Development of friction stir welding techniques for multi-axis machines

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Development of friction stir welding techniques for

multi-axis machines

Thèse

Yousef Imani

Doctorat en génie mécanique

Philosophiae doctor (Ph.D.)

Québec, Canada

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Development of friction stir welding techniques for

multi-axis machines

Thèse

Yousef Imani

Sous la direction de :

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Résumé

Le soudage par friction et malaxage (SFM) est un procédé d'assemblage innovant à l'état solide qui a été inventé en 1993 et qui présente des avantages significatifs par rapport aux techniques de soudage par fusion. En raison des grandes forces appliquées sur l'outil et la nécessité de maintenir un angle constant sur tout le chemin de soudage (angle d'inclinaison), ce processus est normalement effectué sur des machines coûteuses conçues spécifiquement à cette fin. La présente thèse est une tentative de faciliter l'application de soudage par friction malaxage sur les centres d'usinage CNC très communément rencontré en industrie. Une variante peu connue et peu développée de ce processus, à savoir le soudage par friction et malaxage à angle droit dans lequel l'axe d'outil est toujours perpendiculaire à la surface de la pièce a été étudié de près dans cette thèse. Des outils spéciaux pour le soudage par friction et malaxage qui sont appropriées pour cette nouvelle orientation ont été développés et les paramètres de fonctionnement de ces outils ont été mis en place. En outre, des techniques ont été investiguées pour réduire la force axiale par optimisation de la conception de l'outil et des paramètres de soudage. De plus, l'une des principales difficultés qui pourraient survenir durant les applications industrielles du soudage par friction et malaxage est l’alignement horizontal et vertical des pièces pour le soudage de joints aboutés. Une méthodologie est aussi proposée pour effectuer le soudage par friction et malaxage sur des contours 3D. La Méthode Taguchi a été utilisée pour la conception d'expériences et des modèles de réseaux neuronaux artificiels ont été formés pour l'analyse des résultats des expériences et pour l’optimisation.

Il a été démontré que le soudage par friction et malaxage à angle droit a la capacité de faire des soudures saines avec limites ultimes acceptables en utilisant des valeurs plus basses de force axiale d’environ 50% par rapport au soudage par friction et malaxage typique. En outre, les plages utilisables des paramètres de conception de l'outil et des paramètres de fonctionnement ont été trouvées. Elles conduisent à la réduction de la force axiale du soudage par friction et malaxage à angle droit.

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Les erreurs de placement de pièces dans des joints aboutés ont aussi été investiguées conduisant à une définition des plages acceptables d’erreur avec la méthode de soudage à angle droit.

Les techniques développées ont aussi été validées dans la mise en œuvre du soudage par friction et malaxage pour les joints 2D et 3D. De plus, la méthodologie proposée pour le soudage sur contours 3D a été validée avec succès en soudage sur une pièce particulière en utilisant une machine CNC à 5 axes dans les deux configurations de joints aboutés et superposés.

Mots clés- Soudage par friction et malaxage; soudage par friction et malaxage à angle

droit; réduction de la force; précision d’ajustement des pièces; soudage par friction et malaxage 2D et 3D; AA6061-T6; machine CNC; conception d'outils; paramètres de soudage

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Abstract

Friction stir welding (FSW) is an innovative solid state joining method invented at the end of twentieth century and having significant advantages over fusion welding techniques. Due to the high amount of forces applied on the FSW tool and the need to keep a constant angle all over the welding path (tilt angle), this process in normally performed on costly machines designed specifically for it. The present thesis is an attempt to facilitate the implementation of friction stir welding on common CNC machining centers. A less considered variant of this process, namely right angle FSW in which the tool axis is always perpendicular to the surface of workpiece has been closely studied and investigated. Special FSW tools which are appropriate for this new orientation have been developed and operating parameters for these tools have been established. In addition, techniques were developed to reduce the axial force through optimization of tool design and welding parameters. Moreover, one of the major difficulties which could be encountered during industrial applications of FSW, joint fit-up issues have been explored and attempts were made to manage these issues. A methodology has been proposed for FSW over 3D contours. Taguchi method has been used for design of experiments and artificial neural network models have been trained for analysis of results of experiments and optimization. It has been shown that the right angle FSW have the capacity of making sound welds with acceptable UTS employing lower values of axial force in comparison to typical FSW. Furthermore, workable ranges of tool design and welding parameters were found that leads to reduction of axial force within right angle FSW. To tolerate for joint fit-up issues, regions of operating parameters were established that could manage typical values of gap and mismatch. The developed techniques have also been validated and implemented for joining on 2D and 3D paths. In addition, the 3D methodology has been successfully validated in welding a complex part using a 5 axis CNC machine in both butt and lap configurations.

Keywords- Friction stir welding; Right angle FSW; Force reduction; Joint fit-up issues; 2D

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List of Tables

Table 1.1: List of FSW tool materials used for different workpiece materials and

thicknesses... 14

Table 1.2: Properties of different tool materials used for FSW ... 15

Table 1.3: Percentage contribution of process parameters on measurement sensors in FSW of round tubes with different diameters ... 41

Table 1.4: Taguchi Orthogonal Array selection guide ... 54

Table 2.1: Mechanical properties of base material ... 71

Table 2.2: Preliminary convex shoulder tool design ... 73

Table 2.3: Second Taguchi DOE for tool and welding parameters ... 76

Table 2.4: Third Taguchi DOE for tool and welding parameters ... 76

Table 2.5: Complementary tests for CST model retraining ... 77

Table 2.6: Confirming tests to check the accuracy of the CST model ... 78

Table 2.7: FSW tests done using FST3 tool ... 79

Table 2.8: Range of tool design and welding parameters for input data set generation and the ones which lead to acceptable UTS ... 79

Table 3.2: Taguchi L8 design of experiments and results ... 93

Table 3.3: Confirming tests to check the accuracy of the ANN model ... 93

Table 3.4: Range of tool design and welding parameters used to establish Fig. 3.6 ... 94

Table 4.2: Taguchi L16 design of experiments ... 108

Table 4.3: Confirming tests to check the accuracy of ANN model ... 109

Table 4.4: Range of joint fit-up and welding parameters for input data set generation .. 110

Table 5.1: Mechanical characteristic of the base material ... 123

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Table 5.4: Results of tensile testing ... 127

Table 5.5: Average values of UTS for different curvature radii in butt welding ... 132

Table 6.1: Mechanical properties of base materials ... 137

Table 6.2: Taguchi L9 DOE used in FSW of 2.44 mm thick Al6061-T6 ... 139

Table 6.3: Confirming tests to check the accuracy of the ANN model used in FSW of 2.44 mm thick Al6061-T6 ... 140

Table 6.4: Taguchi L9 DOE used in cold FSW of 9.58 mm thick Al6061-T6 ... 145

Table 6.5: Taguchi L9 DOE used in hot FSW of 9.58 mm thick Al6061-T6 ... 146

Table 6.6: Confirming tests to check the accuracy of the ANN models used in cold (test No. 1-3) and hot (test No. 4-6) FSW of 9.58 mm thick Al6061-T6 ... 147

Table 6.7: Acceptable range of parameters for different material thicknesses ... 150

Table 6.8: Tensile test results of 2D FSW experiment ... 154

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List of Figures

Fig. 1.1: Schematic illustration of FSW process ... 3

Fig. 1.2: Basic principle of retractable FSW tool. The pin is withdrawn into the shoulder from left to right ...4

Fig. 1.3: (a) Various microstructural regions in a friction stir welded aluminum. (b) Micrograph of different regions ...6

Fig. 1.4: (a) A void defect beneath the surface. (b) A void defect which breaks through to the weld surface ...7

Fig. 1.5: (a) Macrograph of a weld with joint line remnant defect. (b) Oxide debris which cause joint line remnant flaw ...7

Fig. 1.6: (a) Demonstration of an incomplete root penetration defect. (b) A fracture caused by incomplete root penetration defect ...8

Fig. 1.7: Different types of FSW tools (a) Fixed, (b) Adjustable and (c) Self-retracting (bobbin) ...9

Fig. 1.8: Different possibilities of (a) shoulder shape [5] and (b) shoulder features ... 10

Fig. 1.9: Different possibilities for pin design ... 12

Fig. 1.10: (a) Whorl tool and its different shape possibilities. (b) MX Triflute tool ... 12

Fig. 1.11: (a) Eclipse 500 business-class aircraft. Joining of (b) longitudinal and circumferential slippers and (c) window and door doublers to the fuselage of Eclipse 500 ... 17

Fig. 1.12: Employing FSW in lap configuration for joining of aluminum extrusions to the stampings in assembly of central tunnel of Ford GT ... 18

Fig. 1.13: A FSW machine by ESAB for welding of long extrusions of aluminum to fabricate a panel ... 19

Fig. 1.14: The ISTIR Aero, a 5 axis FSW fabricated by MTS ... 20

Fig. 1.15: (a) Serial and (b) parallel robot manipulators ... 20

Fig. 1.16: Specifications of FSW machines produced by Manufacturing Technology, Inc. . 22

Fig. 1.17: FSW tool pin profiles used by Elangovan et al. ... 23

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Fig. 1.20: Illustration of material displacement using threaded and smooth cylindrical pins

... 25

Fig. 1.21: Acceptable range of process and tool design parameters found by Rajakumar et al. ... 27

Fig. 1.22: Relationship between force and UTS... 28

Fig 1.23: Effect of tool rotation and travel speeds on axial force in FSW ... 29

Fig. 1.24: Relationship between travel speed and tool force and torque using FEM ... 29

Fig. 1.25: Schematic illustration of ultrasonic assisted FSW ... 30

Fig. 1.26: Influence of ultrasonic addition on axial force in FSW of Al6061-T6 using rotation and travel speeds of 1500 rpm and 25 mm/min ... 31

Fig. 1.27: Influence of ultrasonic addition (20 kHz) on axial force in FSW of 1018 steel using rotation speed of 650 rpm and travel speed of (a) 25 and (b) 50 mm/min ... 31

Fig. 1.28: Schematic illustration and experimental setup of electrically assisted FSW ... 32

Fig. 1.29: Preheating system used to increase aluminum temperature prior to FSW ... 33

Fig. 1.30: Effect of preheating on axial force in FSW of Al 6061-T6511 ... 34

Fig. 1.31: Variation of axial force with travel speed in FSW of Al6061 and Al2195 ... 35

Fig. 1.32: Joint fit-up issues in butt FSW, (a) gap, (b) mismatch, (c) misalignment ... 36

Fig. 1.33: (a) Illustration of positive and negative misalignment, (b) Effect of misalignment on UTS and elongation ... 37

Fig 1.34: (a) Effect of gap and (b) mismatch on UTS and elongation ... 37

Fig. 1.35: Effect of gap width and machine compliance on UTS of FSWed joints using a 3 deg tilt angle ... 38

Fig. 1.36: Variation of joint efficiency with plunge depth and tilt angle in FSW with 2 mm gap ... 38

Fig. 1.37: Effect of misalignment on UTS in FSW of 0.125” plates of Al7075-T73 ... 39

Fig. 1.38: Effect of gap on UTS in FSW of 0.125” plates of Al7075-T73 ... 40

Fig. 1.39: (a) 3D curvature welding path, (b) Illustration and (c) deviation of tool inclination angles in A and C shortcut method ... 42

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Fig. 1.40: (a) FSWed sample appearance using A and C shortcut method, Magnified view of corner using (b) tool inclination angle maintaining, (c) welding speed maintaining, and

(d) A and C shortcut methods ... 43

Fig. 1.41: (a) Contact condition of tool with workpiece over linear and curved path, (b) modification of tilt angle to ensure sufficient contact of back of the tool with workpiece ... 44

Fig. 1.42: Variation of tool tilt angle in curved zones ... 44

Fig. 1.43: (a) Outer defect generated using a non-optimized welding path and (b) a sound weld in a curved zone using the proposed strategy... 45

Fig. 1.44: Thermal energy distribution W/m2 (right) at the tool workpiece interface and temperature field (left) for tool rotation speeds of (a) 600 rpm, (b) 800 rpm, and (c) 1000 rpm ... 46

Fig. 1.45: Temperature distribution predicted by thermal model in four different times during FSW... 47

Fig. 1.46: Temperature field during (a) plunging and (b) welding in FSW ... 48

Fig. 1.47: Distribution of (a) temperature, (b) effective strain, (c) strain rate, (d), (e), and (f) longitudinal, transverse and normal residual stresses respectively ... 50

Fig. 1.48: Comparison of FEM and experimental longitudinal residual stresses ... 51

Fig. 1.49: Comparison of crack length for base material, experimental, and numerical FSW joints ... 51

Fig. 1.50: Effect of pin angle and tool travel speed on (a) maximum temperature, (b) minimum grain size ... 52

Fig. 1.51: Effect of pin angle on material flow pattern, (a) cylindrical pin, (b) 30 deg conical pin, (c) 40 deg conical pin (d) to (f) helical movement using 30 deg conical pin ... 53

Fig. 1.52: Illustration of a neural network [1] ... 56

Fig. 1.53: Demonstration of the backpropagation algorithm ... 56

Fig. 2.1: Schematic illustration of friction stir welding process ... 68

Fig. 2.2: Different shoulder features ... 69

Fig. 2.3: Dimensions of tensile test specimen (mm) ... 71

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Fig. 2.6: (a) FSW sample with a groove on surface and (b) FSW sample without any groove

... 73

Fig. 2.7: (a) First design of the flat shoulder tool (FST1), (b) second design (FST2), and (c) FST3 tool ... 74

Fig. 2.8: Relationship between tool design and welding parameters with respect to UTS, (a), (b) and (c) using working diameters of 15, 16 and 17 mm and rotation speed of 2600 rpm, travel speed of 1300 mm/min and penetration of 3.195 mm, (d), (e) and (f) using tool penetration of 3.18, 3.195 and 3.21 mm respectively and pin angle of 5.5˚, shoulder angle of 28˚ and shoulder working diameter of 16 mm ... 80

Fig 2.9: Relationship between tool traverse and rotation speeds with respect to UTS ... 81

Fig. 3.2: Different types of shoulder ... 88

Fig. 3.3: a) Schematic illustration of tool design parameters and b) effect of shoulder angle increasing ... 90

Fig. 3.4: tools used to make FSWs, left to right: XR1, XR2, XR3, and XR4 ... 91

Fig. 3.6: Relationship between tool design, welding parameters, axial force and UTS, (a) thru (d) using rotation and travel speeds of 3000 rpm and 800 mm/min, (a) and (c) using pin angle of 23⁰, (b) and (d) using pin angle of 27⁰ ... 95

Fig. 3.6 (Continued): (e) thru (h) using shoulder angle of 4⁰ and pin angle of 25⁰, (e) and (g) using SWD of 13 mm, (f) and (h) using SWD of 17 mm ... 96

Fig. 4.2: Joint fit-up issues in butt FSW (a) gap, (b) mismatch, and (c) misalignment ... 104

Fig. 4.3: FSW tool used to make tests ... 106

Fig. 4.4: A bended FSW sample ... 107

Fig. 4.6: Schematic illustration of positive and negative mismatch and shoulder penetration plane ... 108

Fig. 4.7: (a) and (b) Effect of tool rotation and travel speeds in tolerating joint fit up issues using shoulder penetration of 0 and 0.15 mm respectively in presence of gap of 1 mm and mismatch of +0.4 mm, (c) and (d) 2D illustration of (a) and (b) ... 111

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Fig. 4.7 (Continued): (e) and (f) change in UTS in variable gap and mismatch in a constant rotation and travel speeds of (2000 rpm, 125mm/min) and (3300 rpm, 350 mm/min)

respectively using penetration of 0.15 mm, (g) and (h) change in force in (e) and (f) ... 112

Fig. 5.1: Schematic illustration of FSW process ... 119

Fig. 5.2: Contact condition between the tool and the workpiece with (a) the convex and (b) the concave curvature ... 121

Fig. 5.3: Demonstration of the strategies applied for (a) and (b) the convex curvature and (c) the concave curvature ... 121

Fig. 5.4: Schematic illustration of the application of the first and second strategies on a convex curvature ... 122

Fig. 5.5: (a) Workpiece and the die used to form sheets and (b) welding set-up ... 124

Fig. 5.6: Overall shape of the flat shoulder tools used in the present study ... 125

Fig. 5.7: Dimensions of the tensile testing specimen (mm) ... 125

Fig. 5.8: Sample welded using constant tool penetration and zero tilt angle ... 126

Fig. 5.9: Butt and lap welding samples and tensile testing samples as well as their locations ... 128

Fig. 5.10: Variation of axial welding force and joint strength in butt welding (a) with curvature type and radius and (b) in top convex and bottom concave points ... 129

Fig. 5.11: Variation of axial welding force and joint strength in lap welding a) with curvature type and radius and (b) in top convex and bottom concave points ... 130

Fig. 6.1: Tools for FSW of (a) 2.44 and (b) 9.58 mm thick material ... 137

Fig. 6.2: Schematic illustration of ANN model used for analysis and optimization ... 139

Fig. 6.3: (a) A welded sample and (b) tensile test samples of FSW of 2.44 mm thick sheets ... 140

Fig. 6.4: Variation of UTS and axial force with process parameters in penetration of (a) and (b) 2.4 mm, (c) and (d) 2.45 mm, (e) and (f) 2.5 mm in FSW of 2.44 mm thick sheets ... 143

Fig. 6.5: Fracture surface of welds made with rotation speed of (a) 5000 rpm, (b) 6000 rpm, and (c) 7000 rpm ... 144

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Fig. 6.8: Variation of UTS and axial force with process parameters in penetration of (a) and (b) 9.5 mm, (c) and (d) 9.55 mm, mm (e) and (f)9.6 mm in hot FSW of 9.58 mm thick

plates ... 151

Fig. 6.9: Optimized convex shoulder FSW tool used for welding of 2D paths ... 152

Fig. 6.10: Welding components used for 2D contours experiments ... 153

Fig. 6.11: Schematic illustration of welding path ... 153

Fig. 6.12: (a) A welded sample and the location of tensile test specimens, (b) and (c) top and side view of broken tensile test specimens (decrease of curvature radius from 1 to 6) ... 154

Fig. 6.13: An example of a welding path for 2D FSW that starts and ends outside of workpieces’ joining line ... 155

Fig. 6.14: Effect of curvature radius on UTS of 2D FSWs ... 156

Fig. 6.15: Variation of axial forces along the weld length ... 157

Fig. 6.16: Welding of flat, concave, and convex surface profiles using (a) flat shoulder, (b) convex shoulder, (c) concave shoulder, and (d) concave shoulder with 3⁰ tilt angle... 158

Fig. 6.17: An example of (a) a joint reachability issue and (b) a possible solution ... 159

Fig. 6.18: Conversion of joint type to make the welding feasible ... 159

Fig. 6.19: Design of components to avoid stiff fixturing and the need for backing plate (a) butt joint and (b) lap joint ... 160

Fig. 7.1: Fanuc M900iA robot used in this study ... 167

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To my parents from whom

I learned many valuable lessons

To my love for

her encouragement and patience!

“In the middle of difficulty lies opportunity”

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Acknowledgments

I would first like to thank my supervisor Prof. Michel Guillot, from whom I learned not only science and technology, but also patience, dedication, and perseverance. As a thesis supervisor, Prof. Michel Guillot supported and helped me in all aspects of this thesis and always gave me constant encouragement and advice, despite his busy agenda. The door to Prof. Michel Guillot’s office was always open whenever I ran into a trouble spot or had a question about my research or writing. He consistently allowed this project to be my own work, but steered me in the right direction whenever he thought I needed it.

I also thank the employees of the department of mechanical engineering’s workshop specially Mr. Pierre Carrier for his contribution and support in experimental parts of this project. I also would like to thank Mr. Antoine Tremblay for his assistance in certain experimental parts of this project.

I also want to express my appreciation for my wife Elham Esmaeilpour Khalili who stuck with me during the long months of writing and re-writing. These past several years have not been an easy ride, both academically and personally. I truly thank Elham for sticking by my side and giving me positive encouragement.

Finally I want to thank the many individuals from all sectors of Québec city who are committed to making the many exciting possibilities of this city into positive and solid realities.

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Chapter 1

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1.1. General background

Friction stir welding (FSW) is a solid state joining process invented in 1991 by Wayne Thomas at TWI (The Welding Institute). Although this method has the capability to be used for joining of wide range of materials, it is mostly used for welding of low melting point materials such as aluminum, magnesium, and copper alloys. Since the process has many advantages over fusion welding techniques including absence of filler material, shielding gas, fumes and intensive light, higher strength and fatigue life, better microstructure, lower temperature gradient, lower residual stress, much lower weld defects and discontinuities, lower energy consumption, lower net cost, and environmentally friendly, it is increasingly gaining interest from different industries such as shipbuilding, aerospace, automotive, and railway. For instance, Boeing Company has extensively used FSW in welding of Delta II and IV rockets; and its FSW specific design of these two rockets resulted in 60% cost saving and reduction of manufacturing time from 23 to only 6 days [1].

1.2. Review of the FSW process

1.2.1. The process

As shown in Fig. 1.1, in FSW process a non-consumable rotating tool consisting of a shoulder and a pin with different shape possibilities penetrates into the butt line of two sheets or plates that are tightly fixed together on a thick plate, with higher strength which is called backing plate, until its shoulder touches the surface of the materials and produces friction that rises the temperature. This allows the pin to plasticise and stir, at about 80-90% of melting temperature, the metal located on both sides of the joint. Then the tool starts moving along the joint at a constant speed. A combination of rotational and linear movement of the tool along joint moves the softened material from the front to the back of the tool. Normally, there is an angle between the tool axis and the plate normal direction which is called tilt angle. The forging action of the tool shoulder is due to the tilt angle of the tool that is approximately 1-3 degrees. It can be seen from Fig. 1.1 that two sides of the joining line possess different names; the side in which the tool rotation and

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traverse are in the same direction is called advancing side and the other side in which the tool rotation and traverse are opposite to each other is called retreating side.

Fig. 1.1: Schematic illustration of FSW process [2]

1.2.2. Advantage and limitations

FSW process has several advantages compared to traditional fusion welding techniques which make it a state of the art technology for joining different materials especially some difficult to weld aluminum alloys like 2xxx and 7xxx. In addition to being a solid state joining method which is the main advantage of FSW, other advantages include:

x Better mechanical properties such as strength, fatigue life, and hardness x Better metallurgical properties

x Reduced number of parameters (only about 5 major parameters: Axial welding force, tool rotation and travel speed, penetration, and tilt angle) which is much less than other welding techniques such as MIG, Laser, and Plasma

x Absence of filler material, shielding gas, fumes and intensive light x Lower temperature gradient

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x Lower net cost

x Possibility to weld in different positions (horizontal, vertical, overhead, and etc.) because of the absence of weld pool

x No need for seam preparation and post-process operations such as cutting off slag x Environmentally friendly

Besides having mentioned benefits, the process has a number of limitations. These challenges are generation of an exit hole at the end of weld, high process forces, robust fixturing requirement, difficulties in using for non-linear joints, and sensitivity to misalignment and geometric errors. Some of these challenges have been carried off. For instance, the generation of exit hole is overcome using a special retractable welding tool. Fig. 1.2 demonstrates the basic principle of this type of tool in which the pin length is adjustable, which gives the possibility of avoiding exit hole.

Fig. 1.2: Basic principle of retractable FSW tool. The pin is withdrawn into the shoulder from left to right [3]

1.2.3. Microstructure

Microstructure of a friction stir weld includes four main regions, namely base material, heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and nugget zone (or stir zone) as shown in Fig. 1.3. Having been involved in different thermal and/or

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mechanical cycles, each of these regions has different mechanical and metallurgical characteristic as described below:

x Base material: The material in this zone is not affected by welding process and there is no change in microstructure and mechanical properties.

x Heat affected zone: Similar to other welding procedures, the material in this zone is affected by the thermal cycles. These thermal cycles could result in dissolution and coarsening of precipitations in case of precipitation hardening alloys such as 2xxx, 6xxx, and 7xxx aluminum alloys or the reduction in density of dislocations in case of work hardening alloys such as 1xxx, 3xxx, and 5xxx aluminum alloys. As a result, the mechanical properties are affected although there is no deformation.

x Thermo-mechanically affected zone: The zone in which material has undergone high temperature gradient and plastic deformation without recrystallization due to low levels of deformation strain. Consequently, the mechanical and microstructural properties are affected. The grain size in this zone is similar to that of base material.

x Nugget Zone: This is the zone which has fine-grain microstructure due to severe mechanical deformation and frictional heating. Normally, the thickness of nugget is slightly larger than pin diameter. Due to direct contact with the tool (pin and shoulder), the material experience high levels of strain and is dynamically recrystallized.

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Fig. 1.3: (a) Various microstructural regions in a friction stir welded aluminum. (b) Micrograph of different regions [4]. Note: micrographs are taken on a plane perpendicular to the welding

direction

1.2.4. Defects and residual stresses

In general, there are three common types of defects associated with FSWs, namely voids, joint line remnants, and incomplete root penetration.

x Voids: As shown in Fig. 1.4, voids are normally found in advancing side of the weld and can be located beneath the weld surface or break through to the weld surface. There are a number of reasons for this kind of weld flaws including insufficient forging pressure, excessive travel speed, and excessive gap in the weld seam (insufficient clamping) [5]. These problems could lead to generation of insufficient heat and therefore deformed material will not be hot enough to flow and fill the space in the trailing edge of the tool especially in last part of welding which is the advancing side. Consequently, the void will form.

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x Joint line remnants: A joint line remnant which is also called entrapped oxides, residual oxides, lazy S, or kissing bond is the remnant of the initial oxide layer that was located on faying surface of the welding material (Fig. 1.5). The main reasons for having this remnant oxide layer are insufficient cleaning of faying surfaces of parts prior to welding and insufficient disruption of the initial oxide layer due to improper position of tool with respect to joint line, excessive travel speed, and oversized shoulder [5].

x Incomplete root penetration: As its name explains, incomplete root penetration is the discontinuities and lack of sufficient bonding on the bottom surface of the material which can deteriorate mechanical properties of the weld including tensile strength and fatigue life (Fig. 1.6). Major causes of this kind of defect are insufficient pin length, inappropriate plunge depth, variations in plate thickness, and poor alignment of tool with respect to joint line [5].

Fig. 1.4: (a) A void defect beneath the surface. (b) A void defect which breaks through to the weld surface [5]. Note: micrographs are taken on a plane perpendicular to the welding direction

Fig. 1.5: (a) Macrograph of a weld with joint line remnant defect. (b) Oxide debris which cause joint line remnant flaw [3]. Note: micrographs are taken on a plane perpendicular to the welding

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Fig. 1.6: (a) Demonstration of an incomplete root penetration defect. (b) A fracture caused by incomplete root penetration defect [3]. Note: micrographs are taken on a plane perpendicular to

the welding direction

1.2.5. FSW tool

FSW tool is one of the most important parameters of the process which determines not only the quality of the resulted joint but also the amount of required welding force and the characteristics of FSW machine. As mentioned before, a FSW tool consists of a shoulder and a pin or probe. In general there are three types of FSW tools, namely fixed, adjustable, and self-reacting (Fig. 1.7). In the fixed type of tool, shoulder and pin are made in a single part and therefore cannot move relative to each other. Consequently, each tool is appropriate for welding of workpieces with a constant thickness. The adjustable FSW tool is composed of two separate parts, a shoulder and a pin, which could be made from different materials as needed. As its name implies, the pin length is adjustable so that different thicknesses could be welded using this category of tool. Furthermore, it gives the possibility of filling the exit hole at the end of welding. While both fixed and adjustable tools need a backing plate, the self-retracting tool (also called bobbin tool) offers the possibility of making FSWs without a backing plate. The self-retracting tool consists of three parts, a top shoulder, an adjustable pin, and a bottom shoulder which operates as a backing plate. A drawback of the bobbin tool is that it can only operate perpendicular to the workpiece surface.

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Fig. 1.7: Different types of FSW tools (a) Fixed, (b) Adjustable and (c) Self-retracting (bobbin) [6]

The main function of the shoulder is to generate enough heat to plasticise the material, to keep it in weld chamber, and to apply the forging force on backside of the tool. As demonstrated in Fig. 1.8(a), there are three types of shoulder, flat, concave, and convex. Concerning fabrication, flat shoulder is the simplest type of shoulders. The main disadvantage of flat shoulders is the absence of a cavity under the shoulder which make it unable to entrap the deformed material and serve as a reservoir. This drawback has been overcome by addition of some features as shown in Fig. 1.8(b). These features can lead to high quality FSWs by increasing the material deformation and generated frictional heat. Concave shoulder is the most common type of shoulder currently used [3]. As can be seen from Fig. 1.8(a), this kind of shoulder includes a cavity which is made by inclining the flat shoulder a small angle of 6-10 degrees. A tilt angle of 1-3 degrees is required for proper operation of the concave shoulder. Therefore, non-linear welds using this type of shoulder is only possible by employing multi-axis FSW machines to keep a constant tilt angle all over the weld seam. Another possible type of shoulder profile is convex. Early attempts at TWI to employ convex shoulder for FSW were unsuccessful because the shoulder profile tried to push the material away from the pin. This problem is solved by adding some features to the surface of convex shape shoulder, which direct the material from the outside diameter toward the pin. The main advantage of convex shape is that the shoulder can engage with workpiece at any level along the convex end, which is beneficial in welding of workpieces with different thicknesses or with mismatch errors. Since engagement of tool with workpiece at any level along the convex end leads to a sound

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Fig. 1.8: Different possibilities of (a) shoulder shape [5] and (b) shoulder features [3]

The FSW pin is designed to disrupt the faying surface of the workpieces, shear the material in front of the tool, and direct it to the trailing edge. The pin produces the main part of heat in welding of thick materials while the shoulder produces majority of heat in welding of thin sheets. The pin design highly influences tool travel speed and the depth of material deformation [3]. Therefore, a large number of various pin designs have been developed since the invention of FSW. Fig. 1.9 demonstrates different possibilities for pin design. Regarding pin end shape, there are two possibilities, domed and flat. Although the domed bottom has a number of advantages including reduction in plunging force and tool wear, and higher quality of weld root directly at the bottom of pin, flat bottom pin is the most commonly used design because of the ease of manufacturing and much higher surface velocity in bottom of the pin which leads to higher material deformation [3]. There are two possibilities for the outer shape of the pin, cylindrical and tapered. The cylindrical shape is normally used for welding of aluminum alloys of up to 12 mm thick [3]. Having larger contact area with the material and therefore higher frictional heat and hydrostatic pressure, tapered pin is typically used for thicker materials [6]. Addition of different features such as flats, flutes and threads (Fig. 1.9) to the surface of the pin can promote

a)

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the material deformation. For instance, added flats act as paddles and produce local turbulent flow of the plasticised material [7], which leads to an increase in the nugget area and the workpiece temperature. In a study done by Colligan et al. [8] it is clarified that the increase in the number of flats reduces the traverse force and tool torque. Flats and flutes also trap the material in front side of the tool and release it behind the tool. Threads are one of the most common types of features made on FSW pin; especially a left hand thread used under clockwise rotation directs the material down toward weld root and eliminates the chance of void initiation. Threaded pins are not typically used for welding of high strength materials since the threads can be worn away easily.

TWI has developed a number of complex features for the pin to encourage FSW of very thick plates. Demonstrated in Fig. 1.10, Whorl and MX Triflute pins are two of the most efficient designs which can produce sound welds in aluminum alloys of up to 50-60 mm thick [9-11]. Furthermore, higher travel speeds are achievable employing these pin designs because of increased material deformation and heat generation as well as decreased welding forces. For instance, Cederqvist [12] reported an increase of 2.5 times in travel speed by use of a MX Triflute tool.

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Fig. 1.9: Different possibilities for pin design [6]

Fig. 1.10: (a) Whorl tool and its different shape possibilities. (b) MX Triflute tool [9, 10]

Tool material is one of the most important characteristics of FSW tool which can affect its design and dimensions. Selection of tool material depends mainly on the workpiece material and thickness as well as expected tool life. An appropriate candidate should typically have the following characteristics [3]:

1) Sufficient compressive strength in ambient and elevated temperatures to be able to withstand compressive forces applied during plunging and FSW respectively 2) Sufficient dimensional and strength stability in elevated temperatures to prevent

fracture of tool or change in its shape and dimensions that cause poor creep and thermal fatigue strength, overaging, and annealing

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3) Good wear resistance to withstand different wear mechanisms such as abrasive, adhesive, and chemical which can change the shape of tool through elimination of various delicate features of tool and result in defective joints

4) No detrimental reaction with the workpiece or the environment that can lead to change in surface properties of the tool and its wear resistance as well as production of toxic substances

5) Acceptable fracture toughness that is highly critical during plunging and first contact of the tool with the workpiece since high stresses and strains are applied to the tool in this phase of the process, which causes highest damages to the FSW tool

6) Low differences in coefficient of thermal expansion between the pin and the shoulder in case these are made of different materials, to eliminate detrimental effects of expansion of one relative to another such as high stresses and fracture 7) Good machinability characteristics to be able to machine different features on

shoulder and pin into the tool

8) Uniform microstructure and density since even minor variations in microstructure and density could create fragile areas within the tool that can give rise to tool failure

9) Available and affordable cost especially in a production environment

A list of tool materials currently used for FSW of different materials and their characteristics are shown in Tables 1.1 and 1.2. Having good material properties and low cost as well as good machinability and availability, tool steel is the most common tool material especially for FSW of aluminum alloys. For high strength materials such as steel, stainless steel, nickel and titanium alloys, materials that can maintain high strength in elevated temperatures of more than 1000 ⁰C such as tungsten alloys, tungsten carbides (WC), and polycrystalline cubic boron nitride (PCBN) is used as tool material. Regarding tools made from PCBN, only the shoulder and the pin of the tool are made from this

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using a superalloy locking collar. Since PCBN has low fracture toughness, the FSW machine’s spindle should exhibit low eccentricity to reduce the chance of tool fracture.

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es of diff erent tool mat erials used f o r FSW [ 6 ]

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1.2.6. Welding parameters

The most significant welding parameters of FSW are tool rotation speed and tool travel speed. Tool rotation can take place in clockwise (CW) or counter clockwise (CCW) direction depending on twist direction of threads or spirals on the shoulder and pin; Clockwise tool rotation for a left hand thread or spiral and counter clockwise for the right hand ones. Tool rotation generates sufficient heat through friction and material deformation to increase the temperature of material. Increasing tool rotation speed results in more heat generation, so makes it possible for the tool to move with higher travel speeds. It should be mentioned that the heat generation does not increase steadily with increasing tool rotation speed because of the change in friction coefficient between tool and workpiece by increasing tool rotation speed. Increasing travel speed is limited by the heat distribution in the material; Heat cannot be distributed in the material in very high travel speeds, and consequently tool starts to cut the material instead of stirring. In addition to the tool rotation and travel speeds, other welding parameters include shoulder penetration into the material and tool tilt angle. Setting an appropriate amount of tool penetration is one of the most critical tasks in FSW process. The process is very sensitive to even very small changes in shoulder penetration. Too high of shoulder penetration creates excessive flash and results in thinning of material in welded zone and worsening of mechanical properties. Too low of shoulder penetration leads to insufficient contact of shoulder with the material and therefore inappropriate mixing of material. Consequently, a number of defects including tunnel and surface groove will be formed. Nominal values for tilt angle are approximately 1-3 degrees. Using suitable tilt angle ensures that the material has been entrapped under shoulder and moved effectively from the front to the back of the pin. Zero degree tilt angle is utilized in welding with flat and convex shoulder tools.

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1.2.7. Applications

From its invention in 1991 until today, FSW process is continually increasing its applications in industry. Some of the industries using the technique extensively include shipbuilding and marine construction, aerospace, automotive and railway. Freezer panels, deck panels and helicopter landing platforms have been commercially manufactured using FSW at Sapa and Marine Aluminium Aanensen companies respectively. The process has been used for lap joining of longitudinal and circumferential stiffeners; and window and door doublers to the fuselage in fabrication of Eclipse 500 aircraft (Fig. 1.11). Airbus has also confirmed the use of FSW in its A350 and new versions of A340 including A340-500 and A340-600 [3].

Tailor welded blanks and B-column of the Audi R8, bonnet and rear doors of the Mazda RX-8, boot lid of Toyota Prius, aluminium engine cradles and suspension struts of the Lincoln Town Car and the central tunnel assembly of the Ford GT (Fig. 1.12) are some of the examples of application of the FSW in automotive industry. Recently Apple has used the FSW in manufacturing of personal computers to join the bottom to the back of the device.

a) b)

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Fig. 1.12: Employing FSW in lap configuration for joining of aluminum extrusions to the stampings in assembly of central tunnel of Ford GT [3]

1.2.8. FSW machines

In general, FSW machines are classified into three categories: custom-built machines, robots, and modified machining centres. Choosing between them depends on a number of factors such as force capability, stiffness, intelligence, and flexibility.

Custom-built machines are built to meet the requirements of their application. Depending on the application, they can have low to high force capacity, stiffness, intelligence and flexibility. Consequently their cost also have a large range from lower than $100,000 to millions of dollars [3]. For instance, a machine manufactured by ESAB for FWS of long extrusions of aluminum to fabricate a panel is shown in Fig. 1.13. Considering the requirements of the application, high travel speed and high force capability, this machine

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has high force capability, moderate intelligence (force control), and low flexibility (only one axis). Another example is a 5 axis FSW machine fabricated by MTS for FSW of complex welding contours (Fig. 1.14). This machine is used for FSW in Eclipse aviation. To meet the needs of the application, this machine has moderate force capability, high flexibility, high stiffness, and high intelligence [3].

Fig. 1.13: A FSW machine by ESAB for welding of long extrusions of aluminum to fabricate a panel [3]

In comparison to custom-built machines, robots have limited force and stiffness capability but higher flexibility and intelligence as well as much lower cost. At first, robots were not considered an option for FSW because of their very low force capability. With increased force and stiffness capabilities, application of robots for FSW have been gradually increased; now with payloads of up to 1350 kg (3000 lbs), robots are going to be major competitors of custom-built machines. In general, robots for FSW are available in two categories, serial and parallel manipulators (Fig. 1.15). Parallel robot manipulators have higher force capability in comparison to serial robots but their limited working envelope

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Fig. 1.14: The ISTIR Aero, a 5 axis FSW fabricated by MTS [13]

Fig. 1.15: (a) Serial and (b) parallel robot manipulators

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The last alternative for a FSW machine is a modified machining center. Since FSW is highly similar to the machining process, a machining center can be used for FSW if it can possess a number of characteristics. The first characteristic to be verified is its force capability as FSW applies high forces to the tool. The need for implementation of force control method in many FSW applications creates the second requirement of machine which is a controller with an open architecture or with the possibility of communication with a second controller. The number of degrees of freedom of the machine is also important especially in FSW using a non-zero tilt angle or over 2D and 3D contours. The final verification is regarding thermal measures. As a friction welding method, FSW generates a huge amount of heat which can be transferred to the tool and machine spindle. Therefore, measures should be taken to manage the heat and prevent possible problems to the machine [3].

1.3. Problems

Typical machines used in industry to make FSWs are custom-built machines that are especially designed for FSW and demand significant investment costs. Therefore, the majority of companies employing FSW are large companies such as Boeing, Airbus, Hitachi, and Sapa. There are a number of reasons for employing this kind of machines for FSW. Some of the most important reasons include: (1) high process forces particularly in axial direction (Fig. 1.16) which also requires robust fixturing, (2) the need to keep a constant angle between the FSW tool axis and the normal to the workpiece surface (tool tilt angle) which cause difficulties in welding of non-linear joints, and (3) sensitivity of the process to misalignment and geometric errors which requires high accuracy and stiffness. Fig. 1.16 represents specifications of FSW machines fabricated by Manufacturing Technology, Inc. It can be seen that the maximum axial force capability of FSW machines is in the range of 15-200 kN which is a significant amount of force. These values of axial force are more remarkable considering the fact that the peak axis thrust of most CNC

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tilt angle all over the welding path requires use of at least five axes for FSW over even a planar path.

Fig. 1.16: Specifications of FSW machines produced by Manufacturing Technology, Inc. [15]

Solving these problems will make it possible to utilize machines with less stiffness and force capabilities such as CNC machines and industrial robots for FSW. Consequently, this green technology will be readily available for every company which has a common CNC machine; therefore, applications of FSW in different industries will increase significantly.

1.4. Literature review

1.4.1. Optimisation of tool design and welding parameters

Tool design and welding parameters are two major determinants of weld quality in FSW. A tremendous amount of research is available in finding the appropriate values of these parameters for FSW of various materials especially using a tilt angle. Different methods are employed to attain this goal such as Taguchi technique, response surface methodology (RSM) and artificial neural networks (ANN). In a work done by Elangovan et al. [16], the effect of pin shape and shoulder diameter on the formation of friction stir processing zone and tensile strength of welds in constant welding parameters is investigated. Analysis of joints fabricated by different pin shapes shown in Fig. 1.17 demonstrated higher strength for the welds produced by the square pin shape. It is also found that the shoulder diameter of 18 mm results in the maximum tensile strength for all pin shapes. Higher shoulder diameters produced tunnel defect for cylindrical and tapered pin shapes because

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of excessive heat generation and material deformation. In case of smaller shoulder diameters, a tunnel defect is generated in the lower portion of weld for all pin shapes due to insufficient heat generation. The same kind of analysis is done by Elangovan et al. on the effect of rotational [17] and travel [18] speeds in FSW. Maximum strength is achieved using rotational speed of 1600 rpm and travel speed of 0.76 mm/sec. Similar to shoulder diameter, higher and lower values of rotational and travel speeds led to the joints with defects or lower strength.

Fig. 1.17: FSW tool pin profiles used by Elangovan et al. [16]

In an attempt to increase the travel speed, Trimble et al. [19] utilized pin and shoulder shapes, and tool rotational speed as variables. Having lower tendency to lift away from material surface and generating more deformational and frictional heating, scroll shoulder (Fig. 1.18) enabled them to achieve higher travel speeds of up to 355 mm/min in comparison to concave shoulder with the highest travel speed of 125 mm/min (Fig. 1.19). Regarding concave shoulder, travel speeds of more than 125 mm/min produce surface defects as a result of tool lifting away from material surface in higher travel speeds. From the three pin shapes investigated in this study (Fig. 1.18), the triflute pin followed by square pin resulted in more plastic deformation and stirring of the material; and

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Obtaining lower traverse welding force is another indicator of better performance of triflute and square pins. Using the best tool design, scrolled shoulder and triflute pin, the authors made an attempt to further increase the travel speed by variation of rotational speed. Increasing rotational speed, they found a rotational speed of 450 rpm in which the traverse force has its minimum regardless of travel speed. Although it is expected that further increasing rotational speed would lead to more heat generation and therefore lesser traverse forces, the results demonstrated higher traverse speeds because of less heat generation. This phenomenon can be explained by the fact that increasing rotational speeds increases the material temperature close to the solidus temperature where the flow stress of material changes rapidly and consequently higher rotational speeds lead to less heat generation [20]. Thus, there is an optimum rotational speed in which the highest heat generation and therefore the highest travel speed could be obtained.

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Fig. 1.19: Effect of welding speed in FSW using (a) the concave and (b) the scrolled shoulders [19]

In another work by Trimble et al. [21], influence of pin shape on material deformation and quality of resulted joints is studied using threaded and smooth cylindrical pins. Their results illustrated that the threaded pin produces higher material deformation and mixing (Fig. 1.20) in comparison to the smooth pin; and consequently higher tensile strength and lower welding forces are reported by threaded pin. According to the authors, this can be attributed to the screwing action of threads which force the material away from the tool and produce more material deformation and stirring.

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In an extensive study, Rajakumar et al. [22] carried out a number of experiments to optimize process (tool rotational and welding speeds and axial force) and tool design parameters (shoulder and pin diameters and tool material hardness) to achieve a maximum tensile strength in FSW of AA7075-T6 aluminum alloy rolled plates of 5 mm thick. For each of these parameters, they found a limited range which leads to a sound weld. As can be seen from Fig. 1.21, choosing parameters out of the acceptable range produces various defects such as worm hole, tunnel defect, pin hole, and kissing bond. Using RSM they developed an empirical relationship between input parameters and tensile strength, and then found the optimum value of each parameter. To identify critical parameters and their degree of importance, a sensitivity analysis is carried out. The results of this analysis ranked rotational speed as the most sensitive parameter, followed by welding speed, axial force, tool material hardness, pin diameter, and shoulder diameter.

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Fig. 1.21: Acceptable range of process and tool design parameters found by Rajakumar et al. [22]

1.4.2. Force reduction

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significant influence on the formation of a sound weld. Effect of axial welding force in the quality of joints is investigated in literature. For instance, Rajakumar et al. [22] found a limited range for the axial force which leads to the absence of defects in resulted joints. As shown in Fig. 1.21, insufficient amount of axial force, lower than 5 kN, results in less heat generation and therefore tunnel defect; from the other side, large values of axial force, more than 11 kN, brings about excessive heat input into the weld and consequently thinning of material and creation of warm hole defect. From the range of 5-11 kN, finally the optimum value of axial force to achieve maximum tensile strength is found to be 8.29 kN for FSW of 5 mm thick rolled plates of AA7075-T6. The same kind of relationship between force and strength of welds in FSW of 6 mm thick plates of AA6061-T6AlNp is

reported by Kumar et al. [23]. Fig. 1.22 demonstrates variation of ultimate tensile strength (UTS) and percentage of elongation with force where the percentage of elongation has a descending behavior and UTS has a quadratic behavior with a maximum value in 5 kN axial force. It should be mentioned that the range of travel speeds used for the mentioned study is between 25 and 85 mm/min.

Fig. 1.22: Relationship between force and UTS and percentage of elongation [23]

Welding parameters including tool rotation and travel speeds have a significant effect on axial force. Cook et al. [24] investigated effect of these parameters on axial force and spindle power. As shown in Fig. 1.23, increasing tool rotation speed reduces the axial force for all travel speeds; also increasing travel speed increases axial force for all values of

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rotation speed. Similar relationship between travel speed and force is found by Trimble et al. [21] using finite element modeling. As demonstrated in Fig. 1.24, increasing travel speed increases axial and traverse welding forces as well as tool torque. According to the authors, increasing travel speed reduces the heat input into the weld and therefore the material becomes harder and demands higher force and torque to stir.

Fig 1.23: Effect of tool rotation and travel speeds on axial force in FSW [24]

Fig. 1.24: Relationship between travel speed and tool force and torque using FEM [21]

To implement FSW for high temperature materials such as steel and titanium as well as eliminate the need for use of large FSW equipment, many researchers made attempts to

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of energy such as ultrasonic, electrical, laser, and plasma. Park [25] employed ultrasonic vibration to reduce axial force in FSW of aluminum and steel. As demonstrated in Fig. 1.25, ultrasonic energy is transmitted to the FSW tool through ultrasonic horns with frequencies of 20 and 40 kHz. Results of addition of ultrasonic vibration to FSW of aluminum and steel sheets of 3.175 mm thick are shown in Fig. 1.26 and Fig. 1.27 respectively. In FSW of aluminum, addition of ultrasonic vibration with frequency of 40 kHz does not seem to have any effect on the amount of axial force, while frequency of 20 kHz moderately reduces the second peak force and the force during tool translation. Since the frequency of 20 kHz leads to better results, it is the only frequency used for FSW of steel. In case of steel, the reduction in force is considerable with a value of 20 % in travel speed of 25 mm/min and 12.5 % in travel speed of 50 mm/min. Although there is no indication for higher travel speeds, it can be seen that increasing tool travel speed reduces the effect of addition of ultrasonic energy in FSW.

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Fig. 1.26: Influence of ultrasonic addition on axial force in FSW of Al6061-T6 using rotation and travel speeds of 1500 rpm and 25 mm/min [25]

Fig. 1.27: Influence of ultrasonic addition (20 kHz) on axial force in FSW of 1018 steel using rotation speed of 650 rpm and travel speed of (a) 25 and (b) 50 mm/min [25]

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As shown in Fig. 1. 28, Ferrando et al. [26] developed the concept of electrically assisted FSW by adding an electric current of 300-450 amperes to the conventional FSW through tool tip. This electric current increases temperature of the material in vicinity of the FSW tool and therefore facilitates stirring of the material. Consequently lower forces will be required to mix the material and form a sound weld. According to the author, addition of an electric current of 300 amperes reduces the axial force to about 100 lbs in FSW of Al5083 sheets of 3.175 mm thick. It should be mentioned that the travel speed used to obtain this amount of force reduction is not reported.

Fig. 1.28: Schematic illustration and experimental setup of electrically assisted FSW [26]

Another method for force reduction is preheating the welding material prior to FSW which is done by Sinclair [27]. As shown in Fig. 1.29, a ceramic heating strip is placed under the backing plate to preheat Al6061-T6511 plates of 0.25” thick up to 300 ⁰C prior to FSW. Similar to electrical assisted FSW, the idea is to reduce aluminum’s yield strength and overcome the resistance of material to stirring. Consequently sound welds could be obtained using lower axial forces. Fig. 1.30 demonstrates variation of axial force with increasing initial temperature of material at different travel speeds. It can be seen that increasing initial temperature of material reduces the axial force up to a value for each travel speed and then with higher initial temperatures the force increases. According to

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the author, further increase of temperature, however, results in retrieval of the reducing trend of force. Another observation is that the effectiveness of preheating in reducing axial force decreases by increasing travel speed. For instance, the maximum force reduction at travel speed of 5 ipm is found to be 43%, while it possesses a value of only 21% at higher travel speed of 14 ipm. This last point could limit the advantages of preheating welding material prior to FSW considering the fact that additional costs is accompanied by added system.

Fig. 1.29: Preheating system used to increase aluminum temperature prior to FSW [27] Aluminum plates

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Fig. 1.30: Effect of preheating on axial force in FSW of Al 6061-T6511 [27]

In a work by Melendez et al. [28], forces applied to the FSW tool are studied. In this study, effect of tool design (shoulder diameter and geometry) and welding parameters (rotation and travel speeds) on amount of axial, traverse, and longitudinal forces is investigated in FSW of 0.25” plates of Al6061 and Al2195 using a tilt angle of 1 degree. Their results showed that the highest force is in the axial direction and it decreases by decreasing tool plunge depth, increasing rotation speed and shoulder diameter, and addition of flutes and channels. For instance, addition of flutes reduced the axial force from 12.9 kN to 7.2 kN. In addition, although it is expected that axial force increases with increasing travel speed due to having cooler material in front of tool, it surprisingly decreases in FSW of Al6061 as can be seen in Fig. 1.31. According to the authors more work is needed to explain this behavior.

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Fig. 1.31: Variation of axial force with travel speed in FSW of Al6061 and Al2195 [28]

1.4.3. Tolerating for joint fit-up issues

Having many advantages over fusion welding techniques such as lower distortion, better mechanical properties, no need for consumables, lower energy input and environmentally friendly, FSW is a right choice for the industry. One of the significant aspects of the process is its sensitivity to joint fit-up issues including gap, mismatch, and misalignment (Fig. 1.32). These issues can be emanated from material thickness variation, high process forces, and improper fixturing. Moving towards industrialization, these issues need to be studied and managed. A number of researchers [29-37] tried to find the effect of aforementioned issues on different aspects of the process and manage them as much as possible.

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Fig. 1.32: Joint fit-up issues in butt FSW, (a) gap, (b) mismatch, (c) misalignment [29]

In an extensive study done by Cole et al. [29], effect of three joint fit-up issues shown in Fig. 1.32 as well as the lack of penetration in FSW of 5-mm-thick Al5083-H111 is addressed. As demonstrated in Fig. 1.33, depending on the side towards which the tool is misaligned, the FSW process is able to tolerate for misalignments of 1.5 mm and 2 mm towards retreating and advancing sides respectively. This indicates lower capability of the process in tolerating for misalignment towards retreating side which could be explained by the fact that most defects in FSW take place in advancing side and therefore presence of tool in advancing side is significant for a sound weld. They also found the effect of gap and mismatch on UTS and elongation (Fig. 1.34). Gaps of more than 1 mm reduce the UTS of the resulted joint adversely. Contrary to the misalignment, results of tests with mismatch represent a symmetrical shape for UTS at advancing and retreating sides.

To characterize the interaction between welding parameters (rotation and travel speeds, and tool tilt angle) and joint fit-up issues, an analysis of variance is carried out in which a multi-factor and multi-level design of experiments is used. From this analysis, the authors were able to find out the most critical parameters as misalignment, tilt angle and gap width. Furthermore, they concluded that modification of rotation and travel speeds to manage joint fit-up issues could not be an effective method.

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Fig. 1.33: (a) Illustration of positive and negative misalignment, (b) Effect of misalignment on UTS and elongation [29]

Fig 1.34: (a) Effect of gap and (b) mismatch on UTS and elongation [29]

Effect of FSW machine compliance and tool tilt angle as well as tool penetration into material in managing gap between plates in FSW of 5-mm-thick Al5083-H111 is investigated by Shultz et al. [30]. As indicated in Fig. 1.35, higher gap widths are more difficult to be compensated for and lead to significant reduction of UTS. Regarding the effect of compliance of FSW machine, it can be seen that a more rigid milling machine

a) b)

b) a)

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industrial robot with a compliance of 1 mm/kN. This is of especial importance in presence of large gap width of 2 mm. According to the model they developed to estimate the joint efficiency in FSW with gaps, lower penetrations demand higher tilt angles to achieve acceptable joint efficiency (Fig. 1.36). For instance, tool penetration of 4 mm requires a tilt angle of 20.11 degrees while bigger penetration of 6 mm requires a tilt angle of only 2.8 degrees to achieve approximately same level of joint efficiency.

Fig. 1.35: Effect of gap width and machine compliance on UTS of FSWed joints using a 3 deg tilt angle [30]

Fig. 1.36: Variation of joint efficiency with plunge depth and tilt angle in FSW with 2 mm gap [30] b)

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Widener et al. [31] carried out a research to determine the influence of a number of joint fit-up issues including gaps and misalignments in the weld seam on mechanical properties of joints and formation of defects in FSW of 0.125” thick plates of Al7075-T73. As demonstrated in Fig. 1.37, UTS does not respond in a symmetrical manner to offset towards advancing and retreating sides. Similar kind of asymmetrical behavior is reported by Cole et al. [29] with the exception that in their work higher offset towards advancing side is tolerable while in the work of Widener et al. higher offset towards retreating side is tolerable. Effect of gap on UTS can be observed in Fig. 1.38 where two curves indicate strength of the joint measured using plate thickness on nugget zone and HAZ zone. Since the plate is thinned in nugget zone to fill the gap, the strength calculated using thickness of nugget zone is higher than HAZ zone which has constant thickness. The highest amount of tolerable gap is found to be 0.032”; larger gaps lead to formation of volumetric defects in the nugget zone. It should be considered that higher values of tolerable offset and gap in work of Cole et al. [29] in comparison to the work of Widener et al. [31] could be the result of different material thicknesses used in two cases. Therefore, it could be concluded that higher offsets are tolerable for thicker materials.

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Fig. 1.38: Effect of gap on UTS in FSW of 0.125” plates of Al7075-T73 [31]

In a study by Takahara et al. [32], the influence of gap and misalignment on the quality of joints is determined in FSW of 3 mm thick plates of Al5058-O aluminum alloy. According to their results, gaps of less than 2 mm do not affect strength and ductility of joint; a gradual reduction in mechanical properties is reported with gaps of larger than 2 mm. The tolerable gap of 2 mm is comparable with the value of 0.81 mm obtained by Widener et al. [31] for approximately the same thickness of material. Different characteristics of materials used in two studies (Al5058-O and Al7075-T73) could be a good reason for different tolerable gaps. Regarding the misalignment, they concluded that the deviation of half of the tool pin diameter towards retreating side is tolerable without reduction in weld quality and other values of deviation lead to the rapid reduction in tensile properties. This last point is not in concordance with what Cole et al. [29] and Widener et al. [31] have found regarding misalignment.

1.4.4. FSW over three dimensional contours

Although FSW has the capability to be employed for welding of 3D contours, it is only utilized for linear or at the most planar joints in most of industrial applications since its invention. A limited number of studies [38-45] are performed on implementation of FSW over 3D paths. Van Niekerk et al. [38] studied the effect of process parameters (rotation and travel speeds, plunge depth, and tilt angle) on the measurement sensors (force,