The study of defect formation and identification is important to the further application of friction stir welding in industry. To better understand the topic, a systematic study was undertaken to describe material flow effects on the formation of defects, to list the various types of defects encountered across a parameter window, and to identify features in the weld force data that can then be used to recognize defects within the weld without destructive testing. Tracer studies were used to determine the impact of the material flow on defect formation with a determination that proper shoulder contact is necessary to obtain sufficient material flow to fully consolidate the weld. A series of welds across a range of rotational speeds was used to identify mechanisms that led to variations in the mechanical properties of the welded panels. A balance between the x- and yorces on the tool is needed to produce robust welds that were defect free. UMF was shown to identify regions of changing material flow conditions; however, the identification of intermittent defects was not as successful.

Thesis (M.S.)--Missouri University of Science and Technology, 2008. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed December 2, 2008) Includes bibliographical references.

Load controlled cyclic fatigue testing was conducted on base metal (BM) and friction stir welded (FSW) magnesium (Mg) alloy AZ31B compact tension (CT) specimens in laboratory air and a 0.05 wt. % NaCl fog environment in efforts to delineate the effects of salt fog and stir welding on fatigue performance under tension-tension loading conditions. FSW beads were produced on single piece AZ31B sheet product, simulating a pristine friction stir butt weld. Optical and electron microscopy, as well as X-ray diffraction was employed to observe the features and characteristics of fracture surfaces. The resulting stress vs. number of cycles to failure (S-N) curves demonstrated a reduction in fatigue life in a salt fog environment and an increase in fatigue life for FSW specimens compared to equivalently loaded BM specimens. Tensile frame displacement data indicated that the salt fog environment had an immediate effect on the BM samples, with an increased displacement required to meet the load control criteria. Fatigue cracks that propagated in CT samples that were notched in the direction of FSW tool travel consistently propagated towards the retreating side (RS) of the stir zone. / Thesis / Master of Applied Science (MASc)

A l'heure où les matériaux composites prennent une part toujours plus importante dans les structures aéronautiques, l'utilisation de l'alliage d'aluminium-lithium 2050 assemblé par Friction Stir Welding (FSW) est envisagé pour remplacer les structures rivetées en alliages d'aluminium traditionnels. L'objectif global de ce travail de thèse était d'étudier le comportement en corrosion et en corrosion sous contrainte de joints soudés FSW de l'alliage d'aluminium-lithium 2050 mais aussi l'influence d'un post-traitement thermique sur les comportements observés. Dans ce but, la microstructure, le comportement mécanique et le comportement en corrosion et en corrosion sous contrainte de l'alliage 2050 servant de matériau de base à l'élaboration des joints soudés FSW ont d'abord été analysés ; ensuite, nous nous sommes focalisés sur le comportement en corrosion et en corrosion sous contrainte des structures FSW proprement dites, en analysant bien entendu la microstructure complexe de ces joints soudés et les propriétés mécaniques résultantes. Les résultats obtenus montrent que l'alliage 2050, bien que sensible à certaines formes de corrosion, semble bien plus performant que les alliages d'aluminium traditionnellement utilisés en aéronautique. Sa sensibilité à la corrosion sous contrainte notamment est très faible. Les structures FSW, après traitement thermique post soudage, ont également un très bon comportement en corrosion et en corrosion sous contrainte. Les joints soudés FSW de l'alliage d'aluminium-lithium 2050 apparaissent donc comme une solution très prometteuse pour l'industrie aéronautique. / Today, composite materials are more and more used for aircraft structures; however, the aluminium-lithium 2050 alloy joined using a Friction Stir Welding (FSW) process could be used to replace riveted structures made of traditional aluminium alloys. The aim of this PhD work was to study the corrosion behaviour and stress corrosion cracking (SCC) behaviour of FSW joints of aluminium-lithium 2050 alloy and also the influence of a post welding heat treatment on the behaviours observed. Therefore, the microstructure, the mechanical behaviour and the corrosion and SCC behaviour of the 2050 alloy used for the FSW joints were first analyzed; then, the work was focused on the corrosion and SCC behaviours of the FSW joints. The complex microstructure of the FSW joints and the resultant mechanical properties were studied. The results showed that the 2050 alloy was more corrosion resistant than traditional aluminium alloys used for aircrafts even if it was susceptible to intergranular and/or intragranular corrosion. Its susceptibility to SCC was very low. The FSW joints, after a post welding heat treatment, also show a low susceptibility to corrosion and SCC. As a conclusion, the FSW joints of 2050 alloy seem to be a relevant solution to replace riveted structures of traditional aluminium alloys.

Out of the known aerospace metal and alloys, Aluminium (Al) and Titanium (Ti) are important due to their unique combination of properties, such as strength, ductility and corrosion resistance etc. For these reasons, welding of these two materials, especially in the butt and lap configuration, has a significant impact for structural applications. However, welding of Al to Ti is a challenge due to wide differences in their physical properties and properties of the brittle intermetallic that are formed. Such problems in Ti-Al weld can be minimized if the temperature of welding is reduced. Therefore, many solid-state welding processes have been introduced for this system in the past few decades. Amongst these processes, Friction Stir Welding (FSW) is among the most appropriate for dissimilar materials in the butt and lap configuration, as this process involves lower temperature of processing. The present thesis is an attempt to address the issues pertaining to the friction stir welding of commercially pure Al and Ti. Though these commercially pure materials are seldom used in actual applications, where alloys such as Ti-6Al-4V and Al 2219 (and their variants) are used, this work is done to get a fundamental understanding of the underlying mechanisms during Friction Stir Welding (FSW). The study has been extended to the effect of using a thin strip of other metallic materials between Al and Ti. These inserts are likely to play a role in the formation of intermetallic and control the after effects of the formation of these intermetallic. Two metals have been chosen for this purpose, namely Zinc (Zn) and Niobium (Nb). The thesis has 8 chapters that attempts to systematically understand the process of FSW of cp-Al to cp-Ti. In Chapter 1 of the thesis, the FSW process is introduced with an emphasis on important parameters that control the welding process. In addition, a brief introduction of Al-Ti binary system is also given. Literature related to conventional solid state welding processes and friction stir welding process is presented in Chapter 2. In this chapter, previous works on the FSW of various materials is reviewed, with more emphasis on welding of aluminium to titanium. At the end of the chapter the scope and motivation of the present investigation has been outlined Chapter 3 includes the experimental details involved in the present study. In addition to the details of the processes and various characterization techniques used in the present investigation, the basic principles involved in various techniques, names as X-ray tomography, Scanning Electron Microscopy (SEM) with Electron Back-Scattered Diffraction (EBSD), X-Ray Diffraction (XRD) and Electron Probe Micro-Analysis (EPMA) have also been given. Micro-hardness and tensile tests results are also reported in this chapter. A detailed study on FSW of Al and Ti is presented in chapter 4 of the thesis. The effect of process parameters on the evolution of microstructure and mechanical properties has been reported. A bottom-up approach on experimentally determining the “process window” is presented. The results emphasises on the distribution of titanium fragments and intermetallic particles in the nugget zone and their influence on mechanical properties of the weld. The microstructural evolution in the matrix is also detailed. The most noteworthy observation is substantial grain refinement in the nugget zone due to the presence of fine fragments of titanium and intermetallic. Cross-tensile tests of the samples welded under the optimised conditions fail in the retreating side of the aluminium material and has strength more than the parent material. The last section in this chapter deals with thermal stability of the microstructures. Chapter 5 deals with the use of Zn as interlayer between Al and Ti. The microstructural evolution and its effect on the mechanical properties have been examined. The investigations clearly show that FSW of Al and Ti with Zn interlayer has superior mechanical properties compared to Al-Ti welds without interlayer. The resulting microstructure has a better thermal stability. The use of Nb as interlayer has been studied in chapter 6. The microstructural investigation of the nugget zone reveals that Nb interlayer does not readily form solid solution with any of the base materials and Nb gets distributed more heterogeneously compared to Ti itself. This has led to a reduction in the strength of the weld, however, the ductility increases The thermal stability of the microstructure is poor compared to FSW of Al to Ti with Zn interlayer. In chapter 7, salient features of the different micro-mechanism operating during FSW of the investigated combinations has been discussed in detail. Finally, the outcome of the thesis has been summarized and scope for future investigation is outlined in chapter 8.

Joining two dissimilar metals, specifically Mg and Al alloys, using conventional welding techniques is extraordinarily challenging. Even when these alloys are able to be joined, the weld is littered with defects such as cracks, cavities, and wormholes. The focus of this project was to use friction stir welding to create a defect-free joint between Al 2139 and Mg WE43. The stir tool used in this project, made of H13 tool steel, is of fixed design. The design included an 11 mm scrolled and concave shoulder in addition to a 6 mm length pin comprised of two tapering, threaded re-entrant flutes that promoted and amplified material flow. Upon completion of this project an improved experimental setup process was created as well as successful welds between the two alloys. These successful joints, albeit containing defects, lead to the conclusion that the tool used in project was ill fit to join the Al and Mg alloy plates. This was primarily due to its conical shaped pin instead of the more traditional cylindrical shaped pins. As a result of this aggressive pin design, there was a lack of heat generation towards the bottom of the pin even at higher (800-1000 rpm) rotation speeds. This lack of heat generation prohibited the material from reaching plastic deformation thus preventing the needed material flow to form the defect free joint.

<p>Friction Stir Welding (FSW) is a new technology which joins materials by using frictional heat. Inthe first part of this thesis, a profound literature study is performed. The basic principles, therobotic implementation and possibilities to use FSW for high strength titanium alloys areexamined. In the next phase, a FSW-tool is modelled and implemented on an industrial robot in arobot simulation program. Reachability tests are carried out on car body parts and jet engineparts. By using a simulation program with embedded collision detection, all possible weldinglocations are determined on the provided parts. Adaptations like a longer FSW-tool and amodified design are suggested in order to get a better reachability. In different case studies, thenumber of required robots and the reduction of weight and time are investigated and comparedto the current spot welding process.</p>

This dissertation presents the results of an investigation into the corrosion resistance of friction stir welding (FSW) for aerospace structures. Two of the most common aerospace aluminum alloys, 2024 and 7075, were investigated. In the as-welded condition, both alloys were found to be highly susceptible to exfoliation corrosion, and 7075 was found to be susceptible to stress corrosion cracking as well. The goal of this research was to identify proper initial temper selection and postweld aging treatments for enhancing the corrosion resistance of both 2024 and 7075 alloys, and their dissimilar joints. A large number of heat treatments were investigated for 7075 in the T6 and T73 tempers, including retrogression re-aging (RRA). Heat treatments were also investigated for 2024-T3 and 2024-T81. Samples were evaluated for resistance to exfoliation corrosion using optical microscopy. Microhardness, electrical conductivity, tension, and fatigue crack propagation tests were also performed on the samples. Beneficial heat treatments were found for both alloys as well as for their dissimilar joints. / "December 2005." / Thesis (Ph.D.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering

Friction Stir Welding (FSW) is a new technology which joins materials by using frictional heat. Inthe first part of this thesis, a profound literature study is performed. The basic principles, therobotic implementation and possibilities to use FSW for high strength titanium alloys areexamined. In the next phase, a FSW-tool is modelled and implemented on an industrial robot in arobot simulation program. Reachability tests are carried out on car body parts and jet engineparts. By using a simulation program with embedded collision detection, all possible weldinglocations are determined on the provided parts. Adaptations like a longer FSW-tool and amodified design are suggested in order to get a better reachability. In different case studies, thenumber of required robots and the reduction of weight and time are investigated and comparedto the current spot welding process.

In this study, the energy conservation equation in a cylindrical coordinate system and the moving heat source from the tool are used to establish a steady-state three-dimensional heat transfer model for the friction stir welding (FSW). Then, the simplified momentum conservation equation is employed to predict the material flow model for the FSW. Combining the effects of heat transfer and material flow, this numerical model successfully predicts the weld temperature field and the material flow for the FSW. Numerical results show that increasing the welding or translational speed of the tool has the effect of decreasing the magnitude of the temperature within the workpiece, while increasing the rotating speed has the opposite effect. During the feeding process, the material located on the back of the tool pin has higher temperature than that on the front. Moreover, the temperature profile are asymmetrical between the advancing and retreating sides due to the material flow stirred by the tool, and this temperature difference depends on the speed of material flow under the tool shoulder.