Khảo sát ảnh hưởng của biên dạng chân đến độ bền kéo của hàn khuấy ma sát

VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY TRA NGOC TIEN DAT INVESTIGATION ON THE EFFECT OF PIN PROFILE ON TENSILE STRENGTH OF FRICTION STIR

VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

TRA NGOC TIEN DAT

INVESTIGATION ON THE EFFECT OF PIN PROFILE ON TENSILE STRENGTH OF FRICTION STIR WELDING

Major Subject: Engineering Mechanics

Codes: 8520101

MASTER THESIS

HO CHI MINH CITY, January 2023

THIS RESEARCH IS COMPLETED AT

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY – VNU HCM Instructor: Ass Prof PhD Vu Cong Hoa

PhD Duong Dinh Hao

Examiner 1: PhD Mai Duc Dai

Examiner 2: PhD Nguyen Tuong Long

Master’s thesis is defended at HCMC University of Technology, VNU HCM on January 11, 2023

The Board of Master’s thesis defense council includes:

1 Chairman: Prof PhD Ngo Kieu Nhi

2 Secretary: PhD Pham Bao Toan

3 Counter-Argument member 1: PhD Mai Duc Dai

4 Counter-Argument member 2: PhD Pham Tan Hung

5 Council Member: PhD Nguyen Tuong Long

Verification of the chairman of the master’s thesis defense council and the Dean of the Faculty of Applied Science after the thesis is correct (if any)

CHAIRMAN OF THE COUNCIL

DEAN OF FACULTY OF APPLIED SCIENCE

(Full name and signature) (Full name and signature)

VIETNAM NATIONAL UNIVERSITY HCMC

VNUHCM UNIVERSITY OF TECHNOLOGY

SOCIALIST REPULIC OF VIETNAM Independent – Liberty – Happiness

MASTER’S THESIS ASSIGNMENTS

Full name: Tra Ngoc Tien Dat Leaner ID: 2070067

Date of birth: 10/10/1997 Place of birth: Quang Ngai

I TITLE:

INVESTIGATION ON THE EFFECT OF PIN PROFILE ON TENSILE STRENGTH OF FRICTION STIR WELDING/ KHẢO SÁT ẢNH HƯỞNG BIÊN DẠNG CHỐT HÀN ĐẾN ĐỘ BỀN KÉO MỐI HÀN MA SÁT KHUẤY

II ASSIGMENTs:

- Research about the principle of friction stir welding process

- Design and fabricate friction stir welding tools with different pin profile

- Initialize the simulation model for friction stir welding process in Abaqus software

- Conduct the friction stir welding experiment with aluminum alloy T6

AA6061 Evaluate and analyze the effect of pin profiles on the tensile strength of the welds and other mechanical properties

III ASSIGMENT DELIVERY DATE: September 05, 2022

IV ASSIGMENT COMPLETING DATE: December 18, 2022

V INSTRUCTOR: Ass Prof PhD VU CONG HOA

PhD DUONG DINH HAO

Ho Chi Minh City, January 11, 2023

INSTRUCTOR 1

(Full name and signature)

Ass Prof PhD Vu Cong Hoa

INSTRUCTOR 2

(Full name and signature)

PhD Duong Dinh Hao

HEAD OF DEPARTMENT

(Full name and signature)

Ass Prof PhD Vu Cong Hoa

DEAN OF FACULTY

(Full name and signature)

Ass Prof PhD Truong Tich Thien

LỜI CẢM ƠN

Lời đầu tiên, tôi xin chân thành cảm ơn đến hai người thầy đã hướng dẫn tôi trong luận văn này là PGS TS Vũ Công Hoà và TS Dương Đình Hảo Cảm ơn hai thầy đã cho tôi những định hướng, phương pháp nghiên cứu, cũng như sự hỗ trợ hết sức nhiệt tình trong suốt thời gian thực hiện đề tài

Tôi cũng xin gửi lời cảm ơn đến:

- Bộ môn Cơ kỹ thuật, khoa Khoa học ứng dụng, trường Đại học Bách Khoa

TP HCM đã cung cấp những kiến thức và kinh nghiệm trong thời gian 2 năm tôi theo học tại đây

- Thầy PGS TS Trần Hưng Trà và phòng thí nghiệm Hàn ma sát khuấy, trường Đại học Nha Trang, đã hỗ trợ trong việc thực hiện thí nghiệm hàn, cũng như việc đánh giá chất lượng mối hàn

- Những đồng nghiệp, bạn bè ở Khoa Cơ khí, cũng như trường Đại học Bách Khoa, đã hỗ trợ trong việc thiết kế, chế tạo các chốt hàn

Cuối cùng, xin gửi lời cảm ơn đén gia đình, người thân, đã luôn đồng hành, hỗ trợ tôi trong suốt thời gian qua

Người thực hiện

Trà Ngọc Tiến Đạt

ABSTRACT

Friction stir welding is now widely applied in material joining applications, in many fields such as aerospace, marine, railway In this method, tool pin profile has great impact on the quality of the weld This study will present the investigation on the effect of different pin profiles on the quality of FSW joint, particularly in tensile strength The numerical model is developed with Coupled Eulerian Lagrangian technique in Abaqus software The heat generation is validated by experimental work The evaluation of mechanical properties of the weld, including tensile strength, bending strength and hardness was performed The macrostructure and microstructure analysis were also conducted As a result, FSW joint had been successful made in which investigated tools with joint efficiency around 78% However, the effect of different tool pin profiles on the tensile strength is not clearly observed The numerical model is good for prediction the temperature distribution in FSW process with a quite accuracy compared with experimental results

TÓM TẮT LUẬN VĂN THẠC SĨ

Hàn ma sát khuấy ngày càng được áp dụng rộng rãi trong ứng dụng hàn hay kết nối vật liệu với nhau, ứng dụng trong các ngành hàng không, vũ trụ, tàu thuỷ, … Trong phương pháp hàn ma sát, biên dạng chốt hàn đóng vai trò quan trọng trong việc tạo

ra một mối hàn đạt chất lượng Luận văn này sẽ trình bày khảo sát ảnh hưởng của các biên dạng chốt hàn khác nhau lên chất lượng của mối hàn ma sát, cụ thể là độ bền kéo Mô hình mô phỏng số cũng được xây dựng bằng phần mềm Abaqus, sử dụng kỹ thuật mô hình hoá Coupled Eulerian Lagrangian Sự sinh nhiệt trong quá trình hàn,

sẽ được kiểm tra bằng kết quả thí nghiệm Cơ tính mối hàn, gồm độ bền kéo, độ bền uốn và độ cứng mối hàn được đánh giá Việc khảo sát cấu trúc thô đại và cấu trúc tế

vi của mối hàn cũng đã được thực hiện Với các biên dạng đầu hàn dùng trong khảo sát này, các mối hàn ma sát khuấy đã được tạo ra với chất lượng đạt yêu cầu, với hiệu suất hàn khoảng 78% Tuy nhiên, ảnh hưởng của các biên dạng chốt hàn khác nhau chưa được thấy rõ Mô hình mô phỏng đã cho ra sự phân bố nhiệt độ trong quá trình hàn ma sát khuấy với độ tin cậy cao

GUARANTEE

I hereby declare that the master’s thesis, “Investigation on The Effect of Pin Profile

on Tensile Strength of Friction Stir Welding” is my independent scientific study

The numerical and experimental data in this thesis was conducted based on my effort, with strong support from Assoc Prof Vu Cong Hoa and Dr Duong Dinh Hao All references have been clearly cited

The research data in the topic is honest and absolutely not copy or use the results of

other research topics If there is a copy found, I will fully assume all responsibility

Author

Tra Ngoc Tien Dat

CONTENT

CONTENT vii

List of Figures x

List of Tables xiii

CHAPTER 1 INTRODUCTION 1

1.1 Background 1

1.2 Problem statement 1

1.3 Scope 3

1.4 Work Plan and Objectives: 3

1.5 Organization of the Thesis 3

CHAPTER 2 LITERATURE REVIEW 5

2.1 Fundamentals of Friction Stir Welding 5

2.2 Heat Generation in Friction Stir Welding 9

2.3 Friction Stir Welding Tool 9

2.4 Coupled Eulerian Lagrangian Formulation 12

2.4.1 Lagrangian and Eulerian Analysis [13] 12

2.4.2 Using Coupled Eulerian – Lagrangian Formulation in FSW 13

2.5 Evaluation of FSW Weld Properties 14

2.5.1 FSW Quality-related Parameters 14

2.5.2 Tensile Strength 16

2.5.3 Bend Testing 17

CHAPTER 3 MODELING DESCRIPTION 18

3.1 Tool Pin Profile Design 18

3.2 Geometric Modeling 19

3.3 Material Model 19

3.4 Meshing 21

3.5 Contact Interaction and Boundary Conditions 22

3.6 Mass Scaling Technique 24

CHAPTER 4 EXPERIMENTAL WORK 26

4.1 Aluminum Alloy 6061 – T6 26

4.2 Tool Fabrication 27

4.3 Fabrication of FSWed butt-joint 27

4.4 Analysis process 29

4.4.1 Temperature distribution 29

4.4.2 Microstructure 29

4.4.3 Hardness distribution 30

4.4.4 Tensile and bending test 31

CHAPTER 5 RESULT AND DISCUSSION 33

5.1 Temperature Distribution 33

5.1.1 Heat generation 33

5.1.2 Numerical Model Validation 38

5.2 Inspection of the butt-joints 39

5.3 Microstructure 42

5.4 Hardness distribution 44

5.5 Tensile Strength Testing Result 44

5.6 Bend Testing Result 48

CHAPTER 6 CONCLUSION AND FUTURE WORK 52

6.1 Summary the Results 52

6.2 Conclusion 52

6.3 Limitation 53

6.4 Recommendation for Future Work 53

REFERENCES 54

APPENDIX A - Al6061_T6 Material Certification 56

APPENDIX B – FSW Tool Drawing 57

List of Figures

Figure 2-1 A schematic drawing of friction stir welding in a butt joint configuration

6

Figure 2-2 Typical conventional FSW transverse section in 25.4-mm thick 2195 aluminum-lithium plate 8

Figure 2-3 Schematic of FSW process 8

Figure 2-4 Image courtesy of Stirweld company 10

Figure 2-5 Shoulder geometries [10] 11

Figure 2-6 FSW Pin Profiles 12

Figure 2-7 DOE FSW Process schematic 16

Figure 2-8 Specimen dimension for tensile testing 17

Figure 2-9 3-point flexure test [19] 17

Figure 3-1 Investigated tool pin profiles 18

Figure 3-2 Tool and workpiece assembly with void and material assigned regions 19

Figure 3-3 Eulerian Mesh (Workpiece) 21

Figure 3-4 Velocity boundary conditions on the workpiece 23

Figure 4-1 Aluminum plates before welding 26

Figure 4-2 Fabricated FSW Tools 27

Figure 4-3 Mazak V500 machine 28

Figure 4-4 Overview of fabricating FSWed butt-joint 28

Figure 4-5 Image of FSW process 29

Figure 4-6 Temperature measurement equipment: (a) Datalogger; (b) Thermocouple type K 29

Figure 4-7 Microstructure analysis equipment 30

Figure 4-8 Hardness measurement in Nha Trang University 30

Figure 4-9 Cutting layout for testing specimens 31

Figure 4-10 Tensile strength testing specimens 31

Figure 4-11 Specimens for bend testing 32

Figure 4-12 Tensile testing process 32

Figure 4-13 Bending testing process 32

Figure 5-1 Temperature recording position 33

Figure 5-2 Heat generation - Tool 1 33

Figure 5-3 Heat generation - Tool 2 34

Figure 5-4 Heat generation - Tool 3 34

Figure 5-5 Heat generation - Tool 4 35

Figure 5-6 Modeling of the temperature distribution at 31s (a) Tool 1, (b) Tool 2, (c) Tool 3, (d) Tool 4 36

Figure 5-7 Modeling of the temperature distribution at 31s (a) Tool 1, (b) Tool 2, (c) Tool 3, (d) Tool 4 36

Figure 5-8 Modeling of the temperature distribution at 60s (a) Tool 1, (b) Tool 2, (c) Tool 3, (d) Tool 4 37

Figure 5-9 Temperature distribution along the cross-section at the end of plunging phase (step time: 31s) 37

Figure 5-10 Experimental result and FEM result comparison 38

Figure 5-11 A finished FSW joint 40

Figure 5-12 Face of the welds 41

Figure 5-13 Root of the welds 41

Figure 5-14 Macrostructure of cross-section of the welds 42

Figure 5-15 Representative microstructures of cross-section of specimen produced by Tool 3 (a) Cross-section macrostructure; (b) Microstructure of Base metal zone; (c) Microstructure of Stir zone; (d) Microstructure of Thermal-mechanically affected zone; (e) Microstructure of Heat affected zone 42

Figure 5-16 Comparison in microstructure at SZ and HAZ produced by various tools 43

Figure 5-17 Hardness distribution measured along center weld at various tools 44

Figure 5-18 Stress-strain curve of specimens produced by various tools 45

Figure 5-19 Fracture position of tested specimens (welded by Tool 1) 45

Figure 5-20 Fracture position of tested specimens (welded by Tool 2) 46

Figure 5-21 Fracture position of tested specimens (welded by Tool 3) 46

Figure 5-22 Fracture position of tested specimens (welded by Tool 4) 46

Figure 5-23 Tensile strength property comparison and joint efficiency of the welds 47

Figure 5-24 After bending specimens - Face bend 48

Figure 5-25 After bending specimens – Root bend 48

Figure 5-26 Force – Displacement Bending Curve of Tool 1 49

Figure 5-27 Force – Displacement Bending Curve of Tool 2 49

Figure 5-28 Force – Displacement Bending Curve of Tool 3 50

Figure 5-29 Force – Displacement Bending Curve of Tool 4 50

Figure 5-30 Bending properties of FSW joints with 4 pin profiles 51

List of Tables

Table 1 Dynamic/static volume of tool pin profiles 18

Table 2 Constants used in the Johnson Cook material model [21] 20

Table 3 Variation of the physical properties of AA6061-T6 with temperature [20] 20 Table 4 H13 steel properties [22] 21

Table 5 Chemical composition of AA6061-T6 (mass percentage) 26

Table 6 Mechanical properties of AA6061-T6 26

Table 7 Mazak V550 specifications 27

Table 8 Numerical and experimental result of temperature 38

Table 9 Tensile strength test results (in MPa) 47

Table 10 Bend testing results (in MPa) 51

Glossary of Term

FSW – Friction Stir Welding

CEL – Coupled Eulerian – Lagrangian

AA – Alloy Aluminum

HAZ – Heat-affected zone

TMAZ – Thermal-mechanically affected zone

CHAPTER 1 INTRODUCTION

Metals joining processes can be divided into four basis categories: fusion welding; brazing and soldering; adhesive bonding; and solid-state bonding The solid-state bonding processes rely on deformation and atom diffusion to produce the bond, with

no melting of either the base metal or a filler metal Friction stir welding (FSW) is a solid-state joining technique, was invented at The Welding Institute (TWI) of UK in

1991 and was initially applied to aluminum alloys (Thomas et al 1991; Dawes and Thomas 1995) [1] FSW is a cold-welding technology, without fusion, without material input and welding in a “pasty” state It is used to weld high-strength aluminum alloys and dissimilar materials Now, Friction Stir Welding is one of the most popular trends in welding technology due to its advantages than traditional welding

In the last decade many researchers have been done on various FSW parameters In order to obtain the optimum FSW process, parameters such as rotation, traverse speed, axial force, tool geometry were evaluated in many studies Some of them are concentrated on tool pin profiles

Elangovan et al [2] had many research on pin profiles on aluminum alloys They investigated the influence of pin profile and rotational speed, traversing speed, shoulder diameter, and axial force on the formation of friction stir processing zone Furthermore, there are many other researchers who investigated the influence of pin profiles on microstructural and mechanical properties of the weld joint [3]

Not only the experimental works, but some researchers also focused on numerical study They tried to model different pin profiles and investigation the effect of pin profiles, as well as other parameters in FSW, on the heat generation, stress distribution, and the mechanical properties of FSW joints With the numerical

analysis, it saved a large amount of time, effort, and money in reducing the number

of experiments

In Vietnam, Friction stir welding process is a potential technology with a wide range

of industrial application Studies on this technology are gradually increasing in quantity as well as quality Some of them can be mentioned as follow:

- Study on the effect of welding parameters on tensile properties of friction stir welding AA7075 aluminum alloy plate, by Duong Dinh Hao, Tran Hung Tra and Vu Cong Hoa [4] The study provides the initial basis for the selection of process parameters in friction stir welding of 7075 aluminum alloy

- PhD thesis of Mai Dang Tuan, Effect of welding parameters on mechanical properties and microstructural features of aluminum alloy plates joint produced by friction stir welding [5] This reseach uses the DOE (desigin of experiment) method to find the optimum process parameters for different thickness of aluminum plates

- Friction stir welding research group by Prof Tran Thien Phuc had conducted several studies on process parameter to improve the weld quality [6] [7] These studies focused on DOE method, investigated on many factors such as welding speed, rotation speed, plunge depth, tilting angle, …

- Master graduation thesis of Do Huynh Nhu, Analysis the influence of threaded pin profile in friction stir welding by numerical simulation [8] This thesis use the numerical method to model the friction stir welding process Different tool pin profiles were investigated to analyze the effect of pin features to the temperature distribution as well as weld defect formation However, the analysis is just the prediction by numerical work The experimental works had not been conducted

Regarding the review above, it is necessary to have more investigation on various pin profiles in FSW And the combination of numerical and experimental work should

be used for the study

1.3 Scope

In recent years, FSW emerges in many industrial applications, so many innovations have been made to improve the quality of the welds One of these innovations is optimization the FSW Tool design, to create more stable weld, improve the weld quality, especially in tensile strength of welds

This thesis details the investigation on the effect of different FSW tool profiles on the tensile strength of the welds The investigation will be conducted with four tools of different profiles for the FSW of AA6061 3mm thickness butt joints These tools were selected based on the review of previous studies, as well as their machineability Details of FSW tool will be mentioned on the following chapters

In order to fulfill the requirement of the thesis, work plan and objectives were systematically addressed List of works are as follow:

- Establish a comprehensive literature review of the recent development of FSW tools

- Review and select materials

- Design, manufacture, and inspection FSW tools

- Finite element modeling of friction stir welding process – thermal analysis

- Conduct the experiment and evaluate the properties of joint welded by different tools

- Validate the heat generation by FEM model and experimental model

1.5 Organization of the Thesis

Following the present chapter: Introduction, the second chapter is Literature Review The fundamental of FSW, and a comprehensive literature review about tool profile

in FSW and its influence on the quality of FSW welds is established Additionally, the modeling technique focused on Coupled Eulerian Lagrangian method, and the method to evaluate quality of the welds, are detailed

Chapter 3 will describe the design of FSW tools and the characterization of Tool’s material and aluminum weld alloy The meshing technique and modeling technique will be presented in this chapter

Chapter 4 covers the set up and the implementation of the FSW experiment Experiment result and the quality of the welds will be detailed in chapter 5 The temperature in each experiment is monitored and compared with the simulation results The results of experiments will be presented with mechanical properties and microstructure analysis of the welds

Chapter 6 summarizes the work and comprises recommendations for the future work

CHAPTER 2 LITERATURE REVIEW 2.1 Fundamentals of Friction Stir Welding

The process of FSW is one of the most important recent discoveries in the field of joining material Friction Stir Welding was invented in 1991 by Wayne Thomas and his colleagues at TWI [4] This invention has opened up numerous new possibilities for making high quality welds and avoiding defects that are common in more conventional fusion welding method

Friction Stir Welding is a solid-state welding process It joins materials without melting them The process welds by rotating a non-consumable tool inside the material to be welded in order to soften them locally using heat generated by friction and plastic deformation Once softened, the joint surfaces are stirred and joined, still

in their solid state, as the material do not reach their melting temperatures This increases the weld quality compared to fusion welding as it avoids many problems associated with the fusion welding process, such as changes in volume, gas solubility, distortion, and residual stress [12]

FSW has numerous other benefits unrelated to the quality of welds It has been shown

to nearly reduce the emission of hazardous fumes during welding, therefore having less environmental impact than other more traditional welding techniques The process can be used in all orientations as gravity has negligible impact during FSW The process is typically mechanized by high axial force of the machine so requires the high equipment cost but lower the skill requirement and cost of operators [12] Figure 2-1 illustrates process definition for the tool and the workpiece in a butt join configuration The definitions of all terms are as following:

- Shoulder: the region of tool in contact with the workpiece surface

- Pin: (also referred to as probe in some literature) is inserted in the workpiece and it influences the horizonal material flow from to back, as well as vertical material flow from top to bottom

- Advancing side: where the tool pin surface rotation and the tool traverse direction have the same vectorial sense

- Retreating side: where the tool pin surface rotation direction and the tool traverse direction have the opposite vectorial sense

Figure 2-1 A schematic drawing of friction stir welding in a butt joint configuration

In Figure 2-1, the FSW tool rotates in the counterclockwise direction and travels into the page The advancing side is on the right where the tool rotation direction (sense

of tangential velocity) is the same as tool travel direction and the retreating side is on the left where the tool rotation (sense of tangential velocity) is opposite the tool travel direction

- Leading edge: is the front side of the tool The tool shoulder meets the cold workpiece material in this region

- Trailing edge: the back side of the tool The trailing part keeps increasing heat

in the workpiece after pin has crossed the region This influences the microstructural evolution after the pin created deformation

- Tool rotation speed: the speed at which the tool rotates This parameter has major contribution to heat generated and material flow

- Tool traverse speed: or welding speed, is the travel speed of the tool This parameter impacts the overall thermal cycle

- Tilt angle: the angle between the plane normal of workpiece and the spindle shaft Typically, an angle between 0˚ and 3˚ is selected

- Plunge rate: the movement rate of tool in plunging phase This rate affects the heat build-up and the force during the beginning phase of the process

- Plunge depth: the depth value measured from bottom surface of tool to the top surface of the workpiece

- Plunge force: the downward force on the tool when the shoulder meets the top surface of workpiece

A traverse section from a typical, conventional FSW joint is shown in Figure 2-2 The weld is bounded on either side by the base metal (BM) zone Although BM near the weld zone was affected by the temperature increasing during welding, this material maintains essentially the same properties as the workpiece in the as-received condition

Closer to the weld is the heat-affected zone (HAZ): in this region, the material has experienced a thermal cycle that has modified the microstructure and/or the mechanical properties The alteration of properties in the HAZ may include changes

in the strength, ductility, corrosion, and toughness of the workpiece However, there

is no plastic deformation in the area

Thermo-mechanically affected zone (TMAZ): in this region, the friction stir welding tool has plastically deformed material and the heat from the process will also have exerted some influences on the material properties In the case of aluminum alloy, it’s possible to get significant plastic strain without recrystallization in this region, and there is generally a distinct boundary between the recrystallized zone (weld nugget) and the deformed zones of the TMAZ

Weld Nugget: The fully recrystallized area, sometimes called the stir zone, referring

to the zone previously occupied by the tool pin

Figure 2-2 Typical conventional FSW transverse section in 25.4-mm thick 2195 aluminum-lithium plate

The FSW process can be divided into three phases, which is illustrated in Figure 2-3

- Plunging phase: where the weld is initiated

- Dwelling phase: tool keeps rotating at plunge position

- Welding phase: where the weld is made

Figure 2-3 Schematic of FSW process

In plunging phase: the rotating welding tool is gradually moving downwards, with specified speed and downward force Due to friction and pressure, the material is displaced and deforms around the pin as it is entering the base materials The pin is

usually plunged into the interface of the materials to be joined but sometimes, especially in FSW of dissimilar materials; it is plunged into either material at the side

of the interface line and then moved towards it When the target depth is reach, the tool is kept rotating for a few second to reach the required temperature, this phase is called dwelling phase During the welding phase, the rotating tool is moved along the joint Plastic deformation and friction generate heat in order to soften the materials

so it may flow around the pin Once the tool has reached the end of the joint path, it

is stopped, and the tool is moved up out of the materials This leaves a keyhole in the material at the end of the weld that can make it unfit for use This is usually solved

by using a run-off tab where the tool is withdrawn which is then cut off to avoid leaving the welded materials with a keyhole [12]

In general, there are two main phenomena involved in FSW process: heat generation

by friction and material flow by stirring These phenomena will be presented in the following section

2.2 Heat Generation in Friction Stir Welding

The heat generation in FSW arises from two main sources: friction between the tool and workpiece surfaces and heat generated during plastic deformation in stirred material

2.3 Friction Stir Welding Tool

A FSW Tool consists of two main parts: the shoulder, and the pin The role of two parts relates to the name of this technology:

- A shoulder to heat the material by FRICTION

- A pin to STIR the material

Figure 2-4 Image courtesy of Stirweld company

The quality of the joint in FSW is highly dependent on the tool design The geometry

of the tool pin effects the heat generation rate, torque, traverse force and the material flow The suitable material for FSW tool needs to be considered so that it can withstand the forces and stress it experiences during the process Otherwise, the tool could quickly wear out or fail Most tools for Aluminum Alloy joining are made from steel

Tool’s pin creates heat and the stirring action on the material The main heat source

in FSW is created by the shoulder The shoulder also prevents the plasticized material from escaping the welded area

In FSW, tool shoulder generates the main heat source in the process It also plays a role in preventing the plasticized material from escaping the welded area The main influencing parts of the shoulder is the contact area between the shoulder and the welded material The three most commonly used shoulder end surfaces are either flat, concave or convex The simplest end surface is featureless but features such as scrolls and grooves are commonly used to influence and improve the material flow properties and the heat generated during welding Scrolled shoulders either, depending on the tool rotation, move material into or out from the weld center as the geometry guides the material into certain flow paths Flat shoulders can pose a problem, especially when high forging loads are used as the flat surface does not efficiently keep the flowing material constrained under the shoulder, which can result in weld defects

when the material escapes the weld Concave shoulders do not have this problem They are operated using a small tool tilt of 2-4° When the tool moves inside the materials with an angle, the front end of the shoulder does not push through the material but is positioned a small distance above it The surface material gathers inside the concave shoulder and is then deposited back into the weld from under the

back end of the shoulder Error! Reference source not found

Figure 2-5 Shoulder geometries Error! Reference source not found

Pin profile of FSW tool can also be variable:

on the probe increase turbulent flow in the weld, which promotes better mixing and the breakup of the oxides of the materials These forces new chemically active surfaces of the two materials to form, which promotes the bonding of the materials

Figure 2-6 FSW Pin Profiles

Based on the above reviews about FSW design, especially in pin profiles, four designs will be chosen to be investigated in the thesis Detail of designs will be mentioned in

Chapter 3

2.4.1 Lagrangian and Eulerian Analysis [13]

In a traditional Lagrangian analysis nodes are fixed within the material, and elements deform as the material deforms Lagrangian elements are always 100% full of a single material, so the material boundary coincides with an element boundary

By contrast, in an Eulerian analysis nodes are fixed in space, and material flows through elements that do not deform Eulerian elements may not always be 100% full

of material - many may be partially or completely void The Eulerian material boundary must, therefore, be computed during each time increment and generally

does not correspond to an element boundary The Eulerian mesh is typically a simple rectangular grid of elements constructed to extend well beyond the Eulerian material boundaries, giving the material space in which to move and deform If any Eulerian material moves outside the Eulerian mesh, it is lost from the simulation

Eulerian material can interact with Lagrangian elements through Lagrangian contact; simulations that include this type of contact are often referred to

Eulerian-as coupled Eulerian-Lagrangian (CEL) analyses This powerful, eEulerian-asy-to-use feature

of Abaqus/Explicit general contact enables fully coupled multi-physics simulation such as fluid-structure interaction

2.4.2 Using Coupled Eulerian – Lagrangian Formulation in FSW

Numerical modeling of FSW is an efficient way to examine the material behavior during the process During FSW process, temperature distribution, material flow and defect formation are greatly affected by welding parameters, tool geometry and joint design By using numerical modeling, one can reduce the effort to have the prediction

on the behavior of material, as well as optimizing the process parameter

Various modeling such as computational fluid dynamics (CFD) and Arbitrary Lagrangian Eulerian formulation (ALE) can be applied to analyze the FSW process [14] In CFD method, Eulerian mesh is used, and because material and mesh are independent from each other, there is no mesh distortion problem Analysis of FSW process with CFD technique provides comprehensive details on frictional heat generation, temperature distribution and material flow [15] However, the CFD method presents difficulty in capturing information about, and accurately defining the material boundary Another difficulty with this technique is that it is impossible

to take into account elastic properties and material hardening behavior In the simulation of FSW with CFD, the workpiece to be welded can only be simplified as rigid visco-plastic material Moreover, the assumption that exact stick conditions exist in the interaction between tool and workpiece causes a significant simulation fault in tool reaction loads and weld temperature

In contrast, the ALE method can apply sliding boundary conditions to describe the interaction between tool and workpiece The ALE method can also simultaneously take into account the material hardening behavior, the rate dependence of the material and the material temperature Nevertheless, in the ALE technique, the Lagrangian bodies used are unable to function with voids during deformation and must be completely filled with material to ensure continuousness Therefore, severe mesh distortion may be encountered when ALE technique is used in simulation of excessive deformation problems, such as FSW process [16][17]

The Coupled Eulerian Lagrangian (CEL) formulation proposed using both Eulerian and Lagrangian domains throughout the control volume In the CEL formulation, Lagrangian mesh comes into contact with Eulerian mesh The CEL formulation can

be used in processes with excessive material deformation It obviates the difficulties

of modeling involving friction in the CFD method and large deformation in Lagrangian bodies Because CEL formulation has the properties of both Eulerian and Lagrangian meshes [18], the CEL formulation has advantages over the previously mentioned CFD and ALE numerical models Perhaps the most important is the ability

to clearly predict defect formation during the FSW process

The quality of the FSW Weld need to be assessed and evaluated with the desired application of the joints Depending upon the application, weld quality measurement may include micrograph cross-section analysis, tensile strength, bending, fatigue, fracture toughness, … testing

2.5.1 FSW Quality-related Parameters

In order to get more understanding about the quality of FSW Weld, it is necessary to

go through which parameter that generally affect weld quality

- Part material consistency - The material used for process development should

be consistent with the end production material This includes the application

of any material coatings Differences from one lot production of material to

another should be understood, especially with thin gauge materials, as variations can affect the weld parameter control window

- Part gap and mismatch – part tolerance variations are guaranteed with any manufacturing processes The more one pays, the tighter the tolerances can be made Welding should be done to characterize the impact of tolerance variation to make sure that the desired quality is achievable for the expected tolerance range

- Sealants – In the case where sealants need to be applied (e.g., lap welds for aircraft structure), tests should be run to verify that the sealant can be welded through or applied post weld Often corrosion testing techniques such as 30-, 60-, and 90-day exposure tests are used to measure corrosion protection During process development care should be taken to test for the variations in sealant application that might affect the weld characteristics

- Pin tool – The FSW pin tool design is a key piece of equipment to the FSW process as discussed earlier, the design of the pin tool is done to maximize the mixing of the joint material for the specific configuration Pin tool wear needs

to be taken into consideration as changes to the pin tool features will impact the quality of the weld for a given set of weld parameters

- Tooling – Proper part fit-up and hold-down fixturing is essential to maintain consistent quality welds

- Process parameters: parameters in FSW process are critical to weld quality, and the capability to achieve the repeatable welds

With the perspective of DOE (Design of Experiment) method, the following schematic (Figure 2-7) will present the relation between input factor (process parameters) and output (weld quality) in FSW

Figure 2-7 DOE FSW Process schematic

In the investigation of this thesis, tensile strength and bending strength of welds will

be evaluated

2.5.2 Tensile Strength

Tensile test is a destructive test where a tensile force is applied to a test specimen, which is pulled apart at a constant rate until fracture The specimen is machined with two wide “gripping ends” and a thinner “neck” in the middle, where the fracture is intended to take place

Dimension of testing specimen is followed by the ASTM E8-04

Figure 2-8 Specimen dimension for tensile testing

2.5.3 Bend Testing

Bend testing, sometimes called flexure testing, is used to measure the behavior of materials subjected to simple beam loading In FSW, bending is an important property that evaluate the bending ability of the weld joint

The most common bend testing method is the 3-point test The schematic of this method is shown in Figure 2-9 The specimen to be tested is mounted on top of two supporting spans and the vertical concentrated force is applied at the middle position

Figure 2-9 3-point flexure test [19]

CHAPTER 3 MODELING DESCRIPTION 3.1 Tool Pin Profile Design

Based on the literature review, four pin profiles were chosen in the investigation:

- Tool 1 (cylindrical thread)

- Tool 2 (tapered thread)

- Tool 3 (tri-flat tapered thread)

- Tool 4 (cylindrical)

Figure 3-1 Investigated tool pin profiles

In FSW tool design, an important parameter is the ratio of dynamic volume (material volume swept by the pin during rotation) to static volume (volume of the pin itself)

Table 1 Dynamic/static volume of tool pin profiles

3.2 Geometric Modeling

A CEL model has been developed using Abaqus/CAE 2020 software The workpiece

is modelled as Eulerian domain having cuboid body with a volume of 130 x 110 x 5

mm3 The workpiece was divided into two partitions to define the void region and material assigned regions, as show in Figure 3-2 No material was assigned on the void region (red color) Void region was defined to visualize the flash formation during FSW Thickness of void region is 2 mm and the material assigned region is 3

mm Defining void region was necessary because during plunging material beneath the tool would flow sideward and provision should be made to capture it Also, void region helps in achieving better convergence of the solution [20]

Figure 3-2 Tool and workpiece assembly with void and material assigned regions

The tool is defined as rigid Lagrangian domain and was constrained at reference point

to define the material properties and boundary conditions for different phase of FSW

In FSW process, the material flow is complex, so the accuracy of numerical simulation result is highly dependent on the correct determination of the flow stress Therefore, Johnson Cook material model is used to define the material flow behavior Flow stress is defined as the function of the temperature, strain rate and strain Also, temperature-dependent mechanical and thermal properties of the workpiece material are defined The material model is shown as Equation (1):

( )

0

m p

- A is the material initial yield strength

- B is the hardening modulus

- C is the strain-rate dependency coefficient

- m is the thermal softening coefficient

- n is the strain hardening coefficient

Table 3 shows the temperature-dependent of mechanical and thermal properties in the range from room temperature to near solidus temperature

Table 2 Constants used in the Johnson Cook material model [21]

A (MPa) B (MPa) C m n Troom (ºC) Tmelt (ºC)

Specific heat (J/kg/ºC)

Young modulus (GPa)

Poisson’s ratio

Thermal conductivity (W/m/ºC)

Density (kg/m 3 )

Thermal conductivity (W/m/ºC)

Specific heat (J/kg/ºC)

3.4 Meshing

The workpiece is defined as an Eulerian domain having an element type of eightnoded thermally coupled linear Eulerian brick elements (EC3D8RT) In order to increase the accuracy of the results, a fine mesh was created in the central section of the workpiece where it connects with the rigid tool In addition, to minimize the computational time, a coarse mesh was created for the rest region Figure 3-3 A total

31900 elements were created for Eulerian domain

Figure 3-3 Eulerian Mesh (Workpiece)