VED-dependent HIP densification of PBF-LB Inconel 718: Defect closure thresholds, microstructural homogenization, and nano-mechanical response

Defining the initial porosity limits within which hot isostatic pressing (HIP) can effectively densify laser powder bed fusion (PBF-LB) components remains a critical gap for industrial qualification of additively manufactured nickel-based superalloys. This study systematically investigates the influence of volumetric energy density (VED) on initial defect population in Inconel 718 (IN718) and its role in densification response during HIP. Specimens with varying initial porosity (0.01–17.76%) were fabricated by intentionally varying laser power, scan speed, and hatch spacing. All samples were subjected to identical HIP conditions (1160 °C, 100 MPa, 4 h) incorporating a combined in-situ solution treatment, followed by double ageing in a vacuum furnace. Porosity evolution, pore morphology, microstructural transformations, and nano-mechanical response were characterized before and after HIP. Three distinct densification regimes were identified based on initial porosity. Below 0.1%, the absolute densification benefit is negligible due to argon-filled gas pores resistant to closure. Between 0.1% and 10%, closure efficiencies of 87–98% are achieved, representing the optimal HIP densification window. Between 10% and 17%, the response degrades progressively, and above ~17% densification efficiency becomes negative owing to open-pore network formation and argon gas expansion during subsequent vacuum treatments. Furthermore, HIP at 1160 °C fully dissolved Nb-rich laves phases, eliminated melt pool boundaries, and promoted recrystallization from columnar to equiaxed grain structure. Double ageing produced a uniform distribution of fine γ′/γ″ strengthening precipitates. These findings establish quantitative defect acceptance criteria for pre-HIP qualification of PBF-LB nickel superalloy components, supporting cost-effective post-processing for aerospace and energy applications.