The Effect Of Different Heat Treatment On Surface Retrogression On Aluminum Alloy 7075

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

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

(Engineering Materials) (Hons.)

by

HAFIZUDDIN BIN ZUHAIMI B050910304

870508035335

FACULTY OF MANUFACTURING ENGINEERING 2013

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA

TAJUK: The Effect of Different Heat Treatment on Surface Retrogression on Aluminum Alloy 7075

SESI PENGAJIAN: 2012/13 Semester 2

Saya HAFIZUDDIN BIN ZUHAIMI

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)

HAFIZUDDIN BIN ZUHAIMI

Alamat Tetap: Lot 717, Kg Resak, Tendong, 17030 Pasir Mas, Kelantan.

Tarikh: 09.06.2013

Disahkan oleh:

PM Dr MOHD WARIKH BIN ABD RASHID

Tarikh: _______________________

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I hereby, declared this report entitled “THE EFFECT OF DIFFERENT HEAT TREATMENT ON SURFACE RETROGRESSION ON ALUMINUM ALLOY

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

Signature : ……….

Author’s Name : HAFIZUDDIN BIN ZUHAIMI

Date : 09.06.2013

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

...………

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ABSTRACT

Aluminum alloy 7075 have been widely used as structural materials in aeronautical industries due to comprehensive properties such as low density and high strength. However aluminum alloy 7075 are susceptible to local corrosion such as stress corrosion cracking. The corrosion resistance of aluminum alloy 7075 can be improved by developing new heat treatment process in order to obtain optimum mechanical properties and susceptibility to corrosion. In the sample preparation, aluminum alloy 7075 will go to solution heat treatment at 470 °C for one hour and followed by cold water quenching. The quenched samples have preheated for age hardening at 120 °C for 24 hours. Then, the surface retrogression and re-aging process has applied to the sample in the T6 condition to improve the resistance to stress corrosion cracking (SCC) and oil is use as medium for retrogression process. Temperature (170˚C, 190˚C and 210˚C) and time (10 minutes and 30 minutes) is the variable that use for the SRRA process. Samples that have done with surface retrogression method then have re-aging at 120 °C for 24 hours. Hardness value of SRRA is 88.5 HRB that reduced 1% compared to T6(89.4 HRB). Maximum stress of SRRA sample is 634.17 N/mm2 that is sacrificed around 3% when compared to T6 (655.53 N/mm2). Corrosion penetration rate when sample not subjected to force for SRRA sample was improved that is 0.003 mpy compare to T73 (0.011 mpy) and when sample is subjected to 55 kN force the corrosion penetration rate show the increased value which SRRA is 0.0020 mpy and T73 is 0.029 mpy. This properties is influence by precipitation of η-phase and η'-phase inside grain and also inside grain boundary.

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ABSTRAK

.Aluminium aloi 7075 telah digunakan secara meluas sebagai bahan binaan untuk industri penerbangan kerana mempunyai sifat yang menarik seperti berkepadatan rendah dan juga berkekuatan tinggi. Namun begitu aluminium aloi 7075 senang terdedah kepada kakisan seperti kakisan tekanan retak. Rintangan terhadap kakisan aluminium aloi 7075 boleh diperbaiki dengan proses rawatan haba yang bertujuan untuk mendapatkan sifat mekanikal yang optimun dan kakisan yang rendah. Dalam penyediaan sampel, aluminium aloi 7075 akan dirawat dengan proses "solution heat treatment" pada suhu 470°C selama 1 jam dan diikuti dengan pelindap-kejutan mengunakan ais. Sampel yang telah terlindap-kejut akan dipanaskan kembali pada suhu 120 °C selama 24 jam. Kemudian, proses SRRA dilakukan pada sampel untuk meningkatkan keupayaan melawan kakisan tekanan retak dengan mengunakan minyak pelincir sebagai medium untuk proses "retrogression". Pembolehubah yang dikawal adalah suhu (170 °C, 190 °C dan 210 °C) dan masa (10 dan 30 minit). Sampel kemudian dipanaskan kembali utuk proses penuaan selama 24 jam pada suhu 120 °C. Nilai kekerasan SRRA adalah 88.5 HRB iatu pengurangan sebanyak 1% berbanding T6(89.4 HRB). Niali tegasan maksimun SRRA adalah 634.17 N/mm2 iatu pengurangan sebanyak 3% berbanding T6(655.53 N/mm2). Kadar penembusan kakisan apabila sampel SRRA tidak dikenakan daya semakin baik iatu 0.003 mpy bila dibandingkan dengan T73(0.011 mpy) dan apabila daya dikenakan sebanyak 55 kN pada sampel, ia menunjukkan kadar penembusan kakisan meningkat iatu untuk SRRA adalah 0.0020 mpy dan T73(0.029 mpy). Sifat-sifat ini adalah kesan pengaruh pemendakan fasa-η dan fasa-η' ke dalam bijiran dan juga sempadan bijiran.

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DEDICATION

To my father; Zuhaimi Bin Zakaria, my mother; Norma Binti Ismail, my siblings and my friends. Your love is my driving force.

To my supervisor, PM Dr. Mohd Warikh Bin Abd Rashid

To Alans Ng that mostly guide me in this project

Thank you for all your supports, guidance, helps and co-operation, directly or in directly.

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ACKNOWLEDGEMENT

Praised to Allah S.W.T, with His grace we are able to complete this Final Year Project (FYP) and also this report. First and foremost, thanks to God for giving us an opportunity in completing the whole system throughout the period. We would like to highly grateful for those who have helped us from the beginning until the project accomplished.

Furthermore, I would like to extend my personal appreciation to PM Dr. Mohd Warikh Bin Abd Rashid for his guidance, concerns and patience in leading me the way and not to be forgotten to Mr. Alans Ng who is master student, to other lecturers, my friends, technician and other person whose name is not mention here.

Last but not least, sincere thanks to our parents that have given us strength from many sides. We really appreciate how much all of you have helped us in our life that had brings us here. Thanks you also to all who have had directly and indirectly involved in completing this project

Thank you. .

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TABLE OF CONTENT

Abstract i

Abstrak ii

Dedication iii

Acknowledgement iv

Table of Content v

List of Tables ix

List of Figures xi

List of Abbreviations, Symbols and Nomenclatures xvi

CHAPTER 1: INTRODUCTION

1.1 Background of This Project 1

1.2 Problem Statement 3

1.3 Objectives 5

1.4 Scope of This Study 5

CHAPTER 2: LITERATURE REVIEW

2.1 Introduction Heat Treatment of Aluminum Alloy 7

2.1.1 Aluminum Alloy Designation 9

2.1.2 Temper Designation 11

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2.1.3 Aluminum Alloys 7xxx Series 12 2.1.4 Precipitation Hardening of Aluminum Alloy 13

2.1.5 Solution Heat Treatment 15

2.1.6 Quenching 15

2.1.7 Aging 16

2.1.8 T6 and T7 17

2.1.9 Retrogression and Re-Aging 19

2.2 Mechanical Properties 21

2.2.1 Hardness Properties 22

2.2.2 Tensile Strength 25

2.3 Microstructure Analysis 26

2.3.1 Optical Microscope 27

2.4 Corrosion in Aluminum Alloys 7075 28

2.4.1 Stress Corrosion Cracking (SCC) 29

2.4.2 Electrochemical Testing 30

CHAPTER 3: METHODOLOGY

3.1 Introduction 33

3.2 Flowchart of Overall Process 34

3.3 Material Preparation 35

3.3.1 Specimen Specification 35

3.4 Solution Heat Treatment Process & Quenching 36

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3.4.1 T6 Tempering 38

3.4.2 T73 Tempering 40

3.4.3 Retrogression & Re-Aging 41

3.5 Mechanical Testing 43

3.5.1 Tensile Test 43

3.5.2 Hardness Test 44

3.6 Microstructure Analysis 46

3.6.1 Optical Microscope 46

3.7 Corrosion Testing 49

3.7.1 Stress Corrosion Cracking Test 49

3.7.2 Electrochemical Testing 50

CHAPTER 4: RESULT AND DISCUSSION

4.1 Designation for Heat Treated Aluminum Alloy 7075 52

4.2 Mechanical Properties Analysis 53

4.3 Electrchemical Testing Analysis (Without Force) 61

4.4 Stress Corrosion Cracking Analysis 65

4.5 Effect of Different Heat Treatment on Microstructure 69

4.6 Rating Analysis 73

CHAPTER 5: CONCLUSION & RECOMMENDATION

5.1 Conclusion 76

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5.2 Recommendation 77

References 78

Appendix

A Gantt Chart

B Drawing of Specimen C Microstructure

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LIST OF TABLES

Table Title Pages

2.1 Designation and Attributes For Aluminum 9

Wrought Alloys

2.2 Temper Designation of Aluminum Alloys 11

2.3 Heat Treated Subdivisons 12

2.4 Chemical Composition Alloying Elements in 13

Aluminum Alloy 7075

2.5 Properties of T7 Which Different Aging 17

Time

2.6 Comparison of Hardness Tests 23

2.7 The Scale of Microstructural Features 27

2.8 Advantages and Disadvantages Tafel 32

Extrapolation Method

3.1 Specification of The Specimen 36

3.2 Keller's Etch Composition. ` 47

4.1 Total Anodic Current, Corrosion Current Density, 64 Corrosion Potential and Corrosion Penetration Rate

for Different Heat Treated Sample (Without Force)

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4.2 Total Anodic Current, Corrosion Current Density, 66 Corrosion Potential and Corrosion Penetration Rate

for Different Heat Treated Sample (With Force)

4.3 Rating for Different Heat Treated Sample of 74 Aluminum Alloy 7075

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LIST OF FIGURES

Table Title Pages

2.1 Schematic Phase Diagram For Age Hardenable 8 Alloy

2.2 Comparison of Phase Present In Al-Mg-Zn 8

Alloy, Solution Treated and Aged at 120 °C and 175 °C

2.3 Step of Precipitation Hardening 13

2.4 Sequence Precipitation 7075 Aluminum Alloy 14 2.5 Effect of Aging Time on Yield Strength 7xxx alloy. 17

2.6 TEM Microstructure of T6 18

2.7 TEM Microstructure of Over-Aged T7 Temper 19

2.8 Schematic of Retrogression and Re-Aging 20

2.9 TEM Microstructure After Retrogression at 200 ˚C 21

2.10 TEM Microstructure After RRA 21

2.11 Variation in Rockwell Hardness (B Scale) as a 24 Function of Retrogression Time of Samples

Subjected to Retrogression and of Samples Subjected to RRA Treatment at Various Retrogression

Temperatures

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2.12 Mechanical Properties of Alloy After 100 °C Aging 26 Treatment.

2.13 Principle of Magnification in a Microscope 28 2.14 Graphical Representation of Butler-Volmer 31

Relationship Between Potential and Current in a Mixed Potential System

3.1 Flowchart of Overall Process 34

3.2 Aluminum Alloy 7075 35

3.3 Graph of SHT & Quenching Time Vs Temperature 37

3.4 Sample in The Furnace 37

3.5 Specimen Quench in Ice Water 38

3.6 Flowchart of Solution Heat Treatment and 38

Quenching Process

3.7 Aging Graph Which is Time Versus Temperature 39

3.8 Aging in Mechanical Oven 39

3.9 Flowchart of Aging Process 40

3.10 Two Stage Aging Process Which is Time Versus 41 Temperature

3.11 Flowchart of Two Stage Aging 41

3.12 Surface Retrogression Process For Heat Treated 42 the Sample

3.13 Three Sample Have Done Surface Retrogression 42 Process

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3.14 Flowchart of Surface Retrogression and 43 Re-Aging Process

3.15 Tensile Testing by Using Shimadzu AG-1 44

Universal Testing Machine

3.16 Flowchart of Tensile Test 44

3.17 Hardness Testing 45

3.18 Indentation Hole at Head of Sample 45

3.19 Flowchart of Hardness Test 46

3.20 Sample Cut by Using Diamond Cutter Machine 47

3.21 Specimen After Done Sample Preparation 48

3.22 Optical Microscope 48

3.23 Flowchart for Microstructure Observation by 48 Using OM

3.24 SCC Test Setup, Where Forced Applied to the 50 Specimen in the 3.5 wt% NaCl Solution

3.25 Flowchart for SCC Test 50

3.26 Electrochemical Corrosion Test Setup 51

3.27 Sample Was Tape by Using Masking Tape 51

3.28 Flowchart of Electrochemical Test 51

4.1 Example of Specimen Declaration 52

4.2 HRB Values for T6 Treated Sample 54

4.3 HRB Values for T73 Treated Sample 54

4.4 HRB Values for SRRA 171 Treated Sample 55

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4.10 HRB Values for Different Heat Treated Sample 58

4.11 Maximum Stress Value for Different Heat 60

Treatment Process of Aluminum Alloy 7075

4.12 % Elongation Before Break for Different Heat 60 Treatment of Aluminum Alloy 7075

4.13 Tafel Extrapolation Graph for T6 and T73 Heat 62 Treated Sample.

4.14 Tafel Extrapolation Graph for SR171 and SR173 62 Heat Treated Aluminum Alloy 7075 Sample

4.17 Tafel Extrapoaltion Graph for T6 and T73 Heat 66 Treated Aluminum Alloy 7075 With Force

4.18 Tafel Extrapolation Graph for SR171 and SR173 67 Heat Treated Aluminum Alloy 7075 With Force

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4.21 OM Microstructure (200x) of T6 Temper 70 4.22 OM Microstructure (200x) of T73 Temper. 70

4.23 OM Microstructure (200x) of SR171 71

4.29 Point to Take Temperature During Surface 75 Retrogression Process

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LIST OF ABBREVIATIONS, SYMBOLS AND

NONENCLATURES

AA - Aluminum Alloy

Al - Aluminum

ASTM - American Society for Testing and Material

Cr - Chromium

Cu - Copper

Fe - Ferum

GP - Guinier Presston

HNO3 - Nitric Acid

Mg - Magnesium

Mn - Manganese

MPa - Mega Pascal

OM - Optical Microscope

RRA - Retrogression and Re-Aging

Sc - Scandium

SCC - Stress Corrosion Cracking SEM - Scanning Electron Microscope SHT - Solution Heat Treatment

Si - Silicon

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SRRA - Surface Retrogression and Re-Aging TEM - Transmission Electron Microscope

Ti - Titanium

UTM - Universal Testing Machine

Zn - Zinc

Wt% - Weight percent

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CHAPTER 1 INTRODUCTION

1.1 Background of This Project

Aluminum is one of most prevalent metallic elements in the solid portion of the earth's crust, comprising approximately 8%. It is always present in a combined form, usually a hydrated oxide, of which bauxite is the primarily ore (Schweitzer 2007). Aluminum also have many outstanding attributes that lead to a wide range applications including; good corrosion and oxidation resistance; high electrical and thermal conductivity; low density; high reflectivity; high ductility and reasonable high strength; and relatively low cost (Campbell 2008).

Aluminum alloy 7075, with nominal composition of 5.6% Zn, 2.5% Mg, 1.6% Cu, 0.3% Cr, has one of the highest attainable strengths of all aluminum alloys. Aluminum alloy 7075 have been widely used in as material in aeronautical, sports and transportation due to comprehensive properties such as low density, ductility, high strength, toughness and also their resistance to fatigue. However, Aluminum alloys 7075 have several failure such as sensitive to localized corrosion such as exfoliation corrosion, inter-granular cracking and also stress corrosion cracking (SCC) same as another 7xxx-series. (Li et al., 2008). Corrosion is a major concern involving the structural integrity of aircraft structure.

Heat treatment T6 treatment is the process to improved corrosion resistance of Aluminum Alloys 7075 that posses high strength but the ability to localized corrosion resistance is very weak. To overcame this problem, over-aging treatment is develop such as T73, T76 and T74. Aluminum alloys 7075 have new ability which

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are high strength and also high stress corrosion resistance after do retrogression and re-aging. Usually heat treatment for Aluminum alloy 7075 is T6, T73 and RRA by using furnace. But sometimes oil is used as medium for treating AA 7075. Specimen is put in the oil and then heat the oil.

In order to improve the compromise between mechanical strength and corrosion resistance, it has been proposed to replace the traditional two-step heat treatment of these alloy by a three-step heat treatment called RRA which has been shown to offer a stress corrosion resistance as good as that T73 and offering a strength comparable of that T6 temper. (Marlaud et al., 2010).

Several studies reported that the main microstructural changes during retrogression are partial dissolution of GP zones and fine η' MgZn2 precipitated in the aluminum

matrix grains that are re-precipitated during the re-aging process, while the η MgZn2

precipitated in the grain boundaries is allowed to form and grow. The microstructure result from RRA is fine and homogenous distributed η' MgZn2 precipitated in the

aluminum matrix grains and η MgZn2 precipitates in the grain boundaries. This

combination results in good performance on both SCC and mechanical strength. (Yan et al., 2011).

This project is to study about the effect of different heat treatment on surface retrogression on Aluminum alloy 7075. Three different type of heat treatment process, namely T6, T73 and Surface Retrogression are applied on aluminum alloy 7075. For observe microstructure after heat treatment process, Optical Microscope (OM) and Scanning Electron Microscope is done to Aluminum Alloy 7075. Tensile test by using hardness test (Rockwell) is applied to determine the mechanical properties of the specimen.

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1.2 Problem Statement

The aluminum alloy 7075-T6 has been widely used for structural application in many aircraft design. The addition of Cu greatly improve the mechanical strength of this alloy by precipitation hardening. Meanwhile, 7075 alloys are susceptible to stress corrosion cracking (SCC) particularly when stress through thickness. SCC of 7075-T6 has frequently occur in service. To combat the SCC problem, a number of T7 temper have been develop for 7xxx alloys but in comparison to 7075-T6, there is strength reduce around 10-15%. An alternative is to perform the retrogression and aging (RRA) heat treatment on part purchased in T6 condition. Retrogression and re-aging (RRA) is heat treatment process performed on aluminum alloy 7075 in the T6 temper condition to improve its resistance to corrosion while at same time maintaining the high strength level required for aircraft structure application. There is small lost in strength, e.g. the RRA yield strength is typically 515 MPa compared to 530 MPa for same material in T6 condition. The corrosion resistance measured by stress corrosion cracking (SCC) are significantly better than T6 condition and approach that over-aging T73.

One of the problem with 7075 and similar alloys when they are heat treated to the peak-aged T6 temper has been stress corrosion cracking (SCC). Thick plate, forging and extrusions of these alloys are particularly vulnerable when stress in the through-the-thickness (short transverse) direction. In response to these in-service in failure, a number of over-aged T7 temper have been develop but there is some sacrifice in strength properties. Additional over-aged temper T77 have been develop that provide strength, SCC and exfoliation corrosion resistance. This tamper is a variation of a treatment called retrogression and re-aging. (Campbell, 2008).

According to Yan et al., (2011), adding Sc as alloying element can impart an attractive combination of strength, ductility and enhance crack growth resistance. These property improvement are attributed to Al3Sc dislocation interactions.

However, the poor corrosion resistance of aluminum alloy 7075 has not been fully overcome by adding Sc to aluminum alloy 7075. Some literatures reported that SCC resistance could improved by over-aging treatment but loss strength around 10-15 %.