Modelling of Ti-6Al-4V linear friction welds.

Linear friction welding (LFW) is a solid-state joining process that is finding increasing industrial interest for the fabrication of Ti-6Al-4V preforms. The fundamental science behind the process needs to be better understood to aid further process implementation. In practice, many aspects of the process are difficult to measure experimentally. Consequently, many researchers use computational models to provide an insight to the process behaviour, such as the thermal cycles and flash formation. Despite these recent research efforts, the effects of the workpiece geometry and process inputs on Ti-6Al-4V linear friction welds are still not fully understood. This thesis focuses on the development and validation of computational models to address this issue. Two and three-dimensional (2D/3D) computational models were developed using the finite element analysis software DEFORM. The models were validated with a systematically designed set of experimental welds. The validated models and experimental data were used to characterise the effects of the process inputs and workpiece geometry on the: thermal fields, material flow, flash morphology, interface contaminant removal, microstructure, energy usage, welding forces, coefficients of friction and welding times. The results showed that there is a benefit to using larger pressures and oscillating the workpieces along the shorter of the two interface-contact dimensions when producing Ti- 6Al-4V welds. This is because the burn-off required to remove the interface contaminants is reduced. Hence for the same burn-off, the factor of safety on contaminant removal is greater. Furthermore, these conditions can also reduce the interface temperature and refine the weld microstructure, which may offer additional benefits, such as reduced residual stresses and improved mechanical properties. In conclusion, the thesis aim was successfully addressed, therefore increasing understanding of the LFW process. The work showed that although the 3D models captured the full multi-directional flow behaviour, 2D models were better suited to parametric and geometric studies.