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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

[email protected]

Telephone 1-334-844-3315Fax 1-334-844-3422

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