Keyhole gas tungsten arc welding: a new process variant

The topic of this thesis is the discovery and development of a robust 'keyhole mode' of the Gas Tungsten Arc Welding (GTAW) process. This process variant represents a significant departure from conventional fusion welding processes, and is not explicable by traditional models of keyhole formation and behaviour. To be specific, the well-known keyhole processes of plasma, laser and electron beam welding are dependent on the generation of significance ablation (or 'recoil') pressure. Furthermore, there has been an acceptance that this is an essential characteristic of all keyholes. The power densities associated with GTAW are known to be too low to achieve significant ablation, and consequently this process is regarded as incapable of conventional keyhole operation, unless the circumstances are exceptional. This thesis therefore challenges the established views on two counts: • Keyholes welding is practical with existing GTAW technology; and • Keyholes can be stable even in the absence of significant surface ablation. Defence of these claims necessarily raises discussions ranging from the very practical aspects of work-place applications through to theoretical considerations such as the geometry of surfaces. While this work has endeavoured to address the various issues as they have arisen, emphasis has been placed on the development of a broad appreciation of the topic. It is acknowledged that in doing so it has failed to fully explore many of the areas that have been presented. For example, the relationship between GTAW keyhole surfaces and minimal surfaces may lead to fresh insights in weld pool mechanics. In practical terms such a study might lead to a much better appreciation of various forms of porosity, or potential control strategies based on the detection and interpretation of plasma or surface oscillations. The work begins with an experimentally based exploration of the process before addressing the questions of keyhole stability, formation and the process dependencies. It is hoped this approach will provide an efficient means of presentation, and might provide fresh insights into the physics of gas tungsten arc welding.