PSI - Issue 34

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

140

6

compressive. Recently, Beretta and al. (2020) measured residual stresses in specimens of similar geometry and orientation and produced with the same equipment and process parameters. They reported positive residual stress  rs = 167 MPa +/- 20 MPa for the Type C geometry and very low residual stresses (-14 MPa +/- 9.4 MPa) in Type B specimens. These important results suggested the development of a process simulation study of the directional fabrication of the miniature specimens. A thorough investigation was performed using the inherent strain method as implemented in Amphion upon process calibration with ad-hoc artifact fabrication (Moroni 2020). The main simulation results in terms of longitudinal residual stresses were as follows: Type A-  rs = 10 MPa; Type B  rs = 45 MPa and Type C  rs = 200 MPa. Therefore, in all cases the residual stresses in the directional specimens are tensile, although very different in magnitude. The correlation with the limited experimental evidence of Beretta et al. (2020) is good and Type C orientation is confirmed to be associated to high tensile residual stresses. Therefore, the directional behavior of as built L-PBF AlSi10Mg is explained as due to the combined effect of residual stresses and surface roughness that especially affects negatively the vertical direction. 4. Conclusions Three sets of specimens of L-PBF AlSi10Mg oriented in different directions with respect to build direction were fabricated in an industrial AM system and tested in the as-built condition. They showed significant directional effect of the fatigue response. The quality of the as-built surfaces in the different specimens was characterized in terms of surface roughness and near surface microstructure on etched metallographic sections demonstrating important structural differences among them. Surface roughness alone did not adequately explain the directional fatigue behavior. Experimental evidence and process simulation of specimen L-PBF fabrication revealed the directional nature of residual stresses in the as-built specimens. Specifically, the vertical direction of specimens involves a high tensile residual stress that negatively affects fatigue crack initiation and considerably lowers the directional fatigue strength. Therefore, fatigue testing of vertical specimens in as-built state may significantly underestimate the fatigue performance of L-PBF AlSi10Mg. Acknowledgements The company BEAM-IT, Fornovo Taro, Italy is gratefully acknowledged for specimen production and the VEGA grant agency for the support by the grant No. 1/0463/19. References Baek M-S et al., 2021. Influence of heat treatment on the high-cycle fatigue properties and fatigue damage mechanism of selective laser melted AlSi10Mg alloy, Materials Science and Engineering: A 819, 141486. Beretta S. et al., 2020. Fatigue strength assessment of “as built” AlSi10Mg manufactured by SLM with different build orientations. International Journal of Fatigue 139, 105737. Brandl E, Heckenberger U, Holzinger V, Buchbinder D., 2012. Additive manufactured AlSi10Mg samples using Selective Laser Melting (SLM): microstructure, high cycle fatigue, and fracture behavior. Mater Des; 34:159–69. https://doi.org/10. 1016/j.matdes.2011.07.067. Mower TM, Long MJ., 2016. Mechanical behavior of additive manufactured, powder-bed laser-fused materials. Mater Sci Eng A 651:198–213. https://doi.org/10.1016/j.msea.2015.10.068. Moroni M. 2020. M.S. Thesis in Mechanical Engineering, University of Parma. Nicoletto G., 2017. Anisotropic high cycle fatigue behavior of Ti-6Al-4V obtained by powder bed laser fusion. International Journal of Fatigue, 94, 255-262. Uzan NE, Shneck R, Yeheskel O, Frage N., 2017. Fatigue of AlSi10Mg specimens fabricated by additive manufacturing selective laser melting (AM-SLM). Mater Sci Eng A 704:229–37. Yadollahi A., Shamsaei N., 2017. Additive manufacturing of fatigue resistant materials: Challenges and opportunities, International Journal of Fatigue, 98, 14-31.