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.

In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that {10-12} twinning only occurs at the early stage of deformation. As the strain increases, the {10-12} twinning is suppressed while {10- 11} twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, {10-11} and {10-12} twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of {10-11} compressive twinning and the break point for the subgrain size. The presence of {10-11} twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30° to CD and another 45° to CD. / QC 20100819