PSI - Issue 34

Radomila Konecna et al. / Procedia Structural Integrity 34 (2021) 135–140 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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different surface orientations besides the vertical orientation used with standard specimens. Three sets of specimens L-PBF AlSi10Mg alloy were fabricated. Fig. 1a shows the specimens orientations considered here with respect to build direction. Type C is vertically oriented and provides reference data for comparison to standard specimen data.

Fig. 1. a) view of miniature specimen orientations on the building plate; b) scheme of individual specimen testing.

Fig. 1b shows schematically the bending loading condition of a miniature specimen. The maximum effective local stress  max on the flat surface is computed from the maximum bending moment M, the resistance modulus W = 20.8 mm 3 and a miniature specimen geometry factor C mg = 0.91 as follows, Nicoletto (2017)  max = C mg M/W. Fatigue testing was performed in an electro-mechanical fatigue testing machine operating at a frequency of 25 Hz in fixed rotation-control mode. The applied stress ratio was R =  min /  max = 0. After testing, the fracture surface of selected specimens was investigated in the SEM to identify initiation and propagation of fatigue cracks. 3. Results and discussion This section initially shows how the near surface microstructure and surface of the directional specimens is influenced by the printing strategy. Then, the dependence of the longitudinal surface roughness on the different orientations is presented along to the directional fatigue behavior of the three sets of specimens. The discussion ties to connect the experimental results in a unique framework that clarifies the observed directional fatigue behavior of as-built L-PBF AlSi10Mg. Microstructure The local near-surface microstructure at the location of crack initiation identified in Fig. 1b was investigate in the different miniature specimens. Fig. 2 provides a composite overview of the range of microstructures observed in the different specimens. In the case of as-built specimens the surface quality and the local microstructure are directly responsible of the mechanism of fatigue crack initiation. On the other hand, the printing strategy adopted in specimen fabrication that is two contours followed by hatch scanning of the internal area within the individual layer results in very different near surface structures. In the case of Type B and Type C specimens, see Fig. 2, the peak stress is reached in the contours, which has a different structure than the internal hatched material because of different process parameters. However, the contour stress in Type B specimens is applied parallel to the layers while in Type C specimens the peak stress acts perpendicular to the transformed layers. The surface morphology is very different in the two cases: in one case Type B the profile is straight with partially melted powder particles mainly contributing to roughness while in the other case the profile is the result of layer-by-layer generation. A completely different situation is found in the Type A- 3.1. Near-surface material characterization