Application of the Hot Isostatic Pressing on Inconel 738 above its solidus temperature for microstructural enhancement

Nickel-based alloys are commonly used in demanding applications such as aerospace, nuclear, oil and gas production. The production of components made of these materials is often highly complicated because of their high melting temperature, low castability and forgeability. This condition makes Additive Manufacturing (AM) so attractive for these alloys. Nevertheless, the defectiveness and the microstructure of additively manufactured components differ from traditional ones. Hot Isostatic Pressing (HIP) for these materials is often compulsory and reported in international standards because of the stringent safety levels typically adopted. Nonetheless, the standards still refer to traditional cast or wrought microstructure and adapt poorly to the AM materials, and this is where the thesis finds its focal point. The main scope of this work is to assess the feasibility of a dedicated HIP treatment on Laser Powder Bed Fusion (LPBF) Inconel 738 performed above its solidus temperature, i.e. within the incipient melting region of the alloy. A traditional heat treatment performed in this high-temperature range would be detrimental due to the formation of large porosities and damages. On the other hand, applying high pressures, as in the case of a HIP process, allows entering the incipient melting region without forming new pores. Indeed, the exceptionally high temperature enhances solid-state diffusivity in the material, allowing to form special microstructures. During the soaking stage at such temperatures, the out-of-equilibrium microstructure of AM samples is eliminated and substituted with an equiaxial one. Furthermore, the grain boundaries can coarse very effectively, making the samples potentially more resistant to creep. Moreover, after the HIP cycle, material density results close to the theoretical one because defects have been effectively healed. Based on the results obtained in this work, using HIP above the material solidus temperature is an effective heat treatment for AM components designed for high-temperature applications where creep is the main reason for failure, as in the case of turbine blades.