Mechanical And Microstructure Characterization Of Oil Treated Aluminium Alloy 7075.
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
MECHANICAL AND MICROSTRUCTURE
CHARACTERIZATION OF OIL TREATED ALUMINUM
ALLOY7075
This report submitted in accordance with requirement of the Universiti Teknikal
Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering
(Engineering Materials) (Hons.)
by
MOHAMAD IZZUDIN B. ABDUL HAMID
B050810048
890215-07-5147
FACULTY OF MANUFACTURING ENGINEERING
2012
DECLARATION
I hereby, declared this report Entitled “Mechanical and Microstructure Characterization
of Oil Treated Aluminum Alloy 7075” is the results of my own research except as cited
in references.
Signature
:
_________________________________
Author’s Name :
MOHAMAD IZZUDIN B. ABDUL HAMID
Date
06 JUNE 2012
:
ABSTRAK
Tujuan kajian ini dijalankan ialah untuk menyiasat kesan proses kemunduran dan
penuaan semula terhadap struktur mikro, tekanan kakisan retak, kekonduksian dan
ciri mekanikal untuk aloi aluminium 7075 bila dikenakan proses kemunduran di
dalam minyak. Dalam kajian ini, aloi aluminium 7075 akan digunakan sebagai bahan
mentah berdasarkan sifatnya yang mempunyai paras kekuatan tertinggi di kalangan
aloi tempa dan mempunyai rintangan tekanan kakisan yang baik. Ketika penyediaan
sampel, aloi aluminium 7075 dikenakan solusi rawatan haba dengan dipanaskan
sampel di dalam relau pada suhu 470 °C selama 1 jam dan diikuti dengan penyejukan
mendadak pada suhu bilik menggunakan air. Sampel penyejukan mendadak akan
dipanaskan semula untuk pengerasan penuaan pada suhu 120 °C selama 24 jam.
Keputusan menunjukkan sampel yang menjalankan dengan kemunduran minyak
pada 165oC selama 10 minit mempunyai sifat-sifat mekanikal tertinggi dicapai
menghampiri kekuatan T6 berbanding dengan parameter sampel lain. Dengan
parameter yang semakin meningkat pada masa dan suhu, kekuatan mekanikal secara
beransur-ansur berkurangan kepada nilai kekuatan T73. Mikrostruktur aluminium
aloi 7075 dirawat banyak mempengaruhi sifat mekanik spesimen. Pembentukan fasa
η yang hadir sebagai pengukuhan mendakan menyumbang kepada kekuatan sampel.
Perbandingan untuk kerentanan kakisan aloi aluminium dirawat 7075, keputusan
menunjukkan kerentanan SCC boleh dipertingkatkan dengan rawatan RRA.
Kemunduran
dan
penuaan
semula
(RRA)
menunjukkan
rawatan
mampu
menghasilkan kombinasi sifat T6 dan T73 dengan pengurangan kekuatan T6 untuk
meningkatkan kerentanan SCC mendekati T73 tempers.
i
ABSTRACT
The objectives of this research are to investigate the effect of retrogression and reaging (RRA) process on microstructure, corrosion susceptibility (both in applied
force (SCC) and without applied force condition) and mechanical properties of
aluminum alloy 7075 when performed by oil retrogression. In this research,
aluminum alloy 7075 have used as raw material of sample according to the highest
attainable strength levels of all forged alloys and is capable of good stress corrosion
resistance. In this sample preparation, aluminum alloy 7075 was have solution heat
treatment by heated samples in the furnace to the temperature of 470 °C for one hour
and followed by water quenching at room temperature. The quenched samples have
preheated for age hardening at 120 °C for 24 hours. Then, the RRA process has
applied to the sample in the T6 condition to improve its stress corrosion cracking
(SCC) by immerse it into oil. Temperature and time is the variable for the RRA
process. Samples that have done with oil retrogression then have re-aging at 120 °C
for 24 hours. Result shows the samples that perform with oil retrogression at 165oC
for 10 minutes have highest attainable mechanical properties approximate the T6
strength compared to others parameters of sample. With increasing parameters of
time and temperature, the mechanical strength gradually decreases to the values close
to the T73 tempers. Microstructures of the treated aluminium alloy 7075 majorly
influence the mechanical properties of the specimens. Formation of the η phase that
presents as strengthening precipitates contributes most to strength of the samples.
Comparison to the corrosion susceptibility of the treated aluminium alloy 7075, the
results showed SCC susceptibility can be enhanced by RRA treatment. Retrogression
and re-aging (RRA) treatment shows able to produce combination of T6 and T73
properties by make reduction of T6 strength to improve SCC susceptibility
approached T73 tempers.
ii
DEDICATION
To my father; Abdul Hamid Bin Abd Razak, my mother; Noriah Binti Chik, siblings
and friends. Your love is my driving force.
To my supervisor, Dr. Mohd Warikh Bin Abd Rashid
To my Love
To Alans Ng that mostly guide me in this project
Thank you for all your supports, guidance, helps and co-operation, directly or in
directly.
iii
ACKNOWLEDGEMENTS
Alhamdulillah, thanks to Allah S.W.T for giving me a chance to complete my Final
Year Project until the end. This journey is not an easy path, but due to the help and
supports from all the people around, the burdens is lessen. First of all, it is very
appropriate for me to thanks my family as their undivided love, undeniable support
and that had been beacons to motivate me through the hardest situations and it will
also spur me on to greater achievement in the future. My most appreciation is to my
supervisor, Dr. Mohd Warikh Bin Abd Rashid for his guidance, concerns and
patience in leading me the way. Special thanks to the technician and staff in Faculty
of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka (UTeM) who
had involved in. Lastly, thank you very much to all my friends for their help and
support. Thanks to all of you.
iv
TABLE OF CONTENT
Abstrak
i
Abstract
ii
Dedication
iii
Acknowledgement
iv
Table of Content
v
List of Tables
viii
List of Figures
ix
List Abbreviations, Symbols and Nomenclatures
xii
CHAPTER 1 :
INTRODUCTION
1.1
Background of study
1
1.2
Problem Statement
3
1.3
Objectives
4
1.4
Scope
5
CHAPTER 2 :
2.1
2.2
LITERATURE REVIEW
Introduction heat treatment of aluminum alloy
6
2.1.1
Aluminum alloy temper designation
7
2.1.1.1
Basic temper designation
7
2.1.1.2
Heat treating T temper codes
8
Strengthening by Heat Treatment
9
2.2.1
Solution heat treatment
9
2.2.2
Time of temperature
10
2.2.3
Quenching
11
2.2.4
Age hardening
12
2.2.4.1
Natural aging
12
Precipitation hardening (Artificial
13
2.2.4.2
aging)
v
2.2.5
2.3
2.4
2.5
2.6
Retrogression and re-aging (RAA) process
14
Aluminum Alloys
15
2.3.1
Aluminum alloy designation
16
2.3.2
Aluminum alloy 7075
17
Mechanical Properties
18
2.4.1
Tensile properties
18
2.4.2
Hardness properties
21
2.4.2.1 Rockwell hardness test
22
Microstructure Investigation
23
2.5.1
23
Optical microscope (OM)
Corrosion Test
25
2.6.1
Tafel extrapolation method
25
2.6.2
Stress corrosion craking (SCC)
26
CHAPTER 3 :
METHODOLOGY
3.1
Introduction
28
3.2
Flowchart for Overall Process
29
3.3
Material Preparation
30
3.3.1
31
3.4
Heat Treatment Process
31
3.4.1
Sample preparation
31
3.4.1.1
Heat treatment
31
3.4.1.2
Microstructure investigation
33
3.4.2
3.5
Aluminum Alloy 7075 preparation
Solution heat treatment (SHT)
33
3.4.2.1
T6 temper
33
3.4.2.2
T73 temper
33
3.4.3
Quenching
34
3.4.4
Retrogression and re-aging treatment process
36
Mechanical Properties
36
3.5.1
36
Tensile test
vi
3.5.2
3.6
3.7
Hardness test
37
Microstructure Investigation
38
3.6.1
38
Optical microscope (OM)
Corrosion Test
38
3.7.1
Tafel Extrapolation Method
38
3.7.2
Stress Corrosion Cracking
39
CHAPTER 4:
RESULTS AND DISCUSSION
4.1
Characterization of aluminum alloy 7075
40
4.2
Hardness properties of treated Aluminum Alloy 7075
42
4.2.1
42
4.3
4.4
4.5
Hardness Properties
Tensile properties of treated Aluminum Alloy 7075
43
4.3.1
Tensile Properties
42
4.3.1.1
Ultimate Tensile Strength
46
4.3.1.2
Ductility (Percentage of elongation)
47
Microstructure of treated Aluminum Alloy 7075
48
4.4.1
Microstructure observation analysis
48
Corrosion properties of treated Aluminum Alloy 7075
52
4.5.1
Tafel Extrapolation Method
52
4.5.2
Stress Corrosion Cracking (SCC) Susceptibility
53
CHAPTER 5:
CONCLUSION AND
RECOMMENDATIONS
5.1
Conclusion
58
5.2
Recommendations
59
REFERENCES
60
APPENDICES
A
Gantt chart of PSM 1
B
Gantt chart of PSM 2
C
Drawing of material specimen
D
Results of Tensile test
vii
E
Results of Hardness Test
F
SCC results using Tafel software
G
Machine used and setup equipment
viii
LIST OF TABLES
Figure
Titles
Page
2.1
Basic Temper Designation
7
2.2
T Temper Codes
8
2.3
Chemical Composition of Aluminum Alloy 7075
17
2.4
Physical Properties of AA 7075
17
4.1
Heat treatment of specimens
41
4.2
Standards Mechanical values for Aluminum Alloy 7075
41
4.3
Hardness values for aluminum alloy 7075.
42
4.4
Tensile properties values for aluminum alloy 7075
44
4.5
Results of Icorr , i corr ,E corr and CPR
52
4.6
Results of total anodic current (I corr ), corrosion current density (i corr )
53
and corrosion potential values (E corr ).
ix
LIST OF FIGURES
Figure
Titles
Page
2.1
Schematic of the Retrogression and Re-aging Process
14
2.2
Stress-strain curve
19
2.3
TEM microstructure of the T6 tempered aluminum alloy 7075
23
2.4
TEM microstructure of the T7 tempered aluminum alloy 7075
24
2.5
TEM microstructure of the RRA tempered aluminum alloy 7075
24
2.3
Tafel Extrapolation Method
25
3.1
Flowchart of Process
29
3.2
Specimen of Aluminum Alloy 7075
30
3.3
Sample preparation for microstructure observation
32
3.4
Specimens quenched in ice water.
34
3.5
Illustration setup for oil retrogression
35
3.6
Equipment setting for oil RRA treatment.
36
3.7
Specimen Dimension
37
SCC test setup, where force applied to the specimens in the NaCl
39
3.8
4.1
4.2
4.3
4.4
solution.
Specimens declaration
40
Hardness results for heat treated specimens of xaluminium alloy
42
7075
Tensile strength comparison for O612 (165oC, 10min.) and O633
44
(165oC, 30min)
Tensile strength comparison for O813 (185oC, 10min.) and O833
45
(185oC, 30min.)
4.5
Tensile strength comparison for O211 (205oC, 10min.) and O233
(205oC, 30min.)
45
4.6
Ultimate Tensile Strength (UTS) values
46
4.7
Percentage elongation of the specimens
47
Microstructure observation of Aluminum Alloy 7075 in various
51
4.8
heat treatments at 100 x magnification using Optical Microscope.
x
4.9
Comparison of T6 and T73
54
4.10
Comparison of SCC O614 and SCC O634
55
4.11
Comparison of SCC O815 and SCC O834
55
4.12
Comparison of SCC O214 and SCC O234
56
4.13
Result of Tafel Extrapolation Method in no applied force conditions
56
Result of Tafel Extrapolation Method in applied force conditions
57
4.14
(55kN)
xi
LIST OF ABBREVIATIONS, SYMBOLS AND
NOMENCLATURES
RRA
-
Retrogression and Re-Aging
SCC
-
Stress Corrosion Cracking
OM
-
Optical Microscope
AA7075
-
Aluminum Alloy 7075
PCT
-
Process-critical temperature
FAA
-
Federal Aviation Administration
ASTM
-
American Testing for Testing and Materials
IACS
-
International Annealed Copper Standard
Cu
-
Copper
Si
-
Silicon
Fe
-
Iron
Mn
-
Manganese
Cr
-
Chromium
Mg
-
Magnesium
Zn
-
Zinc
Al
-
Aluminum
H
-
Hydrogen
GP
-
Guiner-Preston
SiC
-
Silicon Carbide
NaCl
-
Sodium Chloride
xii
CHAPTER 1
INTRODUCTION
1.1
Background of Study
Aluminum is one of the most elements in earth with appearance in range from silvery to
dull gray that have properties like low density, ease of fabrication, good corrosion
resistance, high strength-to-weight ratio and high fracture toughness. Because of these
properties, aluminum used widely according to it most economical and structurally
effective material for commercial applications. But, usually pure aluminum alloyed with
several elements to improve properties required. Aluminum is a big family of material
due to different alloying element that produces different series of aluminum alloy.
Nowadays, application of the aluminum alloy in automotive, aircraft, military and
industry cannot be deny. Among the others series of aluminum, alloy 7075 commonly
used in aircraft industry. Alloy 7075 listed with others in family of 7xxx series alloys.
Mitchell (2004) state that alloy 7075, has one of the highest attainable strength levels of
all forged alloys and is capable of good stress corrosion resistance. For that reason,
AA7075 widely used in aircraft industry function well in aircraft body structure and
component parts.
Alloy 7075 have characteristic that fulfill the requirement of aircraft body structure
where a combination of high strength, moderate toughness and good corrosion resistance
as required. Besides that, the optimum strength, toughness and others desired
characteristic can be occupied by heat treatment and others heat treating process like
1
retrogression and re-aging (RRA) process. Alloy 7075-T6 heat treatment condition has
been widely used for aircraft structural applications. Corrosion damage is often the
reason why 7075-T6 components are replaced.
According to Wallace et al., (2006), the concept of retrogression and re-aging (RRA)
was first developed by Cina and his colleagues at the Israel Aircraft Industries in 1974.
Cina et al., (1974) state that T6-treated alloy of AA7075 has high strength, but have
poor stress corrosion resistance. Improvement of corrosion resistance for T6 required
and the methods to improve this corrosion resistance is over-aging AA7075 to get T73
tempers. But by over-aging, the stress corrosion resistance can be improved, but the
strength may be decreased by 10%−15% (Zhang et al., 2010). This precludes the use of
the T73 temper when high strength required. RRA that applied to 7075 alloy in T6
temper shown the result of stress corrosion cracking (SCC) resistance can be enhanced
with minimal tradeoff in strength approaching T73 condition.
According to Quench Factor analysis, developed by Staley, there have relationship
between the cooling rate and the mechanical properties of age-hardable aluminum alloy
(Ma, 2006). During heat treatment, heating and cooling operation are performed that
functions to alter the mechanical properties, microstructure, or residual stress state of the
alloy. Alloys that have age-hardenable involve solutionizing, quenching, and then aging
at room temperature (natural aging) or at the elevated temperature (artificial aging).
Mechanical properties depend on high quench rates after the solutionizing treatment to
maximize the artificial aging (precipitation hardening) response.
The precipitation treatment used to develop these tempers requires two-stage artificial
aging, the second stage of which is done at a higher temperature than that used to
produce T6 tempers. During the preliminary stage, fine high-density precipitation
dispersion is nucleated, producing high strength. The second stage is then used to
develop resistance to SCC and exfoliation. During age-hardening, or also known as
precipitation hardening, precipitates form in a super saturated solid solution and strength
increases as the number and size of precipitates increase until a maximum critical value
2
is reached and the material becomes overaged and coarsening begins, reducing
mechanical properties, Nowill (2007). The final microstructure was found to be highly
dependent on the retrogression treatment, and the final microstructure of T6 and T73
tempers was both unique and significantly different from each others.
1.2
Problem Statement
Nowadays, aluminum alloys have been widely used in automotive and aircraft industries
due to its good properties and high strength-to-weight ratio. Components and structure
body of aircraft continuously expose to thermal shock, extreme weather, corrosion agent
and others that tends it to corrosion damage. Because of that, the parts of aircraft
required high corrosion resistance beside high strength. To fulfill the requirement of
corrosion resistance, applications of over-aged temper T73 look suitable because
performed high corrosion resistance, but in comparison to 7075-T6, there is a strength
penalty of 10 to 15%, which precludes the use of the T73 temper, when high strength is
required, (Peeler et al., 2002).
Stress corrosion cracking occurs commonly undetectable and not apparent at the first
place. There initiates with microcracks, where the cracks normally occur at the grain
level. The corrosion takes place on the surfaces of the material, forming surface
discontinuities that eventually becomes the stress raiser or notch to crack propagation at
microstructural level, but the mechanism of crack propagation in such phenomenon is
not merely caused by the atomic dislocation due to the stress, but it is also caused by the
chemical attack on the crack tip, causing inter-granular crack along the grain boundaries
of the material. Hence, this cracks potentially causing the failure of the material
according to chemical attacks and mechanical factors.
Extensive researches on RRA treatment were done before that mostly focuses on the
effects of retrogression heating rate temperature and retrogression time on the
3
microstructures, corrosion resistant and mechanical properties. Retrogression can be
applied in air furnace and oil bath. Mostly research used the air furnace as retrogression
medium. But, this research focused on the effects of retrogression process in oil bath
with different parameter of retrogression temperature and time on the microstructures,
corrosion resistant and mechanical properties of AA7075. Peeler et al., (2002) state RRA
process can be applied to improve corrosion resistance of T6 to levels approaching those
of the T73 condition while maintaining the strength. However, RRA heat treatment
result reduction of 5 to 7 percent of tensile strength for T6 temper. Different
retrogression medium absolutely gives different result and effects to 7075 alloys.
So, this investigation is to improve SCC resistance of aluminum alloy 7075-T6
approaching T73 while maintaining the strength at levels at or slightly below the T6
condition by retrogression it in oil medium. Affects of retrogression process in oil also
will be analyzed experimentally in this research. Experiments have done to investigate
the microstructure change of the RRA sample and interface behavior of mechanical
behavior. Microstructure features has determine to compare with other alloy and found
the formation of microstructure after RRA.
1.3
Objectives
The objectives of the study are listed below:
i. To investigate the effect of oil Retrogression and Re-aging (RRA) in different
parameter of temperature and time to aluminum alloy 7075.
ii. To analyze the microstructure of sample after retrogression in oil bath.
iii. To determine the mechanical properties and corrosion resistant of AA7075-T6
after retrogression in oil bath.
4
1.4
Scope of Study
The scope of this analysis is to investigate the role of heat treatment to aluminum alloy
7075. Completely understand the heat treatment process especially retrogression and reaging (RRA) effects that can improve the aluminum alloy to desire properties. Extensive
researches on RRA treatment were done before that mostly focuses on the effects of
retrogression heating rate, but this analysis focused on the effects of retrogression in oil
in different parameter of temperature and retrogression time. Improvement mechanical
properties and alteration of microstructure covered in this study. Optical microscope
(OM) used to observe the microstructure change after RRA process. Hardness and
tensile test used to determine the mechanical properties. Corrosion resistance of the
aluminum alloy 7075 was measured in term of normal corrosion rate (no load applied)
and corrosion rate with applied load (SCC) by using Tafel Extrapolation Method to find
the SCC properties of the 7075 alloy.
5
CHAPTER 2
LITERATURE REVIEW
2.1
Introduction Heat treatment of Aluminum Alloy
Aluminum alloys fall into two general categories: heat-treatable and non-heat treatable.
Heat treatment can be either of the heating or cooling operations that performed for the
purpose of changing the mechanical properties, the metallurgical structure, and the
residual stress state of alloy. Wrought and casting alloy is restricted to the specific
operations employed to increase strength and hardness by precipitation hardening thus
the term heat treatable serves to distinguish the heat treatable alloys from those alloys in
which no significant strength improvement can be achieved by heating and cooling.
According to Quench Factor analysis, developed by Staley, there have relationship
between the cooling rate and the mechanical properties of age-hardable aluminum alloy
(Ma, 2006).
Series 7xxx alloys, considered the high strength aircraft alloy family, are heat treatable
by solution and aging. Various aging cycles produce desired attributes such as maximum
attainable strength (T6 temper) or stress corrosion resistance (T73 temper). Either way,
the alloy must go through solution treatment, the goal of which is to completely dissolve
into solid solution all alloy elements responsible for subsequent precipitation hardening
and retained by rapid quenching. Subsequent heat treatment at tower temperatures i.e.
ageing or natural ageing at room temperature allows for a controlled precipitation of the
constituents thereby achieving increased hardness and strength. The degree of hardening
obtained depends on the size, number and relative strength of the precipitates.
6
2.1.1 Aluminum Alloy Temper Designations
The physical properties exhibited by aluminum alloys are significantly influenced by the
treatment of the sample. A standardized system has been developed to designate these
treatments. First character in the temper designation are defined and described as
follows:
2.1.1.1 Basic Temper Designation
Table 2.1: Basic Temper Designation
F
As Fabricated – This designation is used for cast products made by any casting
process and refers to the condition of the casting as it comes from the mold
without any further thermal or mechanical treatment. No special control has
been performed to the heat treatment or strain hardening after the shaping
process such as casting, hot working, or cold working.
O
Annealed – This designation is used for cast alloys that are annealed. For
casting, the treatment (high-temperature stabilization and recrystallization
treatment) may be used both to improve ductility and increase dimension
stability. This is the lowest strength, highest ductility temper.
H
Strain Hardened - (applied to wrought products only) Used for products that
have been strengthened by strain hardening, with or without subsequent heat
treatment. The designation is followed by two or more numbers as discussed
below.
7
Solution Heat Treated - This is seldom encountered because it is an unstable
temper that applies only to alloys that spontaneously age at ambient temperature
W
after heat treatment.
T
Solution Heat Treated - Used for products that have been strengthened by heat
treatment, with or without subsequent strain hardening. The designation is
followed by one or more numbers as discussed below.
2.1.1.2 Heat Treating T Temper Codes
Table 2.2: T Temper Codes
T1
Cooled from an elevated temperature shaping process and naturally aged to a
substantially stable condition.
T2
Cooled from an elevated temperature shaping process, cold worked, and
naturally aged to a substantially stable condition.
T3
Solution heat treated, cold worked, and naturally aged to a substantially stable
condition.
T4
Solution heat treated, and naturally aged to a substantially stable condition.
T5
Cooled from an elevated temperature shaping process then artificially aged.
T6
Solution heat treated then artificially aged.
T7
Solution heat treated then and over-aged/stabilized
T8
Solution heat treated, cold worked, and then artificially aged.
T9
Solution heat treated, artificially aged, and then cold worked.
T10
Cooled from an elevated temperature shaping process, cold worked, then
artificially aged.
8
2.2
Strengthening by Heat Treatment
Heat treating process for aluminum alloys are precision processes. They must be carried
out in furnace properly and build to provide the thermal conditions required, and
equipped with control instruments to insure the desired continuity and uniformity of
temperature and time. Process details must be established and controlled carefully to
ensure it occupied the final desired characteristics.
Based on Federal Aviation Adminstation (2008), the hardening of aluminum alloy by
heat treatment consists of four-step:
•
Solution heat treatment: heating to predetermine temperature and soaking for a
period of time
•
Quenching: rapidly quenching to relatively low temperature
•
Age hardening: precipitation of solute atoms either at room temperature (natural
aging) or elevated temperature (artificial aging or precipitation heat treatment).
•
Retrogression and re-aging (RRA): is an intermediate heat treatment which when
applied to Al 7075-T6 material results in improved corrosion resistance
approaching the T73 temper, while maintaining the high strength of the T6
temper.
2.2.1 Solution Heat Treatment
To take advantage of the precipitation hardening reaction, it is necessary first to produce
a solid solution. The process by which this is accomplished is called solution heat
treating, and its objective is to take into solid solution the maximum practical amounts of
the soluble hardening elements in the alloy. The principles and procedures for heat
treating wrought and cast alloys are similar. The process consists of heating and holding
the alloy at a temperature sufficiently high and for a long enough period of time to
achieve a nearly homogenous solid solution.
9
MECHANICAL AND MICROSTRUCTURE
CHARACTERIZATION OF OIL TREATED ALUMINUM
ALLOY7075
This report submitted in accordance with requirement of the Universiti Teknikal
Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering
(Engineering Materials) (Hons.)
by
MOHAMAD IZZUDIN B. ABDUL HAMID
B050810048
890215-07-5147
FACULTY OF MANUFACTURING ENGINEERING
2012
DECLARATION
I hereby, declared this report Entitled “Mechanical and Microstructure Characterization
of Oil Treated Aluminum Alloy 7075” is the results of my own research except as cited
in references.
Signature
:
_________________________________
Author’s Name :
MOHAMAD IZZUDIN B. ABDUL HAMID
Date
06 JUNE 2012
:
ABSTRAK
Tujuan kajian ini dijalankan ialah untuk menyiasat kesan proses kemunduran dan
penuaan semula terhadap struktur mikro, tekanan kakisan retak, kekonduksian dan
ciri mekanikal untuk aloi aluminium 7075 bila dikenakan proses kemunduran di
dalam minyak. Dalam kajian ini, aloi aluminium 7075 akan digunakan sebagai bahan
mentah berdasarkan sifatnya yang mempunyai paras kekuatan tertinggi di kalangan
aloi tempa dan mempunyai rintangan tekanan kakisan yang baik. Ketika penyediaan
sampel, aloi aluminium 7075 dikenakan solusi rawatan haba dengan dipanaskan
sampel di dalam relau pada suhu 470 °C selama 1 jam dan diikuti dengan penyejukan
mendadak pada suhu bilik menggunakan air. Sampel penyejukan mendadak akan
dipanaskan semula untuk pengerasan penuaan pada suhu 120 °C selama 24 jam.
Keputusan menunjukkan sampel yang menjalankan dengan kemunduran minyak
pada 165oC selama 10 minit mempunyai sifat-sifat mekanikal tertinggi dicapai
menghampiri kekuatan T6 berbanding dengan parameter sampel lain. Dengan
parameter yang semakin meningkat pada masa dan suhu, kekuatan mekanikal secara
beransur-ansur berkurangan kepada nilai kekuatan T73. Mikrostruktur aluminium
aloi 7075 dirawat banyak mempengaruhi sifat mekanik spesimen. Pembentukan fasa
η yang hadir sebagai pengukuhan mendakan menyumbang kepada kekuatan sampel.
Perbandingan untuk kerentanan kakisan aloi aluminium dirawat 7075, keputusan
menunjukkan kerentanan SCC boleh dipertingkatkan dengan rawatan RRA.
Kemunduran
dan
penuaan
semula
(RRA)
menunjukkan
rawatan
mampu
menghasilkan kombinasi sifat T6 dan T73 dengan pengurangan kekuatan T6 untuk
meningkatkan kerentanan SCC mendekati T73 tempers.
i
ABSTRACT
The objectives of this research are to investigate the effect of retrogression and reaging (RRA) process on microstructure, corrosion susceptibility (both in applied
force (SCC) and without applied force condition) and mechanical properties of
aluminum alloy 7075 when performed by oil retrogression. In this research,
aluminum alloy 7075 have used as raw material of sample according to the highest
attainable strength levels of all forged alloys and is capable of good stress corrosion
resistance. In this sample preparation, aluminum alloy 7075 was have solution heat
treatment by heated samples in the furnace to the temperature of 470 °C for one hour
and followed by water quenching at room temperature. The quenched samples have
preheated for age hardening at 120 °C for 24 hours. Then, the RRA process has
applied to the sample in the T6 condition to improve its stress corrosion cracking
(SCC) by immerse it into oil. Temperature and time is the variable for the RRA
process. Samples that have done with oil retrogression then have re-aging at 120 °C
for 24 hours. Result shows the samples that perform with oil retrogression at 165oC
for 10 minutes have highest attainable mechanical properties approximate the T6
strength compared to others parameters of sample. With increasing parameters of
time and temperature, the mechanical strength gradually decreases to the values close
to the T73 tempers. Microstructures of the treated aluminium alloy 7075 majorly
influence the mechanical properties of the specimens. Formation of the η phase that
presents as strengthening precipitates contributes most to strength of the samples.
Comparison to the corrosion susceptibility of the treated aluminium alloy 7075, the
results showed SCC susceptibility can be enhanced by RRA treatment. Retrogression
and re-aging (RRA) treatment shows able to produce combination of T6 and T73
properties by make reduction of T6 strength to improve SCC susceptibility
approached T73 tempers.
ii
DEDICATION
To my father; Abdul Hamid Bin Abd Razak, my mother; Noriah Binti Chik, siblings
and friends. Your love is my driving force.
To my supervisor, Dr. Mohd Warikh Bin Abd Rashid
To my Love
To Alans Ng that mostly guide me in this project
Thank you for all your supports, guidance, helps and co-operation, directly or in
directly.
iii
ACKNOWLEDGEMENTS
Alhamdulillah, thanks to Allah S.W.T for giving me a chance to complete my Final
Year Project until the end. This journey is not an easy path, but due to the help and
supports from all the people around, the burdens is lessen. First of all, it is very
appropriate for me to thanks my family as their undivided love, undeniable support
and that had been beacons to motivate me through the hardest situations and it will
also spur me on to greater achievement in the future. My most appreciation is to my
supervisor, Dr. Mohd Warikh Bin Abd Rashid for his guidance, concerns and
patience in leading me the way. Special thanks to the technician and staff in Faculty
of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka (UTeM) who
had involved in. Lastly, thank you very much to all my friends for their help and
support. Thanks to all of you.
iv
TABLE OF CONTENT
Abstrak
i
Abstract
ii
Dedication
iii
Acknowledgement
iv
Table of Content
v
List of Tables
viii
List of Figures
ix
List Abbreviations, Symbols and Nomenclatures
xii
CHAPTER 1 :
INTRODUCTION
1.1
Background of study
1
1.2
Problem Statement
3
1.3
Objectives
4
1.4
Scope
5
CHAPTER 2 :
2.1
2.2
LITERATURE REVIEW
Introduction heat treatment of aluminum alloy
6
2.1.1
Aluminum alloy temper designation
7
2.1.1.1
Basic temper designation
7
2.1.1.2
Heat treating T temper codes
8
Strengthening by Heat Treatment
9
2.2.1
Solution heat treatment
9
2.2.2
Time of temperature
10
2.2.3
Quenching
11
2.2.4
Age hardening
12
2.2.4.1
Natural aging
12
Precipitation hardening (Artificial
13
2.2.4.2
aging)
v
2.2.5
2.3
2.4
2.5
2.6
Retrogression and re-aging (RAA) process
14
Aluminum Alloys
15
2.3.1
Aluminum alloy designation
16
2.3.2
Aluminum alloy 7075
17
Mechanical Properties
18
2.4.1
Tensile properties
18
2.4.2
Hardness properties
21
2.4.2.1 Rockwell hardness test
22
Microstructure Investigation
23
2.5.1
23
Optical microscope (OM)
Corrosion Test
25
2.6.1
Tafel extrapolation method
25
2.6.2
Stress corrosion craking (SCC)
26
CHAPTER 3 :
METHODOLOGY
3.1
Introduction
28
3.2
Flowchart for Overall Process
29
3.3
Material Preparation
30
3.3.1
31
3.4
Heat Treatment Process
31
3.4.1
Sample preparation
31
3.4.1.1
Heat treatment
31
3.4.1.2
Microstructure investigation
33
3.4.2
3.5
Aluminum Alloy 7075 preparation
Solution heat treatment (SHT)
33
3.4.2.1
T6 temper
33
3.4.2.2
T73 temper
33
3.4.3
Quenching
34
3.4.4
Retrogression and re-aging treatment process
36
Mechanical Properties
36
3.5.1
36
Tensile test
vi
3.5.2
3.6
3.7
Hardness test
37
Microstructure Investigation
38
3.6.1
38
Optical microscope (OM)
Corrosion Test
38
3.7.1
Tafel Extrapolation Method
38
3.7.2
Stress Corrosion Cracking
39
CHAPTER 4:
RESULTS AND DISCUSSION
4.1
Characterization of aluminum alloy 7075
40
4.2
Hardness properties of treated Aluminum Alloy 7075
42
4.2.1
42
4.3
4.4
4.5
Hardness Properties
Tensile properties of treated Aluminum Alloy 7075
43
4.3.1
Tensile Properties
42
4.3.1.1
Ultimate Tensile Strength
46
4.3.1.2
Ductility (Percentage of elongation)
47
Microstructure of treated Aluminum Alloy 7075
48
4.4.1
Microstructure observation analysis
48
Corrosion properties of treated Aluminum Alloy 7075
52
4.5.1
Tafel Extrapolation Method
52
4.5.2
Stress Corrosion Cracking (SCC) Susceptibility
53
CHAPTER 5:
CONCLUSION AND
RECOMMENDATIONS
5.1
Conclusion
58
5.2
Recommendations
59
REFERENCES
60
APPENDICES
A
Gantt chart of PSM 1
B
Gantt chart of PSM 2
C
Drawing of material specimen
D
Results of Tensile test
vii
E
Results of Hardness Test
F
SCC results using Tafel software
G
Machine used and setup equipment
viii
LIST OF TABLES
Figure
Titles
Page
2.1
Basic Temper Designation
7
2.2
T Temper Codes
8
2.3
Chemical Composition of Aluminum Alloy 7075
17
2.4
Physical Properties of AA 7075
17
4.1
Heat treatment of specimens
41
4.2
Standards Mechanical values for Aluminum Alloy 7075
41
4.3
Hardness values for aluminum alloy 7075.
42
4.4
Tensile properties values for aluminum alloy 7075
44
4.5
Results of Icorr , i corr ,E corr and CPR
52
4.6
Results of total anodic current (I corr ), corrosion current density (i corr )
53
and corrosion potential values (E corr ).
ix
LIST OF FIGURES
Figure
Titles
Page
2.1
Schematic of the Retrogression and Re-aging Process
14
2.2
Stress-strain curve
19
2.3
TEM microstructure of the T6 tempered aluminum alloy 7075
23
2.4
TEM microstructure of the T7 tempered aluminum alloy 7075
24
2.5
TEM microstructure of the RRA tempered aluminum alloy 7075
24
2.3
Tafel Extrapolation Method
25
3.1
Flowchart of Process
29
3.2
Specimen of Aluminum Alloy 7075
30
3.3
Sample preparation for microstructure observation
32
3.4
Specimens quenched in ice water.
34
3.5
Illustration setup for oil retrogression
35
3.6
Equipment setting for oil RRA treatment.
36
3.7
Specimen Dimension
37
SCC test setup, where force applied to the specimens in the NaCl
39
3.8
4.1
4.2
4.3
4.4
solution.
Specimens declaration
40
Hardness results for heat treated specimens of xaluminium alloy
42
7075
Tensile strength comparison for O612 (165oC, 10min.) and O633
44
(165oC, 30min)
Tensile strength comparison for O813 (185oC, 10min.) and O833
45
(185oC, 30min.)
4.5
Tensile strength comparison for O211 (205oC, 10min.) and O233
(205oC, 30min.)
45
4.6
Ultimate Tensile Strength (UTS) values
46
4.7
Percentage elongation of the specimens
47
Microstructure observation of Aluminum Alloy 7075 in various
51
4.8
heat treatments at 100 x magnification using Optical Microscope.
x
4.9
Comparison of T6 and T73
54
4.10
Comparison of SCC O614 and SCC O634
55
4.11
Comparison of SCC O815 and SCC O834
55
4.12
Comparison of SCC O214 and SCC O234
56
4.13
Result of Tafel Extrapolation Method in no applied force conditions
56
Result of Tafel Extrapolation Method in applied force conditions
57
4.14
(55kN)
xi
LIST OF ABBREVIATIONS, SYMBOLS AND
NOMENCLATURES
RRA
-
Retrogression and Re-Aging
SCC
-
Stress Corrosion Cracking
OM
-
Optical Microscope
AA7075
-
Aluminum Alloy 7075
PCT
-
Process-critical temperature
FAA
-
Federal Aviation Administration
ASTM
-
American Testing for Testing and Materials
IACS
-
International Annealed Copper Standard
Cu
-
Copper
Si
-
Silicon
Fe
-
Iron
Mn
-
Manganese
Cr
-
Chromium
Mg
-
Magnesium
Zn
-
Zinc
Al
-
Aluminum
H
-
Hydrogen
GP
-
Guiner-Preston
SiC
-
Silicon Carbide
NaCl
-
Sodium Chloride
xii
CHAPTER 1
INTRODUCTION
1.1
Background of Study
Aluminum is one of the most elements in earth with appearance in range from silvery to
dull gray that have properties like low density, ease of fabrication, good corrosion
resistance, high strength-to-weight ratio and high fracture toughness. Because of these
properties, aluminum used widely according to it most economical and structurally
effective material for commercial applications. But, usually pure aluminum alloyed with
several elements to improve properties required. Aluminum is a big family of material
due to different alloying element that produces different series of aluminum alloy.
Nowadays, application of the aluminum alloy in automotive, aircraft, military and
industry cannot be deny. Among the others series of aluminum, alloy 7075 commonly
used in aircraft industry. Alloy 7075 listed with others in family of 7xxx series alloys.
Mitchell (2004) state that alloy 7075, has one of the highest attainable strength levels of
all forged alloys and is capable of good stress corrosion resistance. For that reason,
AA7075 widely used in aircraft industry function well in aircraft body structure and
component parts.
Alloy 7075 have characteristic that fulfill the requirement of aircraft body structure
where a combination of high strength, moderate toughness and good corrosion resistance
as required. Besides that, the optimum strength, toughness and others desired
characteristic can be occupied by heat treatment and others heat treating process like
1
retrogression and re-aging (RRA) process. Alloy 7075-T6 heat treatment condition has
been widely used for aircraft structural applications. Corrosion damage is often the
reason why 7075-T6 components are replaced.
According to Wallace et al., (2006), the concept of retrogression and re-aging (RRA)
was first developed by Cina and his colleagues at the Israel Aircraft Industries in 1974.
Cina et al., (1974) state that T6-treated alloy of AA7075 has high strength, but have
poor stress corrosion resistance. Improvement of corrosion resistance for T6 required
and the methods to improve this corrosion resistance is over-aging AA7075 to get T73
tempers. But by over-aging, the stress corrosion resistance can be improved, but the
strength may be decreased by 10%−15% (Zhang et al., 2010). This precludes the use of
the T73 temper when high strength required. RRA that applied to 7075 alloy in T6
temper shown the result of stress corrosion cracking (SCC) resistance can be enhanced
with minimal tradeoff in strength approaching T73 condition.
According to Quench Factor analysis, developed by Staley, there have relationship
between the cooling rate and the mechanical properties of age-hardable aluminum alloy
(Ma, 2006). During heat treatment, heating and cooling operation are performed that
functions to alter the mechanical properties, microstructure, or residual stress state of the
alloy. Alloys that have age-hardenable involve solutionizing, quenching, and then aging
at room temperature (natural aging) or at the elevated temperature (artificial aging).
Mechanical properties depend on high quench rates after the solutionizing treatment to
maximize the artificial aging (precipitation hardening) response.
The precipitation treatment used to develop these tempers requires two-stage artificial
aging, the second stage of which is done at a higher temperature than that used to
produce T6 tempers. During the preliminary stage, fine high-density precipitation
dispersion is nucleated, producing high strength. The second stage is then used to
develop resistance to SCC and exfoliation. During age-hardening, or also known as
precipitation hardening, precipitates form in a super saturated solid solution and strength
increases as the number and size of precipitates increase until a maximum critical value
2
is reached and the material becomes overaged and coarsening begins, reducing
mechanical properties, Nowill (2007). The final microstructure was found to be highly
dependent on the retrogression treatment, and the final microstructure of T6 and T73
tempers was both unique and significantly different from each others.
1.2
Problem Statement
Nowadays, aluminum alloys have been widely used in automotive and aircraft industries
due to its good properties and high strength-to-weight ratio. Components and structure
body of aircraft continuously expose to thermal shock, extreme weather, corrosion agent
and others that tends it to corrosion damage. Because of that, the parts of aircraft
required high corrosion resistance beside high strength. To fulfill the requirement of
corrosion resistance, applications of over-aged temper T73 look suitable because
performed high corrosion resistance, but in comparison to 7075-T6, there is a strength
penalty of 10 to 15%, which precludes the use of the T73 temper, when high strength is
required, (Peeler et al., 2002).
Stress corrosion cracking occurs commonly undetectable and not apparent at the first
place. There initiates with microcracks, where the cracks normally occur at the grain
level. The corrosion takes place on the surfaces of the material, forming surface
discontinuities that eventually becomes the stress raiser or notch to crack propagation at
microstructural level, but the mechanism of crack propagation in such phenomenon is
not merely caused by the atomic dislocation due to the stress, but it is also caused by the
chemical attack on the crack tip, causing inter-granular crack along the grain boundaries
of the material. Hence, this cracks potentially causing the failure of the material
according to chemical attacks and mechanical factors.
Extensive researches on RRA treatment were done before that mostly focuses on the
effects of retrogression heating rate temperature and retrogression time on the
3
microstructures, corrosion resistant and mechanical properties. Retrogression can be
applied in air furnace and oil bath. Mostly research used the air furnace as retrogression
medium. But, this research focused on the effects of retrogression process in oil bath
with different parameter of retrogression temperature and time on the microstructures,
corrosion resistant and mechanical properties of AA7075. Peeler et al., (2002) state RRA
process can be applied to improve corrosion resistance of T6 to levels approaching those
of the T73 condition while maintaining the strength. However, RRA heat treatment
result reduction of 5 to 7 percent of tensile strength for T6 temper. Different
retrogression medium absolutely gives different result and effects to 7075 alloys.
So, this investigation is to improve SCC resistance of aluminum alloy 7075-T6
approaching T73 while maintaining the strength at levels at or slightly below the T6
condition by retrogression it in oil medium. Affects of retrogression process in oil also
will be analyzed experimentally in this research. Experiments have done to investigate
the microstructure change of the RRA sample and interface behavior of mechanical
behavior. Microstructure features has determine to compare with other alloy and found
the formation of microstructure after RRA.
1.3
Objectives
The objectives of the study are listed below:
i. To investigate the effect of oil Retrogression and Re-aging (RRA) in different
parameter of temperature and time to aluminum alloy 7075.
ii. To analyze the microstructure of sample after retrogression in oil bath.
iii. To determine the mechanical properties and corrosion resistant of AA7075-T6
after retrogression in oil bath.
4
1.4
Scope of Study
The scope of this analysis is to investigate the role of heat treatment to aluminum alloy
7075. Completely understand the heat treatment process especially retrogression and reaging (RRA) effects that can improve the aluminum alloy to desire properties. Extensive
researches on RRA treatment were done before that mostly focuses on the effects of
retrogression heating rate, but this analysis focused on the effects of retrogression in oil
in different parameter of temperature and retrogression time. Improvement mechanical
properties and alteration of microstructure covered in this study. Optical microscope
(OM) used to observe the microstructure change after RRA process. Hardness and
tensile test used to determine the mechanical properties. Corrosion resistance of the
aluminum alloy 7075 was measured in term of normal corrosion rate (no load applied)
and corrosion rate with applied load (SCC) by using Tafel Extrapolation Method to find
the SCC properties of the 7075 alloy.
5
CHAPTER 2
LITERATURE REVIEW
2.1
Introduction Heat treatment of Aluminum Alloy
Aluminum alloys fall into two general categories: heat-treatable and non-heat treatable.
Heat treatment can be either of the heating or cooling operations that performed for the
purpose of changing the mechanical properties, the metallurgical structure, and the
residual stress state of alloy. Wrought and casting alloy is restricted to the specific
operations employed to increase strength and hardness by precipitation hardening thus
the term heat treatable serves to distinguish the heat treatable alloys from those alloys in
which no significant strength improvement can be achieved by heating and cooling.
According to Quench Factor analysis, developed by Staley, there have relationship
between the cooling rate and the mechanical properties of age-hardable aluminum alloy
(Ma, 2006).
Series 7xxx alloys, considered the high strength aircraft alloy family, are heat treatable
by solution and aging. Various aging cycles produce desired attributes such as maximum
attainable strength (T6 temper) or stress corrosion resistance (T73 temper). Either way,
the alloy must go through solution treatment, the goal of which is to completely dissolve
into solid solution all alloy elements responsible for subsequent precipitation hardening
and retained by rapid quenching. Subsequent heat treatment at tower temperatures i.e.
ageing or natural ageing at room temperature allows for a controlled precipitation of the
constituents thereby achieving increased hardness and strength. The degree of hardening
obtained depends on the size, number and relative strength of the precipitates.
6
2.1.1 Aluminum Alloy Temper Designations
The physical properties exhibited by aluminum alloys are significantly influenced by the
treatment of the sample. A standardized system has been developed to designate these
treatments. First character in the temper designation are defined and described as
follows:
2.1.1.1 Basic Temper Designation
Table 2.1: Basic Temper Designation
F
As Fabricated – This designation is used for cast products made by any casting
process and refers to the condition of the casting as it comes from the mold
without any further thermal or mechanical treatment. No special control has
been performed to the heat treatment or strain hardening after the shaping
process such as casting, hot working, or cold working.
O
Annealed – This designation is used for cast alloys that are annealed. For
casting, the treatment (high-temperature stabilization and recrystallization
treatment) may be used both to improve ductility and increase dimension
stability. This is the lowest strength, highest ductility temper.
H
Strain Hardened - (applied to wrought products only) Used for products that
have been strengthened by strain hardening, with or without subsequent heat
treatment. The designation is followed by two or more numbers as discussed
below.
7
Solution Heat Treated - This is seldom encountered because it is an unstable
temper that applies only to alloys that spontaneously age at ambient temperature
W
after heat treatment.
T
Solution Heat Treated - Used for products that have been strengthened by heat
treatment, with or without subsequent strain hardening. The designation is
followed by one or more numbers as discussed below.
2.1.1.2 Heat Treating T Temper Codes
Table 2.2: T Temper Codes
T1
Cooled from an elevated temperature shaping process and naturally aged to a
substantially stable condition.
T2
Cooled from an elevated temperature shaping process, cold worked, and
naturally aged to a substantially stable condition.
T3
Solution heat treated, cold worked, and naturally aged to a substantially stable
condition.
T4
Solution heat treated, and naturally aged to a substantially stable condition.
T5
Cooled from an elevated temperature shaping process then artificially aged.
T6
Solution heat treated then artificially aged.
T7
Solution heat treated then and over-aged/stabilized
T8
Solution heat treated, cold worked, and then artificially aged.
T9
Solution heat treated, artificially aged, and then cold worked.
T10
Cooled from an elevated temperature shaping process, cold worked, then
artificially aged.
8
2.2
Strengthening by Heat Treatment
Heat treating process for aluminum alloys are precision processes. They must be carried
out in furnace properly and build to provide the thermal conditions required, and
equipped with control instruments to insure the desired continuity and uniformity of
temperature and time. Process details must be established and controlled carefully to
ensure it occupied the final desired characteristics.
Based on Federal Aviation Adminstation (2008), the hardening of aluminum alloy by
heat treatment consists of four-step:
•
Solution heat treatment: heating to predetermine temperature and soaking for a
period of time
•
Quenching: rapidly quenching to relatively low temperature
•
Age hardening: precipitation of solute atoms either at room temperature (natural
aging) or elevated temperature (artificial aging or precipitation heat treatment).
•
Retrogression and re-aging (RRA): is an intermediate heat treatment which when
applied to Al 7075-T6 material results in improved corrosion resistance
approaching the T73 temper, while maintaining the high strength of the T6
temper.
2.2.1 Solution Heat Treatment
To take advantage of the precipitation hardening reaction, it is necessary first to produce
a solid solution. The process by which this is accomplished is called solution heat
treating, and its objective is to take into solid solution the maximum practical amounts of
the soluble hardening elements in the alloy. The principles and procedures for heat
treating wrought and cast alloys are similar. The process consists of heating and holding
the alloy at a temperature sufficiently high and for a long enough period of time to
achieve a nearly homogenous solid solution.
9