Analysis the influence of threaded pin profile in friction stir welding by numerical simulation

ABSTRACT Friction stir welding is a solid-state welding process in which heat is generated by friction between the rotating tool and the workpiece material, resulting in the contact surf

VIET NAM NATIONAL UNIVERSITY HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

PJK烏O"X影"NW一P"X;P"VJ萎E"U

J丑"v‒p"j丑e"xk‒p<""A姥"JW PJ"PJ姶 MSHV: 1970360

Pi {."vj pi."p<o"ukpj<""08/05/1997 P挨k"ukpj<"" Long An

Chuyên ngành: E挨"M悦"vjw壱v O«"u嘘"< 8520101

ACKNOWLEDGEMENTS

Firstly, I would like to express thanks to professor, Assoc Prof.Dr Vu Cong Hoa for his unwavering support, supervision, and helpful recommendations during this research project Besides that, he usually encouraged and gave me good advice when the Covid-19 is spreading to my accommodation On top of that, he gave a chance to

be able to work in Nha Trang University Secondly, I would like to thank the teachers working at the Faculty of Applied Science for supporting me in publishing a scientific article Thirdly, I would like to thank and apologize my family, often shared my feelings and gave me timely advice during a long time of studying However, I made

my parents worry when my dorm had a lot of people infected with Covid-19 Last but not least, I am grateful to all of my best friends and my boyfriend who contributed to help me overcome my internship

ABSTRACT

Friction stir welding is a solid-state welding process in which heat is generated by friction between the rotating tool and the workpiece material, resulting in the contact surface being heated to ductile state Friction stir welding process is widely used for joining aluminum alloy in many fields such as aerospace, railway, marine, and many other modern industries This welding method allows to improve the weld strength

of aluminum alloys in groups as 5xxx, 6xxx and 7xxx In this study, the numerical model of the FSW process with different threaded tools based on Coupled Eulerian-Lagrangian formulation is investigated by ABAQUS software The reliability of this method is demonstrated by comparing the results of the temperature distribution between the numerical simulation method and the published experimental method

An important point in this research is the investigation of friction weld on 7075-T6 aluminum alloy material with threaded pin profile combined with three flats Based

on void information and temperature field distribution, this study would recommend

a new pin profile that provides the best welding quality among the pin configurations tested

As a result, 14% is the average error of welding temperature between simulation and experiment Although this error is quite high, behavior of this simulation model is slightly similar to experimental model Besides that, there are several reasons why it has such difference as the thermal properties of material, parameters in process, assumption of the heat transfer coefficient With four profiles including TF0, TF30, TF60 and TF90 investigated, it is observed that TF60 brings more advantages in joining materials as a smaller void, not much difference in temperature between two sides of material and a larger stirring zone

There are four chapters in this master thesis

Chapter 1: Introduction to Friction stir welding process and many recent studies related to this problem

Chapter 2: Theoretical basis

Chapter 3: Research methodology

Chapter 4: Results and conclusions

TÓM T 溢T LU一P"X;P"VJ萎E"U

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s詠 ma sát gi英a d映ng c映 hàn và v壱t li羽u hàn, nó làm cho v壱t li羽w"j p"p„pi"n‒p"8院n

tr衣ng thái d飲o Nh運 vào s詠 khu医y tr瓜n và áp l詠c c栄a d映ng c映 j p"8吋 t衣o m嘘i liên k院t

gi英a hai t医m v壱t li羽w"pj逢pi"mj»pi"n o"p„pi"ej違y v壱t li羽u Công ngh羽 hàn ma sát khu医{"p {"8逢嬰c áp d映ng r瓜ng r«k"8吋 hàn các h嬰p kim nh姻 trong các ngành công nghi羽p

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c違i thi羽p" 8欝 b隠n m嘘i hàn c栄a các h嬰p kim nhôm nhóm 5xxx, 6xxx, 7xxx Trong nghiên c泳u này, mô hình mô ph臼ng s嘘 quá trình hàn ma sát khu医y v噂i nhi隠u ch嘘t ren

mj e"pjcw"8逢嬰c kh違o sát trên ph亥n m隠o"CDCSWU0"A瓜 tin c壱y c栄c"rj逢挨pi"rj r"o»"

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tr丑ng nh医t trong nghiên c泳u này là kh違o sát hàn ma sát khu医y trên v壱t li羽u AA T6 v噂i ch嘘t hàn ren k院t h嬰p nhi隠u góc m荏 khác nhau c栄c"Ðvjtgg"hncvuÑ0"F詠a vào ph亥n

7075-b嘘 nhi羽v"8瓜."vj»pi"n逢嬰ng nhi羽t và s詠 hình thành khuy院t t壱t trong m嘘k"j p"8吋 8隠 xu医t

ch嘘t hàn ren m噂i mang l衣i ch医v"n逢嬰ng m嘘i hàn t嘘v"j挨p"vtqpi"e e"nq衣i ch嘘v"j p"8逢嬰c

kh違o sát

K院t qu違 cho th医y là 14% là sái s嘘 gi英a k院t qu違 th詠c nghi羽m và k院t qu違 mô ph臼ng v噂i

t益 s嘘 v壱n t嘘c quay và v壱n t嘘c di chuy吋n c栄a d映ng c映 hàn là /y p ?2000 / 180 vg/mm

M員c dù, k院t qu違 sai s嘘 khá cao tuy nhiên 泳ng x穎 quá trình hàn ma sát khu医y gi英a mô

ph臼ng và th詠c nhi羽m có ph亥n ph嘘i nhi羽v"8嘘 e„"zw"j逢噂pi"v逢挨pi"8嘘i gi嘘ng nhau Và sai s嘘 p {"e pi"e„"pjk隠u nguyên nhân khách quan d磯p"8院p"pj逢"j羽 s嘘 ma sát, h羽 s嘘 truy隠n nhi羽t và các th嘘ng s嘘 th運i gian trong các gi違k"8q衣n c栄a quá trình hàn này V噂i

b嘘n biên d衣ng ch嘘v"tgp"8逢嬰c kh違o sát TF0, TF30, TF60 và TF90 thì TF60 mang l衣i

ch医v"n逢嬰ng m嘘i hàn t嘘t nh医t vì chênh l羽ch nhi羽v"8瓜 gi英a hai bên m嘘i hàn th医p và d詠 hình thành kho違ng tr嘘ng nh臼0"A¤{"e pi"n "d p"8衣r"8吋 b逸v"8亥u nghiên c泳w"u¤w"j挨p"

sw "vt·pj"j p"p {"8吋 8逢c"tc"pj英pi"8隠 xu医t t嘘v"j挨p"vtqpi"泳ng d映ng th詠c t院

Lu壱p"x<p"p {"8逢嬰c trình bày v噂i 4 ej逢挨pi

Ej逢挨pi"3<"V鰻ng quan gi運i thi羽u v隠 công ngh羽 hàn ma sát khu医y và các nghiên c泳u liên quan

Ej逢挨pi"2<"E挨"u荏 lý thuy院t

Ej逢挨pi"3<"Rj逢挨pi"rj r"pijk‒p"e泳u

Ej逢挨pi"4: K院t qu違 và bàn lu壱n

GUARANTEE

I hereby declare that the ocuvgtÓu thesis, ÐCpcn{uku"vjg"kphnwgpeg"qh"vjtgcfgf"rkp"

rtqhkng"kp"htkevkqp"uvkt"ygnfkpi"d{"pwogtkecn"ukowncvkqpÑ is my independent scientific

research The scientific research data and research results of the thesis are honest, and the references have been clearly cited

If I find any copying from other research results or errors in research data, I will take full responsibility before the school and the board

Author

A厩 Hu pj"Pj逢

CONTENT

List of Figures ix

List of Tables xi

Glossary of Terms xii

List of Symbols xiii

CHAPTER 1 INTRODUCTION 1

1.1 The Friction Stir Welding (FSW) Process and Reasons for Study 1

1.1.1 Historical Development of Friction Stir Welding 1

1.1.2 Reasons for Study 2

1.2 Object and Scope of Study 4

1.2.1 Objectives 4

1.2.2 Scope of Study 5

1.3 Overview of Master thesis 5

1.4 Literature review 5

CHAPTER 2 THEORETICAL BASIS 11

2.1 Working Principle of Friction Stir Welding 11

2.1.1 Principle of Operation 11

2.1.2 Material flow and Mechanism of Joining 12

2.1.3 The force Analysis of Friction Stir Welding 14

2.1.4 Friction Stir Welding tool 15

2.2 Using Coupled Eulerian Î Lagrangian formulation in Friction Stir Welding17 2.2.1 Introduction to the Coupled Eulerian Î Lagrangian (CEL) 17

2.2.2 Model description in ABAQUS/ Explicit in FSW process 20

2.2.3 Johnson Î Cook material model used to simulate FSW process 22

CHAPTER 3 RESEARCH METHODOLOGY 25

3.1 Introduction to the model and results of the friction stir welding experiment for Aluminum Alloy 6082 Î T6 25

3.2 Setting computational domain and material data 25

3.2.1 Thread pin tool 26

3.2.2 Workpiece 27

3.3 Interaction, loads and boundary conditions 28

3.3.1 Contact 28

3.3.2 Welding parameters introduced in the model 29

3.4 Mesh size and mesh type 30

3.5 Analysis results compared with experimental results available in previous study [3] 32

CHAPTER 4 RESULTS AND CONCLUSIONS 35

4.1 Results 35

4.1.1 Temperature results 35

4.1.2 Prediction of void formation 42

4.2 Conclusions 44

List of Publications 47

References 48

List of Figures

Figure 1-1 Mechanism of Friction Stir Welding [6] 2

Figure 1-2 Diagram of parameters influencing on FSW joint quality 3

Figure 1-3 Tool design showing geometric parameter [9] 6

Figure 1-4 FWS tool pin profiles are used in this research 6

Figure 1-70"Ocvgtkcn"Þqy"xgnqekv{"*c+"Wpvjtgcfgf"rkp."*d+"vjtgcfgf"pin.[11] 7

Figure 1-80"Ecnewncvgf"jgcv"Þwz"cv"vqqn-workpiece interface (a) Unthreaded pin, (b) threaded pin.[11] 7

Figure 1-7 Temperature profiles of the Grooved pin obtained from the experimental measurement and numerical simulation, (a) AA 6061-T6 (advancing side), (b) AA 7075-T6 aluminum alloy (retreating side) 8

Figure 1-8 CNC milling machine are used to set up experimental models in friction stir welding [14] 9

Figure 1-9 Friction stir welding process is conducted by author Duong Dinh Hao [14] 10

Figure 1-10 The comparison of the weld surface with various y/ vratios [14] 10

Figure 1-11 Experimental design model of author Than Trong Khanh Dat [15] 10

Figure 2-1 Schematic Diagram of FSW [16] 11

Figure 2-2 The Friction Stir Welding Process, (a) preparing to plunge, (b) plunging pin into material, (c) force applied on shoulder, and (d) transverse along the weld line[17] 12

Figure 2-3 Stream traces on different horizontal planes (a) 0.35 mm, (b) 1.59 mm and (c) 2.28 mm below the top surface [16] 13

Figure 2-4 Main weld zones (schematic); BM = base metal [18] 14

Figure 2-5 The kinds of force on the tool in FSW process [16] 15

Figure 2-6 Parts of friction stir welding tool [5] 16

Figure 2-90"Fkhhgtgpv"v{rgu"qh"vqqn"rkp"rtqÝngu 16

Figure 2-8 a) Largrangian analysis, b) Eulerian Analysis, c) Split operator for the CEL formulation [21], [22] 19

Figure 2-9 Stress-strain curve [25] 23

Figure 2-10 A graph of the linear relationship between ln(u0/A)and lngpl [26].23 Figure 3-1 The welding machine is used to investigate weldments with AA 6082 Î T6 materials [3] : a) general overview; b) work area 25

Figure 3-2 Dimension of thread pin tool referring to author M Iordache [3] 26

Figure 3-3 Cross section of tool pin profiles to be used in the present study 26

Figure 3-4 Movement direction of pin tool 27

Figure 3-5 Displacement boundary conditions of model in ABAQUS software 30

Figure 3-6 Assembly and mesh in model in ABAQUS software 31

Figure 3-7 Coupled Eulerian-Lagrangian model in this study 32

Figure 3-8 Temperature distributions in workpiece by ABAQUS software 33

Figure 3-9 Temperature distributions along cross-section in FSW weldment after dwelling stage 34

Figure 4-1 Temperature distribution field of four pin profile tested in ABAQUS software a) TF0, b) TF30, c) TF60, d) TF90 36

Figure 4-2 Temperature between advancing side (right) and retreating side (left) in welding phase for the TF0 profile 38

Figure 4-3 Temperature between advancing side (right) and retreating side (left) in welding phase for the TF30 profile 38

Figure 4-4 Temperature between advancing side (right) and retreating side (left) in welding phase for the TF60 profile 39

Figure 4-5 Temperature between advancing side (right) and retreating side (left) in welding phase for the TF0 profile 39

Figure 4-6 Peak temperature in the middle of the weld (oC) 40

Figure 4-7 The process of heating and softening the material in dweller phase 40

Figure 4-8 Heat flux in Friction stir welding process investigated in this study 41

Figure 4-9 Void formation of four pin profiles at the end of dweller phase 43

Figure 4-10 Void formation of four pin profiles at the beginning of welding phase 44

List of Tables

Table 2-1 Summary of tool materials [5] 17Table 3-1 Properties of steel H13 [28] 27Table 3-2 Material properties and constants in Johnson Î Cook model of AA 6082-T6 and AA 7075-T6 [12], [29], [30] 28Table 3-3 Parameters are used in this study 29Table 3-4 Number of tetra elements in model used in this study 30Table 3-5 Temperature is measured in the experiment of author M Iordache [3] 33Table 4-1 Comparing the temperature difference between two material plates in the cases of using TF0, TF30, TF60 and T90 profiles 37

Glossary of Terms

AA Î Alloy Aluminum

ALE Î Arbitrary Lagrangian Eulerian

CEL Î Coupled Eulerian Î Largragian

CFD Î Computational Fluid Dynamic

EVF Î Eulerian Volume Fraction

FEMs Î Finite Element Methods

FSW Î Friction Stir Welding

HAZ Î Heat-affected zone

M The discrete mass matrix

C The viscous damping matrix

K The stiffness matrix

u The nodal displacement

u and u Velocity and acceleration vectors

n The train hardening exponent

m Thermal softening exponent

A, B, and C Constants in Johnson-Cook model

E [qwpiÓu"oqfwnwu"*ORc+

p Poisson coefficient

CHAPTER 1 INTRODUCTION

As mentioned in [1], the Sumerian civilization first used the welding to weld copper utensils After the First World War, the significance of the welding was increased sharply Because the Treaty of Versailles provided that all building ships heavier than 10,000 tons were prohibited for all countries Therefore, German engineers began to study the joining methods for warships and then they could decrease by 1000 tons for the ship weight Since then, weight has become an important consideration in ship design According to the recent news, people have been facing several global problems such as the greenhouse effect and reducing natural resources As a result, there is a growing need to reduce fuel usage and employ ecologically friendly technology Thus, much research has been focused on developing new joining methods and assess their capabilities for manufacturing lightweight structures

FSW is a solid-state welding process in which heat is generated by friction between the rotating tool and the workpiece material, resulting in the contact surface being heated to a ductile state While the tool is both moved along the seam and rotated around its axis, it mixes the softened material of the two metal pieces and forms a bond at the weld surface through mechanical pressure This process was invented at The Welding Institute (TWI), UK in 1991 [2] It has been used for welding similar and dissimilar metals and alloys with different thickness, physical, chemical, and mechanical properties [3], [4] FSW is the opposite of other traditional welding methods, it does not melt the material and does not require sub-materials to create the weld On cooling, a solid phase bond is created between the workpieces [5] (Figure 1-1) This is also a reason why most scientists in the world consider this process a ÐitggpÑ"ygnfkpi"vgejpqnqi{0 The FSW process minimizes the types of defects that tend to arise during solidification as in other welding methods The defects of friction stir welding occur mainly due to insufficient temperature to soften the weld material

or poor welding conditions

Figure 1-1 Mechanism of Friction Stir Welding [6]

Many potential advantages of FSW over conventional fusion-welding processes have been identified: [6]

‚ Can use without sub-materials, harmful gases, toxic fumes and the spatter of molten material

‚ Low environmental and human impact

‚ Can be automated and mechanized

Besides that, FSW has some disadvantages such as exit hole left when tool is withdrawn, and large down forces required

Friction Stir Welding is the most significant development in joining in a decade and

it iu" c" ÐitggpÑ" vgejpqnqi{" dgecwug" qh" kvu" gpgti{" ghhkekgpe{." htkgpfn{" gpxktqpogpv"impact The process is suitable for welding of ferrous and non-ferrous materials of the same kind or dissimilar, especially recommended for soft materials namely aluminum, copper, nickel, titanium, and others [7] Recently, AA 7075 Î T6 have been using usual and commonly in the aviation, maritime, and automotive industries, because of its advanced mechanical and material properties It becomes a preferred choice when the best possible combination of toughness, strength, stress corrosion

cracking resistance is required for the utilization of aluminum alloys AA 7075 Î T6 material is reasonably appropriate for welding of high strength Therefore, AA7075

Î T6 and Friction Stir Welding is the best combination for industries However, in this technology, the parameters play a vital role in affecting the mechanical properties

of weldment [8] The primary parameters are traverse speed, rotational speed, tool geometry (pin profile) The secondary parameters are thickness of the workpiece, workpiece material and welding tool material, (See figure 1-2)

Figure 1-2 Diagram of parameters influencing on FSW joint quality

In the last decade, lots of research have done on different FSW parameters Inside, there are many studies that focused on pin profile The tool profile affects the mechanical properties of the weld joint directly In this study, the tool pin profiles are investigated, especially the threaded pin tool because it manages the material movement and significantly influence the flow of plasticized material Furthermore, most of all studies in which threaded shapes are most effective on tool performance [9] In comparison to previous published research, investigation of friction weld on 7075-T6 aluminum alloy material with pin profiles threaded cylindrical-three flats with various opening angles is a new aspect of research From that, the research

results will evaluate and analyze the influence of each thread pin profile and create a reference base for related studies

Therefore, the topic is called ÐCpcn{uku" vjg" kphnwgpeg" qh" vjtgcfgf" rkp" rtqhkng" kp" htkevkqp"uvkt"ygnfkpi"d{"pwogtkecn"ukowncvkqpÑ0

The research objective is to choose a great threaded pin profile to increase the quality

of the weld and minimize defects in process of friction stir welding via predicting weld defects and temperature field distribution Besides that, this study presents the steps of the numerical simulation using the Coupled Eulerian Î Lagrangian (CEL) methods for this process The design of many different threaded pin profiles through SOLIDWORKS software and simulation model on ABAQUS/ Explicit Moreover, the research results will discuss the advantages and disadvantages of the study method

Specific objectives as:

- Firstly, research on the theoretical basis of Friction Stir Welding (FSW) process principles, many parameters impact, heat transfer, and heat generation

by friction in this process

- Secondly, research coupled thermo-mechanical finite element procedure, setting model, and simulating FSW process with AA 6082 Î T6 via CEL methods The computation results are compared with the experimental results available in previous researchers' papers Based on the comparison results, it will conclude the reliability of study method

- Thirdly, research and design lots of thread pin profiles, using the study method

to simulate FSW of AA 7075 Î T6 with different threaded pins From that, recommend the best threaded pin of the evaluated pins

1.2.2 Scope of Study

The focus of the research work will be concentrated in the coupled mechanical finite element during FSW process, weld defects, distribution temperature field Model has 65mm x 60 mm x 5mm thick sheet Aluminum (AA

thermo-7075 Î T6) using different threaded pin profiles

Chapter 1 introduces Friction Stir Welding Process, the reason why to choose this topic, present objectives, and scope of the study clearly And then, it presents detail many recent studies to find gaps in the FSW process

Chapter 2 presents the FSW process principles, coupled thermo-mechanical finite element procedure and elastic-plastic Johnson Î Cook of material workpiece

Chapter 3 presents setting model, and simulating FSW process the computation results are compared with the experimental results available in previous researchers' paper To use the study method to simulate FSW of AA 7075 Î T6 with different designed threaded pins

Chapter 4 will recommend the best threaded pin of the evaluated pins and discuss the advantages and disadvantages of the study method

As cited in the above chapter, there are many parameters affecting the mechanical properties of weldment in FSW such as velocity along the weld, rotational speed, tool geometry (pin profile), thickness of the workpiece, workpiece material, welding tool material and other things However, the tool/pin shape and size are important variables in FSW There are several simulation methods in FSW like Computational Fluid Dynamic (CFD), Arbitrary Lagrangian Eulerian (ALE) formulation and Couple Eulerian-Lagrangian modeling (CEL)

There are some recent studies related to the FSW process by numerical simulation analysis such as:

G Buffa et al [10] kpxguvkicvgf"vjg"kpÞwgpeg"qh"rkp"cping"qp"vjg"ocvgtkcn"Þqy"cpf"grain size by using a 3D FEM model The obtained result allows using a conical pin with pin angle of 40o speed and speed equal to 100 mm/min to get the best joint strength (Figure 1-3)

Figure 1-3 Tool design showing geometric parameter [10]

Based on this study, it can be concluded that a tool pin angle of 40o and an advancing speed of 100 mm/min is the ideal combination for achieving the best joint strength, taking into account grain size, nugget area, tool wear, and welding forces

K Elangovan et al [11] investigated effect of tool pin profile and tool shoulder diameter in AA6061 aluminum alloy by experiment

Figure 1-4 FWS tool pin profiles are used in this research

Qpg"qh"vjg"Ýxg"vqqn"rkp"rtqÝngu"wugf"kp"vjku"kpxguvkicvkqp to make the joints, square rkp"rtqÝngf"vqqn"rtqfwegf"fghgev"htgg"HUR"tgikqp."kttgurgevkxg"qh"ujqwnfgt"fkcogvgt"

of the tools In addition, one of the three tool shoulder diameters used in this investigation to fabricate the joints, a tool with 18 mm shoulder diameter produced fghgev"htgg"HUR"tgikqp."kttgurgevkxg"qh"vqqn"rkp"rtqÝngu, (Figure 1-4)

Z Sun et al [12] studied the effect of pin thread on heat generation, temperature distribution and material field by a numerical model Pin thread is proposed when it comes to use FSW process because material flow velocity and area of TMAZ (thermal-mechanically affected zone) for pin thread are bigger than for pin unthread, (Figure 1-5, Figure 1-6)

htkevkqp"kp"ygnfogpvÓu"xqkf"hqtocvkqp"d{"wukpg CEL simulation method This study concluded that using friction coefficient lower, the void is larger According to this author, CFD simulation has many disadvantages Firstly, contact conditions are usually assumed to be full sticking, leading to over-estimation of weld peak vgorgtcvwtg"cpf"vqqn"tgcevkqp"nqcfu0"Hwtvjgtoqtg."EHF"oqfgnu"kipqtgf"ocvgtkcnÓu"elasticity For the ALE technique [13], it can take into consideration temperature However, the Lagrangian elements cannot deal with the void formation so serious mesh distortion will occur in the large deformation analysis The Eulerian elements

is based on the volume-of-fluid method where the material is being tracked as it flows through the mesh through its Eulerian Volume Fraction (EVF) Therefore, the Eulerian elements can handle multiple materials as well as void formations, which is

an advantage, (Figure 1-7)

Figure 1-7 Temperature profiles of the Grooved pin obtained from the experimental measurement and numerical simulation, (a) AA 6061-T6 (advancing side), (b) AA 7075-T6 aluminum alloy (retreating side)

Bahman Meyghani et al [1] reviewed different FEMs in the Thermal Analysis of FSW According to this study there are some significant issues and subjects regarding FSW thermal analysis remain to be addressed, and future work prospects are offered For example, the lack of a mathematical model in which the material property can be estimated is important in order to reduce the expenses of the experimental test, because the material property is normally obtained through experimental data, which

is always expensive Besides that, the numerical understanding of joining of similar

and dissimilar aluminum alloys is still lacking, therefore it remains one of the major objectives that many academics are interested in

S Emamian et al [14] gave information about influence of tool pin profile on the friction stir welding of AA6061 There are four pin profiles used to make joints with different rotation and traverse speed Result in this study is showed that threaded cylindrical produced better welds

In addition to foreign studies, there are also domestic friction stir welding studies as research of author Duong Dinh Hao working at Nha Trang University His master -T6thesis research mechanical behavior of aluminum alloy (AA7075) in friction stir welding process by experiment in 2014 This study shows that if the ratio between rotational speed and translational speed of the welding tool (y/ v) is equal to around

4 ÷ 10 rev/mm, weld quality will be better The tensile strength and elongation of AA7075 material in friction stir welding are about 68% and 78%, and it is proportional to this ratio - y/ v Besides that, Fatigue of welds was evaluated and compared with base material [15] There are lots of pictures in this study shown below:

Figure 1-8 CNC milling machine are used to set up experimental models in friction stir welding [15]

Figure 1-9 Friction stir welding process is conducted by author Duong Dinh Hao [15]

Figure 1-10 The comparison of the weld surface with various y/ vratios [15] Besides that, Than Trong Khanh Dat was a graduate student in Bach Khoa University studying the influence of tool tilt angle on the weld quality in friction stir welding process through construction mathematical models and experimental design models

in 2014 The study results provided the technological parameters for friction stir welding processes on aluminum alloy as [16]:

- Rotational speed arranges from 800 to 2000 (Rpm)

- Translation speed arranges from 10 to 100 (mm/min)

- Tool tilt angle arranges from 1o to 5o

Figure 1-11 Experimental design model of author Than Trong Khanh Dat [16]

CHAPTER 2 THEORETICAL BASIS

Figure 2-1 Schematic Diagram of FSW [17]

First, two sheets of material are held in place and clamped together In the second step, the welding pin is mounted in a high-speed rotating chuck placed on the weld to heat the material by friction Heat is generated as a result of the friction, which raises the temperature of the plates and reduces the material's strength Once the pin is fully inserted, it is pushed in the welding direction at a minor angle An advancing side and

a retreating side are formed as the softened and heated material flows around the pin

to its backside, where it is consolidated to create a solid-state weld, thanks to forward and rotational movement of the pin and shoulder of the tool along the weld, (Figure 2-1) [17]

The rotation of the welding pin plays two important roles First, it creates friction to generate heat high enough to soften the material but not melt it Second, the rotation

of the welding tool causes the two materials to be mixed together to create a weld with a new mixed material, (Figure 2-2)

Friction Stir Welding has fewer process elements to regulate, which is a considerable advantage Many process variables must be managed in a Fusion weld, including purge gas, voltage and amperage, wire feed, travel speed, shield gas, and arc gap [17] However, there are only three process variables to manage in Friction Stir Weld: rotation speed, travel speed, and pressure, all of which are simple to control

Figure 2-2 The Friction Stir Welding Process, (a) preparing to plunge, (b) plunging pin into material, (c) force applied on shoulder, and (d) transverse along the weld line [18]

The rotating tool creates a continuous hot working action by moving material around

a pin at temperatures below the melting point of the materials to be processed, plasticizing metal within a restricted zone and conveying metal from the leading face

to the trailing edge of the pin The substance is believed to be in a plastic condition, allowing it to be moved about The energy provided by the pin pressure and torque is used to generate a substance with an apparent viscosity that allows it to flow around the pin Because the fluidic layer serves as a boundary layer between the solid parent material and the pin, it is easy to imagine this boundary layer being pushed away or having its thickness decreased at the pin's leading edge, (Figure 2-3) [17]

Figure 2-3 Stream traces on different horizontal planes (a) 0.35 mm, (b) 1.59 mm and (c) 2.28 mm below the top surface [17]

As a result of M Grujicic et al [19] showed four zones in the weldment affected by this FSW process (Figure 2-4):

a) The heat-c振gevgf" |qpg" *JC¥+ in which just the temperature fluctuations

related with FSW affect the materials' microstructure and characteristics (due

to the material-urgekÝe" rtqeguugu" uwej" cu" tgeqxgt{." tget{uvcnnk|cvkqp." itckp"growth, precipitate coarsening, precipitate dissolution, etc.)

b) The thermo-ogejcpkecnn{"c振gevgf"|qpg"*VOC¥+."in which the original grains

undergo significant form changes, such as elongation, flattening, and bending, and the microstructural alterations seen in the HAZ are more pronounced

c) The stir zone (the innermost zone of an FSW joint), in which the soft workpiece material is exposed to the most severe plastic deformation and high-temperature conditions Small fragments of the harder-workpiece material are frequently found in this zone, and the surface layer of these fragments typically contains intermetallic compounds (IMCs) of varying thickness and size d) The mix zone, in which the most mixing of the two weld faces is combined The degree of inter-material bonding in this zone, as determined by the creation of metals, is often what determines the weld's overall strength

Figure 2-4 Main weld zones (schematic); BM = base metal [19]

During friction stir welding, there are different parameters effects on the process and

on the forces on the pin such as:

- Material alloy

- Grain direction

- Rotational velocity of Shoulder and Pin (rpm)

- Dwell time: The time the solder pin rotates in place is fixed for the material to

be heated and softened - the first stage in the friction stir welding process

- Traverse rate (mm/min)

- Pin profiles

These variables have an impact on micro structural changes, mechanical characteristics, and forces acting on the FSW tool

There are three types of force act on the tool in FSW process shown below, (Figure 2-5)

Figure 2-5 The kinds of force on the tool in FSW process [17]

a) The Longitudinal Force (X-force)

In the traverse direction, the traverse force acts parallel to the tool motion and is positive Since this force arises due to the material's resistance to tool motion, this force can decrease as the temperature of the material around the tool increases

b) The Vertical Force (Y-force)

A downwards force is necessary to maintain the position of the tool in welding process

c) The Lateral Force (Z-axis)

The lateral force may act perpendicular to the tool traverse direction and is defined here as positive towards the advancing side of the process A torque is required to rotate the tool, the value of which will depend on the down force and friction coefficient (sliding friction) and/or the flow strength of the material in the surrounding region (sticking friction) [17]

The FSW tool is made up two main components, the pin and the shoulder shown in figure 2-6

Figure 2-6 Parts of friction stir welding tool [5]

The quality of the weld is highly dependent on the design of the shoulder and pin The tool's pin creates heat and stirs the welded material, but the shoulder also plays a role by providing addition frictional treatment and preventing the plasticized material from escaping from the weld area Different materials and thickness of workpiece will obviously typically require different profile pins, and welds can be made from just one side or by welding half the thickness and then turning over to finish the other side

2.1.4.1 Profiles of pin

Friction stirring pins heat the joint surfaces through deformation and friction ÐThe pin is designed to disrupt the faying, or contacting surfaces of the workpiece, shear material in front of the tool, and move material behind the toolÑ [5] In addition, the profile tools and tool traverse rate are very important as well Some of the different pin profiles are described in figure 2-7

Figure 2-7 Different types of tool pin rtqÝngu

2.1.4.2 Tool Material Selection

Friction stirring is a thermomechanical deformation technique in which the tool temperature approaches the base metal's solidus temperature When choosing a material, keep the following aspects in mind:

Table 2-1 Summary of tool materials [5]

Aluminum Alloys 3 Î 50 Tool steels, Co-WC composite

Magnesium alloys 3 Î 10 Tool steel, WC composite

Copper Alloys 3 Î 50 Ni-alloys, W-alloys, PCBN, Tool

steels Titanium Alloys 3 Î 10 W-alloys

Stainless Steels 3 Î 10 PCBN, W-alloys

Low-alloy Steels 3 Î 10 WC composite, PCBN

There are several simulation methods in FSW like Computational Fluid Dynamic (CFD), Arbitrary Lagrangian Eulerian (ALE) formulation and Couple Eulerian-Lagrangian modeling (CEL) M Safari et al [13] used CEL modeling of material flow