February 2010 Master's Thesis
A Study on the Weldability and Mechanical Characteristics of
Al6061T6/Ti6Al4V Butt Joints by TIGFSW Hybrid Welding
Graduate School of Chosun University
Department of Naval Architecture and Ocean Engineering
Hyun-Jong Song
A Study on the Weldability and Mechanical Characteristics of Al6061T6/Ti6Al4V Butt Joints by
TIG-FSW Hybrid Welding
접합기술을 이용한 TIG-FSW Hybrid
Al6061-T6/Ti-6Al-4V
접합부의 접합성 및 기계적 특성에 관한 연구
February 25, 2010
Graduate School of Chosun University
Department of Naval Architecture and Ocean Engineering
Hyun-Jong Song
A Study on the Weldability and Mechanical Characteristics of Al6061T6/Ti6Al4V Butt Joints by
TIG-FSW Hybrid Welding
Advisor : Professor Han-Sur Bang
A Thesis submitted for the degree of Master of Engineering
October 2010
Graduate School of Chosun University
Department of Naval Architecture and Ocean Engineering
Hyun-Jong Song
Master's thesis written by Hyun-Jong Song was approved by the following review committee members.
Department of Naval Architecture & Ocean Engineering at Chosun University
375 Seosuk-dong, Dong-gu, Gwang-ju Republic of Korea
Ph.D. Hee-Seon Bang
Department of Naval Architecture & Ocean Engineering at Chosun University
375 Seosuk-dong, Dong-gu, Gwang-ju Republic of Korea
Ph.D. Han-Sur Bang
Automotive Components Center,
Korea Institute of Industrial Technology 111-9 Oryong-dong, Buk-gu, Gwang-ju Republic of Korea
Ph.D. Ik-Hyun Oh
November 2010
Graduate School of Chosun University
의 을 함 宋炫鍾 碩士學位論文 認准
委員長 朝鮮大學校 敎授 房 熙 善 印
委 員 朝鮮大學校 敎授 房 漢 瑞 印
委 員 韓國生産技術硏究院 博士 印
2010 年 11 月
朝鮮大學校 大學院
CONTENTS
List of Tables ··· Ⅴ List of Figures ··· Ⅵ Abstract ··· Ⅷ
Chapter 1. Introduction
1 . 1 Research Background and Purpose ··· 1
1 . 2 Research Methodology ··· 3
Chapter 2. Theoretical background
2 . 1 Principles and characteristics of FSW ··· 42.1.1 Principles of FSW ··· 4
2.1.2 Process definitions of FSW ··· 7
2.1.3 Characteristics of FSW ··· 7
2 . 2 Principles and characteristics of GTAW ··· 9
2.2.1 Principles of GTAW ··· 9
2.2.2 Characteristics of FSW ··· 10
2 . 3 Characteristics of TIG-FSW hybrid welding process ···
11
2 . 4 Characteristics of object materials ···
12
2 . 5 Type of characteristics of aluminium alloy ···
13
2.5.1 Type of aluminum alloys ··· 13
2.5.2 Characteristics of 6061 aluminum alloy ··· 14
2 . 6 Type of characteristics of titanium alloy ···
16
2.6.1 Type of titanium alloys ··· 16
2.6.2 Characteristics of Ti-6Al-4V alloy ··· 18
Chapter 3. Experiment method of TIG-FSW hybrid welding process
3 . 1 Details of TIG-FSW hybrid welding ··· 203.1.1 TIG-FSW hybrid welding equipment and experimental setup ··· 20
3.1.2 objective material ···
223 . 2 Description of tool material and shape ··· 23
3 . 3 Experimental procedure ··· 24
3.3.1 TIG-FSW hybrid welding condition ···
243.3.2 TIG preheating condition ···
263 . 4 Evaluation of mechanical and metallurgical characteristic ··· 27
3.4.1 Tensile test ···
273.4.2 Hardness test ···
283.4.3 Microstructure analysis ···
29Chapter 4. Mechanical and metallurgical characteristics
of TIG-FSW hybrid welded joints
4 . 1 Bead profiles of dissimilar material with welding
processes ··· 30
4.1.1 Experiment by TIG welding ··· 30
4.1.2 Experiment by Friction stir welding ··· 31
4.1.3 Experiment by TIG-FSW hybrid welding ··· 35
4 . 2 Tensile test results ···
39
4.2.1 Tensile strength of similar material welded joints(Al6061-T6) ··· 39
4.2.2 Tensile strength of dissimilar materials FSW welded joints ··· 40
4.2.3 Tensile strength of dissimilar materials TIG-FSW hybrid welded joints ··· 45
4 . 3 Hardness test resultst ··· 51
4 . 4 Microstructure analysis ···
53
4.4.1 Microstructure of FSW welded joints ··· 53
4.4.2 Microstructure of TIG-FSW hybrid welded joints 55
4 . 5 Analysis of weld specimen using SEM and EDS observation ··· 574.5.1 SEM of fractured specimens ··· 57
4.5.2 SEM and EDS observation of dissimilar joint by TIG-FSW ··· 60
Chapter 5. Conclusion ···
64Reference ··· 67
List of Tables
Table 2.1 Chemical compositions in Al6061-T6 and Ti-6Al-4V ··· 12
Table 2.2 Mechanical properties of Al6061-T6 and Ti-6Al-4V ··· 13
Table 2.3 The alloy composition of 6061··· 15
Table 2.4 Type and Mechanical properties of Titanium··· 17
Table 2.5 Typical mechanical properties for Ti-6Al-4V··· 18
Table 3.1 Chemical composition and mechanical properties for Al6061-T6 and Ti-6Al-4V ··· 22
Table 3.2 Dimensions of welded specimens ··· 22
Table 3.3 Chemical compositions and mechanical properties of tool ··· 23
Table 3.4 Conditions for TIG preheating ··· 26
Table 3.5 Dimension of tensile test specimen ··· 27
Table 4.1 Bead profiles of dissimilar materials welded joints by TIG welding ··· 30
Table 4.2 Bead profiles of dissimilar materials welded joints by FSW ··· 32
Table 4.3 Bead profiles of dissimilar materials welded joints by TIG-FSW hybrid welding ··· 36
Table 4.4 Tensile strength of similar material FSW welded joints ··· 39
Table 4.5 Tensile strength of dissimilar materials FSW welded joints 41 Table 4.6 Stress-strain curve of dissimilar materials FSW welded joints ··· 43
Table 4.7 Tensile strength of dissimilar materials TIG-FSW hybrid welded joints ··· 46
Table 4.8 Stress-strain curve for dissimilar materials TIG-FSW hybrid welding joints ··· 48
List of Figures
Fig. 1.1 Application of friction stir welding in industries ··· 2
Fig. 2.1 Schematic diagram of friction stir welding ··· 4
Fig. 2.2 Schematic of friction stir welding process ··· 5
Fig. 2.3 Microstructural regions in FS welded Al alloys ··· 6
Fig. 2.4 Schematic diagram of GTAW ··· 9
Fig. 2.5 Schematic diagram of TIG-FSW hybrid welding ··· 12
Fig. 2.6 Classification of Aluminum ··· 14
Fig. 3.1 Equipment and specifications of FSW and TIG welding ··· 21
Fig. 3.2 Experimental set-up for TIG-FSW hybrid welding ··· 21
Fig. 3.3 Dimensions and shape of tool ··· 23
Fig. 3.4 Schematic diagram and welding parameters of TIG-FSW hybrid welding ··· 25
Fig. 3.5 Schematic diagram of TIG heat source and tool position ··· 25
Fig. 3.6 Process of tensile test ··· 27
Fig. 3.7 Micro vickers hardness tester ··· 28
Fig. 3.8 Hardness measurement points of welded specimen ··· 28
Fig. 3.9 Optical microscope ··· 29
Fig. 4.1 Stress-strain curve ··· 39
Fig. 4.2 Comparison of tensile strength with welding process ··· 50
Fig. 4.3 Hardness distribution of dissimilar materials welded joint both welding process ··· 52
Fig. 4.4 Comparison of hardness distribution of FSW and TIG-FSW hybrid welding joints ··· 52
Fig. 4.5 Microstructure of FS welded joints ··· 54
Fig. 4.6 Microstructure of TIG-FSW hybrid joints ··· 56
Fig. 4.7 SEM of dissimilar materials FS welded joints··· 58 Fig. 4.8 SEM of dissimilar materials TIG-FSW hybrid welded joints· 59 Fig. 4.9 SEM of TIG-FSW hybrid welded joints··· 61 Fig. 4.10 EDS of dissimilar materials TIG-FSW hybrid welded joints·· 63
ABSTRACT
접합기술을 이용한 TIG-FSW Hybrid
접합부의 접합성 및 Al6061T6/Ti6Al4V
기계적 특성에 관한 연구
Song, Hyun-Jong
Advisor : Prof. Bang, Han-sur, Ph.D.
Department of Naval Architecture and Ocean Engineering,
Graduate School of Chosun University
최근 전 세계적으로 대기오염 및 지구온난화 그리고 자원고갈로 인하여 세기를 맞이한 현대사회의 각종 산업분야에서 최대의 핵심 과제는 친환경
21 “
적 녹색성장 일 것이다 특히 수송기기 철도차량 항공기 선박 자동차 등” . , ( , , , ) 산업에서는 환경보호와 에너지 절감에 대한 요구가 높아짐에 따라 이산화탄
소 배출 및 중량을 최소화 하기위한 노력이 이루어지고 있다 무엇보다 수송.
기기 차체의 중량은 연비 및 이산화탄소 배출에 직접적인 영향을 미치기 때 문에 경량화의 요구가 급증하였고 차체에 경량 부재의 적용이 빠른 속도로 확대되고 있다.
경량 소재 중에서도 알루미늄 합금은 경량이고 비중이 강의 1/3로서 비강
도가 높고 특히 각종 원소와의 결합을 통한 합금성이 우수하다 또한 소성, . , 가공성이 좋고 전기전도도 및 대기 중에서 내식성과 내마모성이 우수하여 구 조 및 기능성금속으로서 다른 경량 소재에 비해 적용범위 및 활용도가 높은
비철재료이다 하지만 기존의 용융용접방법으로는 알루미늄 합금을 용접하는.
경우 용접 열에 의한 변형 및 기공 균열 등의 결함이 발생하기 쉬우며 접합,
부의 강도저하가 비교적 쉽게 발생하여 이에 대한 대책이 요구되고 있을 뿐 만 아니라 물리적 특성이 상이한 재료의 접합일 경우에도 접합이 매우 어렵 거나 불가능한 실정이므로 이에 대한 대책이 요구되고 있다.
이러한 문제점 등을 개선한 접합방법으로 마찰교반접합(FSW : Friction
기술이 적용되어지고 있다 이 기술은 년 영국
Stir Welding) . 1991 TWI(The
에서 처음으로 개발된 고상접합법의 하나로 회전하는 Welding Institute)
을 피접합체에 삽입하여 마찰발열에 의해 접합체를 연화시켜 소성유동을 Tool
통해 접합하는 비소모성 접합방법으로서 용접에 의한 변형이 적고 비소모성
접합방법일 뿐만 아니라 용접결함 흄 유해광선의 발생 없이 우수한 품질의, ,
접합부를 얻을 수 있으며 기계적 특성 또한 우수하기 때문에 환경친화형 접,
합방법으로 많이 각광을 받고 있다 또한. , 기존의 이종재료 접합방법으로 리벳 팅 및 바이메탈을 이용한 용융 용접을 적용하였으나 이에 소요 공정 시간의 증 가 및 용접열에 의한 용접결함 변형 잔류응력 응고균열 기공 산화 등 뿐만( , , , , )
아니라 금속간 화합물생성으로 접합부의 강도저하로 건전한 접합부를 얻기가,
어렵다.
하지만 이러한 장점에도 불구하고, FSW를 이용하여 이종재료를 접합할 경
우 물리적인 성질이 상이하여 동종재료의 FSW접합 방식으로는 건전한 접합
부를 쉽게 얻을 수 없어 두 재료 중 연질인 재료에 마찰교반접합 Tool을 삽 입하여 건전한 접합부를 얻은 결과가 보고된바 있으나 아직은 연구가 많이
미흡한 실정이다 특히 알루미늄과 티타늄 합금의 이종소재 접합기술의 부재. ,
로 국내외 산업응용분야에서의 실제 구현은 거의 이루어지지 않고 있다.
따라서 본 연구는 저밀도 이면서 비강도가 우수한 알루미늄 합금과 비강도 는 물론 내식성 및 고강도를 갖고 현재 공업용으로 많이 사용되고 있는 티타
늄 합금의 이종재료를 FSW에 TIG를 결합한 하이브리드 접합기술을 적용하
였으며 아울러 기계적 특성 인장시험 경도시험, ( , ) 및 금속학적 특성 광학현미( 경, SEM, EDS)을 파악하여 이종재료의 접합성을 평가하고 산업분야의 적용 가능성을 고찰하고자 하였다.
Chapter 1 INTRODUCTION
1.1 Research Background and Purpose
There are many kinds of approach to meet the demands where considers the environmental problem and the resource exhaustion. Among them, Highest performance and concurrent weight and cost reduction become more and more important in transportation industries such as automobile, aircraft, vessel and railway vehicle. So researchers are focusing on fabricating light weight structures which are economical and environmentally friendly. Therefore, materials manufacturers are developing the lighter materials with a higher mechanical property by various methods like design and processing of alloy.
Dissimilar joints are used in structures where light weight and high strength are desirable. Riveting, and bimetallic strip joining techniques for dissimilar material joint increases manufacturing cost and require more work time.
To solve this problem, Friction stir welding(FSW) is a novel solid state joining process that was invented in 1991. It can avoid many problems associated with conventional fusion welding methods, there by producing defect free welds with excellent properties, even in some materials with poor fusion weldability.
Due to its many advantages, FSW attracts a great deal of attention in the industrial fields, and is successfully applied to the joining of various aluminum alloys, magnesium and copper alloys. In recent years, FSW of high melting temperature materials such as steels and titanium and nickel alloys has become a research carefully. However, that kinds of joint are difficult to obtain on high melting temperature materials, especially
titanium and nickel and their alloys, because of tool limitations.
Thus, to overcome this problem for joining dissimilar joint, TIG preheating can be implemented. Hence, this work intends to establish the possibility of joining Al6061-T6 and Ti-6Al-4V joints by TIG-FSW Hybrid. For this, weldability, thermal characteristics, mechanical characteristics and metallurgical characteristics of dissimilar joints are studied. Successfully obtaining the process of dissimilar metal joining by TIG-FSW Hybrid and analysis of the experimental data influence the industrial and national competitiveness.
Fig. 1.1 Application of friction stir welding in industries
1.2 Research Methodology
The methodology in this study followed for this work are as follows:
1) FSW experiment on dissimilar materials to find optimum welding conditions.
In experiments, 120 trials has been carried out to obtain the optimum welding condition by Friction Stir Welding. Tilt angle, tool geometry, thickness, room temperature were fixed and tool travel speed and tool rotation speed were changed.
2) TIG-FSW hybrid on dissimilar materials to find the optimum welding conditions
Considering the optimum conditions obtained from FSW experiments TIG preheating on titanium side was carried out. The TIG focus distance and shielding gas composition was varied to obtain the optimum welding condition.
3) Mechanical tests and microstructural analysis
Mechanical tests were carried out to test the weld strength of dissimilar joint and the microstructural analysis were carried out to study the change in microstructure on HAZ, TMAZ and SZ and compared with base metal using SEM data and EDS.
4) Comparison of numerical simulation result with experiment(infra red camera).
Thermal camera was used to measure the temperature at the tool-work piece. The thermal camera result was used to compare with the numerical simulation results.
Chapter 2
THEORETICAL BACKGROUND
2.1 Principles and characteristics of FSW
2.1.1 Principles of FSW
Friction-stir welding (FSW) is a solid-state joining process (meaning the metal is not melted during the process) and is used for applications where the original metal characteristics must remain unchanged as far as possible(Fig. 2.1). This process is primarily used on aluminium, and most often on large pieces which cannot be easily heat treated post weld to recover temper characteristics. It was invented and experimentally proven by Wayne Thomas and a team of his colleagues at The Welding Institute (TWI) UK in December 1991. TWI holds a number of patents on the process, the first being the most descriptive.
Fig. 2.1 Schematic diagram of Friction Stir Welding
The first is the plunging period, where the probe is fully and shoulder is partially plunged into the joint line of the work piece. The second action is in the dwell period during which the tool keeps on rotating at the plunge point. In this phase the material around the tool is heated due to the friction between the probe and matrix surfaces due to sliding action. Due to this thermo-mechanical action, materials around the tool get plasticized. The third phase of action is in the steady state welding period, during which the rotating tool is traversed along the welding line. This is followed by a second dwell period, which is the fourth phase of action. The last and the fifth phase of action is in the releasing period during which the rotating tool is raised up from the weld line leaving behind a pin cavity in the work piece(Fig 2.2).
Fig. 2.2 Schematic of Friction Stir Welding Process
Fig.2-3 shows the general classification of FSW regions with microstrucutral characteristics and FSW regions can be devided by four zone. The stir zone (also nugget, dynamically recrystallised zone) is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly equiaxed and often an order of magnitude smaller than the grains in the parent material. A unique feature of the stir zone is the common occurrence of several concentric rings which has been referred to as an
‘onion-ring’ structure. The precise origin of these rings has not been firmly established, although variations in particle number density, grain
size and texture have all been suggested. The flow arm zone is on the upper surface of the weld and consists of material that is dragged by the shoulder from the retreating side of the weld, around the rear of the tool, and deposited on the advancing side. The thermo-mechanically affected zone (TMAZ) occurs on either side of the stir zone. In this region the strain and temperature are lower and the effect of welding on the microstructure is correspondingly smaller. Unlike the stir zone the microstructure is recognizably that of the parent material, albeit significantly deformed and rotated. Although the term TMAZ technically refers to the entire deformed region it is often used to describe any region not already covered by the terms stir zone and flow arm. The heat-affected zone (HAZ) is common to all welding processes. As indicated by the name, this region is subjected to a thermal cycle but is not deformed during welding. The temperatures are lower than those in the TMAZ but may still have a significant effect if the microstructure is thermally unstable. In fact, in age-hardened aluminium alloys this region commonly exhibits the poorest mechanical properties.
Fig. 2.3 Microstructural regions in FS Welded Al alloys
2.1.2 Process Definitions of FSW
1) Advancing side : The tool advancing side is the side of the tool where the local direction of the tool surface due to tool of rotation and the direction of traverse are in the same direction.
2) Backing plate : The backing plate, this is the fixture that the weld rests on. The weld is formed between the backing plate and the tool.
3) Plunge depth : The plunge depth is the maximum depth that the pin penetrates into the material. As the tool is tilted at an angle this is usually in the trailing edge of the tool.
4) Retreating side : The tool retreating side is the side of the tool where the local direction of the tool surface due to tool of rotation and the direction of traverse are in the opposite direction.
5) Rotation speed : The tool rotation speed is the rate of angular rotations per minute (rpm) of the tool around its axis.
6) Tool pin : part of the welding tool which rotates; it is normally shaped as a truncated cone. The pin extends from the shoulder and enters the joint line.
7) Tool shoulder : Part of the welding tool, which rotates and is normally disk shaped. The shoulder forms the weld cap.
8) Traveler speed : The welding speed is the speed(mm/s) of the tool traverse through the weld joint-line.
2.1.3 Characteristics of FSW
1) Advantages of Friction stir welding
Good mechanical properties in the as welded condition Improved
⋅
safety due to the absence of toxic fumes or the spatter of molten material.
No consumables - conventional steel tools[clarification needed] can
⋅
weld over 1000m of aluminium and no filler or gas shield is required for aluminium.
Easily automated on simple milling machines - lower setup costs
⋅
and less training.
Can operate in all positions (horizontal, vertical, etc), as there is no
⋅
weld pool.
Generally good weld appearance and minimal thickness under/over
⋅
-matching, thus reducing the need for expensive machining after welding.
Low environmental impact.
⋅
2) Disadvantages of Friction stir welding Exit hole left when tool is withdrawn.
⋅
Large down forces required with heavy-duty clamping necessary to
⋅
hold the plates together.
Less flexible than manual and arc processes (difficulties with
⋅
thickness variations and non-linear welds).
Often slower traverse rate than some fusion welding techniques
⋅
although this may be offset if fewer welding passes are required.
2.2 Principles and characteristics of GTAW
2.2.1 Principles of GTAW
Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon and helium), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors know as a plasma.
GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and titanium alloys. The process grants the operator greater control over the weld than competing procedures such as shielded metal arc welding and gas metal arc welding, alloying for stronger, higher quality welds.
Fig. 2.4 Schematic diagram of GTAW
2.2.2 Characteristics of FSW
1) Advantages of TIG Welding
Precise control of welding variables (heat).
⋅
Welds can be made with or without filler metal.
⋅
Weld composition is close to that of the parent metal.
⋅
High quality weld structure.
⋅
Slag removal is not required (no slag).
⋅
Thermal distortions of work pieces are minimal due to concentration
⋅
of heat in small zone.
2) Disadvantages of TIG Welding Excessive electrode consumption.
⋅
Arc wandering.
⋅
Oxidized weld deposit.
⋅
Difficult arc starting.
⋅
Low welding rate.
⋅
Relatively expensive.
⋅
Requres high level of operators skill.
⋅
3) Shielding Gases Argon.
⋅
Argon/Helium.
⋅
Oxidized weld deposit.
⋅
2.3 Characteristics of TIG-FSW hybrid welding process
FSW Heating is created within the workpiece both by friction between the rotating tool probe and shoulder and by severe plastic deformation of the workpiece. The localized heating softens material around the probe and, combined with the tool rotation and translation, lead to movement of material from the front to the back of the probe, thus filling the hole in the tool wake as the tool moves forward. The tool shoulder restricts metal flow to a level equivalent to the shoulder position, that is, approximately to the initial workpiece top surface.
As a result of the tool action and influence on the workpiece, when performed properly, a solid-state joint is produced, that is, no melting.
Because of various geometrical features on the tool, material movement around the probe can be complex, with gradients in strain, temperature, and strain rate.
To overcome these kinds of FSW weakness, a TIG-FSW Hybrid system(Fig. 2.4) has been developed. Laser power is used to preheat the workpiece at a localized area ahead of the rotating tool, thus plasticizing a volume of the work piece ahead of the tool. The work piece is then joined in the same way as in the conventional FSW process. The high temperature ahead of the rotating tool softens the workpiece and enables joining with out strong clamping fixtures. Less force is needed to move the welding tool forward, hence, wear is reduced. A further advantage of TIG energy for this process is ability to weld at higher rates without causing excessive wear to the welding tool.
Fig. 2.5 Schematic diagram of TIG-FSW hybrid welding
2.4 Characteristics of Object Materials
The materials used for this study are Al6061-T6 and Ti-6Al-4V. To minimize the mechanical effect in welds such as contraction and expansion in weldment, specimens with dimension 200×100×3.5mm was made to conduct the welding experiment. Chemical composition and mechanical properties of base metals are given in Table 1.1 and 1.2 respectively.
Table 2.1 Chemical compositions in Al6061-T6 and Ti-6Al-4V
Table 2.2 Mechanical properties of Al6061-T6 and Ti-6Al-4V
Al6061-T6 has good mechanical properties and exhibits good weldability, and thus have been applied widely in construction of aircraft structures, yacht construction, including small utility boats, automotive parts etc.
Ti-6Al-4V has high specific strengths and good erosion resistance, and thus have been applied widely in the aerospace, chemical and nuclear industries. Study of welding structure characteristics is essential for its promotion and application.
2.5 Type of characteristics of aluminum alloy
2.5.1 Type of aluminum alloys
Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat treatable and non heat treatable(Fig.
2.5). About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important
cast aluminium alloy system is Al-Si, where the high levels of silicon (4.0~13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.
Fig. 2.6 Classification of Aluminum
2.5.2 Characteristics of 6061 aluminum alloy
Alloy 6061 is one of the most widely used alloys in the 6000 series.
This standard structural alloy, one of the most versatile of the heat treatable alloys, is popular for medium to high strength requirements and has good toughness characteristics. Because It has good mechanical properties and exhibits good weldability. Also, it has excellent corrosion resistance to atmospheric conditions and good corrosion resistance to sea
water. So it is one of the most common alloys of aluminum for general purpose use.
It is commonly available in pre-tempered grades such as, 6061-O (solutionized), 6061-T6 (solutionized and artificially aged), 6061-T651 (solutionized, stress-relieved stretched and artificially aged).
Table. 2.1 shows the alloy composition of 6061. It can be changed depending on the temper.
Table 2.3 The alloy composition of 6061
1) Mechanical properties
The mechanical properties of 6061 depend greatly on the temper, or heat treatment, of the material. Young's Modulus is 10x10^6 psi (69 GPa) irrespective of temper.
· 6061-O : Annealed 6061 (6061-O temper) has maximum tensile strength no more than 18,000 psi (125 MPa), and maximum yield strength no more than 8,000 psi (55 MPa). The material has elongation (stretch before ultimate failure) of 25-30 %.
· 6061-T4 : T4 temper 6061 has an ultimate tensile strength of at least 30,000 psi (207 MPa) and yield strength of at least 16,000 psi (110 MPa).
It has elongation of 16%.
· 6061-T6 : T6 temper 6061 has an ultimate tensile strength of at least 42,000 psi (290 MPa) and yield strength of at least 35,000 psi (241 MPa).
More typical values are 45,000 psi (310 MPa) and 40,000 psi (275 MPa), respectively. In thicknesses of 0.250 inch (6.35 mm) or less, it has
elongation of 8% or more; in thicker sections, it has elongation of 10%.
T651 temper has similar mechanical properties.
2) Application
6061 is widely used for construction of aircraft structures, such as
⋅
wings and fuselages, more commonly in homebuild aircraft than commercial or military aircraft.
6061 is used for yacht construction, including small utility boats.
⋅
6061 is commonly used in the construction of bicycle frames and
⋅
components.
6061 is also used in automotive parts, such as wheel spacers.
⋅
6061 is also used in the manufacture of aluminum cans for the
⋅
packaging of foodstuffs and beverages.
2.6 Type of characteristics of titanium alloy
2.6.1 Type of titanium alloys
The high strength, low weight and outstanding corrosion resistance possessed by titanium and titanium alloys have led to a wide and diversified range of successful applications in aerospace, chemical plant, power generation, oil and gas extraction, medical, sports, and other industries. Welding of titanium by various arc welding processes is widely practised, and good serviceperformance of welds is proven. Newer joining methods, such as laser welding, have been successfully adapted for titanium. Application of appropriate welding technology to the design, manufacture and application of titanium products is as important a step in design as the specification of the alloy. Titanium is a unique material;
as strong as steel but half its weight, with excellent corrosion
resistance.
There are basically four types of alloys distinguished by their microstructure. Also Table listed the type and mechanical properties of titanium;
· Pure titanium : Commercially pure (98 to 99.5% Ti) or strengthened by small additions of oxygen, nitrogen, carbon and iron. The alloys are readily fusion weldable.
· Alpha alloys : These are largely single-phase alloys containing up to 7% aluminium and a small amount (< 0.3%) of oxygen, nitrogen and carbon. The alloys are fusion welded in the annealed condition.
· Alpha-beta alloys : These have a characteristic two phase microstructure formed by the addition of up to 6% aluminium and varying amounts of beta forming constituents - vanadium, chromium and molybdenum. The alloys are readily welded in the annealed condition.
· Beta Alloys ; which are metastable and which contain sufficient beta stabilisers (such as molybdenum, silicon and vanadium) to allow them to maintain the beta phase when quenched, and which can also be solution treated and aged to improve strength.
Table 2.4 Type and Mechanical properties of Titanium
2.6.2 Characteristics of Ti-6Al-4V alloy
Ti-6Al-4V is known as the "workhorse" of the titanium industry because it is by far the most common Ti alloy, accountin for more than 50% of total titanium usage. It is an alpha-beta alloy that is heat treatable to achieve moderate increases in strength. Ti-6Al-4V is recommended for use at service temperatures up to approximately 350 .℃ Also Ti-6Al-4V which calls Grade 5 titanium offers excellent resistance to many marine and offshore oil and gas environments.
Titanium and its alloys resist a wide range of acid conditions being highly resistant to oxidising acids, prossessing useful resistance to reducing acids and offering good resistance to most organic acids at lower concentrations and temperatures. Titanium should not be used with red fuming nitric acid and is rapidly attacked by hydrofluoric acid.
Table 2.5 Typical mechanical properties for Ti-6Al-4V
1) Characteristics of Ti-6Al-4V
Alpha Beta alloy are heat treatable and most are weldable. Typical properties include:
· Strength levels are medium to high
· High temperature creep strength is not as good as nost alpha alloys
· Cold froming may be limited but hot forming qualities are normally good
· Many alloys can be superplastically formed
The most commonly used alpha beta is Ti 6Al-4V, developed in many variations of the basic formulation for the widest possible choice of key properties and for many widely differing applications.
2) applications of Ti-6Al-4V
Direct Manufacturing of parts and prototypes for racing and aerospace industry
· Biomechanical applications, such as implants and prosthesis
· Marine applications
· Chemical industry
· Gas turbines
Chapter 3
EXPERIMENT METHOD OF TIG-FSW HYBRID WELDING PROCESS
3.1 Details of TIG-FSW hybrid welding
3.1.1 TIG-FSW hybrid welding equipment and experimental setup
To conduct TIG-FSW Hybrid experiment, WINXEN FSW gantry type system together with DAIHEN Inverter ELECON 500P TIG welding machine is used in this welding experiment. FSW Tool combined with TIG torch was arranged in order to conduct the welding experiment in X, Y and Z directions. Fig. 3.1 shows the specification of FSW(a) and TIG(b) equipments.
(a) Equipment and specifications of FSW system
(b) Equipment and specifications of TIG system
Fig. 3.1 Equipment and specifications of FSW and TIG welding
TIG welding torch to preheat the titanium alloy material was attached adjacent to the FSW tool shoulder, inclined at 45 degrees. The electrode tip of TIG is placed at a distance of about 20mm from the FSW tool probe. When the TIG torch is placed near to the tool probe, ie, less than 20mm, the current affects the tool surface and thus the desired preheating is not achieved. Fig. 3.2 shows the experimental setup for TIG-FSW hybrid welding.
Fig. 3.2 Experimental set-up for TIG-FSW hybrid welding
3.1.2 Objective material
The chemical compositions and mechanical properties of the materials for Al6061-T6 abd Ti-6Al-4V used in the experiment are given at Table 3.1. Specimens of size 200mm(L) × 100mm(W) × 3mm(T) were made and the edge preparation at the contact side of the specimens is done by milling process.(Table 3.2.) The welding surface was wiped with Methyl Alcohol to remove the grease before welding process.
Table 3.1 Chemical composition and mechanical properties for Al6061-T6 and Ti-6Al-4V
Table 3.2 Dimensions of welded specimen
3.2 Description of tool material and shape
The tool material is made of 12%Co tungsten carbide (WF20) to prevent the tool wear-resistant due to frictional contact with titanium plate while conducting FSW process.
Tool prove shape is of smooth frustum type and shoulder is designed to obtain the proper mixing at the stir zone with good plastic flow of the material. The shoulder is made concave with 3° clearance to act as an escape volume for the material displaced by the probe during the plunge action. The dimensions of shoulder and probe and tool shape to obtain substantial improvements in productivity and quality is shown in Fig 3.3. Table 3.3 shows the Chemical composition and mechanical properties of tungsten carbide tool.
Fig. 3.3 Dimensions and shape of tool
Table 3.3 Chemical compositions and mechanical properties of tool
3.3 Experimental procedure
3.3.1 TIG-FSW hybrid welding condition
In the experiments for TIG-FSW hybrid welding, TIG leading FSW process was carried out where the TIG was focused 45° to the surface of the weld specimen. TIG focus depth was adjusted to penetrate 1~1.5mm on the specimen. Welding parameters such as TIG current and position, tool rotating direction, distance between FSW and TIG, tilt angle were kept fixed whereas FSW tool rotating speed, welding speed were varied.
From the previous researches on FSW of dissimilar materials, it is relevant that join dissimilar plates with tool position at the welding center line is impossible. The difference in hardness and mechanical properties of the materials causes tool wear due to frictional heat at harder material and therefore tool plunge position is adjusted accordingly to conduct TIG-FSW experiment. The plunge position was kept such that the probe outer face is at a distance 2mm away from the weld centre line to Ti-6Al-4V side and remaining part of the tool plunges at Al6061-T6 side.
The actual welding process was carried out with tool rotating direction clock wise (ccw) placing Ti-6Al-4V in the advancing side and Al6061-T6 in the retreating side. The TIG position was placed at 2mm away from weld centre line to SS400 side and 20mm ahead from the probe centre. Fig. 3.4 shows the schematic diagram of TIG-FSW process and the welding parameters used. Fig. 3.5 shows the schematic diagram of TIG and tool position.
Fig. 3.4 Schematic diagram and welding parameters of TIG-FSW hybrid welding
Fig. 3.5 Schematic diagram of TIG heat source and tool position
3.3.2 TIG preheating condition
Experiment to obtain optimum TIG preheating was carried out on Ti-6Al-4V plate and the results are given in Table 3.4 From the bead shapes obtained and macro image observation, TIG current of 60A found optimum to give preheating on Ti-6Al-4V plate. At this TIG current, a penetration depth of 2mm and bead with of 5mm was obtained. Later this TIG preheating parameters was found accurate to obtain sound dissimilar weld joint by TIG-FSW hybrid welding.
Table. 3.4 Conditions for TIG preheating
3.4 Evaluation of mechanical and metallurgical characteristic
3.4.1 Tensile test
Tensile test was carried out with Dongil-Simaz Universal Testing Machine (EHF-EG200KN-40L) using WINSERVO program. Fig. 3.6 shows the EHF-EG200KN-40L and tensile testing setup.
The specimens are fabricated in accordance with the korean standards (KS0801-13-B). The specimen dimensions are given in Table 3.5 Tensile test was done with Load speed 0.033mm/sec and stress-strain curve was obtained.
Fig. 3.6 Process of tensile test
Table 3.5 Dimension of tensile test specimen
3.4.2 Hardness test
The hardness of welded specimen was measured using Akashi HM-112 Vickers Hardness tester as shown in Fig. 3.7. The indenter employed in the Vickers test was a square-based pyramid whose opposite sides meet at the apex at an angle of 136° with load 500g applied for 10 sec.
Fig. 3.8 shows hardness measurement points of welded specimen. The hardness test was carried out on the welded specimen at 0.5gap at three different positions at 1mm distance apart.
Fig. 3.7 Micro vickers hardness tester
Fig. 3.8 Hardness measurement points of welded specimen
3.4.3 Microstructure analysis
The cross section TIG assisted FSW specimen was cut perpendicular to the welding direction. It was polished with 6,3 and 1um diamond paste. Alumina powder was used as the final polishing solution, and then the specimen was chemically etched in a Kroll's reagent(2mml HF+
10mml HNO3+ 94mml H2O) to observe the microstructure and macro of the dissimilar joint.
The prepared specimen was mounted on OLYMPUS optical microscope to observe the micro structure as shown in Fig. 3.9.
Fig. 3.9 Optical microscope
Chapter 4
MECHANICAL AND METALLURGICAL CHARACTERISTICS OF TIG-FSW HYBRID
WELDED JOINTS
4.1 Bead profiles of dissimilar material with welding processes
4.1.1 Experiment by TIG welding
Table 4.1(a) shows the TIG welding parameters used for this experiments. The material properties of Al6061-T6 and Ti-6Al-4V are different and the melting point of Ti-6Al-4V is over two times higher than Al6061-T6. Therefore during welding, melting of Al6061-T6 occurs prior to Ti-6Al-4V. Thus it was considered that it makes impossible to join dissimilar weld with TIG welding process. The bead appearance obtained for dissimilar weld with TIG is given in Table 4.1(b).
Table 4.1 Bead profiles of dissimilar maerials welded joints by TIG welding
(a) TIG welding condition
(b) Bead appearance of dissimilar materials welded joints
4.1.2 Experiment by Friction stir welding
From many trials of experiment carried out on dissimilar joints by FSW, best 20 trials was taken in to account and is tabulated as given in Table 4.2(a). The bead shapes of the best 20 trials are shown in table 4.2(b). Rotation speed of the tool, tool travel speed and tool rotating direction were varied to obtain better results. Initial trials were done for obtaining better surface beads with proper exit holes. and several preliminary experiments are conducted to determine the appropriate offset of the probe to the butt line. Over 600RPM, from the macro images, it was observed that the presence of Ti-6Al-4V deposits are more in the aluminium stir zone which can seriously affect the mechanical characteristics of weld joint. From the FSW experiments and macro images, tool rotation at 400 and 500RPM at tool travel speed 1.0~1.2mm/s was found good and considered to carry out TIG-FSW hybrid experiments. Between 600~700RPM more heat was generated due to friction at the tool-workpiece interface and thus higher amount of Ti-6Al-4V inclusions were found in the Al6061-T6 stir zone.
Table 4.2 Bead profiles of dissimilar materials welded joints by FSW
(a) Welding condition
Material Adv.
side
Rotating speed (rpm)
Travel speed (mm/s)
Rotation direction
Al-Ti Ti 300
0.6
C C W 0.8
1.0 1.2 1.4
Al-Ti Ti 350
0.6 0.8 1.0 1.2 1.4
Al-Ti Ti 400
0.6 0.8 1.0 1.2 1.4
Al-Ti Ti 500
0.6 0.8 1.0 1.2 1.4
(b) Beads appearance and cross section of dissimilar materials FS welded joints
Rotation speed (rpm)
Travel speed (mm/s)
Bead appearance Cross section
300
0.6
0.8
1.0
1.2
1.4
350
0.6
0.8
1.0
1.2
1.4
400
0.6
0.8
1.0
Rotation speed (rpm)
Travel speed (mm/s)
Bead appearance Cross section
400
1.2
1.4
500
0.6
0.8
1.0
1.2
1.4
4.1.3 Experiment by TIG-FSW hybrid welding
Table 4.3 (a) and (b) shows the TIG-FSW hybrid parameters and obtained bead appearance for dissimilar joints of Al6061-T6 and Ti-6Al-4V butt joints. For TIG-FSW process, the best welding parameters obtained from the FSW experiments and TIG preheating BOP test conditions were considered respectively. 20 trials was carried out by varying different welding parameters to achieve the optimum welding condition. At, tool rotation speed between 350~500PM, TIG current of 60(A) and shield gas flow rate of 15ℓ/min, excellent bead appearance was obtained and no weld defects was found in the cross section appearance. Above 500RPM tool rotation speed with TIG preheating, more heat was generated due to friction at the tool-workpiece interface and thus higher amount of Ti-6Al-4V inclusions were found in the Al6061-T6 stir zone.
Table 4.3 Bead profiles of dissimilar materials welded joints by TIG-FSW hybrid welding
(a) TIG-FSW hybrid welding condition
Material
Rotating speed (rpm)
Travel speed (mm/s)
Rotation direction
TIG current
(A)
TIG pulsed current (A)
Shield gas (l/min)
Torch angle (°)
Al-Ti
300
0.8
ccw 60 40 15 45
1.0 1.2 1.4 1.6
350
0.8 1.0 1.2 1.4 1.6
400
0.8 1.0 1.2 1.4 1.6
500
0.8 1.0 1.2 1.4 1.6
(b) Beads appearance and cross section of dissimilar materials TIG-FSW hybrid welded joints
Rotation speed (rpm)
Travel speed (mm/s)
Bead appearance Cross section
300
0.8
1.0
1.2
1.4
1.6
350
0.8
1.0
1.2
1.4
1.6
400
0.8
1.0
1.2
Rotation speed (rpm)
Travel speed (mm/s)
Bead appearance Cross section
400
1.4
1.6
500
0.8
1.0
1.2
1.4
1.6
4.2 Tensile test results
4.2.1 Tensile strength of similar material welded joints (Al6061-T6)
Tensile test of similar joint of Al6061-T6 was carried out to understand the tensile strength of FSW joint made at 500RPM and 1.6 and 4.0mm/sec tool travel speed (Table 4.4). Testing was carried out as per korean standards. This result was used as a reference for the testing of TIG-FSW dissimilar joints. The tensile test results were shown in Fig4.1. The tensile strength of the FS welded joints was obtained as 256.4 and 254.0MPa which is over 76% of the tensile strength of base metal (330MPa).
Table 4.4 Tensile strength of similar material FSW welded joints
Fig. 4.1 Stress-Strain curve
4.2.2 Tensile strength of dissimilar materials FSW welded joints
Testing of tensile strength of dissimilar weld joints by FSW was carried out as per korean standards. The tensile test results (Table 4.5) reveals that fracture occurs at the dissimilar joining interface. The fracture was occurred at the Al6061-T6 stir zone where Ti-6Al-4V inclusions are more.
From stress-strain curve it is evident that the specimen is subjected to brittle fracture(Table 4.6). Good tensile strength is obtained for the welded joints made at tool rotation 400RPM and travel speed 1.2mm/s.
Table 4.5 Tensile strength of dissimilar materials FSW welded joints Rotation
speed (rpm)
Travel speed (mm/s)
Fractured specimen Cross section
T.S (MPa)
300
0.6 142.3
0.8 222.5
1.0 211.8
1.2 162.1
1.4 163.7
350
0.6 201.4
0.8 224.1
1.0 226.5
1.2 214.8
1.4 198.9
400
0.6 205.0
0.8 202.6
1.0 268.1
Rotation speed (rpm)
Travel speed (mm/s)
Fractured specimen Cross section
T.S (MPa)
400
1.2 276.7
1.4 227.9
500
0.6 229.4
0.8 272.4
1.0 212.1
1.2 215.7
1.4 188.6
Table 4.6 Stress-strain curve of dissimilar materials FSW welded joints
Rotation speed (rpm)
Travel speed(mm/s)
0.6 0.8 1.0
300 1.2 1.4 -
-
350
0.6 0.8 1.0
1.2 1.4 -
-
Rotation speed (rpm)
Travel speed(mm/s)
0.6 0.8 1.0
400 1.2 1.4 -
-
500
0.6 0.8 1.0
1.2 1.4 -
-
4.2.3 Tensile strength of dissimilar materials TIG-FSW hybrid welded joints
Table 4.7 shows the cross sections and fractured specimens of dissimilar joint by TIG-FSW hybrid. As observed from the tensile test results, the fracture occurred at the joining interface in specimens welded at 300 and 400RPM tool rotation speed. Above 400RPM the fracture occurred little across the Al6061-T6 weld nugget. The tensile strength of dissimilar joint by TIG-FSW was found 80~90% than that of Al6061-T6 base metal. The good tensile strength obtained was 301.5MPa (Al6061-T6 base metal tensile strength 330MPa) i.e, about 91% than that of base metal at tool rotation speed 350RPM, travel speed 1.2mm/s and TIG current 60A. The stress-strain curve (Table 4.8) shows that the specimen is fractured in a brittle manner with little plastic deformation.
Table 4.7 Tensile strength of dissimilar materials TIG-FSW hybrid welded joints
Rotation speed (rpm)
Travel speed (mm/s)
Fractured specimen Cross section
T.S (MPa)
300
0.8 174.7
1.0 211.9
1.2 226.2
1.4 222.1
1.6 199.2
350
0.8 231.3
1.0 261.9
1.2 301.5
1.4 243.5
1.6 182
400
0.8 212.4
1.0 222.3
1.2 260.5
Rotation speed (rpm)
Travel speed (mm/s)
Fractured specimen Cross section
T.S (MPa)
400
1.4 211.0
1.6 187.6
500
0.8 253.0
1.0 256.5
1.2 278.8
1.4 220.3
1.6 217.6
Table 4.8 Stress-strain curve for dissimilar materials TIG-FSW hybrid welding joints
Rotation speed (rpm)
Travel speed(mm/s)
0.8 1.0 1.2
300 1.4 1.6 -
-
350
0.8 1.0 1.2
1.4 1.6 -
-
Rotation speed (rpm)
Travel speed(mm/s)
0.8 1.0 1.2
400 1.4 1.6 -
-
500
0.8 1.0 1.2
1.4 1.6 -
-
(a) FSW (b) TIG-FSW hybrid welding
Fig. 4.2 Comparison of tensile strength with welding process
Comparing the results of FSW and TIG-FSW hybrid welding, tensile strength of TIG-FSW hybrid welding shows better than FSW(Fig. 4.2).
Tensile strength of above 80% of base metal is obtained 400 and 500RPM in case of FSW joint whereas TIG-FSW hybrid weld joint made at 350RPM and 500RPM at welding speeds 1.0~1.2mm/sec gives better tensile strength. The tensile strength of TIG-FSW hybrid was 80~90% of base metal and best tensile strength value (301.5MPa) is obtained at a welding speed of 350RPM and 1.2mm/sec welding speed.
Therefore, weld joint with better tensile strength can be made at higher welding speed by TIG-FSW hybrid than by FSW
4.3 Hardness test results
The hardness distributions of dissimilar joints by TIG-FSW is shown in Fig.4.3. A drop in hardness from base metal hardness is evident in the welding zone for Al6061-T6. Precipitation hardening alloys such as the Al6061-T6 show a loss of hardness in the HAZ, with some recovery in the nugget because a lower hardness due to the absence of strengthening precipitates at heat affected zone(HAZ) and the dissolved precipitates do re-precipitate or recrystallize subsequently at higher temperatures in the nugget zone. The hardness of Al6061-T6 at the thermo-mechanically affected zone(TMAZ) and stir zone(SZ) is more than HAZ because of the mechanical effect of plastic flow during weld formation.
On Ti-6Al-4V, the hardness value at weld zone is more than that of base metal due to work hardening effect by TIG preheating and frictional heating. The base metal hardness of Al6061-T6 and Ti-6Al-4V are 97HV and 375HV respectively. The hardness values HAZ, TMAZ and SZ of Al6061-T6 are 62, 73 and 78HV respectively.
The hardness of the weld nugget shows variable values because of the presence of the fine or coarse dispersed stainless steel particles in the weld nugget.
When comparing hardness results(Fig. 4.4) of TIG-FSW hybrid with FSW at each top position, hardness values are found more in TIG-FSW.
This is because, plastic flow is increased by TIG preheating making finer recrystallized grains at the TMAZ and SZ.
(a) FSW (b) TIG-FSW hybrid welding Fig. 4.3 Hardness distribution of dissimilar materials welded joints both
welding process
Fig. 4.4 Comparison of hardness distribution of FSW and TIG-FSW hybrid welding joints
4.4 Microstructure analysis
4.4.1 Microstructure of FS welded joints
Microstructure of FSW dissimilar joint is shown in Fig. 4.4. The base metal (point 'a') and HAZ (point 'b') exhibits almost similar microstructure. Grain size little bigger in HAZ than base metal. The TMAZ (point 'c') is characterized by a highly deformed structure. The base metal elongated grains were deformed in an upward flowing pattern around the nugget zone. The Al6061-T6 alloy in the weld nugget consists of fine, equiaxed, recrystallized grains. The fine recrystallized grains in the stirred zone are attributed to the generation of high deformation and temperature during FSW.
The weld nugget exhibits a mixture of Al6061-T6 alloy and Ti-6Al-4V particles pulled away by forge of tool probe from the Ti-6Al-4V surface. Therefore, the weld nugget has a composite structure of Ti-6Al-4V particles reinforced Al6061-T6 alloy.
Ti-6Al-4V particles inclusions were more in the Al6061-T6 weld nugget and have an irregular shape and inhomogeneous distribution within the weld nugget.
Fig. 4.5 Microstructure of FS welded joints
4.4.2 Microstructure of TIG-FSW hybrid welded joints
Microstructure of TIG-FSW hybrid joints are shown in Fig. 4.5. From the microstructure image of Al6061-T6 base metal, it is clear that the base metal have the same microstructure with homogeneous grain distribution as in FSW. Compared to FSW process, only finer particles of Ti-6Al-4V inclusions were found in TIG-FSW microstructure. At nugget zone, coarse grain size is appeared in TIG-FSW hybrid when compared to FSW because of more plastic flow due to TIG preheating effect. The microstructure of acicular α and α′(dark) is exhibited at point f. Also precipitate in β grains(light) at fusion zone(point f)
Fig. 4.6 Microstructure of TIG-FSW hybrid welded joints
4.5 Analysis of weld specimen using SEM and EDS observation
4.5.1 SEM of fractured specimens(FSW and TIG-FSW hybrid)
The SEM microstructures of the fractured specimens have been illustrated in Fig. 4.6. The images of top, middle and bottom side of the fractured surface of Al6061 and Ti-6Al-4V were observed for weld joints by FSW. The fracture surface shows a dimple pattern on all points. But middle and bottom points are subjected to brittle fracture. At Ti-6Al-4V side, Top point is subjected to ductile fracture and middle point is subjected to ductile and brittle fracture, where as at bottom point, complete brittle fracture is occurred.
From the SEM image observation of as-received dissimilar joint by TIG-FSW hybrid (Fig. 4.7.), dimple pattern is observed at the fractured surface in Al6061-T6 side associated with ductile fracture. Titanium inclusions were found in the middle and bottom part of Al6061-T6 side.
At Ti-6Al-4V side, top side shows dimple patterns associated with ductile fracture wherein middle side mixed mode of cleavage area and ductile fracture was observed. At bottom side, is subjected to ductile and brittle fracture.
(a) Top (b) Middle (c) Bottom
(a) FSW - Al 6061-T6 side
(a) Top (b) Middle (c) Bottom
(b) FSW - Ti-6Al-4V side
Fig. 4.7. SEM of dissimilar materials FS welded joints
(a) Top (b) Middle (c) Bottom
(a) TIG-FSW hybrid welding - Al 6061-T6 side
(a) Top (b) Middle (c) Bottom
(b) TIG-FSW hybrid welding - Ti-6Al-4V side
Fig. 4.8. SEM of dissimilar materials TIG-FSW hybrid welded joints
4.5.2 SEM and EDS observation of dissimilar welded joints by TIG-FSW hybrid
The SEM and EDS photographs of the welded joints have been illustrated in Figs 4.8. 4.9 and 4.10 respectively. The possibility of appearing inter-metallic compounds in the interface between titanium and aluminum is studied from the SEM images. Judging from SEM and EDS analysis, no inter-metallic compounds was observed on the check points.
But, previous research works report the possibility of appearing TiAl3
mainly at the upper part of dissimilar joint interface of aluminum and titanium. Here, only Wt% of Al and Ti at aluminum side where titanium inclusion are found at weld interface has been analysed.
(a) (b) (c)
(d) (e) (f)
Fig. 4.9. SEM of TIG-FSW hybrid welded joints
(a) (b)
(c) (d)
(e) (f)
(g) (h)
Fig. 4.10. EDS of dissimilar materials TIG-FSW hybrid welded joints
Chapter 5 CONCLUSION
TIG-FSW hybrid welding was successfully carried out to join dissimilar materials (Al6061-T6 and Ti-6Al-4V). In order to investigate the weldability of dissimilar materials TIG-FSW hybrid welded joints, mechanical test(tensile test, hardness test) and microstructral analysis have been carried out. Moreover, a comparative study has been carried out between TIG-FSW hybrid welded joints and FSW welded joints.
From this study conclusions are made as follows :
1) Because of the maximum plunge force area, the heat generation in FSW of dissimilar joints was maximum at Al6061-T6 side which lead to brittle fracture of the weld joints. Hence, TIG-FSW hybrid welding process helps to balance the heat generation at both Al6061-T6 and Ti-6Al-4V which increased the tensile strength of the dissimilar materials welded joints.
2) At a tool rotation speed of 350 and 500rpm with welding speed 1.2mm/sec, good tensile strength was obtained by TIG-FSW hybrid welding. But comparing with FSW, overall welding experimental results of TIG-FSW hybrid welding is evident that good weldability can be made between rotation speed 350~500rpm and welding speed 1.0~1.2mm/s.
3) The tensile strength of FSW dissimilar materials welded joints is found to be lower than that of TIF-FSW hybrid welding at optimum welding conditions. The maximum tensile strength obtained was 276.7MPa and 30.15MPa for FSW and TIF-FSW hybrid welding respectively.
4) The hardness values were found different in the weld nugget of
dissimilar materials welded joints depending upon the material properties. The hardness value at the retreating side sharply decreased towards the weld nugget from the level of TMAZ in the titanium at advancing side of welded joints. The hardness at the weld nugget across Al6061-T6 bond line are not seriously affected by presence of Ti-6Al-4V deposits.
5) Due to increase in plastic flow and formation of finer recrystallized grains at the TMAZ and SZ by TIG preheating in TIG-FSW hybrid welding, the hardness values are more in TIG-FSW hybrid welding compared to FSW.
6) From SEM image observation of fractured face of the specimen by FSW and TIG-FSW hybrid welding, dimple pattern with ductile and brittle fracture was found to occur at joint interface. But more brittle fracture is found in FSW welded specimens. In TIG-FSW hybrid dissimilar materials welded joints ductile mode of fracture is found to occur at Al6061-T6 side with fewer brittle particles. Mixed mode of cleavage area and ductile fracture was observed at Ti-6Al-4V side.
As for welding processes, in terms of weldability, TIG-FSW hybrid welded joint seems to be advantageous compared to FS welded joints.From the results of experiment, it could comfirm, it is more reasonable to apply TIG-FSW hybrid welding for butt joint of dissimilar materials Al6061-T6/Ti-6Al-4V
REFERENCES
[1] G.H. Jeon, A Study on Weldability of TIG Assisted Friction Stir Dissimilar Welding of Al6061 Alloy and STS Sheet, 2010
[2] Ulrike Dressler, Gerhard Biallas, Ulises Alfaro Mercado. "Friction stir welding of titanium alloy Ti6Al4V to aluminium alloy AA2024-T3", 2009
[3] Y.J.Chao, and X.Qi: Heat transfer and thermo-mechanical analysis of Friction Stir joining of Al6061-T6 plates, 1st International
Symposium on Friction Stir Welding, (1999).
[4] Hidetoshi Fujii, Takahiro Tatsuno, Takuya Tsumura " Hybrid Friction Stir Welding of Carbon Steel" Material Science Forum Vols.580-582 (2008) P.393~396
[5] Yu Zhang, Yutaka S. Sato, Hiroyuki Kokawa, Seung Hwan C.Park, Satoshi Hirano "Microstructural characteristics and mechanical properties of Ti-6Al-4V friction stir welds. (2008)
[6] W.B. Lee, S.B. Jung, Gehard Biallas, Martin Schmuecker. "Joint properties and Interface Analysis of Friction Stir Welded Dissimilar Materials between Austenite Stainless Steel and 6013 Al Alloy"
Journal of the Korean Welding Society, Vo.23, No.5, pp.469
~476, October 2005.
[7] A.William, Using Gleeble flow stress data to establish optimum FSW processing parameter in Aluminium Alloys, Advanced material
processing centre, (2002).
[8] H.S. Bang. "Study on The Mechanical Behaviour of Welded part in thick Plate - Three-dimensional Thermal Elasto-Plastic Analysis Baseon Finite Element Method." Journal of the Korean Welding Society, Vol.10, No.4, pp.37~43, December 1992.
[9] Rajesh S.R., "Development of mathematical model for Thermo - mechanical behavior of Friction Stir Welding an introduction
Soldering by using FEM/BEM", 2007
[10] M. Balasubramanian, V. Jayabalan, V. Balasubramanian. "Effect of microstructure on impact toughness of pulsed current GTA welded α -β titanium alloy", 2008
[11] H. Kokawa, S.H.C. Park, Y.S. Sato, K. Okamoto, S. Hirano, M.
Inagaki : Microstructures in Friction Stir Welded 304 Austenitic Stainless Steel, Welding in the World, 49(3/4), 2005, pp.34-40 [12] M. Ramulu, P.D. Edwards, D.G. Sanders, A.P. Reynolds, T.Trapp :
Tensile properties of friction stir welded and friction stir welded superplastically formed Ti-6Al-4V butt joints. (2010)
[13] AWS, WELDING HANDBOOK, Vol.1, Eighth Edition, 1987.
[14] AWS, WELDING HANDBOOK, Vol.2, Eighth Edition, 1991.
[15] W.M. Thomas, P.L.Threadgill, and E.D. Nicholas : Feasible of Friction Stir Welding Steel, Science and Technology of Welding and Joining, 4(6), 1999, pp. 365-372
[16] L.E.Murr, et al.: Solid-state flow association with the friction stir welding of dissmilar metals, Fluid flow phenomena in metals processing (1999), 31-40
[17] Huseyin Uzun, Claudio Dalle Donne, Alberto Argagnotto,
Tommaso Ghidini, Carla Gambaro.:Friction stir welding of dissimilar Al 6013-T4 To X5CrNi18-10 stainless steel, Materials and Design 26 (2005) 41 .46–
[18] L. Zhou, H.J. Liu, Q.W. Liu : Effect of rotation speed on microstructure mechanical properties of Ti-6Al-4V friction stir welded joints.(2010)
[19] H.S. Bang, G.Y. Han, "The plane-deformation thermal elasto -plastic analysis during welding of plate", The society of naval architects of korea p33~40, Apr.1994
[20] S. Katayama, et al.: Proc.5th Int.Conf.on TRENDS IN WELDING RESEARCH, Georgia, June (1998), pp.467-472.
Acknowledgments
지난 2년간의 연구실 생활을 돌이켜 한 편의 논문으로 대신하기엔 아쉬움
이 많이 남지만 무한한 배움의 터였던 조선대학교 대학원 생활을 잘 마무리,
할 수 있게 되어서 무엇보다 뜻 깊게 생각하고 이 논문이 완성되기까지 저,
를 도와주시고 지도해 주신 분들께 적은 지면을 빌려 감사의 마음을 전하고 자 합니다.
우선 항상 부족함이 많은 제자를 세심한 관심과 배려로 지도해 주시고 때,
로는 사랑으로 때로는 질책으로 학문의 길을 이끌어주신 방한서 지도교수님
께 깊은 감사를 드립니다 또한 격려와 지도편달을 해주신 방희선 교수님과. ,
논문에 세심한 관심을 가져주신 오익현 박사님 그리고 대학원생으로써 새롭
게 다짐하고 도전할 수 있는 자극과 가르침을 주셨던 이창우 박사님 미흡한,
저의 논문을 심사해 주셨던 이귀주 교수님과 박제웅 교수님 그리고 권영섭
교수님께도 감사를 드립니다 그리고 대학원 과정 동안 저에게 많은 가르침.
을 주신 금속공학과 여러 교수님들 및 선후배님에게 감사를 드립니다.
그리고 지난 대학원 생활 2년 동안 함께 가족 같이 서로 도움을 주고 받으
면서 소중한 추억을 간직하게 해 준 연구실 동료들에게도 고마운 마음을 전
하고자 합니다 묵묵함과 성실함을 몸소 보여주시고 논문에도 많은 도움을.
주신 스승이자 친형 같은 근홍이형 영어로 대화하기 힘들었지만 먼 이국땅,
에 와서 많은 도움을 주고 항상 남을 배려하는 비조이 친누나같이 항상 동,
생들 챙겨주시는 정미누나 항상 순발력과 재치로 웃음을 선사해주신 계성이,
형 연구실 터줏대감 같은 준형이형 용혁이형 항상 침착함과 자기관리를 잘, , ,
하신 세민이형 모든 일에서 깔끔한 두송이에게 고마움을 전하고 함께 연구, ,
하진 못했지만 멀리서 많은 도움을 준 기상이 실험하느라 고생한 주연이 언, ,
제나 형들 도와주느라 고생한 정한이 힘든 상황에서도 자신감이 부족했던,
저에게 오히려 용기와 힘을 주었던 금속공학과 정수형 태영이형 원빈이형, , , 강선이형 그리고 많은 정보를 공유해주신 윤재형 성묵이형 형준이 입신양, , , ,
면으로 대학원에 인도해준 만물박사 승현이 연구실 새내기인 경학이 희준이, ,
도 고마움을 같이 하고 싶고 열심히 대학원 생활하라고 전하고 싶습니다.
무엇보다도 믿음으로 저에게 큰 힘이 되어주시고 물심양면으로 고생하신 아버지 어머니 그리고 주희 주미 누나에게도 감사함과 고마움을 전합니다, , . 그리고 힘들 때나 지칠 때나 늘 저의 활력소가 되어주었던 사랑하는 여자 친
구 설이에게도 감사한 마음을 전달하고 싶고 항상 저를 믿어주시고 챙겨주,
시는 여자 친구 어머니 아버지 모두에게 깊은 감사를 드립니다 마지막으로, .
늘 함께 하고 싶은 고향친구 이영이 원준이 영호 그리고 해병대 식구들에, , , 게도 감사하다는 말과 함께 변치않는 우정을 기약하고 싶습니다.
길의 끝은 언제나 또 다른 길의 시작을 의미합니다 대학원을 마치고 어떠.
한 길이 제 앞에 나타나더라도 저에게 힘을 주는 분들이 있기에 이제는 그
길을 해쳐나갈 수 있는 용기가 생겼습니다 대학원에서 보고 배우고 느낀 것.
들을 디딤돌 삼아 이 세상에 소금과 같은 존재가 되고 싶습니다 모두 감사.
합니다.
년 월
2010 12 배움의 터 연구실에서
송 현 종 올림