Porosity closure during hot isostatic pressing of additively manufactured Ni-based superalloy IN718 produced by LPBF and EBM

Abstract

Inconel 718 is a precipitation hardened Ni-based superalloy for aerospace applications where corrosion, creep and fatigue resistance are required. Additive manufacturing (AM) enables production of new designs of weight-optimised components with features not possible with traditional manufacturing techniques. However, components produced by additive manufacturing contain some defects such as gas porosity and lack of fusion defects (LOFD) which deteriorate the mechanical performance of the components. Hot Isostatic Pressing (HIP) utilises a high pressure up to 200 MPa in combination with a high temperature of up to 2000°C in a gas atmosphere to heal the defects in additively manufactured parts. In this project, focus was placed on defect elimination in case of Inconel 718 components, produced by powder bed fusion technologies. To do so, two sets of cylinders (5 and 10 mm in diameter) were produced by laser powder bed fusion (LPBF) and electron beam melting (EBM). The cylinders contain a designed 1-mm sphere filled by un-molten powder in the centre of the sample. Pores contain processing gas, that is Ar in case of LPBF and gas residues from the vacuum in case of EBM. The specimens experienced 5 variants of HIPing cycles followed by 2 variants of heat treatments. All HIPing cycles were done at 150 MPa pressure and temperature of 1120 and 1180°C for duration between 5 and 360 minutes in 5 different combinations. Standard heat treatment for IN718 was performed inside HIP (pressurised heat treatment) or at atmospheric pressure. The LPBF and EBM specimens responded differently to the HIPing cycles. The HIPing cycles were not able to fully heal all gas porosities in LPBF specimens at applied conditions. Specimens from EBM were fully dense after HIPing for 1 hour. The LPBF specimens experience a re-growth of Ar porosity during atmospheric heat treatment compared to pressurised heat treatment in HIP. The microstructure inside the designed sphere is analysed in terms of oxide films, nitride inclusions, and prior powder boundaries. The designed sphere filled by powders was seen as a relatively large lack of fusion defect, and this gave the possibility of following this defect during HIP and HT. The grain structure and precipitation of different phases are compared to the solidification microstructure achieved by laser and electron beam processes.