In this work, the Ni-based super alloy Inconel 718 manufactured via electron beam melting is investigated. Typical microstructure of Inconel 718, which was processed by electron beam melting, consists of columnar oriented dendritic structure with strong texture along building direction in hatch region, whereas microstructure differs in contour region, which is supposed to influence mechanical properties in the as-built state. As highly complex geometries are possible to manufacture with additive manufacturing techniques, the influence of miniaturization and surface roughness on microstructural and mechanical properties has to be understood in detail. Therefore, samples were processed with different initial sizes and subsequently tested in as-built and polished condition. Before performing mechanical tests, process-induced microstructure was determined by scanning electron microscope as well as distribution of defects and geometrical deviations by microfocused computed tomography. To characterize the mechanical properties, different testing methods, both tensile and fatigue tests, were carried out. Present investigations show almost similar microstructures in large-scale and small-scale Inconel 718 volumes. However, small-scale volumes show higher number of defects in the form of surface and near-surface defects. Furthermore, as-built specimens show geometrical deviations when compared to initial CAD diameter, which makes the implementation of an average equivalent diameter mandatory. It can be demonstrated that small-scale as-built volumes have reduced mechanical properties, whereby ultimate tensile strength is reduced by 60% and fatigue strength is reduced by 75%, showing that increased defect density and as-built surface roughness have a higher impact on fatigue properties and are the dominating reason for early failure in the as-built state due to multiple crack initiation.
Additive manufacturing Electron beam melting Ni-based superalloy Fatigue behavior Fatigue life estimation
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The authors would like to thank Thomas Niendorf and Tizian Arold (University of Kassel) for providing the investigated material. Further on, the authors thank the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft) for its financial support within the research project “Damage tolerance evaluation of electron beam melted cellular structures by advanced characterization techniques” (NI 1327/13-1, WA 1672/32-1).
Sochalski-Kolbus LM, Payzant EA, Cornwell PA, Watkins TR, Babu SS, Dehoff RR, Lorenz M, Ovchinnikova O, Duty C (2015) Comparison of residual stresses in Inconel 718 simple parts made by electron beam melting and direct laser metal sintering. Metall Mater Trans A 46:1419–1432. https://doi.org/10.1007/s11661-014-2722-2CrossRefGoogle Scholar