Borrelli, Domenico (2025) Methodology for Selective Laser Melting of thin-walled Inconel 718 propulsion systems and optimization of surface quality through chemical polishing techniques. [Tesi di dottorato]

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Abstract

This research focuses on the development and validation of an integrated manufacturing process, based on Additive Layer Manufacturing (ALM) technology, for high-performance Inconel 718 superalloy components intended for aerospace propulsion systems. The evolution of the New Space Economy imposes a growing demand for reusable, lightweight, and efficient systems, requiring materials capable of withstanding extreme operating conditions. Although Inconel 718 possesses the ideal properties, its processing via Laser Powder Bed Fusion (L-PBF) presents significant challenges that limit its adoption for the most critical flight applications. The main challenges addressed in this thesis are twofold. The first is the difficulty in obtaining dense, integral, and non-permeable thin walls, a crucial geometric feature for maximizing heat exchange efficiency in regenerative cooling channels. The second is the high surface roughness of "as-built" parts, which acts as a primary limiting factor: surface irregularities create stress concentrations that drastically reduce fatigue life and increase the risk of undesirable interactions with reactive propellants. This challenge is exacerbated by the impossibility of treating internal surfaces—a key design advantage of ALM—with conventional methods. The first phase of the work established a robust methodology for optimizing printing parameters through a systematic Design of Experiments (DoE) approach. The application of parameters optimized on standard specimens to a complex geometry with very thin walls revealed severe permeability issues, even after a HIP treatment. A subsequent, targeted DoE campaign made it possible to identify the "Body-Contour Offset" as a critical interface parameter, the precise adjustment of which proved fundamental to resolving "sub-skin" porosity problems and ensuring the leak-tightness of the thin walls. The final printing process yielded a component that passed leakage tests at pressures up to 20 bar and was used to provide a complete characterization of the material with and without full heat treatment. The second phase focused on the development of an optimized post-print surface finishing strategy. After quantitatively demonstrating the limitations of mechanical techniques, an innovative multi-stage chemical polishing process was developed. Two complementary acid solutions were formulated: one based on HNO₃/HF/NH₄HF₂ for the primary etch, and a second based on FeCl₃/HCl for a refining etch. An experimental campaign demonstrated that while the process is slow in a static regime, it is drastically accelerated by convection (agitation) and temperature, but that the optimal procedure to ensure uniformity on complex geometries is a static, temperature-controlled regime. Finally, a rotating bending fatigue testing campaign validated the process's effectiveness by comparing the performance of "as-built," sandblasted, and chemically polished specimens. The results show a clear hierarchy: sandblasting offers the best performance due to the introduction of compressive residual stresses. Chemical polishing significantly improves fatigue life compared to the "as-built" condition by eliminating printing defects, but its effectiveness is partially limited by the introduction of pitting "pits" that act as new crack initiation sites. In summary, this thesis qualifies and quantifies the benefits of a new chemical polishing procedure, validating it as a fundamental enabling technology for finishing complex internal surfaces where mechanical processes are inapplicable or ineffective.

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