Many metal structures are assembled from thin plate with welded supports for stiffness to resist local loadings. However, welded joints, which require large heat input, may incur significant distortion in the finished plate. Although the causes of distortion are known, and have been the focus of number of studies, there is still a lack of fundamental understanding of process and physical parameters in causing distortion. The overall aim of this work is to identify the interaction of process and physical parameters in causing distortion of welded ferritic thin steel plates. Experimental measurements and the finite element method are used to identify the relationship between distortion and the influence of pre-existing (residual) stresses in the plates. Effect of onset of transformation temperature on distortion is examined. An improved comprehension of the mechanisms causing distortion, and a readily useable model to explore alternatives has significant potential in wide range of industries and thus is a major driving force for continued research. The ability to predict with reasonable certainty the geometry of distortion will enable users to evaluate alternative design and production parameters. The work is divided into eight chapters: The first chapter gives an introduction and lists the objectives of the research. A theoretical exploration of the problem in addition to a survey of relevant work with regard to the welding of ferritic steels, weld microstructures, residual stresses, finite element modelling (FEM) and an overview of experimental techniques including transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), neutron diffraction (ND) is given in chapter 2. In chapter 3 experimental investigation results including both post-weld and in-situ microstructure observation and residual stress distribution are presented and discussed [1, 2]. To provide a qualitative insight into fundamental understanding of development of residual stress, a finite element model that considers both the thermal and the transformation strains caused by solid-state phase transformation was developed and is presented in the chapter 4 [3]. A validated finite element model for computation of residual stresses is presented in the chapter 5 [4]. Special emphasis was placed on the effect of transformation temperature on residual stress development in both the actual weld and the model. Most of the modelling results were validated against experimental measurements. Chapter 6 presents a sensitivity study on the effect of parameter changes on distortion. An attempt was made to elucidate both the effect of transformation start temperature and the initial distortion on the final distortion [5]. Chapter 7 details application of a model for variant selection [6], which is based on work published in [7, 8] to actual welds. This work was performed to elucidate the effect of texture on residual stress. Finally the last chapter draws together the major conclusions of the thesis, and suggests future avenues of investigation to progress the research discussed here.

Zr-2.5 wt % Nb (Zr-2.5Nb) is the main alloy used in the pressure tubes of CANDU nuclear reactors, which are manufactured by hot extrusion. Pressure tubes are subjected to high irradiation fields and corrosion, in addition to the applied stress at operating temperatures of around 330°C, which leads to irradiation creep that is often life-limiting; re-tubing the reactors, is a source of significant through-life cost of the reactor system. However, significant variability in performance is observed between tubes and stations, which is felt to be due to variability in the fabrication and operation conditions. The performance of Zr-2.5Nb is sensitive to both microstructure and texture and therefore it is desirable to be able to understand the extrusion conditions more fully. In this thesis, the extrusion of Zr-2.5Nb is examined, along with commercially pure titanium (CP Ti), commercially pure magnesium (CP Mg) and AA2014. The effect of extrusion ratio, die geometry and rod versus tube conditions are examined. The resulting microstructures and textures are rationalised with the aid of a finite element model for the process. After the introduction and literature review (Chapters 1-2), the modelling procedure and extrusion theory are examined (Chapter 3). Constitutive data (including friction conditions) are gathered and a Norton-Hoff constitutive model is generated in Chapter 4. It is found that adiabatic heating can be important at high strain rates and low temperatures, particularly in CP Ti and CP Mg. Recrystallization during deformation can be observed in the flow curves, particularly in CP Mg and AA2014 at low strain rates and high temperatures. The extrusion of AA2014 tubes is examined in Chapter 5. It is found that satisfactory textures and microstructures can be obtained, and that the model can reproduce the observed load curves. Partially extruded gridded billets are also used to verify the flow conditions predicted by the model and to obtain textures and microstructures part-way through the extrusion process. The extrusion of CP Mg and CP Ti are examined in Chapters 6 and 7, respectively. It was found that CP Mg recrystallized very easily, dominating the microstructures and textures observed. The CP Ti extrusions were performed in the [alpha]+[beta] regime in order to match Zr-2.5Nb conditions. The high extrusion ratio rod textures were dominated by the [beta]->[alpha]transformation, while those in the tubes were more characteristic of deformation of the [alpha] phase. Zr-2.5Nb extrusion is examined in Chapter 8. Satisfactory microstructures with elongated grains surrounded by thin ligaments of [beta] were obtained in the tube extruded through a flat-faced die, with the expected texture for this ratio of wall to diametral reduction (paragraph 2.5.7.1, Figure 2.25). The microstructures obtained were found to be a product of the temperature in the die and the cooling rate of the material. Excessive cooling rates lead to the production of basket-weave microstructures, and breakup of the grain boundary [alpha] to very fine microstructures. Again, the extrusion modelling allowed the results obtained to be rationalised. Finally, the reader is referred in Chapter 9 for a discussion of the obtained result. Conclusions drawn and suggestions for further work can be found in Chapter 10, together with recommendations for the industrial modelling of tube extrusion and for industrial practice.

Joining technology has played an important role in manufacturing since the industrial revolution. Welding methods are under constant development in response to real demands. Laser welding is considered an effective joining method that can provide high quality and cost effective results to bring economical benefits to industry. Nowadays, fibre lasers have the capability to fill some of the roles of the CO2 and Nd:YAG lasers in industrial welding applications because of their excellent characteristics such as higher energy density and superior beam quality. However, up to now, few quantitative evaluations of its performance against more traditional lasers have been conducted in laser material processing. This thesis presents an investigation into the fibre laser welding of dissimilar materials processes. The challenges in the welding of dissimilar materials are mainly related to the large differences in the physical and chemical properties of the welding materials. These differences readily cause residual stresses, intermetallic phases and chemical composition gradients. The aim of this work is to understand and explain mechanisms occurring in single mode fibre laser welding of dissimilar materials. The first part of this work addressed fibre laser butt welding of Ti-6Al-4V titanium alloy to Inconel 718 nickel alloy. Here, the weld quality was evaluated in terms of the weld geometry, microstructures, hardness distributions and the formation of intermetallic phases. Results showed that the offset position of the laser beam was an important factor affecting the weld quality. Furthermore, the thermal history of the weld was simulated using analytical modelling analysis and this was used to identify a parameter window for crack-free welding. The second part of this work focused on fibre laser lap welding of Zn-coated steel to Al alloy with different laser power delivery modes (pulsed wave and continuous wave). The relationship between the weld quality and process parameters, such as: pulse frequency, laser power, welding speed, the shielding gas type and number of welding passes, were investigated. The mechanical properties, metallurgical effects and corrosion performances of welds were analysed. Results showed that the shielding gas type and the number of welding passes were key factors in controlling the weld quality in the fibre laser welding of Zn-coated steel to Al alloy process. Finally, the common features, characteristics and the potential of fibre laser welding of dissimilar materials are presented.

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