The microstructures developed in the heat affected zone of low alloy steel weldments are critically dependent upon the alloy composition and the thermal cycles introduced by the welding process. Review of available literature indicates that although a significant amount of research has been performed examining microstructural development, the bulk of this work has either been aimed at steels for low temperature application or has been used to overcome reheat cracking in ½ Cr- ½ Mo- ¼ V steels. In general it appears that the measures taken to prevent reheat cracking which occurs early in the life of a weld, i.e., limiting the carbon and vanadium content and adopting procedures which result in significant refinement of weldment structures, may well have been responsible for promoting the long term, in-serve creep damage. Thus in the present programme, research has concentrated on establishing the factors affecting the formation of austenite on reheating, the growth of these austenite grains as well as considering microstructural transformations on cooling with the view to optimising both short term and long term performance.

The purpose of this thesis is to develop numerical techniques to model and analyse metal forming operations involving material removal and ductile fracture. Due to the diversity and complexity of the physical phenomena involved, several different computational aspects of the problem have been addressed such as: computational strategies for general thermo-mechanical coupled problems accounting for heat generation due to plastic and frictional work, thermal contact, thermal strains and temperature dependent properties; ductile fracture criteria for damaged and conventional <I>J<SUB>2</SUB></I> elasto-plastic materials; and transfer operators for thermo-mechanical coupled problems and error estimates for damaged and conventional <I>J<SUB>2</SUB></I> elasto-plastic materials. The above techniques made possible studies on the following subjects: application of ductile fracture concepts to material separation in incipient chip formation and blanking; and application of error estimates and re-meshing procedures to high-speed machining. The technique developed in this thesis provide useful computational tools in the analysis of the phenomena involved in chip formation processes and constitute an advance with respect to numerical simulation of orthogonal machining.

High Speed Resistance Welding (HSRW) is a high-volume manufacturing route for the production of welded steel cans. Full optimisation of such a process requires a scientific understanding of the relationships between process variables and thermal effects occurring in the weld zone and heat affected zone. This body of work describes the development of a predictive electro-thermal FE computer model to provide a more profound understanding of the HSRW process. The model has been used to investigate the thermal effects of welding speed and A.C. frequency on the macro-scale evolution of the nuggets developed during welding. Factors affecting weld quality and previous attempts to model the process are highlighted and the evolution of the model is described in detail determining the unsuitability of finite difference to the final Windows based user friendly interface for the validated finite element model. The model formulation and generated results are described and a novel method of using Re-melted Tin Zone measurements for validation is presented.

The current research was commissioned to assess the feasibility of fabricating bonds between a range of commercially available titanium based aerospace alloys, using a low-cost direct resistance heating technique. A novel methodology has been devised and implemented which utilises a Gleeble facility to produce controlled resistance bonds. The optimisation of bonding parameters required to produce high quality joints is discussed i.e. the effects of surface treatment, pressure, time, temperature and environment. Optimised bond conditions have been defined for joining Ti 6/4 to itself and TNB (gamma titanium aluminide) to itself, negating the requirement for specialised surface treatments. In addition, a collaborative venture with Birmingham University was established to incorporate novel surface treatments using an electrical discharge machining procedure. In this respect, Ti 6/4 and TNB bonding couples utilising integral copper recast surface braze layers have been suitably optimised. Finally, the bonding of the dissimilar alloy combination Ti 6/4 to Ni-Ti (shape memory alloy) has been attempted using butt welding (with and without a Cu-Ni interlayer) and eutectic bonding procedures. However, bonding of this material combination proved difficult and this specific pairing may be incompatible using the present technique. Metallographic sections, chemical composition and micro-hardness traverses across all bond lines are presented as evidence of the form and integrity of the resulting joints. The mechanical performance of the bonds was assessed under monotonic tensile loading conditions.

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