Study and characterisation of Ni-based superalloys produced by laser additive manufacturing

The laser additive manufacturing techniques (LAM) are of growing interest in different industries due to their capacity to produce near net shape components in a single step. Particularly in the case of the Ni-based superalloys, LAM processes can produce highly complex shape components more cheaply with respect to the traditional technologies. However, nowadays, there is still a lack of knowledge on the study of the process parameters on the microstructure and densification levels and the study of tailored heat treatments to meet standard qualification as well as precise industrial requirements. This thesis presents the research performed on two Ni-based superalloys produced by LAM processes. Inconel 625 (IN625) fabricated by laser powder bed fusion (LPBF) and directed energy deposition (DED) and Hastelloy X (HX) built by LPBF. For LPBF IN625 alloy was studied the effect of different process parameters on the hardness and densification level, obtaining dense samples (relative density over 99.8 %). Afterward, it was studied in detail the microstructure, mechanical and thermomechanical properties of as-built IN625. The results showed that the tensile behaviours of as-built IN625 state are higher than minimum values requirements for wrought IN625 alloys, due to very fine dendritic structures (mainly less than 1.5 micron) coupled with high dislocation density. However, the characterisation of as-built IN625 samples revealed that heat treatments are necessary to decrease the residual stresses, reduce elements segregation, and produce mechanical properties suitable for industrial applications. For this reason, the microstructural evolution of LPBF IN625 under different heat treatments was investigated. According to the performed heat treatment, the mechanical properties of heat-treated IN625 were influenced by phases precipitation (mainly γʺ phases and carbides) and/or recrystallisation, grain growth and dissolution of dendritic structures. For DED IN625 alloy, the aim of this work was to determine the impact of different process parameters on the densification level (relative density over 99.7 %), hardness and microstructure in order to select the appropriate parameters for industrial production. Finally, LPBF HX alloy was studied in a research collaboration with GE AVIO s.r.l. The target of this work was to investigate the microstructure of as-built and post-processed LPBF HX alloy. The as-built HX samples showed different microcracks caused by the LPBF process, so some samples were hot isostatically pressed (HIPed) to close the microcracks. The microstructure, grain size, phases and hardness of the as-built, heat-treated and HIPed HX samples were investigated. The results revealed that in case microcracks appear during production, they can be removed after specific post processing, generating LPBF HX parts with grain size similar to standard solution heat-treated (SHT) wrought HX alloy