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Metal Cutting theory applied to Thermal
Mapping of the
Friction Stir Welding Process
Vishnu Vardhan ChandrasekaranDepartment of Mechanical Engineering
Auburn University
Contact author: Dr. Lewis N. Payton, Department of Mechanical Engineering, Auburn University
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Introduction
Solid state welding process
Green technique
Developed by The Welding Institute, UK
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Advantages
Does not generate fumes of any type
Quickly train operators (In hours)
Easily automated
Does not use any filler materials and hence adds no additional
weight
Does not change alloy content
Interlaces alloys without melting
Better mechanical properties
(more ductile and tougher)
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FSW Tool
Most referred tool in the literature, called the
Shouldered Pin Tool
Shoulder
Pin
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FSW process
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Basic FSW process
Work pieces to be joined are placed together either overlapping or like butt
joint.
A rotating pin tool is inserted at one end and moved transversely at a
uniform feed.
The pin dislocates the material without melting it forming a weld pattern.
The shoulder helps in containing the dislocated material within the cavity
to form the weld.
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FSW Nomenclature
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Importance of temperature
As the tool passes through, the material flows around the tool
without melting involving a large amount of deformation.
Deformation at high strain rates, temperature (heat generated)
and grinding lead to changes in the grain structure
Improper grain structure leads to defects in welds
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Thermal Studies So Far
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Objectives
Gain better understanding of the thermal field surrounding the FSW
To develop a thermal mapping device to map thermal fields
To statistically validate the thermal data thus obtained
Compare the thermal data with the existing thermal models
Simulate the thermal field using physics based simulation
techniques and compare it with the real world data obtained
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Initial Experiments
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Draw backs of Initial Experiments
Attaching and positioning of thermocouples is tiring
and time consuming
Precise and accurate location of holes had to be
drilled every time a sample was prepared every time
Lead to higher standard deviation of observations and
not a statistically sound process
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Work Holder
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Statistical validation
The work holder was statistically validated for
Sensitivity
Repeatability
Reliability
Capable of detecting difference in temperature
measurements up to +/- 2 C for up to 5 replicates witha statistical power of 95%
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Temperature measurement
Thermocouples were used to measure the temperature at each
point of interest.
Labview software was used to read, convert and plot the
thermocouple input into real world temperature data
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Placement of thermocouples
1. Shoulder S1 (Ad)2. Shoulder S2 (Re)
3. Pin P3 (Ad)
4. Pin P4 (Re)
5. Bottom of pin
(P Under)
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Placement of thermocouples
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Temperature data collection
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Temperature data collection
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Start Up experimental setup
RPM: 350 and 450
Shoulder size: 0.9, 0.7 and 0.5
Traverse speed: 4/min and 7/min
12 Factor level combinations 7 experiments were done for each factor level combinations. A
total of 84 experiments done.
Shoulder Material: Titanium (AMS 4928N by TIMETAL)
Work piece Material: AL 6061
Holder Material : Steel 1018
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Experimental setup
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Experimental setup
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Experimental setup
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Results (sample shoulder temperatures)
S9 Vs S10
0
100
200
300
400
1 100 199 298 397
Time (Secs)
Temperature
(Celsius)
S9
S10
S10-Advancing side
S9-Retreating side
Peak Temperature of S10> Peak temperature of S9
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Results (sample Pin temperatures)
P3 Vs P4
0
100
200
300
400
1 100 199 298 397
Time (Secs)
Temperature(Celsius)
P3
P4
P4-Advancing side
P3-Retreating side
Peak Temperature of P4> Peak temperature of P3
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ResultsDistribution of Temperature recorded by 5 thermocouples located at
each of the 7 stations along the length of weld under consideration.
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Statistical validation
Advancing (Shoulder and pin) was hotter than retreating
side with a statistical power of 90%
Results from the analysis of variance done on the factors
involved shows that
Shoulder size has more impact during the entry of the tool
Traverse speed has more impact during the traverse of the tool
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Need for thermal field models
Helps, operators to chose the running
parameters (milling parameters like feed, RPM
etc.,) in order to target a sweet spot or
temperature range for alloy considered.
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Previous thermal field models
All researchers have the same heat equation for both sides of
the tool(Advancing and Retreating) for their thermal models.
Payton developed a thermal model for FSW based on Shaws
metal cutting equations for a slot milling operation
In the history of FSW, only Paytons model best describes the
process as an asymmetric process
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Comparison of FSW with Slot Milling
FSW Slot Milling
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Paytons model
In Machining theory, the leading side or the up milling side of the
work piece would be hotter than the trailing or down milling side.
This is due to the velocity of the tool tip being maximum on the
leading side and minimum on the trailing side.
In FSW, the leading side is the advancing side and the trailing side
is the retreating side.
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Paytons model
Equations Nomenclature
HP - Specific Horsepower
V - Cutting speed
k - Coefficient of thermal conductivity
pc - Volume specific heat of the work
material
nRPM
R - Radius of the tool
r - Radius of Pin
y - Depth of cut.
*[16,148.58]
*( )
s
V tT HP
k pc
1
2 22 [{ } 2 ]V n R r ry
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Physics based Simulations
In the past due to computational power constraints, study of
only one physical behavior at an instance like structural
integrity, aerodynamic behavior etc., was possible.
With the advent of high power computing software, it is now
possible to model and study the Multiphysics behavior at a
time for any problem under consideration.
Software used here are COMSOL and ANSYS
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Simulations done with COMSOL
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Simulations done with ANSYS
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Simulations done with ANSYS
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Comparison of Results
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Conclusions
The highest temperatures of the friction stir welding
process occur during the relatively long insertion into the
specimen.
Once the transit starts, temperatures fall and reach as
lower steady state transient temperature.
For aluminum 6061-T6, this occurs within 2 diameters
of the tool shoulder (on average).
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Conclusions
The advancing side of the tool is always hotter than the retreating
side of the tool, with a statistical power of at least 90% for all the
experiments and factor level combinations done.
The temperature always peaks following passage of the trailing
edge of the shoulder. This is an important consideration in
applications where a run out tab is not used.
The weld properties at the extraction point may be very different
because of the lower temperatures at that point.
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Conclusions
Temperature rose linearly as RPM and shoulder
diameter increased.
Temperature decreased inversely as the traverse
speed increased.
Temperature changed with material being welded, all
other things constant.
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Conclusions
Paytons model, being the only model in the literature
which describes the asymmetric nature of the temperature
field, was used to simulate the process using a multi
physics finite element analysis tool.
The results obtained were compared to the real world data
for the shoulder with excellent results. Software limitationsprecluded a good simulation of the pin tool area
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Future work
Can rapidly collect large amount of statistically reliable thermal
data on a new alloy combination with standard inexpensive stocks
Classic metal cutting theory from the 1950s and 1960s has been
successfully applied to a joining process.
This suggests that classic extrusion theory and software such as
DEFORM 3D might also be very beneficially applied to Friction
Stir Welding.
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DATA AVAILABLE AT
WWW.ENG.AUBURN.EDU/~PAYTOLN
Thank You
Contact author:
Dr. Lewis N. Payton
Director, Design and Manufacturing Laboratory
Department of Mechanical Engineering
270 Ross Hall
Auburn University, AL 36849
Telephone 1-334-844-3315Fax 1-334-844-3422