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 L-PBF technology can be successfully exploited if is used to produce complex geometries, such as lattice structures or intricated cooling channels, which present several features that can act as notches. In the critical areas of such components, it is unfeasible to machine the component to remove the surface roughness produced by the L-PBF process. The effects produced by the macroscopic notch coexist with the ones introduced by the local defects of the as-built surface. The paper is aimed at investigating the elevated temperature HCF behavior and notch sensitivity of Inconel 718 produced via L-PBF. Three V-notch geometries were defined through FEM analyses, to obtain theoretical stress concentration factors ranging from 1.5 to 3.5. These values were selected to reproduce the typical stress concentrations that are present in structural components produced in conventional Inconel 718 alloy, such as turbine-disk fir tree connections. As-built smooth and V-notched round specimens were subjected to elevated temperature HCF tests, carried out with a load ratio of 0.1. The tests were carried out at a constant temperature of 650°C, chosen in agreement with the literature data and the operating temperatures of industrial components produced in Inconel 718, [5–7]. Metallographic and fractographic investigations pointed out the features of the local notch geometry and the crack onset site for the different specimen geometries. 2. Material and methods 2.1. Specimen design and production A round specimen having a minimum diameter of 5 mm was adopted, Fig. 1 a). A smooth specimen and three geometries of 90° V-notched specimens were employed. The smooth specimen features a uniform diameter along the gauge length, as it allows having a wide iso-stressed region exposed to the surface and microstructural defects that can be introduced by the L-PBF technology. The geometry of the notched specimens was defined via a Finite Element (FE) model. It was prepared a 2D axisymmetric model, featuring quadratic elements with a minimum element size defined on the basis of a mesh convergence analysis. A linear elastic model was implemented employing material properties obtained from previous experimental campaigns on the same material, [8,9]. A 90° V-notch was selected to avoid supports in the notch downskin region. The notch radius was adjusted to obtain the desired theoretical stress concentration factors ( k t ): 1.5, 2.0, and 3.5. The notched specimens feature the same minimum diameter as the smooth ones, while the notch depth ratio versus the specimen outer diameter was equal to 0.3, to enhance the intensity of the singular stress term ahead of the notch, [12].

Fig. 1. (a) Geometry of the smooth and notched specimens, as produced by the L-PBF process. The arrow BD points out the build direction. (b) Experimental setup for isothermal elevated temperature fatigue tests.