Application of Hot Isostatic Pressing to Tailor the Microstructure of Additively Manufactured Nickel-Based Superalloy (In939)

The object of this study is the characterization of a nickel superalloy, Inconel 939, processed by Laser Powder Bed Fusion (LPFB), an Additive Manufacturing (AM) technique. This kind of metallic material finds its application in crucial fields such as the aerospace sector and energy production, particularly in the production of turbine blades. Here the specific properties required for the service, such as mechanical stability at high temperatures, meet those of nickel superalloys. Whitin this group of materials, IN939 has excellent mechanical characteristics and resistance to corrosion and oxidation at high temperatures. However, Ni superalloys exhibit high melting temperature, low castability and forgeability. These conditions make additive manufacturing one of the most attractive choices for the production of these alloys. Indeed, AM allows to build components with a layer-by-layer process, using a 3D model. This allows for components with a more complex geometry and also to a completely different defectiveness compared to those shown by traditional methods. The critical aspect is that with this technology the final microstructure is composed of extremely fine grains, condition that is excellent for fatigue resistance but results in an issue for creep resistance. This is where the thesis focuses. The main scope is to assess the impact on the microstructure of non-standard heat treatments and HIP treatment above the solidus temperature of IN939. The primary objective of this study is maximizing the grain size to enhance the high-temperature performance, specifically the creep resistance, of the superalloy through microstructural control. While the high-temperature heat treatments can promote grain growth, their practical application is fundamentally limited by the incipient melting temperature, which constrains the maximum usable processing temperature. The observation of liquid phase formation, however, suggests an alternative processing route. This study proposes investigating the potential of Hot Isostatic Pressing (HIP) as a means to circumvent this limitation. The central hypothesis is that the application of high isostatic pressure during HIP may suppress the thermodynamic driving force for melting, inducing a shift in the phase equilibrium and allowing the material to be processed within or near the conventional incipient melting region without the catastrophic formation of liquid phase. Alternatively, the pressure may significantly delay its formation, permitting access to higher temperatures than are achievable at atmospheric pressure. Consequently, this research aims to systematically explore this hypothesis. The goal is to determine whether the synergistic combination of high temperature and high isostatic pressure can facilitate extreme grain coarsening without causing microstructural damage.