An Investigation Of Fracture Toughness On Heat Treated Aluminum Alloy 7075 With Effect Of Corrosion.

(1)

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

AN INVESTIGATION OF FRACTURE TOUGHNESS ON HEAT

TREATED ALUMINUM ALLOY 7075 WITH EFFECT OF

CORROSION

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

(Engineering Materials) with Honours.

SITI KHADHIJAH BINTI MOHAMAD

FACULTY OF MANUFACTURING ENGINEERING 2009/2010

(2)

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA

TAJUK: AN INVESTIGATION OF FRACTURE TOUGHNESS ON HEAT TREATED ALUMINUM ALLOY 7075 WITH EFFECT OF CORROSION

SESI PENGAJIAN: 2009/10 Semester 2 Saya SITI KHADHIJAH BINTI MOHAMAD

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.

√ 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: 193 Jalan Melati, 23000 Dungun,

Terengganu Darul Iman. Tarikh: 25 May 2010

Disahkan oleh:

Cop Rasmi:

Tarikh: 25 May 2010

** 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.

(3)

DECLARATION

I hereby, declared this report entitled “An Investigation of Fracture Toughness on Heat Treated Aluminum Alloy 7075 With Effect of Corrosion” is the results of my

own research except as cited in references.

Signature : ………

Author’s Name : Siti Khadhijah Binti Mohamad

(4)

APPROVAL

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) with Honours. The member of the supervisory committee is as follow:

……… (Principal Supervisor) (Mr Mohamad Haidir Bin Maslan)

(5)

ABSTRACT

The aim of this project is to investigate the fracture toughness of heat treated Aluminum Alloy 7075 (AA7075) with effect of corrosion. As we know, characteristics of Aluminum are soft, light weight, and high corrosion resistant, but low strength. In this research, AA7075 is used as raw material. This AA7075 will be heated to T6, T73, and retrogression and re-aging (RRA) at proper temperature. After heat treatment, several specimens will be corroded by doing immersion in chemical solution corrosion process. But the important thing is we need to record the reading of hardness of each specimen before and after corrosion process. Finally, the specimen will be test by doing fracture toughness test, hardness test, and microstructure observation. The new properties of AA7075 are developing by heat treatment. We can compare the hardness, microstructure of phases and fracture surface and then decide which process is better to improve and build an excellent properties for future application.

(6)

ABSTRAK

Tujuan utama kajian ini adalah untuk mengkaji kekuatan kegagalan pada aloi aluminium 7075 (AA7075) yang telah diubahsuai sifat semulajadi melalui kaedah pemanasan haba dan dikenakan kesan pengaratan pada bahan. Seperti yang diketahui umum, sifat asli aluminium adalah lembut, ringan, dan mempunyai ketahanan kesan karat yang tinggi, tetapi memimiliki sifat daya tahan yang lemah. Di dalam kajian ini, AA7075 digunakan sebagai bahan utama kajian. AA7075 dipanaskan untuk mebghasilkan sampel ujikaji yang berlainan suhu, dan masa pemanasan. Selepas proses pemanasan, sampel yang telah siap akan didedahkan pada proses pengaratan melalui kaedah rendaman dalam larutan kimia. Perkara yang penting adalah bacaan kekerasan bahan perlu dicatat sebelum dan selepas proses pengaratan. Akhir sekali, sampel akan dianalisis melalui ujian kekuatan kegagalan, ujian kekerasan, dan pemerhatian struktur bahan. Sifat baru bahan akan dihasilkan melalui kaedah pemanasan haba. Di akhir kajian, kita dapat membuat perbandingan pada sifat kekerasan bahan, susun atur struktur bahan, permukaan kegagalan bahan, dan sifat bahan itu sendiri, yang telah diubahsuai dan akan digunakan untuk aplikasi pada masa akan datang

(7)

iii

ACKNOWLEDGEMENT

First and foremost, I’m so grateful to Allah S.W.T for giving me a chance, time, and

strength in finishing my Projek Sarjana Muda (PSM) technical report. Special thanks to my supervisor, Mr. Mohamad Haidir Bin Maslan for his full commitment, guidance, valuable contributions to advice, and suggestion in carrying out the technical knowledge to complete this report. Then, I would like to extent my sincere appreciation to Mr. Sivarao a/l Subramoniam as the PSM Coordinator of Faculty Manufacturing Engineering of University Technical Malaysia Malacca (UTeM). Lastly, I would like to express my highest gratitude and appreciation to all individual, who have contributed a lot towards the success of my PSM especially to my parents, and my fellow friends in UTeM especially to my classmates. Thank you for the support, guidance, and contribution to the achievement of my PSM.

(8)

LIST OF TABLES

2.1 Mechanical Properties of Aluminum 5

2.2 Alloy Series and Their Principle Alloying Element 6

2.3 Combination of Elements of Alloy in Aluminum Alloy 7075 8

2.4 Properties of Aluminum Alloy 7075 9

2.5 Basic Temper Designation System 11

2.6 Types of Heat Treatable Aluminum Alloys 12

2.7 Values of Fracture Toughness 23

2.8 Different Types Of Atmosphere Affect Corrosion Rate 28

3.1 Mechanical Properties of Aluminum Alloy 7075 32

3.2 List of Samples 36

4.1 Result of Tensile Strength of AA7075 47

(9)

LIST OF FIGURES

2.1 Steps of ductile material failure, extensive plastic deformation takes place before fracture

2.2 Fractograph of Ductile Material 21

2.3 Fractograph of Brittle Material 21

2.4 Fracture Of an Aluminum Crank (brittle) 21

2.5 Ductile Failure Of A Specimen Strained 22

2.6 Applying A Force to Enable a Crack propagate 23

2.7 Specimen for Fracture Toughness Test 24

2.8 Reaction and Fracture Direction during Fracture Toughness Test 24

2.9 Reaction between water, rust, and oxygen create pitting corrosion 25

2.10 Corrosion rate influence the aggressive environment and temperature 27

3.1 Flow Chart of Research Design 31

3.2 Dimension of Specimen for heat treatment and corrosion process. 33

3.3 Flow Chart Process for Heat Treatment 34

3.4 Immersion Sample in Chemical Solution 37

3.5 Surface of Sample after Corrosion Process. 38

3.6 Sample for Fracture Toughness Test 39

3.7 Dimension of Specimen for Toughness Test 40

3.8 Principal Types of Load Displacement for Toughness Test 41

3.9 Direction of Specimen During Testing 42

(10)

LIST OF FIGURES

3.11 Direction of Gage During The Measurement 43

3.12 Vickers Hardness Testing Machine for Measuring Hardness 44

3.13 Optical Microscope 45

4.1 Graph Of Fracture Toughness of AA7075 49

4.2 Comparison Graph of Hardness Value between Samples in Different Temperature of Heat Treatment

4.3 Comparison Graph of Hardness Value of Across Material between Samples in Different Temperature of Heat Treatment

4.4 Comparison Graph of Hardness Value of Surface Material between Samples in Different Temperature of Heat Treatment

4.5 Microstructure of A1 Sample, As a Received Material of AA7075 55

4.6 Microstructure of A3 Sample, As a T6 Sample of AA7075 56

(11)

vii

LIST OF ABBREVIATIONS, SYMBOLS,

SPECIALIZED NOMENCLATURE

UTeM - University Technical Malaysia Malacca AA7075 - Aluminum Alloy 7075

RRA - Retrogression and Re-Aging Treatments SCC - Stress- Corrosion Cracking

Al - Aluminum

Zn - Zinc

Mg - Magnesium

Cu - Copper

FCC - Face Centered Cubic

SSSS - Supersaturated Solid Solution

GP - Guinier-Preston

CT - Compact Test

SENB - Single Edge Notch Bend

Cl - Chloride

SEM - Scanning Electron Microscope UTM - Universal Tester Machine

ASTM - American Society for Testing and Materials LEFM - Linear-Elastic Fracture Mechanics

EPFM - Elastic-Plastic Fracture Mechanics

KIc - Toughness

K - Stress Intensity

MPa - Mega Pascal

m - meter

nm - nanometer

(12)

viii

LIST OF ABBREVIATIONS, SYMBOLS,

SPECIALIZED NOMENCLATURE

EXCO - exfoliation corrosion NaCl - sodium chloride KNO3 - potassium nitrate

(13)

Abstract i

Acknowledgement ii

List of Tables iii

List of Figures iv

List Of Abbreviations, Symbols, Specialized Nomenclature vi

Table Of Contents vii

CHAPTER 1 : INTRODUCTION

1.1 Introduction 1

1.2 Problem Statement 2

1.3 Objectives 3

1.4 Scope 3

CHAPTER 2 : LITERATURE REVIEW

2.1 Introduction of Aluminum 4

2.1.1 Aluminum Alloys 5

2.1.2 Aluminum Alloys 7075 7

2.2 Heat Treatment 10

2.2.1 Temper Designation System 11

2.2.2 System For Heat Treatable Aluminum Alloys 12

2.2.2.1 Solution Heat Treatment 13

2.2.2.2 Quenching 14

2.2.2.3 Age Hardening 15

(14)

2.3 Fracture Mechanics 18

2.3.1 Linear-elastic fracture mechanics (LEFM) 19

2.3.2 Fractography 20

2.3.2.1 Fracture Toughness 22

2.4 Corrosion 25

2.4.1 Corrosion in Aluminum Alloy 7075 26

2.4.2 Stress Corrosion Cracking of Aluminum Alloys 28

CHAPTER 3 : METHODOLOGY

3.1 Introduction 30

3.2 Research Design 31

3.3 Material Selection 32

3.4 Sample preparation 33

3.5 Heat Treatment 34

3.5.1 T6 temper 35

3.5.2 RRA Treatment 35

3.5.3 List of Specimen 36

3.6 Corrosion 37

3.7 Notch Preparation 39

3.8 Testing And Analysis 40

3.8.1 Fracture toughness 40

3.8.2 Hardness 44

3.8.3 Microscope Observation 45

(15)

CHAPTER 4 : RESULT AND DISCUSSION

4.1Fracture toughness properties of heat treated AA7075 46 4.1.1Tensile Strength properties of AA7075 47

4.1.2Fracture toughness data 48

4.2Hardness properties of AA 7075 50

4.2.1Hardness properties of heat treated and corroded AA 7075 51

4.3Microstructure of Aluminum Alloy 7075 54

4.3.1 Microstructure of heat treated Aluminum Alloy 7075 55

CHAPTER 5 : CONCLUSION AND RECOMMENDATIONS

5.1 Conclusions: 58

5.2 Recommendations 61

REFERENCES 63

APPENDICES

A: Gantt Chart 69

(16)

CHAPTER 1 INTRODUCTION

In this chapter, it will present briefly about heat treated Aluminum Alloy 7075, and problem that influence on AA7075 in aircraft application. We also briefly explain about the objectives and scope of this research.

1.1 Introduction

Aluminum is soft and lacks strength, but it can be alloyed with small amounts of copper, magnesium, silicon, manganese, and other elements to impart a variety of useful properties. It is much lighter and more corrosion resistant than plain carbon steel, but not as corrosion resistant as pure aluminum. The Aluminum Alloy 7075 (AA7075) in the peak aged heat treatment condition T6 has been widely used for structural applications in many aircraft. T6 temper was produced high strength of AA7075 but poor in corrosion resistant. Corrosion damage is often the reason why 7075-T6 components are replaced in older aircraft. The retrogression and re-aging treatments (RRA) was able to provide good corrosion protection and an excellent of strength (J. P. Auger, 1976). After heat treatment, the property of AA7075 was change in terms of structural, characteristics, and others.

(17)

1.2 Problem Statement

On 1988, incident on Boeing 737-200 is on an internal flight in Hawaii suffered sudden structural failure. The mechanism by which fatigue propagates in a structure is the well known crack. Cracks propagate because the geometry of a crack produces a very high concentration of stress at the end of the crack and eventually, if a growing crack goes undetected, fracture will occur. Nearly 6 meters of cabin skin and structure aft of the cabin entrance door and above the passenger floor line separated from the aircraft. The subsequent investigation found that the bonding and fatigue damage which had led to the failure. The aircraft involved had completed 89,680 flight cycles with an average flight time of only 25 minutes, almost all of them in the marine environment of the Hawaiian Islands, where a typical service life which was considered to have allowed corrosion to increase the failure rate. The factors that affect the corrosion behavior included the temperature, pressure, metallurgy, velocity and galvanic effects. In general, corrosion rates increase with increasing temperature or pressure. The retrogression and re-aging treatments can be improving the SCC of Aluminum Alloy 7075 at the same time it can be maintain the mechanical property of T6 temper Aluminum Alloy 7075.

(18)

1.3 Objectives

The objectives of this research are to

(a) Investigate the fracture toughness of heat treated AA7075 with effect of corrosion.

(b) Investigate the effect of heat treatment T6 and retrogression and re-aging treatments on fracture toughness.

(c) Correlate the morphological characteristics at Aluminum Alloy 7075 exposed to the different types of heat treatment and corrosion environment to the fracture toughness.

1.4 Scope

The scope and limitation of this project is we only used Aluminum Alloy 7075 as our raw material. We apply heat treatment process by using furnace as our main equipment to complete heating process. For corrosion process, we decide to use Chloride solution to develop corrosion behavior on our material. We use Universal Tester Machine (UTM) to perform fracture toughness test and Vickers Hardness Testing Machine for hardness testing. Finally, we will be used Optical Microscope to complete our investigation on morphology study.

(19)

CHAPTER 2 LITERATURE REVIEW

In this chapter, it will present about properties of Aluminum and Aluminum Alloy 7075. We also discuss the effects of different types of heat treatment process that will involve on this research. Besides that, we also discuss about the effect of heat treatment and corrosion into the properties of the material.

2.1 Introduction of Aluminum

Aluminum is a white mineral and an extremely useful metallic element. It is a shiny and silvery metal and ductile member of the boron group of chemical elements. Aluminum atoms are arranged in a FCC structure. Pure aluminum has excellent atmospheric corrosion resistance and is used extensively as cladding materials. It has own protected thin film when it exposed to the air and the surface of the metal reacts with oxygen to form an aluminum oxide coating, preventing further corrosion of the metal. (David R. Lide, CRC Handbook of Chemistry and Physics, 2007)

(20)

We can change their properties by doing heat treatment at proper temperature, in terms of strength, corrosion resistance, and their grain size. Aluminum is a relatively light metal compared to other metals such as steel, nickel, brass, and copper.

2.1.1 Aluminum Alloys

Aluminum alloys are mixtures of aluminum with other metals that we called an alloy, which are often with copper, zinc, manganese, silicon, or magnesium. This combination is much lighter and more corrosion resistant than plain carbon steel, but not as corrosion resistant as pure aluminum. Aluminum alloys can be categorized into a number of

groups based on the particular material’s characteristics such as its ability to respond to

thermal and mechanical treatment and the primary alloying element added to the aluminum alloy.

Melting Point: 660 °C

Elastic Modulus: 70-79 GPa

Poisson's Ratio: 0.33

Tensile Strength: 230-570 MPa Yield Strength: 215-505 MPa

Percent Elongation: 10-25%

Table 2.1: Mechanical Properties of Aluminum

(21)

The wrought and cast aluminums have different systems of identification. The wrought system is a 4-digit system and the castings having a 3-digit and 1-decimal place system (Hatch, 1984).

Alloy Series Principal Alloying Element

1xxx 99.000% Minimum Aluminum

2xxx Copper

3xxx Manganese

4xxx Silicon

5xxx Magnesium

6xxx Magnesium and Silicon

7xxx Zinc

8xxx Other Elements

Table 2.2: Alloy Series and Their Principle Alloying Element

(22)

2.1.2 Aluminum Alloy 7075

Aluminum alloy 7075 (AA7075) is one of the highest strength aluminum alloys available. It may be formed in the annealed condition and subsequently heat treated. Aluminum Alloy 7075 defined as wrought aluminum. Introduced in 1943, alloy 7075 has been the standard workhorse series alloy within the aerospace industry ever since. It was the first successful Al-Zn-Mg-Cu high strength alloy using the beneficial effects of the alloying addition of chromium. Although other alloys have since been developed with improved specific properties, AA7075 remains the baseline with a good balance of properties required for aerospace applications.

General 7075 characteristics is very high strength material and used for highly stressed structural parts. This material is used as aircraft fittings, aircraft, aerospace and defense applications. 7075 is widely used for construction of aircraft structures, such as wings and fuselages. Its strength and light weight are also desirable in other fields. These alloys are often used in high performance applications such as aircraft, where a combination of high strength with moderate toughness, light weight, and corrosion resistance are required.

(23)

Component W.t %

Al 87.1-91.4

Cr 0.18-0.28

Cu 1.2-2.0

Fe Max. 0.5

Mg 2.1-2.9

Mn Max. 0.3

Si Max. 0.4

Ti Max. 0.2

Zn 5.1-6.1

Table 2.3: Combination of Elements of Alloy in Aluminum Alloy 7075

(24)

Terms: Metric:

Physical Properties

Density 2.81 g/cm3

Mechanical Properties

Hardness, Brinell 60

Hardness, Knoop 80

Hardness, Vickers 68

Ultimate Tensile Strength 228 MPa

Yield Strength 500 MPa

Elongation at Break 17 %

Modulus of Elasticity 71.7 GPa

Shear Modulus 26.9 GPa

Shear Strength 152 MPa

Thermal Properties

Heat Capacity 0.96 J/g-°C

Melting Point 477 - 635 °C

Table 2.4: Properties of Aluminum Alloy 7075

(1)

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction of Aluminum

(2)

2.1.1 Aluminum Alloys

groups based on the particular material’s characteristics such as its ability to respond to

thermal and mechanical treatment and the primary alloying element added to the aluminum alloy.

Melting Point: 660 °C

Elastic Modulus: 70-79 GPa

Poisson's Ratio: 0.33

Tensile Strength: 230-570 MPa

Yield Strength: 215-505 MPa

Percent Elongation: 10-25%

Table 2.1: Mechanical Properties of Aluminum

(3)

The wrought and cast aluminums have different systems of identification. The wrought system is a 4-digit system and the castings having a 3-digit and 1-decimal place system (Hatch, 1984).

Alloy Series Principal Alloying Element

1xxx 99.000% Minimum Aluminum

2xxx Copper

3xxx Manganese

4xxx Silicon

5xxx Magnesium

6xxx Magnesium and Silicon

7xxx Zinc

8xxx Other Elements

Table 2.2: Alloy Series and Their Principle Alloying Element

(4)

2.1.2 Aluminum Alloy 7075

(5)

Component W.t %

Al 87.1-91.4

Cr 0.18-0.28

Cu 1.2-2.0

Fe Max. 0.5

Mg 2.1-2.9

Mn Max. 0.3

Si Max. 0.4

Ti Max. 0.2

Zn 5.1-6.1

Table 2.3: Combination of Elements of Alloy in Aluminum Alloy 7075

(6)

Terms: Metric:

Physical Properties

Density 2.81 g/cm3

Mechanical Properties

Hardness, Brinell 60

Hardness, Knoop 80

Hardness, Vickers 68

Ultimate Tensile Strength 228 MPa

Yield Strength 500 MPa

Elongation at Break 17 %

Modulus of Elasticity 71.7 GPa

Shear Modulus 26.9 GPa

Shear Strength 152 MPa

Thermal Properties

Heat Capacity 0.96 J/g-°C

Melting Point 477 - 635 °C

Table 2.4: Properties of Aluminum Alloy 7075

An Investigation Of Fracture Toughness On Heat Treated Aluminum Alloy 7075 With Effect Of Corrosion.

UNIVERSITI TEKNIKAL MALAYSIA MELAKA