MECHANICAL TESTING AND EVALUATION OF HIGH-SPEED AND LOWSPEED FRICTION STIR WELDS
A Thesis by
Nitin Banwasi
Bachelor of Engineering, Bangalore University, Bangalore, India 2000
Submitted to the College of Engineering
and the faculty of the Graduate School of
Wichita State University in partial fulfillment of
the requirements for the degree of
Master of Science
Fall 2005
EXPERIMENTAL TESTING AND EVALUATION OF HIGH-SPEED AND LOWSPEED FRICTION STIR WELDS
I have examined the final copy of this thesis for form and content and recommend that it
be accepted in partial fulfillment of the requirements for the degree of Master of Science,
with a major in Mechanical Engineering.
George E. Talia, Committee Chair
We have read this thesis and recommended its acceptance:
Dr. Hamid M. Lankarani, Department Chair, Committee Member
Dr. Krishna K. Krishnan, Committee Member
ii
DEDICATION
To My Parents
iii
ACKNOWLEDGEMENTS
I am grateful to all that are part of my efforts during my work both academically and
personally. I am thankful to my committee chair, Dr.George E.Talia, for being not only
supportive in my endeavors but also patient and informative. I appreciate the involvement
of both Dr. Hamid M. Lankarani and Dr. Krishna K. Krishnan for their involvement in its
fulfillment. I also want to remember fellow student’s help and suggestions in making it
possible with gratitude.
iv
ABSTRACT
The potential of the Friction Stir Welding (FSW) process is easily observed in the
creation of defect free welds in almost all of the Aluminum alloys. The success and
applicability of the process, however, will depend on the performance of the welds
compared to other joining processes. Experimental testing and evaluation are necessary
for the determination of the mechanical response of Friction Stir Welds and vital to the
development and optimization of the FSW process. The goal of this experimental testing
of Friction Stir Welds is to obtain the data necessary to begin understanding the effects of
the FSW process. An attempt has been made to systematically examine the effects of
FSW process parameters and alloy on the weld properties. An attempt has been made to
evaluate and compare High Speed and Low Speed Friction Stir Welds.
v
TABLE OF CONTENTS
1. INTRODUCTION
1
1.1.
Welding
3
1.2.
The physical nature of joining
4
1.3.
Welding, from a metallurgical point of view
4
1.4.
A metallurgical classification of the welding processes
5
1.5.
Types of welding
5
1.6.
Solid state welding
7
1.7.
Friction welding
7
1.8.
Rotary friction welding
8
2. ALUMINUM WELDING
2.1.
Introduction
9
2.2.
Characteristics of Aluminum
10
2.3.
Aluminum alloy designation – wrought alloys
11
2.4.
Nonheat treatable Aluminum alloys
12
2.5.
Heat treatable Aluminum alloys
12
2.6.
Wrought Aluminum alloys
12
2.7.
Welding Aluminum
14
3. FRICTION STIR WELDING
3.1.
Introduction
17
3.2.
Different parameters in FSW
18
3.3.
Process advantages
20
3.4.
Microstructure classification
22
vi
3.5.
Joint geometries
24
3.6.
Applications
25
4. MECHANICAL TESTING AND METALLOGRAPHY
4.1.
Testing
28
4.2.
Tensile Test
28
4.3.
Tensile Specimens
29
4.4.
Hardness Test
30
4.5.
Types of hardness tests
31
4.6.
Factors for selection of hardness testing methods
32
4.7.
Rockwell hardness test
32
4.8.
Metallographic specimen preparation basics
34
5. EXPERIMENTAL PROCEDURE
5.1.
Tensile test
37
5.2.
Hardness test
37
5.3.
Metallographic analysis
38
6. RESULTS AND DISCUSSIONS
6.1.
Effect of changing welding speed at constant weld pitch
40
6.2.
Temper effects on required loads and weld energy
41
6.3.
Alloy effects on specific weld energy
42
6.4.
Low-speed friction stir weld 1
43
6.5.
Low-speed friction stir weld 2
47
6.6.
Low-speed friction stir weld 3
53
6.7.
Low-speed friction stir weld 4
64
vii
6.8.
Low-speed friction stir weld 5
69
6.9.
Low-speed friction stir weld 6
73
6.10.
Low-speed friction stir weld 7
77
6.11.
Low-speed friction stir weld 8
81
6.12.
Low-speed friction stir weld 9
88
6.13.
Low-speed friction stir weld 10
92
6.14.
Tool geometry effects
97
6.15.
High-speed friction stir weld 11
100
6.16.
High-speed friction stir weld 12
104
6.17.
High-speed friction stir weld 13
107
6.18.
High-speed friction stir weld 14
110
6.19.
High-speed friction stir weld 15
116
6.20.
High-speed friction stir weld 16
118
6.21.
High-speed friction stir weld 16
119
7. CONCLUSIONS AND FUTURE SCOPE
121
8. REFERENCES
125
viii
LIST OF FIGURES
1.1. Friction stir welding process
2
1.2. Master chart of welding and allied processes
6
1.3. Friction stir welding and processing technologies
8
3.1. Friction stir welding
18
3.2. Microstructure of a friction stir weld
20
4.1. Tensile test specimen
29
4.2. Rockwell principle
33
6.1. Required energy and specific weld energy at constant weld pitch
40
6.2. X-axis force for welds made at constant weld pitch
41
6.3. Specific weld energy as a function of welding speed
42
6.4(a). Hardness graph – across the weld
46
6.4(b). Microstructure of the weld
47
6.5(a). Hardness graph – across the weld @ 10”/min
52
6.5(b). Hardness graph – across the weld @ 15”/min
52
6.6(a). Hardness graph – across the weld @ 10”/min
58
6.6(b). Hardness graph – across the weld @ 15”/min
58
6.6(c). Peak/Yield stress of the weld – 10”/min @ 750, 760 & 600 rpm
59
6.6(d). Break stress of the weld – 10”/min @ 750, 760 & 600 rpm
60
6.6(e). Weld 2 – change in stress due to change in welding pitch
61
6.6(f). Weld 3 – change in stress due to change in welding pitch
62
6.6(g). Weld 2 & weld 3 – change in hardness due to change in welding pitch
63
6.7(a). Hardness graph – across the weld
67
ix
6.7(b). Microstructure of the weld nugget
68
6.8(a). Hardness graph – across the weld
72
6.8(b). Microstructure of the weld
73
6.9(a). Hardness graph – across the weld
76
6.10(a). Hardness graph – across the weld
80
6.11(a). Hardness graph – across the weld
84
6.11(b). Peak/yield stress variation
85
6.11(c). Break stress variation
85
6.11(d). Variation in hardness
86
6.12(a). Hardness graph – across the weld
91
6.12(b). Microstructure of the weld nugget
92
6.13(a). Hardness graph – across the weld
95
6.13(b). Microstructures of the weld
96
6.14(a). Specific weld energy as a function of welding speed and tool geometry
97
6.14(b). Required weld power as a function of tool geometry and welding speed
98
6.14(c). Transverse tensile strength of the welds as a function of tool geometry
and welding speed
99
6.14(d). X axis force as a function of tool geometry and welding pitch
99
6.15(a). Hardness graph – across the weld
103
6.15(b). Microstructure of the weld
103
6.16(a). Microstructure of the weld nugget
106
6.17(a). Microstructure of the weld nugget
109
6.18(a). Hardness graph - across the weld
113
x
6.18(b). Microstructure of the weld
113
6.18(c). Variation in peak and yield stress
114
6.18(d). Variation in break stress
114
6.18(e). Variation in hardness
115
xi
LIST OF TABLES
6.4(a). Tensile test data of parent 1 – Alclad 2024-T3
43
6.4(b). Tensile test data of parent 2 - Al 7075-T6
44
6.4(c). Tensile test data of the weld
44
6.4(d). Hardness test data – along the weld
45
6.4(e). Hardness test data – across the weld
46
6.5(a). Tensile test data of parent 1 – Alclad 2024-T3
48
6.5(b). Tensile test data of parent 2 – Al 7075-T6
48
6.5(c). Tensile test data of weld @ 10”/min
49
6.5(d). Tensile test data of the weld @ 15”/min
49
6.5(e). Hardness test data – along the weld
51
6.5(f). Hardness test data – across the weld
51
6.6(a). Tensile test data of parent1 – Alclad 2024-T3
54
6.6(b). Tensile test data of parent 2 - Al 7075-T6
54
6.6(c). Tensile test data of the weld @10"/min
55
6.6(d). Tensile test data of the weld @ 15"/min
55
6.6(e). Hardness test data - along the weld
57
6.6(f). Hardness test data - across the weld
57
6.7(a). Tensile test data of parent1 - Alclad 2024-T3
64
6.7(b). Tensile test data of parent 2 - Al 7075-T6
65
6.7(c). Tensile test data of the weld
65
6.7(d). Hardness test data- along the weld
66
6.7(e). Hardness test data - across the weld
67
xii
6.8(a). Tensile test data of parent1 – Alclad 2024-T3
69
6.8(b). Tensile test data of parent2 - Al 7075-T6
69
6.8(c). Tensile test data of the weld
70
6.8(d). Hardness test data – along the weld
71
6.8(e). Hardness test data – across the weld
71
6.9(b). Tensile test data of the parent – Al 6061-T6
74
6.9(b). Tensile test data of the weld
74
6.9(c). Hardness test data – along the weld
75
6.9(d). Hardness test data – across the weld
75
6.10(a). Tensile test data of the parent – Al 6061-T6
77
6.10(b). Tensile test data of the weld
78
6.10(c). Hardness test data – along the weld
79
6.10(d). Hardness test data – across the weld
79
6.11(a). Tensile test data of the parent – Al 6061-T6
81
6.11(b). Tensile test data of the weld
82
6.11(c). Hardness test data - along the weld
83
6.11(d). Hardness test data – across the weld
83
6.12(a). Tensile test data of the parent – Al 2024-T3
88
6.12(b). Tensile test data of the weld
89
6.12(c). Hardness test data – along the weld
90
6.12(d). Hardness data – across the weld
90
6.13(a). Tensile test data of the parent – Alclad 2024-T3
93
6.13(b). Tensile test data of the weld
93
xiii
6.13(c). Hardness test data – along the weld
94
6.13(d). Hardness test data – across the weld
95
6.14. Tool geometry
97
6.15(a). Tensile test data of the parent - Al 7075-T6
100
6.15(b). Tensile test data of the weld
101
6.15(c). Hardness test data – along the weld
102
6.15(d). Hardness test data – across the weld
102
6.16(a). Tensile test data of the parent Al 7075-T6
104
6.16(b). Tensile test data of the weld
105
6.16(c). Hardness test data – along the weld
106
6.17(a). Tensile test data of the parent Al 7075-T6
107
6.17(b). Tensile test data of the weld
108
6.17(c). Hardness test data – along the weld
109
6.18(a). Tensile test data of the parent Al 7075-T6
110
6.18(b). Tensile test data of the weld
111
6.18(c). Hardness test data – across the weld
112
6.18(d). Hardness test data – across the weld
112
6.19(a). Tensile test data of the parent Al 7075-T6
116
6.19(b). Tensile test data of the weld (tilted).
117
6.19(c). Tensile test data of the weld (untilted).
117
6.20(a). Hardness test data of the weld
119
6.21(a). Hardness test data of the weld
120
xiv
CHAPTER 1
INTRODUCTION
Friction Stir Welding (FSW) was developed at and patented by The Welding Institute
(Cambridge, UK) in 1991. Since the time of its invention, the process has been
continually improved and its scope of application expanded. Friction Stir Welding is a
solid state joining process combining deformation heating and mechanical work to obtain
high quality, defect free joints. Friction stir welding is especially well suited to joining
Aluminum alloys in a large range of plate thickness and has particular advantages over
fusion welding when joining of highly alloyed Aluminum is considered [1].
Because of many demonstrated advantages of FSW over fusion welding techniques, the
commercialization of FSW is proceeding at a rapid pace. Much of the work done to bring
FSW to production applications has been of a very practical nature, driven primarily by
the pressing industrial need. Industry, federal laboratories and universities have been
investigating this technique for joining Aluminum, Steel, Titanium, Metal matrix
composites and even hard metals. Research and engineering is rapidly progressing across
many fronts. Fundamental research is investigating critical phenomenon through process
modeling, microstructure studies, properties and tool wear. FSW has matured to a point
where laboratory research is beginning to transition to Aluminum alloy structural
applications [1].
1
In principle, Friction Stir Welding is a very simple process. The two plates to be welded
are butted together (lap and other configurations are also possible) and clamped to a rigid
backing plate. The rotating FSW tool is plunged into the plates at the joint line and
traversed along the line, forming the joint.
Fig 1.1: Friction stir welding process [34].
Because the FSW process has only recently become a subject of wide study, there are
currently no large databases of weld properties and, in fact, no specifications on how to
make or test friction stir welds currently exist. In general, the process is robust and a wide
range of processing parameters and tool designs can be used to make metallurgically
sound welds in a given alloy and plate thickness. While weld free of defects may be made
using a wide range of processing parameters, the chosen process parameters may
significantly affect the mechanical properties of the weld either through direct
modification of the weld microstructure or by indirect influence (e.g. by modification of
residual stress state) [1].
2
1.1. Welding
Welding can be defined as the joining of two components by a coalescence of the
surfaces in contact with each other. This coalescence can be achieved by melting the two
parts together – fusion welding – or by bringing the two parts together under pressure,
perhaps with the application of heat, to form a metallic bond across the interface. This is
known as solid phase joining [2].
Welding is by no means a new science. According to some researchers, its origin dates
back to the very beginning of the technology of metals. For instance, some welded copper
utensils have been traced to the days of the Sumerian civilization (14th century B.C).
Also, welding is mentioned by the prophet Isaiah in the Old Testament, by the Greek
historian Herodotus in his “Clio”, by the Latin writer Pliny the Elder in his “Naturalis
Historia”, and by many other prominent contributors to ancient history. Coming down
through the ages, welding and its application progressed rather slowly, principally
because of the limitations of the primitive methods used and of the empirical technical
knowledge available. However, toward the end of the 19th century and the beginning of
the 20th century, the art and science of welding began to advance at a very rapid pace.
Today it constitutes, by far, one of the most important and widely used tools for the
joining of metals [9].
Welding is the most economical and efficient way to join metals permanently. Welding
ranks high among industrial processes and involves more sciences and variables than
those involved in any other industrial process. In many cases welding is the most cost
effective and structurally sound joining technique. Welding can be performed almost any
3
where out doors, indoors, under sea or in space. Some of the processes cause sparks
where as others do not even require extra heat. Most of the things we use in our daily life
are welded.
1.2. The Physical Nature of Joining [5]
Theoretically, to produce a weld, one need only bring the atoms on the opposing metallic
surfaces close enough to establish the spontaneous attractive forces. Ideally, two perfectly
plane surfaces, if treated in this fashion, would be drawn together spontaneously until the
distance separating them corresponds to the equilibrium interatomic spacing. At this
point, perfect “coalescence” would result and the two objects would merge to comprise a
single solid body.
1.3. Welding, From a Metallurgical Point of View [5]
The forces inherent in the metallic objects can bring about perfect coalescence only if:
•
The oxides and other non-metallic films present on real metallic surfaces can
either be removed or completely dispersed from the areas being joined.
•
The distance separating the metallic atoms on one surface of the proposed joint
from those on the opposing surface of the joint can be reduced consistently to a
value approximately the equilibrium atomic spacing for the metal, thus producing
a metallic bond.
4
1.4. A Metallurgical Classification of the Welding Processes [5]
Basically, it is convenient to divide the welding processes into two major categories:
•
The Pressure Welding Processes, in which externally applied forces play an
important role in the bonding operation, whether consummated at room or
elevated temperature.
•
The Fusion Welding Processes, in which the joining operation involves melting
and solidification, and any external forces applied to the system play no active
role in producing coalescence.
1.5. Types of Welding
1. Arc welding
•
Shielded Metal Arc Welding
•
Submerged Arc Welding
•
Gas Metal Arc and Flux Cored Arc Welding
•
Gas Tungsten Arc Welding
•
Plasma Arc Welding
•
Electroslag and Electrogas Welding
2. Resistance Welding
3. Flash Welding
4. Oxyfuel Gas Welding
5. Solid State Welding
•
Friction Welding
5
•
Friction stir welding
•
Diffusion Welding
6. Electron Beam Welding
7. Laser Beam Welding
8. Brazing
9. Soldering
10. Induction welding
Fig 1.2: Master chart of welding and allied processes [30].
6
1.6. Solid State Welding (SSW)
Solid state welding is "a group of welding processes which produces coalescence at
temperatures essentially below the melting point of the base materials being joined
without the addition of a brazing filler metal. Pressure may or may not be used".
The oldest of all welding processes forge welding belongs to this group. Others include
cold welding, diffusion welding, explosion welding, friction welding, hot pressure
welding, and ultrasonic welding. These processes are all different and utilize different
forms of energy for making welds [30].
1.7. Friction Welding
Friction, which requires relative motion, pressure and time, is an efficient thermal energy
source for the welding of materials. Friction welding is a solid state welding process
which produces coalescence of materials by the heat obtained from mechanically induced
sliding motion between rubbing surfaces. The work parts are held together under
pressure. This process usually involves the rotating of one part against another to
generate frictional heat at the junction. When a suitable high temperature has been
reached, rotational motion ceases and additional pressure is applied and coalescence
occurs [33].
7
Fig 1.3: Friction welding and processing technologies [32].
1.8. Rotary Friction Welding
Two variants of the rotary friction welding process have been developed. These are
known as conventional ‘continuous drive friction welding’ and stored energy friction
welding where the most widely adopted is inertia friction welding. In both these methods,
friction welds are made by holding a rotating component in contact with a non-rotating
component while under a constant or increasing axial load. The interface reaches the
appropriate welding temperature, at which point rotation is stopped and the weld
completed [31].
8
CHAPTER 2
ALUMINUM WELDING
2.1. Introduction
Aluminum is the most abundant metal in nature. Some 8% by weight of the Earth’s crust
is Aluminum. Many rocks and minerals contain a significant amount of Aluminum.
Unfortunately, Aluminum does not occur in nature in the metallic form. In rocks,
Aluminum is present in the form of silicates and other complex compounds. The ore from
which most Aluminum is presently extracted, Bauxite, is a hydrated Aluminum oxide [1].
The existence of Aluminum was postulated by Sir Humphrey Davy in the first decade of
the nineteenth century and the metal was isolated in 1825 by Hans Christian Oersted. It
remained as somewhat of a laboratory curiosity for the next 30 years when some limited
commercial production began, but it was not until 1886 that the extraction of Aluminum
from Bauxite became a truly viable industrial process [2].
Pure Aluminum is a silvery-white metal with many desirable characteristics. It is light,
nontoxic (as the metal), nonmagnetic and nonsparking. It is somewhat decorative. It is
easily formed, machined and cast. Pure Aluminum is soft and lacks strength, but alloys
with small amounts of Copper, Magnesium, Silicon, Manganese and other elements have
very useful properties [4].
Aluminum is the most difficult metal to weld. Aluminum oxide should be cleaned from
the surface prior to welding. Aluminum comes in heat treatable and non heat treatable
alloys. Heat treatable aluminum alloys get their strength from a process called ageing.
9
Significant decrease in tensile strength can occurs when welding aluminum due to over
aging [3].
2.2. Characteristics of Aluminum [2]
Listed below are the main physical and chemical Characteristics of Aluminum, contrasted
with those of Steel:
•
The difference in the melting points of the two metals and their oxides. The
oxides of Iron all melt close or below the melting point of the metal; Aluminum
oxide melts at 20600 C, some 14000 C above the melting point of Aluminum.
•
The oxide film on Aluminum is durable, highly tenacious and self-healing. This
gives the Aluminum alloys excellent corrosion resistance.
•
The coefficient of thermal expansion of Aluminum is approximately twice that of
Steel.
•
The coefficient of thermal conductivity of Aluminum is six times that of Steel.
•
The specific heat of Aluminum – the amount of heat required to raise the
temperature of a substance – is twice that of Steel.
•
Aluminum has high electrical conductivity, only three-quarters that of Copper but
six times that of Steel.
•
Aluminum does not change color as its temperature rises.
•
Aluminum is non-magnetic.
•
Aluminum has a modulus of elasticity three times that of Steel.
10
•
Aluminum does not change its crystal structure on heating and cooling, unlike
Steel which undergoes crystal transformations or phase changes at specific
temperatures.
2.3. Aluminum Alloy Designation – Wrought Alloys [6]
Pure Aluminum is readily alloyed with many other metals to produce a wide range of
physical and mechanical properties. This means by which the alloying elements
strengthen Aluminum are used as the basis to classify Aluminum alloys into two
categories: nonheat treatable and heat treatable.
1. First digit – Principal alloying constituent(s)
2. Second digit – Variations of initial alloy
3. Third and fourth digits – Individual alloy variations
•
1xxx – Pure Al (99.00% or greater)
•
2xxx – Al-Cu alloys
•
3xxx – Al-Mn alloys
•
4xxx – Al-Si alloys
•
5xxx – Al-Mg alloys
•
6xxx – Al-Mg-Si alloys
•
7xxx – Al-Zn alloys
•
8xxx – Al + other elements
•
9xxx – Unused series
11
2.4. Nonheat Treatable Aluminum Alloys [6]
The initial strength of the nonheat treatable Aluminum alloys depends primarily upon the
hardening effect of alloying elements such as Silicon, Iron, Manganese and Magnesium.
These elements affect increase in strength either as dispersed phases or by solid solution
strengthening. The nonheat treatable alloys are mainly found in the 1xxx, 3xxx, 4xxx,
and 5xxx alloy series depending upon their major alloying elements.
2.5. Heat Treatable Aluminum Alloys [6]
The initial strength of Aluminum alloys in this group depends upon the alloy
composition, just as the nonheat treatable alloys. Heat treatable Aluminum alloys develop
their properties by solution heat treating and quenching, followed by either natural or
artificial aging. The heat treatable alloys are found primarily in the 2xxx, 6xxx and 7xxx
alloy series.
2.6. Wrought Aluminum Alloys [6, 2]
1xxx: This series represent the commercially pure Aluminum, ranging from the baseline
1100 (99% min Al) to relatively purer 1050/1350 (99.5% min Al) and 1175 (99.75% min
Al).These grades of Aluminum are characterized by excellent corrosion resistance, high
thermal and electrical conductivities, low mechanical properties, and excellent
workability. Moderate increases in strength may be obtained by strain hardening. Iron
and silicon are the major impurities.
12
2xxx: The major alloying element in 2xxx series alloys is Copper. The alloys in this
series are heat treatable and possess good combinations of high strength (especially at
elevated temperatures), toughness and in specific cases, weldability. They are not
resistant to atmospheric corrosion and so are usually painted or clad in such exposures.
3xxx: The major alloying element in 3xxx series alloys is Manganese. These alloys are
strain hardenable, have excellent corrosion resistance and are readily welded, brazed and
soldered.
4xxx: The major alloying element in 4xxx series alloys is Silicon, which can be added in
sufficient quantities (up to 12%) to cause substantial lowering of the melting range. For
this reason, Aluminum-Silicon alloys are used in welding wire and as brazing alloys for
joining Aluminum, where a lower melting range than that of the base metal is required.
These alloys have good flow characteristics and medium strength.
5xxx: The major alloying element is Magnesium and when it is used as a major alloying
element or with Manganese, the result is a moderate-to-high-strength work-hardenable
alloy. Magnesium is considerably more effective than Manganese as a hardener, about
0.8% Mg being equal to 1.25% Mn, and it can be added in considerably higher
quantities. Alloys in this series possess excellent corrosion resistance even in salt water
and very high toughness even at cryogenic temperature to near absolute zero.
6xxx: Alloys in the 6xxx series contain Silicon and Magnesium. Although not as strong
as most 2xxx and 7xxx alloys, 6xxx series alloys have relatively good formability,
13
weldability, machinability, and relatively good corrosion resistance, with medium
strength.
7xxx: Zinc, in amounts of 1 to 8% is the major alloying element in 7xxx series alloys.
These alloys are heat treatable and possess very high strength.
8xxx: The alloys in this series have high conductivity, strength and hardness.
2.7. Welding Aluminum
GTAW Welding
Gas Tungsten Arc Welding (GTAW) is frequently referred to as TIG welding. TIG
welding is a commonly used high quality welding process. TIG welding has become a
popular choice of welding processes when high quality, precision welding is required.
In TIG welding an arc is formed between a non consumable tungsten electrode and the
metal being welded. Gas is fed through the torch to shield the electrode and molten weld
pool. If filler wire is used, it is added to the weld pool separately.
MIG Welding
Gas Metal Arc Welding (GMAW) is frequently referred to as MIG welding. MIG
welding is a commonly used high deposition rate welding process. Wire is continuously
fed from a spool. MIG welding is therefore referred to as a semiautomatic welding
process.
14
Flux Cored Welding
Flux Cored Arc Welding (FCAW) is frequently referred to as flux cored welding. Flux
cored welding is a commonly used high deposition rate welding process that adds the
benefits of flux to the welding simplicity of MIG welding. As in MIG welding wire is
continuously fed from a spool. Flux cored welding is therefore referred to as a
semiautomatic welding process.
Self shielding flux cored arc welding wires are available or gas shielded welding wires
may be used. Flux cored welding is generally more forgiving than MIG welding. Less
precleaning may be necessary than MIG welding. However, the condition of the base
metal can affect weld quality. Excessive contamination must be eliminated.
Stick Welding
Shielded Metal Arc Welding (SMAW) is frequently referred to as stick or covered
electrode welding. Stick welding is among the most widely used welding processes.
The flux covering the electrode melts during welding. This forms the gas and slag to
shield the arc and molten weld pool. The slag must be chipped off the weld bead after
welding. The flux also provides a method of adding scavengers, deoxidizers, and alloying
elements to the weld metal.
Resistance Welding
Resistance Spot Welding (RSW), Resistance Seam Welding (RSEW), and Projection
Welding (PW) are commonly used resistance welding processes. Resistance welding
15
uses the application of electric current and mechanical pressure to create a weld between
two pieces of metal. Weld electrodes conduct the electric current to the two pieces of
metal as they are forged together.
The welding cycle must first develop sufficient heat to raise a small volume of metal to
the molten state. This metal then cools while under pressure until it has adequate strength
to hold the parts together. The current density and pressure must be sufficient to produce
a weld nugget, but not so high as to expel molten metal from the weld zone.
Electron Beam Welding
Electron Beam Welding (EBW) is a fusion joining process that produces a weld by
impinging a beam of high energy electrons to heat the weld joint. Electrons are
elementary atomic particles characterized by a negative charge and an extremely small
mass. Raising electrons to a high energy state by accelerating them to roughly 30 to 70
percent of the speed of light provides the energy to heat the weld.
The electron beam is always generated in a high vacuum. The use of specially designed
orifices separating a series of chambers at various levels of vacuum permits welding in
medium and no vacuum conditions. Although, high vacuum welding will provide
maximum purity and high depth to width ratio welds.
16
CHAPTER 3
FRICTION STIR WELDING
3.1. Introduction [11]
Conventional friction welding has been around for many years, but relies on relative
motion between the parts to be joined while pressure is applied. The need to move one or
both parts restricts the conventional friction process between relatively simple shapes –
thus joining plate or sheet is almost impossible. In Friction Stir Welding (FSW), a
cylindrical, shouldered tool with a profiled probe is rotated and slowly plunged into the
joint line between two pieces of sheet or plate material, which are butted together. The
parts have to be clamped onto a backing bar in a manner that prevents the abutting joint
faces from being forced apart. Frictional heat is generated between the wear resistant
welding tool and the material of the work pieces. This heat causes the latter to soften
without reaching the melting point and allows traversing of the tool along the weld line.
The plasticized material is transferred from the leading edge of the tool to the trailing
edge of the tool probe and is forged by the intimate contact of the tool shoulder and the
pin profile. It leaves a solid phase bond between the two pieces. The process can be
regarded as a solid phase keyhole welding technique since a hole to accommodate the
probe is generated, then filled during the welding sequence.
17
Figure 3.1: Friction stir welding [11].
3.2. Different parameters in FSW [11]
The whole of rotating device between the machine spindle and the work piece is referred
to as the ‘tool’. The part of the tool, which is embedded in work piece during welding, is
referred to as the ‘probe’. The part of the tool, which is pressed onto the surface of the
work piece during welding, is referred to as the ‘shoulder’.
In a non-cylindrical tool the terms ‘leading edge’ (front face of shoulder during welding)
and ‘trailing edge’ (rear face of shoulder during welding) are used, whereas in cylindrical
tools there is clearly no edge, and so the terms ‘leading face’ and ‘trailing face’ may be
preferred. ‘Probe leading face’ is the front face of the probe during welding. Similarly
‘probe trailing face’ is the rear face of the probe during welding.
As the tool may in some circumstances be tilted through a small angle, part of the
shoulder may be embedded deeper into the work piece. That part of the shoulder which
experiences the greatest penetration is referred to as the ‘heel’ and the maximum depth of
the shoulder penetration below the work piece surface is defined as the ‘heel plunge
18
depth’. The angle of tilt is referred to as the ‘tilt angle’, or ‘travel angle’. In some
instances the tool is tilted sideways, and in this case the angle is described as the
‘sideways tilt angle’ or ‘work angle’.
The side of the weld where the local direction of the tool is the same as the traversing
direction or the side of the weld where direction is the same as the direction of rotation of
the shoulder is called the ‘advancing side’. Similarly, the side where the directions are
opposite and the local movement of the shoulder is against the traversing direction or side
of the weld where direction of travel is opposed to direction of rotation of shoulder is
called the ‘retreating side’. The total area of the tool on the work piece surface is
described as the ‘tool shoulder footprint’.
The term ‘Welding speed’ is preferred to traversing speed or traversing rate, which is the
rate of travel of tool along joint line. ‘Tool Rotation speed’ is the rotation speed of the
friction stir welding tool. ‘Clockwise Rotation’ is when viewed from above the tool,
looking down onto the work piece.
Forces are an important part of friction stir welding technology. The force applied
parallel to the axis of rotation of the tool (Z-direction) is the ‘down force’, and the force
applied parallel to the welding direction (X-direction) is the ‘traversing force’. The force
developed in a direction perpendicular to both X and Z forces is ‘Side force’ (Ydirection).
19
3.3. Process Advantages [11]
The key benefits of this newly developed welding process include an increase in joint
efficiency and process robustness, as well as a greater range of applicable alloys that can
be welded. Friction Stir Welding will permit production-welding opportunities relative to
dissimilar alloys and materials previously thought to be "unweldable" such as Aluminum
alloy. Composite materials are also candidate materials for this welding process. Friction
Stir Welding's solid-phase, low distortion welds are achieved with relatively low costs,
use simple energy efficient mechanical equipment, and require minimal operator
expertise and training. The process advantages result from the fact that the FSW process
(as all Friction Welding of metals) takes place in the solid phase below the melting point
of the materials to be joined. The benefits therefore include the ability to join materials
that are difficult to fusion weld, for example 2000 and 7000 Aluminum. Other advantages
are as follows:
•
Low distortion, even in long welds
•
Excellent mechanical properties as proven by fatigue, tensile and bend tests
•
No fume, No porosity
•
No spatter
•
Low shrinkage
•
Can operate in all positions
•
Energy efficient
20
Friction Stir Welding can use existing and readily available machine tool technology. The
process is also suitable for automation and adaptable for robot use. Its main advantages
are:
•
Non-consumable tool, No filler wire
•
One tool can typically be used for up to 1000m of weld length in 6000 series
aluminum alloys
•
No gas shielding for welding aluminum
•
No welder certification required
•
Some tolerance to imperfect weld preparations - thin oxide layers can be accepted
•
No grinding, brushing or pickling required in mass production
The limitations of the FSW process are being reduced by intensive research and
development. However, the main limitations of the FSW process are at present:
•
Welding speeds are moderately slower than those of some fusion welding
processes (up to 750mm/min for welding 5mm thick 6000 series aluminum alloy
on commercially available machines)
•
Work pieces must be rigidly clamped
•
Backing bar required
•
Keyhole at the end of each weld
The repeatable quality of the solid-phase welds can improve existing products and lead to
a number of new product designs previously not possible. Welds with the highest quality
can be achieved by Friction Stir Welding. The crushing, stirring and forging action of the
21
FSW tool produces a weld with a finer microstructure than the parent material. The weld
metal strength can be, in the as welded condition, in excess of that in the thermomechanically affected zone.
3.3. Microstructure Classification [11]
The first attempt at classifying microstructures was made by P L Threadgill (Bulletin,
March 1997). This work was based solely on information available from Aluminum
alloys. However, it has become evident from work on other materials that the behavior of
Aluminum alloys is not typical of most metallic materials, and therefore the scheme
cannot be broadened to encompass all materials. It is therefore proposed that the
following revised scheme is used. This has been developed at TWI, but has been
discussed with a number of appropriate people in industry and academia, and has also
been provisionally accepted by the Friction Stir Welding Licensees Association. The
system divides the weld zone into distinct regions as follows:
Figure 3.2: Microstructure of a friction stir weld [11].
A. Unaffected material or parent metal
B. Heat affected zone (HAZ)
22
C. Thermo-mechanically affected zone (TMAZ)
D. Weld Nugget
Unaffected material or parent metal: This is material remote from the weld, which has
not been deformed, and which although it may have experienced a thermal cycle from the
weld is not affected by the heat in terms of microstructure or mechanical properties.
Heat affected zone (HAZ): In this region, which clearly will lie closer to the weld
centre, the material has experienced a thermal cycle, which has modified the
microstructure and/or the mechanical properties. However, there is no plastic deformation
occurring in this area. In the previous system, this was referred to as the "thermally
affected zone". The term heat affected zone is now preferred, as this is a direct parallel
with the heat affected zone in other thermal processes, and there is little justification for a
separate name.
Thermo-mechanically affected zone (TMAZ): In this region, the Friction Stir Welding
tool has plastically deformed the material, and the heat from the process will also have
exerted some influence on the material. In the case of aluminum, it is possible to get
significant plastic strain without recrystallisation in this region, and there is generally a
distinct boundary between the recrystallised zone and the deformed zones of the TMAZ.
In the earlier classification, these two sub-zones were treated as distinct micro structural
regions. However, subsequent work on other materials has shown that aluminum behaves
in a different manner to most other materials, in that it can be extensively deformed at
high temperature without recrystallisation. In other materials, the distinct recrystallised
23
region (the nugget) is absent, and the whole of the TMAZ appears to be recrystallised.
This is certainly true of materials, which have no thermally induced phase transformation,
which will in itself induce recrystallisation without strain, for example pure Titanium, b
Titanium alloys, Austenitic Stainless Steels and Copper. In materials such as Ferritic
Steels and a-b Titanium alloys (e.g.Ti-6Al-4V), understanding the microstructure is made
more difficult by the thermally induced phase transformation, and this can also make the
HAZ/TMAZ boundary difficult to identify precisely.
Weld Nugget: The recrystallised area in the TMAZ in Aluminum alloys has traditionally
been called the nugget. Although this term is descriptive, it is not very scientific.
However, its use has become widespread, and as there is no word, which is equally
simple with greater scientific merit, this term has been adopted. It has been suggested
that the area immediately below the tool shoulder (which is clearly part of the TMAZ)
should be given a separate category, as the grain structure is often different here. The
microstructure here is determined by rubbing by the rear face of the shoulder, and the
material may have cooled below its maximum. It is suggested that this area is treated as a
separate sub-zone of the TMAZ.
3.4. Joint Geometries
The process has been used for the manufacture of butt welds; overlap welds, T-sections,
fillet, and corner welds. For each of these joint geometries specific tool designs are
required which are being further developed and optimized. Longitudinal butt welds and
circumferential lap welds of Al alloy fuel tanks for space flights have been Friction Stir
Welded and successfully tested.
24
The FSW process can also cope with circumferential, annular, non-linear, and threedimensional welds. Since gravity has no influence on the solid-phase welding process, it
can be used in all positions, viz:
1. Horizontal
2. Vertical
3. Overhead
4. Orbital
3.5. Applications
Shipbuilding and marine industries
The shipbuilding and marine industries are two of the first industry sectors, which have
adopted the process for commercial applications. The process is suitable for the following
applications:
•
Panels for decks, sides, bulkheads and floors
•
Aluminum extrusions
•
Hulls and superstructures
•
Helicopter landing platforms
•
Offshore accommodation
•
Marine and transport structures
•
Masts and booms, e.g. for sailing boats
•
Refrigeration plant
25
Aerospace industry
At present the aerospace industry is welding prototype parts by Friction Stir Welding.
Opportunities exist to weld skins to spars, ribs, and stringers for use in military and
civilian aircraft. This offers significant advantages compared to riveting and machining
from solid, such as reduced manufacturing costs and weight savings. Longitudinal butt
welds and circumferential lap welds of Al alloy fuel tanks for space vehicles have been
friction stir welded and successfully tested. The process could also be used to increase the
size of commercially available sheets by welding them before forming. The Friction Stir
Welding process can therefore be considered for:
•
Wings, fuselages, empennages
•
Cryogenic fuel tanks for space vehicles
•
Aviation fuel tanks
•
External throw away tanks for military aircraft
•
Military and scientific rockets
•
Repair of faulty MIG welds
Railway industry
The commercial production of high-speed trains made from Aluminum extrusions which
may be joined by friction stir welding has been published. Applications include:
•
High speed trains
•
Rolling stock of railways, underground carriages, trams
•
Railway tankers, goods wagons and Container bodies
26
Land transportation
The friction stir welding process is currently being experimentally assessed by several
automotive companies and suppliers to this industrial sector for its commercial
application. A joint EWI/TWI Group Sponsored Project is investigating representative
joint designs for automotive lightweight structures. Potential applications are:
•
Engine, chassis cradles and wheel rims
•
Attachments to hydro formed tubes
•
Tailored blanks, e.g. welding of different sheet thicknesses
•
Space frames, e.g. welding extruded tubes to cast nodes
•
Truck bodies, Tail lifts for lorries, Mobile cranes
•
Armor plate vehicles and Fuel tankers
•
Ships, buses and airfield transportation vehicles
•
Motorcycle, bicycle frames and Repair of aluminum cars
•
Articulated lifts and personnel bridges
•
Magnesium and magnesium/aluminum joints
Construction industry
The use of portable FSW equipment is possible for:
•
Aluminum bridges
•
Facade panels made from aluminum, copper or titanium
•
Window frames and Aluminum pipeline.
27
CHAPTER 4
MECHANICAL TESTING AND METALLOGRAPHY
4.1. Testing [7]
Mechanical testing of materials is generally performed for one of the following reasons:
1. Test development: to create or refine the test method itself.
2. Design: to create or select materials for specific applications.
3. Quality control: to verify that incoming material is acceptable.
4.2. Tensile Test [7]
Uniaxial tensile test is one of the most frequently performed mechanical tests. This type
of test generally involves gripping a specimen at both ends and subjecting it to increasing
axial load until it breaks. Recording of load and elongation data during the test allows the
investigator to determine several characteristics about the mechanical behavior of the
material.
There are several reasons for performing tensile tests. The results of tensile tests are used
in selecting materials for engineering applications. Tensile properties frequently are
included in material specifications to ensure quality. Tensile properties often are
measured during development of new materials and processes, so that different materials
and processes can be compared. Finally, tensile properties often are used to predict the
behavior of a material under forms of loading other than uniaxial tension.
28
The strength of a material often is the primary concern. The strength of interest may be
measured in terms of either the stress necessary to cause appreciable plastic deformation
or the maximum stress that the material can withstand. Also of interest is the material’s
ductility, which is a measure of how much it can be deformed before it fractures. Low
ductility in a tensile test often is accompanied by low resistance to fracture under other
forms of loading.
4.3. Tensile Specimens [7]
The figure below shows a typical tensile test specimen. It has enlarged ends or shoulders
for gripping. The important part of the specimen is the gage section. The cross-sectional
area of the gage section is reduced relative to that of the remainder of the specimen so
that deformation and failure will be localized in this region. The gage length is the region
over which measurements are made and is centered within the reduced section. The
distances between the ends of the gage section and the shoulders should be great enough
so that the larger ends do not constrain deformation within the gage section.
Figure 4.1: Tensile test specimen [11].
29
A tensile test involves mounting the specimen in a machine and subjecting it to tension.
The tensile force is recorded as a function of the increase in gage length. When forceelongation data are converted to engineering stress and strain, a stress-strain curve that is
identical in shape to the force-elongation curve can be plotted. The advantage of dealing
with stress versus strain rather than load versus elongation is that the stress-strain curve is
virtually independent of specimen dimensions.
4.4. Hardness Test [8]
Hardness has a variety of meanings. To the metal industry, it may be thought of as
resistance to permanent deformation. To the metallurgist, it means resistance to
penetration. To the lubrication engineer, it means resistance to wear. To the design
engineer, it is a measure of flow stress. To the mineralogist, it means resistance to
scratching. To the machinist, it means resistance to machining. Hardness may also be
referred to as mean contact pressure. All of these characteristics are related to the plastic
flow stress of materials.
Hardness test is one of the most valuable and widely used mechanical tests for evaluating
the properties of metals as well as certain other materials. The hardness of a material
usually is considered resistance to permanent indentation. In general, an indenter is
pressed into the surface of the metal to be tested under a specific load for a definite time
interval and a measurement is made of the size or depth of the indentation. Hardness is
not a fundamental property of a material. Hardness values are arbitrary and there are no
absolute standards of hardness. Hardness has no quantitative value, except in terms of a
30
given load applied in a specific manner for a specified duration and a specified penetrator
shape.
The principal purpose of the hardness test is to determine the suitability of a material for
a given application or the particular treatment to which the material has been subjected.
The importance of hardness testing has to do with the relationship between hardness and
other properties of material. The hardness test is simple, easy and relatively
nondestructive.
Hardness test is divided into two categories: Macrohardness and Microhardness.
Macrohardness refers to testing with applied loads on the indenter of more than 1 Kg and
covers, for example, the testing of tools, dies and sheet material in the heavier gages. In
microhardness testing, applied loads are 1 Kg and below and material being tested is very
thin (down to 0.0125 mm). Applications include extremely small parts, thin superficially
hardened parts, plated surfaces and individual constituents of materials.
4.5. Types of Hardness Tests [8]
1. Indentation tests
2. Microhardness testing
3. Scratch hardness test
4. Special indentation tests
5. Rebound principle
6. Abrasion and erosion testing
7. Laboratory wear tests
31
8. Service tests
9. Electromagnetic testing
4.6. Factors for Selection of Hardness Testing Methods [8]
1. Hardness range of the test material
2. Size of the workpiece
3. Shape of the workpiece
4. Degree of flatness of the workpiece
5. Surface condition of the workpiece
6. Nature of the test material: homogeneous or nonhomogeneous
7. Effect of indentation marks
8. Number of identical pieces to be tested
9. Equipment availability
4.7. Rockwell Hardness Test [15]
The Rockwell hardness test method consists of indenting the test material with a diamond
cone or hardened steel ball indenter. The indenter is forced into the test material under a
preliminary minor load F0 usually 10 kgf. When equilibrium has been reached, an
indicating device, which follows the movements of the indenter and so responds to
changes in depth of penetration of the indenter, is set to a datum position. While the
preliminary minor load is still applied an additional major load is applied with resulting
increase in penetration. When equilibrium has again been reach, the additional major load
is removed but the preliminary minor load is still maintained. Removal of the additional
32
major load allows a partial recovery, so reducing the depth of penetration. The permanent
increase in depth of penetration, resulting from the application and removal of the
additional major load is used to calculate the Rockwell hardness number.
HR = E - e
F0 = preliminary minor load in kgf
F1 = additional major load in kgf
F = total load in kgf
e = permanent increase in depth of penetration due to major load F1 measured in units of
0.002 mm
E = a constant depending on form of indenter: 100 units for diamond indenter, 130 units
for steel ball indenter
HR = Rockwell hardness number
D = diameter of steel ball
Figure 4.2: Rockwell Principle [15]
33
Advantages of the Rockwell hardness method include the direct Rockwell hardness
number readout and rapid testing time. Disadvantages include many arbitrary non-related
scales and possible effects from the specimen support anvil.
4.8. Metallographic Specimen Preparation Basics [14]
Metallography is the study of a materials microstructure. Analysis of a materials
microstructure aids in determining if the material has been processed correctly and is
therefore a critical step for determining product reliability and for determining why a
material failed. The basic steps for proper metallographic specimen preparation include:
Documentation - Metallographic analysis is a valuable tool. By properly documenting
the initial specimen condition and the proceeding microstructural analysis, metallography
provides a powerful quality control as well as an invaluable investigative tool.
Sectioning and Cutting - Following proper documentation, most metallographic samples
need to be sectioned to the area of interest and for easier handling. Depending upon the
material, the sectioning operation can be obtained by abrasive cutting (metals and metal
matrix composites), diamond wafer cutting (ceramics, electronics, biomaterials,
minerals), or thin sectioning with a microtome (plastics).
Proper sectioning is required to minimize damage, which may alter the microstructure
and produce false metallographic characterization. Proper cutting requires the correct
selection of abrasive type, bonding, and size; as well as proper cutting speed, load and
coolant.
34
Mounting - The mounting operation accomplishes three important functions (1) it
protects the specimen edge and maintains the integrity of a materials surface feature (2)
fills voids in porous materials and (3) improves handling of irregular shaped samples,
especially for automated specimen preparation. The majority of metallographic specimen
mounting is done by encapsulating the specimen into a compression mounting compound
(thermosets - phenolics, epoxies, diallyl phthalates or thermoplastics - acrylics), casting
into ambient cast able mounting resins (acrylic resins, epoxy resins, and polyester resins),
and gluing with a thermoplastic glues.
Planar Grinding - or course grinding is required to planarize the specimen and to reduce
the damage created by sectioning. The planar grinding step is accomplished by
decreasing the abrasive grit/ particle size sequentially to obtain surface finishes that are
ready for polishing. Care must be taken to avoid being too abrasive in this step, and
actually creating greater specimen damage than produced during cutting (this is
especially true for very brittle materials such as silicon).
The machine parameters which effect the preparation of metallographic specimens
includes: grinding/polishing pressure, relative velocity distribution, and the direction of
grinding/polishing.
Rough Polishing - The purpose of the rough polishing step is to remove the damage
produced during cutting and planar grinding. Proper rough polishing will maintain
specimen flatness and retain all inclusions or secondary phases. By eliminating the
previous damage and maintaining the microstructural integrity of the specimen at this
35
step, a minimal amount of time should be required to remove the cosmetic damage at the
final polishing step.
Rough polishing is accomplished primarily with diamond abrasives ranging from 9
micron down to 1 micron diamond. Polycrystalline diamond because of its multiple and
small cutting edges, produces high cut rates with minimal surface damage, therefore it is
the recommended diamond abrasive for metallographic rough polishing on low napped
polishing cloths.
Final Polishing - The purpose of final polishing is to remove only surface damage. It
should not be used to remove any damage remaining from cutting and planar grinding. If
the damage from these steps is not complete, the rough polishing step should be repeated
or continued.
Etching - The purpose of etching is to optically enhance microstructural features such as
grain size and phase features. Etching selectively alters these microstructural features
based on composition, stress, or crystal structure. The most common technique for
etching is selective chemical etching and numerous formulations have been used over the
years. Other techniques such as molten salt, electrolytic, thermal and plasma etching have
also found specialized applications.
36
CHAPTER 5
EXPERIMENTAL PROCEDURE
5.1. Tensile test
The most common measure of FSW quality after visual inspection for surface breaking
defects may be the transverse tensile test (loading direction perpendicular to the welding
direction) [1]. The objective of transverse tensile test is to determine whether or not the
weld is suitable for its intended use. The Low-Speed Friction Stir welds and the HighSpeed Friction Stir welds were cut into straps perpendicular to the welding direction.
These straps were then cut or machined into a ‘dog-bone’ shape, according to the ASTM
standards. The tensile tests were carried out at room temperature at a crosshead speed of
1 mm/min using a computer controlled testing machine. Load and strain ranges were
selected so that the test will fit the range. The tensile properties of each weld were
evaluated by a number of samples or tensile specimens cut from the same weld. The data
obtained from the tensile test such as Peak load, Break stress, Peak stress, Yield stress
and the elongation were recorded. These properties of the weld were then compared with
that of its parent metal to obtain the weld or joint efficiency.
5.2. Hardness Test
Rockwell hardness tester was employed to measure the hardness of the welds. The
hardness was measured in two ways:
37
Along the weld or longitudinal hardness of the weld: The hardness values of the weld
were recorded on the weld, from the starting point of the weld to the end point. This
shows the variation of the weld hardness from beginning to the end of the weld.
Across the weld or transverse hardness of the weld: Hardness measurements were made
across a weld, in a line, perpendicular to the welding direction. Made on the top surface
of the weld, a hardness traverse is a useful tool to help identify the weak zones of a weld.
The hardness values of the weld were then compared with that of its parent metal to
obtain the relative hardness of the weld or the hardness joint efficiency.
5.3. Metallographic Analysis
Metallographic analyses of the welds were carried out to detect weld defects present and
the microstructure of the weld zone. The welds were cross-sectioned perpendicularly to
the welding direction for metallographic analyses. These samples were then mounted in a
transparent epoxy. These mounted samples were then grinded and polished on abrasive
silicon carbide sheets. The samples were then fine polished, etched with Keller’s reagent
and observed by optical microscopy.
38
CHAPTER 6
RESULTS AND DISCUSSIONS
In the early days of Friction Stir Welding, most welding was performed on modified
machine tools. The level of instrumentation available for process monitoring was often
minimal leading to a great deal of speculation on the quantitative effects of process
variable and tool geometry changes. Generally, these changes were correlated either with
improved joint strength or the ability to make a weld with greater speed: these are at best
indirect correlations with the true, physical, process changes. As the sophistication of
FSW equipment has increased, so has our ability to quantify the effects of process
variables. Correlation between the indirect effects of process changes and the direct
effects will greatly enhance our understanding of the process. With sufficient
understanding of these effects and high fidelity models of the process developed using
this understanding, we may in time be able to predict many of the effects which must
now be determined via trial and error [21].
Because the FSW process has only recently become a subject of wide study, there are
currently no large databases of weld properties and, in fact, no specifications on how to
make or test friction stir welds currently exist. In general, the process is robust and a wide
range of processing parameters and tool designs can be used to make metallurgically
sound welds in a given alloy and plate thickness. While weld free of defects may be made
using a wide range of processing parameters, the chosen process parameters may
significantly affect the mechanical properties of the weld either through direct
39
modification of the weld microstructure or by indirect influence (e.g. by modification of
residual stress state) [1].
6.1. Effect of Changing Welding Speed at Constant Weld Pitch [21]
A series of welds was made in 6.4 mm thick plate at a weld pitch of 0.43 mm/rev. In
order to maintain constant weld pitch, the welding speed and the rpm were increased by
the same factors for the various welds. Z axis load was varied to accommodate the
Power (watts) or Energy (J/mm)
varying weld speeds.
3000
2500
2000
Specific weld energy
1500
power
1000
500
0
0
2
4
6
welding speed, mm/s
Fig 6.1: Required power and specific weld energy at constant weld pitch.
In the Fig 6.1, the relationship of weld power and the specific weld energy to welding
speed at constant weld pitch is illustrated. Weld energy decreases and the required power
increases with increasing welding speed at constant weld pitch. This indicates that weld
pitch is not, as sometimes suggested, a very good indicator of weld energy. Neither does
an advance per revolution that produces good welds at one speed guarantee good welds at
another.
40
12
X-axis force, kN
10
8
6
4
2
0
0
1
2
3
4
5
6
Welding speed, mm/s
Fig 6.2: X-axis force for welds made at constant weld pitch.
In the Fig 6.2, the relationship of X axis force to welding speed at constant weld pitch is
illustrated. The increase in X axis force with increasing welding speed at constant weld
pitch may indicate that the material being involved in the process is in different stages of
evolution when welded using different speeds. It is also interesting to note the lack of
profound effect of the Z axis force on the weld energy and power.
6.2. Temper Effects on Required Loads and Weld Energy [21]
Alloys 7075 and 7050 were each welded in three different tempers (0, T6 and T7). The
plate thickness was 9.5 mm and the same tool was used for all welds. All six welds were
made using 240 rpm tool rotation rate and 2.4 mm/s welding speed. For all six welds, the
x axis forces did not vary by more than 12%. The weld energies for both 7050 and 7075
varied by less than 3%. These results indicate that the composition is critical and the
welding forces and torques may be independent of starting microstructure.
41
6.3. Alloy Effects on Specific Weld Energy [21]
Al 6061-T6, Al 7075-T6 and Al 2024-T3 were welded at three different welding speeds
of 1.3 mm/s, 2.4 mm/s and 3.3 mm/s. For each weld, the specific weld energy decreases
with increase in welding speed. This effect may be observed in Fig 6.3.
Specific weld energy, J/mm
2500
2000
7075-T6
1500
6061-T6
1000
2024-T3
500
0
0
1
2
3
4
Welding Speed, mm/s
Fig 6.3: Specific weld energy as a function of welding speed.
The highest energy per unit weld length is observed when welding alloy 6061. This is
probably because of the relatively high thermal conductivity of the alloy and hence,
thermal energy would diffuse away from the weld zone at the greatest rate in 6061. The
relative changes in weld energy associated with decreasing welding speed are essentially
the same for all the alloys tested.
42
6.4. LOW-SPEED FRICTION STIR WELD 1
Weld Specifications
Materials: Alclad 2024-T3 (0.080”) & Al 7075-T6 (0.040”)
Type of Joint: Lap
Welding Speed: 10”/min
Rotational Speed: 750 rpm
Tool Shoulder Diameter: 0.375 in
6.4.1. Tensile Test – Parent 1 (Alclad 2024-T3)
SI
Peak
Load
(lb)
1
1121.63
2
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
37387.70 35441.30
16.20
37387.70
15.2
735124
1121.63
56081.60 53648.60
16.00
56081.60
13.8
1084599
3
1121.63
56081.60 53648.60
15.00
56081.60
13.7
998141
Avg
1121.63
49850.30
15.70
49850.30
14.2
939288
47579.5
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.4(a): Tensile test data of parent 1 – Alclad 2024-T3.
43
6.4.2. Tensile Test – Parent 2 (Al 7075-T6)
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
1
722.61
72261.30 70314.90
11.10
72261.30
8.8
2324608
2
712.88
71288.10 68368.50
10.30
71288.10
8.8
2035341
3
693.42
69341.70 67395.30
10.20
69341.70
8.6
2086943
Avg
709.64
70963.70 68692.90
10.50
70963.70
8.6
2148964
Table 6.4(b): Tensile test data of parent 2 - Al 7075-T6.
6.4.3. Tensile Test - Weld
Break
Elongation
(%)
Yield
Stress
(psi)
54581.20 54256.80
7.30
54581.20
7.30
911848
1715.29
57176.50 56852.10
8.10
57176.50
8.10
924842
3
1520.65
50688.40 50364.00
6.20
50688.40
6.20
980258
4
1549.85
51661.60 51661.60
7.00
51661.60
7.10
843674
Avg
1566.53
52217.73 51986.01
7.10
52217.73
7.1
941437
SI
Peak
Load
(lb)
1
1637.44
2
Peak
Stress
(psi)
Break
Stress
(psi)
Table 6.4(c): Tensile test data of the weld.
44
Yield
Tangent
Elongation Modulus
(%)
(psi)
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent 1
(Alclad 2024-T3) are 1.09 and 1.05 for Break stress and Peak / Yield stress
respectively. This implies that the tensile strength of the weld is more than parent
1.
•
The joint efficiencies of the weld with respect to parent 2 (Al 7075-T6) are 0.76
and 0.74 for Break stress and Peak / Yield stress respectively. This implies that
the tensile strength of the weld is less than parent 2.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
All the weld tensile test specimens fractured in the TMAZ on the retreating side.
•
Parent 2, even though has a better tensile properties than parent 1, was the first to
fracture. This might be due to the constant contact of parent 2 with tool shoulder.
6.4.4. Hardness Test – Along the Weld
Parent
Weld
SI No
1
2
3
4
Avg
Al 7075-T6
Alclad 2024-T3
84.30
84.00
84.70
84.00
84.25
70.00
69.40
68.90
70.20
69.63
77.00
74.60
74.60
76.50
75.68
Table 6.4(d): Hardness test data – along the weld.
45
6.4.5. Hardness Test – Across the Weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
83.80
81.10
73.40
73.80
72.50
76.80
83.70
84.30
77.80
68.90
76.20
73.80
75.70
84.80
84.10
81.90
73.40
77.00
74.70
69.50
84.30
84.07
80.27
71.90
75.67
73.67
74.00
84.27
Table 6.4(e): Hardness test data – across the weld.
1. HARDNESS TEST ( HRB )
86.00
84.00
82.00
HRB
80.00
78.00
76.00
74.00
72.00
70.00
0
1
2
3
4
5
6
7
8
Figure 6.4(a): Hardness graph – across the weld.
•
As seen in the Table 6.4(d), the weld has a better hardness value compared to
parent 1 and is softer than parent 2. The hardness joint efficiencies are 1.10 and
0.90 for parent 1 and parent 2 respectively.
•
It can be seen from Figure 6.4(a) that a hardness degradation region (i.e. softened
region) has occurred in each joint [10].
46
•
There are two low hardness zones on the two sides of the weld center, but the
minimum hardness value exists in the low hardness zone on the retreating side,
accordingly the joint is fractured on the retreating side [10]. This implies that the
tensile properties and fracture locations are related to the hardness profile of the
weld.
6.4.6. Microstructure
Figure 6.4(b): Microstructure of the weld.
6.5. LOW-SPEED FRICTION STIR WELD 2
Weld Specifications
Materials: Alclad 2024-T3 (0.080”) & Al 7075-T6 (0.040”)
Type of Joint: Lap
Welding Speed: 10”/min & 15”/min
Rotational Speed: 760 rpm
Tool Shoulder Diameter: 0.375 in
47
6.5.1. Tensile Test – Parent 1 (Alclad 2024-T3)
SI
Peak
Load
(lb)
Peak
Stress
(psi)
1
1121.63
2
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
37387.70 35441.30
16.20
37387.70
15.2
735124
1121.63
56081.60 53648.60
16.00
56081.60
13.8
1084599
3
1121.63
56081.60 53648.60
15.00
56081.60
13.7
998141
Avg
1121.63
49850.30
15.70
49850.30
14.2
939288
47579.5
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.5(a): Tensile test data of parent 1 – Alclad 2024-T3.
6.5.2. Tensile Test – Parent 2 (Al 7075-T6)
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
1
722.61
72261.30 70314.90
11.10
72261.30
8.8
2324608
2
712.88
71288.10 68368.50
10.30
71288.10
8.8
2035341
3
693.42
69341.70 67395.30
10.20
69341.70
8.6
2086943
Avg
709.64
70963.70 68692.90
10.50
70963.70
8.6
2148964
Table 6.5(b): Tensile test data of parent 2 – Al 7075-T6.
48
6.5.3. Tensile Test – Weld (10”/min)
Break
Elongation
(%)
Yield
Stress
(psi)
51986.00 51986.00
6.70
51986.00
6.70
969705
1452.53
48417.50 48093.10
6.50
48417.50
6.50
836114
3
1559.58
51986.00 51337.20
6.90
51986.00
6.80
895946
Avg
1523.90
50796.50 50472.10
6.70
50796.50
6.70
900588
SI
Peak
Load
(lb)
1
1559.58
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.5(c): Tensile test data of weld @ 10”/min.
6.5.4. Tensile Test – Weld (15”/min)
SI
Peak
Load
(lb)
1
1374.67
2
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
45822.30 32521.70
5.60
45822.30
4.80
1013254
1384.40
46146.70 45822.30
5.90
46146.70
5.80
844730
3
1403.87
46795.50 46795.50
6.50
46795.50
6.50
773184
Avg
1387.65
46254.83 41713.17
6.00
46254.83
5.70
877056
Table 6.5(d): Tensile test data of the weld @ 15”/min.
49
•
The joint efficiencies (weld / parent ratio) of the weld @ 10”/min with respect to
parent 1 (Alclad 2024-T3) are 1.06 and 1.02 for Break stress and Peak / Yield
stress respectively. This implies that the tensile strength of the weld is more than
parent 1.
•
The joint efficiencies of the weld @ 10”/min with respect to parent 2 (Al 7075T6) are 0.73 and 0.72 for Break stress and Peak / Yield stress respectively. This
implies that the tensile strength of the weld is less than parent 2.
•
The average break and yield elongation of the weld @ 10”/min is much lower
than the average break and yield elongation of its parents.
•
The joint efficiencies (weld / parent ratio) of the weld @ 15”/min with respect to
parent 1 (Alclad 2024-T3) are 0.88 and 0.93 for Break stress and Peak / Yield
stress respectively. This implies that the tensile strength of the weld is less than
parent 1.
•
The joint efficiencies of the weld @ 15”/min with respect to parent 2 (Al 7075T6) are 0.61 and 0.65 for Break stress and Peak / Yield stress respectively. This
implies that the tensile strength of the weld is less than parent 2.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
All the weld tensile test specimens fractured in the TMAZ on the retreating side.
•
Parent 2, even though has a better tensile properties than parent 1, was the first to
fracture. This might be due to its constant contact with tool shoulder.
•
The weld @ 10”/min has better tensile properties than the weld @ 15”/min.
50
6.5.5. Hardness Test – Along the Weld
Weld
Parent
SI No
1
2
3
4
5
Avg
Al 7075-T6
Alclad 2024-T3
Weld -10”/min
Weld – 15”/min
83.70
84.00
83.40
84.10
83.70
83.78
69.60
69.00
68.80
68.80
69.50
69.14
78.70
80.50
79.50
79.10
81.20
79.80
77.70
76.70
78.80
76.30
77.70
77.44
Table 6.5(e): Hardness test data – along the weld.
6.5.6. Hardness Test – Across the Weld
SI No
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
15”/min
83.70
77.70
77.40
78.40
76.80
77.20
83.20
Interface
83.60
74.90
74.60
77.70
71.20
74.20
83.80
10”/min
83.90
79.00
78.60
80.60
77.00
77.30
83.50
Avg
83.73
77.20
76.87
78.90
75.00
76.23
83.50
Table 6.5(f): Hardness test data – across the weld.
51
Weld2 Hardness Test (HRB) 10"/min
85.00
84.00
83.00
82.00
HRB
81.00
80.00
79.00
78.00
77.00
76.00
0
1
2
3
4
5
6
7
8
Figure 6.5(a): Hardness graph – across the weld @ 10”/min.
Weld2 Hardness Test (HRB) 15"/min
85.00
84.00
83.00
82.00
HRB
81.00
80.00
79.00
78.00
77.00
76.00
0
1
2
3
4
5
6
7
8
Figure 6.5(b): Hardness graph – cross the weld @ 15”/min.
•
As seen in the Table 6.5(e), the weld @ 10”/min has a better hardness value
compared to parent 1 and is softer than parent 2. The hardness joint efficiencies
are 1.15 and 0.95 for parent 1 and parent 2 respectively.
52
•
As seen in the Table 6.5(e), the weld @ 15”/min has a better hardness value
compared to parent 1 and is softer than parent 2. The hardness joint efficiencies
are 1.12 and 0.92 for parent 1 and parent 2 respectively.
•
The hardness profiles of both the welds are almost the same, as seen in Figures
6.5(a) and 6.5(b), with the minimum hardness value existing in the TMAZ on the
retreating side. Accordingly all the tensile specimens fractured in the TMAZ on
the retreating side.
•
The interface between weld @ 10”/min and weld @ 15”/min has hardness values
lower than the two welds.
6.6. LOW SPEED FRICTION STIR WELD 3
Weld Specifications
Materials: Alclad 2024-T3 (0.080”) & Al 7075-T6 (0.040”)
Type of Joint: Lap
Welding Speed: 10”/min & 15”/min
Rotational Speed: 600 rpm
Tool Shoulder Diameter: 0.375 in
53
6.6.1. Tensile Test – Parent 1 (Alclad 2024-T3)
Peak
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
37387.70 35441.30
16.20
37387.70
15.2
735124
1121.63
56081.60 53648.60
16.00
56081.60
13.8
1084599
3
1121.63
56081.60 53648.60
15.00
56081.60
13.7
998141
Avg
1121.63
49850.30
15.70
49850.30
14.2
939288
SI
Peak
Load
(lb)
1
1121.63
2
Break
Stress
(psi)
47579.5
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.6(a): Tensile test data of parent1 – Alclad 2024-T3.
6.6.2. Tensile Test – Parent 2 (Al 7075-T6)
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
1
722.61
72261.30 70314.90
11.10
72261.30
8.8
2324608
2
712.88
71288.10 68368.50
10.30
71288.10
8.8
2035341
3
693.42
69341.70 67395.30
10.20
69341.70
8.6
2086943
Avg
709.64
70963.70 68692.90
10.50
70963.70
8.6
2148964
Table 6.6(b): Tensile test data of parent 2 - Al 7075-T6.
54
6.6.3. Tensile Test - Weld (10"/min)
SI
Peak
Load
(lb)
1
1364.94
2
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
45497.90 39334.20
10.00
45497.90
5.30
889318
1413.60
47119.90 44952.70
5.70
47119.90
5.50
907242
3
1306.54
43551.40 39009.80
9.90
43551.40
5.00
891491
Avg
1369.80
45660.08 40657.73
8.90
45660.08
5.30
908639
Table 6.6(c): Tensile test data of the weld @10"/min.
6.6.4. Tensile Test - Weld (15"/min)
SI
Peak
Load
(lb)
1
1111.90
2
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
37063.30 30575.20
6.70
35116.90
3.60
994281
1296.81
43227.00 39658.60
9.50
43227.00
4.30
1042696
3
1209.22
40307.40 31548.40
7.60
40307.40
4.60
879070
Avg
1206.79
40226.28 35035.80
7.90
39739.68
4.40
937781
Table 6.6(d): Tensile test data of the weld @ 15"/min.
55
•
The joint efficiencies (weld / parent ratio) of the weld @ 10”/min with respect to
parent 1 (Alclad 2024-T3) are 0.64 and 0.92 for Break stress and Peak / Yield
stress respectively. This implies that the tensile strength of the weld is less than
parent 1.
•
The joint efficiencies of the weld @ 10”/min with respect to parent 2 (Al 7075T6) are 0.44 and 0.64 for Break stress and Peak / Yield stress respectively. This
implies that the tensile strength of the weld is less than parent 2.
•
The average break and yield elongation of the weld @ 10”/min is much lower
than the average break and yield elongation of its parents.
•
The joint efficiencies (weld / parent ratio) of the weld @ 15”/min with respect to
parent 1 (Alclad 2024-T3) are 0.74, 0.81 and 0.80 for Break stress, Peak stress
and Yield stress respectively. This implies that the tensile strength of the weld is
less than parent 1.
•
The joint efficiencies of the weld @ 15”/min with respect to parent 2 (Al 7075T6) are 0.51, 0.57 and 0.56 for Break stress, Peak stress and Yield stress
respectively. This implies that the tensile strength of the weld is less than parent 2.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
All the weld tensile test specimens fractured in the TMAZ on the retreating side.
The parent 2 of two of the tensile specimen fractured in HAZ.
•
Parent 2, even though has a better tensile properties than parent 1, was the first to
fracture. This might be due to its constant contact with tool shoulder.
•
The weld @ 10”/min has better tensile properties than the weld @ 15”/min.
56
6.6.5. Hardness Test - Along the Weld
Parent
Weld
SI No
1
2
3
4
5
Avg
Al 7075-T6
Alclad 2024-T3
Weld -10”/min
Weld – 15”/min
82.80
83.50
82.40
82.90
82.90
82.78
69.30
68.30
68.80
69.80
69.60
69.16
77.00
77.80
76.70
77.70
75.10
74.20
74.80
75.00
77.30
74.78
Table 6.6(e): Hardness test data - along the weld.
6.6.6. Hardness Test - Across the Weld
SI No
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
15"/min
82.90
78.90
75.40
75.00
74.20
77.50
82.50
Interface
83.20
78.20
73.60
74.80
73.00
75.00
82.90
10"/min
83.30
79.80
76.80
77.70
76.40
78.80
82.90
Avg
83.13
78.97
75.27
75.83
74.53
77.10
82.77
Table 6.6(f): Hardness test data - across the weld.
57
Hardness - across the Weld 3 @ 10"/min
84.00
83.00
82.00
HRB
81.00
80.00
79.00
78.00
77.00
76.00
0
1
2
3
4
5
6
7
8
Figure 6.6(a): Hardness graph - across the weld @ 10"/min.
Hardness - across the Weld 3 @ 15"/min
84.00
83.00
82.00
81.00
80.00
HRB
79.00
78.00
77.00
76.00
75.00
74.00
73.00
0
1
2
3
4
5
6
7
8
Figure 6.6(b): Hardness graph - across the weld @ 15"/min.
•
As seen in the Table 6.6(e), the weld @ 10”/min has a better hardness value
compared to parent 1 and is softer than parent 2. The hardness joint efficiencies
are 1.12 and 0.93 for parent 1 and parent 2 respectively.
•
As seen in the Table 6.6(e), the weld @ 15”/min has a better hardness value
compared to parent 1 and is softer than parent 2. The hardness joint efficiencies
are 1.08 and 0.90 for parent 1 and parent 2 respectively.
58
•
The hardness profiles of both the welds are almost the same, as seen in Figures
6.6(a) and 6.6(b), with the minimum hardness value existing in the TMAZ on the
retreating side. Accordingly all the tensile specimens fractured in the TMAZ on
the retreating side.
•
The interface between weld @ 10”/min and weld @ 15”/min has hardness values
lower than the two welds.
6.6.7. Comparison - Weld 1, Weld 2 and Weld 3
54000
52000
50000
Peak / Yield
Stress
48000
(psi)
46000
44000
42000
10"/min @ 10"/min @
750 rpm
600 rpm
10"/min @
760 rpm
Figure 6.6(c): Peak/yield stress of weld - 10"/min @ 750,760 & 600rpm.
59
60000
Break Stress (psi)
50000
40000
30000
20000
10000
10"/min @
600 rpm
10"/min @
750 rpm
10"/min @
760 rpm
0
Figure 6.6(d): Break stress of weld - 10"/min @ 750, 760 & 600 rpm.
•
It can be seen from Figures 6.6(c) and 6.6(d) that the tensile properties of each
joints change considerably with the change in the welding pitch (the ratio of the
rotational speed to the welding speed).
•
When the welding pitch is smaller than 75 r/in, the tensile properties of the joints
increases with the increase in the welding pitch.
•
When the welding pitch is greater than 75 r/in, all tensile properties tend to
decrease with the increase in the welding pitch.
•
These results indicate that a softening effect has taken place in the joint. The
softened levels or the tensile properties of the joints are significantly affected by
the welding parameters. For example, the welding pitch of 75 r/in, corresponding
to the rotational speed of 750 rpm and the welding speed of 10"/min, is optimum
for the tensile properties of the joints in case of welds 1, 2 & 3.
60
•
None of the tensile test specimens failed on the advancing side of the joint, which
implies that the tensile properties of the welds are not the same on either sides of
the weld center. This also implies that the retreating side of the joint is weaker
than the advancing side.
There are two low hardness zones on the two sides of the weld center, but the
minimum hardness value exists in the low hardness zone on the retreating side,
accordingly the joint is fractured on the retreating side. This implies that the
tensile properties and fracture locations are related to the hardness profile of the
weld.
60000
50000
Stress (psi)
•
40000
Peak / Yield Stress (psi)
30000
Break Stress (psi)
20000
10000
0
10"/min
15"/min
Welding Speed (in/min)
Figure 6.6(e): Weld 2 - change in stress due to change in welding pitch.
61
It can be seen from Figure 6.6(e) that the tensile properties of weld 2 change considerably
with change in its welding pitch. Break, Peak and Yield stress of weld 2 decrease with the
decrease in the welding pitch from 76 r/in to 50.67 r/in (i.e. increase in welding speed
Stress (psi)
from 10"/min to 15"/min).
50000
45000
40000
35000
30000
25000
20000
15000
10000
5000
0
Peak Stress (psi)
Break Stress (psi)
Yield Stress (psi)
10"/min
15"/min
Welding Speed (in/min)
Figure 6.6(f): Weld 3 - change in stress due to change in welding pitch.
It can be seen from Figure 6.6(f) that the tensile properties of weld 3 too changes
considerably with change in its welding pitch. Peak and Yield stress of the weld decrease
and its Break stress increases with the decrease in the welding pitch from 60 r/in to 40
r/in (i.e. increase in welding speed from 10"/min to 15"/min).
62
80
Hardness (HRB)
79.5
79
78.5
Weld 2
78
77.5
77
76.5
76
10"/min
15"/min
Hardness (HRB)
Welding Spee d (in/m in)
78
77.5
77
76.5
76
75.5
75
74.5
74
73.5
73
Weld 3
10" /min
15"/min
W e lding Spee d (in/min)
Figure 6.6(g): Weld 2 & weld 3 - change in hardness due to change in welding pitch
Figure 6.6(g) shows the change in the hardness of weld 2 and weld 3, along the weld,
with change in the welding pitch. In weld 2, as the welding pitch decreases from 76 r/in
to 50.67 r/in, there is a decrease in the hardness of the weld. In weld 3, the hardness along
the weld reduces with the decrease in the welding pitch from 60 r/in to 40 r/in. These
63
results indicate that softening effect takes place in the weld with an increase in its
welding speed.
6.7. LOW-SPEED FRICTION STIR WELD 4
Weld Specifications
Materials: Alclad 2024-T3 (0.080”) & Al 7075-T6 (0.063”)
Type of Joint: Lap - Double pass
Welding Speed: 10”/min
Rotational Speed: 760 rpm
Tool Shoulder Diameter: 0.375 in
6.7.1. Tensile Test – Parent 1 (Alclad 2024-T3)
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
1
2217.39
55434.80
53260.9
29.30
55434.80
Yield
Tangent
Elongation Modulus
(%)
(psi)
27.2
Table 6.7(a): Tensile test data of parent1 - Alclad 2024-T3.
64
1880680
6.7.2. Tensile Test – Parent 2 (Al 7075-T6)
SI
Peak
Load
(lb)
2156.52
1
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Elongation
(%)
Tangent
Modulus
(psi)
20.10
68461.00
18.6
1088848.00
68461.00 65976.50
Table 6.7(b): Tensile test data of parent 2 - Al 7075-T6.
6.7.3. Tensile Test - Weld
SI
Peak
Load
(lb)
1
973.91
2
Break
Elongation
(%)
Yield
Stress
(psi)
13621.20 13621.20
3.90
13621.20
3.90
381015
947.83
13256.30 13256.30
5.80
13256.30
5.80
396472
3
965.22
13499.50 13377.90
4.10
13499.50
4.10
363561
4
947.83
13256.30 13256.30
4.00
13256.30
4.00
358545
973.91
13621.20 13499.50
4.10
13621.20
4.00
364711
961.72
13450.90 13402.24
4.38
13450.90
4.36
372861
5
Avg
Peak
Stress
(psi)
Break
Stress
(psi)
Table 6.7(c): Tensile test data of the weld.
65
Yield
Tangent
Elongation Modulus
(%)
(psi)
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent 1
(Alclad 2024-T3) are 0.25 and 0.24 for Break stress and Peak / Yield stress
respectively. This implies that the tensile strength of the weld is less than parent1.
•
The joint efficiencies of the weld with respect to parent 2 (Al 7075-T6) are 0.20
and 0.19 for Break stress and Peak / Yield stress respectively. This implies that
the tensile strength of the weld is less than parent 2.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
All the weld tensile test specimens fractured in the TMAZ.
•
Parent 2, even though has a better tensile properties than parent 1, was the first to
fracture.
6.7.4. Hardness Test – Along the weld
SI No
1
2
3
4
5
Avg
Parent
Al 7075-T6
Alclad 2024-T3
88.50
88.60
88.60
89.10
89.10
88.78
70.20
69.30
70.00
70.00
70.40
69.98
Weld
77.60
77.80
78.30
78.30
78.60
78.12
Table 6.7(d): Hardness test data- along the weld.
66
6.7.5. Hardness Test - Across the Weld
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
86.30
83.50
76.10
79.20
77.00
84.50
88.50
85.20
80.50
75.80
78.70
78.00
79.40
89.00
87.20
83.30
76.30
78.20
77.60
82.00
86.00
86.23
82.43
76.07
78.70
77.53
81.97
87.83
Table 6.7(e): Hardness test data - across the weld.
HARDNESS TEST ( HRB )
90.00
88.00
86.00
84.00
HRB
SI No
82.00
80.00
78.00
76.00
74.00
0
1
2
3
4
5
6
7
8
Figure 6.7(a): Hardness graph - across the weld.
67
•
As seen in the Table 6.7(d), the weld has a better hardness value compared to
parent 1 and is softer than parent 2. The hardness joint efficiencies are 1.12 and
0.88 for parent 1 and parent 2 respectively.
•
The hardness of the weld does not vary much on both sides of the weld center,
with the minimum hardness value existing in the TMAZ. Accordingly all the
tensile specimens fractured in the TMAZ.
•
Though the weld has good hardness values, it displays very poor tensile
properties.
6.7.6. Metallography
Fig 6.7(b): Microstructure of the weld nugget.
68
6.8. LOW-SPEED FRICTION STIR WELD 5
Weld Specifications
Materials: Alclad 2024-T3 (0.080”) & Al 7075-T6 (0.063”)
Type of Joint: Lap – Double pass
Welding Speed: 15”/min
Rotational Speed: 760 rpm
Tool Shoulder Diameter: 0.375 in
6.8.1. Tensile Test – Parent 1 (Alclad 2024-T3)
Peak
Stress
(psi)
SI
Peak
Load
(lb)
1
2191.30
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
31.10
54782.60
54782.60 52391.30
Yield
Tangent
Elongation Modulus
(%)
(psi)
28.30
780511
Table 6.8(a): Tensile test data of parent1 – Alclad 2024-T3.
6.8.2. Tensile Test – Parent 2 (Al 7075-T6)
SI
1
Peak
Load
(lb)
2173.91
Peak
Stress
(psi)
Break
Stress
(psi)
69013.10 67080.70
Break
Elongation
(%)
Yield
Stress
(psi)
19.70
69013.10
Yield
Tangent
Elongation Modulus
(%)
(psi)
17.70
Table 6.8(b): Tensile test data of parent2 - Al 7075-T6.
69
1277115
6.8.3. Tensile Test - Weld
SI
Peak
Load
(lb)
1
730.43
2
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
10215.90 10215.90
3.60
10215.90
3.60
306131
756.52
10580.70 10459.10
3.80
10580.70
3.70
319643
3
713.04
9972.60
9729.40
3.50
9972.60
3.40
345836
Avg
733.33
10256.40 10134.80
3.63
10256.40
3.57
323870
Table 6.8(c): Tensile test data of the weld.
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent 1
(Alclad 2024-T3) are 0.19 and 0.18 for Break stress and Peak / Yield stress
respectively. This implies that the tensile strength of the weld is less than parent1.
•
The joint efficiencies of the weld with respect to parent 2 (Al 7075-T6) are 0.15
and 0.15 for Break stress and Peak / Yield stress respectively. This implies that
the tensile strength of the weld is less than parent 2.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
All the weld tensile test specimens fractured in the TMAZ.
70
6.8.4. Hardness Test – Along the weld
SI No
1
2
3
4
Avg
Parent
Al 7075-T6
Alclad 2024-T3
88.70
89.30
87.70
87.90
88.40
Weld
68.60
69.00
68.10
70.40
69.03
79.60
80.20
83.90
83.40
81.78
Table 6.8(d): Hardness test data – along the weld.
6.8.5. Hardness Test – Across the Weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
86.70
83.80
75.10
77.00
75.90
78.20
87.40
87.90
81.50
76.40
78.50
77.10
82.20
87.70
87.00
83.60
72.00
77.00
73.60
80.40
88.50
87.20
82.97
74.50
77.50
75.53
80.27
87.87
Table 6.8(e): Hardness test data – across the weld.
71
HARDNESS TEST ( HRB )
90.00
88.00
86.00
84.00
HRB
82.00
80.00
78.00
76.00
74.00
72.00
0
1
2
3
4
5
6
7
8
Figure 6.8(a): Hardness graph - across the Weld.
•
As seen in the Table 6.8(d), the weld has a better hardness value compared to
parent 1 and is softer than parent 2. The hardness joint efficiencies are 1.18 and
0.93 for parent 1 and parent 2 respectively.
•
The hardness of the weld does not vary much on both sides of the weld center,
with the minimum hardness value existing in the TMAZ. Accordingly all the
tensile specimens fractured in the TMAZ.
•
Though the weld has good hardness values, it displays very poor tensile
properties.
72
6.8.6. Metallography
Fig 6.8(b): Microstructure of the weld.
6.9. HIGH-SPEED FRICTION STIR WELD 6
Weld Specifications
Materials: Al 6061-T6 (0.125”)
Type of Joint: Butt
Welding Speed: 15”/min
Rotational Speed: 1500 rpm
Tool Shoulder Diameter: 0.375 in
73
6.9.1. Tensile Test – Parent (Al 6061-T6)
SI
Peak
Load
(lb)
Peak
Stress
(psi)
1
4037.50
2
Avg
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
64600.00 59502.80
20.80
64600.00
17.70
1367795
4019.80
64316.90 59361.20
20.80
64316.90
18.00
1320557
4028.65
64458.45 59432.00
20.80
64458.45
17.85
1344176
Table 6.9(b): Tensile test data of the parent – Al 6061-T6.
6.9.2. Tensile Test - Weld
SI
Peak
Load
(lb)
1
3232.21
2
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
51715.40 49520.80
8.20
51715.40
7.30
1283741
3218.94
51503.00 49308.40
8.80
51503.00
7.80
1324307
3
3223.36
51573.80 49166.80
8.20
51573.80
7.10
1294124
4
3218.94
51503.00 49096.00
8.40
51503.00
7.40
1295279
Avg
3226.52
51624.37 49450.00
8.50
51624.37
7.59
1283318
Table 6.9(b): Tensile test data of the weld.
74
Yield
Tangent
Elongation Modulus
(%)
(psi)
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
6061-T6) are 0.83 and 0.80 for Break stress and Peak / Yield stress respectively.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
All the weld tensile test specimens fractured in the HAZ.
6.9.3. Hardness test – along the Weld
SI NO
PARENT
WELD
1
51.50
21.80
2
49.90
21.00
3
46.90
20.00
Avg
49.43
20.93
Table 6.9(c): Hardness test data – along the weld.
6.9.4. Hardness Test – across the weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
49.80
16.40
27.70
21.80
29.80
20.20
51.50
48.40
14.80
27.70
21.00
27.70
18.20
49.90
50.50
13.80
26.80
20.00
26.70
18.80
49.00
49.60
15.00
27.40
20.93
28.10
19.10
50.13
Table 6.9(d): Hardness test data – across the weld.
75
Hardness (HRB) across the Weld, Al6061-T6 @ 15"/min
60.00
50.00
HRB
40.00
30.00
20.00
10.00
0.00
0
1
2
3
4
5
6
7
8
Figure: 6.9(a): Hardness graph - across the weld.
•
As seen in the Table 6.9(c), the weld is softer than the parent. The hardness joint
efficiency is 0.42.
•
It can be seen from Figure 6.9(a) that a hardness degradation region (i.e. softened
region) has occurred in each joint, with the weld nugget having a hardness value
less than the TMAZ hardness values.
•
There are two low hardness zones on the two sides of the weld center, but the
minimum hardness value exists in the low hardness zone on the retreating side,
accordingly the joint is fractured on the retreating side. From Figure6.6(c) it can
be found that the minimum hardness occurs in the HAZ adjacent to the TMAZ on
the retreating side. Therefore, the joint is fractured in the HAZ on the retreating
side and the fracture surface is parallel to the TMAZ / HAZ interface on the
retreating side [10].
76
6.10. LOW-SPEED FRICTION STIR WELD 7
Weld Specifications
Materials: Al 6061-T6 (0.125”)
Type of Joint: Butt
Welding Speed: 20”/min
Rotational Speed: 1500 rpm
Tool Shoulder Diameter: 0.375 in
6.10.1. Tensile Test – Parent (Al6061-T6)
SI
Peak
Load
(lb)
1
3921.30
2
Avg
Break
Elongation
(%)
Yield
Stress
(psi)
67242.20 60970.00
23.10
67242.20
19.60
1563554
3830.93
65692.60 59346.60
21.50
65692.60
18.30
1566628
3876.12
66467.40 60158.30
22.30
66467.4
18.95
1565091
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.10(a): Tensile test data of the parent – Al 6061-T6.
77
6.10.2. Tensile Test - Weld
Break
Elongation
(%)
Yield
Stress
(psi)
55952.20 53369.50
9.00
55952.20
8.10
1366549
3262.91
55952.20 54402.50
8.40
55952.20
7.60
1425574
3262.91
55952.20 53886.00
8.70
55952.20
7.85
1396062
SI
Peak
Load
(lb)
1
3262.91
2
Avg
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.10(b): Tensile test data of the weld.
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
6061-T6) are 0.90 and 0.84 for Break stress and Peak / Yield stress respectively.
This implies that the tensile strength of the weld is less than parent.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
All the weld tensile test specimens fractured in the HAZ.
78
6.10.3. Hardness Test – Along the weld
SI NO
PARENT
WELD
1
50.90
28.50
2
49.80
28.50
3
48.90
27.00
Avg
49.87
28.00
Table 6.10(c): Hardness test data – along the weld.
6.10.4. Hardness Test – Across the Weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
49.80
26.70
29.78
28.50
34.30
29.80
49.60
51.70
25.80
30.40
28.50
32.40
31.40
50.40
50.00
27.00
29.60
27.00
34.40
30.60
48.70
50.50
26.50
29.93
28.00
33.70
30.60
49.60
Table 6.10(d): Hardness test data – across the weld.
79
Hardness (HRB) across the Weld Al6061-T6, 20'/min
60
50
HRB
40
30
20
10
0
0
1
2
3
4
5
6
7
8
Figure 6.10(a): Hardness graph - across the weld.
•
As seen in the Table 6.10(c), the weld is softer than the parent. The hardness joint
efficiency is 0.56.
•
It can be seen from Figure 6.10(a) that a hardness degradation region (i.e.
softened region) has occurred in each joint, with the weld nugget having a
hardness value less than the TMAZ hardness values.
•
There are two low hardness zones on the two sides of the weld center, but the
minimum hardness value exists in the low hardness zone on the retreating side,
accordingly the joint is fractured on the retreating side. From Figure 6.7(c) it can
be found that the minimum hardness occurs in the HAZ adjacent to the TMAZ on
the retreating side. Therefore, the joint is fractured in the HAZ on the retreating
side and the fracture surface is parallel to the TMAZ / HAZ interface on the
retreating side.
80
6.11. LOW-SPEED FRICTION STIR WELD 8
Weld Specifications
Materials: Al 6061-T6 (0.125”)
Type of Joint: Butt
Welding Speed: 25”/min
Rotational Speed: 1500 rpm
Tool Shoulder Diameter: 0.375 in
6.11.1. Tensile Test – Parent (Al6061-T6)
Break
Elongation
(%)
Yield
Stress
(psi)
64677.90 58303.90
23.00
64677.90
19.60
1494830
4051.22
64819.50 58445.60
23.40
64819.50
20.10
1373849
3
4006.96
64111.30 57737.30
23.30
64111.30
19.90
1292505
4
4042.37
64677.90 57879.00
23.20
64677.90
20.00
1395983
Avg
4035.73
64571.65 58091.45
23.23
64571.65
19.90
1389292
SI
Peak
Load
(lb)
1
4042.37
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.11(a): Tensile test data of the parent – Al 6061-T6.
81
6.11.2. Tensile Test - Weld
Break
Elongation
(%)
Yield
Stress
(psi)
54054.60 52921.50
9.30
54054.60
8.40
1295614
3382.84
54125.40 52496.50
9.90
54125.40
9.00
1317391
3
3378.41
54054.60 52638.20
9.20
54054.60
8.40
1313819
4
3391.96
54267.10 53133.90
9.40
54267.10
8.60
1293661
Avg
3382.84
54125.43 52797.53
9.45
54125.43
8.60
1305121
SI
Peak
Load
(lb)
1
3378.41
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.11(b): Tensile test data of the weld.
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
6061-T6) are 0.91and 0.84 for Break stress and Peak / Yield stress respectively.
This implies that the tensile strength of the weld is less than parent.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
The weld tensile test specimens fractured in the HAZ, with one of the specimen
fracturing in the weld nugget due to crack like defect in the joint.
82
6.11.3. Hardness Test – Along the Weld
SI NO
PARENT
WELD
1
50.00
27.60
2
50.40
28.80
3
52.40
23.20
Avg
50.93
26.53
Table 6.11(c): Hardness test data - along the weld.
6.11.4. Hardness Test – Across the Weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
50.40
24.80
28.60
26.60
28.80
27.80
50.00
51.50
24.80
29.00
27.60
29.70
28.20
52.40
51.20
25.20
28.30
27.00
28.60
28.60
50.80
51.03
24.93
28.63
27.10
29.03
28.20
51.10
Table 6.11(d): Hardness test data – across the weld.
83
Hardness (HRB) across the Weld, Al 6061-T6 @ 25"/min
60
50
HRB
40
30
20
10
0
0
1
2
3
4
5
6
7
8
Figure 6.11(a): Hardness graph - across the weld.
•
As seen in the Table 6.11(c), the weld is softer than the parent. The hardness joint
efficiency is 0.52.
•
It can be seen from Figure 6.11(a) that a hardness degradation region (i.e.
softened region) has occurred in each joint, with the weld nugget having a
hardness value less than the TMAZ hardness values.
•
There are two low hardness zones on the two sides of the weld center, but the
minimum hardness value exists in the low hardness zone on the retreating side,
accordingly the joint is fractured on the retreating side. From Figure 6.11(c) it can
be found that the minimum hardness occurs in the HAZ adjacent to the TMAZ on
the retreating side. Therefore, the joint is fractured in the HAZ on the retreating
side and the fracture surface is parallel to the TMAZ / HAZ interface on the
retreating side.
84
6.11.5. Comparison - Weld 6, Weld 7 & Weld 8
Peak/Yield Stress (psi)
57000
56000
55000
54000
53000
52000
51000
50000
20"/min
15"/min
25"/min
49000
Welding Speed (in/min)
Figure 6.11(b): Peak/yield stress variation.
55000
Break Stress (psi)
54000
53000
52000
51000
50000
49000
48000
15"/min
20"/min
25"/min
47000
Welding Speed (in/min)
Figure 6.11(c): Break stress variation.
85
•
It can be seen from Figures 6.11(b) and 6.11(c) that the tensile properties of each
joints change considerably with the change in the welding pitch.
30
Hardness (HRB)
25
20
15
10
5
15"/min
20"/min
25"/min
0
Welding Speed (in/min)
Figure 6.11(d): Variation in hardness.
•
It can be seen from Figures 6.11(d) that the hardness values of each joints change
considerably with the change in the welding pitch.
•
When the welding pitch is smaller than 75 r/in, the tensile properties and the
hardness of the joints decreases with the decrease in the welding pitch.
•
When the welding pitch is greater than 75 r/in, all tensile properties and the
hardness of the joints tend to increase with the decrease in the welding pitch.
•
These results indicate that a softening effect has taken place in the joint. The
softened levels or the tensile properties of the joints are significantly affected by
the welding parameters. For example, the welding pitch of 75 r/in, corresponding
to the rotational speed of 1500 rpm and the welding speed of 20"/min, is optimum
for the tensile properties of the joints in case of welds 6, 7 & 8.
86
•
None of the tensile test specimens failed on the advancing side of the joint, which
implies that the tensile properties of the welds are not the same on either sides of
the weld center. This also implies that the retreating side of the joint is weaker
than the advancing side.
•
The hardness of the weld center is lower than TMAZ, but the minimum hardness
value exists in the HAZ on the retreating side, accordingly the joint is fractured in
HAZ on the retreating side. This implies that the tensile properties and fracture
locations are related to the hardness profile of the weld.
87
6.12. LOW-SPEED FRICTION STIR WELD 9
Weld Specification
Materials: Al 2024-T3 (0.090”)
Type of Joint: Butt
Welding Speed: 20”/min
Rotational Speed: 955 rpm
Tool Shoulder Diameter: 0.375 in
6.12.1. Tensile Test – Parent (Al 2024-T3)
SI
Peak
Load
(lb)
1
3917.32
2
Break
Elongation
(%)
Yield
Stress
(psi)
87051.60 84100.70
28.40
86658.10
22.90
1826437
3873.06
86068.00 83707.20
27.60
85674.50
22.30
1838938
3
3917.32
87051.60 84297.40
27.20
87051.60
22.30
1805899
Avg
3904.04
86756.53 83658.05
28.05
86559.78
23.55
1830592
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.12(a): Tensile test data of the parent – Al 2024-T3.
88
6.12.2. Tensile Test - Weld
Break
Elongation
(%)
Yield
Stress
(psi)
76723.40 76625.10
13.30
76723.40
13.30
1661615
3518.95
78198.90 78002.20
14.80
78198.90
14.70
1661268
3
3496.82
77707.10 77707.10
14.70
77707.10
14.70
1656029
Avg
3494.61
77657.90 77559.55
14.45
77657.90
14.40
1642937
SI
Peak
Load
(lb)
1
3452.55
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.12(b): Tensile test data of the weld.
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
2024-T3) are 0.93 and 0.90 for Break stress and Peak / Yield stress respectively.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
The weld tensile test specimens fractured in the TMAZ.
89
6.12.3. Hardness Test – Along the Weld
SI NO
PARENT
WELD
1
73.80
72.80
2
72.00
71.60
3
74.10
65.50
4
74.40
67.50
Avg
73.70
68.70
Table 6.12(c): Hardness test data – along the weld.
6.12.4. Hardness Test – Across the Weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
73.80
70.80
66.60
68.70
65.90
71.90
74.10
73.70
68.00
64.30
69.60
64.40
68.30
73.80
72.90
73.50
64.00
71.00
68.30
70.00
73.70
73.46
70.76
64.96
69.76
66.20
70.06
73.86
Table 6.12(d): Hardness data – across the weld.
90
Hardness (HRB) - Across the Weld.
75.00
74.00
73.00
72.00
71.00
HRB
70.00
69.00
68.00
67.00
66.00
65.00
64.00
0
1
2
3
4
5
6
7
8
Figure 6.12(a): Hardness graph - across the weld.
•
As seen in the Table 6.12(c), the weld is softer than the parent. The hardness joint
efficiency is 0.93.
•
It can be seen from Figure 6.12(a) that a hardness degradation region (i.e.
softened region) has occurred in each joint.
•
There are two low hardness zones on the two sides of the weld center, but the
minimum hardness value exists in the low hardness zone on the retreating side,
accordingly the joint is fractured on the retreating side. From Figure 6.12(c) it can
be found that the minimum hardness occurs in the TMAZ on the retreating side.
Therefore, the joints fractured in the TMAZ on the retreating side.
91
6.12.5. Metallography
Fig 6.12(b): Microstructure of the weld nugget.
6.13. LOW-SPEED FRICTION STIR WELD 10
Weld Specifications
Materials: Alclad 2024-T3 (0.080”)
Type of Joint: Butt
Welding Speed: 10”/min
Rotational Speed: 500 rpm
Tool Shoulder Diameter: 0.375 in
92
6.13.1. Tensile Test – Parent (Alclad 2024-T3)
SI
Peak
Load
(lb)
1
3788.96
2
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
94723.90 91846.80
28.80
94723.90
26.90
1549111
3749.12
93728.00 91182.90
27.30
93728.00
25.00
1624823
3
3762.40
94060.00 91293.50
29.10
94060.00
26.90
1658996
4
3802.24
95055.90 92178.80
28.80
95055.90
27.30
1547559
Avg
3775.68
94391.95 91625.50
28.50
94391.95
26.53
1595122
Table 6.13(a): Tensile test data of the parent – Alclad 2024-T3.
6.13.2. Tensile Test - Weld
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
1
2642.53
66063.30
3.50
66063.30
66063.30
3.50
1988631
2
2629.25
65731.30
3.50
65731.30
65731.30
3.50
1977220
3
2646.96
66174.00
3.60
66174.00
66174.00
3.60
1937763
Avg
2635.89
65897.33
3.55
65897.33
65897.33
3.55
1952399
Table 6.13(b): Tensile test data of the weld.
93
Yield
Tangent
Elongation Modulus
(%)
(psi)
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
clad 2024-T3) are 0.72 and 0.70 for Break stress and Peak / Yield stress
respectively. This implies that the tensile strength of the weld is less than parent.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
All the weld tensile specimens failed in the weld nugget due to crack like defects
in the joint.
6.13.3. Hardness Test – Along the Weld
SI NO
Parent
Weld
1
70.20
57.00
2
69.94
58.80
3
70.00
57.40
Avg
70.09
57.85
Table 6.13(c): Hardness test data – along the weld.
94
6.13.4. Hardness Test – Across the Weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
70.40
66.90
62.70
57.40
62.40
69.20
70.00
70.00
68.80
61.80
58.70
65.70
70.90
70.20
70.40
68.20
62.30
57.80
64.40
68.30
69.98
70.26
67.96
62.26
58.00
64.16
69.46
70.06
Table 6.13(d): Hardness test data – across the weld.
Hardness (HRB) - across the Weld
80.00
70.00
60.00
HRB
50.00
40.00
30.00
20.00
10.00
0.00
0
1
2
3
4
5
6
7
8
Figure 6.13(a): Hardness graph - across the weld.
•
As seen in the Table 6.13(c), the weld is softer than the parent. The hardness joint
efficiency is 0.83.
•
It can be seen from Figure 6.13(a) that a hardness degradation region (i.e.
softened region) has occurred in each joint.
95
•
The hardness values decrease gradually across the weld, with the minimum
hardness value in the weld center or weld nugget. Hence all the tensile specimens
failed at the weld nugget.
6.13.5. Metallography
Fig 6.13(b): Microstructures of the weld.
96
6.14. Tool Geometry Effects [21]
For a given alloy and plate thickness, the required z-axis load for production of a sound
weld is primarily a function of shoulder diameter.
Tool Number
Pin Dia (mm)
Shoulder Dia (mm)
1
10
25
2
8
25
3
12
25
4
10
20
5
10
30
6
10
28
7
7.2
20
Table 6.14: Tool geometry.
In Table 6.14, tools 1, 2 and 3 all required approximately the same loads. Tool number 5,
with the largest shoulder, required z-axis loads from 5-10% greater while tool number 4,
with the smallest shoulder, needs loads as much as 40% lower than the other tools.
Specific weld energy, J/mm
2500
2000
3.3 mm/s welding
speed
1500
2.4 mm/s welding
speed
1000
1.3 mm/s welding
speed
500
0
0
2
4
6
Tool number
Fig 6.14(a): Specific weld energy as a function of welding speed and tool geometry.
97
In Fig 6.14(a), the specific weld energy as a function of welding speed and tool geometry
is shown. For each tool, increased welding speed (for the rpm’s chosen in this study)
results in reduced weld energy. In addition, for each welding speed, tool number 4
(smallest shoulder) results in the lowest weld energy and tool number 5 (largest shoulder)
results in the highest weld energy. The use of a large diameter pin (tool 3) also increases
the weld energy and the required power, but this effect is less than that associated with
the shoulder diameter changes.
Required power, watts
3500
3000
2500
1.3 mm/s welding
speed
2000
1500
2.4 mm/s welding
speed
1000
3.3 mm/s welding
speed
500
0
0
2
4
6
Tool number
Fig 6.14(b): Required weld power as a function of tool geometry and welding speed.
In Fig 6.14(b), the required power for the three welding speeds is plotted against the tool
number. In this case, it is shown that the highest welding speed requires the greatest
power delivery from the machine and again, the shoulder size appears to be the primary
determinant of the power requirement for a given set of welding parameters (rpm and
welding speed).
98
Transverse tensile strength, MPa
430
425
420
415
410
405
400
395
390
385
380
375
1.3 mm/s welding
speed
2.4 mm/s welding
speed
3.3 mm/s welding
speed
0
2
4
6
Tool number
Fig 6.14(c): Transverse tensile strength of the welds as a function of tool geometry and
welding speed.
Fig 6.14(c) shows the transverse tensile strength of the welds as a function of tool
geometry and welding speed. For each tool, it is shown that increasing the weld speed
results in a small but consistent increase in the tensile strength of the weld.
12
X-axis force, kN
10
8
1.3 mm/s welding
speed
6
2.4 mm/s welding
speed
3.3 mm/s welding
speed
4
2
0
0
2
4
6
Tool number
Fig 6.14(d): X axis force as a function of tool geometry and welding speed.
Fig 6.14(d) shows X axis force as a function of tool geometry and welding speed. The
force on the welding tool in the X direction is a critical variable in that, if it becomes too
large the tool will break due to bending stresses. For each tool, the X axis force increases
with increasing welding speed even though the intermediate welding speed weld has the
highest advance per revolution.
99
6.15. HIGH-SPEED FRICTION STIR WELD 11
Weld specification
Materials: Al 7075-T6 (0.062”)
Type of Joint: Butt
Welding Speed: 1”/min
Rotational Speed: 12000 rpm
Tool Shoulder Diameter: 0.375 in
Pin length: 0.053 in
Shoulder angle: 3.7730
6.15.1. Tensile Test – Parent Al 7075-T6
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(psi)
(%)
1
3279.93 104957.70
99008.70
10.40
104957.70
8.70
2425350
2
3293.21 105382.60 100141.80
10.30
105382.60
8.80
2340873
3
3293.21 105382.60
99433.60
10.50
105382.60
9.20
2331391
Avg 3288.78 105240.97
99528.03
10.40
105240.97
8.90
2365871
Table 6.15(a): Tensile test data of the parent - Al 7075-T6.
100
6.15.2. Tensile Test of the Weld
Break
Elongation
(%)
Yield
Stress
(psi)
27496.20 27496.20
4.80
27496.20
4.80
1207796
416.67
26881.70 26881.70
4.00
26881.70
4.00
1305856
3
311.90
20122.90 20122.90
1.90
20122.90
1.90
2130791
Avg
384.92
24833.60 24833.60
3.60
24833.60
3.60
1548148
SI
Peak
Load
(lb)
1
426.19
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.15(b): Tensile test data of the weld.
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
7075-T6) are 0.25 and 0.24 for Break stress and Peak / Yield stress respectively.
This implies that the tensile strength of the weld is less than parent.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
The weld tensile test specimens fractured in the weld nugget.
101
6.15.3. Hardness Test – Along the Weld
SI NO
Parent
Weld
1
88.80
55.20
2
88.90
51.50
3
88.80
51.90
4
88.80
Avg
88.83
52.90
Table 6.15(c): Hardness test data – along the weld.
6.15.4. Hardness Test – Across the Weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
89.90
60.80
73.50
55.40
72.30
58.50
90.00
90.00
62.90
70.80
50.90
77.80
61.60
90.00
90.00
66.70
72.80
51.50
74.10
63.80
90.00
90.00
63.50
72.40
52.60
74.73
61.30
90.00
Table 6.15(d): Hardness test data – across the weld.
102
Hardness across the weld
100
90
80
H a rd n e s s (H R B )
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
Figure 6.15(a): Hardness graph – across the weld.
•
As seen in the Table 6.15(c), the weld is softer than the parent. The hardness joint
efficiency is 0.60. It can be seen from Figure 6.15(a) that a hardness degradation
region (i.e. softened region) has occurred in each joint.
•
The hardness values decreases across the weld, with the minimum hardness value
in the weld center or weld nugget. Hence all the tensile specimens failed at the
weld nugget.
6.15.5. Microstructure
Figure 6.15(b): Microstructure of the Weld nugget.
103
6.16. HIGH-SPEED FRICTION STIR WELD 12
Weld specification
Materials: Al 7075-T6 (0.062”)
Type of Joint: Butt
Welding Speed: 1”/min
Rotational Speed: 12000 rpm
Tool Shoulder Diameter: 0.375 in
Pin length: 0.053 in
Shoulder angle: 9.8860
6.16.1. Tensile Test – Parent Al 7075-T6
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
1
3279.93 104957.70
99008.70
10.40
104957.70
8.70
2425350
2
3293.21 105382.60 100141.80
10.30
105382.60
8.80
2340873
3
3293.21 105382.60
99433.60
10.50
105382.60
9.20
2331391
Avg 3288.78 105240.97
99528.03
10.40
105240.97
8.90
2365871
Table 6.16(a): Tensile test data of the parent Al 7075-T6.
104
6.16.2. Tensile Test of the Weld
Break
Elongation
(%)
Yield
Stress
(psi)
35483.90 35483.90
4.00
35483.90
4.00
1648618
207.14
13364.10 12749.60
1.90
13364.10
1.80
3225932
3
292.86
18894.00 18279.60
2.50
18894.00
2.50
1366332
4
473.81
30568.40 30568.40
3.80
30568.40
3.80
1345966
Avg
381.00
24577.60 24270.38
3.05
24577.60
3.03
1896712
SI
Peak
Load
(lb)
1
550.00
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.16(b): Tensile test data of the weld.
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
7075-T6) are 0.24 and 0.23 for Break stress and Peak / Yield stress respectively.
This implies that the tensile strength of the weld is less than parent.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parent.
•
The weld tensile test specimens fractured in the weld nugget.
105
6.16.3. Hardness Test – Along the Weld.
SI NO
Parent
Weld
1
88.80
56.8
2
88.90
62.7
3
88.80
62.6
4
88.80
Avg
88.83
60.70
Table 6.16(c): Hardness test data – along the weld.
•
As seen in the Table 6.16(c), the weld is softer than the parent. The hardness joint
efficiency is 0.68.
6.16.4. Microstructure
Fig 6.16(a): Microstructure of the weld nugget.
106
6.17. HIGH-SPEED FRICTION STIR WELD 13
Weld specification
Materials: Al 7075-T6 (0.062”)
Type of Joint: Butt
Welding Speed: 1”/min
Rotational Speed: 12000 rpm
Tool Shoulder Diameter: 0.375 in
Pin length: 0.053 in
Shoulder angle: 15.7860
6.17.1. Tensile Test – Parent Al 7075-T6
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(psi)
(%)
1
3279.93 104957.70
99008.70
10.40
104957.70
8.70
2425350
2
3293.21 105382.60 100141.80
10.30
105382.60
8.80
2340873
3
3293.21 105382.60
99433.60
10.50
105382.60
9.20
2331391
Avg 3288.78 105240.97
99528.03
10.40
105240.97
8.90
2365871
Table 6.17(a): Tensile test data of the parent Al 7075-T6.
107
6.17.2. Tensile Test of the Weld
Break
Elongation
(%)
Yield
Stress
(psi)
28725.00 28725.00
3.40
28725.00
3.40
1949039
350.00
22580.60 22580.60
2.90
22580.60
2.90
1439180
3
216.67
13978.50 13978.50
1.50
13978.50
1.50
1526225
4
435.71
28110.60 28110.60
3.40
28110.60
3.40
1418963
5
369.05
23809.50 23809.50
3.30
23809.50
3.30
1268191
Avg
363.34
23440.84 23440.84
2.90
23440.84
2.90
1520320
SI
Peak
Load
(lb)
1
445.24
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.17(b): Tensile test data of the weld.
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
7075-T6) are 0.24 and 0.22 for Break stress and Peak / Yield stress respectively.
This implies that the tensile strength of the weld is less than parent.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
The weld tensile test specimens fractured in the weld nugget.
108
6.17.3. Hardness Test – Along the Weld
SI NO
Parent
Weld
1
88.80
70.10
2
88.90
66.70
3
88.80
66.50
4
88.80
72.50
Avg
88.83
68.95
Table 6.17(c): Hardness test data – along the weld.
•
As seen in the Table 6.17(c), the weld is softer than the parent. The hardness joint
efficiency is 0.78.
6.17.4. Microstructure
Fig 6.17(a): Microstructure of the weld nugget.
109
6.18. HIGH-SPEED FRICTION STIR WELD 14
Weld specification
Materials: Al 7075-T6 (0.062”)
Type of Joint: Butt
Welding Speed: 1”/min
Rotational Speed: 12000 rpm
Tool Shoulder Diameter: 0.375 in
Pin length: 0.053 in
Shoulder angle: 21.3540
6.18.1. Tensile Test – Parent Al 7075-T6
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
1
3279.93 104957.70
99008.70
10.40
104957.70
8.70
2425350
2
3293.21 105382.60 100141.80
10.30
105382.60
8.80
2340873
3
3293.21 105382.60
99433.60
10.50
105382.60
9.20
2331391
Avg 3288.78 105240.97
99528.03
10.40
105240.97
8.90
2365871
Table 6.18(a): Tensile test data of the parent Al 7075-T6.
110
6.18.2. Tensile Test of the Weld
Break
Elongation
(%)
Yield
Stress
(psi)
33640.60 33640.60
3.80
33640.60
3.80
1471862
671.43
43318.00 43318.00
6.00
43318.00
6.00
1134024
3
376.19
24270.40 24270.40
3.00
24270.40
3.00
1449471
4
652.38
42089.10 42089.10
3.40
42089.10
3.40
1302887
5
595.24
38402.50 38402.50
3.40
38402.50
3.40
1125304
6
471.43
30414.70 30414.70
3.30
30414.70
3.30
1418458
Avg
548.02
35355.88 35355.88
3.82
35355.88
3.82
1317001
SI
Peak
Load
(lb)
1
521.43
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.18(b): Tensile test data of the weld.
•
The joint efficiencies (weld / parent ratio) of the weld with respect to parent (Al
7075-T6) are 0.36 and 0.34 for Break stress and Peak / Yield stress respectively.
This implies that the tensile strength of the weld is less than parent.
•
The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
•
The weld tensile test specimens fractured in the weld nugget.
111
6.18.3. Hardness Test – Along the Weld
SI NO
Parent
Weld
1
88.80
64.70
2
88.90
39.10
3
88.80
72.5
4
88.80
26.2
Avg
88.83
50.62
Table 6.18(c): Hardness test data – across the weld.
6.18.4. Hardness Test – Across the Weld
SI No
Weld
Avg
Parent
HAZ
TMAZ
Nugget
TMAZ
HAZ
Parent
89.90
61.30
71.30
55.40
71.80
64.40
90.00
90.00
65.90
73.60
64.70
74.30
65.90
90.00
88.90
63.50
63.90
49.50
40.30
70.20
90.00
89.60
63.60
69.60
56.53
62.13
66.83
90.00
Table 6.18(d): Hardness test data – across the weld.
112
Hardness across the weld
100
90
80
H a rd n e s s (H R B )
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
Figure 6.18(a): Hardness graph - across the weld.
•
As seen in the Table 6.18(c), the weld is softer than the parent. The hardness joint
efficiency is 0.57. It can be seen from Figure 6.18(a) that a hardness degradation
region (i.e. softened region) has occurred in each joint.
•
The hardness values decreases across the weld, with the minimum hardness value
in the weld center or weld nugget. Hence all the tensile specimens failed at the
weld nugget.
6.18.5. Microstructure
Figure 6.18(b): Microstructure of the weld.
113
6.18.5. Comparison – Weld 11, Weld 12, Weld 13 and Weld 14
`
Peak/Yield Stress (psi)
40000
21.354 deg
35000
30000
25000
3.773 deg
9.886 deg 15.786 deg
20000
15000
10000
5000
0
Shoulder Angles (deg)
Figure 6.18(c): Variation in peak & yield stress.
40000
21.354 deg
Break Stress (psi)
35000
30000
3.773 deg
9.886 deg 15.786 deg
25000
20000
15000
10000
5000
0
Shoulder Angle (deg)
Figure 6.18(d): Variation in break stress.
•
It can be seen from Figures 6.18(b) and 6.18(c) that the tensile properties of each
joints change considerably with the change in the shoulder angles.
114
80
15.786 deg
Hardness (HRB)
70
60
9.886 deg
3.773 deg
21.354 deg
50
40
30
20
10
0
Shoulde r Angle (deg)
Figure 6.18(e): Variation in hardness.
•
It can be seen from Figures 6.18(d) that the hardness values of each joints change
considerably with the change in the shoulder angles.
•
When the shoulder angle is smaller than 15.7860, the tensile properties decreases
with increase in the shoulder angle and the hardness of the joints increases with
the increase in the shoulder angle.
•
When the shoulder angle is greater than 15.7860, all tensile properties increase
with the increase in the shoulder angle and the hardness of the joints tend to
decrease with the increase in the shoulder angle.
•
The joints have the best tensile properties and lowest hardness value, of the four
welds, when the shoulder angle is 21.3540 and highest hardness value and lowest
tensile properties, of the four welds, when the shoulder angle is 15.7860.
•
The minimum hardness value exists in the weld nugget, accordingly the joints
fractured in the weld nugget.
115
6.19. HIGH-SPEED FRICTION STIR WELD 15
Weld specification
Materials: Al 7075-T6 (0.125”)
Type of Joint: Butt – Tilted and Untilted
Welding Speed: 2”/min
Rotational Speed: 12000 rpm
Tool Shoulder Diameter: 0.375 in
6.19.1. Tensile Test – Parent (Al 7075-T6)
SI
Peak
Load
(lb)
Peak
Stress
(psi)
Break
Stress
(psi)
Break
Elongation
(%)
Yield
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
1
3279.93 104957.70
99008.70
10.40
104957.70
8.70
2425350
2
3293.21 105382.60 100141.80
10.30
105382.60
8.80
2340873
3
3293.21 105382.60
99433.60
10.50
105382.60
9.20
2331391
Avg 3288.78 105240.97
99528.03
10.40
105240.97
8.90
2365871
Table 6.19(a): Tensile test data of the parent Al 7075-T6.
116
6.19.2. Tensile Test – Weld (Tilted)
Break
Elongation
(%)
Yield
Stress
(psi)
20396.60 20396.60
3.10
20396.60
3.10
976818
699.36
22379.60 22379.60
3.10
22379.60
3.10
1119785
3
885.00
28328.70 28328.70
2.60
28328.70
2.60
2331391
Avg
740.58
23701.63 23701.63
2.93
23701.63
2.93
1059765
SI
Peak
Load
(lb)
1
637.39
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.19(b): Tensile test data of the weld (tilted).
6.19.3. Tensile Test – Weld (Untilted)
Break
Elongation
(%)
Yield
Stress
(psi)
12889.50 12889.50
1.70
12889.50
1.70
793020
358.53
11473.10 11473.10
1.50
11473.10
1.50
808284
3
309.84
9915.00
9915.00
0.80
9915.00
0.80
1262390
Avg
357.06
11425.87 11425.87
1.33
11425.87
1.33
954565
SI
Peak
Load
(lb)
1
402.80
2
Peak
Stress
(psi)
Break
Stress
(psi)
Yield
Tangent
Elongation Modulus
(%)
(psi)
Table 6.19(c): Tensile test data of the weld (untilted).
117
•
The joint efficiencies (weld / parent ratio) of the weld (tilted) with respect to
parent (Al 7075-T6) are 0.24 and 0.23 for Break stress and Peak / Yield stress
respectively. This implies that the tensile strength of the weld is more than the
parent.
•
The joint efficiencies of the weld (untilted) with respect to parent (Al 7075-T6)
are 0.12 and 0.11 for Break stress and Peak / Yield stress respectively. This
implies that the tensile strength of the weld is less than parent.
•
The average break and yield elongation of the weld (tilted) and weld (untilted) are
much lower than the average break and yield elongation of its parent.
•
All the weld tensile test specimens fractured in the weld nugget.
•
The weld (tilted) has better tensile properties than the weld (untilted).
118
6.20. HIGH-SPEED FRICTION STIR WELD 16
Weld specification
Materials: Al 7075-T6 (0.125”)
Type of Joint: Butt – Ceramic tool
Welding Speed: 1”/min
Rotational Speed: 15000 rpm
Tool Shoulder Diameter: 0.38 in
6.20.1. Hardness Test – Along the Weld
SI NO
Parent
Welds
1
2
3
1
88.80
65.00
69.90
60.10
2
88.90
71.70
68.40
48.10
3
88.80
62.50
63.80
43.80
4
88.80
Avg
88.83
66.40
67.37
50.67
Table 6.20(a): Hardness test data of the weld.
•
The hardness weld efficiency (weld/parent) obtained are 0.75, 0.76 and 0.57 for
welds 1, 2 and 3 respectively.
119
6.21. HIGH-SPEED FRICTION STIR WELD 17
Weld specification
Materials: Al 7075-T6 (0.125”)
Type of Joint: Butt – Ceramic tool
Welding Speed: 1”/min
Rotational Speed: 15000 rpm
Tool Shoulder Diameter: 0.50 in
6.21.1. Hardness Test – Along the Weld
SI NO
Parent
Welds
1
2
3
1
88.80
46.10
66.20
56.30
2
88.90
44.30
42.00
60.00
3
88.80
53.50
57.80
32.40
4
88.80
Avg
88.83
47.97
55.33
49.57
Table 6.21(a): Hardness test data of the weld.
•
The hardness weld efficiency (weld/parent) obtained are 0.54, 0.62 and 0.56 for
welds 1, 2 and 3 respectively.
120
CHAPTER 7
CONCLUSIONS AND FUTURE SCOPE
1. The average break and yield elongation of the weld is much lower than the
average break and yield elongation of its parents.
2. None of the tensile test specimens failed on the advancing side of the joint, which
implies that the tensile properties of the welds are not the same on either sides of
the weld center. This also implies that the retreating side of the joint is weaker
than the advancing side.
3. The flow patterns on the advancing side and retreating sides of the weld are
different. The FSW can be roughly described as an in-situ extrusion process
where the tool shoulder, the pin, the backing plate and the cold base material form
an extrusion die. Near the top of the weld, because of the shape of the tool, a
substantial amount of material is moved from the retreating side of the weld to the
advancing side. This movement of material causes vertical mixing in the weld and
a complex circulation of material around the longitudinal axis of the weld. This
may be one of the reasons for the failure of the weld in the retreating side [1].
4. The microstructure of the advancing side is characterized by a sharp boundary
between the nugget and TMAZ. The retreating side of the joint has a more
complex microstructure, with no clear boundary between the nugget and TMAZ.
Also the texture is the strongest on the advancing side and weak on the retreating
side. This may be another reason for the failure of the weld in the retreating side
[35].
121
5. There are two low hardness zones on the two sides of the weld center, but the
minimum hardness value exists in the low hardness zone on the retreating side,
accordingly the joints fractured on the retreating side. This implies that the tensile
properties and fracture locations are related to the hardness profile of the weld.
6. The Al 7075-T6 and Alclad 2024-T3 Lap weld tensile test specimens fractured in
the TMAZ on the retreating side.
7. The tensile properties of Al 7075-T6 and Alclad 2024-T3 Lap joints change
considerably with the change in the welding pitch. When the welding pitch is
smaller than 75 r/in, the tensile properties of the joints increases with the increase
in the welding pitch. When the welding pitch is greater than 75 r/in, all tensile
properties tend to decrease with the increase in the welding pitch.
8. The hardness values of Al 7075-T6 and Alclad 2024-T3 Lap joints reduce with a
decrease in the welding pitch. This indicates that softening effect takes place in
the weld with an increase in its welding speed (i.e., decrease in the welding pitch).
9. The Al 6061-T6 Butt joints fractured in the Heat Affected Zone (HAZ). In this
case the minimum hardness zone occurs in HAZ and the fracture surface is
parallel to the TMAZ / HAZ interface on the retreating side.
10. The properties of Al 6061-T6 Butt joints change considerably with the change in
the welding pitch. When the welding pitch is smaller than 75 r/in, the tensile
properties and the hardness of the joints decreases with the decrease in the
welding pitch. When the welding pitch is greater than 75 r/in, all tensile properties
and the hardness of the joints tend to increase with the decrease in the welding
pitch.
122
11. In Al 2024-T3 Butt joints the minimum hardness occurs in the TMAZ on the
retreating side. Therefore, the joints fractured in the TMAZ on the retreating side.
12. All the Alclad 2024-T3 Butt weld tensile specimens failed in the weld nugget due
to crack like defects in the joint and the weld nugget is the minimum hardness
zone in this case.
13. The properties of the welds change considerably with the change in the shoulder
angles. When the shoulder angle is smaller than 15.7860, the tensile properties
decreases with increase in the shoulder angle and the hardness of the joints
increases with the increase in the shoulder angle. When the shoulder angle is
greater than 15.7860, all tensile properties increase with the increase in the
shoulder angle and the hardness of the joints tend to decrease with the increase in
the shoulder angle.
14. Weld energy decreases and the required power increases with the increase in
welding speed at constant weld pitch [21].
15. X axis force increases with the increase in the welding speed at constant welding
pitch. This may indicate that the material being involved in the process is in
different stages of evolution when welded using different speeds [21].
16. The highest energy per unit weld length is required when welding Al 6061-T6.
This is probably because of the relatively high thermal conductivity of the alloy
[21].
123
Because the Friction Stir Welding process has only recently become a subject of wide
study, there are currently no large databases of weld properties and, in fact, no
specifications on how to make or test friction stir welds exist. In general, the process is
robust and a wide range of processing parameters and tool designs can be used to make
metallurgically sound welds in a given alloy and plate thickness. In this thesis, only a few
friction stir welds, welded in a limited range of welding speed, rotational speed, were
tested for their mechanical properties and fracture locations. Tool parameters of the welds
considered were not extensive. Further studies have to be done, considering most of the
welding parameters, on a wider range of values. Different types of tools, tool parameters
have to be considered to determine the effect they have on the resultant welds. Fatigue
analysis, shear tests have to be conducted. Some of the tables in this thesis report may not
contain all the experimentally obtained values. But the average value in the tables was
calculated considering all the values obtained experimentally.
124
REFERENCES
125
1. Anthony P. Reynolds, ‘Friction stir welding of Aluminum alloys’, Handbook of
Aluminum, 2004.
2. Gene Mathers, ‘The welding of Aluminum and its alloys’, 2002.
3. www.weldingengineer.com
4. www.world-aluminum.org
5. Allan R. Pfluger, Richard E. Lewis, ‘Weld imperfections’, Proceedings of a
symposium at Lockheed Palo Alto Research Laboratory, Palo Alto, California,
1996.
6. Mandal, N.R., ‘Aluminum welding’, 2002.
7. Patricia Han, ‘Tensile testing’, 1992.
8. Harry Chandler, ‘Hardness testing’, 1999.
9. Boniface E. Rossi, ‘Welding Engineering’, 1954.
10. Huijie Liu, Fujii, H., Maeda, M., Nogi, K., ‘Tensile properties and fracture
locations of friction-stir welded joints of 6061-T6 Aluminum alloy’, Journal of
materials science letters, 22, 2003.
11. www.twi.co.uk
12. Harry Udin, Edward R. Funk, John wulff, ‘ Welding for engineers’, 1954.
13. Tylecote, R. F., ‘The solid phase welding of metals’, 1968.
14. www.metallographic.com
15. www.gordonengland.co.uk
16. Kenneth Budinski, ‘Engineering materials: properties and selection’, 1983.
17. Carl E. Cross, David L. Olson, Stephen Liu, ‘Aluminum welding’, handbook of
Aluminum, 2004.
126
18. Fletcher, M. J., ‘Friction welding’, 1972.
19. Reynolds, A. P., Lockwood, W. D., Seidel, T.U., ‘Processing property correlation
in friction stir welds’, Materials science forum, Vol 331-337, 2000.
20. Cederqvist, L., Reynolds, A. P., ‘Factors affecting the properties of friction stir
welded Aluminum lap joints’, Welding research supplement.
21. Reynolds, A. P., Wei Tang, ‘Alloy, tool geometry, and process parameter effects
on friction stir weld energies and resultant FSW joint properties’, Friction stir
welding and processing, TMS, 2001.
22. Ulysse, P., ‘Three dimensional modeling of friction stir welded process’,
International journal of machine tools & manufacture, 42, 2002.
23. Lippold, J. C., Ditzel, P. J., ‘Microstructure and properties of Aluminum friction
stir welds’, Materials science forum, vol 426-432, 2003.
24. Fonda, R. W., Bingert, J. F., ‘Texture and microstructure development in the heat
affected zone of friction stir welds’, Friction stir welding and processing II, TMS,
2003.
25. Beate Heinz, Birgit Skrotzki, Gunther Eggeler, ‘Microstructural and mechanical
characterization of a friction stir welded Al alloy’, Materials science forum, vol
331-337, 2000.
26. Rhodes, C. G., Mahoney, M. W., Bingel, W. H., Spurling, R. A., Bampton, C. C.,
‘Effects of friction stir welding on microstructure of 7075 Aluminum’, Scripta
materialia, vol 36, 1997.
127
27. Liu, G., Murr, L. E., Niou, C. S., Mclure, J. C., Vega, F. R., ‘Microstructural
aspects of the friction stir welding of 6061-T6 Aluminum’, Scripta materialia, vol
37, 1997.
28. Benavides, S., Li, Y., Murr, L. E., Brown, D., Mclure, J. C., ‘Low temperature
friction stir welding of 2024 Aluminum’, Scripta materialia, vol 41, 1999.
29. Thomas, W. M., Nicholas, E. D., Kallee, S. W., ‘ Friction based technologies for
joining and processing’, Friction stir welding and processing, TMS, 2001.
30. www.arconweld.com
31. Thomas, W. M., Nicholas, E. D., Kallee, S. W., ‘ Friction based technologies for
joining and processing’, Friction stir welding and processing, TMS, 2001.
32. Thomas, W. M., Dolby, R. E., ‘ Friction stir welding developments’, TWI,
Cambridge, UK.
33. www.key-to-metals.com
34. www.msm.cam.ac.uk
35. www.hkltechnology .com
128