PSI - Issue 68

Matias Jaskari et al. / Procedia Structural Integrity 68 (2025) 480–485 M. Jaskari et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Inconel 718 is a nickel-based superalloy widely used in specialized applications requiring excellent high temperature resistance, such as in aerospace, energy, and automotive industries. Its combination of high strength, toughness, and superior creep resistance under elevated temperatures makes it an ideal material for demanding environments. However, the processing and fabrication of Inconel 718 is costly, primarily due to its expensive alloying elements, high solid solution strength, and reliance on precipitation hardening for enhanced performance. This makes near net-shape fabrication methods, such as Additive Manufacturing (AM), an attractive option to reduce material waste and production costs. In principle, AM presents new opportunities for optimizing component designs, enabling weight reduction while maintaining or even enhancing material properties (Thompson et al., 2016). Among various AM methods, laser powder-bed fusion (PBF-LB) is frequently employed for Inconel 718 fabrication. However, the process is prone to several manufacturing defects, including surface roughness, porosity, and brittle phase formation, largely due to the complex interplay of large number of factors influencing process quality (Oliveira et al., 2020). These defects, especially in dynamic applications, can severely undermine mechanical performance. A significant challenge is the relatively low fatigue resistance of PBF-LB structures compared to their wrought counterparts, a problem well documented for alloys such as AISI 316L (Jaskari et al., 2019; Kumar et al., 2020), Ti6Al4V (Bhandari & Gaur, 2022), AlSi10Mg (Beretta et al., 2020), and Inconel 718 (Rautio et al., 2023; Witkin et al., 2019). In particular, PBF-LB alloys exhibit a notable sensitivity to fatigue failures driven by surface quality and internal defects, and lack-of-fusion defects, which create stress concentrations or even work as a pre-crack, often dominate fatigue behavior (Volpato et al., 2022). Moreover, the fine substructure of PBF-LB alloys is associated with high notch sensitivity, resulting in low threshold stress intensity factor (ΔK th ) values. These microstructural characteristics contribute to premature crack initiation and propagation in high-cycle fatigue (HCF) regimes (Pei et al., 2019). This study aims to assess the influence of surface quality on the fatigue performance of commercially heat-treated PBF-LB Inconel 718. Axial fatigue tests were conducted on both as-built samples with poor surface quality and electropolished samples. The goal is to demonstrate the detrimental effect of rough surfaces on the fatigue limit, even in the absence of significant internal defects, highlighting the critical role of surface treatment in optimizing fatigue resistance in PBF-LB components. 2. Experimental Samples were printed from unused powder supplied by EOS GmbH. in vertical orientation using EOS M290 PBF LB equipment with standard 40 μm layer thickness printing parameters. For the study, round axial fatigue samples shown in Fig. 1a were fabricated, with diameter and test length of 6 and 12 mm, respectively. Post-fabrication, samples underwent heat treatment, including solution annealing at 960 °C for 2 hours, followed by double aging treatment at 720 and 620 °C for 8 hours each, as is illustrated in Fig. 1b. Surface roughness reduction was achieved through meticulous machining and subsequent electropolishing with commercial DLyte 100HF+ electropolishing equipment, resulting an enhancement of surface roughness R a from 5.2 to 0.3 µm. Additionally, R z values decreased from 28.1 to 4.9 µm, as is depicted in Fig. 2. The polished samples were then subjected to uniaxial load-controlled fatigue testing using Step-Lab UD20 uniaxial dynamic testing equipment, with a stress ratio of -1 and a cut-off cycle limit of 2x10 6 cycles. Metallographic examination was conducted for mechanically polished cross-sections using JEOL JSM-7900F field emission scanning electron microscope (FE-SEM), equipped with Oxford Instruments Symmetry S2 Electron Backscatter Detector (EBSD). Hardness was measured from cut and polished cross-sections of axial fatigue samples before and after fatigue, using Zwick ZHU 2.5 hardness tester, utilizing Vicker’s intender and 10 kg load.