DECLARATION

I hereby declare that this report entitled “Investigation of damage process due to fatigue

on heat treated Aluminum 7075 with effect of corrosion” is the result of my own

research except as cited in the references.

Signature : ……………………………………………..

Author’s Name: Mohd Fasihin b Abd Satar

Date : 21th

April 2010

APPROVAL

This report is submitted to the faculty of Manufacturing Engineering of UTem as a

partial fulfillment of the requirement for the degree of Bachelor of Manufacturing

Engineering (Material Engineering).

MOHD HAIDIR B. MASLAN

(PSM Supervisor)

i

ABSTRACT

The objective of this research are to investigate the damage process due to the fatigue

failure on heat treated aluminum alloy 7075 with the effect of corrosion. In these

research, the aluminum alloy 7075 been used because of it was the highest attainable

strength level of all forged alloy and have a good corrosion resistance. Moreover, the

aluminum alloy 7075 being used widely in automotive and aircraft industry. Many

research on this material behavior being discuss nowadays because of it is important in

safety and reliable. Many damaged from this material are related to the corrosion effect.

In this research, three condition of specimen being prepared. The specimens is non

heated, heated (T6 and RRA) and the heated (T6 and RRA) with effect of corrosion. The

sample been tested mechanically to investigate the damage process to the aluminum

alloy 7075. Here, the result of each specimen of tensile testing, hardness properties and

damaged due to the fatigue been discuss to investigate the damaged process. Then the

failure sample been cut and polished to investigate the microstructure properties and the

evolution of the grain boundary changed using optical microscope. From the analysis, it

showed the behavior of each type of specimen, the grain boundaries, and material

properties. The heated specimen score the best properties from the fatigue failure

followed corrosion specimen. The damaged in corrosion specimen is tend to crack faster

because of the attack in the pitting region had damaged the grain boundary and produce

starting tip that lead to the failure mechanism.

ii

ABSTRAK

Tujuan utama kajian ini dilakukan adalah untuk mengkaji kegagalan lesu pada

aluminium aloi 7075 yang telah dibaiki sifatnya melalui pemanasan haba dengan kesan

pengaratan. Di dalam kajian ini, aluminium aloi telah dipilih kerana mempunyai tahap

kekuatan yang terbaik daripada kelasnya serta mempunyai sifat tahan pada pengaratan.

Tambahan pula, aluminium 7075 ini telah digunakan dengan meluas dalam industri

automobil dan penerbangan. Dewasa ini, banyak kajian telah dilakukan pada bahan ini

kerana kepentingannya terhadap keselamatan dan kebolehpercayaannya. Kebanyakan

kerosakan yang berlaku kepada bahan ini adalah disebabkan oleh kesan pengaratan.

Untuk kajian ini, tiga jenis keadaan sampel telah digunakan. Sampel tersebut adalah

tanpa pemanasan haba, dengan pemanasan haba dan sampel yang melalui pemanasan

haba tetapi dikaratkan. Sampel ini kemudiannya diuji secara mekanikal untuk mengkaji

proses kegagalan pada aluminum 7075. Disini, setiap data yang diperolehi melalui ujian

terikan, ujian kekerasan dan kegagalan pada ujian lesu telah dianalisis dan disiasat.

Kemudian, sampel tersebut telah dipotong dan dikilatkan supaya dapat mengkaji proses

kegagalan pada bahan melalui mikroskop. Daripada analisis yang dilakukan, ianya

menunjukkan sifat setiap sample, sempadan ira dan sifat bahan itu sendiri. Daripada data

yg diperolehi, sampel yg melalui proses pamanasan haba iaitu T6 dan RRA mempunyai

sifat bahan yang paling bagus. Manakala sampel berkarat lebih terdedah pada kegagalan

kerana kerosakan pada struktur bahan itu sendiri.

iii

AKNOWLEDGEMENT

Firstly, thankfull to Allah because of him, I finally finish my PSM II. I would like to

give my deepest thanks to my respective supervisor, Mr Haidir b.Maslan for helping and

giving me so much motivation and support to fulfill my this PSM report. It is a privilege

for me to complete my research as to fulfill my degree requirement.

Thanks also to my friend for supporting and helping me when I had faced with problem

during doing my PSM report. I appreciate all of their kind, and support to complete these

report. Hence, I would like to my housemate that really supported, sharing information

together amd always ready for helping me.

Lastly, I would like to thanks each and every individual that who have either directly or

indirectly helped me throughout the effort of this report be it in the form of

encouragement, advice or kind reminder. A special thanks to my beloved family

members, I extend my gratitude for their support, understanding and love.

iv

TABLE OF CONTENTS

ABSTRACT…………………………………………………………………………… i

ABSTRAK……………………………………………………………………………..ii

DEDICATION………………………………………………………………………...iii

ACKNOWLEDGEMENT……………………………………………………………..iv

TABLE OF CONTENT………………………………………………………………..v

LIST OF TABLE…………………………………………………………………......viii

LIST OF FIGURE…………………………………………………………………….ix

LIST OF ABBREVIATION…………………………………………………………..x

1.0 INTRODUCTION 1

1.1 Background 1

1.2 Statement of problem 2

1.3 Objective 3

1.4 Scope 4

2.0 LITERATURE REVIEW 5

2.1 Introduction 5

2.2 Aluminum alloy 6

2.2.1 Engineering use 7

2.2.2 Wrought Alloy 8

2.2.3 Wrought aluminum-zinc-magnesium alloy 7075 9

2.3 Introduction to Fatigue 11

2.3.1 Basic Concept 11

v

2.3.2 Factor that effect fatigue 13

2.3.3 Characteristic of Fatigue 14

2.4 Corrosion 15

2.4.1 Pitting Corrosion 16

2.4.2 Galvanic Corrosion 17

2.4.3 Corrosion in Alumunum Alloy 7075 17

2.4.4 Solution Potential 18

2.4.5 Effect of Composition and Microstructure on Corrosion 19

2.4.6 Stress Corrosion Cracking 19

2.5 Heat Treatment 22

2.5.1 Basic Aluminum Heat Treatment Designations 22

2.5.2 Heat Treatment of Aluminum Alloy 25

2.5.2.1 Quenching 26

2.5.2.2 Age Hardening 27

2.5.2.3 T6 Temper 28

2.5.2.4 Introduction to Retrogression and Re-aging 29

3.0 METHODOLOGY 30

3.1 Introduction 30

3.2 Research Design 31

3.3 Material Composition 33

3.4 Heat Treatment process 33

3.4.1 Material Preparation 34

3.4.2 Sample preparation 34

3.4.3 T6 heat treatment process 35

3.4.4 Retrogression and Re aging 36

3.5 Corrosion Process 36

3.6 Fatigue 37

3.6.1 Degradation Analysis 38

vi

3.7 Testing 40

3.7.1 Mechanical Testing 40

3.7.2 Tensile Test 40

3.7.3 Optical Microscope 41

3.7.4 Hardness Test 42

3.7.5 Fatigue Test 43

3.8 Analysis 44

4.0 RESULT AND DISCUSSION 45

4.1 Characterization of aluminum alloy 45

4.1.1 Non heated aluminum alloy 7075 46

4.1.2 T6 + RRa heat treatment 47

4.1.3 T6 + RRa heat treatment corrosion exposure 48

4.2 Hardness properties of heat treated aluminum alloy 7075 49

4.2.1 Hardness Properties 50

4.2 Tensile properties of the heat treatment process on AA 7075 52

4.2.1 Tensile Properties 52

4.2.2. Microstructure Properties for Tensile 58

4.3 Fatigue properties of aluminum alloys 60

4.3.1 Low Cycle Fatigue 60

4.3.2 High cycle fatigue 61

4.3.3 Fatigue properties on un-heated Aluminum Alloy 7075 62

4.3.4 Fatigue properties on heated Aluminum Alloy 7075 64

4.3.5 Fatigue properties on heated Aluminum Alloy 7075 67

with effect with corrosion

4.4 Comparison on fatigue failure for various conditions of 71

Aluminum Alloy 7075

vii

5.0 CONCLUSION AND RECOMMENDATION 76

5.1 Conclusion 76

5.2 Recommendations 79

REFERENCES 80

APPENDICES

A Gantt chart for PSM I and II

B Tensile

C Fatigue

D Hardness

E Microstructure

viii

LIST OF TABLE

Table 2.1 : Properties of Aluminum Alloy 10

Table 2.2 : Type of fatigue stress 12

Table 2.3: Basic Temper Designation 23

Table 2.4: Subdivisions of “T” Temper Heat Treatable Alloys 24

Table 2.5: Subdivisions of “H” Temper Heat Treatable Alloys 24

Table 4.1 : List of hardness value of each specimens 50

Table 4.2 : List of the results for Load at Break, Young Modulus, Maximum 53

Load, Tensile Stress at maximum Load and Elongation to break for

Tensile Test

Table 4.3: Fatigue test data for heated Aluminum Alloy 7075 65

Table 4.4: Fatigue test data for heated Aluminum Alloy 7075 with effect of 68

corrosion

Table 4.5: Comparison for different condition AL7075 on the number of cycles 71

for failed, rate of Young Modulus drop, and value of Young Modulus

ix

LIST OF FIGURES

Figure 2.1:The formation of salt bridge of corrosion 16

Figure 3.1: Process sequence for the whole project 31

Figure 3.2: Experimental flow chart of the whole research 32

Figure 3.3: Specimen been immersed in the EXCO solution 37

Figure 3.4: Specimen dimension for Fatigue test 38

Figure 3.5 :Pitting damage in the aluminum showing flat bottomed 39

pit development

Figure 3.6 :SEM picture of initiation pit cluster produced 39

Figure 3.7: Universal testing Machine 41

Figure 3.8: Specimen dimension for tensile test 41

Figure 3.9: Optical microscope 42

Figure 3.10: Rockwell Hardness testing Machine 43

Figure 3.11: Instron fatigue machine 43

Figure 3.12 : The initiation phase in the fatigue testing 44

Figure 4.1 :Microstructure of non heated AA7075 using optical microscope 46

Figure 4.2 :Microstructure of heated AA7075 using optical microscope 47

Figure 4.3 :Microstructure of corrosion AA7075 using optical microscope 49

Figure 4.4: The comparison of hardness value between each specimens 50

Figure 4.5: Tensile graph for un-heated and heated (T6-RRA) aluminum 52

alloy 7075

Figure 4.6: The result for elongation to break of the AA7075 with different 54

condition.

Figure 4.7: The result for the tensile strength of the AA7075 with different 55

condition

Figure 4.8: The result for the Young’s Modulus of the AA7075 with different 56

x

condition

Figure 4.9: The result for the yield strength of the AA7075 with different 57

condition.

Figure 4.10: Microstructure for un-heated tensile aluminum alloy 7075 58

Figure 4.11: Microstructure for heated (T6-RRA) tensile aluminum alloy 7075 58

Figure 4.1 : Microstructure for corrosion (T6-RRA) tensile aluminum 59

alloy 7075

Figure 4.13 : The crack of low cycle fatigue in tested sample of AA7075 60

Figure 4.14: Crack of high cycle fatigue in tested sample of AA7075 62

Figure 4.15: Graph of Young Modulus Vs Num. of Cycles for un-heated 63

Aluminum Alloy 7075

Figure 4.16 : Microstructure of non-heated AA7075 63

Figure 4.17: Graph of Young Modulus Vs Num. of Cycles for heated 66

Aluminum Alloy 7075

Figure 4.18 : Microstructure fatigue failure of heated AA7075 67

Figure 4.19: Graph of Young Modulus Vs Num. of Cycles for corrosion 69

Aluminum Alloy 7075

Figure 4.20 : Microstructure of corrosion effect AA7075 at the center 70

of specimen.

Figure 4.21 : Microstructure of corrosion effect AA7075 at the edge of specimen. 70

Figure 4.22: Graph of Young Modulus Vs Num. of Cycles for un-heated 72

and heated Aluminum Alloy 7075

Figure 4.23: Shows schematically a portion of the surface containing a 74

flaw before and after load application.

xi

LIST OF ABBREVIATIONS, SYMBOLS, SPECIALIZED

NOMENCLATURE

A -Cross sectional area

AA -Aluminum Alloy

Al -Aluminum

ASTM -American Standard Test Method

Cº -Degree Celsius

CF -Corrosion Fatigue

CFCP -Corrosion Fatigue Crack Propagation

Cr -Chromium

Cu -Copper

E -Young Modulus

EXCO -Exfoliation Corrosion

Fe -Ferrous

GP -Guiner Preston Zones

HNO3 - Nitric Acid

Hr -Hour

HT -Heat Treatment

KNO3 -Potassium Nitride

L -Length

Mg -Magnesium

Mn -Manganese

Mpa -Mega Pascal

N -Newton

NaCl -Sodium Chloride

RRA -Retrogression and Re-ageing

SCC -Stress Corrosion Cracking

SSSS -Super Saturated Solid Solution

SHT -Solution Heat Treatment

xii

T -Temper

Wt. % -Weight of percentages

Zn -Zinc

σ -Stress

ε -Strain

η -Stable MgZn2

η’ - Metastable precipitation

1

CHAPTER 1

INTRODUCTION

1.1 Background of Project

Engineering system may fail in many different ways. If only the behavior of the

structural members is considered, some of the damage will occur such as large elastic or

inelastic deformation, buckling, loss of bonds in material structure and others. The

treatment of corrosion 7075 aluminum alloy fracture process by numerical technique is

the aim for this research, and it is therefore worthwhile to look at the phenomenological

aspect of fracture process before it start to failure.

One of the most striking origin for the failure is metal fatigue. It is the result of

application of load variations the properties of metallic material can change. These

change would be observe on a micro scale (dislocation formation and movement) and

macro scale (growth of defect). Depending on the scale of observation we can

distinguish between damage process and crack process. In general, a region can be

indicated that can be discussed using arguments applicable to the micro domain as well

as using argument from the macro regime.

2

1.2 Problem Statement

Aluminum Alloy 7075 been used widely in aircraft technology because of it properties.

Year by year, the discussion about the capabilities of this 7075 Aluminum Alloy been

discussed widely among scientist and manufacturer. In the aircraft design the safe

service life of metallic components subjected to fatigue load and corrosion effect can be

demonstrate by adopting the Safe Life concept (Brit, H.J.et al., 2002). This paper

describes work on the effect of corrosion on the fatigue and fracture behaviour of a

7178-T6 aluminum alloy Although a substantial work has been reported on heat

treatment and recrystallization behavior of 7xxx series aluminum alloys a limited studies

have been reported on fracture and impact-toughness behavior of the alloys by use of

impact test especially on sheet and plate that have small thickness. This research paper,

reports a comparative analysis of tensile and fatigue behavior of cold-worked and

recrystallized 7075 aluminum alloy.

The 7xxx series aluminum alloys have been widely used as structural materials due to

their attractive comprehensive properties, such as low density, high strength, ductility,

toughness and resistance to fatigue. The 7075 aluminum alloy is one of the most

important engineering alloys and has been utilized extensively in aircraft structures

because of its high strength-to-density ratio. Metal alloys, which have undergone

extensive plastic deformation by rolling or extrusion, exhibit a significant anisotropy of

mechanical properties. Even in the case of untextured metals showing isotropic or

almost isotropic yielding behavior, ductility can be very anisotropic.

Corrosion damage is often found on structural components subjected to fatigue loading.

Many structures are expected to endure long lives while exposed to corrosive

environments. It is well known that corrosion and fatigue damage can occur

simultaneously and they have a combined effect much more severe than each one

occurring on its own [3,5–7]. However, even the simpler problem of fatigue cracks

initiating from preexisting corrosion damage is still not well understood. One of the

3

major objectives of the current investigation is to determine an appropriate methodology

for assessing the remaining fatigue life of an aircraft or similar component containing

corrosion damage (Mohammad et al, 2001).

The corrosion fatigue life of structural elements could be divided into two phases: the

corrosion fatigue crack initiation life and the corrosion fatigue crack propagation

(CFCP) life to be investigated. The CFCP life of typical structure elements is a portion

of total corrosion fatigue and especially for large structure elements, is dominant. In

addition, the CFCP rate is based on damage tolerance design of typical structure

elements in a corrosive environment using a fracture mechanics approach. Therefore,

more attention has been paid to CFCP rate investigations in steels and aluminum alloys.

It is recognized that the CFCP mechanism is similar to those proposed for stress

corrosion cracking. The CFCP rate is regarded as an enhanced crack propagation rate

promoted in a certain corrosive environment by cyclic loading (Wang, 1996).

1.3 Objective

The objectives of this project are:

a) To investigated the Damage Process due to Fatigue on Heat Treated Aluminum

7075 with effect of the corrosion.

b) To study the changing in mechanical properties in AA7075 as a result from the

mechanical testing.

c) To understand the phenomenons of failure process just before it start to form a

crack or damage.

d) To study the Young’s Modulus of the material and to related it with the damaged

of material

4

1.4 Scope

To ensure the objective was successfully achieved, there are several elements that

needed to follow as well.

a) The understanding of the physical and mechanical characteristic of aluminum

alloy of 7075 series.

b) The behavior of fatigue failure process in AA7075 in effect of corrosion.

c) The study of retrogression and re-ageing (RRA) treatment process that suitable

for the AA7075.

d) Some of mechanical testing must be carried out for investigate the tensile,

hardness and fracture properties of the aluminum alloy 7075.

e) The damage process of AA7075 after being pointed with the load in mechanical

testing.

f) To investigate the influence of fatigue damage due to the corrosion effect on the

AA7075.

5

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

7xxx series aluminum alloy are widely use in automotive and aircraft industry due to

their high strength to density ratio. Such as AA7075 with T6 heat treatment process can

reach their peak strength through proper T6 aging treatment. However they are very

susceptible to intergranular stress corrosion cracking (SCC) in the T6 temper, especially

in chloride-containing media in the short transverse direction of thick section. Overaging

treatments (like T73 temper) were develop to improve the short transverse SCC

resistance by increasing the grain boundary precipitate size and spacing, but with some

reduction of tensile strength from the T6 temper.

However , it cannot be assumed that alloy and temper with goo SCC resistance would

show good resistance to corrosion fatigue (CF) as CF failure from aluminum alloy are

characteristically transgranular. For example Jacko and Duquette reported that no

significant different in total fatigue lives could be detected between 7075-T6 and T73

aluminum alloy when they were tested in the form of smooth axial fatigue specimens in

aerated 0.5 N NaCL solution.

This chapter will explain general information that related to the work. We will discuss

about the aluminum alloy and it wrought which will choose to determine the material

and heat treatment that used. When aluminum alloy being heat treatment, it basically

6

will strengthen their hardness, tensile strength and electrical conductivity that make it

more suitable for the manufacture of aircraft components.

Moreover, this chapter will explain about the fatigue which includes how the process of

fatigue failure happen from the S-N curve observation and basic term in corrosion that

effect the crack propagation process. There also will explain about S-N curve and factor

that affect the fatigue life

2.2 Aluminum Alloy

Aluminum alloys are mixtures of aluminum with other metals (called an alloy), often

with copper, zinc, manganese, silicon, or magnesium. They are much lighter and more

corrosion resistant than plain carbon steel, but not as corrosion resistant as pure

aluminum. Bare aluminum alloy surfaces will keep their apparent shine in a dry

environment due to the formation of a clear, protective oxide layer. Galvanic corrosion

can be rapid when aluminum alloy is placed in electrical contact with stainless steel, or

other metals with a more negative corrosion potential than the aluminum alloy, in a wet

environment. Aluminum alloy and stainless steel parts should only be used together in

water-containing systems or outdoor installations if provision is made for either

electrical or electrolytic isolation between the two metals.

Aluminum alloy compositions are registered with the Aluminum Association. Many

organizations publish more specific standards for the manufacture of aluminum alloy,

including the Society of Automotive Engineers standards organization, specifically its

aerospace standards subgroups, and ASTM International.

7

2.2.1 Engineering use

Aluminum alloys with a wide range of properties are used in engineering structures.

Alloy systems are classified by a number system (ANSI) or by names indicating their

main alloying constituents (DIN and ISO). Selecting the right alloy for a given

application entails considerations of strength, ductility, formability, weldability and

corrosion resistance to name a few. A brief historical overview of alloys and

manufacturing technologies is given in Ref.[2]

Aluminum is used extensively in modern

aircraft due to its high strength to weight ratio.

Improper use of aluminum may result in problems, particularly in contrast to iron or

steel, which appear "better behaved" to the intuitive designer, mechanic, or technician.

The reduction by two thirds of the weight of an aluminum part compared with a

similarly sized iron or steel part seems enormously attractive, but it must be noted that

this replacement is accompanied by a reduction by two thirds in the stiffness of the part

(Miller et al, 2002). Therefore, although direct replacement of an iron or steel part with a

duplicate made from aluminum may still give acceptable strength to withstand peak

loads, the increased flexibility will cause three times more deflection in the part.

Where failure is not an issue but excessive flex is undesirable due to requirements for

precision of location, or efficiency of transmission of power, simple replacement of steel

tubing with similarly sized aluminum tubing will result in a degree of flex which is

undesirable; for instance, the increased flex under operating loads caused by replacing

steel bicycle frame tubing with aluminum tubing of identical dimensions will cause

misalignment of the power-train as well as absorbing the operating force. To increase

the rigidity by increasing the thickness of the walls of the tubing increases the weight

proportionately, so that the advantages of lighter weight are lost as the rigidity is

restored.

8

2.2.2 Wrought alloys

The International Alloy Designation System is the most widely accepted naming scheme

for wrought alloys. Each alloy is given a four-digit number, where the first digit

indicates the major alloying elements.

1000 series are essentially pure aluminum with a minimum 99% aluminum

content by weight and can be work hardened

2000 series are alloyed with copper, can be precipitation hardened to strengths

comparable to steel. Formerly referred to as duralumin, they were once the most

common aerospace alloys, but were susceptible to stress corrosion cracking and

are increasingly replaced by 7000 series in new designs.

3000 series are alloyed with manganese, and can be work-hardened.

4000 series are alloyed with silicon. They are also known as silumin.

5000 series are alloyed with magnesium, derive most of their strength from work

hardening. It is suitable for cryogenic applications and low temperature work.

However is susceptible to corrosion above 60°C.

6000 series are alloyed with magnesium and silicon, are easy to machine, and

can be precipitation-hardened, but not to the high strengths that 2000, and 7000

can reach.

7000 series are alloyed with zinc, and can be precipitation hardened to the

highest strengths of any aluminum alloy.

8000 series is a category mainly used for lithium alloys