Residual stress in plasma transferred arc (PTA) of Ti-6Al-4V for additive manufacturing (AM)

Additive Manufacturing (AM) is known as an alternative method to conventional manufacturing processes, which enables design freedom, light-weighting solutions and resource efficiency for the high value manufacturing sector. As a valuable but costly material, titanium and its alloys have been the material of the choice for most of the metal-based AM processes. Plasma Transferred Arc (PTA) has been used as a high deposition rate AM technique to manufacture near net-shape parts. PTA AM of titanium alloys have presented a great business case to be adopted by different industrial sectors. However, similar to all metal-based AM techniques, PTA AM parts are affected by high thermal gradients and suffer from residual stress and associated distortion. The evolution of residual stress therefore remains a main challenge in commercialising PTA AM. Understanding the effect of the manufacturing process on the final state of the residual stress could result in manufacturing optimisation. This thesis investigates the effects of the three main PTA AM process parameters, deposition strategy, dwell-time and energy density, on the evolution of residual strain/stress in PTA AM parts. For the first time, this work links manufacturing process and the final state of the residual stress, for PTA AM parts. Three residual stress measurement techniques, including two non-destructive diffraction-based techniques and one destructive contour method, are used to determine the level and the variation of the residual stress within the PTA AM parts, with different process parameters. The results provide an understanding of the effects of the three process parameters (and their combinations) on the residual stress evolution in PTA AM parts. The applicability of the stress measurement techniques for PTA AM parts is demonstrated, where the contour method is shown to be a viable and straightforward technique to determine residual stress. Conversely, it is seen that diffraction techniques have experimental limitations for titanium PTA AM components, leading to increased measurement error. The residual stress/strain data show that the deposition strategy is the dominant process parameter. However, the dwell-time and the energy density are also contributor factors to the evolution of residual stress in the final component. The data presented identifies the critical combinations of the process parameters and their levels on the evolution of residual stress in PTA AM parts