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1
STUDYING THE EFFECT OF SHOULDER GEOMETRY IN
FRICTION STIR WELDING OF AA1100 ALUMINIUM ALLOY
BASED ON THERMAL IMAGING
Sridhar Iyer
2
‘STUDYING THE EFFECT OF SHOULDER GEOMETRY IN
FRICTION STIR WELDING OF AA1100 ALUMINIUM ALLOY
BASED ON THERMAL IMAGING
Thesis submitted to
Indian Institute of Technology Kharagpur
For the award of the degree
of
Master of Technology
in
Manufacturing Science and Engineering
by
Sridhar Iyer
Roll No. 08ME3111
Under the guidance of
Prof. Surjya K. Pal
DEPARTMENT OF MECHANICAL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
April 2013
3
APPROVAL OF THE VIVA-VOCE BOARD
Certified that the thesis entitled Studying the effect of shoulder geometry in friction stir
welding of AA1100 aluminium alloy based on thermal imaging submitted by Sridhar Iyer
(08ME3111) to the Indian Institute of Technology, Kharagpur, for the award of the degree
Master of Technology in Manufacturing Science and Engineering has been accepted by the
external examiner and that the student has successfully defended the thesis in the viva-voce
examination held today.
(Supervisor)
(External Examiner)
Department of Mechanical Engineering
Indian Institute of Technology Kharagpur
4
CERTIFICATE
This is to certify that the thesis entitled Studying the effect of shoulder geometry in friction
stir welding of AA1100 aluminium alloy based on thermal imaging, submitted by Sridhar
Iyer (08ME3111) to Indian Institute of Technology, Kharagpur, is a record of bona fide research
work under my supervision and I consider it worthy of consideration for the award of the degree
of Master of Technology in Manufacturing Science and Engineering.
(Supervisor)
Date:
Department of Mechanical Engineering
Indian Institute of Technology Kharagpur
5
DECLARATION
I certify that
a. The work contained in the thesis is original and has been done by myself under
the general supervision of my supervisor(s).
b. The work has not been submitted to any other Institute for any degree or diploma.
c. I have followed the guidelines provided by the Institute in writing the thesis.
d. I have conformed to the norms and guidelines given in the Ethical Code of Conduct of the
Institute.
e. Whenever I have used materials (data, theoretical analysis, and text) from other sources, I
have given due credit to them by citing them in the text of the thesis and giving their details
in the references.
f. Whenever I have quoted written materials from other sources, I have put them under
quotation marks and given due credit to the sources by citing them and giving required
details in the references.
Signature of the Student
6
Abstract
Frictions stir welding is a relatively new solid state joining process. Over the past decade a lot of
research has been done on optimization of FSW parameters by analyzing the mechanical
properties. A few papers have analyzed the process using finite element methods, though the
process has a lot of lacunae as the onion ring formation and tool work-piece interaction haven’t
been fully incorporated in the model yet. In this study, a thermal imaging camera was used to
obtain temperature profiles while welding. The images obtained by the thermal imager were
processed in MATLAB to obtain the temperature isotherms. The welded joints were further
analyzed for tensile strength and micro-hardness. Moreover, a new tool with different shoulder
geometry was used and the results were compared to that of the conventional FSW tool.
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Table of Contents
Abstract ......................................................................................................................................................... 6
Friction Stir Welding ..................................................................................................................................... 8
Process parameters .................................................................................................................................. 9
Micro-structural changes ........................................................................................................................ 10
Literature survey ......................................................................................................................................... 12
Weld Characteristics ............................................................................................................................... 12
Process Optimization .............................................................................................................................. 13
Temperature Measurement Techniques ................................................................................................ 13
Typical friction Stir Welding Defects ....................................................................................................... 15
Objective ..................................................................................................................................................... 17
Experimental Setup ..................................................................................................................................... 18
Machines/Instruments used during experiments .................................................................................. 18
Experimental Procedure ............................................................................................................................. 23
Experimental work .................................................................................................................................. 23
Micro-hardness ....................................................................................................................................... 25
Temperature Measurements .................................................................................................................. 25
Tensile Strength ...................................................................................................................................... 25
Results and discussion ................................................................................................................................ 26
Thermal Analysis ..................................................................................................................................... 26
Measurement of power .......................................................................................................................... 29
Tensile properties ................................................................................................................................... 32
Optical macrograph for weld cross section and tensile test specimen .................................................. 36
Micro-hardness ....................................................................................................................................... 38
Conclusion ................................................................................................................................................... 40
References .................................................................................................................................................. 41
8
Friction Stir Welding Friction stir welding (FSW) is a solid state joining process. This means that the weld material
never changes to liquid state and the welding takes place below the welding temperature offering
excellent mechanical properties. FSW finds its application in the aircraft and automobile industry
for welding of aluminium plates. In FSW a rotating non consumable tool is transversely moved
along the welding line. The nib, i.e. the tool pin is slightly shorter than the depth of welding
required, with the shoulder in contact with the work piece. [1] Due to the rotating motion of the
tool, frictional heat is generated at the tool work piece interface. The weld obtained is the
combined result of this localized heat generated and the mechanical deformation due the plunged
tool tip. 95% of this heat generated flows to the work piece [2]. This causes softening of material
around the pin and the tool rotation leads to the flow of material from the front end to the back
end, thereby producing a weld in solid state with dynamic re-crystallization of the work material
At the start of joining, tool plunge, the tool undergoes only rotation motion till the shoulder
comes in contact with the work piece, this is called the dwelling period of the tool. After the
dwell phase longitudinal feed is provided and the tool moves along the weld direction as shown
in figure 2. The tool is then withdrawn after the welding is complete. [1]
Friction stir welding has several advantages over fusion welding method as there is no change of
state involved. Issues such as porosity, solidification and liquation cracking do not arise in the
case of FSW and the concentration of defects is low. It has been increasingly used to weld
difficult-to-weld aluminium alloys [6-9]
Figure 1 Schematic diagram FSW [3] Figure 2 Stages in FSW [4]
9
Some key benefits of FSW are [10]:
Low distortion of the work piece
Good dimension stability and repeatability
No loss of alloying elements
Improved safety due to the absence of toxic fumes or the spatter of molten material.
Easily automated on simple milling machines — lower setup costs and less training.
Can operate in all positions (horizontal, vertical, etc.), as there is no weld pool.
No Filler material ; Cost reduction
Process parameters
For a successful weld, the temperature of the tool should be hot enough to enable extensive
plastic deformation of the work piece; else voids or other flaws may be present in the work piece.
At the same time the increase in temperature also needs to be controlled to insure that the
temperature does not increase beyond the melting point in materials with low melting point such
as aluminium. Stirring at the plastic region also plays an important role in determining the
quality of weld.
Tool Rotation and welding speed
The rotation of tool causes stirring and mixing of material along the rotating pin. Higher tool
rotation generates higher temperature because of higher frictional heating and results in more
intense stirring and mixing of material. The translation of the tool moves the stirred material
from front to back of the pin. With constant rotational speed, high welding speed results in lower
heat input per unit length of the weld, causing lack of stirring in the friction stir processing zone
leading to poor tensile properties.
Tool Design
Tool design is also an important welding parameter as a good tool design can not only improve
the quality of the weld but also the maximum possible welding speed. Material selection,
shoulder diameter and tool pin height are some of the aspects that need to me considered while
selecting a tool. In the initial stage of the tool plunge, heating is a result of friction between the
10
work piece and tool pin. Thus the relative size of the pin plays an important role. The tool
shoulder not only provides a confinement for the heated volume of the material but also stirs and
moves the material as stated above. The uniformity of the micro-structural properties is governed
by the tool design.
Micro-structural changes
The welding parameters plays very significant role in deciding the temperature distribution
which produce a number of micro structural changes. These changes include diffusion and
diffusion less phase transformations, grain refinement, coarsening, and recrystallization. These
properties significantly alter the microstructure, thereby changing the properties of the material
near the weld zone.
There is an inverse relationship between delta yield strength and grain size, given by [11]
where is the strengthening coefficient and both and are material specific.
The relation between yield stress and grain size is described mathematically by Hall-Petch
equation [12]
Where is the yield stress, is a materials constant for the starting stress for dislocation
movement (or the resistance of the lattice to dislocation motion), is the strengthening
coefficient (a constant unique to each material), and is the average grain diameter.
The yield doesn’t keep increasing with decrease in grain size because at very small grain size
grain boundary sliding is observed.
11
The micro-structure can be broken into the following zones [1,13-14]
Dynamically recrystallized zone (DXZ) or stir zone: the high temperatures and high levels of
deformation result in full dynamic recrystallization and the formation of a fine-grain equiaxed
microstructure. This zone is indicative of hot-working produced by the combination of heat
induced by the rotating tool as well as the forging action produced by the shoulder of the tool as
it traverses along the joint. This is in contrast to a fusion-welded joint, which shows a dendritic
structure representative of a solidification process. Several concentric rings, called the “onion
rings”, are observed, the precise origin of which has not been completely understood.
Thermo mechanically affected zone (TMZ): This region occurs on either side of the stir zone
where the deformation is sufficient to cause grain reorientation and rotation; however, the grains
are not recrystallized.
Heat-affected zone (HAZ): A well-defined HAZ is present in heat-treatable alloys; this zone is
adjacent to the TMZ. In this zone, only thermal effects are present; no deformation is observed
during welding. However the temperature may have significant effect if the microstructure is
unstable.
Flow arm zone: It consists of material that is dragged by the shoulder from the retreating side
and deposited on the advancing side.
12
Literature survey
Weld Characteristics
James and Mahoney [ 16 ] measured the residual stresses of friction stir welds by means of X-ray
diffraction. It was observed that the residual stresses in all the FSW welds were quite low
compared to those generated by fusion welding. This was attributed to the low heat input. Peel et
al [17] observed that the nugget zone was in tension both in longitudinal and transverse
directions. Moreover, the longitudinal stress increased with increasing tool traverse speed
whereas the transverse residual stresses do not exhibit any dependence on transverse speed.
For heat treatable alloys, Sato et al [18] examined the hardness profiles associated with the
microstructure in friction stir welding of 6063Al-T5. The hardness profile was fouond to be
strongly affected by precipitate distribution rather than grain size in the weld. For solid-solution-
hardened aluminium alloys, FSW does not result in softening of welds [19, 20].
Mahoney et al [21] investigated the effect of FSW on room-temperature tensile properties of Al
7074-T651. The weld samples showed a reduction in yield and ultimate strengths in the weld
nugget, while elongation was unaffected. This reduced strength was attributed to reduction in
pre-existing dislocations and elimination of the very fine hardening precipitates. Biallas et al [22]
observed that for a constant ratio of tool traverse speed/rotation rate, both yield and ultimate
strengths increase with increasing tool rotation rate.
Elangovan et al [23] studied the effect of tool pin profiles and welding speed on the formation of
friction stir processing zone in AA2219 aluminium alloy. It was observed that square pin profile
tool produced defect free FSP region irrespective of the welding speed. Zhou et al [24] observed
that the fatigue life of 5083 aluminium alloy was longer than that of MID-pulse welds. The
fatigue characteristic value of each weld increased from 39.8MPa for MIG to 67.3MPa for FSW.
13
Process Optimization
Rajakumar et al [25] studied the influence of friction stir welding process and tool parameters on
strength properties of AA7075-T6 aluminium alloy joints. The strength properties of square butt
joints were evaluated and correlated with microstructure and micro-hardness of the weld nugget.
It was observed that the joint fabricated with tool RPM as 1400, welding speed of 60mm/min
and axial force of 8kN, using tool of 15mm shoulder diameter and 5 mm pin diameter yielded the
best strength properties.
Hans et al [26] evaluated the mechanical characteristics of friction stir welds for aluminium 5082
alloy. A tunnel type void was observed from an early stage to the end point at 1800 RPM. These
defects are rough with imperfect joining due to excessive rotation speed and high physical force.
The optimum FSW conditions were observed when the welding speed was 124 mm/min and
rotation speed was 800 RPM. A systematic approach to optimizing FSW process parameters
through consideration of power input was presented by Lombard et al. [27] Frictional power
governs the tensile strength and fatigue life of 5083 aluminium alloy through its effect on plastic
flow processes in the thermo-mechanically affected zone (TMAZ)
Design of experiments has been found to an effective tool for parameter optimization during
friction stir welding. Two level factorial design was used by Grover et al [28] to investigate the
interaction effects of parameters on the required response. Nourani et al[29] used Taguchi
optimization to minimize the HAZ distance to the weld line and peak temperatures. Rotational
speed was observed to be the most significant parameter among axial force, rotational speed and
welding speed based on ANOVA analysis.
Temperature Measurement Techniques
Since the temperature distribution within and around the stirred zone directly influences the
Micro structure of the welds, the range of maximum temperature within the stirred zone can be
estimated form the microstructure of the weld. [30-31] In most cases the temperatures generated
in the FSW zone exceeded the precipitate solvus temperatures. As a result, precipitate dissolution
and re-precipitation was observed in some specific alloys. An investigation of micro-structural
evolution in 7075Al-T651 during FSW by Rhodes et al. [30] showed dissolution of larger
14
precipitates and re-precipitation in the weld center. Therefore, they concluded that maximum
process temperatures are between about 400 and 480°C in FSW 7075Al- T651. On the hand,
Murr et al [31] indicated that some of the precipitates were not dissolved during welding and
suggested that the temperature rises to roughly 400°C in FSW 6061Al. Sato et al. [18] estimated
the peak temperatures at different distances away from the weld center by studying the micro
structural evolution of 6063Al during FSW using transmission electron microscopy (TEM) and
comparing it with the simulated weld thermal cycles.
Another technique used for measuring the temperatures during welding is by using embedded
thermocouples in the region to be welded. Tang et al. [33] measured the heat input and
temperature distribution within friction stir weld of 6061Al-T6 aluminum plates with a thickness
of 6.4 mm using this technique. The thermocouples were embedded into the back surface of the
work piece at three different depths. It was found that the thermocouples were not destroyed by
the pin during welding but changed position due to plastic flow of the material ahead of the pin.
Further, the weld temperature was found to increase with increase in weld pressure and tool
rotation. Kwon et al. [34] and Hashimoto et al. [35] also measured the temperature rise in the
weld zone by embedded thermocouple technique. Kwon et al. reported that in FSW 1050Al, the
peak temperature in the FSP zone increased linearly from 190 to 310°C with increasing tool
rotation rate from 560 to 1840 rpm at a constant tool traverse speed of 155 mm/min. Hashimoto
et al reported that the peak temperature in the weld zone increases with increasing the ratio of
tool rotation rate/traverse speed for FSW of 2024Al-T6, 5083Al-O and 7075Al-T6. A peak
temperature >550°C was observed in FSW 5083Al-O at a high ratio of tool rotation rate/traverse
speed.
Another technique for temperature measurement which has gained prominence over the last few
years is the use of infra red (IR) thermal imager. Many commercially available IR imaging
systems now support excellent temperature precision for fast frame rates and small spatial
resolutions. Hamilton et al [36] used a Mikron M7815 thermal imaging camera to record the
temperature profile using welding. The thermal emissivity was calibrated by heating the
aluminium surface to 460°C and adjusting the emissivity value till the recorded temperature of
the camera matched the reference temperature. The emissivity was determined to be .285. FSW
15
research team from National R&D Institute for Welding and Material Testing – ISIM Timisoara,
demonstrated the possibilities of using infrared thermo graphic technique for monitoring friction
stir welding process (FSW) [37]
Typical friction Stir Welding Defects
The process parameters in FSW cause too hot or too cold welds. Too cold weld condition lead to
insufficient material flow and gives rise to defects like void formation and nonbonding. Too hot
weld condition may lead to liquation of low melting point phase or material expulsion with flash
formation and the collapse of the nugget within the stir zone. [37]
Defects from too hot welds
The defects which are generated under such processing conditions are visually identified through
the surface appearance of the welded joint. The improper parameter settings cause too much
thermal softening. The surface of welded joint appears to contain blisters or surface galling.
Furthermore, excessive heat generation can lead to thermal softening in the work piece material
beyond the boundary of tool shoulder. Therefore, the tool shoulder, rather than actively
participating as a means for material containment, gives rise to material expulsion in the form of
excessive flash formation. This leads to the thinning of the work piece material. The work piece
material below the tool shoulder may reaches a point where it is no longer able to support the
axial load placed upon it, which causes production of excessive flash.
A weld nugget collapse under too hot welding condition is another serious defect in FSW joints.
It is not always expected that the increase of tool rotational speed at constant tool travel speed
will increase the size of weld nugget.[38]
Colegrove et al [39] has observed that the nugget region for an Al-Cu-Mg-Mn 2024 alloy can
actually decrease in size rather than increase in size when tool rotational speed is sufficiently
increased. The thermal softening brought about by very hot processing condition can lead to slip
between the tool pin and the workpiece material, and thus decreases strain rates within the
immediate vicinity of the tool pin. The weld nugget appears distorted. This weld nugget collapse
is generally occurred in the retreating side of the stir zone.
16
Defects from too cold welds
Too cold welding condition results in work hardening of the workpiece material. This causes dry
slip between the tool pin and the workpiece material. The lack of surface fills or voids and
channel defect are the main defects arising due to insufficient heat generation. The insufficient
heat generation causes improper material mixing and thus responsible for non-bonding.
Cavaliere et al [40] studied FSW joint cross-sections and SEM observations of the fractured
surfaces to characterize the weld performances. He studied the effect of the welding speed on the
fractured surface of the tensile and fatigue tested specimens. The work-piece material
investigated was AA 6082. The fractured surface appeared populated with very fine dimples
revealing a very ductile behavior of the material before failure. All the fatigue tested specimens
were observed to fracture in the advancing side of the tool. It was observed that, at higher
stresses the fatigue cracks started from the surface. Such big defects were often associated with
the vortex formed in the material in the advancing side where a more chaotic flow is formed
leading to the presence of voids of the mean dimension of hundreds of microns that represent the
site of fatigue cracks initiation. By decreasing the stress amplitude a strong change in the crack
behaviour was detected, the crack appear to start from the forging defects inside the joints which
are always present in this kind of welding. The failure was also related to the coalescence of
many small voids and defects in the material. The presence of dimples on the surface revealed a
local ductile behaviour of the material prior to fracture. This is the case of such conditions in
which the optimal solution between material mixing and grain refinement is obtained. By
increasing the advancing speed of the tool the material is extruded too fast (high strain rates) and
do not reached the conditions for the optimal mixing. The coupling of a high rotation speed and
high advancing speed leads to a good material mixing but to a non-optimal grain structure.
17
Objective Friction stir welding process involves hot-working due to heat produced by the combination of
friction at the rotating shoulder and plastic deformation produced by the tool pin as it traverses
along the joint. The variation in weld quality with changing parameter could be understood by
analyzing the temperature variation during various weld parameters and further analysis of weld
properties.
The objectives of this research project are as follows:
Development of temperature isotherms for various weld parameters during friction stir
welding by careful temperature measurements using infrared imaging, and correlating
these measurements with the experimental results of tensile strength
Designing a new tool for friction stir welding, with different shoulder geometry, and
comparing the variation in weld properties between the welds obtained by this tool and
the conventional FSW tool.
To sum it up the research aims to understand how the thermal fields produced during FSW
influences the physical properties of FSW joints. The variation in tensile strength and micro-
hardness would be used study the effect of change in parameters on the friction stir welds.
18
Experimental Setup
Machines/Instruments used during experiments
VF3.5 Knee Type Milling Machine
Friction stir welding was performed in the VF3.5 Knee Type Milling Machine shown in the
figure. The friction stir welding setup was mounted over this knee type milling Machine. The
machine has a RPM range from 50 to 1800 RPM and traverse speed range from 16 to 800
mm/min.
Figure 3 FSW Setup
19
Power Sensor
The power input to the VF3.5 Knee Type Milling Machine during the welding procedure was
measured by the power sensor PS100-DGM as shown in figure 4. The PS100-DGM is a power
sensor with on-board microprocessor controlled gain, offset, and non-linear filtering.
Sensor Type Active power sensor
Range of Measurements Current supply: 4.2A to 100A
Voltage supply: 230 to 460VAC
Range of input frequency: 0 – 1000Hz
Output Signal 0-10V DC analog
Dimensions 114 x 232 x 33
Temperature Range Operation: up to 55 °C
Storage: -40 to 100 °C
Relative Humidity 0 to 95 %, non-condensing
Diameter Cable Lead-trough 16mm
National Instrument and LABview Software
A NI card was used for data acquisition for the measurement of power during welding
experiment. Power sensor was connected to a personal computer through one channel of a data
acquisition system.
The experimental set up for power measurement, acquisition system and computer LabVIEW are
displayed in figure 5 and figure 6.
Table 1. Technical data of Power Sensor
Figure 4 NI card
20
FLIR A-320 camera for Thermal Imaging
An Infrared camera measures and images the emitted radiation from an object. The fact that
radiation is a function of object surface temperature makes it possible for the camera to calculate
and display this temperature.
FLIR R&D software was used to analysis the video feed from the camera. A sample image taken
during the welding process is shown in the figure. The FLIR A320 camera has optimum
resolution of 320x240, frequency of 30Hz and 8X interpolating zoom. The object temperature
range was set as 200○C to 1200
○C
The radiation measured by the camera does not only depend on the temperature of the object but
is also a function of the emissivity. This was calculated as 0.28. For calculating this a black
Figure 5 Power Sensor circuit Figure 6 Capturing the reading by using LabVIEW
Figure 7 FLIR A320 Camera Figure 8 Sample video feed
21
coating with known high emissivity was used and the temperature of aluminium plate was
calibrated to this temperature.
LEICA DFC-295 for Macrostructure
Macrostructure analysis was carried out using light optical microscope (LEICA DFC-295). The
images were analyzed in QWin-V3 software.
Figure 9 LEICA DFC-295
22
Vicker’s micro-hardness testing apparatus
A diamond indenter was used to make micro-indentations on the polished aluminium surface.
The indentations were measured optically and converted to hardness value. Test load of 50 gf
and 15s dwell tome was used.
INSTON (Universal Testing Machine)
Tensile tests were carried out on a computer controlled universal tensile testing machine
INSTRON at a crosshead speed of 2 mm/min. Transverse tensile test specimen were prepared
and tested according to ASTM E8M-04 guidelines.
Figure 10 Leco LM-700 Micro-indenter Figure 11 Universal Testing
23
Experimental Procedure
Experimental work
The rolled plates of 2.5 mm thickness, aluminium alloy, were cut into the required size
(200mm×100mm×2.5mm) by power hacksaw cutting. The initial joint configuration is obtained
by securing the plates in position using mechanical clamps. The direction of welding is normal to
the rolling direction. Single pass welding procedure has been used to fabricate the joints. No
special treatment was carried out before welding and testing. Non-consumable tool made of
stainless steel SS316 was used to as the machining tool. The chemical composition of work piece
material was analysed by energy dispersive X-ray spectroscopy.
Chemical Composition of the work-piece material (AA1100)
Composition Weight Percentage
Silicon 0.76
Iron 0.850
Copper 0.01
Manganese 0.006
Aluminium 98.3
Chemical Composition of the tool material (SS316)
Composition Weight Percentage
Iron 72.01
Chromium 16.29
Manganese 8.95
Silicon 2.13
Phosphorus 0.27
Nickel 0.20
Table 2 Composition of the workpiece material
Table 3 Composition of the tool material
24
Mechanical properties of the work piece material
Properties Value
Ultimate tensile strength 106.27 MPa
Yield strength 94.25 MPa
Percentage elongation 21.27
Vickers micro hardness 50
Figure 12 and figure 13 show the tools which were used for FSW, the dimensions for the same
are given in table 5.
Experiments were performed at 3 different RPM and feed rates. The RPM that were selected
were 900, 1400 and 1800 and the feed rates were 16mm/min, 31.5mm/min and 63mm/min.
Tool Dimensions Value
Tool Shoulder Diameter(D), mm 16
Tool Pin Diameter(d), mm 5
Tool Pin length(L), mm 2.1
D/d ratio 3.2
Table 5 FSW Tool Specifications
Table 4 Mechanical Properties of work piece
Figure 12 Tools used for Friction stir Figure 13 Top View Cylindrical Tool Ribs and Flat Cylindrical
tool
25
Micro-hardness
The micro-hardness profiles of friction stir welding joints were measured in the cross sections in
order to evaluate the material behavior as a function of the different welding parameters. The
specimen for the same was prepared by grinding the specimen cross section using emery paper
and then using a diamond paste to polish the samples
Temperature Measurements
While conducting experiments FLIR A320 camera was used to obtain the temperature
distribution over the entire field of view. The camera was placed on a tripod stand and placed in
a stable position in front of the milling machine bed. This camera was connected to a laptop to
capture the video feed and further process it in propriety FLIR software.
The thermal image thus obtained was analyzed in MATLAB to obtain temperature isotherms.
Tensile Strength
Test specimens for tensile testing were produced by wire EDM. Tensile tests were performed in
100 KN; electro-mechanical controlled Universal Testing Machine (INSTRON). The specimen
fails with necking and various data such as yield strength, ultimate tensile strength and
percentage elongation are recorded. The specimen is loaded at the strain rate of 2mm/min as per
ASTM specification and extensometer is attached to the specimen.
Figure 14 FLIR Camera on Tripod
Stand
Figure 15 Field of view of the camera
26
Results and discussion
Thermal Analysis
In FSW heat is generated by combination of friction and plastic dissipation during deformation
of the metal. The dominating heat generation mechanism is influenced by weld parameters,
thermal conductivities of the work piece, pin tool, backing anvils and the weld tool geometry.
Hotter welds are generated with high rpm and low weld speed and colder welds with low rpm
and high weld speed. A too-low temperature around the joint line will make it difficult for the
pin to traverse the work piece, which can result in breaking of the pin. A higher temperature can
decrease the flow stress of the work piece and make the work piece material stick.
During plunging and dwelling there is rapid increase in the temperature w. r. t. time. As the
temperature increases during plunging and dwelling, the metal softens and friction between tool
and work piece reduces, this results in decrease of rate of temperature increase during tool
traverse i.e. during actual welding. The max temperature reduces after a while as the heat loss
due to conduction increases. Similarly, as the tool reaches the other end of the work piece there
is yet another sharp rise in the temperature as the area for the heat loss by conduction around the
tool decreases. The temperature versus time graph for cylindrical tool weld speed of 16 mm/min
and rotational speed of 1800pm is shown in Graph 1.
0
100
200
300
400
500
600
700
0 2 4 6 8 10 12
Tem
pe
ratu
re (
○C
)
Time (min)
Temperature Vs Time
Graph 1 Variation in for cylindrical tool with RPM 1800 & feed
rate 16mm/min
27
It was found that as the RPM increases the surface temperature increases. The percentage
increase in temperature from 900 RPM to 1400 RPM is greater than the corresponding increase
from 1400 to 1800 RPM. This is because as the temperature of the weld metal rises, the metal
softens, torque is reduced, and less heat is imparted to the metal by mechanical work, friction
phenomenon gets replaced by stick-slide phenomenon between tool and work piece.
The steady state temperature recorded for each welding process is given in table 6
Experimental Parameters Steady state temperature for
plain cylindrical tool
Steady state temperature of
cylindrical tool with ribs
Feed Rate : 16 mm/min
RPM
900 320 300
1400 400 480
1800 440 500
Feed Rate: 31.5 mm/min
RPM
900 330 320
1400 380 430
1800 420 480
Feed Rate: 63 mm/min
RPM
900 320 330
1400 360 410
1800 400 470
Table 6 Steady state temperature readings
28
200
250
300
350
400
450
500
700 900 1100 1300 1500 1700 1900
Tem
pe
ratu
re
RPM
For Cylindrical Tool Average Temperature VS RPM
16
31.5
63
Graph 2 Variation in temperature with RPM for Flat
Cylindrical Tool
The variation in temperature for both tools is shown in the following graphs.
200
250
300
350
400
450
500
550
700 900 1100 1300 1500 1700 1900
Tem
pe
ratu
re
RPM
For Tool woth Ribs Average Temperature VS RPM
16
31.5
63
Graph 3 Variation in temperature with RPM for Cylindrical
Tool with Ribs
29
The temperature readings obtained were processed in MATLAB to obtain temperature isotherms
for various weld parameters.
Measurement of power
The FSW process consists of several steps i.e. plunging and dwelling, actual welding and pulling
the tool out of work piece. During the plunge stage a non-consumable rotating tool is slowly
plunged into the joint line between the two materials to be welded. Once the tool reaches the
required depth it is held in position for some time while still rotating, this is called the dwell
stage. The purpose of this stage is to soften the work piece material ahead of the tool before
welding. During the translational stage the tool traverses along the joint line of the two work
piece plates leaving behind a fully consolidated weld. The power used during FSW of aluminium
alloy plate of 2.5 mm thickness with cylindrical tool with ribs at weld speed of 16 mm/min and
rotational speed 1800 RPM as recorded is shown in Graph 4. Power reaches to peak values
during the plunging and dwelling of the tool into the work piece when the shoulder reaches the
target depth. The power then reaches a relatively steady state during translation motion of the
tool. Further, the input power decreases sharply when the welding is completed and the tool is
withdrawn.
Figure 16 Temperature isotherm for Cylindrical tool with RPM
1800 and feed rate 16mm/min
30
Graph 4 Variation in Power Consumed
RPM:- 1800 Feed Rate:- 16 mm/min for flat cylindrical tool with ribs
The power consumption for varying RPMs and at a feed rate of 16mm/min was calculated while
welding. The results have been tabulated below
Experiment Parameters Steady state power measurement
for tool with ribs (in Watts)
Steady state power measurement
for flat cylindrical tool ( Watts)
Feed Rate: 16 mm/min
RPM
900 2275 2000
1400 2750 2500
1800 3500 3000
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 100 200 300 400 500 600 700
Po
we
r (W
att)
Time (seconds)
Power Vs Time
Table 7 Variation of power with RPM at
16mm/min feed rate
31
From the table it can observed that the power consumption for cylindrical tool with ribs is greater
than that for the flat cylindrical tool. This can be attributed to the greater plastic deformation
produced while using the cylindrical tool with ribs. The power consumption also increases with
increase in RPM. Graph 5 shows the variation in power consumption with changing RPM.
0
500
1000
1500
2000
2500
3000
3500
4000
0 500 1000 1500 2000
Po
we
r (W
att)
RPM
Power Vs RPM
Cylindrical tool with ribs
Flat cylindrical tool
Graph 5 Variation of power with RPM at 16mm/min feed rate for
both tools
32
Tensile properties
The parent material was machined by wire EDM to prepare a sample for tensile testing.
American Society for testing materials (ASTM E8M-04) guidelines was followed for preparing
the test specimens.
Figure 17 Tensile test specimen
The Ultimate tensile strength and yield strength was obtained as 106.27 MPa and 94 MPa
respectively. The percentage elongation for the aluminium alloy was recorded as 21.27%.
0
20
40
60
80
100
120
0 0.05 0.1 0.15 0.2 0.25
Ten
sile
Str
ess
(M
pa)
Tensile Strain
Stress Vs Strain
Graph 6 Stress-strain curve for AA1120
33
Influence of tool rotational speed and shoulder geometry on tensile properties
Traverse tensile properties of Friction stir welds such as yield strength and ultimate tensile
strength were obtained for a constant feed rate of 16mm/min and varying RPM. The values
obtained are given in table 8.
Feed Rate: 16 mm/min
RPM
UTS (MPa) Yield Strength
(MPa)
UTS (MPa) Yield Strength
(MPa)
900 90 71 75 71
1400 90.24 57.89 98 65
1800 91 58 93 54
A plot of UTS Vs RPM at 16mm/min shows that for cylindrical tool with ribs a sharp
increase in UTS is observed when the RPM is increased from 900 to 1400, thereafter the
value of UTS decreases as the RPM is further increased to 1800. Not much variation in UTS
is observed for flat cylindrical tool at a feed rate of 16mm/min
50
60
70
80
90
100
110
700 900 1100 1300 1500 1700 1900
UTS
(M
Pa)
RPM
UTS Vs RPM
Flat
Ribs
Experiment Parameters Flat cylindrical tool Cylindrical tool with Ribs
Table 8 UTS and Yield Strength at feed rate of
16mm/min
Graph 7 Variation of UTS with RPM for a constant feed
rate of 16mm/min
34
The yield strength for the weld samples shows a decrease with increase in RPM. At higher
RPM the steady state temperature is more, this leads to a larger grain size which in turn
causes decrease in yield strength. Moreover, the increase in grain size is also accompanied
by decrease in elongation, i.e. the ductility of the welded material. The values for the
elongation percentage are given in table 9
40
45
50
55
60
65
70
75
700 900 1100 1300 1500 1700 1900
Yie
ld S
tre
ngt
h (
MP
a)
RPM
Yield Strength Vs RPM
Flat
Ribs
Experiment Parameters Percentage Elongation for Flat
Cylindrical tool
Percentage Elongation for
Cylindrical tool with ribs
Feed Rate: 16 mm/min
RPM
900 14 7
1400 17.53 18
1800 18 21
Graph 8 Variation of yield strength with RPM for
a constant feed rate of 16mm/min
Table 9 Variation in % elongation with
varying RPM
35
Influence of weld speed and shoulder geometry on tensile properties
Table 10 shows the variation in ultimate tensile strength (UTS) and yield strength at a
constant RPM of 900 and varying feed rate.
RPM: 900
Feed Rate: UTS (MPa)
Yield Strength
(MPa) UTS (MPa)
Yield Strength
(MPa)
16 mm/min 90 71 75 71
31.5 mm/min 91 86 94 90
63 mm/min 95 71.94 83 78.29
It was observed that the yield strength increases with feed rate as it increases from
16mm/min to 31.5mm/min, but with further increase in feed rate the yield strength again
decreases. Moreover, the yield strength for cylindrical tool with ribs was found to be greater
than flat cylindrical tool. However, since UTS is depends on both yield strength and
hardness no specific trend is observed in UTS for the two given tools at constant RPM and a
varying feed rate. A plot for yield strength Vs feed rate at a RPM of 900 is shown below.
50
55
60
65
70
75
80
85
90
95
0 10 20 30 40 50 60 70
Yie
ld S
tre
ngt
h (
MP
a)
Feed Rate (mm/min)
Yield Strength Vs Feed Rate
Flat
Ribs
Experiment Parameters Flat cylindrical tool Cylindrical tool with Ribs
Graph 9 Variation of Yield strength with feed rate for
a constant RPM of 900
Table 10 UTS and Yield Strength at 900 RPM
36
Optical macrograph for weld cross section and tensile test specimen
Friction stir welds are prone to defects such as pin hole, tunnel defect, etc. The ultimate tensile
strength of the weld shows dependence on yield strength as well as the quality of the weld. Table
11 shows macrograph for weld cross section and tensile test specimen obtained using flat
cylindrical tool, at a constant feed rate of 31.5 mm/min and varying RPM.
RPM Macrostructure
Cross section
Macrostructure
Tensile specimen
Quality and Defect
900
No defects
No necking, fracture
along the centre line
Yield Strength:
86MPa
Elongation: 7%
1400
Tunnel defect found
on the weld cross
section
Fractured by necking
from retreating side
Yield Strength:
64MPa
Elongation: 15 %
1800
Pin hole defect.
Fractured by necking
from retreating side
Yield Strength:
57MPa
Elongation: 16%
Table 11 Macrographs of weld cross section &
tensile specimen: Flat Cylindrical tool
37
For a given feed rate the yield strength decreases with increase in RPM, as the welding
temperature increases, leading to increase in grain size as explained earlier. Further, it was
observed that most of the welds failed from the retreating side during tensile loading.
Table 12 shows macrographs for weld cross section and tensile test specimen obtained using
cylindrical tool with ribs, at a constant feed rate of 16 mm/min and varying RPMs. As in the
previous case, very small elongation is observed for welds obtained at 900 RPM which
ultimately leads to low UTS.
RPM Macrostructure :
Cross section
Macrostructure :
Tensile specimen
Quality and Defect
900
Tunnel Defect
No necking, fracture
along the centre line
Yield Strength:
71MPa
Elongation: 7%
Very low UTS of
75MPa
1400
No defects
Fractured by necking
from retreating side
Yield Strength:
65Mpa
Elongation: 18%
High UTS of 98
1800
No macroscopic
defects.
Fractured by necking
from retreating side
Yield Strength:
54MPa
Elongation: 21%
Table 12 Macrographs of weld cross section & tensile
specimen: Cylindrical tool with Ribs
38
20
25
30
35
40
45
50
-4 -2 0 2 4
Har
dn
ess
Distance from weld center (mm)
Flat Cylindrical Tool; Feed Rate : 31.5mm/min
1400
1800
900
20
25
30
35
40
45
50
-3 -2 -1 0 1 2 3
Har
dn
ess
Distance from weld center (mm)
Cylindrical Tool with Ribs; Feed Rate : 16mm/min
1800 RPM
1400 RPM
900 RPM
Micro-hardness
Micro-hardness values were obtained for the polished weld cross sections. Graph 10 shows the
variation of hardness across the cross section of weld joint obtained using cylindrical tool with
ribs at a feed rate of 16mm/min. It was observed that the hardness in the nugget zone did not
show much variation with change in RPM at constant feed rate. However, in the TMAZ and
HAZ it was observed that the hardness decreases with increase in RPM. This increase in
hardness at lower RPM is accompanied by decrease in % elongation during tensile testing as
show previously.
Graph 10 Variation of Micro-hardness, Feed rate: 16mm/min
(Cylindrical tool with Ribs)
Graph 11 Variation of Micro-hardness, Feed rate: 31.5 mm/min
(Flat Cylindrical tool )
39
20
25
30
35
40
45
50
-4 -3 -2 -1 0 1 2 3 4
Har
dn
ess
Distance from weld center (mm)
RPM 900 and Feed rate 31.5mm/min
Flat Cylindrical Ribs
Graph 11 shows the effect of change in RPM when the feed rate is kept constant at 31.5mm/min.
As in the case of Cylindrical tool with ribs the hardness in the nugget zone did not show much
variation with change in RPM. Small variation in hardness across the cross section was observed
at 1400 RPM and 1800 RPM. At 900 RPM maximum hardness was observed.
Graph 12 shows the variation in micro-hardness for welds obtained at 900 RPM and
31.5mm/min feed rate using the two tools. The hardness obtained using cylindrical tools with
ribs was found to be greater.
Graph 12 Effect of shoulder geometry on
micro-hardness
40
Conclusion
From the experimental data it was observed that the maximum temperature during friction stir
welding increases with corresponding increase in the rotational speed of the tool for a constant
feed rate. Further a sudden rise in temperature was observed at the end of each pass, this was due
to the fact that the area for heat loss by conduction around the tool had reduced, and as a result
the temperature increase is greater. Melting point temperature of aluminium alloy is greater than
~640°C. Temperature measured during experiments ranges from 250 to 550 °C. The cylindrical
tool with ribs (tool 2) showed greater temperature variation as well as greater temperature at
steady state as compared to the flat cylindrical tool (tool 1).
The temperature isotherms were plotted from the thermal images obtained by the infrared
camera. These isotherms can give an idea, how the heat is dissipated in and around the weld
zone.
The power consumption in case of tool 2 was found to be more than tool 1 when the welding
parameters were kept same. This result can be attributed to the fact that the energy for plastic
deformation and the frictional heat generated are both greater in case of the second tool.
The tensile strength of welds obtained using the cylindrical tool with ribs was found to be greater
than those with flat cylindrical tool. For cylindrical tool with ribs the ultimate tensile strength
was found to be highest at 1400 RPM.
For both the tools it was observed that the yield strength was highest at 31.5mm/min among the
three feed rate. When the feed rate was kept constant, the value of yield strength decreases with
increase in RPM. Thus, maximum value of yield strength was obtained at an RPM of 900 and
feed rate of 31.5mm/min and is equal to 86MPa and 94MPa for flat cylindrical tool and tool with
ribs respectively.
41
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