The influence of Hot Isostatic Pressing performed above solidus temperature of LPBFed Inconel 939 for microstructural improvement

Ni-based superalloys are a particular class of metallic materials primarily used in the energy production field (nuclear, oil and gas) and in the aerospace sector, where they are particularly employed in the production of turbine blades and casings. This is because Ni-based superalloys retain their mechanical stability at high temperatures, as they can operate up to 80-85% of their melting temperature without experiencing a significant loss in strength. Moreover, they exhibit excellent high-temperature oxidation resistance, a crucial aspect for sectors with extreme temperature conditions or contact with corrosive fluids. However, these alloys are challenging to process due to their high melting temperature, poor castability, and forgeability, which only allows the production of components with simple geometries. This makes additive manufacturing a highly appealing process for these alloys. Using additive manufacturing, it is possible to create components with more complex geometries and a defect structure and microstructure that differ completely from traditional methods, such as casting and wrought. The only issue with using this technology is the resulting microstructure at the end of the process; specifically, the grain structure is extremely fine, which is excellent for fatigue resistance but not effective for creep resistance, a crucial consideration for this type of alloy. This is where hot isostatic pressing (HIP) is used as a treatment, combining high temperature and high pressure to address this issue. Literature indicates that this treatment primarily pertains to components not produced with AM technology but rather with traditional processes, and this is where the following thesis finds its focal point. The main purpose of this thesis is to determine the feasibility of a dedicated HIP treatment on Laser Powder Bed Fusion (LPBF) Inconel 939, conducted above its solidus temperature, i.e. within the incipient melting region of the alloy. This is because if a standard high-temperature heat treatment were applied, it would reach the incipient melting region but would also cause uncontrolled porosity, damaging the material. By applying high pressure as well, it is possible to enter the incipient melting region without forming the liquid. Additionally, the use of HIP helps reducing the crystalline anisotropy typical of the LPBF process, favoring a more equiaxed structure, effectively coarsening grain boundaries, thus improving creep resistance. Furthermore, material density increases to almost the theoretical value since defects are easily healed. The results obtained in this study show that using HIP above the solidus temperature allows for AM components with a crystalline structure more suitable for high-temperature applications, where creep resistance is a critical factor to consider.