Study on the welding features of the weld joint between aluminum alloy to stainless steel and aluminum alloy to dual phase steel by using tig and friction stir welding

應用 TIG 與摩擦攪拌焊接法探討鋁合金與不銹鋼及鋁合金與雙相鋼間焊接接頭之焊接特性 Study on the Welding Features of the Weld Joint between Aluminum Alloy to Stainless Steel and Aluminum Alloy to Dual Phase Steel by Using TIG

國 立 高 雄 科 技 大 學 機械工程系博士班

中華民國 107 年 12 月

應用 TIG 與摩擦攪拌焊接法探討鋁合金與不銹鋼及鋁合

金與雙相鋼間焊接接頭之焊接特性

Study on the Welding Features of the Weld Joint between Aluminum Alloy to Stainless Steel and Aluminum Alloy to Dual

Phase Steel by Using TIG and Friction Stir Welding

國 立 高 雄 科 技 大 學 機械工程系博士班 博士論文

Department of Mechanical Engineering National Kaohsiung University of Science and Technology

In Partial Fulfillment of the Requirements for the Degree of Doctor of

Philosophy in Mechanical Engineering

December 2018 Kaohsiung, Taiwan, Republic of China

中華民國 107 年 12 月

應用 TIG 與摩擦攪拌焊接法探討鋁合金與不銹鋼及鋁合

金與雙相鋼間焊接接頭之焊接特性

研究生：阮文一 指導教授：黃世疇 教授

國立高雄科技大學 機械工程系博士班

摘要

由於重量輕，耐腐蝕性和高抗氧化等優點，鋁合金與不銹鋼，鋁合金和雙相鋼之間的焊接在工業上有廣泛的應用。然而，要將金屬焊接在一起，仍然存在許多挑戰。 例如，鋼的熔點遠大於鋁的熔點，機械性能和化學成分的差異。 特別是在不銹鋼與焊縫之間的界面處，容易形成金屬間化合物（IMC）層的脆性和裂縫。 這些問題將對接頭的強度和焊接質量產生負面影響。 為了防止 IMC 層的形成和焊接接頭質量缺陷的產生，必須發展合宜的焊接方法與焊接參數。

在本研究中，摩擦攪拌焊接（FSW）和鎢極惰性氣體（TIG）焊接分別用於焊接鋁AA6351 / DP800 鋼和鋁 A6061-T6 / SUS304L 鋼。

鋁合金和不銹鋼，鋁合金和雙相鋼之間的銲件有許多優點，如重量輕，耐腐蝕性和耐氧化性高，因而得到了更廣泛的工業應用 然而，要將金屬焊接在一起，仍然存在許多挑戰 例如，鋼的熔點遠大於鋁的熔點，機械性能和化學成分的差異 特別是在不銹鋼與焊縫之間的界面處，容易形成金屬間化合物（IMC）層的脆性和裂縫 這些問題將對接頭的強度和焊接質量產生負面影響 因此有必要有一個適當的焊接

方法和設定的焊接參數來防止 IMC 層的形成和發展並形成缺陷，從而提高焊接接頭的質量 在這項研究中，摩擦攪拌焊（FSW）和鎢惰性氣體（TIG）焊接分別用於焊

接鋁 AA6351 / DP800 鋼和鋁 A6061-T6 / SUS304L 鋼

通過攪拌摩擦焊方法成功地進行了 AA6351 與 DP800 鋼之間的搭接。利用掃描電子顯微鏡（SEM）和 X 射線衍射（XRD）技術研究焊縫的顯微組織特徵。調查結果表明，在鋼和鋁合金之間的界面出現的金屬間化合物層的厚度小於 7 微米，並進行相

分佈與金屬間化合物層的形成之間的關係

分析了採用 TI G 焊和 ER4047 填充金屬對鋁與鋼對接的特點 並使用光學顯微鏡（OM），掃描電子顯微鏡（SEM），能量色散 X 射線衍射（EDS）來顯示微觀結構 試驗結果表明，焊縫外觀良好，無缺陷，且熱影響區非常小 此外，在鋼與焊縫之

頭的機械性能。結果，不銹鋼，焊縫和金屬間層中的硬度的平均值分別為 218HV，79HV 和 411HV。最大抗拉強度達到 226.5 Mpa，斷裂位置發生在焊接釬焊表面。

關鍵字：鎢極惰性氣體工藝，攪拌摩擦焊工藝，填充金屬，DP800 鋼金屬間化合物層，

微觀結構，機械性能，熱循環。

Study on the Welding Features of the Weld Joint between Aluminum Alloy to Stainless Steel and

Aluminum Alloy to Dual Phase Steel by Using TIG and

Friction Stir Welding

Student: Van Nhat Nguyen Advisor: Shyh-Chour Huang

Department of Mechanical Engineering

National Kaohsiung University of Science and Technology

Abstract

Due to the advantages such as light weight, corrosion resistance and high oxidation resistance, the connection between aluminum alloys and stainless steel, aluminum alloys and dual phase steel has used more widely in industrial applications However, to weld the metal together, there are still many challenges Such as, the melting point of steel is much larger than that of aluminum, the difference in mechanical properties and chemical composition Especially at the interface between the stainless steel and weld seam easily form an intermetallic compound (IMC) layer brittle and cracks These problems will have a negative impact on the strength of the joint and the quality of the weld To prevent the formation and development of the IMC layer and the formation of defects improving the quality of the welding joint, it

is necessary to have a proper welding method and set of welding parameters In this study, Friction Stir Welding (FSW) and Tungsten Inert Gas (TIG) welding were used

to weld Aluminum AA6351/DP800 steel and aluminum A6061-T6/SUS304L steel, respectively

Lap joint between AA6351 to DP800 steel was carried out successfully by a friction stir welding method The scanning electron microscopy (SEM) and X-ray diffraction (XRD) technique was utilized to investigate the microstructural characteristics of the weld The survey results showed that at the interface between steel and aluminum alloy have appeared intermetallic compound layer with a thickness less than of 7μm, and the phases

examined to show the relationship between the distributions of temperature with the formation of the intermetallic layer

The characteristics of Butt joint between aluminum and steel by using TIG welding with ER4047 filler metal were analyzed The optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-Ray diffraction (EDS) have been done to demonstrate the microstructure of the weld Test results illustrated that the appearance of the weld good, no defects, and the heat-affected zone is very small Further, an intermetallic compound layer and cracks was also found at the interface between the steel and the welding seam, its thickness of 2 µm The new phases formed in an intermetallic layer comprising

explored by means of a Vickers hardness test and tensile test method As a result, the average value of hardness in the stainless steel, in the welding seam, and in the intermetallic layer is

218 HV, 79 HV, and 411 HV, respectively Maximum tensile strength reached 226.5 Mpa and the fracture location occurred at the welding-brazing surface

Keywords: Tungsten Inert Gas (TIG) process, Friction Stir Welding (FSW) process, Filler

metal, DP800 steel Intermetallic compound layer (IMC), Microstructure, Mechanical properties, Thermal Cycles

Acknowledgments

During my studies and research at the National Kaohsiung University of Science and Technology, I received great support from my teachers, my family, my friends, and my coworkers Through this opportunity, I want to present my deep sincere thanks

In addition, I would like to choose this opportunity to indicate my sincere thanks

to the members of the Computer Aided Engineering Application and Design LAB has helped me very enthusiastic during my time at National Kaohsiung University of Science and Technology

Finally, I would also like to send my sincere thanks to I would like to send my sincere thanks to my younger brother Van Hai Nguyen and my younger sister Thi Lan Anh Nguyen They have replaced me with care, raised my mother, and help me to solve all the work in the family A very special thanks and respect for my wife, who has been with me overcome many difficulties, she has replaced me with care and education for my children

Contents

摘要 IV

Acknowledgments VIII Contents VIII List of Tables XI List of Figures XII

Chapter 1 Introduction 1

1.1 Overview 1

1.2 Scope and Objectives of the Dissertation 3

1.3 Dissertation Outline 4

Chapter 2 Literature Reviews 5

2.1 Overview of Previous Research 5

2.2 Overview of Some Welding Methods 8

2.2.1 Tungsten Inert Gas Welding 8

a Current Models Used in TIG Welding 9

b The Shielding Gases and Gas Mixtures Used in TIG Welding 11

c Electrodes 13

2.2.2 Gas Metal Arc Welding 13

2.2.3 Friction Stir Welding 15

a Microstructural Characteristics 16

b Tools of Friction Stir Welding Process 17

2.2.4 Laser Welding 19

2.2.5 Ultrasonic Welding 22

2.2.6 Resistance Welding 24

2.2.7 Explosive Welding 26

2.3 Welding Defects 27

2.3.1 Cracks 28

2.3.2 Porosity 29

2.3.3 Undercutting 30

2.3.4 Lack of Fusion 32

2.4 Heat Transfer during Welding 32

2.5 Measurement Methods 33

2.5.1 Optical Microscope 33

2.5.2 Scanning Electron Microscopy 34

2.5.3 X-Ray Diffraction 35

2.5.4 Vickers Hardness Test 36

2.5.5 Tensile Testing 37

2.6 Summary 38

Chapter 3 Experimental Procedure 39

3.1 The Influence of Welding Parameters on the Quality of TIG Weld 39

3.1.1 Welding Current 39

3.1.2 Welding Voltage 39

3.1.3 Filler Metals 40

3.1.4 Shielding Gas 40

3.2 The Influence of Welding Parameters on the Quality of Friction Stir Weld 41 3.2.1 Rotational Speed and Travel Speed 41

3.2.2 Welding Tool 42

3.3 Experimental Preparation 42

3.3.1 Tungsten Inert Gas Welding Process 43

3.3.2 Friction Stir Welding Process 48

3.3.3 Preparation for Microstructure Measurement 51

3.3.4 Preparation for the Tensile Test 52

3.4 Summary 53

Chapter 4 Tungsten Inert Gas Welding Results and Discussion 54

4.1 The Appearance of Welding Joint 54

4.2 The Microstructure of Welding Joint 55

4.3 Tensile Strength Test 62

4.4 Hardness Test 65

4.5 Summary 67

Chapter 5 Friction Stir Welding Results and Discussion 68

5.1 The microstructure of Lap Joints 68

5.2 Shear Tensile Test 73

5.3 Comparison between Tungsten Inert Gas Welding Method and Friction Stir Welding Method 74

5.4 Summary 75

Chapter 6 Conclusions and Future Works 76

6.1 Conclusions 76

6.2 Suggestion for Future Works 77

References 79

List of Tables

Table 3 1 Chemical compositions and mechanical properties of the A6061-T6 alloy [75] 44Table 3 2 Chemical compositions and mechanical properties of SUS304L [76]

Table 4 1 The chemical composition of the alloying elements appears at the EDS test positions……… 57 Table 4 2 Tensile strength test results of the welding joint between aluminum alloy A6061-T6 and stainless steel SUS304L 63

List of Figures

Figure 2 1 Tungsten inert gas welding process 8

Figure 2 2 Schematic diagram of the direct current TIG welding; 10

Figure 2 3 TIG welding cycle with pulse current [35] 11

Figure 2 4 The relationship between the depth penetration and the shielding gas Figure 2 5 Gas metal arc welding process 14

Figure 2 6 Schematic of the friction stir welding process 16

Figure 2 7 Schematic of transverse cross-section zone of the FSW weld [36] 16

Figure 2 8 Welding tools used in the FSW process 19

Figure 2 9 Diagram of the laser beam welding method 20

Figure 2 10 Schematic of the ultrasonic welding principle 22

Figure 2 11 Resistance welding process (a) Spot welding; (b) Seam Welding 26 Figure 2 12 Explosive welding method 26

Figure 2 13 Types of welding defects 28

Figure 2 14 Location of the cracks 29

Figure 2 15 Model of Porosity 29

Figure 2 16 Undercut defect 31

Figure 2 17 Illustrates the location of lack of fusion 32

Figure 2 18 Optical microscope equipment 34

Figure 2 19 Scanning electron microscopy equipment 35

Figure 2 20 X-ray diffraction equipment 36

Figure 2 21 Micro-hardness tester machine 37

Figure 2 22 Tensile test process 37

Figure 3 1 (a) X6332B milling machine; (b) Base material plates 46

Figure 3 2 (a) Type of welding joint (Unit: mm); 47

Figure 3 3 Friction Stir Welding Setup (Unit: mm) 50

Figure 3 4 Diagram of the preparation process for microstructural examination samples; (a) Cutting samples; (b) Casting samples; (c) Grinding and polishing samples 52

Figure 3 5 The shape and dimension of the tensile specimen (Unit: mm) 53

Figure 4 1 (a) Image for weld bead appearance; (b) The sample cross-section 55 Figure 4 2 SEM images of the weld joint between Al/Steel; 57

Figure 4 3 (a-c) SEM images of spectrum1, spectrum2, and spectrum3; (d-f) the appearance of the alloying elements in spectrum1, spectrum2, and spectrum3 59

Figure 4 4 (a) Location of linear scanning; (b) The result of the linear scanning test 59

Figure 4 5 (a) Mapping image; (b-f) the distribution of Fe, Cr, Mn, Si, C, Al, Ni elements 60

Figure 4 6 XRD results of the weld specimen 61

Figure 4 7 The Fe-Al binary phase diagram [42] 61

Figure 4 8 The tensile test results 62

Figure 4 9 (a) Image of the fracture position of the tensile test; (b) Surface of fault in stainless steel side; (c) Surface of fault in the aluminum side 64

Figure 4 10 Defects on the fracture surface of the tensile test 64

Figure 4 11 EDS analysis results on the fracture surface of tensile specimens; (a, b) Electron microscopy image; (c) Corresponding amounts at spectrum1; (d)

Corresponding amounts at spectrum2 65

Figure 4 12 The location of hardness testing process 66

Figure 4 13 Microhardness profile 67

Figure 5 1 Thermal cycle for (a) 800 rpm 25 mm/min and (b)……… 69

Figure 5 2 The appearance of intermetallic layer phases on the interface of the welded specimens are generated by different welding parameters 71

Figure 5 3 The intermetallic layer thickness changes in different welding joints.71 Figure 5 4 The X-ray test results of the welds created with different welding parameters; (a) 800 rpm 25 mm/min; (b) 800 rpm 50 mm/min; 800pm and 75 mm/min; (d) 400 rpm 25 mm/min; (e) 400 rpm 50 mm/min and (f) 400 rpm/75 mm/min 72

Figure 5 5 The shear tensile test results of lap joints 73

Figure 5 6 The fracture surface of the shear test specimen (a) DP800 steel side and (b) AA6351 alloy side 74

Chapter 1 Introduction 1.1 Overview

Welding is a technological process used to connect two or more parts together

by using a temperature source to heat the connection point to a molten state or plasticizer and then the molten metal self-crystallization or using a pressure to form welds The filler metals have been added and combined with basic metals to form

a pool of molten metal, which then crystallizes into a welded joint Weld joints usually have a tensile strength equal to or higher than the base metal The welding state can be liquid, flexible, and even cool In the welding process if the metal reaches a liquid state, in most cases the self-formed weld without pressure Sometimes in some welding processes, it is not necessary to use extra pressure to form the weld because the metal at the welded position only needs to reach the plastic state Welding process usually has some advantages and limitations as follows:

The welding joints are characterized by continuity and are not removable With the ability to work the joints made by welding method allows saving more than 15-20% compared to the bolt joints and compared with the casting method, the welding method saves more than 50% of the volume of metal

Welding allows fabrication of complex structures, super, overweight from the same materials or materials with very different properties Suitable for different conditions and environments Welding creates strong bonding and high tightness

to meet the working requirements of important structures such as hull, tanks, boilers, pressure equipment

The welding process is simple, easy to implement, and high productivity compared to other technologies, easy to carry out mechanics, automation in the production process The residual stresses and deformations are usually generated

in the weld structure so that it affects the shape, size, and workability of the structure

Friction stir welding process has been invented in recent years The appearance

of this process has solved many difficulties in the welding process of aluminum and aluminum alloy that conventional welding methods cannot pass through The welding bond of this process is formed in a solid state with high quality without the use of complementary metal This method has quickly become the first choice

in manufacturing structures in industries such as trains, airplanes, boats, shipping This welding process uses a cylindrical tool, which is forced onto the parts or work piece with a fixed-shoulder placement The tool will rotate and move along the joint to be fixed so that the heat-by-friction between the tool and the work piece will soften and yield without transformation during the phase (i.e., from solid to molten) The heat flow that is moving from the leading edge of the tool onto the following edge has the distinct possibility of bringing about material deformation This method can be used for welding of the same material or welding of dissimilar materials [1-4]

Another name for tungsten inert gas welding is gas tungsten arc welding and

it uses argon, helium or Ar + He gas as a shield gas to protect the molten metal and filler rod throughout the welding process This method uses a non-melting tungsten electrode and transfers electrons from the power source to the surface of the work-piece Electric arc generated between the welding surface and the electrode generates heat to melt the filler metal and metal to form the weld Due to the simple implementation process, the weld was easily formed so this method was used to weld for almost of metals [5-8] It plays an important role in the production of structures by the welding method

This dissertation presents studies on the characteristic of welding joints between A6061-T6 / SUS304L and AA6351 / DP800 produced by TIG welding and friction stir welding method, respectively

1.2 Scope and Objectives of the Dissertation

The bonding between dissimilar materials not only provides the good properties of the materials but also reduces the volume of the structure, reduces the cost of the products, and increases the life of the structure Therefore, the welded joint between aluminum alloy and steel has been gradually increasing in recent years [9, 10] However, due to the great difference in physical properties, mechanical properties, and the formation of an intermetallic layer at the interface between the steel surface and the welding seam [11-17] To solve these problems, there are different welding methods have been studied and used [18-23] Among, the friction stir welding method and TIG welding method have been used as potential methods for welding between aluminum alloy to dual phase steel, and aluminum alloy to stainless steel

The purpose and objects of this dissertation are:

Investigates on lap joint between AA6351 alloy and DP800 steel by using friction stir welding process Microstructure and mechanical properties of butt joint between the A6061 alloys to SUS304L stainless steel by TIG welding with filler wire ER4047 were examined

Determine the relationship between the thermal cycler with the formation and development of inter-metallic layer Developed a numerical model to simulate temperature field, residual stress, and distortion of the dissimilar welded joint Investigated the influence of the combination of welding parameters on the change of thickness of the intermetallic layer Examination of microhardness and tensile strength of welding joints Examined the microstructure and the formation

of defects inside welds The formation of a new phase in the inter-metallic layer

1.3 Dissertation Outline

The contents of the chapters of the dissertation show the related theories and the conclusions of the experimental analysis of the welding joint of dissimilar materials by using the TIG welding method and the friction stir welding method There are six chapters in this dissertation and the contents presented in the chapters are summarized as follows:

Chapter 1 indicates the introduction, scope and objectives, and dissertation outline of this study

Chapter 2 presents an overview of previous research on welding of different materials It provides an overview of issues related to welding processes such as

an overview of some welding methods, welding defects, heat transfer during welding In addition, the characteristics of the welding materials used in welding processes are also discussed in this chapter It also briefly describes the measurement methods used to test the properties of the weld after welding

Chap 3 describes the influence of welding parameters on the quality of the TIG welding joint and the friction stir welding joint The preparation of experimental welding process and preparation the test samples after welding are also presented

in this chapter

Chapter 4 presents the results obtained during the survey on the microstructure and mechanical properties of the butt joint between A6061-T6 alloy and SUS304L steel by TIG welding method

Chapter 5 shows that the results and discussions in welding joints between AA6351 alloys and DP800 steel by the friction stir welding process This chapter also donates a comparison of the properties of welding bond between different materials produced by TIG welding method and FSW method

Chapter 6 presents the main conclusions and recommendations for further research direction

Chapter 2 Literature Reviews 2.1 Overview of Previous Research

Welded joints between aluminum alloy and stainless steel have been used extensively in industrial applications There are many reports were presented on the characteristics of welding joints between them such as microstructures, mechanical properties, residual stress, deformation, and the factors affecting on the formation and development of the IMC layer

The reports that can be pointed out as follows: Liu et al [24] concluded that the butt joint between aluminum alloy Al6061 and TRIP 780/800 steel with a thickness of 1.4 mm was successfully welded by a friction stir welding method In this report, the tensile strength and composition of the interlayer have been investigated The results showed that the tensile strength was 85% compared to the basic tensile strength of aluminum and the thickness of the interlayer formed at the Al-Fe interface was less than 1µm Lan et al [25] also utilized friction stir welding process to weld Al6061 alloy and TRIP 7800 Steel under butt configuration They investigated the macrostructure and microstructure of the welded joint obtained with different welding conditions An intermetallic compound layer was formed at the Al-Fe interface with a thickness of less than 1µm The fault locations of all specimens occur in the aluminum heat affected zone

The various welding mode parameters of the friction welding method have been used by Taban et al [26] to create a welding joint between A6061-T6 aluminum alloy and AISI 108 steel The microstructure and mechanical properties

of the joints have been analyzed scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray elemental mapping, focused ion beam (FIB) with ultra-high-resolution SEM and transmission electron microscopy (TEM) in TEM and STEM modes, tensile test, and micro-hardness testing The analysis results pointed out that the tensile strength of weld joint was 250 MPa and the FeAl

and Fe2Al5 phases were found in an intermetallic layer 250 nm thick Movahedi et

al [27] studied the effects of annealing temperature and duration on the tensile strength of the welding joint between Al-5083 alloy sheet and St-12 steel sheet This report has exhibited that the joint strength increases when the annealing temperature of the bonded joints at 300 degree C and 350 degree C increases The strength of the welded joint is significantly reduced when the annealing temperature of 400 degree C and the annealing duration of 60 minutes

Chang et al [28] considered on the microstructure and mechanical properties

of 6082 aluminum alloy and SYG960E ultrahigh strength steel welded joints formed by MIG welding with filler metals such as (ER5087) Al-Mg and (ER2319) Al-Cu They divided the cross-section of the weld joint into three regions, the weld zone (WZ), the bond zone (BZ), and interface zone (IZ) The needle-like Fe4Al13

and lath-shaped Fe2Al5 layers were found in the interface zone They also show that the formation and development of the intermetallic layer at the interface zone have been suppressed by Cu element and the tensile strength of the bonded welds made by the Al-Cu filler wire higher than the tensile strength of solder joint created

by Al-Mg filler wire The microstructure and mechanical properties of the aluminum-steel weld are investigated by Cui et al [29] The tensile strength of the weld joint made by the Zn-Al filler rod obtains of 133.6 MPa was illustrated in this report In addition, they also found that the Fe2Al5 phase was present in both joints create by Zn-Al and Al-Si-Cu filler wire

Shah et al [30] utilized TIG welding method and two different types of filler metal ER4043 and ER4047 to weld aluminum AA6061-O and galvanized iron in

a lap configuration A comparison of the microstructure and mechanical properties

of the solder specimens obtained with the two-filler metals was carried out As results have indicated that the thickness of the interlayer formed in the weld joint with the ER4047 filler metal (20 - 40 μm) thicker than that of the IMC layer made

in the weld joint with the ER4043 (4 - 7μm) The tensile strength of the specimen made by ER4043 filler wire higher than the specimen made by ER4047 filler wire

Song et al [31] analyzed the spreading behavior and microstructure characteristics

of the welded joint of the 5A06 aluminum alloy to SUS321 stainless steel by using TIG welding The spreading behavior of the filler wire divided into two parts, one covering the upper surface of the joint, the other flowing down to the underside of the bond At the aluminum side formed fusion welding joint due to the very low melting point of aluminum, at the stainless steel side forming the braze weld due

to the melting point of the steel is high Interlayer formed in the cross-section of the weld specimen is uneven From the weld to the stainless steel, the interlayer is divided into three sections The average tensile strength of the butt joint obtained 120.0Mpa In the report [32] Lin et al studied dissimilar tungsten inert gas welding between 5A06 aluminum alloy and SUS321 austenite stainless steel sheets with a thickness of 3mm At the interface of the welded joint and the stainless steel, a thin interlayer has been found with a thickness of 3-5 μm The mechanical properties result measurement of the weld joint indicate that the average microhardness values of the IMC layer, welded seam, and stainless steel are 644.7 HV, 104.5 HV

200 HV respectively A tensile strength of the welding joint reaches 172.5 Mpa and developmental cracks start from the bottom of the bonding to the welding seam

The laser welding-brazing process with ER4043 filler wire has been used by Sun et al [33] for bonding AA6061 aluminum alloy and Q235 low-carbon steel of 2.5mm thickness The microstructure and mechanical properties of the weld joints with different bevel angles of the steel sheet surface have been analyzed in this work The results of the report indicate the following: an intermetallic layer and a zinc-containing zone were found at the brazing interface Thickness and shape of the intermetallic layer are not uniform and vary along the weld surface The fracture position during the tensile test takes place in the brazing area, and the tensile strength values of the welded joints with bevel angles steel sheet of 450,

300 is obtained at 110 Mpa, 150 Mpa, respectively

2.2 Overview of Some Welding Methods

A Russian scientist discovered the electric arc phenomenon in 1802 and pointed out that it could be used to melt the metal Until today, it has many different welding methods are found and applied to weld for a variety of different materials They include the following methods: Resistance Welding, Metal Inert Gas Welding, Tungsten Inert Gas Welding, Submerged Arc Welding, Plasma Welding, Laser Welding, Ultrasonic Welding, and Explosive Welding

2.2.1 Tungsten Inert Gas Welding

Tungsten inert gas (TIG) welding is a method of molten soldering using an electric arc, which is formed between the non-melting electrode and the welding zone The welding pool and arc are protected by an inert gas such as Argon (Ar), Helium (He), or Ar + He gas to prevent harmful effects of oxygen and nitrogen in the air Welding arc region has very high temperatures can reach up to 61000C The TIG welding process is described in Figure 2.1

Figure 2 1 Tungsten inert gas welding process

TIG welding methods have some characteristics as follows:

TIG welding produces high-quality welds for most metals and alloys

No splashes during welding

Can be weld anywhere in space

The high concentration of welding process allows the welding speed to be increased, reducing the distortion caused by welding

It is difficult to protect the welding pool in a windy environment and easily automate the welding process

TIG welding is used in many different manufacturing sectors; it can weld carbon steel, stainless steel, brazing, and aluminum welding In addition, TIG welding can also be welded to the plate, bar, tube, welded pressure vessels

a Current Models Used in TIG Welding

Direct current (DC): During the TIG welding by direct current (DC), the welding electrode is connected directly to the former of the power supply, the anode of the welding power connected to the welding material Welding arc is formed between the welding electrodes and welding materials in the environment argon, helium, or a mixture of argon + helium was ionized The temperature at the center of the electrode and at the arc near the welding electrode is very high up to several thousand degrees centigrade, which quickly melts the base metal and filler metal to form a welded pool The schematic diagram of the DC/TIG welding process is demonstrated in Figure 2.2

Figure 2 2 Schematic diagram of the direct current TIG welding;

(a) DCEN; (b) DCEP

The choice of electrode type and size depends on the relationship between the operating mode and the current With advantages such as high durability, high current resistance, good anti-contamination, electronic emission, and easy to induce arc So that the Wolfram with 1.2% thorium electrode has been selected to use in the direct current TIG welding process In addition, tungsten electrodes with lanthanum oxide or cerium oxide are also used because lanthanum oxide and cerium oxide make the arc formation easier, the arc's stability is higher and low electrode corrosion The diameter of the electrodes used in the direct current electrode negative (DCEN) welding process must be greater than the direct current electrode positive (DCEP) welding process During the welding process, the temperature of the arc is very large and it can melt the electrode

Alternating current (AC): The TIG welding process by using alternating current is suitable for use in the welding of aluminum, magnesium and their alloys because at the molten solder the aluminum oxide layer forms very quickly In the welding process, a positive half cycle was used to bombard the oxide film on the base metal surface and cleans its surface The negative half cycle has the effect of heating the base metal

Pulsed direct current (PDC): This method uses direct current with pulsed cycle interruption and the weld is formed from the individual point of overlap This

process is suitable for automating TIG welding in all positions for perimeter welds and which are carried out on thin tubes The TIG welding method used pulse direct current is illustrated in Figure 2.3

Figure 2 3 TIG welding cycle with pulse current [34]

b The Shielding Gases and Gas Mixtures Used in TIG Welding

The types of shielding gases used in TIG welding process include Ar, He, or

Ar + He Shielding gas is utilized to protect the electrode and molten metal in the welding zone from the harmful effects of ambient air In the case of welding of stainless steel, ferrous metals and alloys need to use protective gas at the bottom

of the weld to protect the weld from oxidation, reducing the cracking and porosity defects

* Argon

Argon is a chemical element in the periodic table system with symbol Ar; it

is an inert gas, colorless, odorless, non-flammable and non-explosive Argon gas

was prepared by air liquefaction process, its purity used in the welding process need to reach 99.95%, the rest is impurities such as nitrogen and oxygen In the aluminum welding process, the purity of Ar gas using to protect welding seam should reach 99.97% Ar Ar gas is contained in high-pressure cylinders or in a liquid state with a temperature of -184degree C Ar has the ability to stabilize the arc the same when welding by alternating current or DC current, good cleaning ability with alternating current While helium gas allows stable arc with direct current, but the arc's stability and cleanliness for the alternating currents are relatively low Can employ the Ar and He gas mixtures with a He content of 75% when need to balance the characteristics of these two gases

* Helium

Helium has a purity of at least 99.99%, which is very expensive so it is less widely used in the welding process and particularly during hand welding Helium was taken from nature and stored in high-pressure vessels of about 150 at Because

it is about 10 times lighter than Ar, when it comes out of the gas lens nozzle it tends to rise to form a vortex around the arc Therefore, to protect the welding area, the flow rate of He must be 2-3 times higher than Ar With the same arc length and the same current, the He gas requires higher arc voltage than Ar gas The arc in Helium has a higher temperature than Argon arc, so Helium is often used to weld materials of high thickness, high conductivity, or high melting metal The depth penetration of the weld was protective by He is higher than the weld was protective

by Ar and display in Figure 2.4

Figure 2 4 The relationship between the depth penetration and the shielding gas

2.2.2 Gas Metal Arc Welding

The Battelle Memorial Institute invented gas metal arc welding method in the late 1940s Originally, it was used for welding aluminum and other non-ferrous materials However, due to the simple welding process, faster welding speed, better welding quality than other welding methods, the method of GMAW was soon applied for welding steel In this welding process, a consumable wire electrode is supplied continuously to the weld pool; the liquid metal droplets from this electrode are transferred to the weld pool and mixed with the base metal melting to form the weld The molten weld pool is protected by an inert gas or

reactive gas depending on the type of base metal being welded GMAW is divided into two main categories: If the liquid metal tank is protected by activated gas, it

is called Metal Active Gas (MAG) welding If the liquid metal tank has been protected by an inert gas, it is called Metal Inert Gas (MIG) welding A robot can

do the gas metal arc welding process manually, semi-automatically or automatically The robotic arms carry the torch and perform the movement of the welding process to form the welding seam Today, GMAW method is most commonly used in industries such as the automotive industry, shipbuilding industry, and construction Unlike other welding methods during MIG / MAG welding, molten metal weld should be protected by shielding gas, so it is rarely used for welding outdoors The gas metal arc welding process is displayed in Figure 2.5

Figure 2 5 Gas metal arc welding process

The quality of the weld joint during welding GMAW depends on the choice

of welding parameters, protective gas, and wire electrode type suitable for basic materials, thickness and types of the welding joint In particular, it depends on the choice of the metal transfer modes Metal transfer modes during GMAW welding

include short-circuiting transfer, globular transfer, spray transfer, and pulsed spray transfer

2.2.3 Friction Stir Welding

Friction stir welding (FSW) method is a method of welding in the solid state

In 1991, Thomas and his colleagues at the Welding Institute invented the friction stir welding method Friction stir welding is one of the latest inventions of the welding process and has played an important role in the process of bonding metals

in recent years Figure 2.6 shows that a diagram of the friction stir welding process has been presented The friction welding process is commonly used to weld sheet materials and it is only used to weld with the butt joint, lap joint In the friction stir welding process, a tool rotates with a profiled pin is slowly plunged into the surface between two base work piece, until the shoulder contact with the working surface This rotary tool has a constant rotational speed, and the travel speed remains constant, it moves along the interface between the two working plates The friction between the base materials and the welding tool generated frictional heat; this frictional heat causes the material at the welding position to reach the plastic state During welding, the plastic material around the welding tool flows from the front

to the back and under the effect of a force, which forms a welding seam The combination of rotational speed and transverse motion of the welding tool produces an advancing side and retreating side with velocity differences Compared with other fusion welding processes, the friction stir welding method has more advantages as the mechanical properties of the welding seam under the same conditions are better, easier to automate, do not pollute the environment, less damage to workers, eliminating defects such as porosity, solidification cracking, liquation cracking, undercut, and especially do not create distortion after welding However, the friction stir welding method still has some disadvantages examples creating a hole at the end of the weld, requiring a large clamping force to keep the

base metals from moving during welding, less flexible than other welding processes

Figure 2 6 Schematic of the friction stir welding process

a Microstructural Characteristics

Figure 2 7 Schematic of transverse cross-section zone of the FSW weld [35]

As seen in Figure 2.7, the cross-section of the weld joint formed by the friction stir welding method is divided into regions such as Weld nugget or stir zone (SZ), flow arm zone, thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ) and base metal (BM) [35]

The flow arm zone is the metal formed at the upper surface of the working plates This area forms around the welding tool and its size is the diameter of the tool shoulder

The relative motion of a weld pin and the base material generates welding nugget or dynamically recrystallized zone In this area, the crystallization occurs strongly, which makes the granular structure in this region very fine and the size

of the particles is smaller than that of the parent metal The structure of this area consists of many concentric circles like the rings

Thermo-mechanically affected zone is formed between the weld nugget and the heat-affected zone In this region, the influence of welding on the microstructure is negligible because the temperature and deformation here are relatively low So in this area do not occur recrystallization that occurs only plastic deformation

The heat-affected zone is shown in Figure 2.7, which is the metal region adjacent to the base metal Because this area is affected only by the temperature of the welding process, the chemical composition of the metal in this region does not change, but changes in the structure and size of the particles

b Tools of Friction Stir Welding Process

Welding tool plays an important role in the friction welding process The structure of the welding tool consists of a welding shoulder and a threaded or non-threaded probe When using the friction stir welding method to weld the same material, the welding seam is made easily However, it is a challenge when welding dissimilar materials because they have a great difference in physical properties, chemical composition, and the difference in melting point Especially the formation of a brittle intermetallic layer at the interface of the work piece Therefore, a suitable welding tool is chosen that prevents the formation of the interlayer and improves the quality of the welding joint [36-38] In recent years,

there have been many published works for the use of friction stir welding method

to weld dissimilar materials example aluminum alloy and steel, aluminum/copper, aluminum/magnesium, aluminum/ stainless steel, aluminum/titanium [39-44] The choice of materials for welding tools depends on the type of material, thickness, type of welding machine Besides, how to achieve the weld without defects, reducing the intermetallic layer thickness, enhancing the tool life, as well

as the requirements for tooling materials In the welding of dissimilar materials by friction stir welding, steel tools such as H13, SKD51, O1 are usually chosen as welding tools Because they have a wear resistance, good elevated-temperature strength, and thermal fatigue resistance Welding tools made of Tungsten-based will be the most suitable choice for the welding of high hardness materials and the probe tool is subjected to the great friction conditions Welding tools are often used

as tungsten-rhenium (W-25Re) and tungsten carbide (WC-10%Co)

The tool geometry during friction stir welding also plays a decisive role in the formation of the microstructure and the mechanical properties of the joint between the same material and the weld joint of dissimilar materials The geometry of the friction stir welding tool also prevents the formation of defects and the formation

of the intermetallic layer in welding joints between dissimilar materials The formation of IMC layer in the welding process of dissimilar materials is unavoidable If the thickness of the IMC layer is optimized, the mechanical properties of the welded joint are good Therefore, when designing the tool geometry, it should be noted to it can reduce the formation of the interlayer Generally, when designing the tool geometry to weld dissimilar materials, to note some important factors, such as the characteristics of the shoulder, the characteristics of the probe The convex scrolls and concave are characteristics of the tool shoulder While the pin features include flats, threads, and steps The welding tool are pointed out in Figure 2.8

Figure 2 8 Welding tools used in the FSW process

is widely used in various industries; it is applied to weld both the same material and dissimilar materials In the laser welding process, the heat input to the molten weld metal pool is small and very concentrated, so it reduces the metallurgical process in the heat-affected zone, the deformation of the bonding after welding is optimal The quality of the welds obtained by laser welding is very good with materials up to 32mm in thickness without filler wire Compared with the arc welding method, the magnetic field does not affect the laser welding process

However, in the laser welding process, the joint need to be clamped and make sure that the joint does not move out of the welding area Weld ability of aluminum alloys, copper alloys will be restricted in the laser welding process because the reflectivity and thermal conductivity thereof is high The porosity may appear in laser weld joints and brittle welding joints due to the rapid solidification of the weld metal pool The laser beam welding process was illustrated in figure 2.9

Figure 2 9 Diagram of the laser beam welding method

Laser welding methods can weld all types of joints for example Butt joint, Lap joint and Tee joint Beam YAG laser with the continuous wave (CW) and pulse wave (PW) modes was commonly used to weld but and lap joints The formation

of a weld in the shape of a keyhole or heat conductor depends on the type of power density and the duration of the laser irradiation During the laser beam shattering the metal surface, the laser beam energy is converted to thermal energy and it heats

the surface of the metal sheet Keyhole formation will greatly increase the laser's energy absorption capacity and the effectiveness of the joining will be better when the keyhole penetrates deep into the weld Spot welding with a pulsed laser has created small parts When the pulse width near the focus of the lens is increased,

it will easily create a deep penetration welds The porosity defects will easily be formed if the depths of sound laser spot welds exceed of 1.5mm or 3mm The deep penetration of the weld joint is easily attained in high power laser welding with Ar protective gas The weld has deeply penetrated and porosity are closely related to laser energy When laser welding with high power, large penetrates and porosity defects are prevented The penetration is shallow and the porosity defects is easily formed when the laser power source is low In addition, the defects such as cracking, underfilling, humping, and spattering also occur under certain conditions

The CO2 laser beam has a length of 10.6mm and it is 10 times longer than a solid-state laser Power conversion efficiency from electrical to optical beam about 8-15% The conversion efficiency of diode lasers higher than the conversion efficiency of the CO2 laser beam and the conversion efficiency of rod type Nd: YAG lasers lower than the conversion efficiency of the CO2 laser beam Compared

to some other laser systems, the CO2 laser beam has some of the following advantages: over unalterable welding, low device cost, and high beam feature In addition, in the welding using CO2 laser beam has some limitations such as high absorption of metals, the high reflectivity of metals, and high absorption of laser energy in laser-induced plasma

Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) is a new discovery

of solid state laser sources, it uses the same laser medium The Nd: YAG has been used in many different applications In some industrial applications, especially when using artificial robots, the Nd: YAG’s ability to transport with fiber lasers is greater than that of CO2 lasers Nd: YAG laser sources are widely used for welding

of various materials because of their high stability and high efficiency There are

two types of laser welding process commonly used include the conduction mode and deep penetration welding

By the 1990s, disk laser welding was invented but until 2003, the first industrial products made by this process were created In the disk laser to obtain the highest output power with the best laser beam quality, the disk needs to be placed on a heat sink, which will help cool down and cool the disk faster To reflect the laser beam and the light injected into the back of the disc is designed with reflective function and acts as a mirror folding Only a fraction of the passing rays

is absorbed because the disk thickness is very thin Therefore, a couple of turning mirrors and parabolic mirrors have been designed inside the cavity to allow the beam to be pumped through the disk more

2.2.5 Ultrasonic Welding

Figure 2 10 Schematic of the ultrasonic welding principle

Ultrasonic metal welding (USMW) was invented in the 1950s In recent years, this method has received considerable attention and it is widely used in the welding

of industrial applications Ultrasonic metal welding is a welding process pressure, use the mechanical energy of ultrasonic vibrations make plastic deformation localized in the surface joints, make the elements diffuse, absorbent and bonded

together to form a weld Ultrasonic metal welding takes place with a series of processes as follows plastic distortion, work hardening, fracturing of contaminant layers, crack establishment and propagation, crack, the creation of heating by friction and plastic deformation, recrystallization and interdiffusion Besides the quality of the joint ultrasonic welding metal in the solid state is also determined by the sliding mechanism and plastic deformation of individual touch points Ultrasonic metal welding can solve all the difficulties encountered when welding various materials that other welding methods do not solve All types of assembly and shapes of work pieces can be welded by ultrasonic metal welding Further, this process has been used successfully to weld materials such as plastics, ceramics or glass Compared with some methods such as resistance welding, friction stir welding, ultrasonic metal welding is more efficient because it consumes less energy, the energy generated during welding is concentrated to the welding position and it has higher stability in certain cases In recent times, ultrasonic metal welding techniques applied more widely in the welding of metals and their alloys such as copper and copper alloys, aluminum and aluminum alloys, magnesium, gold, and silver

The schematic diagram of the ultrasonic welding process is illustrated in Figure 2.10 For maximum performance in a conventional welding system, it depends on the amplification condition of the system Frequency ranges from 15

to 70 kHz are commonly used for metal welding; however, in the case of micro bonding, the frequency should be above 100 kHz An ultrasonic welding system uses piezoelectric ceramic disks to convert electrical power into mechanical power through a transducer, but the wavelengths obtained at the working surface of the probe are typically in the range of 10-30mm with a low amplitude A vibration amplitude control system is also used in ultrasonic welding systems, which will produce high stress at the joining points because it increases the temperature at the contact surface During welding, the horn has a role to transfer the ultrasonic energy from the transducer to the welding surface Depending on the design of the

welding tools and the power of the generator, it is possible to obtain different vibration amplitude

Ultrasonic welding equipment is usually designed according to certain models, for example, the first model is designed according to the time of operation, the second model is designed according to the energy, and the third welding model involves the welding distance and allows for welding with different weld depths Ultrasonic metal welding and ultrasonic plastic welding are the two major applications of ultrasonic welding During ultrasonic metal welding, ultrasonic oscillation moves in parallel with the base metal surface to form a welded joint and the welding bond in this process is a solid-state bond In contrast to the ultrasonic metal welding process to form a welding joint, the ultrasonic oscillation is moving perpendicular to the welding surface However, so far the number of studies on plastic ultrasonic welding has been published more than the ultrasonic metal welding process

2.2.6 Resistance Welding

Resistance welding is a type of pressure welding, which uses a high current to flow through interface contact between the work pieces, where the high current generates a sufficiently high heat to make the metal at the contact position reaches the plastic state, and then use a large enough pressure to force the contact surfaces together and forming the weld

When a high current is passed through, the contact surface between the components is heated up very rapidly because the contact resistance between them

is greater than the resistance of the components The heat generated at the contact place will be proportional to the resistance, with the square of the current intensity and proportional to the time the current pass through the components Joule’s Law determines the heat generated during welding [45]

2

R: Resistance of the work (Ω)

t: Time of current flow (s)

From formula 2.1 it can be seen that the time is inversely proportional to the current Therefore, in order to form a weld, the electrical current required is very large; the welding time is extremely short The very high heat generated during the welding process is transmitted directly to the weld nugget and weld electrode in an extremely short time, as a result, it rapidly damaged the electrode and the metal surface at the welding point melted very seriously The heat generated at the welding point is directly influenced by the following elements: welding current, welding time, welding pressure, welding electrodes, metal surface conditions, and metal composition

Resistance welding process can be divided into spot welding and seam welding:

Spot welding is a type of resistance welding in which the parts are joined together at distinct points At the same time one, two, or multiple points can be welded, the welding parts are squeezed together by two electrodes, which heat up the contact surface of the welding parts and melt a thin layer on the metal surface, while the nearby area is soft in the plastic state Then disconnect the welding current and use a force to squeeze the parts together to form the weld Spot welding can be used to weld metals, alloys and welding materials typically have a thickness thin

Seam welding is also a method of contact resistance welding in which the weld

is a set of continuous welding points At each point will have a welding point is created by the action of currents and pressure through the welding electrodes