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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)

    by

    LENG SIONG CHENG

    B050710018

    FACULTY OF MANUFACTURING ENGINEERING

    2011

  • 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.

  • 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

    Date : 18 MAY 2011

  • 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

  • i

    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.

  • ii

    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.

  • iii

    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.

  • iv

    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 Fatt 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…

  • v

    TABLE OF CONTENTS

    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

    19

    2.2.5 T6 and T7 Tempering 20

  • vi

    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

    34

    2.3.8 Stress Corrosion Testing 35

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