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The subject of welding is challenging because of its complexity and because its applications are in the majority of cases, safety critical. The work presented in this thesis deals with both these aspects from the point of view of welding alloys.

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The purpose of the present work was to characterise and further develop a novel nanostructured type of bainitic steel. Three chemical compositions were considered with different concentrations of Al and Co. The addition of Al and Co is believed to be necessary to produce the desired nanostructure at very low temperatures within a reasonable transformation time. An overview of the mechanical performance of fully bainitic steels vs other steel systems is presented in Chapter 1. An introduction to metallurgical concepts regarding the design and performance of bainite steels is presented in Chapters 1 and 2.Chapter 2 focuses on the design concepts by which the steel chemical composition was optimised, primarily on the basis of cost and the avoidance of carbide precipitation. Chapter 3 deals with the evolution of the microstructure during uniaxial tension, studied using X-ray diffraction. The effect of tempering deformed and undeformed structures, and heating to high temperatures, have also been investigated. In this context, data on bainite-containing steels in the literature are found to be rather limited. Chapter 4 is a comprehensive assessment of the mechanical behaviour of the steels subjected to a variety of processing routes. It is demonstrated that it is possible to outperform current commercially available steels. The microstructural behaviour of strain-aged and as-transformed steels during uniaxial tension studied using in situ neutron diffraction is described in Chapter 5. The evolution of texture with plastic deformation was confirmed as previously observed using conventional X-ray analysis. Evidence regarding the presence of two populations of carbon-depleted and carbon-rich austenite and their response to strain, grain rotation, anisotropy, stress partitioning between phases and the lack of work-hardening to overcome the onset of necking are presented.

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This thesis investigates the modelling of friction stir welding (FSW). FSW is a relatively new welding process where a rotating non-consumable tool is used to join two materials through high temperature deformation. The aim of the thesis is the development of a numerical model to improve process understanding and to assist in the design of new tools. The early part of the thesis describes the process, defines the modelling problem and describes why a computational fluid dynamics package (FLUENT) was selected for the subsequent work. A systematic series of friction stir welding experiments in 7075 aluminium alloy, used to provide validation data for a numerical model of the process, are described in chapter 2. The trials examined how the welding conditions and tool type affected the weld temperature and heat input. From this data a thermal model of the welds was developed that included the convective heat flow due to material mixing. Chapters 3 to 6 describe the model development, from a preliminary model of a standard tool, to a detailed analysis of 2 dimensional profiles incorporating a novel slip boundary condition, and finally to a full 3 dimensional model of a new tool design, including material slip. The preliminary model with a standard tool assumed that the material stuck to the tool surface and included features such as the tool tilt, heat generation and heat flow. The model captured many of the real process characteristics, but gave poor predictions of the welding forces and heat generation. This identified the need for a more complex treatment of the tool-material interface that allowed material slip. The slip model was first implemented in a 2 dimensional study of flow around profiled tooling (chapter 4). This enabled a first order visualisation of the flow and the quantitative comparison of different 2 dimensional pin profiles. In chapter 5 an optimised 2 dimensional pin profile was determined by selecting the shape that minimised the traversing force. Two prototype tools based on this profile were manufactured: the plain 'Trivex™' and the threaded 'MX-Trivex™'. These were tested against a conventional 'MX-Triflute™' tool with the results showing that the traversing force was reduced by 18-25%. Chapter 6 describes 3 dimensional models of the 'Trivex™' and 'Triflute™' tools, which extended the slip model to 3 dimensions. The model correctly predicted that the Trivex™ tool had lower traversing and down forces than its Triflute™ counterpart, as observed experimentally. The thesis successfully demonstrates the application of fluid dynamics modelling to friction stir welding, enhancing visualisation of the flow, and guiding the development of new tooling.

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