PSI - Issue 42

Radomila Konecna et al. / Procedia Structural Integrity 42 (2022) 857–862 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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When the L-PFB process parameters are optimized, the static strength characteristics of the printed alloy (without heat treatment) reach values comparable to those of conventionally produced materials, however the ductility is low and residual stresses are present in the component. As regards the fatigue properties, in particular the fatigue life, it is lower than that of the conventionally manufactured material and it exhibits large scatter. The decisive controlling factors are internal defects, surface quality (roughness), microstructure and residual stresses. The problem of large internal defects, mainly lack of fusion reducing the fatigue life can be solved nowadays by appropriate setting of processing parameters. Moreover, it has been shown, that the influence of remaining small defects, mainly pores, plays a role only when the material surface is machined or polished, Brandão et al. (2017). The quality of the as-built surface has its limits and depends on the surface orientation with respect to the build direction. In the case of complex shape components, surface polishing is expensive and even impossible for geometrically complicated parts. Therefore, an important and yet poorly understood and quantified issue for reliable fatigue life prediction, crucial for many applications, is the effect of notches with as-build surfaces. The microstructure of as-built material is dependent on the build direction. It means that in the vicinity of a notch root differs for different notch orientations to the build direction. And because the crack initiation and propagation are dependent on microstructure, see e.g. Piette et al. (2021), this phenomenon has to be taken into account. The last phenomenon influencing the fatigue life is residual stress present in the as-built material, which influences both the crack initiation and propagation. This paper presents experimental data on the influence of semicircular notches with as-built surface differently oriented to the build direction on fatigue life of L-PBF AlSi10Mg alloy and discusses of the effect of the above mentioned factors on high cycle fatigue life. 2. Material and experiments Specimens for the experimental investigation of the fatigue notch sensitivity of AlSi10Mg alloy were manufactured on an SLM 280 HL system equipped with 400 W yttrium laser. The layer thickness was 50  m, spot diameter 80  m and the hatch spacing 170  m with contour strategy and layer-to-layer hatch rotation. The chamber temperature was 80 °C. No heat treatment was carried out after the building of specimens. Reference mechanical properties were the

following: tensile strength R m = 410 MPa; yield stress R s = 230 MPa; elongation A = 5 %. Miniature fatigue test specimens and testing technique developed by Nicoletto (2017) was successfully used for this type of material. Four sets of notched test specimens were made, differing in notch orientation with respect to the direction of building. Individual orientations are marked A+, A-, B and C, Fig. 1. The specimen dimensions were 22 x 7 x 5 mm 3 with the minimum cross-section 5 x 5 mm. The semicircular 2-mm-radius notch generates a stress concentration factor K t = 1.63 when under bending load. The specimens were cyclically loaded in plane bending at load ratio R =  min /  max = 0 in an electromechanical fatigue testing machine at frequency of 25 Hz, Nicoletto (2017).

The microstructure of the as-built alloy exhibits strong directionality. Details of the microstructure in the notch root for all orientations are shown in Figs. 2 and 3. Moreover, the surface quality depends on the build direction, being the worst in the case of A- orientation where the notch root corresponds to the down-skin surface. The up skin surface of the notch root in A+ orientation exhibits substantially lower roughness. Furthermore, the quantity and size of defects is here substantially lower than in the case A-. The microstructure in the notch root vicinity imaged by EBSD is shown in Fig. 3. Differences in grain size and microstructure, in relation to the notch root surface are pronounced. 3. Results The experimentally determined fatigue life of notched specimens presented as S-N dependences (Wöhler curves) for different notch orientations with respect to the build direction is shown in Fig. 4. The maximum stress  max used to plot the data is determined as  max = K t M/W where M is the maximum bending moment M max divided by the Fig. 1. Notched specimens for fatigue tests.