Investigation Of Damage Process Due To Fatigue On Heat Treated Aluminum 7075.
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
INVESTIGATION OF DAMAGE PROCESS DUE TO FATIGUE ON
HEAT TREATED ALUMINUM 7075
Thesis submitted in accordance with the partial requirements of the Universiti
Teknikal Malaysia Melaka for the Bachelor of Manufacturing Engineering
(Engineering Material) with Honours
By
MOHD KHAIRULNIZAM BIN MUHAMAT NAWAWI
Faculty of Manufacturing Engineering
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)
DECLARATION
I hereby declare that this report entitled “Investigation of damage process due to fatigue
in heat treated Aluminum 7075” is the result of my own research except as cited in the
references.
Signature
: ……………………………………………..
Author’s Name: Mohd Khairulnizam B. Muhamat Nawawi
Date
: 19th April 2009
ABSTRACT
The objectives of this research are to investigate the damage process due to the fatigue
failure on heat treated Aluminum Alloy 7075 on the mechanical properties and
microstructures structure. In this research, the aluminum alloy 7075 being used widely
in automotive and aircraft industry because of it highest in strength level of all alloy.
Besides that, many researches on this material behavior being discuss nowadays because
of it important in safety and reliable. In the sample preparation, the aluminum alloy 7075
will be heated to T6 heat treatment process. Then, the RRA process is applied to the
material in the T6 condition. The sample will be testing mechanically to investigate the
tensile strength, hardness properties and the fatigue damage process to the Aluminum
alloy 7075. Here, the result of the damage process will be discussed and at the same time,
the surface structure of the fracture process for the fatigue failure also will be observed
under optical microscope. From the result, the discussion about microstructure and
damage phenomenon that lead to the degradation process of the aluminum alloy 7075T6 will be covered. Lastly, the conclusion for the whole of the research will be covered
based on the observation from the experimental and discussion.
i
ABSTRAK
Objektif penyelidikan ini adalah untuk mengkaji proses kegagalan disebabkan oleh
kelesuan pada proses pemanasan aluminium aloi 7075 dari segi sifat mekanikal dan
bentuk mikrostrukturnya. Dalam kajian ini, aluminium aloi 7075 telah digunakan secara
meluasnya dalam industri automotif dan kapal terbang disebabkan oleh tahap kekuatan
aloi itu. Selain itu, banyak kajian telah dibincangkan terhadap sifat bahan ini sendiri
kerana kepentingan di dalam keselamatan dan kebolehpercayaan. Di dalam penyediaan
sampel, aluminium aloi 7075 akan dipanaskan kepada proses pemanasan T6. Kemudian
proses RRA dikenakan pada bahan dalam keadaan T6 itu. Sampel akan diuji untuk
menyiasat tentang kekuatan terikan, sifat kekerasan dan proses kegagalan terhadap
aluminium aloi 7075. Selepas itu, keputusan tentang proses kegagalan akan
dibincangkan dan pada masa yang sama struktur permukaan untuk kegagalan
disebabkan oleh kelesuan akan dikaji dibawah mikroskop. Daripada keputusan itu,
perbincangan tentang mikrostruktur dan fenomena kerosakan yang mana mempengaruhi
proses kegagalan aluminium aloi 7075 akan di liputi. Akhir sekali, kesimpulan bagi
keseluruhan ujikaji ini akan dibuat berdasarkan kepada pemerhatian daripada
eksperiment dan perbincangan yang telah dijalankan.
ii
DEDICATION
To my beloved parent, family and friends who have give their attention and help me to
finish this thesis.
iii
ACKNOWLEDGEMENT
First of all, I would like to express my deepest thanks and appreciation to my supervisor,
En. Mohd Haidir B. Maslan who had offered his precious time, attention, experience and
guidance throughout the completion of the investigation thus far.
I also would like to convey my thanks to my friends whose support me when I had faced
problem during doing this final year project. It is a privilege for me to complete my final
year project to fulfill my degree requirement.
Lastly I would like to thank every individual who have helped me throughout the efforts
of this report be it in the form of encouragement, advice or kind reminder. A special
thank 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…………………………………………………………………....ix
LIST OF FIGURE…………………………………………………………………..x
LIST OF ABBREVIATIONS………………………………………………...…….xii
1. INTRODUCTION
1
1.1 Background of project
1
1.2 Background of problem
2
1.3 Statement of problem
2
1.4 Objective
2
1.5 Scope
3
1.6 Important of study
3
1.7 Thesis overview
4
2. LITERATURE REVIEW
5
2.1 Introduction
5
2.2 Aluminum alloy
6
2.2.1 Wrought alloy
6
2.2.2 7075 Aluminum Alloy
8
2.2.3 7075 Aluminum Alloy Applications
8
2.3 Aluminum Alloy Heat Treatment
9
2.3.1 Basic Aluminum Temper Designations
10
2.3.2 Quenching
12
v
2.3.3 Age Hardening
13
2.3.3.1 Precipitation Heat Treating (Artificial Aging)
14
2.3.4 T6 temper of Aluminum Alloy 7075
15
2.3.5 Introduction to the Retrogression and Re-ageing (RRA)
16
2.3.6 Mechanical properties
17
2.3.6.1 Un-heat-treated Aluminum alloy 7075
17
2.3.6.2 Aluminum alloy 7075 with T6 heat treatment
17
2.4 Fatigue
18
2.4.1 Characteristics of fatigue
20
2.4.2 The S-N curve
21
2.4.3 Factors that affect fatigue-life
22
2.4.4 Fatigue failure
23
2.4.4.1 Stress Ratio
23
2.4.5 Design against fatigue
24
2.4.5.1 Stopping fatigue crack
25
2.4.5.2 Material change
25
2.5 Damage due fatigue
26
2.5.1 Crack initiation and propagation
27
2.6 Fatigue Degradation
30
3. METHODOLOGY
34
3.1 Introduction
34
3.2 Research design
35
3.3 Material preparation
37
3.3.1 Aluminum Alloy 7075 preparation
37
3.4 Heat Treatment process
38
3.4.1 Sample preparation
38
3.4.2 T6 Temper
38
3.4.3 RRA Heat Treatment
39
3.5 Fatigue Exposure
40
vi
3.5.1 Degradation analysis
41
3.6 Testing and Analysis
44
3.6.1 Tensile test
44
3.6.2 Hardness test
45
3.6.3 Optical Microscope
46
3.7 Result and Discussion
47
4. RESULT AND DISCUSSION
48
4.1 Introduction
48
4.2 Tensile properties of the heat treatment process on Aluminum Alloy 7075
49
4.2.1 Tensile Properties
49
4.2.2 Microstructure Properties for Tensile
55
4.3 Hardness properties of heat treatment process on Aluminum Alloy 7075
56
4.3.1 Hardness Properties
56
4.4 Microstructure properties on different condition of Aluminum Alloy 7075
58
4.4.1 Microstructure Properties
59
4.5 Fatigue properties on different condition of Aluminum Alloy 7075
61
4.5.1 Fatigue properties on un-heated Aluminum Alloy 7075
61
4.5.2 Fatigue properties on heated Aluminum Alloy 7075
64
4.5.3 Comparison for un-heated and heated Aluminum Alloy 707
67
5. CONCLUSION AND RECOMMENDATION
70
5.1 Conclusion
70
5.2 Recommendation
72
REFERENCES
73
vii
APPENDIX
A
Gantt chart for PSM I and II
B
Tensile
C
Fatigue
D
Hardness
E
Microstructure
viii
LIST OF TABLE
Page
2.1
Basic Temper Designation
10
2.2
Subdivisions of “T” Temper Heat Treatable Alloys
11
2.3
Subdivisions of “H” Temper Heat Treatable Alloys
12
4.1
List of the results for Stress at Break, Young Modulus,
Maximum Stress, Tensile Stress at maximum Stress and
Elongation to break for Tensile Test
4.2
50
List of result for hardness properties of different condition
Aluminum Alloy 7075
56
4.3
Fatigue test data for un-heated Aluminum Alloy 7075
62
4.4
Fatigue test data for heated Aluminum Alloy 7075
64
4.5
Comparison for different condition AL7075 on the number of
cycles for failed, rate of Young Modulus drop, and value of
Young Modulus
67
ix
LIST OF FIGURE
Page
2.1
Types of stress cycles with which loads may be applied
20
2.2
S-N curve for brittle aluminum of 320 Mpa
21
2.3
Fatigue stress variation
24
2.4
Schematic representation showing stages I and II of fatigue crack propagation
28
2.5
Fatigue crack propagation mechanism (stage II) by repetitive crack tip plastic
Blunting and sharpening.
2.6
29
Micrographs of an Al7Cu2Fe constituent particle on the surface of a fatigue
loaded 7075 showing regimes of (i) incubation, (ii) nucleation, and
(iii) microstructurally short crack propagation.
2.7
30
(a) SEM image of 7075-T651 under load, showing multiple, microstructurally
small fatigue cracks. Close-up images show (b) an uncracked particle,
(c) a cracked particle where the crack did not extend into the matrix and
(d) a cracked particle where the crack did extend into the matrix.
2.8
31
Schematic illustration of the reaction-layer fatigue mechanism
for thin-film fatigue.
32
3.1
Process sequence for the whole project
35
3.2
Experimental flow chart of the whole research
36
3.3
Specimen dimension for Fatigue test
41
3.4
Micrographs showing regimes of (i) incubation, (ii) nucleation, and
(iii) microstructurally short crack propagation.
41
3.5
Close-up images show an uncracked particle.
42
3.6
Close-up images show a cracked particle where the crack did not extend
into the matrix.
3.7
42
Close-up images show a cracked particle where the crack did extend
into the matrix.
42
x
3.8
Universal testing Machine
44
3.9
Specimen dimension for tensile test
45
3.10
Rockwell Hardness testing Machine
45
3.11
Optical microscope
46
4.1
Tensile graph for un-heated and heated (T6-RRA) aluminum alloy 7075
49
4.2
The result for elongation to break of the AA7075 with different condition.
51
4.3
The result for the tensile strength of the AA7075 with different condition
52
4.4
The result for the Young’s Modulus of the AA7075 with different condition
53
4.5
The result for the yield strength of the AA7075 with different condition.
54
4.6
Microstructure for un-heated tensile aluminum alloy 7075
55
4.7
Microstructure for heated (T6-RRA) tensile aluminum alloy 7075
55
4.8
The comparison of hardness value of the AA7075 with different condition.
57
4.9
Microstructure for un-heated aluminum alloy 7075
59
4.10
Microstructure for heated (T6-RRA) aluminum alloy 7075
59
4.11
Graph of Young Modulus Vs Num. of Cycles for un-heated
Aluminum Alloy 7075
63
4.12
Graph of Young Modulus Vs Num. of Cycles for heated Aluminum Alloy 7075
66
4.13
Graph of Young Modulus Vs Num. of Cycles for un-heated and heated
Aluminum Alloy 7075
68
xi
LIST OF ABBREVIATIONS, SYMBOLS, SPECIALIZED
NOMENCLATURE
A
-Cross sectional area
AA
-Aluminum Alloy
Al
-Aluminum
ASTM
-American Standard Test Method
Cº
-Degree Celsius
Cr
-Chromium
Cu
-Copper
E
-Young Modulus
Fe
-Ferrous
GP
-Guiner Preston Zones
Hr
-Hour
L
-Length
Mg
-Magnesium
Mn
-Manganese
Mpa
-Mega Pascal
N
-Newton
RRA
-Retrogression and Re-ageing
SSSS
-Super Saturated Solid Solution
SHT
-Solution Heat Treatment
T
-Temper
Wt. %
-Weight of percentages
Zn
-Zinc
σ
-Stress
ε
-Strain
η
-Stable MgZn2
η’
- Metastable precipitation
xii
CHAPTER 1
INTRODUCTION
1.1 Background of project
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 (Feng, 2008). 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. This study have been conducted to
understand the fatigue behavior of aluminum alloys and the fatigue process consists of
crack initiation and crack propagation to failure. Smooth specimens are usually used to
study the crack initiation behavior because fatigue lives are related to the stress and
strain quantities.
The choosing of alloy 7075 for this research due to the characteristic itself that has
application throughout aircraft and aerospace structures where a combination of high
strength will moderate toughness are required. Besides that, many heat treatment and
heat practices are available to develop optimum strength, toughness and others desirable
characteristics for proper application of aluminum alloy 7075 (Hatch, 1983).
1
1.2 Background of problem
Currently, aluminum alloy are commonly used in many types of application and most of
them are for industry purpose. With the increasing popularity of the application, there
are many challenges for the material to stand with other types of new material which
have more advantages and opportunities. As we know, the airplane crack was happened
long time ago and that is happens because the material properties exchange due to the
time and environment factors. The purpose of this project is to investigate damage due
fatigue or the degradation of fatigue life in the aluminum alloy 7075 series that used in
aircraft application.
1.3 Statement of Problem
Base on this study, several question need to be answer at the end of the topic likes;
How to investigate the fatigue failure.
How to determine the growth of crack before it achieved failure.
How to explain about the steps of crack.
How to investigate the fatigue microscopic failure.
1.4 Objective
The objectives of this project are to:
Investigate the effect of heat treatment on the aluminum alloy.
Know about the degradation of fatigue life.
Conduct an experiment to determine the fatigue characteristic.
2
1.5 Scope
The study of this paper covered damage due fatigue of aluminum alloy 70705 that use in
the aircraft body. On that damage degradation, an investigation of the fatigue failure, the
behavior of fatigue had been studied. The fatigue failure was investigated by compare
the drop of young modulus value for each cycle that occurs the elongation of the
material. There is limitation for the material because the material that uses to investigate
the fatigue failure only aluminum alloys 7075 which prepared by some kind of heat
treatment. For tensile test, the yield strength and young‟s modulus had been covered to
see about the properties and the condition of the material. Besides that, the studied about
the hardness of the aluminum alloy 7075 also covered as a tensile test and fatigue
investigation.
1.6 Important of study
This project is an idea to investigate the damage due fatigue and its factor. On the
fatigue failure scope, the steps factor that affects the fatigue failure had been studied in
detail and what pressure the crack to continue. By doing this research, the factors that
affect the fatigue failure will be known and as a result from that, the way to avoid it can
be study and research in the future. By doing this project also, the opportunities for the
aluminum alloy 7075 to used as a widely application will be maintain as before because
this chapter will inform the others user about it weakness and a way to reduce that
weakness.
3
1.7 Thesis Overview
In chapter 1, there are explanation generally about the project that had been research
including it background of problems, objective of the project, scope, and the important
of study. Next, in chapter 2, the discussion about the data collection for the project was
applied. In the data collection there are include about the wrought alloy that used for the
project, it alloys composition, the alloy application and properties. Besides that, this
chapter also explains about the heat treatment that can apply to the alloy based on their
application. For adding issues, this chapter was explaining more specific about the
fatigue and it characteristic and also the process of fatigue failure. At the end of this
chapter, the discussion about the way how to prevent the failure in the future was done.
Moreover, in chapter 3, the research work that done in this field involved many types of
experimental like fatigue test, tensile test, hardness test and optical microscope. In this
chapter also discuss about the material preparation and used for the test. The material
that used in the current experimental investigation was 7075-T6 aluminum alloy. The
specimen dimension for each test also discuss in this chapter which consist of it size and
dimension parameter. Lastly for this chapter, the discussion about the fatigue exposure
that done as the primary result that expected from the experiment was conduct. Then in
chapter 4, the result and the discussion about the testing like microstructure observation
tensile test, hardness test, and fatigue test were covered. Lastly, the conclusion and
recommendation for this thesis were covered.
4
CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
This chapter explains about the aluminum alloy and it wrought which will choose to
determine the material and heat treatment that used. In this chapter also, there were
explanation about 7075 aluminum alloy based on its application and mechanical
properties. Besides that, the explain about the term of heat treatment for the aluminum
alloy and like basic Aluminum Heat Treatment Designations, Heat Treating T Temper
Codes, and H Temper Strain Hardening Codes. Besides that, it was explain about the
Quenching, artificial aging, and retrogression and re-ageing (RRA). Moreover, this
chapter explains about the fatigue which includes the how the process fatigue happen
and what type of force that will allow the process of fatigue. There also explanation
about S-N curve and factor that affect the fatigue life. Lastly, this chapter explains about
the fatigue failure process and how to design against that fatigue failure.
5
2.2 Aluminum Alloy
Aluminum alloys is a material that 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 (Sanders, 2001).
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, we ability and
corrosion resistance to name a few.
2.2.1 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.
6
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
7
mainly
used
for
lithium
all.
2.2.2 7075 Aluminum Alloy
It is be introduced in 1943 and has been the standard workhorse 7XXX 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 to
develop good stress-corrosion cracking resistance in sheet products. Although other
7XXX alloys have since been developed with improved specific properties, alloy 7075
remains the baseline with a good balance of properties required for aerospace
applications. This heat treatable alloy is considered high in strength. Corrosion
resistance and machinability is fair. Rated low on workability and welded only by the
resistance process. Alloy 7075 is available in bare and alclad sheet and plate product
forms in the annealed state as well as several tempers of the T6, T73 and T76 types.
2.2.3 7075 Aluminum Alloy Applications
Alloy 7075 sheet and plate products have application throughout aircraft and aerospace
structures where a combination of high strength with moderate toughness and corrosion
resistance are required. Typical applications are alclad skin sheet, structural plate
components up to 4 inches in thickness and general aluminum aerospace applications.
Besides, it is widely used for construction of aircraft structures, such as wings and
fuselages. Because it properties are strength and light weight, alloy 7075 are also
desirable in other fields likes rock climbing equipment, bicycle components, and hang
glider airframes.
One interesting use for 7075 is in the manufacture of M16 rifle for the American
military. It is also commonly used in shafts for lacrosse sticks and due to its strength,
low density, thermal properties and its polish ability 7075 is widely used in mould tool
manufacture (Sanders, 2001).
8
INVESTIGATION OF DAMAGE PROCESS DUE TO FATIGUE ON
HEAT TREATED ALUMINUM 7075
Thesis submitted in accordance with the partial requirements of the Universiti
Teknikal Malaysia Melaka for the Bachelor of Manufacturing Engineering
(Engineering Material) with Honours
By
MOHD KHAIRULNIZAM BIN MUHAMAT NAWAWI
Faculty of Manufacturing Engineering
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)
DECLARATION
I hereby declare that this report entitled “Investigation of damage process due to fatigue
in heat treated Aluminum 7075” is the result of my own research except as cited in the
references.
Signature
: ……………………………………………..
Author’s Name: Mohd Khairulnizam B. Muhamat Nawawi
Date
: 19th April 2009
ABSTRACT
The objectives of this research are to investigate the damage process due to the fatigue
failure on heat treated Aluminum Alloy 7075 on the mechanical properties and
microstructures structure. In this research, the aluminum alloy 7075 being used widely
in automotive and aircraft industry because of it highest in strength level of all alloy.
Besides that, many researches on this material behavior being discuss nowadays because
of it important in safety and reliable. In the sample preparation, the aluminum alloy 7075
will be heated to T6 heat treatment process. Then, the RRA process is applied to the
material in the T6 condition. The sample will be testing mechanically to investigate the
tensile strength, hardness properties and the fatigue damage process to the Aluminum
alloy 7075. Here, the result of the damage process will be discussed and at the same time,
the surface structure of the fracture process for the fatigue failure also will be observed
under optical microscope. From the result, the discussion about microstructure and
damage phenomenon that lead to the degradation process of the aluminum alloy 7075T6 will be covered. Lastly, the conclusion for the whole of the research will be covered
based on the observation from the experimental and discussion.
i
ABSTRAK
Objektif penyelidikan ini adalah untuk mengkaji proses kegagalan disebabkan oleh
kelesuan pada proses pemanasan aluminium aloi 7075 dari segi sifat mekanikal dan
bentuk mikrostrukturnya. Dalam kajian ini, aluminium aloi 7075 telah digunakan secara
meluasnya dalam industri automotif dan kapal terbang disebabkan oleh tahap kekuatan
aloi itu. Selain itu, banyak kajian telah dibincangkan terhadap sifat bahan ini sendiri
kerana kepentingan di dalam keselamatan dan kebolehpercayaan. Di dalam penyediaan
sampel, aluminium aloi 7075 akan dipanaskan kepada proses pemanasan T6. Kemudian
proses RRA dikenakan pada bahan dalam keadaan T6 itu. Sampel akan diuji untuk
menyiasat tentang kekuatan terikan, sifat kekerasan dan proses kegagalan terhadap
aluminium aloi 7075. Selepas itu, keputusan tentang proses kegagalan akan
dibincangkan dan pada masa yang sama struktur permukaan untuk kegagalan
disebabkan oleh kelesuan akan dikaji dibawah mikroskop. Daripada keputusan itu,
perbincangan tentang mikrostruktur dan fenomena kerosakan yang mana mempengaruhi
proses kegagalan aluminium aloi 7075 akan di liputi. Akhir sekali, kesimpulan bagi
keseluruhan ujikaji ini akan dibuat berdasarkan kepada pemerhatian daripada
eksperiment dan perbincangan yang telah dijalankan.
ii
DEDICATION
To my beloved parent, family and friends who have give their attention and help me to
finish this thesis.
iii
ACKNOWLEDGEMENT
First of all, I would like to express my deepest thanks and appreciation to my supervisor,
En. Mohd Haidir B. Maslan who had offered his precious time, attention, experience and
guidance throughout the completion of the investigation thus far.
I also would like to convey my thanks to my friends whose support me when I had faced
problem during doing this final year project. It is a privilege for me to complete my final
year project to fulfill my degree requirement.
Lastly I would like to thank every individual who have helped me throughout the efforts
of this report be it in the form of encouragement, advice or kind reminder. A special
thank 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…………………………………………………………………....ix
LIST OF FIGURE…………………………………………………………………..x
LIST OF ABBREVIATIONS………………………………………………...…….xii
1. INTRODUCTION
1
1.1 Background of project
1
1.2 Background of problem
2
1.3 Statement of problem
2
1.4 Objective
2
1.5 Scope
3
1.6 Important of study
3
1.7 Thesis overview
4
2. LITERATURE REVIEW
5
2.1 Introduction
5
2.2 Aluminum alloy
6
2.2.1 Wrought alloy
6
2.2.2 7075 Aluminum Alloy
8
2.2.3 7075 Aluminum Alloy Applications
8
2.3 Aluminum Alloy Heat Treatment
9
2.3.1 Basic Aluminum Temper Designations
10
2.3.2 Quenching
12
v
2.3.3 Age Hardening
13
2.3.3.1 Precipitation Heat Treating (Artificial Aging)
14
2.3.4 T6 temper of Aluminum Alloy 7075
15
2.3.5 Introduction to the Retrogression and Re-ageing (RRA)
16
2.3.6 Mechanical properties
17
2.3.6.1 Un-heat-treated Aluminum alloy 7075
17
2.3.6.2 Aluminum alloy 7075 with T6 heat treatment
17
2.4 Fatigue
18
2.4.1 Characteristics of fatigue
20
2.4.2 The S-N curve
21
2.4.3 Factors that affect fatigue-life
22
2.4.4 Fatigue failure
23
2.4.4.1 Stress Ratio
23
2.4.5 Design against fatigue
24
2.4.5.1 Stopping fatigue crack
25
2.4.5.2 Material change
25
2.5 Damage due fatigue
26
2.5.1 Crack initiation and propagation
27
2.6 Fatigue Degradation
30
3. METHODOLOGY
34
3.1 Introduction
34
3.2 Research design
35
3.3 Material preparation
37
3.3.1 Aluminum Alloy 7075 preparation
37
3.4 Heat Treatment process
38
3.4.1 Sample preparation
38
3.4.2 T6 Temper
38
3.4.3 RRA Heat Treatment
39
3.5 Fatigue Exposure
40
vi
3.5.1 Degradation analysis
41
3.6 Testing and Analysis
44
3.6.1 Tensile test
44
3.6.2 Hardness test
45
3.6.3 Optical Microscope
46
3.7 Result and Discussion
47
4. RESULT AND DISCUSSION
48
4.1 Introduction
48
4.2 Tensile properties of the heat treatment process on Aluminum Alloy 7075
49
4.2.1 Tensile Properties
49
4.2.2 Microstructure Properties for Tensile
55
4.3 Hardness properties of heat treatment process on Aluminum Alloy 7075
56
4.3.1 Hardness Properties
56
4.4 Microstructure properties on different condition of Aluminum Alloy 7075
58
4.4.1 Microstructure Properties
59
4.5 Fatigue properties on different condition of Aluminum Alloy 7075
61
4.5.1 Fatigue properties on un-heated Aluminum Alloy 7075
61
4.5.2 Fatigue properties on heated Aluminum Alloy 7075
64
4.5.3 Comparison for un-heated and heated Aluminum Alloy 707
67
5. CONCLUSION AND RECOMMENDATION
70
5.1 Conclusion
70
5.2 Recommendation
72
REFERENCES
73
vii
APPENDIX
A
Gantt chart for PSM I and II
B
Tensile
C
Fatigue
D
Hardness
E
Microstructure
viii
LIST OF TABLE
Page
2.1
Basic Temper Designation
10
2.2
Subdivisions of “T” Temper Heat Treatable Alloys
11
2.3
Subdivisions of “H” Temper Heat Treatable Alloys
12
4.1
List of the results for Stress at Break, Young Modulus,
Maximum Stress, Tensile Stress at maximum Stress and
Elongation to break for Tensile Test
4.2
50
List of result for hardness properties of different condition
Aluminum Alloy 7075
56
4.3
Fatigue test data for un-heated Aluminum Alloy 7075
62
4.4
Fatigue test data for heated Aluminum Alloy 7075
64
4.5
Comparison for different condition AL7075 on the number of
cycles for failed, rate of Young Modulus drop, and value of
Young Modulus
67
ix
LIST OF FIGURE
Page
2.1
Types of stress cycles with which loads may be applied
20
2.2
S-N curve for brittle aluminum of 320 Mpa
21
2.3
Fatigue stress variation
24
2.4
Schematic representation showing stages I and II of fatigue crack propagation
28
2.5
Fatigue crack propagation mechanism (stage II) by repetitive crack tip plastic
Blunting and sharpening.
2.6
29
Micrographs of an Al7Cu2Fe constituent particle on the surface of a fatigue
loaded 7075 showing regimes of (i) incubation, (ii) nucleation, and
(iii) microstructurally short crack propagation.
2.7
30
(a) SEM image of 7075-T651 under load, showing multiple, microstructurally
small fatigue cracks. Close-up images show (b) an uncracked particle,
(c) a cracked particle where the crack did not extend into the matrix and
(d) a cracked particle where the crack did extend into the matrix.
2.8
31
Schematic illustration of the reaction-layer fatigue mechanism
for thin-film fatigue.
32
3.1
Process sequence for the whole project
35
3.2
Experimental flow chart of the whole research
36
3.3
Specimen dimension for Fatigue test
41
3.4
Micrographs showing regimes of (i) incubation, (ii) nucleation, and
(iii) microstructurally short crack propagation.
41
3.5
Close-up images show an uncracked particle.
42
3.6
Close-up images show a cracked particle where the crack did not extend
into the matrix.
3.7
42
Close-up images show a cracked particle where the crack did extend
into the matrix.
42
x
3.8
Universal testing Machine
44
3.9
Specimen dimension for tensile test
45
3.10
Rockwell Hardness testing Machine
45
3.11
Optical microscope
46
4.1
Tensile graph for un-heated and heated (T6-RRA) aluminum alloy 7075
49
4.2
The result for elongation to break of the AA7075 with different condition.
51
4.3
The result for the tensile strength of the AA7075 with different condition
52
4.4
The result for the Young’s Modulus of the AA7075 with different condition
53
4.5
The result for the yield strength of the AA7075 with different condition.
54
4.6
Microstructure for un-heated tensile aluminum alloy 7075
55
4.7
Microstructure for heated (T6-RRA) tensile aluminum alloy 7075
55
4.8
The comparison of hardness value of the AA7075 with different condition.
57
4.9
Microstructure for un-heated aluminum alloy 7075
59
4.10
Microstructure for heated (T6-RRA) aluminum alloy 7075
59
4.11
Graph of Young Modulus Vs Num. of Cycles for un-heated
Aluminum Alloy 7075
63
4.12
Graph of Young Modulus Vs Num. of Cycles for heated Aluminum Alloy 7075
66
4.13
Graph of Young Modulus Vs Num. of Cycles for un-heated and heated
Aluminum Alloy 7075
68
xi
LIST OF ABBREVIATIONS, SYMBOLS, SPECIALIZED
NOMENCLATURE
A
-Cross sectional area
AA
-Aluminum Alloy
Al
-Aluminum
ASTM
-American Standard Test Method
Cº
-Degree Celsius
Cr
-Chromium
Cu
-Copper
E
-Young Modulus
Fe
-Ferrous
GP
-Guiner Preston Zones
Hr
-Hour
L
-Length
Mg
-Magnesium
Mn
-Manganese
Mpa
-Mega Pascal
N
-Newton
RRA
-Retrogression and Re-ageing
SSSS
-Super Saturated Solid Solution
SHT
-Solution Heat Treatment
T
-Temper
Wt. %
-Weight of percentages
Zn
-Zinc
σ
-Stress
ε
-Strain
η
-Stable MgZn2
η’
- Metastable precipitation
xii
CHAPTER 1
INTRODUCTION
1.1 Background of project
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 (Feng, 2008). 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. This study have been conducted to
understand the fatigue behavior of aluminum alloys and the fatigue process consists of
crack initiation and crack propagation to failure. Smooth specimens are usually used to
study the crack initiation behavior because fatigue lives are related to the stress and
strain quantities.
The choosing of alloy 7075 for this research due to the characteristic itself that has
application throughout aircraft and aerospace structures where a combination of high
strength will moderate toughness are required. Besides that, many heat treatment and
heat practices are available to develop optimum strength, toughness and others desirable
characteristics for proper application of aluminum alloy 7075 (Hatch, 1983).
1
1.2 Background of problem
Currently, aluminum alloy are commonly used in many types of application and most of
them are for industry purpose. With the increasing popularity of the application, there
are many challenges for the material to stand with other types of new material which
have more advantages and opportunities. As we know, the airplane crack was happened
long time ago and that is happens because the material properties exchange due to the
time and environment factors. The purpose of this project is to investigate damage due
fatigue or the degradation of fatigue life in the aluminum alloy 7075 series that used in
aircraft application.
1.3 Statement of Problem
Base on this study, several question need to be answer at the end of the topic likes;
How to investigate the fatigue failure.
How to determine the growth of crack before it achieved failure.
How to explain about the steps of crack.
How to investigate the fatigue microscopic failure.
1.4 Objective
The objectives of this project are to:
Investigate the effect of heat treatment on the aluminum alloy.
Know about the degradation of fatigue life.
Conduct an experiment to determine the fatigue characteristic.
2
1.5 Scope
The study of this paper covered damage due fatigue of aluminum alloy 70705 that use in
the aircraft body. On that damage degradation, an investigation of the fatigue failure, the
behavior of fatigue had been studied. The fatigue failure was investigated by compare
the drop of young modulus value for each cycle that occurs the elongation of the
material. There is limitation for the material because the material that uses to investigate
the fatigue failure only aluminum alloys 7075 which prepared by some kind of heat
treatment. For tensile test, the yield strength and young‟s modulus had been covered to
see about the properties and the condition of the material. Besides that, the studied about
the hardness of the aluminum alloy 7075 also covered as a tensile test and fatigue
investigation.
1.6 Important of study
This project is an idea to investigate the damage due fatigue and its factor. On the
fatigue failure scope, the steps factor that affects the fatigue failure had been studied in
detail and what pressure the crack to continue. By doing this research, the factors that
affect the fatigue failure will be known and as a result from that, the way to avoid it can
be study and research in the future. By doing this project also, the opportunities for the
aluminum alloy 7075 to used as a widely application will be maintain as before because
this chapter will inform the others user about it weakness and a way to reduce that
weakness.
3
1.7 Thesis Overview
In chapter 1, there are explanation generally about the project that had been research
including it background of problems, objective of the project, scope, and the important
of study. Next, in chapter 2, the discussion about the data collection for the project was
applied. In the data collection there are include about the wrought alloy that used for the
project, it alloys composition, the alloy application and properties. Besides that, this
chapter also explains about the heat treatment that can apply to the alloy based on their
application. For adding issues, this chapter was explaining more specific about the
fatigue and it characteristic and also the process of fatigue failure. At the end of this
chapter, the discussion about the way how to prevent the failure in the future was done.
Moreover, in chapter 3, the research work that done in this field involved many types of
experimental like fatigue test, tensile test, hardness test and optical microscope. In this
chapter also discuss about the material preparation and used for the test. The material
that used in the current experimental investigation was 7075-T6 aluminum alloy. The
specimen dimension for each test also discuss in this chapter which consist of it size and
dimension parameter. Lastly for this chapter, the discussion about the fatigue exposure
that done as the primary result that expected from the experiment was conduct. Then in
chapter 4, the result and the discussion about the testing like microstructure observation
tensile test, hardness test, and fatigue test were covered. Lastly, the conclusion and
recommendation for this thesis were covered.
4
CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
This chapter explains about the aluminum alloy and it wrought which will choose to
determine the material and heat treatment that used. In this chapter also, there were
explanation about 7075 aluminum alloy based on its application and mechanical
properties. Besides that, the explain about the term of heat treatment for the aluminum
alloy and like basic Aluminum Heat Treatment Designations, Heat Treating T Temper
Codes, and H Temper Strain Hardening Codes. Besides that, it was explain about the
Quenching, artificial aging, and retrogression and re-ageing (RRA). Moreover, this
chapter explains about the fatigue which includes the how the process fatigue happen
and what type of force that will allow the process of fatigue. There also explanation
about S-N curve and factor that affect the fatigue life. Lastly, this chapter explains about
the fatigue failure process and how to design against that fatigue failure.
5
2.2 Aluminum Alloy
Aluminum alloys is a material that 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 (Sanders, 2001).
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, we ability and
corrosion resistance to name a few.
2.2.1 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.
6
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
7
mainly
used
for
lithium
all.
2.2.2 7075 Aluminum Alloy
It is be introduced in 1943 and has been the standard workhorse 7XXX 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 to
develop good stress-corrosion cracking resistance in sheet products. Although other
7XXX alloys have since been developed with improved specific properties, alloy 7075
remains the baseline with a good balance of properties required for aerospace
applications. This heat treatable alloy is considered high in strength. Corrosion
resistance and machinability is fair. Rated low on workability and welded only by the
resistance process. Alloy 7075 is available in bare and alclad sheet and plate product
forms in the annealed state as well as several tempers of the T6, T73 and T76 types.
2.2.3 7075 Aluminum Alloy Applications
Alloy 7075 sheet and plate products have application throughout aircraft and aerospace
structures where a combination of high strength with moderate toughness and corrosion
resistance are required. Typical applications are alclad skin sheet, structural plate
components up to 4 inches in thickness and general aluminum aerospace applications.
Besides, it is widely used for construction of aircraft structures, such as wings and
fuselages. Because it properties are strength and light weight, alloy 7075 are also
desirable in other fields likes rock climbing equipment, bicycle components, and hang
glider airframes.
One interesting use for 7075 is in the manufacture of M16 rifle for the American
military. It is also commonly used in shafts for lacrosse sticks and due to its strength,
low density, thermal properties and its polish ability 7075 is widely used in mould tool
manufacture (Sanders, 2001).
8