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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maximum stresses is however similar to the notch in orientation C, which has a superior surface quality. The previous observations clearly indicate that the surface roughness is not the only factor determining the fatigue life otherwise data of orientation C and A- should be significantly different. Residual stresses are expected to be present in the as-built L-PBF AlSi10Mg. They however depend on many factors including geometry and part orientation. Beretta and al. (2020) reported residual stress measurements in mildly notched AlSi10Mg specimens printed by the same company according to the same process parameters used in the present study. Mesomechanic simulations of directional fabrication of notched specimen by Nicoletto at al. (2019) confirmed the experimental findigs obtained by x-ray diffraction for three of the four orientations shown in Fig. 1. The respective values of near-surface residual stresses are introduced in Tab. 1: it is noted that orientation C is characterized by a high tensile residual stress while the other directions have negligible residual stresses. Therefore, to account for the influence of residual stresses, the directional fatigue strength at R = 0 for the notched configurations reported in Tab. 1 can be converted into the fatigue strength  a, R=-1 at R = -1 using the Goodman-Haigh mean stress equation  a, R = -1 =  a /(1- (  m +  rs )/R m ). (1) The computed local fatigue limits of Tab. 1 are observed to define a different ranking of the notch orientations in fatigue. Namely orientation B is the best and will used as reference. Orientation C is associated to a lower fatigue performance (- 17 % with respect to B) possibly because the applied stress is perpendicular to the layers instead of parallel as in B. The orientation A+, which is characterized by a mild stairstep effect, shows an even lower fatigue limit (- 30 % with respect to B). Finally, orientation A-, i.e. the down skin notch, shows the lowest fatigue performance (- 58 % with respect to B). This ranking appears to reasonably combine the most important effects on notch fatigue behavior of as-built AlSi10Mg alloy. As regards the defects, i.e. shrinkages and mainly gas pores, their density and average size near the notch root is highest in the A- orientation. In other orientations, the defect size and density are comparable. Moreover, the crack initiation was not observed to be directly related to the shrinkages or pores. Observation of fracture surfaces of broken specimens leads to the conclusion that initiation occurs mainly from the free surface, often at the point where the notch meets the side of the specimen. The microstructure generally influences the initiation of fatigue cracks and fatigue life. The effect of loading direction in respect to the build orientation on the fatigue strength of plain specimens was investigated by Larossa et al. (2018). The as-built specimens in vertical build orientation had slightly higher life than specimens build horizontally. Fine grain structures are generally more resistant that coarse grained structures. As regards the crack growth rate, it is highest, when the crack plane is perpendicular to the build direction, Piette et al. (2021). Heat affected zones around interlayer boundaries are the weakest link in the material Di Giovanni et al. (2019). Figs. 2 and 3 document the differences in the microstructure in regions where the fatigue cracks are initiated and where the short cracks propagate. This stage of fatigue damage represents the decisive part of the fatigue lifetime. Notch root area of A+ orientation exhibits a characteristic melt pool solidification structure, with grains elongated in the direction of the main heat flow. In the down-skin orientation A-, apart from the low surface quality, coarse grain structure can be seen in the notch root area, which can have negative consequences on the resulting fatigue resistance of given orientation. In case of orientations B and C, the microstructure in the notch root region is governed by the presence of a contour layer. Contouring results in formation of a specific grain structure, with grains elongated in the build direction. Besides the grain structure itself, the orientation of melt pool boundaries with respect to the crack propagation direction can play an important role. In Fig. 2 it can be seen, that in C orientation, melt pool boundaries of contour layer are parallel to the crack propagation direction, which can be detrimental to the fatigue properties. The EBSD maps of Fig. 3 further show that crack initiation and early propagation occurs in an anisotropic material structure, which in orientation B is a fine grained structure and in orientation C is across columnar grains. Therefore, the highest high cycle fatigue life of B oriented notch is the result of the interaction of low roughness in the root of the notch, the very fine-grained structure of the material and low residual stresses in the plane perpendicular to the crack initiation and growth. The reduced high cycle fatigue life of the C oriented notch shown in Fig. 4 cannot be caused by difference in the notch root quality, which is comparable to the B orientation. Substantial difference is, however, in the microstructure consisting of large grains allowing easier crack initiation and propagation of short