1

EFFECT OF PROCESS PARAMETERS ON TEMPERATURE DISTRIBUTION

DURING FRICTION STIR WELDING

A PROJECT REPORT

SUBMITTED BY

AAYUSH TRIPATHI -CB.EN.U4MEE11103

VENKATA PRITHVI PUPPALA -CB.EN.U4MEE11160

A. SRI PRUDHVI -CB.EN.U4MEE11202

CH. VARUN TEJA -CB.EN.U4MEE11216

In partial fulfillment for the award of the degree

of

BACHELOR OF TECHNOLOGY

in

MECHANICAL ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

AMRITA SCHOOL OF ENGINEERING

AMRITA VISHWA VIDYAPEETHAM

COIMBATORE-641112,

MAY, 2015

2

AMRITA SCHOOL OF ENGINEERING

AMRITA VISHWA VIDYAPEETHAM, COIMBATORE – 641112

DEPARTMENT OF MECHANICAL ENGINEERING

BONAFIDE CERTIFICATE

This is to certify that the thesis entitled “EFFECT OF PROCESS PARAMETERS ON

TEMPERATURE DISTRIBUTION DURING FRICTION STIR WELDING”

submitted by AAYUSH TRIPATHI (CB.EN.U4MEE11103), VENKATA PRITHVI

PUPPALA (CB.EN.U4MEE11160), A. SRI PRUDHVI (CB.EN.U4MEE11202) and

CH. VARUN TEJA (CB.EN.U4MEE11216) for the award of the Degree of Bachelor

of Technology in Mechanical Engineering is a bonafide record of the work carried out

under the my/ our guidance and supervision at Amrita School of Engineering,

Coimbatore.

Dr. R.Padmanaban

Project Advisor

Dept. of Mechanical Engineering

Amrita School of Engineering

Dr. S. Thirumalini

Chairperson

Dept. of Mechanical Engineering

Amrita School of Engineering

This report was examined and the candidates underwent Viva- Voce examination on

____________.

Internal Examiner External Examiner

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AMRITA SCHOOL OF ENGINEERING

AMRITA VISHWA VIDYAPEETHAM, COIMBATORE – 641112

DEPARTMENT OF MECHANICAL ENGINEERING

DECLARATION

We AAYUSH TRIPATHI (CB.EN.U4MEE11103), VENKATA PRITHVI

PUPPALA (CB.EN.U4MEE11160), A. SRI PRUDHVI (CB.EN.U4MEE11202) and

CH. VARUN TEJA (CB.EN.U4MEE11216) hereby declare that the project work

entitled “EFFECT OF PROCESS PARAMETERS ON TEMPERATURE

DISTRIBUTION DURING FRICTION STIR WELDING”, is the record of the

original work done by us, under the guidance of Dr. R. PADMANABAN, Department of

Mechanical Engineering. To the best of our knowledge this work has not formed the

basis for the award of any degree/ diploma/ associateship/ fellowship or a similar award

to any candidate in any university.

AAYUSH TRIPATHI VENKATA PRITHVI PUPPALA

A.SRI PRUDHVI CH.VARUN TEJA

Place: Coimbatore, 641112

Date:

COUNTERSIGNED

Dr. R. PADMANABAN

Project Advisor

Dept. of Mechanical Engineering

Amrita School of Engineering

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ACKNOWLEDGEMENT

There has been a great many people that have played a part in the completion of this

work. Each has contributed in different but equally important ways that must be

acknowledged.

This report is the final year project submitted to the Mechanical Engineering Department

of Amrita School of Engineering. My supervisor, Dr. R. Padmanaban has played a large

role and must be thanked for his guidance and invaluable help throughout the period of

this work.

We would finally like to acknowledge the Amrita School of Engineering for the support

over the duration of this work. Our gratitude goes to all the CAD lab assistants and the

faculty of the Department of Mechanical Engineering for their timely help and valuable

inputs.

Aayush Tripathi

Venkata Prithvi Puppala

A. Sri Prudhvi

Ch. Varun Teja

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Contents

ABSTRACT ...................................................................................................................... 10

CHAPTER 1 Introduction................................................................................................. 11

1.1 Friction Stir Welding ............................................................................................... 11

1.2 Advantages of FSW ................................................................................................ 12

1.3 Disadvantages of FSW ............................................................................................ 12

1.4 Industrial Applications ............................................................................................ 13

1.4.1 Shipbuilding and Marine Construction ............................................................. 13

1.4.2 Aerospace Industry ........................................................................................... 14

1.4.3 Railway Industry............................................................................................... 15

1.4.4Land Transportation .......................................................................................... 15

1.5 PROCESS PARAMETERS .................................................................................... 16

1.5.1 Tool Geometry .................................................................................................. 16

1.5.2. Welding Parameters ......................................................................................... 17

1.5.3 Joint Design ...................................................................................................... 18

1.6 Objective ................................................................................................................. 19

CHAPTER 2: LITERATURE SURVEY.......................................................................... 20

2.2 Residual Stress ........................................................................................................ 24

2.3 Temperature Distribution ........................................................................................ 28

CHAPTER 3 MODELLING OF FSW IN ANSYS .......................................................... 34

3.1 Introduction ............................................................................................................. 34

3.2 Thermal model ........................................................................................................ 34

3.4 Heat Generation....................................................................................................... 35

3.5 Finite Element Model .............................................................................................. 36

3.5.1 SOLID70 Input Summary ................................................................................ 37

3.6 Response surface methodology ............................................................................... 39

Chapter 4: Results and Discussion .................................................................................... 41

4.1 Introduction ............................................................................................................. 41

4.2 Effect of Welding Speed on Temperature ............................................................... 45

4.3 Effect of Tool Rotation Speed on Temperature ...................................................... 46

4.5 Interaction effect of tool rotation and welding speed .............................................. 47

6

CHAPTER 5: CONCLUSION ......................................................................................... 50

REFERENCES ................................................................................................................. 51

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LIST OF FIGURES

Fig 1.1 Schematic drawing of friction stir welding

Fig 1.2 Ship Building

Fig 1.3 Modern Aerospace building

Fig 1.4 Modern high speed trains

Fig 1.5 Armored car for armed forces

Fig 1.6 Schematic Drawing of FSW Tool

Fig 1.7 Joint configurations for friction stir welding

Fig 2.1 Longitudinal residual stress distribution

Fig 2.2 Average, through thickness, longitudinal

and traverse residual stress distribution

Fig 2.3 Peak temperature distribution adjacent to a

friction stir weld

Fig 2.4 Effect of depth on peak temperature as a

function of distance from weld centerline

Fig 2.5 Effect of tool rotation rate on peak

temperature as a function of distance from

weld centerline

Fig 2.6 Effect of tool rotation rate/traverse speed

(v/n) ratio on peak temperature of FSW

Fig 3.1 SOLID70 Geometry

Fig 3.2 FEM model used for simulation

Fig 4.1 Temperature contour for Trial 5

Fig 4.2 Temperature contour (cross section) for

Trial 5

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Fig 4.3 Effect of Welding speed on Maximum

temperature

Fig 4.4 Effect of Tool rotation speed on Maximum

temperature

Fig 4.5 Contour Plot of Temperature

Fig 4.6 Surface plot of temperature

Fig 4.7 Normal Probability plot of residual

Fig 4.8 Variation of Residual with run order

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LIST OF TABLES

Table 2.1 Residual Stress measurements

Table 3.1 Thermal properties used for simulation

Table 4.1 Parameters used and temperatures

Table 4.2 t-test on Coefficients

Table 4.3 Parameters used and temperatures

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ABSTRACT

Friction stir welding with huge potential for varied applications promising technique has

attracted lot of research work. The process parameters play an important role in deciding

the heat generation, temperature distribution, material flow, mechanical properties and

hence the final properties and strength of the joint. In this work a three dimensional finite

element model has been used to predict the effect of tool rotation speed and welding

speed on the temperature distribution. In this work the material flow has been neglected

as a whole and only the heat transfer aspect has been considered. The tool is represented

as a moving heat flux and the clamps are not considered as the heat loss to the clamp is

negligible due to their small area of contact and location. The heat loss to the backing

plate, which is essentially contact conduction, is accounted by using an equivalent

convection coefficient at the bottom of the plates. Temperature dependent material

properties are used for the plates as these give reasonable prediction for transient

temperature fields. To study the effect of process parameters on the temperature

distribution, the simulation trials have been tried as per central composite design for three

variables. Response surface methodology was used to study the effect of process

parameters on the temperature during friction stir welding. The statistical modelling

software MINITAB was used for building the response surface model. The regression

model for the temperature is given below. The Analysis of Variance for model conducted

at 95% Confidence level results show that all the regression coefficients contribute to the

model. The shoulder diameter is kept constant at 18 mm and the dwell time was 30

seconds. The peak temperature in all the cases was found to be lower than the

melting point of the base material. The temperature distribution was found to vary with

both the parameters. The maximum temperature was found to increase with increase in

tool rotation speed and it was found to decrease with welding speed. This is due to faster

heating and cooling at higher welding speeds. The increase in temperature when the

tool rotation speed is increased is due to the high frictional heat generation at higher

tool rotation speeds. Analysis of the contour plots reveal that in general when tool

rotation is increased the corresponding welding sped is to be increased for realising

higher temperatures.

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CHAPTER 1 Introduction

1.1 Friction Stir Welding

Friction stir welding (FSW) is a relatively innovative solid-state joining process. This

joining method is energy efficient, environment friendly, and versatile. In particular, it

can be used to join high-strength aerospace aluminum alloys and other metallic alloys

that are hard to weld by orthodox fusion welding. FSW is deemed to be the most

momentous development in metal joining in a decade. Recently, friction stir processing

(FSP) was developed for microstructural modification of metallic materials. In this

review article, the current state of understanding and development of the FSW and FSP

are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the

formation of welds and microstructural refinement, and (b) effects of FSW/FSP

parameters on resultant microstructure and final mechanical properties. While the bulk of

the information is related to aluminum alloys important results are now available for

other metals and alloys. At this stage, the technology diffusion has significantly outpaced

the fundamental understanding of microstructural evolution and microstructure–property

relationships.

Fig. 1.1: Schematic drawing of friction stir welding

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A constantly rotated non consumable cylindrical-shouldered tool with a profiled probe is

transversely fed at a constant rate into a butt joint between two clamped pieces of butted

material. The probe is slightly shorter than the weld depth required, with the tool shoulder

riding atop the work surface.

Frictional heat is generated between the wear-resistant welding components and the work

pieces. This heat, along with that generated by the mechanical mixing process and

the adiabatic heat within the material, cause the stirred materials to soften

without melting. As the pin is moved forward, a special profile on its leading face forces

plasticized material to the rear where clamping force assists in a forged consolidation of

the weld.

This process of the tool traversing along the weld line in a plasticized tubular shaft of

metal results in severe solid state deformation involving dynamic recrystallization of the

base material.

1.2 Advantages of FSW

FSW is considered to be the most significant development in metal joining in a decade

and is green technology due to its energy efficiency, environment friendliness, and

versatility. As compared to the conventional welding methods, FSW consumes

considerably less energy. No cover gas or flux is used, thereby making the process

environmentally friendly. The joining does not involve any use of filler metal and

therefore any aluminum alloy can be joined without concern for the compatibility of

composition, which is an issue in fusion welding. When desirable, dissimilar aluminum

alloys and composites can be joined with equal ease. In contrast to the traditional friction

welding, which is usually performed on small axisymmetric parts that can be rotated and

pushed against each other to form a joint, friction stir welding can be applied to various

types of joints like butt joints, lap joints, T butt joints, and fillet joints

1.3 Disadvantages of FSW

Exit hole left when tool is withdrawn. Large down forces are required with heavy-duty

fastening essential to grip the plates together. It is also less flexible than manual and arc

processes (difficulties with thickness variations and non-linear welds).Often slower

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traverse rate than some fusion welding techniques, although this may be offset if fewer

welding passes are required.

1.4 Industrial Applications

1.4.1 Shipbuilding and Marine Construction

The shipbuilding and marine industries are two of the first sectors that have adopted the

process for commercial applications.

The process is suitable for the following applications:

Panels for decks, sides, bulkheads and floors

Hulls and superstructures

Helicopter landing platforms

Marine and transport structures

Masts and booms, e.g. for sailing boats

Refrigeration plant

Fig 1.2: Ship Building

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1.4.2 Aerospace Industry

At present the aerospace industry is welding prototype and production 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 in Al alloy fuel tanks for space vehicles have been friction stir

welded and successfully used. 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 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

Fig 1.3: Modern Aerospace building

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1.4.3 Railway Industry

The commercial production of high speed trains made from aluminium extrusions, which

may be joined by friction stir welding, has been established.

Applications include:

High speed trains

Rolling stock of railways, underground carriages, trams

Railway tankers and goods wagons

Fig 1.4: Modern high speed trains

1.4.4Land Transportation

The friction stir welding process is currently being used commercially and is also being

assessed by several automotive companies and suppliers.

Existing and potential applications include:

Engine and chassis cradles

Wheel rims

Attachments to hydroformed tubes

Space frames, e.g. welding extruded tubes to cast nodes

Truck bodies & tail lifts for lorries

Mobile cranes

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Armour plate vehicles

Fuel tankers

Caravans

Fig 1.5: Armored car for armed forces

1.5 PROCESS PARAMETERS

Various parameters are involved in Friction Stir Welding due to complex material

movement. Tool geometry, joint design and welding parameters wield substantial effects

on material flow pattern and temperature distribution, thereby impacting the

microstructural evolution of material.

1.5.1 Tool Geometry

Tool geometry is the most significant feature of process development. The tool geometry

plays a decisive role in material flow and in turn oversees the traverse rate at which FSW

can be directed. An FSW tool comprises of a shoulder and a pin as displayed

schematically in Fig. 5.

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As mentioned prior, the tool has two major functions: (a) localized heating, and (b)

material flow. In the preliminary stage of tool plunge, the heating results predominantly

from the friction between pin and work piece. Further heating results from distortion of

material.

The tool is plunged till the shoulder touches the work piece. The friction, between the

shoulder and workpiece, effects in the biggest constituent of heating. From the heating

characteristic, the relative size of pin and shoulder is vital, and the other design attributes

are not as critical. The shoulder also offers confinement for the heated volume of

material. The secondary function of the tool is to ‘stir’ and ‘move’ the material. The

consistency of microstructure and properties as well as process loads is overseen by the

tool design. Usually a concave shoulder and threaded cylindrical pins are used.

Fig.1.6 Schematic Drawing of FSW Tool

1.5.2. Welding Parameters

For FSW, two parameters are very essential: tool rotation rate (v, rpm) in clockwise or

counter-clockwise direction and tool traverse speed (n, mm/min) along the line of joint.

The rotation of tool ensues in stirring and fusing of material around the rotating pin and

the translation of tool transfers the stirred material from the front to the back of the pin

and terminates welding process. Higher tool rotation rates generate higher temperature

because of higher friction heating and result in more concentrated stirring and mixing of

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material as will be discussed later. However, it should be remarked that frictional

coupling of tool surface with workpiece is going to manage the heating. So, a monotonic

surge in heating with increasing tool rotation rate is not expected as the coefficient of

friction at interface will change with increasing tool rotation rate.

1.5.3 Joint Design

The most suitable joint configurations for FSW are butt and lap joints. A simple square

butt joint is shown in Fig. 1.7a. Two plates or sheets with same thickness are placed on a

backing plate and clamped firmly to prevent the abutting joint faces from being forced

apart. During the initial plunge of the tool, the forces are fairly large and extra care is

required to ensure that plates in butt configuration do not separate. A rotating tool is

plunged into the joint line and traversed along this line when the shoulder of the tool is in

intimate contact with the surface of the plates, producing a weld along abutting line. On

the other hand, for a simple lap joint, two lapped plates or sheets are clamped on a

backing plate. A rotating tool is vertically plunged through the upper plate and into the

lower plate and traversed along desired direction, joining the two plates (Fig. 1.7d). Many

other configurations can be produced by combination of butt and lap joints. Apart from

butt and lap joint configurations, other types of joint designs, such as fillet joints (Fig.

1.7g), are also possible as needed for some engineering applications. It is important to

note that no special preparation is needed for FSW of butt and lap joints. Two clean metal

plates can be easily joined together in the form of butt or lap joints without any major

concern about the surface conditions of the plates.

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Fig.1.7: Joint configurations for friction stir welding: (a) square butt, (b) edge butt, (c) T

butt joint, (d) lap joint, (e) multiple lap joint, (f) T lap joint, and (g) fillet joint.

1.6 Objective

The aim of this project is to predict the effect of process parameters namely, tool rotation

speed and welding speed on the temperature distribution during friction stir welding

using a three dimensional finite element model. The material used is AA1100 aluminium

alloy. The 3D finite element thermal model is built using ANSYS software. By varying

parameters, the effect of process parameters on the peak temperature during friction stir

welding of AA1100 is studied.

.

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CHAPTER 2: LITERATURE SURVEY

Initial attempts at thermal modelling of the FSW process were analytical in nature.

Subsequently, FEM based solid and thermal models and computational fluid dynamics

based viscoplastic flow models have replaced the analytical approach because of the

versatility of these methods. Although none of these efforts has combined all the aspects

of the modelling, namely, the flow visualisation, the material properties, and the thermal

profiles and histories, they have nevertheless contributed towards a greater understanding

of the process.

The earliest efforts on thermal modelling of friction stir welding started with the

Rosenthal equation for a uniformly moving point or line heat source to describe the heat

input. Russell and Shercliff adapted the existing analytical solutions for moving heat

sources and friction welding to model the friction stir welding by replacing the real heat

generation around the tool with equivalent simplified heat inputs such as the point source,

line source, and distributed surface heat input. McClure et al.presented some

experimental results accompanied with an intuitive analytical model similar to the

Rosenthal moving point source solution, and found that microstructures within the weld

zone are characterised by dynamic recrystallisation, grain growth, and complex aging

phenomena that show no evidence of melting.

They found that the maximum temperatures during the FSW process did not exceed

80%of the melting point temperature. They assumed that all the heat was generated by

the friction between the tool shoulder and the workpiece and considered only the

conduction mode of heat transfer. They also assumed that the downward pressure P

applied on the tool shoulder surface is uniform and correlated the heat generation rate

with the downward pressure using the friction coefficient. Using the analytical solution

for the Rosenthal moving point heat source and incorporating the frictional heat

generation as the heat source, they derived an integral equation to define the temperature

weld. In their model all thermo physical properties were assumed to be temperature

independent and the combined parameter µP (where µis the coefficient of friction and P

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is the average uniform pressure at the tool shoulder) was fitted until the model predicted

the correct temperature at the centreline of the weld. They observed that a value of 5480

N (1230 lb) for µP resulted in a reasonable match between the experimental data and

simulated outputs at the weld centreline.

Gould and Feng and Feng et al. presented similar preliminary thermal models for FSW

that are analytical in nature and consider heat generation at the tool shoulder only. They

also used the Rosenthal equation to predict the quasi steady temperature weld due to a

moving point heat source of constant velocity and correlated the heat source with the

friction process at the shoulder.

Tang et al. investigated the heat input and temperature distribution during FSW and

found that the highest temperature in the weld seam was less than 80% of the melting

point for Al6061 – T6 and that, for the specific working conditions of 2 mm/s travel rate

and 300 – 1200 rev/min rotational speed, the temperature did not change appreciably in

the specimen thickness direction. They also observed that the temperature distribution

perpendicular to the weld was nearly isothermal under the tool shoulder, that the

temperature distribution is symmetric about the weld centerline and that increasing the

welding pressure and the rotational speed of the tool increased the peak welding

temperature.

They found an average thickness reduction of about 3% at the start of the weld and noted

that the shoulder of the pin tool played the most significant role in the welding process.

The microstructure of the welded joint showed that there was no melting during the

welding process. They found the grains at the weld centre to be fine and equiaxed

because of the large deformation that facilitates dynamic recrystallisation.

Chao and Qi presented a decoupled heat transfer model to study the temperature fields

developed during the FSW process. They used a FEM based three-dimensional model of

the thermal process in which it was assumed that all the heat was generated at the

frictional interface between the tool shoulder and the workpiece. The rate of heat input

was assumed to vary linearly with the radius of the tool shoulder and set directly

proportional to the coefficient of friction and the downward force exerted by the tool.

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They considered the inner radius of the tool shoulder to be zero to compensate for the

heat generated by the tool pin.

In addition, they did not measure the downward force and consequently were obliged to

fit the combined variable µF(where F is the downward force) along with other estimated

variables. They used convection coefficient of 30 W m-2 K-1at the top surface of the

workpiece and modelled the bottom surface in contact with the backing plate with a

convective heat transfer coefficient determined by a trial and error procedure. They

determined that a convective coefficient of 500 Wm-2K-1correlated best with

experimental results.

Frigaard et al.used the finite difference technique to calculate the two- and three-

dimensional thermal fields during the FSW process and used these data in a separate

module to predict the resulting hardness distributions.

The model predictions of Frigaard et al. showed that the welding cycle has four major

stages: (i) the stationary heating period where the material beneath the shoulder is

preheated to a certain temperature (about 400°C for aluminium alloys) to allow the

plastic deformation during welding, (ii) the transient heating period during which heat

starts to build up around the shoulder as the tool starts to move, (iii) the pseudo-steady

state period where the thermal field around the tool remains essentially constant during

the welding operation, and (iv) the post-steady state period(with approximately three-

quarters of the weld completed)where the reflection of heat from the end plate surface

leads to additional buildup of heat around the tool shoulder and influences the heat

affected zone thermal profile.

They identified a process parameter q0 /vd(kJ mm-2) (where is q0 is the net power, v the

welding speed, and d the plate thickness) that controls the thermal behaviour during the

pseudo-steady state. They found that the heat source during the FSW is axisymmetric and

that the outer contour of the plastically deformed region corresponds approximately to the

450°C isotherm. They accounted for the heat generation due to plastic deformation by

using an elevated average value of the friction coefficient, m~0.4, which falls between

the values for sticky friction and dry sliding.

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Colegrove et al. presented a finite element based thermal model of FSW that included the

backing plate and the tool and a flow model of the process that attempts to predict the

flow of material around the FSW tool. They used a pseudomaterial of reduced thermal

conductivity to simulate the contact between the aluminium and the steel backing plate.

However, they did not measure the power consumption during the FSW process and were

obliged to fit the heat input for overall agreement between modelled and measured

thermal cycles. They found that a shoulder heat input of 3474 W gave the best correlation

with the experimental data for the lower part of the weld. Khandkar and Khan presented a

friction based thermal model of overlap friction stir welding, whereas Khandkar and co-

workers presented some preliminary results on input torque based thermal modelling of

butt joining. In the friction based model of an overlap joint, they used a frictional heat

input at the tool shoulder–workpiece interface and used a value of 3% of the shoulder

flux for the regions that fall within the volume of the pin tool. They also incorporated the

effect of convective transport around the pin into the thermal model for overlap joints. In

an input torque based model of a butt joint, Khandkar et al. presented the novel idea of a

three-dimensional heat input model that emulates the shape of the pin tool in contact with

the workpiece by correlating the heat input with the experimentally measured input

torque.

This latter approach has been investigated and refined further in the present work. Smith

et al. presented the concept of a physics based model of the FSW process in which the

material close to the weld centreline is viewed as a highly viscous non-Newtonian fluid.

Seidel refined the idea further and implemented fluid dynamics based two-dimensional

and preliminary three dimensional models of the weld process. Seidel correlated the non-

Newtonian viscosity with the flow stress of the alloy through the Zener – Hollomon

parameter. The heat generated originates from the viscous dissipation of the laminar flow.

None of the above mentioned works, except those of Khandkar and Khan and Khandkar

et al. incorporated a truly three-dimensional heat source along with the backing plate

attached to the bottom surface of the workpiece offering a thermal contact resistance in

between.

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Although Colegrove et al made such an attempt, it was necessary to fit their heat input

values to match the experimentally determined temperature data. In the present work, a

heat input model that approximates the contour of the profiled tool by correlating the heat

source with the input torque has been explored.

This model is a distinct improvement over previous frictional heating models in which it

was necessary to rely on inverse estimation of the frictional coefficient for compliance

between the modeled heat source and the temperature data. Parametric analyses have also

been presented to study the effects on the thermal profiles of (i) different heat transfer

conditions at the bottom of the workpiece, (ii) backing plates of different material

(different conductivities), and (iii) different contact gap conductances at the workpiece –

backing plate interface.

2.2 Residual Stress

During fusion welding, complex thermal and mechanical stresses develop in the weld and

surrounding region due to the localized application of heat and accompanying constraint.

Following fusion welding, residual stresses commonly approach the yield strength of the

base material. It is generally believed that residual stresses are low in friction stir welds

due to low temperature solid-state process of FSW. However, compared to more

compliant clamps used for fixing the parts in conventional welding processes, the rigid

clamping used in FSW exerts a much higher restraint on the welded plates. These

restraints impede the contraction of the weld nugget and heat-affected zone during

cooling in both longitudinal and transverse directions, thereby resulting in generation of

longitudinal and transverse stresses. The existence of high value of residual stress exerts a

significant effect on the post-weld mechanical properties, particularly the fatigue

properties. Therefore, it is of practical importance to investigate the residual stress

distribution in the FSW welds. James and Mahoney measured residual stress in the FSW

7050Al-T7451, C458 Al–Li alloy, and 2219Al by means of X-ray diffraction sin2 c

method. Typical results obtained in FSW 7050Al- T7451 by pinhole X-ray beam (1 mm)

are tabulated in Table 1. This investigation revealed following findings. First, the residual

stresses in all the FSW welds were quite low compared to those generated during fusion

welding. Second, at the transition between the fully recrystallized and partially

25

recrystallized regions, the residual stress was higher than that observed in other regions of

the weld. Third, generally, longitudinal (parallel to welding direction) residual stresses

were tensile and transverse (normal to welding direction) residual stresses were

compressive.

Table 2.1: Residual Stress measurements (Mpa) in FSW 7050 Al-T6541 weld by pinhole

beam X-Ray (after James and Mahoney)

Location Distance

from weld

centerline

(mm)

Longitudinal

Retreating

side

Longitudinal

Advancing

side

Transverse

Retreating

side

Transverse

Advancing

side

Top surface 2 22 19 -33 -41

4 39 35 -14 -27

6 55 72 -21 -24

7 64 48 -40 -47

8 101 76 -99 -43

Root

surface

1 13 42 28±52 -12

3 36±52 48±54 -71 -19

5 61±30 55 --55±103 -48

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Fig 2.1: Longitudinal residual stress distribution in FSW 6013Al-T4 welds determined by

different measurement methods (tool rotation rate: 2500 rpm, traverse speed: 1000

mm/min, tool shoulder diameter: 15 mm) (after Donne et al.)

The low residual stress in the FSW welds was attributed to the lower heat input during

FSW and recrystallization accommodation of stresses. Recently, Donne et al. measured

residual stress distribution on FSW 2024Al-T3 and 6013Al-T6 welds by using the cut

compliance technique, X-ray diffraction, neutron diffraction and high-energy synchrotron

radiation. Six important observations can be made from their study. First, the

experimental results obtained by these measurement techniques were in good qualitative

and quantitative agreement. Second, the longitudinal residual stresses were always higher

than the transverse ones, independent on pin diameter, tool rotation rate and traverse

speed. Third, both longitudinal and transverse residual stresses exhibited an ‘‘M’’-like

distribution across the weld. A typical longitudinal residual stress distribution is shown in

Fig. 4. Fig. 4 reveals that maximum tensile residual stresses were located ~10 mm away

from the weld centerline, i.e., the HAZ. Small compressive residual stresses were

detected in the parent metal adjacent to the HAZ and the weld seam. Fourth, residual

27

stress distribution across the welds was similar at the top and root sides of the welds.

Fifth, large-diameter tool widened the M-shaped residual stress distribution. With

decreasing welding speed and tool rotation rate, the magnitude of the tensile residual

stresses decreased. Sixth, in the case of the small samples of 30 mm ×80 mm and 60 mm

×80 mm, the maximum longitudinal tensile residual stresses were in the range of 30–60%

of weld material yield strength and 20–50% of base material yield strength. Clearly, the

residual stress values in the FSW welds are remarkably lower than those in the fusion

welds. However, Wang et al. reported that larger values of residual stress may be present

in larger samples of 200 mm × 200 mm. More recently, Peel et al investigated the

residual stress distribution on FSW 5083Al using synchrotron X-ray diffraction.

Following observations can be made from their investigation. First, while longitudinal

residual stress exhibited a ‘‘M’’-like distribution across the weld similar to the results of

Donne et al., transverse residual stresses exhibited a peak at the weld center. Second, the

nugget zone was in tension in both longitudinal and transverse directions. Third, peak

tensile residual stress was observed at ~10 mm from the weld centerline, a distance

corresponding to the edge of the tool shoulder. Fourth, longitudinal residual stress

increased with increasing tool traverse speed, whereas transverse residual stresses did not

exhibit evident dependence on the traverse speed. Fifth, a mild asymmetry in longitudinal

residual stress profile was observed within the nugget zone with the stresses being ~10%

higher on the advancing side.

Fig 2.2: Average, through thickness, longitudinal and traverse residual stress distribution

as a function of distance from the weld centerline in FSW 204L stainless steel (tool

traverse speed: 102 mm/min) (after Reynolds et al)

28

Sixth, similar to the results of Donne et al., maximum residual stresses in longitudinal

direction (40–60 MPa) were higher than that in transverse direction (20–40 MPa).Clearly,

maximum residual stresses observed in various friction stir welds of aluminum alloys

were below 100 MPa. The residual stress magnitudes are significantly lower than those

observed in fusion welding, and also significantly lower than yield stress of these

aluminum alloys. This results in a significant reduction in the distortion of FSW

components and an improvement in mechanical properties. On the other hand, Reynolds

et al.measured residual stress of 304L stainless steel FSW welds by neutron diffraction.

Average through thickness, longitudinal and transverse residual stresses are presented in

Fig. 2.2 as a function of distance from the weld centerline. Fig. 2.2 revealed the following

observations. First, the residual stress patterns observed for FSW are typical of most

welding processes such as fusion welding, namely, high value of longitudinal tensile

residual stress and very low transverse residual stress. Second, the maximum values of

longitudinal residual stress were close to the base metal yield stress, and therefore similar

in magnitude to those produced by fusion welding processes in austenitic stainless steels.

Third, increasing tool rotation rate from 300 to 500 rpm at a constant tool traverse speed

of 102 mm/min did not exert marked effect on the residual stress distribution apart from

slightly widening the range of high values of residual stress. Further, Reynolds et

al.reported that the longitudinal residual stress varied only slightly with depth, whereas

the transverse stress varied significantly through the thickness. The sign of the transverse

residual stress near the weld centerline was in general positive at the crown and negative

at the root. This was attributed to rapid cooling experienced by the weld root due to the

intimate contact between the weld root side and the backing plate. Clearly, the

distribution and magnitude of residual stress in friction stir welds are different for

aluminum alloy and steel. This is likely to be related to the temperature dependence of

the yield strength and the influence of final deformation by the trailing edge of the tool

shoulder.

2.3 Temperature Distribution

FSW results in intense plastic deformation around rotating tool and friction between tool

and workpieces. Both these factors contribute to the temperature increase within and

around the stirred zone. Since the temperature distribution within and around the stirred

29

zone directly influences the microstructure of the welds, such as grain size, grain

boundary character, coarsening and dissolution of precipitates, and resultant mechanical

properties of the welds, it is important to obtain information about temperature

distribution during FSW. However, temperature measurements within the stirred zone are

very difficult due to the intense plastic deformation produced by the rotation and

translation of tool. Therefore, the maximum temperatures within the stirred zone during

FSW have been either estimated from the microstructure of the weld or recorded by

embedding thermocouple in the regions adjacent to the rotating pin. An investigation of

microstructural evolution in 7075Al-T651 during FSW by Rhodes et al. showed

dissolution of larger precipitates and reprecipitation in the weld center. Therefore, they

concluded that maximum process temperatures are between about 400 and 480 8C in an

FSW 7075Al- T651. On the hand, Murr and co-workers indicated that some of the

precipitates were not dissolved during welding and suggested that the temperature rises to

roughly 400 8C in an FSW 6061Al. Recently, Sato et al.studied the microstructural

evolution of 6063Al during FSW using transmission electron microscopy (TEM) and

compared it with that of simulated weld thermal cycles. They reported that the

precipitates within the weld region (0–8.5 mm from weld center) were completely

dissolved into aluminum matrix. By comparing with the microstructures of simulated

weld thermal cycles at different peak temperatures, they concluded that the regions 0–8.5,

10, 12.5, and 15 mm away from the friction stir weld center were heated to temperatures

higher than 402, 353, 302 ºC and lower than 201 ºC, respectively. Recently, Mahoney et

al. conducted friction stir welding of 6.35 mm thick 7075Al-T651 plate and measured the

temperature distribution around the stirred zone both as a function of distance from the

stirred zone and through the thickness of the sheet. Fig. 2.3 shows the peak temperature

distribution adjacent to the stirred zone. Fig.2.3 reveals three important observations.

First, maximum temperature was recorded at the locations close to the stirred zone, i.e.,

the edge of the stirred zone, and the temperature decreased with increasing distance from

the stirred zone.

30

Fig 2.3: Peak temperature distribution adjacent to a friction stir weld in 7075Al-T651.

The line on the right side of figure shows the nugget boundary (after Mahoney et al)

Second, the temperature at the edge of the stirred zone increased from the bottom surface

of the plate to the top surface. Third, a maximum temperature of 475 ºC was recorded

near the corner between the edge of the stirred zone and the top surface. This temperature

is believed to exceed the solution temperature for the hardening precipitates in 7075Al-

T651. Based on these results the temperature within the stirred zone is likely to be above

475 ºC. However, the maximum temperature within the stirred zone should be lower than

the melting point of 7075Al because no evidence of material melting was observed in the

weld

More recently, an attempt was made by Tang et al. to measure the heat input and

temperature distribution within friction stir weld by embedding thermocouples in the

region to be welded. 6061Al- T6 aluminum plates with a thickness of 6.4 mm were used.

They embedded thermocouples in a series of small holes of 0.92 mm diameter at different

distances from weld seam drilled into the back surface of the workpiece. Three depths of

holes (1.59, 3.18, and 4.76 mm) were used to measure the temperature field at one

quarter, one half, and three quarter of the plate thickness. They reported that the

thermocouple at the weld center was not destroyed by the pin during welding but did

change position slightly due to plastic flow of material ahead of the pin. Fig. 2.4 shows

the variation of the peak temperature with the distance from the weld centerline for

31

various depths below the top surface. Three important observations can be made from

this plot. First, maximum peak temperature was recorded at the weld center and with

increasing distance from the weld centerline, the peak temperature decreased. At a tool

rotation rate of 400 rpm and a traverse speed of 122 mm/min, a peak temperature of ~450

ºC was observed at the weld center one quarter from top surface. Second, there is a nearly

isothermal region ~4 mm from the weld centerline. Third, the peak temperature gradient

in the thickness direction of the welded joint is very small within the stirred zone and

between 25 and 40 ºC in the region away from the stirred zone. This indicates that the

temperature distribution within the stirred zone is relatively uniform. Tang et al.further

investigated the effect of weld pressure and tool rotation rate on the temperature field of

the weld zone. It was reported that increasing both tool rotation rate and weld pressure

resulted in an increase in the weld temperature. Fig. 2.5 shows the effect of tool rotation

rate on the peak temperature as a function of distance from the weld centerline. Clearly,

within the weld zone the peak temperature increased by almost 40 ºC with increasing tool

rotation rate from 300 to 650 rpm, whereas it only increased by 20 ºC when the tool

rotation rate increased from 650 to 1000 rpm, i.e., the rate of temperature increase is

lower at higher tool rotation rates.

Fig 2.4. Effect of depth on peak temperature as a function of distance from weld

centerline for a 6061Al-T6 FSW weld made at 400 rpm and 120 mm/min traverse speed

(after Tang et al)

32

Fig 2.5. Effect of tool rotation rate on peak temperature as a function of distance from

weld centerline for a 6061Al-T6 FSW weld made at 120 mm/min traverse speed

Furthermore, Tang et al. studied the effect of shoulder on the temperature field by using

two tools with and without pin. The shoulder dominated the heat generation during FSW

(Fig. 2.6). This was attributed to the fact that the contact area and vertical pressure

between the shoulder and workpiece is much larger than those between the pin and

workpiece, and the shoulder has higher linear velocity than the pin with smaller radius .

Additionally, Tang et al.showed that the thermocouples placed at equal distances from

the weld seam but on opposite sides of the weld showed no significant differences in the

temperature.

Similarly, Kwon et al., Sato et al., and Hashimoto et al.also measured the temperature rise

in the weld zone by embedding thermocouples in the regions adjacent to the rotating in.

Kwon et al. reported that in FSW 1050Al, the peak temperature in the FSP zone

increased linearly from 190 to 310 ºC with increasing tool rotation rate from 560 to 1840

rpm at a constant tool traverse speed of 155 mm/min. An investigation by Sato et al.

indicated that in FSW 6063Al, the peak temperature of FSW thermal cycle increased

sharply with increasing tool rotation rate from 800 to 2000 rpm at a constant tool traverse

speed of 360 mm/min, and above 2000 rpm, however, it rose gradually with increasing

rotation rate from 2000 to 3600 rpm. Peak temperature of >500 ºC was recorded at a high

tool rotation rate of 3600 rpm. 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

33

of 2024Al-T6, 5083Al-O and 7075Al-T6 (Fig. 2.6). A peak temperature >550 ºC was

observed in FSW 5083Al-O at a high ratio of tool rotation rate/traverse speed.

Fig 13: Effect of tool rotation rate/traverse speed (v/n) ratio on peak temperature of FSW

2024Al-T6, 5083Al-O, and7075Al-T6 (after Hashimoto et al.)

34

CHAPTER 3 MODELLING OF FSW IN ANSYS

3.1 Introduction

AA1100 is categorized as Wrought Aluminum Alloy. It is composed of (in weight

percentage) 99.00%(min) Aluminum (Al) and 0.12% Copper (Cu). It can be seen in

forms of sheet, plate, foil, extruded rod bar and wire, extruded shapes, extruded tubes,

cold finished rod bar and wire, drawn tube, and forgings.

Table 3.1 Thermal properties used for simulation

Temperature

ºC

Thermal Conductivity

W/mºC

Heat Capacity

J/KgºC

37.8 162 945

93.3 177 978

148.9 184 1004

204.4 192 1028

260 201 1052

315.6 207 1078

371.1 217 1104

426.7 223 1133

3.2 Thermal model

The governing equation for 3D heat transfer is given by:

𝜌𝑐𝜕𝑇

𝜕𝑡= 𝜆 (

𝜕2𝑇

𝜕𝑥2+

𝜕2𝑇

𝜕𝑦2+

𝜕2𝑇

𝜕𝑧2) +𝑞0

𝑉 (1)

The temperature calculation is based on the Fourier's equation

Where λ is the thermal conductivity, T is the temperature, 𝜌 is thedensity, c is the heat

capacity, and q0 /V is the source term.

The thermal model is created using the Finite element package ANSYS .In this work the

material flow has been neglected as a whole and only the heat transfer aspect has been

35

considered. The tool is represented as a moving heat flux and the clamps are not

considered as the heat loss to the clamp is negligible due to their small area of contact and

location. The plates are represented by a finite element mesh of 8 node thermal brick

elements with temperature as the only degree of freedom. To reduce the computational

requirements, regions around the weld line are meshed with smaller element, while the

element size farther from the weld line is larger. The heat loss into the tool is accounted

by reducing the energy input into the plate. Convection coefficient for all the surfaces

exposed to atmosphere is 30 W/m2 K. The heat loss to the backing plate, which is

essentially contact conduction, is accounted by using an equivalent convection coefficient

at the bottom of the plates. Constant room-temperature properties are used for the plates

as these give reasonable prediction for transient temperature fields.

3.4 Heat Generation

In FSW, heat is generated due to friction and plastic deformation at the tool workpiece

interface and at the TMAZ. The heat generation at the contact surfaces due to friction is

the product of frictional force and the tangential speed of the tool with respect to the

workpiece. The heat generated per unit area due to plastic deformation at the tool-

workpiece interface is the product of shear stress and the velocity of the workpiece

material sticking to the tool as it traverses. This velocity is actually the tangential speed

of the tool. The heat generation due to friction on an elemental area dA at the tool-

workpiece interface, considering high rotational speed compared to traverse speed of the

FSW tool, is given by:

𝑑𝑄𝑓 = (1 − 𝛿)𝜔𝑟𝜇𝑝𝑑A (2)

The heat generated due to plastic shear deformation leading to workpiece material

sticking to the tool is given by:

𝑑𝑄𝑝 = 𝛿𝜔𝑟𝜏𝑦𝑑𝐴 (3)

Therefore, total heat due to friction and plastic deformation is given by:

𝑑𝑄 = 𝑑𝑄𝑓 + 𝑑𝑄𝑝 = 𝜔𝑟𝑑𝐴(𝜇𝑝 − 𝛿𝜇𝑝) + 𝛿𝜏𝑦 (4)

36

Let 𝜏𝑐𝑜𝑛𝑡𝑎𝑐𝑡 = [(𝜇𝑝 − 𝛿𝜇𝑝) + 𝛿𝜏𝑦 (5)

Therefore, 𝑑𝑄 = 𝜔𝑟𝑑𝐴𝜏𝑐𝑜𝑛𝑡𝑎𝑐𝑡 (6)

There is no straightforward mechanism to estimate the extent of slip. At the same time,

with the increase in temperature, the yield strength of the workpiece material decreases,

resulting in reduction in heat generation from plastic deformation. In such a situation, it

was felt more logical to consider pure friction and neglect the heat generation due to

plastic deformation. In the case of pure friction δ = 0. Therefore,

Equation 16 reduces to:

𝜏𝑐𝑜𝑛𝑡𝑎𝑐𝑡 = 𝜇𝑝 (7)

Therefore, from Equations 17 and 18, the expression for heat generation on an elemental

surface area dA at the tool-workpiece interface is given by:

𝑑𝑄 = 𝜔𝑟𝜇𝑝 𝑑𝐴 (8)

i.e.,𝑑𝑄 = 𝜔𝑟 𝑑𝐹 (9)

where 𝑑𝐹 = 𝜇𝑝 𝑑𝐴 (10)

The three distinct tool-workpiece interface surfaces are tool shoulder, tool pin side, and

tool pin tip. However, the contribution of tool tip surface is negligible toward the total

heat generation required for welding. Q1 and Q2 are the components of the respective heat

generated from the tool shoulder and tool pin side interfaces. Therefore, the total heat

generated is given by Qtotal = Q1 + Q2 .

3.5 Finite Element Model

Fig 3.1 : SOLID70 Geometry

37

The geometry, node locations, and the coordinate system for this element are shown in

Figure 3.1: SOLID70 Geometry. The element is defined by eight nodes and the

orthotropic material properties. A prism-shaped element, a tetrahedral-shaped element,

and a pyramid-shaped element may also be formed as shown in Figure 3.1: SOLID70

Geometry. Orthotropic material directions correspond to the element coordinate

directions. The element coordinate system orientation is as described in Coordinate

Systems. Specific heat and density are ignored for steady-state solutions. Properties not

input default as described in Linear Material Properties in the Material Reference.

Element loads are described in Nodal Loading. Convection or heat flux (but not both) and

radiation may be input as surface loads at the element faces as shown by the circled

numbers on Figure 3.1: SOLID70 Geometry.

Heat generation rates may be input as element body loads at the nodes. If the node I heat

generation rate HG(I) is input, and all others are unspecified, they default to HG(I).

The nonlinear porous flow option is selected with KEYOPT = 1. For this option,

temperature is interpreted as pressure and the absolute permeability of the medium are

input as material properties KXX, KYY, and KZZ. Properties DENS and VISC are used

for the mass density and viscosity of the fluid. Properties C and MU are used in

calculating the coefficients of permeability as described in the Mechanical APDL Theory

Reference. Temperature boundary conditions input with the D command are interpreted

as pressure boundary conditions, and heat flow boundary conditions input with the F

command are interpreted as mass flow rate (mass/time).

A mass transport option is available with KEYOPT. With this option the velocities VX,

VY, and VZ must be input as real constants (in the element coordinate system). Also,

temperatures should be specified along the entire inlet boundary to assure a stable

solution. With mass transport, you should use specific heat (C) and density (DENS)

material properties instead of enthalpy (ENTH).

A summary of the element input is given in "SOLID70 Input Summary". A general

description of element input is given in Element Input.

3.5.1 SOLID70 Input Summary

Nodes

I, J, K, L, M, N, O, P

Degrees of Freedom

TEMP

Real Constants

Mass transport effects (KEYOPT(8) = 1):

38

VX - X direction of mass transport velocity

VY - Y direction of mass transport velocity

VZ - Z direction of mass transport velocity

Material Properties

MP command: KXX, KYY, KZZ, DENS, C, ENTH, VISC, MU (VISC and MU

used only if KEYOPT = 1.

Do not use ENTH with KEYOPT = 1.

Surface Loads

Convection or Heat Flux (but not both) and Radiation (using Lab = RDSF) --

face 1 (J-I-L-K), face 2 (I-J-N-M), face 3 (J-K-O-N),

face 4 (K-L-P-O), face 5 (L-I-M-P), face 6 (M-N-O-P)

Body Loads

Heat Generations --

HG(I), HG(J), HG(K), HG(L), HG(M), HG(N), HG(O), HG(P)

Special Features

KEYOPT(2)

Evaluation of film coefficient:

0 -- Evaluate film coefficient (if any) at average film temperature, (TS + TB)/2

1 -- Evaluate at element surface temperature, TS

2 -- Evaluate at fluid bulk temperature, TB

3 -- Evaluate at differential temperature |TS-TB|

KEYOPT(4)

Element coordinate system defined:

0 -- Element coordinate system is parallel to the global coordinate system

1 -- Element coordinate system is based on the element I-J side

KEYOPT

Nonlinear fluid flow option:

0 -- Standard heat transfer element

1 -- Nonlinear steady-state fluid flow analogy element

39

KEYOPT

Mass transport effects:

0 -- No mass transport effects

1 -- Mass transport with VX, VY, VZ

Fig 3.2 The finite element model used for simulation

3.6 Response surface methodology

Response surface methodology (RSM) is a collection of mathematical and statistical

techniques useful for analysing problems in which several independent variables

influence a dependent variable or response, and the goal is to optimize the response

variable. In most situations, the form of the relationship between the response and the

independent input variables is unknown. Thus the first step in RSM is to find a suitable

approximation for the true functional relationship between response and the set of

independent variables. A low-order polynomial in some region of the independent

variables is employed. If the response is well modelled by a linear function of the

independent variables, then the approximating function is the first-order model and if

there is curvature in the system, then a polynomial of higher degree such as the second

order is used. RSM is a sequential procedure. The eventual objective of RSM is to

40

determine the optimum operating conditions for the system, or to determine a region of

the factor space in which the operating specifications are satisfied.

The data generated from the experimentation is used to develop the mathematical models

by regression analysis that gives linear, quadratic and two way interaction effects of the

input parameters. The tensile strength (TS) of the friction stir welded joints is a function

of tool rotational speed (N), welding speed (S), Shoulder diameter (SD) and pin diameter

(PD) and it can be expressed as

𝑌=𝑓 (𝑁, SD, 𝑃, D) (10)

The developed mathematical models are useful for accurately predicting responses (viz.

tensile strength) for the input parameters (viz. tool rotational speed (N), welding speed

(S), shoulder diameter (SD) and pin diameter (PD) in this work).

The second order polynomial (regression equation) used to represent the response surface

for K factors is given by

k k2

o i i ii i ij i j

i 1 i 1 i j

y b b x b x b x x

(11)

Where “bo” is the constant of the regression equation and provides a mean value of the

response factor,"bi" is the linear term, ”bij” is the interaction term, “bii” is the quadratic

term of the polynomial and “e” is the residual error

The coefficients bo, bi, bij and bii are the least square estimates of the polynomial,

representing the response surface. These coefficients represent the strength of the

respective process parameters and their interactions. These are also called the parameters

of the response function. The experiments were designed and analysed using Minitab. For

four factors, the selected polynomial can be expressed as

2 2 2 2

o 1 2 3 4 11 1 22 2 33 3 44 4 12

13 14 23 24 34

TS(MPa) b b (N) b (S) b (SD) b (PD) b x b x b x b x b (N)(S)

b (N)(SD) b (N)(PD) b (S)(SD) b (S)(PD) b (SD)(PD)

(12)

41

Chapter 4: Results and Discussion

4.1 Introduction

Friction Stir Welding of AA1100 was simulated using finite element method. A three

dimension finite element model was built using the finite element package ANSYS. The

element used was solid 70, a three dimensional eight noded brick element. The element

had temperature as the degree of freedom. In this work the material flow has been

neglected as a whole and only the heat transfer aspect has been considered. The tool is

represented as a moving heat flux and the clamps are not considered as the heat loss to

the clamp is negligible due to their small area of contact and location. To reduce the

computational requirements, regions around the weld line are meshed with smaller

element, while the element size farther from the weld line is larger. Convection

coefficient for all the surfaces exposed to atmosphere is 25 W/m2 K. The heat loss to the

backing plate, which is essentially contact conduction, is accounted by using an

equivalent convection coefficient at the bottom of the plates. Temperature dependent

material properties are used for the plates as these give reasonable prediction for transient

temperature fields. The contour plots of temperature distribution in the plates when the

tool has traversed 50 mm from the start of welding are shown in figure 4.1 and Figure

4.2. The tool shoulder was 18 mm and the pin diameter was 6 mm. The tool rotation

speed was 750 rpm and the welding speed was 90 mm/min. A constant dwell time of 30

seconds was used for all the simulations.

42

Figure 4.1 Temperature contour for Trial 5

Figure 4.2 Temperature contour (cross section) for Trial 5

To study the effect of process parameters on the temperature distribution, the simulation

trials have been tried for different process parameters as given below. The corresponding

maximum temperatures are listed in table 4.2.

43

Table 4.1 Parameters used and temperatures

S.No TRS WS TRS WS Temp

1 -1 -1 750 60 455

2 -1 1 1250 60 540

3 1 -1 750 120 381

4 1 1 1250 120 470

5 -1 0 750 90 396

6 1 0 1250 90 482

7 0 -1 1000 60 473

8 0 1 1000 120 401

9 0 0 1000 90 414

10 0 0 1000 90 414

11 0 0 1000 90 414

12 0 0 1000 90 414

13 0 0 1000 90 414

To study the effect of parameters trials were made as per central composite design for

three variables. The design was a face cantered one and had three levels as listed above in

table 4.1.The simulations were run using a computer with Intel core i3 processor and 8

GB Ram. The results are also given in the table 4.1. Response surface methodology was

used to study the effect of process parameters on the temperature during friction stir

welding. The statistical modelling software MINITAB was used for building the response

surface model. The regression model for the temperature is given below.

Peak Temperature = 961.701 -0.630943*TRS-0.88506*WS+0.000396138*(TRS2) +

0.0252874*(WS*WS) +0.000133333333*(TRS*WS)

44

The results of t-test for the estimated Regression Coefficients for Temp are given below

in Table 4.2. The Analysis of Variance for model conducted at 95% Confidence level is

shown in table 4.3. The ANOVA results show that all the regression coefficients

contribute to the model. The normal probability plot and variation of residual are shown

in Figure 4.7 and Figure 4.8 respectively.

Table 4.2: t-test on Coefficients

Term Coef SE T P

Constant 414.069 0.1116 3709.958 0.00001

TRS 43.333 0.1097 394.894 0.00001

WS -36.000 0.1097 -328.065 0.0001

TRS*TRS 24.759 0.1617 153.078 0.0003

WS*WS 22.759 0.1617 140.712 0.00021

TRS*WS 1.000 0.1344 7.441 0.0004

Table 4.3 Analysis of Variance for Temp

Source DF Seq SS Adj SS Adj MS F P

Regression 5 24087.5 24087.5 4817.5 66678.56 0.000

Linear 2 19042.7 19042.7 9521.3 131783.9

1

0.000

TRS 1 11266.7 11266.7 11266.

7

155940.9

1

0.000

WS 1 7776.0 7776.0 7776.0 107626.9

1

0.000

Square 2 5040.8 5040.8 2520.4 34884.82 0.000

TRS*TRS 1 3610.3 1693.0 1693.0 23432.91 0.000

WS*WS 1 1430.5 1430.5 1430.5 19800.00 0.000

45

Interaction 1 4.0 4.0 4.0 55.36 0.000

TRS*WS 1 4.0 4.0 4.0 55.36 0.000

Residual Error 7 0.5 0.5 0.1

Lack-of-Fit 3 0.5 0.5 0.2

Pure Error 4 0.0 0.0 0.0

Total 12 24088

S = 0.268793 PRESS = 4.56900 R-Sq = 99.23% R-Sq(pred) = 99.19% R-Sq(adj) =

99.56%

4.2 Effect of Welding Speed on Temperature

The effect of welding speed on the temperature is shown in the Figure 4.3. The shoulder

diameter is kept constant at 18 mm and the dwell time was 30 seconds. The peak

temperature in all the cases was found to be lower than the melting point of the base

material. For a constant Tool rotation speed of 750 rpm, when the welding speed is

increased from 60 mm/min to 90 mm/min the maximum temperature decreases from

455°C to 396°C. When the welding speed is further decreased to 120 mm/min at a tool

rotation speed of 750 rpm the temperature decreases further to 381°C. The same trend of

temperature variation with welding speed is observed for other tool rotation speeds also.

When the welding speed is very high, the weld area is subjected to more heat generation

and faster cooling, resulting in the temperature drop.

1209060

560

520

480

440

400

WS

Mea

n of

Tem

p

750

1000

1250

TRS

Line Plot of Mean( Temp )

Figure 4.3: Effect of Welding speed on Maximum temperature

46

4.3 Effect of Tool Rotation Speed on Temperature

The effect of tool rotation speed on the temperature is shown in the Figure 4.4. The

operating conditions and tool dimensions remain the same. The peak temperature

was found to be lower than the melting point of the base material. The temperature is

found to increase when the tool rotation speed in increased. For a constant welding

speed of 60 m/min, when the tool rotation speed is increased from 750 rpm to 1000 rpm

the maximum temperature increases from 455°C to 473°C. When the tool rotation speed

is further increased to 1250 rpm at a welding speed of 60 mm/min the temperature

increases creases further to 540°C. The same trend of temperature variation with tool

rotation speed is observed for other welding speeds also. When the tool rotation speed is

very high, the frictional heat generation is more and hence the maximum temperature

increases. It is to be noted that at higher tool rotation speeds may increase the maximum

temperature resulting in a lower force on tool but the same time may result in flash

defects also.

12501000750

560

520

480

440

400

TRS

Me

an

of

Te

mp

60

90

120

WS

Line Plot of Mean( Temp )

Figure 4.4 Effect of Tool rotation speed on Maximum temperature

47

4.5 Interaction effect of tool rotation and welding speed

The surface plot and contour plot showing the variation of temperature with

tool rotation speed and welding speed is shown in figure 4.5. In general better friction stir

welded joints can be obtained when the peak temperature is between 70- 90 % of the

melting temperature of the base material. Analysis of the contour plots reveal that in

general when tool rotation is increased the corresponding welding sped is to be increased

for realising higher temperatures. For example consider Figure 4.5, if higher welding

speeds are desired for production runs, the tool rotation speed has to be increased for an

optimum temperature (say 80% of melting temperature).

TRS

WS

120011001000900800

120

110

100

90

80

70

60

>

–

–

–

–

–

< 400

400 425

425 450

450 475

475 500

500 525

525

Temp

Contour Plot of Temp vs WS, TRS

Figure 4.5 Contour Plot of Temperature

48

120

100400

450

80

500

550

8001000 60

1200

Temp

WS

TRS

Surface Plot of Temp vs WS, TRS

Figure 4.6 Surface plot of temperature

3210-1-2-3

99

95

90

80

70

60

50

40

30

20

10

5

1

Standardized Residual

Pe

rce

nt

Normal Probability Plot(response is Temp)

Figure 4.7 Normal Probability plot of residual

49

13121110987654321

3

2

1

0

-1

-2

Observation Order

Sta

nd

ard

ize

d R

esid

ua

l

Versus Order(response is Temp)

Figure 4.8 Variation of Residual with run order

50

CHAPTER 5: CONCLUSION

Friction stir welding with huge potential for varied applications promising technique has

attracted lot of research work. The process parameters play an important role in deciding

the heat generation, temperature distribution, material flow, mechanical properties and

hence the final properties and strength of the joint. In this work a three dimensional finite

element model has been used to predict the effect of tool rotation speed and welding

speed on the temperature distribution. The temperature distribution was found to vary

with both the parameters. The maximum temperature was found to increase with increase

in tool rotation speed and it was found to decrease with welding speed.

The work can be extended by including the effects of shoulder diameter, pin

diameter, dwell time, backing plate and its material. The simulation of dissimilar friction

stir welding will be a challenging and interesting task to perform. Also fully coupled

thermo mechanical models using arbitrary Lagrangean Eulerian approach will provide

better results on material flow. The effect of parameters on material flow will provide

insight into local conditions around tool during friction stir welding which is tedious to

visualise using experimental methods.

51

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