PSI - Issue 68

Giuseppe Macoretta et al. / Procedia Structural Integrity 68 (2025) 974–980 G. Macoretta et al. / Structural Integrity Procedia 00 (2025) 000–000

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The specimens were produced by using a Renishaw RenAM 500S Flex L-PBF machine (Renishaw S.p.A., Torino, Italy), having a maximum laser power of 500W and a maximum laser speed of 7 m/s, installed in the “Metal Additive Manufacturing” laboratory of the University of Pisa. The process parameters used for the specimen production are the ones that were named “baseline” in our previous works [8–10]. A standard Inconel 718 powder was employed, featuring a chemical composition in compliance with the ASTM F3055 standard, [11]. The specimens were built in vertical direction, namely with the printing direction aligned with the loading axis. The specimens were tested in the surface as-built condition, after being subjected to a solution annealing and double aging heat treatment in an inert atmosphere. The adopted heat treatment cycle, based on the AMS 5663 standard and literature data [12,13], is reported in Fig. 2.

Fig. 2. Post-processing heat treatment in an inert atmosphere adopted for specimen preparation.

2.2. Microstructural and fractographic analysis Microstructural investigations were carried out both in longitudinal and transverse directions. Specimens were prepared following the ASTM E3-11 standard and were etched with Kalling’s II (n.94, ASTM E407). The same examinations were employed to calculate the material density and the features of the pores, by using the ImageJ software. Metallographic investigations were also employed to measure the effective notch radius produced by the L-PBF process and the local geometry in the notch area, [14]. Dedicated specimens were produced and not subjected to heat treatment, to preserve the traces of the L-PBF process. Samples were extracted by cutting the specimens along their longitudinal direction. Particular attention was paid to having the observation surface in proximity to a plane containing the specimen axis. 2.3. Fatigue tests HCF tests were carried out at a constant temperature of 650°C in an axial loading configuration. A Rumul Mikrotron (Russenberger Prüfmaschinen AG) resonant testing machine equipped with a thermal chamber (Rumul Thermotron), shown in Fig. 1 (b), was employed. The tests were carried out in load control, with a load ratio R of 0.1. The test frequency was monitored to detect the macroscopic crack onset and its propagation. A life range between 10 thousand and 10 million cycles was explored. The fatigue notch factor, k f , and the notch sensitivity, q , were calculated from the fatigue strength of the smooth and notched specimens, which was defined as the value calculated from the S-N fitting line at 1 million cycles to failure, considering a 50% probability of failure.