Tailoring thickness debit for high-temperature fatigue resistance of Inconel 718 superalloy fabricated by laser powder bed fusion

The thickness debit often leads to uncertainty regarding the fatigue performance of laser powder bed fusion (LPBF)-fabricated Inconel 718 thin-walled components and restricts the structural design of these components. Aiming to address this issue, fatigue properties of LPBF-fabricated Inconel 718 homogenized at various temperatures were investigated at 650 °C using specimens with different thicknesses. The results reveal a pronounced influence of both the thickness debit and the intricate interplay between the microstructural and geometrical scales of the thin-walled specimens on their fatigue life at 650 °C. The fatigue life of the thin-walled specimens with the same microstructural scale reduces with decreasing the ratio (t/d) of the specimen thickness (t) to the grain length (d). The coupling effect is described by a mechanism model correlated with the geometrical and microstructural scales of the specimens, in which continuous damage mechanics (CDM) and calculation of the yield strength have been considered. Based on the model, a criterion of t/d > 6.2 for the LPBF-fabricated Inconel 718 specimens homogenized at 1100 °C, and t/d > 8.8 for those homogenized at 1065 °C are proven to be satisfied to ensure a longer and more stable fatigue life of the thin-walled specimens serving at 650 °C. Elevating the homogenization temperature from 1065 °C to 1100 °C results in an extension of the fatigue life for specimens of the same thickness. This enhancement is attributed to the improved ability of grains to coordinate local deformation, as well as the reduced prevalence of elongated Laves and other phases, which typically serve as preferential sites for crack initiation and propagation. The finding suggests that the thickness debit in high-temperature fatigue resistance of LPBF-fabricated components can be minimized by tailoring the heat treatment strategy.