MODIFICATION OF MECHANICAL PROPERTIES OF 6351 Al-Mg-Si ALLOY BY AGING HEAT ...

113 Pages·2015·5.17 MB·English

MODIFICATION OF MECHANICAL PROPERTIES OF
6351 Al-Mg-Si ALLOY BY AGING HEAT TREATMENT

By

Ahmed Yehia Ahmed Abd El-Rahman

A Thesis Submitted to the
Faculty of Engineering at Cairo University
in Partial Fulfillment of the
Requirements for the Degree of
MASTER OF SCIENCE
In
Metallurgical Engineering

FACULTY OF ENGINEERING, CAIRO UNIVERSITY
GIZA, EGYPT
2015
MODIFICATION OF MECHANICAL PROPERTIES OF
6351 Al-Mg-Si ALLOY BY AGING HEAT TREATMENT

By
Ahmed Yehia Ahmed Abd El-Rahman

A Thesis Submitted to the
Faculty of Engineering at Cairo University
in Partial Fulfillment of the
Requirements for the Degree of
MASTER OF SCIENCE
In
Metallurgical Engineering

Under the Supervision of

Prof. Dr. Mohamed Mamdouh Ibrahim Prof. Dr. El-Sayed Mahmoud El-Banna

Professor of Metallurgy Professor of Metallurgy
Mining, Petroleum and Metallurgical Mining, Petroleum and Metallurgical
Department Department
Faculty of Engineering, Cairo University Faculty of Engineering, Cairo University

Prof. Dr. Taher Ahmed El-Bitar

Head of Plastic Deformation Department
Central Metallurgical R&D Institute (CMRDI)

FACULTY OF ENGINEERING, CAIRO UNIVERSITY
GIZA, EGYPT
2015
MODIFICATION OF MECHANICAL PROPERTIES OF
6351 Al-Mg-Si ALLOY BY AGING HEAT TREATMENT

By
Ahmed Yehia Ahmed Abd El-Rahman

A Thesis Submitted to the
Faculty of Engineering at Cairo University
in Partial Fulfillment of the
Requirements for the Degree of
MASTER OF SCIENCE
In
Metallurgical Engineering

Approved by the
Examining Committee
____________________________
Prof. Dr. Mohamed Mamdouh Ibrahim, Thesis Main Advisor

____________________________
Prof. Dr. El-Sayed Mahmoud El-Banna, Member

____________________________
Prof. Dr. Taher Ahmed El-Bitar, Member
Central Metallurgical R&D Institute (CMRDI)

___________________________
Prof. Dr. Abd El-Hamid Ahmed Hussein, Internal Examiner

___________________________
Prof. Dr. Mohamed Abd El-WahabWaly, External Examiner
Central Metallurgical R&D Institute (CMRDI)

FACULTY OF ENGINEERING, CAIRO UNIVERSITY
GIZA, EGYPT
2015
Engineer’s Name: Ahmed Yehia Ahmed Abd El-Rahman

Date of Birth: 4/3/1989
Nationality: Egyptian
E-mail: [email protected]
Phone: [phone]
Address: El Qlubia, El Khanka, El Qalag
Registration Date: 1/10/2011
Awarding Date: …./…./……..
Degree: Master of Science ere
Department: Metallurgy Departement

Supervisors: Prof. Mohamed Mamduoh Ibrahim
Prof. Elsayed Mahmoud Elbanna
Prof. Taher Ahmed El-Bitar

Examiners: Prof. Mohamed Abd El-WahabWaly (External examiner)
Central Metallurgical R&D Institute (CMRDI)
Prof. Abdel Hamid Ahmed Hussein (Internal examiner)
Prof. Mohamed Mamdouh Ibrahim(Thesis main advisor)
Prof. Elsayed Mahmoud Elbanna (Member)
Prof. Taher Ahmed El-Bitar (Member)
Central Metallurgical R&D Institute (CMRDI)

Title of Thesis:
MODIFICATION OF MECHANICAL PROPERTIES OF 6351
Al-Mg-Si ALLOY BY AGING HEAT TREATMENT
Key Words:
Artificial Aging; Natural Aging; Pre-aging; XRD; SEM; EDAX

Summary:
The present study is dealing with modification of mechanical properties of Al-Mg-Si alloy
6351 by age hardening. The study investigates the effect of aging temperature, time, natural
aging and pre-aging on artificial aging behavior in terms of mechanical properties and
fractography examination. Artificial aging after solution treatment-water quenched resulted
in a sharp increase in both ultimate tensile strength UTS and yield stress YS, can lead with a
decrease in total elongation. As the time of aging increase the strength increase slightly till
reaches peak strength after that it starts to decrease with increasing time of aging. Better
mechanical properties are observed at lower aging temperature. Natural aging at room
temperature (25 ±3oC) after solution treated-water quenched resulted in a mild increase in
tensile properties with a slight drop in total elongation, natural aging for 170 h and for 1000
h after solution treatment followed by artificial aging of this alloy at 160oC, shifted the time
to reach peak strength to shorter aging time (8- 4 h respectively) in comparison to peak-aged
condition (160oC for 18 h). Pre-aging at 100oC for various times before artificially aging at
160oC for 18 h was investigated. It was found that the pre-aging for 10 min followed by
artificially peak aging led to slight increase in ultimate tensile strength and yield stress YS
associated with a reasonable total elongation.
AKNOWLEDMENT
First and foremost, I have to thank my research supervisors, Prof. Mohamed Mamdouh
Ibrahim, Prof. El-Sayed Mahmoud El-Banna and Prof. Taher Ahmed Al-Bitar. Without their
assistance and dedicated involvement in every step throughout the process, this thesis
would have never been accomplished. I would like to thank you very much for your
support and understanding over these past two years.
I would also like to show gratitude to my committee, including Prof. Mohamed
Mamduoh Ibrahim and Prof. El-Sayed Mahmoud El-Banna were my third-year professor in
metallurgy department at faculty of engineering, Cairo University. Their teaching style and
wide knowledge for different topics made a strong impression on me and I have always
carried positive memories of their classes with me. I discussed early versions of this work
with them. They raised many precious points in our discussion and I hope that I have
managed to address several of them here. Working with Prof. Mohamed Mamduoh Ibrahim
and Prof. El-Sayed Mahmoud El-Banna were an extraordinary experience. Much of the
analysis presented in Section IV and V is owed to my time at physical metallurgy classes I
had been through in the undergraduate level and in the postgraduate level.
I am very grateful to Prof. Taher Ahmed Al-Bitar at the Central Metallurgical Research and
Development Institute (CMRDI) kindly assisted me in my recent work, present all available
methods to accomplish my work and his experience to get a very useful suggestion and
discussion and he was very patient with my knowledge gaps in the area.
I must also thank two colleagues at the Department of Mohamed Hafez and Mustafa Ahmed
Othman, for giving me the retreat to have this thesis rushed to the printer. I would also like to
present a great thankful to Eng. Almosilhy at CMRDI for his helpful in my present work. I do
not forget Mr. Tarek a technician at CMRDI and Mechanical Testing Laboratory staff for
their efforts in preparation and testing my specimen.
Most importantly, none of this could have happened without my family. My father, my
mother and my wife, who offered me encouragement through everything limited devotion to
correspondence. Every time I was ready to quit, you did not let me and I am forever grateful.
This dissertation stands as a testament to your unconditional love and encouragement.

I
Dedication
I dedicate this thesis to my parents, my brother and sisters, my wife their love give
me forces to perform this work.

II
TABLE OF CONTENTS
Page
ACKNOWLEDGMENT………………………………………………………...... I
DEDICATION…………………………………………………………………….. II
TABLE OF CONTENTS…………………………………………………............. III
LIST OF FIGURES………………………………………………………............. V
LIST OF TABLES ………………………………….............................................. XI
ABSTRACT…………………………………………............................................. XII
CHAPTER 1: INTRODUCTION……………………………………………….. 1
CHAPTER 2: LITERATURE SURVEY……………………………………….. 3
2.1 Aluminum…………………………………………………………………… 3
2.1.1 History of Aluminum……………………………………………………. 3
2.1.2 Application……………………………………………………………… 4
2.1.3 Alloy Types………………………………………………………............ 4
2.2 Strength of Metals……………………………………………………………… 6
2.2.1 Dislocations……………............................................................................ 6
2.2.2 Slip………………………………………………………………………. 6
2.2.3 Particle coherency……………………………………………………….. 7
2.2.4 Solute solution hardening……………………………………………….. 8
2.2.5 Precipitation hardening …………………………………………………. 9
2.2.5.1 Precipitation hardening mechanism……………………………… 9
2.2.5.1.1 Cutting versus bowing…………………………………. 10
2.2.5.1.2 Shearing mechanisms of particle strengthening………... 11
2.2.5.1.2.1 Chemical hardening………………...................... 11
2.2.5.1.2.2 Stacking fault hardening……............................... 12
2.2.5.1.2.3 Modulus hardening……………………………... 12
2.2.5.1.2.4 Coherency hardening…………………………… 12
2.2.5.1.2.5 Order hardening………………………………… 13
2.2.5.1.2.6 Dispersion hardening…………………………… 13
2.2.5.1.3 Orowan bowing or bypass mechanism…………............. 14
2.2.5.2 Precipitation hardening in aluminum alloys……………………… 14
2.3 Heat treatment of Aluminum alloys……………………………………………. 17
2.3.1 Solute solubility………………………………………………………….. 19
2.3.2 The usual heat treatment procedure for aluminum……………………….. 19
2.3.2.1 Solution heat treatment (SHT)……………………………………. 20
2.3.2.2 Room temperature storage. (RT-storage)………………………… 21
2.3.2.3 Artificial aging (AA)……………………………………………... 21
2.4 The Al-Mg-Si (6xxx) alloy system…………………………………………. 21
2.4.1 Precipitation Hardening on Al-Mg-Si alloys………………………….. 22
2.4.1.1 Pseudo-binary Al-Mg Si………………………………………… 22
2
2.4.1.2 Precipitation sequence………………………………………… 22
2.5 Factors Affecting the Precipitation Hardening in Al-Mg-Si alloys…………. 26
III
2.5.1 Solution heat treatment……………………………………………............. 26
2.5.2 Aging condition……………………………………………………………. 27
2.5.2.1 Time-Temperature variation………………………………............. 27
2.5.2.2 Two-step aging………………………………………..................... 27
2.5.3 Chemical compositions……………………………………………………. 29
CHAPTER 3: MATERIALS AND EXPRIMENTAL TECHNIQUE…………. 32
3.1 Materials……………………………………………………………………….. 32
3.2 Heat-treatment………………………………………………………………….. 32
3.3 Tensile Test…………………………………………………………………….. 34
3.4 Hardness test …………………………………………………………………… 36
3.5 XRD Analysis ……………………...................................................................... 37
3.6 Microstructure Examination……………………………………………………. 38
3.7 Fractographic Examination (SEM)……………………………………………... 38
3.8 Energy Dispersive X-rays Analysis (EDAX)………………………….............. 39
CHAPTER 4: RESULTS AND DISCUSSION………………………………….. 40
4.1 Effect of Artificial Aging on Tensile Properties……………………………….. 40
4.2 Factors Affecting the Artificial Aging…………………………………………. 52
4.2.1 Natural Aging……………………………………………………………. 52
4.2.1.1 The Influence of Natural Aging Duration on Mechanical
52
Properties……………………………………………….
4.2.1.2 Effect of natural aging time on artificial aging…………………… 59
4.2.2 Pre-aging………………………………….................................................. 67
4.2.2.1 Effect of pre-aging time on artificial peak aging condition………………… 67
4.3 Microstructure Examination and XRD Analysis ………………………………. 72
4.4 Scanning Electron Microscope (SEM) with Energy Dispersive X-rays
79
Analysis (EDAX)…………………………………………………………..
4.5 Fracture behavior………………………………………………...……………... 84
CHAPTER 5: CONCLUSIONS………………………………………………….. 87
REFERENCES……………………………………………………………............. 89
ARABIC SUMMARY ……………………………………………………............ أ

IV
LIST OF FIGURES
Page
Fig. 2.1 AA Designation of wrought Aluminum and its alloys. 5

Fig. 2.2 Illustrations of a line dislocation (a) and a screw dislocation (b). In 7
the case of the line dislocation, Burgers vector can be seen to lie in
the same plane as the plane 1 → 5, while it lies perpendicular to it
in the case of the screw dislocation.

Fig. 2.3 Figure (a) shows a fully coherent particle, figure (b) a coherent 8
particle, figure (c) a partially coherent particle and figure (d) a non-
coherent particle dispersed in the surrounding matrix.

Fig. 2.4 Figure (a) shows a schematic drawing of an atom dispersed in the 9
surrounding matrix which demands more space than the matrix
atoms. Figure (b) shows a schematic drawing of an atom which
requires less space than the surrounding matrix. Both can be seen to
cause coherency strain.

Fig. 2.5 A dislocation held up by a random array of slip-plane obstacles. 10

Fig. 2.6 A dislocation motion through strong and weak obstacles. 10

Fig. 2.7 Variation of yield strength with aging time for typically age- 11
hardening alloys with two different volume fractions of
precipitates.

Fig. 2.8 Schematic representation of the shape change accompanying the 12
movement of a dislocation through a GP zone.

Fig. 2.9 View of edge dislocation penetrating an ordered particle. 13

Fig. 2.10 Shown the precipitation sequence in Al-Mg-Si from the 16
supersaturated solid solution.

Fig. 2.11 GP zones in Al-Cu, Al-Zn and Al-Mg-Si. 17

Fig. 2.12 Coherency in a cubic lattice; [001] section of GP zone. 17

Fig. 2.13 The temper designation scheme of aluminum alloy. 18

V
Fig. 2.14 The phase diagram of silicon and aluminum. Theα phase to the left 19
is silicon fully dissolved in aluminum while the phase to the lower
right is a combination of the α-phase and solid silicon. The
horizontal line at 577oC is the solidus line. All phases above this
line except for the α-phase consists partly or fully of a liquid state.

Fig. 2.15 Schematic drawing of the heat treatment procedure. T , T and 20
RT AA
T denote room temperature (RT), temperature during artificial
SHT
aging (AA) and temperature during solution heat treatment (SHT)
respectively. The symbols t , t and t denote the times for the
RT AA SHT
three steps. The vertical slopes in the temperature indicate assumed
instantaneous changes in temperature as the sample goes from one
treatment to another.

Fig. 2.16 Pseudo-binary diagram of Al-Mg Si. 22
2

Fig. 2.17 Pictures of the β" precipitate taken with conventional TEM. (a) 24
shows the original picture, while (b) shows a filtered version. The
precipitate eyes can be seen as small rings, and denote the unit cell
centers.

Fig. 2.18 Picture of the β' precipitate taken with conventional TEM. The unit 25
cell can be observed to be hexagonal with lattice parameters a = b =
7.05o A.

Fig. 2.19 Picture of the B‟ precipitate taken with conventional TEM. The 25
precipitate eyes can be seen as hexagonal rings, and denote the unit
cell centers. The unit cell can be observed to be hexagonal with
lattice parameters a = b = 10.4˚ A.

Fig. 2.20 Al-Mg Si-Two step aging. 28
2

Fig. 3.1 Heat-treatment furnace 33

Fig. 3.2 Heat-Treatment process 33

Fig. 3.3 Age hardening sequence of Aluminum alloys 34

Fig. 3.4 Tensile Test Specimen according to ASME E8 35

Fig. 3.5 Universal tensile testing machine 35

Fig. 3.6 Hardness Machine test 36

Fig. 3.7 XRD Machine 37
VI