Optimization of the process parameters in CT scan inspections of TiAl blades

This work of thesis, developed in collaboration with Polytechnic of Turin and Avio Aero, is part of a larger industrial project of development and research (R&D), aimed at increasing the emerging technologies, especially the Additive Manufacturing and the Computed Tomography. The focus of the project deals with improving the post processing phase of the EBM manufacturing, for saving time and money. The conventional EBM post-process plan foresees the following steps: heat treatments, machining operations and quality inspections, including the Non-Destructive Tests (NDTs), where the industrial CT, is gaining a growing attention. The project wants to evaluate the consequences and the inherent benefits, that would be obtained by anticipating the CT scan before the heat treatments, to avoid an expensive waste of components. Indeed, if the detected porosities are too many or too large, a pre-defined statistical limit, could be able to highlight those pores, for which a heat treatment would show up useless. The definition of this limit requires a long statistical research, whereby the project has been divided in two branches: as first the CT inspection of a batch with “as-built” blades, while in the second part, after a special HIP treatment, the CT inspection of the same batch, to obtain a final comparison of the porosity’s distributions. Together with the CT Provider, some training days have been organized, with the purpose of defining, optimizing and maximizing some of the process parameters of acquisition, reconstruction and visualization, that will be used on the batch. Moreover, an agreement, about the defect’s properties to be listed in the CT reports and the possible pores’ statistical characterizations, have been finalized. The first part of the thesis is reserved to the AM techniques, the EBM technology and the TiAl properties, to better understand the blade’s nature and the pores’ origin. The central chapters are focused on the CT scan capabilities, physical principles and the CT role in AM applications, where this combination has a still limited knowledge. Then, an in-depth study has been conducted to improve the process control and the technological “know-how” about the CT parameters (voltage, current, focal spot size, number of projections, acquisition time, detector size, etc.) and tradeoffs. Here every parameter has been cataloged and investigated about the effects it has on the other parameters and which ones it is influenced by. Eventually, the last chapters focuse on the porosity characterization and the CT results of the training days, made on blades with known defects, in order to find the most correct set of parameters and to calibrate the CT machine.