetic. xii. Nontoxic.

xiii. One-third the stiffness of steel.

Aluminium alloy 7075 is an aluminium alloy, with zinc as the primary alloying element, as according to the designation registered by US Aluminium Association. The composition of aluminium alloy 7075 includes 5.1-6.1% zinc, 2.1-2.9% magnesium, 1.2-2.0% copper, and less than half a percent of silicon, iron, manganese, titanium, chromium, and other metals.

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It is strong, with the strength comparable to many steels, and has good fatigue strength and average machinability, but has less resistance to chemical corrosion than many other aluminium alloys. Its relatively high cost limits its use to applications where cheaper alloys are not suitable. It is commonly produced in several heat temper grades, 7075-O, 7075-T6, and 7075-T651.

The first aluminium alloy 7075 was developed by Japanese company Sumitomo Metal in 1936. 7075 was used for the Zero fighter's air frame of the Imperial Japanese Navy in pre-war times. Aluminium alloy 7075 is often used in transport applications, including marine, automotive and aviation applications, due to their high strength-to-density ratio. Its strength and light weight are also desirable in other fields. Rock climbing equipment, bicycle components, and hang glider airframes are commonly made from aluminium alloy 7075. One interesting use for 7075 is in the manufacture of M16 rifles for the American military. It is also commonly used in shafts for lacrosse sticks.

Due to its strength, high density, thermal properties and its polishability 7075 is widely used in mould tool manufacture. This alloy has been further refined into other 7000 series alloys for this application namely 7050 and 7020.

2.1. 1 Aluminium Alloys Designation for Wrought Alloys

Aluminium alloy compositions are registered with The Aluminium Association. However, many organizations published more specific standards for the manufacture of aluminium alloys, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups, and ASTM International. Aluminium Association of United States had designated 4-digit code to the aluminium alloys based on their principal alloying elements. Table 2.1 shows the designation code and the representing principal alloying elements.

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For about 40 years as since aluminium alloys begin to gain their wide acceptance in the modern day applications, researches and developments in improving the mechanical properties and the resistance to corrosion had been done vigorously. In the studies done by Li et al. (2007) and Reda et al. (2008), the stress corrosion resistance, in the 7000 series aluminium alloys, including aluminium alloy7075 can be modified by heat-treatment. In the heat treatment done for such purposes, the most common heat treatment methods for aluminium alloy 7075 would be over-aging, as in T7 tempering. Although the aluminium alloy 7075 with T6 tempered, possesses

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upon this problem in T6, over-aging treatments such as T73, T76 and T74 have been developed. However, the strength of these over-aging treated aluminium alloy 7075-T7 is relatively poorer than T6. Retrogression and re-aging, RRA is then developed as it produces a balance of properties for both strength and corrosion resistance for the 7000 series Al alloys. (Reda et. al., 2008) However, the RRA treatment cannot be used for large-section Al alloys due to its very short retrogression time.

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1.2 Objectives and Aims

The objectives of the investigation include:

i. To relate the microstructure of the heat treated aluminium alloy 7075 with material’s stress corrosion behaviour.

ii. To study the effect of various heat treatment on the mechanical properties of aluminium alloy 7075.

1.4 Scope of Project

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CHAPTER 2

LITERATURE REVIEW

2.1 Aluminium Alloy 7075

According to Budinski et al. (2005), aluminium and its alloys are the second only to steel in importance in our modern world. With steels, it is possible to make large structures and tools, but aluminium alloys made large structures possible with a lighter weight.

Budinski et al. (2005) stated that aluminium is a good electrical conductor; it is ductile and can be readily cast or machined. It has a face centred cubic structure, as do other „metallic‟ metals, such as copper, silver, nickel and gold. It is lighter than all other engineering metals, except for magnesium and beryllium. It has the density of about 2990kg/ . Aluminium has the conductivity of about 60% IACS. However due to its lower density, aluminium has a higher conductivity than copper per unit

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In overall, aluminium alloys are best described of having some noteworthy advantages like

i. One-third the weight of steel.

ii. Good thermal and electrical conductivity. iii. High strength-to-weight ratio.

iv. Can be given a hard surface by anodizing and hardcoating. v. Most alloys weldable.

vi. Will not rust. vii. High reflectivity. viii. Can be die cast.

ix. Easily machined. x. Good formability. xi. Nonmagnetic. xii. Nontoxic.

xiii. One-third the stiffness of steel.

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2.1. 1 Aluminium Alloys Designation for Wrought Alloys

An Investigation Of The Effect Of Various Heat Treatment Processes On Microstructure And Stress Corrosion Cracking (SCC) Of Aluminium Alloy 7075.

UNIVERSITI TEKNIKAL MALAYSIA MELAKA