i

Mechanical properties of friction stir processed AA5754 sheet

metal at different elevated temperature and strain rates

M.Tech Thesis submitted to

Indian Institute of Technology Kharagpur

In Partial fulfillment for the award of the degree

Of

Master of Technology

in

Manufacturing Science and Engineering

Submitted By

Mr. Saurabh Suman

11ME31019

Under the guidance of

Dr. S. K. Panda Prof. S. K. Pal

Department of Mechanical Engineering Department of Mechanical Engineering IIT Kharagpur, India IIT Kharagpur, India

DEPARTMENT OF MECHANICAL ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR

JUNE 2016

ii

Department of Mechanical Engineering Indian Institute of Technology

Kharagpur - 721302

CERTIFICATE OF EXAMINATION

This is to certify that we have examined the thesis entitled “Mechanical properties of friction stir

processed AA5754 sheet metal at different elevated temperature and strain rates” submitted

by Saurabh Suman (Roll no. 11ME31019), a dual degree student of Mechanical Engineering with

specialization in Manufacturing Science and Engineering. We hereby accord our approval of it as a

study carried out and presented in a manner required for its acceptance in partial fulfilment for the

degree of Master of Technology. This approval does not necessarily endorse or accept every statement

made, opinion expressed or conclusion drawn as recorded in this thesis. It only signifies the

acceptance of the thesis for the purpose for which it is submitted.

Prof. S.K Pal

Department of Mechanical Engineering

Indian Institute of Technology, Kharagpur

(Supervisor)

Dr. S.K Panda

Department of Mechanical Engineering

Indian Institute of Technology, Kharagpur

(Supervisor)

(External examiner)

Dr. P. Saha

(Course-Coordinator)

Date:

iii

Department of Mechanical Engineering Indian Institute of Technology

Kharagpur - 721302

CERTIFICATE

This is to certify that the thesis entitled “Mechanical properties of friction stir processed AA5754

sheet metal at different elevated temperature and strain rates” submitted by Saurabh Suman

(Roll no. 11ME31019) is a record of bona fide research work carried out by him under my supervision

for the partial fulfilment of the requirements for the degree of Master of Technology in

Manufacturing Science and Engineering during the academic session 2015-16, in the Department of

Mechanical Engineering, Indian Institute of Technology Kharagpur.

Prof. S. K Pal

Department of Mechanical Engineering

Indian Institute of Technology, Kharagpur

(Supervisor)

Dr. S. K Panda

Department of Mechanical Engineering

Indian Institute of Technology, Kharagpur

(Supervisor)

iv

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 supervisors.

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.

Saurabh Suman

v

ACKNOWLEDGEMENT

I would like to express my deepest gratitude to my supervisors Dr. Sushanta Kumar Panda and

Prof. Surjya Kanta Pal who have supported me throughout my thesis with their patience and

knowledge and gave me opportunity to work on an interesting and rewarding project. Their

support, stimulating suggestions and encouragement helped me in all the time of work and

writing of this thesis. I attribute the level of my Master’s degree to encouragement and effort

and without them this thesis, too, would not have been completed or written.. It was a great

experience working under them in the cordial environment. One simply could not wish for a

better or friendlier supervisor. The experience with Prof. Panda at IIT Kharagpur is something

that I will always cherish as it has helped me to grow professionally and intellectually.

I am highly grateful to research scholars Sudhy S. Panicker, Shamik Basak, Lin Prakash, Raju

Prasad mahto, Sajun Prasad and Kanchan kumari of IIT Kharagpur, India for their valuable

suggestion and co-operation throughout the work.

I would also like to thank my faculty advisor Dr. Partha Saha and Head of Department

Prof. Prasanta Kumar Das for the extensive support and help they have offered to me till

date at IIT Kharagpur. I am thankful to Prof. Asimava Roy Chowdhury, Department of

Mechanical Engineering for the polishing, eatching and mounting facility at EDM lab, IIT

Kharagpur

I am thankful to Dr. Alok Kumar Nandi and Mr. Chandan Mondal of Metal Forming

Laboratory, IIT Kharagpur, India for encouraging and helping me in a doubtful situation.

Finally, I am also thankful to all those who directly or indirectly helped me for completion of

this project work.

I sincerely appreciate my colleagues Shishir Dhara, Bijoy Rajak, Md Irshad Ansari and Ravi

Kumar for all their help, support, interest, valuable hints and enjoyable friendship. Finally, I

would like to thank my parents, friends and batchmates, from the bottom of my heart, for their

blessings, constant moral support and motivation they provided till date which has helped in

achieving my targets.

Saurabh Suman

vi

Contents Abstract ................................................................................................................................................... 1

Chapter 1 ................................................................................................................................................. 2

1 Introduction ..................................................................................................................................... 2

1.1 Friction Stir Welding .............................................................................................................. 5

1.1.1 Different weld regions in FSW joint ............................................................................... 6

1.1.2 Terminology in FSW ...................................................................................................... 7

1.1.3 Friction stir welding (FSW) parameters .......................................................................... 8

1.2 Friction stir processing ............................................................................................................ 9

1.2.1 Friction stir processing for superplastic forming .......................................................... 11

1.2.2 Friction stir processing (FSP) - Casting modification ................................................... 13

1.2.3 Friction stir processing (FSP) - Microforming .............................................................. 14

1.2.4 Friction stir processing (FSP) - Powder processing ...................................................... 14

1.2.5 Friction stir processing - Channeling ............................................................................ 15

1.2.6 Friction stir processing - Enhanced low-temperature formability ................................ 15

1.3 Applications of AA5754 in auto-body structures and challenges: ........................................ 15

Chapter 2 ............................................................................................................................................... 17

2 Review of related literature ........................................................................................................... 17

2.1 Friction stir welding a brief review ....................................................................................... 17

2.2 Recent research on friction stir processing (FSP) ................................................................. 18

2.2.1 Influence of temperature and super-plasticity in FSP ................................................... 19

2.2.2 Influence of tool pin profile and shoulder diameter on FSP ......................................... 20

2.2.3 Influence of strain rate on friction stir processed material ............................................ 21

2.3 Application of Johnson Cook model ..................................................................................... 22

Chapter 3 ............................................................................................................................................... 24

3 Objectives of present study ........................................................................................................... 24

Chapter 4 ............................................................................................................................................... 25

4 Methodology ................................................................................................................................. 25

4.1 Selection of sheet material .................................................................................................... 25

4.2 Friction stir processing of thin sheets ................................................................................... 26

vii

4.2.1 Tool design .................................................................................................................... 26

4.2.2 Process parameters ........................................................................................................ 30

4.3 Microstructure of stir zone : .................................................................................................. 31

4.4 The tool design and experimental procedure ........................................................................ 33

4.5 Tensile testing at different elevated temperature and strain rate ........................................... 36

4.6 Formulation using Johnson Cook (JC) model ....................................................................... 37

Chapter 5 ............................................................................................................................................... 40

5 Results and discussions ................................................................................................................. 40

5.1 Tensile testing results ............................................................................................................ 40

5.1.1 Effect of temperature and strain rate on Engg stress strain response of base material…40

5.1.2 Effect of temperature and strain rate on Engg stress strain response of FSP material…42

5.1.3 Effect of temperature and strain rate on mechanical properties:................................ 44

5.1.4 Effect of strain rate and temperature on true stress and true strain response ................ 46

5.2 Prediction of Johnson Cook model ....................................................................................... 48

5.2.1 Evaluation of material constants of Johnson Cook model ............................................ 48

5.2.2 Experimental vs predicted Stress for parent material.................................................... 48

5.2.3 Experimental vs predicted Stress for FSPed AA5754 .................................................. 51

5.3 Fractography ......................................................................................................................... 53

Chapter 6 ............................................................................................................................................... 56

6 Conclusions ................................................................................................................................... 56

7 References ..................................................................................................................................... 57

viii

LIST OF FIGURES

Figure 1: Aluminum body and structural component growth with year [1] ........................................... 2

Figure 2: Aluminum alloy applications in passenger cars [3] .................................................................. 3

Figure 3: Friction stir welding process taking place [5] ........................................................................... 5

Figure 4: Metallurgical processing zones developed during friction stir Joining [5] ............................... 6

Figure 5 : Various microstructural regions in the transverse cross section of a FSW [11] ...................... 7

Figure 6: Schematic of friction stir processing [16] ................................................................................. 9

Figure 7: An illustration of the evolution of microstructural features because of the basic friction stir

process attributes, and its linkage to various emerging friction stir processing technologies [12] ...... 10

Figure 8: A layout depicting the materials science and engineering chain along with five commonly

used design approaches. The design approaches are linked with key materials properties [17] ......... 11

Figure 9: Variation of elongation with (a) strain rate, and (b) temperature, showing high strain rate

super-plasticity [18] .............................................................................................................................. 11

Figure 10: Superplastic elongation non-conventional aluminum alloys with very fine thermally stable

particles [19] ......................................................................................................................................... 12

Figure 11: A comparison of as-cast (a, b, c) and friction stir processed (d, e, f) microstructures of ..... 13

Figure 12: Inner door panels of automobiles made of AA5754 [20] .................................................... 16

Figure 13: True stress-true stain curves of T24steel at different temperatures with strain rate of

(a)0.01s−1 ; (b)0.1s−1; (c)1s−1 and (d)10s−1 [40] ................................................................................ 23

Figure 14: Variation of sliding torque, sticking torque and total torque with shoulder diameter [13] 30

Figure 15 FSP at 1200 RPM and 150mm/min weld velocity ................................................................. 30

Figure 16: Abrasive cutter ..................................................................................................................... 31

Figure 17: polishing machine ................................................................................................................ 31

Figure 18: (a) Finally polished mounted surface (b) Setup for diamond polishing ............................. 32

Figure 19 Stir zone depth and width is clearly visible ........................................................................... 32

Figure 20: Microstructures of the SZ(Stir Zone) observed on AA5754 aluminum alloy ....................... 33

Figure 21 Schematic representation of FSP (all dimensions are in mm) (a) Isometric view of FSP ...... 33

Figure 22 (a) Tool used for friction stir Processing (b) Tool schematic diagram ................................... 34

Figure 23: Friction stir processing machine........................................................................................... 35

Figure 24: During friction stir processing .............................................................................................. 35

Figure 25 Dimension of Tensile specimen (all dimension all in mm)..................................................... 36

Figure 26: The die for cutting tensile specimen .................................................................................... 36

Figure 27 (a)UT-04-0050 ELECTRA 50 Hot Forming machine (b)Tensile specimen shown in hot forming

machine ................................................................................................................................................. 37

Figure 28 Effect of Temperature and strain rate on engineering stress-strain response at 1mm/min

cross head velocity rate......................................................................................................................... 40

Figure 29 Effect of Temperature and strain rate on engineering stress-strain response at 100mm/min

crosshead velocity rate. ........................................................................................................................ 41

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Figure 30: Effect of Temperature and strain rate on engineering stress-strain response at 200mm/min

crosshead velocity rate ......................................................................................................................... 41

Figure 31: Effect of Temperature and strain rate on engineering stress-strain response at 1mm/min

crosshead velocity rate: ........................................................................................................................ 42

Figure 32: Effect of Temperature and strain rate on engineering stress-strain response at 100mm/min

crosshead velocity rate ......................................................................................................................... 43

Figure 33: Effect of Temperature and strain rate on engineering stress-strain response rate at

100mm/min crosshead velocity rate .................................................................................................... 43

Figure 34: A figurative comparison of (a) Base sample at temperature 400°C before and after tensile

failure (b) FSP sample at temperature 400°C before and after tensile failure ...................................... 44

Figure 35: A figurative comparison of (a) Base sample at room temperature before and after tensile

failure (b) FSP sample at room temperature before and after tensile failure ...................................... 45

Figure 36: Effect of temperature and strain rate on FSPAA5754 and base AA5754 (Room Temperature):

true stress-strain response .................................................................................................................... 47

Figure 37: Effect of temperature and strain rate on FSPAA5754 and base AA5754 (300°C): engineering

stress-strain response ........................................................................................................................... 47

Figure 38: Comparison between experimental flow stress and predicted flow stress using Johnson Cook

model in temperature domain 293 K–673K of base metal for elongation rate of 200mm/min ........... 49

Figure 39: Comparison between experimental flow stress and predicted flow stress using Johnson Cook

model in temperature domain 293 K–673K of base metal for elongation rate of 100mm/min ........... 49

Figure 40: Comparison between experimental flow stress and predicted flow stress using Johnson Cook

model in temperature domain 293 K–673K of base metal for elongation rate of 1mm/min ............... 50

Figure: 41 Experimental stress vs Predicted stress for base AA5754 .................................................... 50

Figure 42 Comparison between experimental flow stress and predicted flow stress using Johnson Cook

model in temperature domain 293 K–673K of FSP for elongation rate of 200mm/min ....................... 51

Figure 43: Comparison between experimental flow stress and predicted flow stress using Johnson Cook

model in temperature domain 293 K–673K of FSP for elongation rate of 100mm/min ....................... 52

Figure 44: Comparison between experimental flow stress and predicted flow stress using Johnson Cook

model in temperature domain 293 K–673K of FSP for elongation rate of 1mm/min ........................... 52

Figure 45: Experimental stress vs Predicted stress for friction stir Processed AA5754 ......................... 53

Figure 46: Parent material fractured at 100 cross head velocity and room temp (a) 1000x (b) 2000x 54

Figure 47: Parent material fractured at 100 cross head velocity and 400°C (a) 1000x (b) 2000x ........ 54

Figure 48: FSP tensile sample fractured at 100 cross head velocity and 20°C(a) 1000X (b) 2000X ...... 55

Figure 49 FSP tensile sample fractured at 100 cross head velocity and 400°C (a) 1000X (b) 2000X .... 55

x

LIST OF TABLES

Table 1: Different grade of aluminium alloys and their use [4] .............................................................. 4

Table 2: Chemical composition of AA5754 H-22 alloy [42] .................................................................. 25

Table 3Chemical composition of Stainless steel 316 [43] ..................................................................... 26

Table 4 Mechanical properties of Stainless Steel 316[44] .................................................................... 27

Table 5 pin length variation effect on weld quality .............................................................................. 29

Table 6 Results showing mechanical properties of base as well as FSP AA5754 .................................. 45

Table 7: Johnson Cook model parameter value for base material ....................................................... 48

Table 8: Johnson Cook model parameter value for FSPed material ..................................................... 48

1

Abstract

Recently, the increase in the cost of fuel and level of air pollution have emphasized the use of

aluminum in automobile industry. However, it is very challenging to deform this material to a

critical auto body shape due to its limited formability at room temperature. It was found from

literature survey that formability of most aluminum alloys after friction stir processing

increased significantly after 200°C. In the present work, automotive grade aluminum alloy

AA5754 of 1.5mm thickness was chosen due to its encouraging level of strength to weight ratio

with significant corrosion resistance to saline water. The friction stir processing (FSP) of this

alloy was carried out experimentally in order to improve the mechanical properties. Further the

influence of temperature and strain rate on mechanical properties of FSP sheets were studied.

Tensile tests of friction stir processed (FSPed) as well as Base AA5754 were conducted in

different elevated temperature and strain rates to investigate the effect on yield strength,

ultimate tensile strength and total elongation. It was found that %elongation increased

significantly at elevated temperature. A relationship between mechanical properties of friction

stir processed (FSPed) alloy and Base AA5754 were studied using graphs and tables. The total

elongation in the material increased significantly with rise in temperature, however the strength

decreased significantly. It was found that stress-strain response of this strain rate insensitive

material at room temperature was very sensitive at elevated temperature. For base AA5754 at

400°C, the %elongation was more than 100% at lower strain rates. A Johnson Cook model

incorporating the effect of strain rate, temperature, strain hardening, strain and plastic strain

has been developed to evaluate flow stress theoretically. Fractography of tensile specimen

suggested ductile failure.

Keywords – Friction stir processing (FSP); friction stir welding (FSW); Johnson cook model

(JC model); AA5754-H22; Uniaxial tensile testing (elevated temperature).

2

Chapter 1

1 Introduction

In the past few years, use of aluminums in automobile industry is emphasized because of the

increase in the cost of fuel, the risk of petroleum scarcity and increasing level of air pollution.

It is estimated that a 10% reduction in vehicle weight improves the fuel efficiency by 5.5% [1].

The latest developments of full aluminum car bodies indicate the use of extruded aluminum

tubes in frame structures, aluminum sheet for inner and outer panels. Weight reductions of 50%

have been achieved in automobiles by the substitution of steel by aluminum [1].

In the present, several types of aluminum alloys are used in automobile industry and to

represent each alloy, four digit numbers are used. The major alloying element for each type is

indicated by the first digit, i.e., in 1XXX, ‘1’ indicates aluminum of 99.00% minimum so no

major alloying element in this; in 2XXX, ‘2’ indicates that copper is the main alloying element.

Manganese for 3XXX, silicon for 4XXX, magnesium for 5XXX, magnesium and silicon for

6XXX, zinc for 7XXX. For 8XXXseries in few alloys, iron and silicon are major alloying

element like in 8017 and in some lithium is main alloying element. For 9XXX series, till now

no particular major alloying element is suggested. Figure 1 interprets increasing demand of

Figure 1: Aluminum body and structural component growth with year [1]

3

aluminum in various sectors of automobile with year. It can be observed from Figure 1 that

percentage of aluminum increases about 300% from year 2012 to 2015 in body and bumper of

vehicle. Figure 2 depicts applications of aluminum alloys in fabrications of distinct parts in a

commercial car. Table 1 shows properties and application of different grade of aluminium

alloys. As seen from the figure 2XXX and 7XXX alloy series is widely used in aircraft

industries. The temper designation is also used to show post processing of the aluminium

alloys. It follows the cast or wrought designation number with a dash, a letter, and potentially

a one to three digit number, e.g. 6061-T6. Most commonly used tempers designations are F-

as fabricated, H- strain hardened(cold worked) with or without thermal treatment, T- heat

treated to produce stable tempers, O- full soft (annealed) and W- solution heat treated only.[2]

Figure 2: Aluminum alloy applications in passenger cars [3]

In present work, automotive grade aluminum alloy AA5754 was chosen because of its high

strength to weight ratio and corrosion resistance property especially to saline water. However

it has limited formability at room temperature compare to automotive grade steels which limit

its applications in automobile sector but it is widely used for automotive inner body panels [3]

4

Table 1: Different grade of aluminium alloys and their use [4]

Alloy

series

Alloy Properties Application

1XXX Pure Low strength

,Excellent thermal

/electrical

conduction and

corrosion

resistance ,highly

reflective

Fuel filters, electrical conductors, lighting

reflectors, decorative component

2XXX(age

hardening)

Cu High strength,

relatively good

corrosion

resistance ,good

elevated

temperature

strength

Aircraft skin ,aircraft fitting and wheel ,

ballistic armour, forged and machined

component

3XXX Mn Medium strength,

good formability,

good corrosion

resistance

Storage tank, beverage cane, home appliances,

heat exchangers, pressure vessels, sidling,

gutters

4XXX Si High cast-ability,

high fluidity ,low

ductility, high

machinability

Variety of casting, filler material

5XXX Mg Medium strength

,good formability

,excellent marine

corrosion

resistance

Interior automotive, appliance trim,

Armor plate, marine

and cryogenic component,

6XXX(age

hardening)

Mg+Si Med-high strength

,good corrosion

resistance ,easily

extruded

Exterior automotive, automotive profile,

railcars, piping, marine, screw stock, doors and

windows

7XXX(age

hardening)

Zn Very high strength

,prone to

corrosion

Aircraft construction, truck trailers,

railcars, armor plate, ski holes, tennis racket

5

1.1 Friction Stir Welding

Friction stir welding was patented in 1991 by TWI [5] and is a relatively new manufacturing

technique for joining metals and plastics. This process does not require melting or filler

material unlike fusion welding processes. The basic concept of FSW is remarkably simple. A

non-consumable rotating tool with a specially designed pin and shoulder is inserted into the

abutting edges of sheets or plates to be joined and traversed along the line of joint as shown in

Figure 3. The tool serves two primary functions: (a) heating of workpiece, and (b) movement

of material to produce the joint. The heating is accomplished by friction between the tool and

the workpiece and plastic deformation of workpiece. The localized heating softens the material

around the pin and combination of tool rotation and translation leads to movement of material

from the front of the pin to the back of the pin. As a result of this process a joint is produced in

‘solid state’. Because of various geometrical features of the tool, the material movement around

the pin can be quite complex [6]. During FSW process, the material undergoes intense plastic

deformation at elevated temperature, resulting in generation of fine and equiaxed recrystallized

grains [7-10]. The fine microstructure in friction stir welds produces good mechanical

properties.

Figure 3: Friction stir welding process taking place [5]

Friction stir welding (FSW) can be accurately described as a forging and extrusion or metal

working process. In the process, a cylindrical tool, composed of a pin and shoulder similar to

that shown in Figure 4, is rotated and slowly plunged into the joint line of the materials to be

joined. The pin tool generates heat through friction and plastic strain energy release during

mechanical deformation of the workpiece, which softens the material to be welded. Once the

6

shoulder of the tool is in contact with the material, it is generally hot enough to reach the plastic

region, and the tool begins to traverse along the joint line. The material in front of the tool is

then extruded around the pin where it is deposited behind the pin and forged into a solid-state

joint. As shown, the tool follows the joint line, taking the material from in front of the tool, and

mechanically mixes it together to form a joint. It is important that sufficient down force is

applied to maintain shoulder contact with the material, since the shoulder contact is a critical

component of the forging action that happens behind the tool. The majority of the material flow

in these joints is longitudinal with the weld; however, vertical material flow can also take place

under “hot” processing conditions (slow feed rate with high spindle rotation speed and is aided

through different pin tool geometries such as the addition of threads to the pin.

Figure 4: Metallurgical processing zones developed during friction stir Joining [5]

1.1.1 Different weld regions in FSW joint

Friction stir welding has important effects on the microstructure of the parent material. The

microstructure of a FSW is separated into three principal zones, as shown in Figure 5. These

zones are commonly known as the weld nugget or dynamically recrystallized zone (DXZ), the

thermo-mechanically affected zone (TMAZ), and the heat-affected zone (HAZ). Each zone

exhibits a distinct microstructure. The weld nugget is comparable to the pin diameter in size,

with a microstructure that is composed of equi-axed, dynamically recrystallized grains. The

7

sizes of these grains are substantially smaller than the grains in the parent material, usually less

than 10 micro meter. The grain size of the TMAZ remains similar to that of the parent material,

but the grain orientation is altered by partial mechanical deformation. In addition, the TMAZ

may contain some areas of partial recrystallization. The TMAZ also experiences a coarsening

of precipitates at the grain boundaries, due to the high temperatures experienced, accompanied

by the formation of precipitate free zones (PFZ). At the HAZ/TMAZ boundary, variable grain

size, break up of inter-metallic particles and over-aging cause a loss in hardness. This area of

the weld has been noted in numerous works as the weak point, at which the minimum hardness

and fracture initiation usually occurs. Continued research in the microstructural

characterization of FSW will lead to a better understanding of the process. By understanding

how the microstructure is affected by FSW, many of the current observed FSW anomalies

could be explained, or even controlled.

Figure 5 : Various microstructural regions in the transverse cross section of a friction stir welded material [11]

1.1.2 Terminology in FSW

Various terminologies used in FSW are as follows :

i. Advancing side (AS): It is the side of the plate where tool rotation direction is same as

tool travel direction.

ii. Retreating side (RS): It is the side of the plate where tool rotation direction is opposite

to tool travel direction.

iii. Tool shoulder: Part of the tool which comes in contact with top surface of the workpiece

and is mainly responsible for the amount of heat generation and mixing of the material

at the top surface .

iv. Tool pin: Part of the tool that is impinged along the thickness of the plate and is

responsible for mixing of material in thickness direction.

v. Spindle tilt angle: Angle between the normal to the workpiece top surface and the axis

of the tool. It reduces the amount of flash formation. Generally ranges from 1⁰ to 4⁰.

8

vi. Plunge depth: Distance by which the tool shoulder has impinged the workpiece top

surface.Generally ranges from 0 to 0.4 mm

1.1.3 Friction stir welding (FSW) parameters

There are various parameters on which the quality of weld depends. Proper understanding of

process parameters is necessary to understand their effects on weld quality. Some of the

important process parameters are as follow:

1. Rotational speed of the tool (rpm): Amount of heat generated depends upon tool rotational

speed. Higher the speed, higher is the heat generation and vice versa

2. Welding speed or transverse speed (mm/min): It is responsible for the proper mixing of

material. It should be optimum as lower welding speed reduces the productivity and higher

welding speed might lead to the defect formation due to less time available for heat

generation and material flow.

3. Tool geometry:

(a) Pin profile: Pin helps to stir the material around the tool along the thickness direction. Pin

geometry may be cylindrical, square, tapered, threaded etc.[12]

(b) Tool shoulder: Shoulder is responsible for majority of heat generation on top and sub-

surface through friction and deformation of the material. The shoulder region which may be

concave, convex or flat povides downward forging action necessary for weld

consolidation[12]

(c) Pin diameter, d (mm): Pin diameter is responsible for stir zone, more is the pin diameter

more is the stir zone.

(d) D/d ratio of tool: Here D is the diameter of tool shoulder and d is the pin diameter. Most

common D/d ration is 3([13]).

(e) Pin length (mm): Pin length is responsible for depth of stir zone. If pin length is less

welding will not be proper and if pin length is more, then tool will stick with the work surface

and may penetrate or bend the workpiece.

(f) Tool inclination angle. Tool tilt angle increases the forging pressure and thus helps in high

temperature generation and proper consolidation of the stirring materials. It also reduces the

flash formation.

9

1.2 Friction stir processing

Recently friction stir processing (FSP) was developed by Mishra et al. ([14], [15]) as a generic

tool for microstructural modification based on the basic principles of FSW. In this case, a

rotating tool is inserted in a monolithic workpiece for localized microstructural modification

for specific property enhancement. The intrinsic nature of friction stir process has two basic

components material flow and microstructural evolution. The development of friction stir

processing as a generic metallurgical tool for microstructural modification and a broader

manufacturing technology is connected to these. Even though the adaption of these friction stir

process based technological variants is slow, the potential of these is limitless. Figure 6 shows

schematic of friction stir processing. The processed metal is subjected to high strain that

modifies its dendrite (grain) pattern. The dendrites are smaller and more round in the nugget

zone than in the undeformed regions [5].

Figure 6: Schematic of friction stir processing [16]

Figure 7 serves as basic illustration for understanding application of friction Stir Processing

(FSP). Material flow can be treated as main aspect of all FSP attributes other than temperature.

As shown in figure it has wide demand in superplasticity, room temperature formabilty, casting

modification, surface modification and powder processing. There is one drawback also shown

in figure as wormholes due to FSP.

10

Figure 8 shows this broad framework including dynamic and corrosion properties. A new

keyword that is included is ‘unintended microstructure. This figure tries to emphasize on

achieving design goals (means achieving required strength, toughness, ductility etc) with the

help of FSP. For example suppose we wanted some specific mechanical properties for an alloy

but due to some reasons (for example not able to give tight tolerance, impurity) unintended

microstructure has been created. Number of engineering failures are a result of these

unintended microstructure or microstructural flaws. Friction stir processing can be used to

modify these microstructural features, particularly the regions where finite element modeling

tools show higher vulnerability.

Figure 7: An illustration of the evolution of microstructural features because of the basic friction stir process attributes, and its linkage to various emerging friction stir processing technologies [12]

The following unique features of friction stirring can be utilized to develop new processes :

Low amount of heat generated,

Extensive plastic flow of material,

Very fine grain size in the stirred region,

Random misorientation of grain boundaries in stirred region,

Mechanical mixing of the surface layer,

Large forging pressure, and

Controlled flow of material

11

Figure 8: A layout depicting the materials science and engineering chain along with five commonly

used design approaches. The design approaches are linked with key materials properties [17]

1.2.1 Friction stir processing for superplastic forming

(a) (b)

Figure 9: Variation of elongation with (a) strain rate, and (b) temperature, showing high strain rate super-plasticity [18]

Superplasticity is an ability of a material to exhibit >200 % elongation in tension. Historically,

a key aspect of the superplastic materials is also the low flow stress. In fact, the original

development of superplastic forming in 1960s by Backofen et al. [18] (1964) attracted more

attention because it was gas forming of a metallic component. This gives an impression of ease

of forming because one can conceptually visualize this as equivalent to glass blowing. Of

course, once the formed metallic component cools down to room temperature, its properties

12

are that of any structural metallic material. Superplastic forming grew rapidly in 1970s and

1980s, because of its ability to create ‘unitized’ components. Unitized components lower the

number of parts that is used for a system. Aerospace industries were among the first adopters

in spite of a major drawback of slow forming rate (strain rate of 10−4–10−3𝑠−1). Last 20 years

or so there has been a large increase in efforts related to high strain rate super-plasticity (strain

rate >10−2𝑠−1).

Figure 9 captures the variation of elongation with strain rate and temperature for these alloys.

A remarkable part of this data set is the high strain rate range and wider temperature range over

the superplasticity is observed. The comparison with as-rolled 6.3 mm thick rolled Al7075

alloy sheet is instructive in lack of superplasticity in sheets that are thicker and not particularly

processed for superplasticity. So, fundamentally it highlights two limitations of conventional

superplasticity; lower strain rate and thinner sheets. In the plot against temperature, a region of

abnormal grain growth is marked. This sets the upper limit for superplasticity in friction stir

processed material. The abnormal grain growth aspect is different from conventionally

processed aluminum alloys. It results from unique as-processed friction stir microstructure that

contains strain gradients and microstructural gradients that can lead to microstructural

instability at the higher end of temperature range.

Figure 10 shows results for two non conventional aluminium alloys 𝐴𝑙3𝑆𝑐 and 𝐴𝑙3𝑍𝑟 at strain

rate of 3 × 10−2𝑠−1[19].The graph shows that at 530°C maximum elongation of 810% has

taken place.

Figure 10: Superplastic elongation non-conventional aluminum alloys with very fine thermally stable particles [19]

13

1.2.2 Friction stir processing (FSP) - Casting modification

Casting is a very widely used manufacturing technique because of its unique ability to produce

complex shaped part at low cost. However, its performance is limited by many metallurgical

features, such as, dendritic porosity, particulate oxides/inclusions, secondary dendritic arm

spacing (SDAS), and iron-phase inter-metallics. FSP provides an unique opportunity to embed

‘wrought’ microstructure in ‘cast’ component by localized modification [19]. Such approach

for components requiring higher performance would lead to the best combination, low overall

cost due to casting and higher performance in localized areas due to wrought microstructure.

Figure 11 shows a comparison of cast and friction stir processed microstructure for three

commercially cast components of A356 alloy. The level of porosity and dendritic arm spacing

is different in all the specimens. Also the particles in inter-dendritic regions have different sizes

because of the practice of adding Si modifier. After friction stir processing, the obvious

microstructural changes include, elimination of porosity, refinement of particles and

homogenization of microstructure. It is important to focus on the larger particles. In as-cast

condition more than 10 % particles are above 15μm. During mechanical loading such as tensile

test or fatigue test, larger particles are the local area where failure starts.

Figure 11: A comparison of as-cast (a, b, c) and friction stir processed (d, e, f) microstructures of A356 alloy from three commercial casting. Note the dendritic arm spacing, large particles and

porosity in as-cast condition. FSP refines and homogenizes the microstructure as well close all the porosity [19]

14

1.2.3 Friction stir processing (FSP) - Microforming

The trend of producing more compact/integrated systems demands miniaturization of the

components involved. At the same time, the system should be capable of performing at par or

sometimes better than those macro-systems conventionally available. Micro-

Electromechanical-Systems (MEMS) are good example of such a push. In addition,

requirements of miniaturization can be seen in a very diverse consumer product sectors, be it

smaller and smaller cell phones and consumer electronics, to biomedical implants and tiny

cameras for medical applications, to microturbines and so on. Components employed in MEMS

and similar devices are generally made with traditional techniques such as etching,

photolithography, electroless and electrochemical deposition, and micromachining.

Conventional forming of macrocomponents, such as forging, is widely used because it can

produce large volume of components in cost-efficient manner. Microforming has been difficult

because of frictional effects associated with metal forming processing. For microcomponents

the surface area/volume ratio is large and new concepts are needed to extend forming processes

to micro-levels. Combination of FSP and superplasticity can be enabling technology for

manufacturing of metallic microcomponents by replication. The better formability of

superplastic material is quite evident. This technique will allow fabrication of

microcomponents from common engineering alloys.[12]

1.2.4 Friction stir processing (FSP) - Powder processing

Powder metallurgy is used to make alloys and composites of non-equilibrium compositions.

The processing steps often involve powder compaction and further thermomechanical

processing. For aluminum alloys, three microstructural features are very important; prior-

particle boundaries, microstructural inhomogeneity, and size of primary intermetallic particles.

Breakage of the aluminum oxide film on prior-particle boundaries by extrusion or forging is

critical for ductility, fatigue and fracture toughness. Because of the material flow pattern, some

microstructural inhomogeneity can not be eliminated in forging and extrusion. Friction stir

processing provides opportunity to homogenize microstructure for subsequent forming

operations or produce selectively reinforced regions. Because of the severe plastic deformation

associated with friction stirring, the prior-particle boundaries and any powder scale

microstructural or chemical inhomogeneity are eliminated. Using this approach P/M aluminum

alloy with ~700 MPa strength and >10% ductility has been obtained [5].

15

1.2.5 Friction stir processing - Channeling

Friction stir channeling (FSC) is a new concept to produce integral channels in metallic

materials. There are many applications where heat exchange is needed or desirable. The

conventional approaches of building heat exchangers or incorporating fluid channels can be

broadly divided in two groups. The first group would consist of use of tubes and joining

processes, whereas the second group would involve machining of channels and joining of

several pieces. FSC concept on the other hand can be used to create integral channels in a solid

plate in one step. The shape and size of the channel can be controlled by the tool design as well

as process parameters.

1.2.6 Friction stir processing - Enhanced low-temperature formability

Manufacturing of components from thick plates usually is done by joining, as bending and

shaping is difficult because of limited ductility. In as FSP condition, aluminum alloys exhibit

very high ductility. Mahoney et al. [15] have used this to bend 1” thick 2519 Al plate with just

a partial FSP layer on the tensile side. This would give designers added flexibility of shaping

sheets and plates with localized enhanced formability. With low heat input from FSP, the

region of reduced strength is very limited. In applications where some of the welded joints are

replaced by bends, such design can lead to significantly higher performance.

1.3 Applications of AA5754 in auto-body structures and challenges:

Aluminum 5754 alloy has excellent corrosion resistance especially to saline water and

industrially polluted water. This alloy has potential application in automotive industries

because of its high strength to weight ratio. It has excellent weldability, machinability and high

fatigue strength. It is found from literature survey that formability of AA5754 increases

considerably at elevated temperature. It has face centered cubic structure (FCC) with a melting

point of 600°C. It is widely used in automotive body structures such as interior body panel in

automobiles, shown in Figure 12.

However, AA-5754 has following major limitations:

It has poor formability at room temperature compared to automotive grade low carbon

steels which limits its applications in automobile sector.

Serrated stress-strain response is found at room temperature which results in stretcher marks

on the surface of the fabricated outer panel of the auto-body.

16

Figure 12: Inner door panels of automobiles made of AA5754 [20]

17

Chapter 2

2 Review of related literature

There are several literatures available on friction stir welding/processing parameters selection

and temperature effect on strength and elongation of FSP. Some of the relevant research works

have been discussed in this chapter.

2.1 Friction stir welding a brief review

According to the report of American Welding Society, the practice of friction welding has been

carried out since 1891 which is proved by the fact that first patent of the process was dispensed

in the USA in 1891[21]. In the sixties, this method was further industrialized in USA by

Caterpillar, AMF and Rockwell International. Rockwell fabricated its own set ups for friction

welding to weld spindles to lorry differential casings, AMF created machineries to weld

steering worm shafts, and Caterpillar’s developed machineries to weld turbochargers and

hydraulic cylinders.[22]

Y.J Kwon et al. [23] investigated the friction stir welding between 5052 aluminum alloy plates

with a thickness of 2 mm. The tool rotation speeds were ranging from 500 to 3000 rpm under

a constant traverse speed of 100 mm/min. Welded joints were obtained at tool rotation speed 1

000, 2000 and 3000 rpm. At 500, 1000, and 2 000 rpm onion ring structure was clearly observed

in the friction-stir-welded zone (SZ). The effect of tool rotation speed on the onion rings was

observed. Gain size in the SZ is smaller than that in the base metal and is decreased with a

decrease of the tool rotation speed. The study showed that the strength, tensile strength of the

joint is more than that of the parent metal. The investigation also demonstrated that the joint is

less ductile than the parent alloy.

J. Adamowski et al. [24] analyzed the mechanical properties and microstructural variations in

Friction Stir Welds in the AA 6082-T6 with varying process parameters. Tensile test of the

welds was done and relation among the process parameter was judged. Microstructure of the

weld interface was observed under optical microscope. Also micro hardness of the resulting

joint was measured. It was observed that test welds show resistance to increment of welding

speed, Hardness reduction was observed in weld nugget and heat affected zone (HAZ). The

reason for this occurrence was the kinetic and thermal asymmetry of the FSW process. An

initial stage of a longitudinal, volumetric defect was found at the interface of weld nugget and

18

TMAZ. The hardness was inferior to that of fusion welding. Tunnel (wormhole) defects were

found in the nugget zone.

R. Nandan et al. [25] reviewed the recent trends in FSW process, weldment structure and

properties of the resulting material at the weld joints. This study dealt with the essential

understanding of the process and its consequences in the molecular level. Other characteristics

that are studied are heat generation, heat transfer and plastic flow during welding, components

of tool design, study of defect formations and the structure and properties of the welded

materials. They described important factors that have to be optimized to reduce fracture and

improve the uniformity of weld properties so that FSW can be expanded to new engineering

fields. Principles of heat transfer, material flow, tool-work–piece contact conditions and

properties of various process parameters, efficient tools have been formulated. Uncertain

parameters of FSW like friction coefficient, the extent of slide between the tool and the work–

piece, the heat transfer coefficients for different work–piece surfaces, splitting of the heat

amongst the work–piece and the tool at the tool-work piece boundary are also counted for and

processes to optimize these parameters are discussed.

2.2 Recent research on friction stir processing (FSP)

The mechanical and wear behavior of friction stir processed A-286 steel was studied by O.O.

Tinubu & J.E.Mogonye et al. [26]. The alloy was characterized in different processed

conditions, namely as rolled (AR)+aged and FSP+aged. High frequency reciprocating sliding

wear behavior and wear mechanisms were investigated at room temperature. The Vickers

micro-hardness and wear rates were measured and compared for each processing condition. It

was determined that along with increasing micro-hardness in the stir zone, FSP resulted in

improved wear resistance. Specifically, the wear rate in the stir zone was reduced from

1×10−6 to 6×10−7 mm3/N m due to FSP. Furthermore, cross-sectional focused ion beam

microscopy studies inside the stir zone of the FSP+aged alloy determined that increased micro

hardness was due to FSP-induced microscopic grain refinement resulting in Hall–Petch

strengthening, and the corresponding wear rate decrease was due to even finer wear-induced

grain refinement.

R.S.Mishra et al. [12] have fabricated Al–SiC surface composites with different volume

fractions of particles. The thickness of the surface composite layer ranged from 50 to 200μm.

The SiC particles were uniformly distributed in the aluminum matrix. The surface composites

19

have excellent bonding with the aluminum alloy substrate. The micro-hardness of the surface

composite reinforced with 27vol % SiC of 0.7μm average particle size was ∼173 HV, almost

double of the 5083Al alloy substrate (85 HV). The solid-state processing and very fine

microstructure that results are also desirable for high performance surface composite.

C.I.Chang & C.J.Lee et al.[27] established relationship between grain size and Zener-Holloman

parameter during friction stir processing in AZ31 Mg alloys. The Zener–Holloman parameter

is utilized in rationalizing the relationship. The grain orientation distribution is also studied

using the X-ray diffraction. X-ray diffraction results show that, in the FSP dynamically

recrystallized zone, the (0 0 0 2) basal plane tends to lie on the transverse plane at lower pin

rotation speeds, and approaches to nearly random orientation at higher rotation speeds.

S. k. singh & R.J.Immanuel el al. [28] studied influence of multi-pass friction stir processing

on wear behavior and machinability of an Al-Si hypoeutectic A356 alloys. The wear behavior

of FSPed materials is characterized against metallic and abrasive medium and the machining

studies are done by drilling experiments in dry condition. Study on edge burr formation during

drilling suggests that the entry and exit burrs are minimal for 3 pass FSPed material. A detailed

investigation on the observed results is done in correlation with the microstructural evolution

and mechanical properties.

H. Jingyu et al. [29] studied influence of processing parameters on thermal field in Mg-Nd-Zn-

Zr alloy during friction stir processing. Three groups of processing parameters were applied on

as-cast Mg–Nd–Zn–Zr (NZK) plates during friction stir processing (FSP) and the thermal field

was built through thermocouple measurements. The results concluded that the processing pitch

has an influence on the formation of onion ring, the microstructure evolution, as well as the

related mechanical properties. It was seen that the highest peak temperature in FSP NZ30K is

around 600°C, a combination of Ω increment and v decrement will not arise the peak

temperature but to enlarge the heat affected area. The peak temperature in the center of stir

zone always keeps about 520°C during friction stir processing of NZ30K alloy.

2.2.1 Influence of temperature and super-plasticity in FSP

L.H.Wu & P.Xue et al.[30] have worked on achieving superior low-temperature super-

plasticity for lamellar microstructure in nugget of a friction stir welded TI-6Al-4V joint. A low-

temperature super-plasticity of 442% was achieved at 650 °C for a fully lamellar microstructure

in the nugget of friction stir welded Ti-6Al-4V joint. More importantly it was shown that nugget

showed a comparable super-plasticity > 395%) to the base material at 800 °C and

20

1 × 10− 4 ~ 1 × 10− 3 s− 1; especially at 3 × 10− 4 s− 1, both the elongation and flow stress of the

nugget were similar to those of the base material, which provides the possibility of uniform

superplastic forming of the entire weld. Good super-plasticity in the lamella-structured nugget

was mainly attributed to gradual globularization of the fine lamellae during static annealing

and superplastic deformation.

M.A. García-Bernal & Macro Antonio et al. [31] have worked on Inhibition of abnormal grain

growth during hot deformation behavior of friction stir processed 5083 Al alloys. Friction stir

processing (FSP) has demonstrated to refine the microstructure of different alloys resulting in

superior mechanical properties. Abnormal grain growth (AGG) has been recognized as a

critical issue during hot deformation of FSPed Al alloys. 5083 Al alloys with different Mn

content were subjected to heat treatments at 350 and 535 °C before FSP to avoid AGG during

subsequent hot deformation. As a result, heat treatment of 350 °C was able to retard AGG

significantly. Also, an improvement in the refinement of the microstructure was observed.

Consequently, a better ductility of 861% at elevated temperature was reached in the alloy with

lower Mn content comparing with the same alloy without heat treatment prior to FSP.

2.2.2 Influence of tool pin profile and shoulder diameter on FSP

The design of the tool has been shown to play a decisive role in microstructure modification.

The tool with a larger shoulder area allowed more plastic deformation on the microstructure

generating a more suitable microstructure for high temperature deformation. Super-plasticity

is the ability of a polycrystalline material to exhibit very high tensile elongation prior to failure.

This phenomenon has important implications for sheet metal forming industry. Recently, it was

found that FSPed Al alloys can produce a grain size less than 5μm [32][33][34][35]. FSP is a

severe plastic thermomechanical process originated after the novel friction stir welding (FSW)

technique shaped in the early nineties. FSP is particularly attractive because it obtained ultra-

fine grain regions without changing the thickness of sheet, with potential benefits for the

superplastic forming industry. There is a close relationship between temperature and the

shoulder area; the larger the shoulder area, the higher the maximum temperature [5].

M.A.Garcia-Bernal et al. [36] has investigated the influence of friction stir processing tool

design on microstructure and superplastic behavior of Al-Mg alloys. In this investigation an

attempt has been made to understand the effect of tool pin profile and tool shoulder diameter

on FSP zone formation in AA6061 aluminium alloy. Five different tool pin profiles (straight

cylindrical, tapered cylindrical, threaded cylindrical, triangular and square) with three different

21

shoulder diameters have been used to fabricate the joints. The formation of FSP zone has been

analyzed macroscopically. Tensile properties of the joints have been evaluated and correlated

with the FSP zone formation. From this investigation it is found that the square pin profiled

tool with 18 mm shoulder diameter produced mechanically sound and metallurgically defect

free welds compared to other tool pin profiles. From the macrostructure analysis, it was inferred

that the formation of defect free FSP zone is a function of tool profile and tool shoulder

diameter. The joints fabricated by the tools with shoulder diameter of 18 mm (D/d = 3) have

shown higher tensile strength and elongation compared to the joints fabricated by the tools with

shoulder diameter of 15 mm (D/d = 2.5) and this trend is common for all the tool pin profiles.

Similarly, the joints fabricated by the tools with shoulder diameter of 21 mm (D/d = 3.5) have

also shown lower tensile strength and elongation compared to the joints fabricated by the tools

with shoulder diameter of 18 mm.

Elangovan & V. Balasubramanian et al. [37] have studied the solid state flow visualization of

FSW of AA2024 and AA6013 aluminium alloys and they observed that the flow of the plate

material on the advancing side and the retreating side are different. The material on the

retreating side never enters into the rotational zone near the pin, but the material on the

advancing side forms the fluidized bed near the pin and rotates around it.

2.2.3 Influence of strain rate on friction stir processed material

F. Chai & D. Zhang et al. [38] have worked on High strain rate super-plasticity of a fine-grained

AZ91 magnesium alloy prepared by submerged friction stir processing. The as-cast AZ91 plate

was subjected to normal friction stir processing (processed in air) and submerged friction stir

processing (processed in water, SFSP), and microstructure and superplastic tensile behavior of

the experimental alloys were investigated SFSP results in remarkable grain refinement due to

the enhanced cooling rate compared with normal FSP, with an average grain size of 1.2μm and

7.8μm. The SFSP AZ91 specimen exhibits considerably enhanced superplastic ductility with

reduced flow stress and higher optimum strain rate, as compared to the normal FSP specimen.

The optimum superplastic deformation temperature is found to be 623 K for both the normal

FSP and SFSP AZ91 specimens. An elongation of 990% is obtained at 2×10−2 s−1 and 623 K

for the SFSP specimen, indicating that excellent high strain rate super-plasticity could be

achieved. By comparison, maximum ductility of the normal FSP specimen strained at high

strain rate is 158% Grain boundary sliding is the main mechanism for the superplastic

deformation of the normal FSP and SFSP specimens.

22

Liu & Ma et al. [39] have worked on to achieve exceptionally high super-plasticity at high

strain rates in a micro-grained Al–Mg–Sc alloy produced by friction stir processing. Friction

stir processing (FSP) was applied to extruded Al–Mg–Sc alloy to produce fine-grained

microstructure with 2.6μm grains. A maximum elongation of 2150% was achieved at 450 °C

and a high strain rate of 1 × 10−1 s−1. Although the grains obtained by FSP were much larger

than those by other techniques, such as equal-channel angular pressing, approximately the same

superplasticity was achieved at an even higher strain rate in the FSP alloy. Several previous

investigations indicated that elongation of more than 2000% could be achieved in ultrafine-

grained Al–Mg–Sc alloys with grain sizes 0.2–1μm

2.3 Application of Johnson Cook model

Li & Hong-Ying et.al [40] have worked on to develop Johnson cook model for elevated

temperature flow behavior of T24 steel. The isothermal hot compression tests were carried out

on Gleeble-3500 thermomechanical simulator in the temperature range of 1323–1473 K and

strain rates of 0.01s−1, 0.1s−1, 1 s−1 and 10s−1. Based on the experimental results, a modified

Johnson Cook model has been proposed to describe the flow behavior of T24 steel. The

modified model considers not only the yield and strain hardening portion of the original model

but also the coupled effects of strain and temperature, and of strain rate and temperature on the

flow behaviors. The high temperature deformation behavior of T24steel was characterized

based on the analysis of the stress-strain curves. The results showed that the flow stress

predicted by the proposed model agrees well with the experimental stress which validates the

efficiency of the modified model in describing the deformation behavior of the steel. The true

stress–true strain curves of T24 steel during the compression under different deformation

conditions are shown in Figure 13. At the lower strain rates of 0.01 s−1 and 0.1 s−1, all the true

stress–true strain curves exhibit an obvious peak stress, after which the stress decreases

gradually, showing dynamic softening due to recrystallization. The true stress–true strain

curves exhibit a peak stress at a very small strain, after which the flow stresses decrease until

high strains, showing a typical dynamic recrystallization(DRX) behavior of the steel under the

deformation conditions of lower strain rates. The results indicate that the flow stress of this

steel increases with the increasing of the strain rate and the decreasing of the deformation

temperature. The steel exhibits typical dynamic recrystallization behaviors at lower strain rates.

23

Figure 13: True stress-true stain curves of T24steel at different temperatures with strain rate of

(a)0.01s−1 ; (b)0.1s−1; (c)1s−1 and (d)10s−1 [40]

D. Samantaray & S. Mandal et.al et.al [41] have also worked on Johnson Cook, modified

Zerilli–Armstrong and Arrhenius-type constitutive models to predict elevated temperature flow

behavior in modified 9Cr–1Mo steel. The experimental stress–strain data from isothermal hot

compression tests over a wide range of temperatures (1123–1373 K), strains (0.1–0.5) and

strain rates (0.001–1 𝑠−1) were employed to evaluate the material constants of these

constitutive models.

Suitability of these models were evaluated by comparing the correlation coefficient and

absolute average error of prediction, ability to describe the deformation behavior, number of

material constants involved and the computational time required to evaluate these constants.

Experimental data from isothermal hot compression tests were employed to determine the

material constants of these models. Subsequently, the suitability of these models for predicting

the flow stress of modified 9Cr–1Mo steel over a specified hot working domain was evaluated

24

Chapter 3

3 Objectives of present study

In automotive industries, use of AA5754 is stricted due to its low formability at lower

temperature. FSP can be used to increase formability. So, the aim of current work is to

study effect of different elevated temperature and strain rate on mechanical properties

of friction stir processed (FSPed) AA5754 sheet metals. Hence the following objectives

are identified in the present work.

1. Design and fabrication of FSP tool to successfully fabricate friction stir

processed sheet of AA5754 alloy using suitable process parameter.

2. Characterization of uniaxial tensile properties of both FSPed and base metal

sheets in terms of yield stress, ultimate stress and % elongation at different

elevated temperature and strain rate.

3. Development of Johnson Cook model to predict the flow stress incorporating

the effect of temperature, strain rate, strain hardening and plastic strain.

4. Fractography of FSPed specimens to understand the failure mechanism.

25

Chapter 4

4 Methodology

4.1 Selection of sheet material

AA5754-H22 aluminum alloy is being used for structural and inner body applications because

of its high strength to weight ratio. Also, it has excellent corrosion resistance especially to

saline water and industrially polluted water and excellent weldability with gas, arc and

resistance. Hence, various formability experimental works were carried out for this alloy at

room and elevated temperatures which are discussed in the following sections. Table 2 shows

chemical composition of AA5754- H22 alloy.

Table 2: Chemical composition of AA5754 H-22 alloy [42]

Alloy material % Composition

Aluminum (Al) 94.2 to 97.4 %

Silicon (Si) 0 to 0.4 %

Magnesium (Mg) 2.6 to 3.6 %

Iron (Fe) 0 to 0.4 %

Copper (Cu) 0 to 0.1 %

Manganese (Mn) 0 to 0.5 %

Zinc (Zn) 0 to 0.2 %

Residuals 0 to 0.15 %

Titanium (Ti) 0 to 0.15 %

Chromium (Cr) 0 to 0.3 %

26

4.2 Friction stir processing of thin sheets

FSP involves complex material movement and plastic deformation. Processing parameters, tool

geometry and joint design exert significant effect on the material flow pattern and temperature

distribution, thereby influencing the micro structural evolution of material. In this section, a

few major factors affecting FSW/FSP process, such as tool geometry, welding parameters, joint

design are addressed. The strength of friction stir welding/processing depends on the following

three process parameters spindle speed, feed rate and depth of penetration respectively. First

we will discuss our methodology for tool design.

4.2.1 Tool design

A) Tool material selection

Here we have selected Stainless steel 316 as our tool material. Chemical composition of the

tool is given in the table 3.

Table 3Chemical composition of Stainless steel 316 [43]

Stainless steel 316 % Composition

Iron (Fe) Balance

Nickel (Ni) 12 %

Chromium(Cr) 17 %

Molybdenum (Mo) 2.5%

Silicon (Si) 1%

Manganese (Mn) 2%

Carbon (C) 0.08 %

Phosphorus (P) 0.045 %

Sulfur (S) 0.03 %

27

Weld quality and tool wear were two important considerations in the selection of tool material,

the properties of which may affect the weld quality by influencing heat generation and

dissipation. Apart from the potentially undesirable effects on the weld microstructure,

significant tool wear increases the processing cost of FSW. Owing to the severe heating of the

tool during FSW, significant wear may result if the tool material has low yield strength at high

temperatures. Stresses experienced by the tool are dependent on the strength of the workpiece

Table 4 Mechanical properties of Stainless Steel 316[44]

Property Value

(Metric)

Units

(S.I.)

Value

(English)

Units (English)

Density 8 g/cc 0.289 lb/in3

Hardness 79 BHN 79 BHN

Tensile strength, Ultimate 580 MPa 84100 psi

Tensile strength, Yield 290 MPa 42100 psi

Elongation at Break 50% - 50% -

Modulus of Elasticity 193 GPa 28000 ksi

Charpy impact 105 J 77.4 ft-lb

Melting Point 1370-1400°

C °C 2500 °F °F

Specific Heat capacity 0.5 J/g-°C 0.12 BTU/lb-°F

at high temperatures common under the FSW conditions. Temperatures in the workpiece

depend on the material properties of tool, such as thermal conductivity, for a given workpiece

and processing parameters. The coefficient of thermal expansion may affect the thermal

stresses in the tool. Other factors that may influence tool material selection are hardness,

ductility and reactivity with the workpiece material. The tool hardness is important in

mitigating surface erosion due to interaction with particulate matter in the workpiece. The

28

brittle nature of ceramics such as pcBN may be undesirable if there is a significant probability

of breakage due to vibrations or accidental spikes in loads. Because of their high temperature

strength, pcBN and W based alloys are commonly used tool materials for FSW of harder alloys.

The thermal conductivity of the tool material determines the rate of heat removal and affects

the temperature fields, flow stresses and weld [13].

B) Tool geometry

Tool geometry affects the heat generation rate, traverse force, torque and the thermomechanical

environment experienced by the tool. The flow of plasticized material in the workpiece is

affected by the tool geometry as well as the linear and rotational motion of the tool. Important

factors are shoulder diameter, shoulder surface angle, pin geometry including its shape and

size.

The diameter of the tool shoulder is important because the shoulder generates most of the heat,

and its grip on the plasticized materials largely establishes the material flow field. Both sliding

and sticking generate heat whereas material flow is caused only from sticking. For a good FSW

practice, the material should be adequately softened for flow, the tool should have adequate

grip on the plasticized material and the total torque and traverse force should not be excessive.

Figure 14 shows variation of sliding torque, sticking torque and total torque with respect of

shoulder diameter. Experimental investigations have shown that only a tool with an optimal

shoulder diameter results in the highest strength of the AA 6061 FSW joints. Although the need

to determine an optimum shoulder diameter has been recognized in the literature, the search

for an appropriate principle for the determination of an optimum shoulder diameter is just

beginning. The tool diameter was selected as 15mm and pin diameter was selected as 5mm.

The most commonly used ratio of shoulder-to-probe diameter is 3[13].

C) Pin length determination

TO determine pin length we have done our experiment at different pin length with plunge

depth one 0.1mm and another 0.05 mm as the thickness of the sheet was 1.5mm. We examined

29

Table 5 pin length variation effect on weld quality

the welded workpiece and got to know that at pin length of 1.1mm and plunge depth of 0.1mm

the best weld specimen was obtained. It was seen that at greater plunge depth and pin

penetration the amount of heat generation was very much so the sheet got bend. Table 5 shows

Pin

length

Plunge

depth

Weld

quality

Plate

bending

Remarks picture

1.2mm 0.1mm Poor Yes Sheet was bended

and due to this pin

penetrated the

sheet completely.

1.2mm .05mm Average Yes (but

less than

previous

one)

Optimum FSP

was not achieved,

the amount of

flash was more

and pin

penetration

destroyed lower

part of sheet.

1.12mm 0.05mm Poor Yes Good clamping is

as necessary as

selecting right

parameter. Due to

bending pin

penetrate the

sheet completely

as shown in the

adjacent figure.

1.1mm .1mm Excellent No Excellent FSPed

sample was

achieved and

bending was

minimum in this

case.

30

pin length and plunge depth variation effect on quality of weld. As shown in the table at pin

length 1.2 and plunge depth .1mm the sheet got bent.

Figure 14: Variation of sliding torque, sticking torque and total torque with shoulder diameter [13]

4.2.2 Process parameters

The tool rational speed was set to 900 rpm and travel velocity was set to 125mm/min. The

plunge depth and tilt angle were 0.1mm and 1° respectively [45][46]. Though for obtaining

sound weld we have done welding on other parameters also. At 1000 rpm amount of flash was

found to be more. At weld velocity 150 mm/min and rpm 1200 heat generation was enough to

bend the sheet as shown in Figure 15.

Figure 15 FSP at 1200 RPM and 150mm/min weld velocity

31

4.3 Microstructure of stir zone :

We decided to know stir zone length so that when we do the tensile test only stir zone would

come in tensile testing for better results.

Procedure to find microstructure and stir zone

1. The FSP (friction stir processed AA5754) workpiece was cut using grinding cutter as

shown in the figure 16.

Figure 16: Abrasive cutter

2. After cutting the sample, the sample was cleaned properly and next task was to mount the

sample so that polishing process can take place easily. Now mounting of the sample was

done using Geosyn cold mounting powder.

3. After mounting the sample was polished using sand paper of grit size 200, 400, 600, 800,

1000 and 1200 respectively as shown in figure 17.

Figure 17: polishing machine

4. After doing fine polishing upto 1200 micrometer particle size sand paper the diamond

polishing was done to polish the surface even more polished and without roughness.

32

Diamond paste of grain size 3 micron and 0.25 micron was used on variable speed grinder

polishing machine. Figure 18 shows finally polished FSPed AA5754 surface and setup for

diamond polishing.

(a) (b)

Figure 18: (a) Finally polished mounted surface (b) Setup for diamond

polishing

5. Polished samples were etched by Kellers reagent (200 ml) consisting of 190 ml distilled

water, 5 ml nitric acid, 2ml hydro- fluoric acid and 3ml hydrochloric acid for 5 minutes.

Microstructure were seen using Inverted metallurgical microscope (Leica, DMILM)..

Figure 19 Stir zone depth and width is clearly visible

33

Figure 19 shows the macrostructure of the FSPed sample. As shown in the figure the pin

penetration is clearly visible. It comes nearly 1.2 mm and the width of the stir zone comes near

to 6.5mm. Our tensile specimen width will be 6mm so the tool design is accurate as per our

requirement. Figure 20 shown the microstructure of the stir zone. The stir zone is having small

and uniform grain size which increases its formability.

Figure 20: Microstructures of the SZ(Stir Zone) observed on AA5754 aluminum alloy sheets joined by

FSP with tool rotation speed 900 rpm and tool travel speed 125 mm/min

4.4 The tool design and experimental procedure

(a) (b)

Figure 21 Schematic representation of FSP (all dimensions are in mm) (a) Isometric view of FSP

(b) Cross-sectional view of FSP

34

Rolled Sheets of aluminium alloy AA5754-H22 of dimensions 140mm×80mm×1.5mm is used

for friction stir processing (FSP). Samples surface was polished with 600-grit emery paper and

cleaned with acetone before welding. Figure 21 shows the schematic representation of FSP of

AA 5754–H22 sheets. Figure 22 shows the tool picture containing dimensions, tool schematic

and tool picture.

(a) (b)

(c)

Figure 22 (a) Tool used for friction stir Processing (b) Tool schematic diagram

(c) Tool dimensions

The machine setup for doing FSP is shown in Figure 23. This machine uses the principle of

hydraulic control for actuation. The machine setup is also shown in figure. The tool is fixed in

the machine and using fixtures the AA5754-H22 sheet is clamped. The position of the plate is

determined by the machine and the welding parameters are given in the machine. Figure 23

shows the picture of workpiece and tool during FSP.

35

Figure 23: Friction stir processing machine

Figure 24: During friction stir processing

36

4.5 Tensile testing at different elevated temperature and strain rate

Flat tensile test specimens of AA5754 were fabricated by blanking as per ISO 6892

(International standard ISO 6892–1:2009(E). The exact dimension is shown in the Figure 25.

The samples were cut along the stir zone so that the stir zone would be our lengthening zone.

To fabricate the sample punch and die combination were used as shown in Figure 36. After

removing from punching machine the burrs were removed using file and the tensile samples

were polished using sand paper of grit size 400 and 800 micron. For making the hole of

diameter 13mm drilling machine was used.

Figure 25 Dimension of Tensile specimen (all dimension all in mm)

Figure 26: The die for cutting tensile specimen

37

Total 24 samples were cut to investigate effect of temperature and strain rate on material

mechanical properties. At room temperature total 6 uniaxial tensile testing experiments were

done. Three for base AA5754 and 3 for FSPed AA5754 at elongation rate of 1mm/min,

100mm/min and 200mm/min respectively. Similarly uniaxial tensile testing was done on UT-

04-0050 ELECTRA 50 Hot Forming machine (Figure 27) at room temperature, 200°C, 300°C

and 400°C respectively. The standard tensile properties of parent metal and TWB: 0.2% yield

tensile strength (YTS), ultimate tensile strength (UTS) and percentage elongation were

determined. The graphs were plotted to study the effect of temperature, and strain rate on FSP

as well as base sheet metal.

(a) (b)

Figure 27 (a)UT-04-0050 ELECTRA 50 Hot Forming machine (b)Tensile specimen shown in hot forming

machine

4.6 Formulation using Johnson Cook (JC) model

According to the JC model, the flow stress is expressed as:

38

𝜎 = (𝐴 + 𝐵𝜀𝑛)(1 + 𝐶 ln 𝜀 ∗̇)(1 − 𝑇∗𝑚), (1)

Where σ is the (Von Mises) flow stress, A is the yield stress at reference temperature and

reference strain rate, B is the coefficient of strain hardening, n is the strain hardening

exponent, 𝜀 is the plastic strain, 𝜀 ∗̇ = 𝜀̇/𝜀0̇ is the dimensionless strain rate with 𝜀̇ being the

strain rate and 𝜀0̇ the reference strain rate, and 𝑇∗ is the homologous temperature and

expressed as:

𝑇∗ =𝑇−𝑇𝑟𝑒𝑓

𝑇𝑚−𝑇𝑟𝑒𝑓 , (2)

With T as the current absolute temperature, 𝑇𝑚the melting temperature (600 K for AA5754)

and 𝑇𝑟𝑒𝑓 as the reference temperature (T≥𝑇𝑟𝑒𝑓). The minimum temperature of the test matrix

is taken as the reference temperature. C and m are the material constants that represent the

coefficient of strain rate hardening and thermal softening exponent, respectively. The JC model

considers isotropic hardening, strain rate hardening and thermal softening, but as three

independent phenomena whence these can be isolated from each other. Thus, the total effect of

strain hardening, strain rate hardening and thermal softening on flow stress can be calculated

by multiplying these three terms, i.e. the first, second and third parentheses in Eq. (1).

To predict the flow behavior of friction stir processed AA5754 employing the JC model, 293

K is taken as reference temperature (minimum temperature of test matrix) and .056𝑠−1the

reference strain rate. At reference temperature and reference strain rate, Eq. (1) will reduce to:

𝜎 = 𝐴 + 𝐵𝜀𝑛 (3)

The value of A is calculated from the yield stress (i.e. the stress at 0.056 strain) of the flow

curve at 293 K and 0.056𝑠−1. Substituting the value of A in Eq. (3) and using the flow stress

data at various strains for the same flow curves, ln (𝜎 − 𝐴) vs. ln 𝜀 is plotted. B is calculated

from the intercept of this plot while n is obtained from the slope. At reference temperature,

there is no flow softening term as T* = 0. So, Eq. (1) can be expressed as:

𝜎 = (𝐴 + 𝐵𝜀𝑛)(1 + 𝐶 ln 𝜀 ∗̇) (4)

39

Using the flow stress data for a fixed strain at various strain rates, C is obtained from the slope

of { 𝜎 /(A + B𝜀𝑛)} vs. ln 𝜀 ∗̇ plot. Similarly, at reference strain rate (𝜀̇ = 0.056𝑠−1̇ ) , thermal

softening effect on flow stress can be isolated since ln𝜀 ∗̇ = 0. So, Eq. (1) can be expressed as:

𝜎 = (𝐴 + 𝐵𝜀𝑛)(1 − 𝑇∗𝑚) (5)

Using the flow stress data for a particular strain at different temperatures, the graph of

ln(1 − {𝜎

𝐴+𝐵𝜀𝑛}) vs. ln T* is plotted. The material constant m is obtained from the slope of this

graph. The material constants C and m of the JC model are determined using the least-square

method. A constrained optimization procedure is used to find their optimized values. This

optimization is done by minimizing the average absolute error (∆) between the experimental

and predicted flow stress.

∆=1

𝑁∑ |

𝜎𝑒𝑥𝑝𝑖 −𝜎𝑝

𝑖

𝜎𝑒𝑥𝑝𝑖 |𝑖=𝑁

𝑖−1 × 100 (6)

Where 𝜎𝑒𝑥𝑝 is the experimental flow stress, 𝜎𝑝is the predicted flow stress and N is the total

number of data. A significant deviation in prediction could be observed in most of the loading

conditions. The Predictability of the constitutive equation is also quantified by employing

standard statistical parameters such as correlation coefficient (R) and average absolute error

(D). Correlation coefficient is a commonly used statistic and provides information on the

strength of linear relationship between observed and the computed values. It can be

mathematically expressed as:

𝑅 =∑ (𝜎𝑒𝑥𝑝

𝑖𝑖=𝑁𝑖=1 −𝜎𝑒𝑥𝑝̅̅ ̅̅ ̅̅ ̅)(𝜎𝑝

𝑖 −�̅�𝑝)

√∑ (𝜎𝑒𝑥𝑝𝑖𝑖=𝑁

𝑖=1 −�̅�exp )2 ∑ 𝑖=𝑁𝑖=1 (𝜎𝑝

𝑖 −�̅�𝑝)2 (7)

Where and 𝜎exp and 𝜎𝑝 are the mean values of 𝜎𝑒𝑥𝑝 and 𝜎𝑝 respectively.It should be borne in

mind that higher value of R may not necessarily indicate better performance [47] because of

the tendency of the model/equation to be biased towards higher or lower values. On the other

hand, D is computed through a term by term comparison of the relative error and therefore is

unbiased statistics for measuring the predictability of a model/equation [48].

40

Chapter 5

5 Results and discussions

This section presents the results of the tests conducted as mentioned in the previous section

5.1 Tensile testing results

A total of 12 experiments were conducted each for base metal and friction stir processed

(FSPed) AA5754 at three different cross head velocity of 1mm/min, 100mm/min and

200mm/min (corresponding to different strain rate) and four different temperature room

temperature, 200°C, 300°C and 400°C. In first section we are going to discuss results of base

material and will compare the results at all strain rates and temperature.

5.1.1 Effect of temperature and strain rate on Engg stress strain response of base

material:

The graphs shown in Figure 28, Figure 29 and Figure 30 compare the engineering stress-strain

response at different temperature for base AA5754 conducted at 1mm/min, 100mm/min and

200mm/min cross head velocity respectively. Here CHV signifies cross head velocity means

the rate with which the specimen is elongating per minute and Base signifies parent AA5754

alloy sheet.

Figure 28 Effect of Temperature and strain rate on engineering stress-strain response at 1mm/min

cross head velocity rate

-50

0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Engg

str

ess

Engg strain

Engg stress vs strain_Base_CHV 1mm/min

BASE_CHV_ROOM TEMP

Base15_CHV1_Temp400

Base7_CHV_Temp300

Base5_CHV_temp200

CHV=cross head velocity

41

Figure 29 Effect of Temperature and strain rate on engineering stress-strain response at 100mm/min

crosshead velocity rate.

Figure 30: Effect of Temperature and strain rate on engineering stress-strain response at

200mm/min crosshead velocity rate

As shown in the figure above it was shown that as the temperature increases ductility of base

material increases with the decrease in strength. The effect of cross head velocity was not

perceived at lower temperature as affected at higher temperature. For the cross head velocity

of 1mm/min at room temperature and 200°C elongation was approximately 12% but at 300°C

it was 60% and at 400°C it was 145%. The phenomenal increase in % elongation can be

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Engg

str

ess

Engg strain

BASE_Engg Stress vs Strain _CHV 100mm/min

Base14_CHV100_Temp400

Base12_CHV100_Temp300

Base3_CHV100_Temp200

Base1_CHV100_Room temp

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Engg

str

ess

Engg strain

Engg stress vs strain _Base_CHV 200mm/min

BASE16_CHV200_TEMP400

Base13_CHV200_TEMP300

BASE6_CHV200_TEMP200

Base2_CHV200_Room temp

42

justified by theory of super-plasticity and grain refinedness as discussed in literature survey.

From 300°C to 400°C strength reduces from 150MPa to 50MPa at strain rate of 1mm/min. At

CHV (Cross head velocity) of 1mm/min more elongation came because of dynamic softening

due to recrystallization. At higher CHV effect of increase in temperature reduces strength at

lower rate and consistent pattern. A very interesting thing that came after analyzing the graph

is that material doesn’t loses its strength up to 300°C (approximately) but elongation is

increasing. At lower strain rate and lower temperature strength was found more relative to

lower strain rate but higher temperature or lower temperature but higher strain rate. From graph

it is shown that at lower strain rate elongation is more and at higher strain rate elongation is

less except for CHV of 1mm/min in which at temperature 200°C has more elongation than at

room temperature.

5.1.2 Effect of temperature and strain rate on Engg stress strain response of FSP

material:

The graphs shown in Figure 31, Figure 32 and Figure 33 compare the engineering stress-strain

response at different temperature, conducted at 1mm/min, 100mm/min and 200mm/min

elongation rate respectively. Here CHV signifies cross head velocity means the rate at which

then specimen is elongating per minute and FSP signifies friction stir processed AA5754 alloy

sheet.

Figure 31: Effect of Temperature and strain rate on engineering stress-strain response at 1mm/min

crosshead velocity rate:

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Engg

str

ess

Engg strain

Engg stress vs strain_FSP_ CHV 1mm/min

FSP15_CHV1_TEMP400

FSP6_CHV1_TEMP300

FSP3_CHV1_TEMP200

Fsp0_CHV1_ROOM TEMP

43

Figure 32: Effect of Temperature and strain rate on engineering stress-strain response at 100mm/min

crosshead velocity rate

Figure 33: Effect of Temperature and strain rate on engineering stress-strain response rate at

100mm/min crosshead velocity rate

As shown in the figure above it was shown that as the temperature increases ductility of FSP

material increases with the decrease in strength. The effect of cross head velocity was perceived

at lower temperature as well as at higher temperature. For the cross head velocity of 1mm/min

at room temperature and 200°C elongation was approximately 16% but at 300°C it was 58%

and at 400°C it was 88%. The phenomenal increase in % elongation can be justified by theory

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Engg

str

ess

Engg strain

Engg stress vs strain_FSP_CHV 100mm/min

Fsp10_CHV100_temp400

Fsp7_CHV100_TEMP300

Fsp5_CHV100_Temp200

FSP1_CHV100_ROOM TEMP

CHV= cross head velocity

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Engg

Str

ess

Engg Strain

Engg stress vs strain_FSP_CHV 200mm/min

FSP11_CHV200_TEMP400

Fsp8_CHV200_temp300

Fsp4_CHV200_Temp200

Fsp2_CHV200_room temp

44

of super-plasticity and grain refinedness as discussed in literature survey. From 300°C to 400°C

strength reduces from 140MPa to 48MPa at strain rate of 1mm/min. At higher strain rate the

effect of temperature was less up to 200°C as compared to lower strain rate. At CHV (Cross

head velocity) of 1mm/min more elongation came because of dynamic softening due to

recrystallization. A very interesting thing that came after analyzing the graph is that material

doesn’t loses its strength up to 300°C (approximately) but elongation is increasing. At lower

strain rate and lower temperature strength was found more relative to lower strain rate but

higher temperature or lower temperature but higher strain rate. From graph it is shown that at

lower strain rate elongation is more and at higher strain rate elongation is less except for CHV

of 1mm/min in which at temperature 200°C has more elongation than at room temperature. At

400°C the strength is highly dependent up on strain rate. At lower strain rate strength was found

to be half as compared to higher strain rate. Higher elongation for FSPed material can be

utilized in many hot forming operations. Knowing the strain rate sensitivity and temperature

effect on % elongation and yield strength would help the industries a lot.

5.1.3 Effect of temperature and strain rate on mechanical properties:

(a) (b)

Figure 34: A figurative comparison of (a) Base sample at temperature 400°C before and after tensile

failure (b) FSP sample at temperature 400°C before and after tensile failure

45

(a) (b)

Figure 35: A figurative comparison of (a) Base sample at room temperature before and after tensile

failure (b) FSP sample at room temperature before and after tensile failure

As shown in Figure 35 it compares FSP sample and base sample before and after tensile test at

all three CHV (cross head velocity). The elongation of base sample is more than elongation of

FSPed sample at CHV of 1mm/min. The necking zone clearly suggests ductile failure at all

cases. Figure 47 depicts comparison of base and FSPed sample at room temperature. As shown

in figure strain rate sensitivity of material is very less at room temperature. Formability

behavior is also very less at room temperature. Table 6 shows data of YTS (yield tensile

strength), UTS (ultimate tensile strength) and % elongation base and FSPed AA5754.

Table 6 Results showing mechanical properties of base as well as FSP AA5754

Sample

Specification

Temp

(°C)

Elongation

rate

(mm/min)

% Elongation Yield Strength

(MPa)

Ultimate

strength

(MPa)

1.Base0 20 1 10.72 185.40 246.611

2.Base1 20 100 13.14 211.59 239.45

3.Base2 20 200 12.25 223.45 256.47

4.FSP0 20 1 15.86 148.34 210.24

5.FSP1 20 100 21.27 160.28 208.50

46

Sample

Specification

Temp

(°C)

Elongation

rate

(mm/min)

% Elongation Yield Strength

(MPa)

Ultimate

strength

(MPa)

6.FSP2 20 200 21.69 161.28 211.04

7.Base5 200 1 12.9 220.82 230.80

8.Base3 200 100 8.32 211.86 228.12

9.Base6 200 200 8.84 209.53 222.14

10.FSP3 200 1 34.7 153.29 184.86

11.FSP5 200 100 19.45 159.65 206.19

12.FSP6 200 200 21.27 157.98 199.85

13.Base7 300 1 60.12 147.50 149.41

14.Base12 300 100 15.16 187.30 193.77

15.Base13 300 200 15.79 182.88 187.99

16.FSP6 300 1 59.97 131.62 139.43

17.FSP7 300 100 36.67 148.46 167.78

18.FSP8 300 200 32.4 145.45 166.53

19.Base15 400 1 144 39.72 45.49

20.Base14 400 100 82 93.45 95.52

21.Base13 400 200 75.42 93.42 96.47

22.FSP15 400 1 91.45 47.12 47.75

23.FSP10 400 100 61.45 101.81 104.28

24.FSP11 400 200 65.12 104.21 106.93

5.1.4 Effect of strain rate and temperature on true stress and true strain response

The graphs shown in Figure 36 and Figure 37 compare the true stress-strain response for FSP

AA5754 and base AA5754 at different strain rate conducted at room temperature and 300°C.

As shown in the figure 36 at room temperature strength of base material is more but elongation

of FSP is very good without losing too much strength. For FSP maximum elongation came at

CHV of 200mm/min and same trend followed for base metal. But at higher temperature as

shown in figure 37 maximum elongation came for CHV (cross head velocity) of 1mm/min for

47

both FSP and base metal sheet. For base metal after increase in temperature loss in strength is

more as compare to FSP.

Figure 36: Effect of temperature and strain rate on FSPAA5754 and base AA5754 (Room

Temperature): true stress-strain response

Figure 37: Effect of temperature and strain rate on FSPAA5754 and base AA5754 (300°C):

engineering stress-strain response

0

50

100

150

200

250

300

0 0.05 0.1 0.15 0.2 0.25 0.3

Tru

e St

ress

True strain

True stress vs strain__ Room temperature

FSP2_CHV200_ROOM TEMP FSP0_CHV1_ROOM TEMP FSP1_CHV100_ROOM TEMP

BASE2_CHV200_ROOM TEMP BASE1_CHV100_ROOM TEMP BASE0_CHV1_ROOM TEMP

0

50

100

150

200

250

300

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Tru

e st

ress

True Strain

True Stress vs Strain_temp300

Fsp6_CHV1_temp300 Fsp7_CHV100_Temp300 Fsp8_CHV200_Temp300

BASE7_CHV1_TEMPP300 BASE13_CHV200_TEMP300 Base12_CHV100_temp300

48

5.2 Prediction of Johnson Cook model

5.2.1 Evaluation of material constants of Johnson Cook model

Johnson cook equation derived for base AA5754

𝜎 = (𝐴 + 𝐵𝜀𝑛)(1 + 𝐶 ln 𝜀 ∗̇)(1 − 𝑇∗𝑚)

A = Yield stress at reference temperature in MPa =160

B= Coefficient of strain hardening = 279

n= Strain hardening exponent =0.3436

C= Coefficient of strain rate hardening = 0.039137

m= thermal softening exponent =1.6687

Where σ is the (Von Mises) flow stress, 𝜀 is the plastic strain, 𝜀 ∗̇ = 𝜀̇/𝜀0̇ is the dimensionless

strain rate with 𝜀̇ being the strain rate and 𝜀0̇ the reference strain rate, and 𝑇∗ is the

homologous temperature and expressed as

𝑇∗ =𝑇−𝑇𝑟𝑒𝑓

𝑇𝑚−𝑇𝑟𝑒𝑓

With T as the current absolute temperature, 𝑇𝑚the melting temperature (600 K for AA5754)

and 𝑇𝑟𝑒𝑓 as the reference temperature (T≥𝑇𝑟𝑒𝑓). The minimum temperature of the test matrix

is taken as the reference temperature. Table 7 and Table 8 shows value of Johnson Cook’s

parameters for base material.

Table 7: Johnson Cook model parameter value for base material

Parameter

A(MPa) B(MPa) n c m

Value 160 279 .3436 0.039137 1.6687

Table 8: Johnson Cook model parameter value for FSPed material

Parameter A(MPa) B(MPa) n c m

Value 110 225 0.4051 -0.0068 2.487

5.2.2 Experimental vs predicted Stress for parent material

The Three graphs Figure 38, Figure 39 and Figure 40 show experimental stress vs predicted

stress for base aluminium. X-axis is represented by true strain and Y-axis is represented by true

49

stress. The dots show predicted value and solid line shows experimental value at 293 K, 473 k,

573 k and 673 k respectively.

Figure 38: Comparison between experimental flow stress and predicted flow stress using Johnson

Cook model in temperature domain 293 K–673K of base metal for elongation rate of 200mm/min

Figure 39: Comparison between experimental flow stress and predicted flow stress using Johnson

Cook model in temperature domain 293 K–673K of base metal for elongation rate of 100mm/min

0

50

100

150

200

250

300

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

Tru

e st

ress

True strain

True stress vs strain_Base_JC model_CHV1mm/min

predicted stress Experimental stress

293 k

473 k

573 k

673 k

0

50

100

150

200

250

300

0 0.02 0.04 0.06 0.08 0.1

Tru

e st

ress

True strain

True stress vs strain_Base_JC model_CHV100mm/min

Experimental stress Predicted stress

293 k

673k

473k

573k

50

Figure 40: Comparison between experimental flow stress and predicted flow stress using Johnson

Cook model in temperature domain 293 K–673K of base metal for elongation rate of 1mm/min

As seen from the figure the model gives accurate results for higher strain rate for base metal.

At 400°C temperature the model was not able to predict accurate results. We have got best

prediction for higher strain rate and lower temperature. This is attributed to the inadequacy of

the JC model to incorporate the coupled effects of strain and temperature and of strain rate and

temperature.

Figure: 41 Experimental stress vs Predicted stress for base AA5754

0

50

100

150

200

250

300

0 0.02 0.04 0.06 0.08 0.1

Tru

e st

ress

True strain

True stress vs strain_Base_JC model_CHV1mm/min

Experimental stress Predicted stress

293 k

473 k

573 k

673 k

0

50

100

150

200

250

300

0 50 100 150 200 250 300

Pre

dic

ted

Tru

e s

tres

s

Experimental true Stress

Experimental stress vs Predicted strain_Base

Error= 27.67 %Std dev=47.389R=0.9171

51

As seen from the graph the relationship between Experimental vs predicted true stress is not

following exact y=x type pattern. But approximately we can say it is following linear Y=X type

pattern with some error. The Johnson cook model is basic model. It does not consider the

coupled effect of strain rate and temperature. Arrhenius-type equation and modified Zerilli–

Armstrong (ZA) model for high-temperature application may predict good result.

5.2.3 Experimental vs predicted Stress for FSPed AA5754

The Three graphs Figure 42, Figure 43 and Figure 44 show experimental stress vs predicted

stress for FSPed aluminium. X-axis is represented by true strain and Y-axis is represented by

true stress. The dots show predicted value and solid line shows experimental value at 293 K,

473 k, 573 k and 673 k respectively.

Figure 42 Comparison between experimental flow stress and predicted flow stress using Johnson

Cook model in temperature domain 293 K–673K of FSP for elongation rate of 200mm/min

0

50

100

150

200

250

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

Tru

e st

ress

True Strain

True stress vs strain_JC Model_CHV 200mm/min_FSP

Predicted stress Expeimental Stress

293 K473 K573 K

673 K

52

Figure 43: Comparison between experimental flow stress and predicted flow stress using Johnson

Cook model in temperature domain 293 K–673K of FSP for elongation rate of 100mm/min

Figure 44: Comparison between experimental flow stress and predicted flow stress using Johnson

Cook model in temperature domain 293 K–673K of FSP for elongation rate of 1mm/min

As seen from the figure the model gives accurate results for higher strain rate for base metal.

At 400°C temperature the model was not able to predict accurate results. We have got best

prediction for higher strain rate and lower temperature. This is attributed to the inadequacy of

the JC model to incorporate the coupled effects of strain and temperature and of strain rate and

temperature.

0

50

100

150

200

250

0 0.05 0.1 0.15

Tru

e st

ress

True strain

True stress vs strain_JC model_CHV100mm/min_FSP

Predicted stress Experimental stress

293 K473 K573 K

673 K

0

50

100

150

200

250

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

Tru

e st

ress

True strain

Johnson Cook Model _Elongation rate1mm/min_FSP

Experimental stress Predicted stress

293 K473 K

573 K

673 K

53

Figure 45: Experimental stress vs Predicted stress for friction stir Processed AA5754

As seen from the graph the relationship between Experimental vs predicted true stress is not

following exact y=x type pattern. But approximately we can say it is following linear Y=X type

pattern with some error. The Johnson cook model is basic model. It does not consider the

coupled effect of strain rate and temperature. Arrhenius-type equation and modified Zerilli–

Armstrong (ZA) model for high-temperature application may predict good result.

5.3 Fractography

The fractured surfaces of the FSPed and base plates under tension are shown in Figure 46,

Figure 47, Figure 48 and Figure 49. SEM study showing the presence of microscopic voids of

different sizes and shapes confirms the existence of ductile failure. In this case cup like

depression called dimples are exhibited, and it is termed as dimple rupture. The size of dimples

are bigger as the temperature is high. The fractured surfaces of the tensile samples are populated

with a large number of dimples revealing ductile failure. This type of situation arises due to

severe stirring action causing intense plastic deformation.

0

50

100

150

200

250

0 50 100 150 200 250

Pre

dic

ted

str

ess

Experimental stress

Experimental vs Predicted Stress_FSP

R=0.919Errror= 9.11Std Dev=10.66

54

(a) (b)

Figure 46: Parent material fractured at 100 cross head velocity and room temp (a) 1000x (b) 2000x

(a) (b)

Figure 47: Parent material fractured at 100 cross head velocity and 400°C (a) 1000x (b) 2000x

55

(a) (b)

Figure 48: FSP tensile sample fractured at 100 cross head velocity and room temp (a) 1000X (b)

2000X

(a) (b)

Figure 49 FSP tensile sample fractured at 100 cross head velocity and 400°C (a) 1000X (b) 2000X

56

Chapter 6

6 Conclusions

Friction stir processing has been successfully used to modify mechanical properties of

AA5754. From this experimental study following conclusions can be made:

1. A cylindrical tool of 15mm shoulder diameter and 5mm pin diameter with 1.1mm pin-

length was designed. Friction stir processed samples were successfully fabricated using

900rpm and 125mm/min travel speed.

2. It was found that the %elongation increased from 16% to 92% for FSPed (friction stir

processed) when temperature was increased from room temperature to 4000° C at a

constant cross head velocity of 1 mm/min and there was 77.28% decrease in ultimate

tensile strength. Similar observation was found in base material.

3. The FSPed sample was found to be insensitive to strain rate when cross head velocity

(CHV) was changed from 1mm/min to 200mm/min at room temperature. However,

significant strain rate effect was observed for both parent and FSPed sample at 300° C and

400° C.

4. The Johnson Cook model was successfully developed after evaluating all the material

parameter for predicting flow strength of FSPed and base material at different elevated

temperature and strain rate. The predicted results were found to be reasonable match with

experimental data with regression coefficient (R-value) of 0.919 and 0.9171 for FSP and

base material respectively.

5. All the base metal and FSPed sample failed after localized necking, and the fractograph

studies confirm ductile rupture of the samples.

57

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