PSI - Issue 7

Ana D. Brandão et al. / Procedia Structural Integrity 7 (2017) 58–66

65

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Author name / Structural Integrity Procedia 00 (2017) 000–000

Fig. 7. SEM images of a vertically printed sample, with a 30 µm layer thickness, net shaped and vibratory polished (ID 8 of Table 1), showing an overview of the fracture surface (a)), and the defect which caused the failure during the investigation (b)). This sample survived the lowest number of cycles from ID 8. 4. Conclusions The fatigue properties of additively manufactured samples were assessed. The investigations have shown that the net shaped specimens were greatly affected by the surface finish compared to the milled specimens, even if the net shaped surface was jet blasted or vibratory polished. Parts produced without contour parameter sustained by far the highest number of cycles amongst the net shaped specimens. Still, the cycles to failure were found to be significantly lower than values of milled samples. The applied stress relieving heat treatment had a negative effect on the fatigue properties, as the yield strength was reduced to a level close to the tested stress level, potentially leading to more or earlier plastic deformation of the specimens. The application of a hardening heat treatments as described in the literature seems to be more promising. Nevertheless the milled specimens produced with 30 μm layer thickness were considered to be runouts at 2 × 10 7 cycles although no hardening heat treatment was applied. The defect analysis through X-Ray CT revealed pores and areas of lack of fusion throughout the investigated samples, with the ones produced with 30 μm layer thickness showing the lowest number of defects. Despite the presence of defects, these high quality process parameters (i.e. 30 μm layer thickness) allowed the production of parts with a fatigue life behaviour at least comparable to materials produced with conventional manufacturing methods. The fracture surface analysis has shown that the fatigue cracks initiated at the surface for all investigated net shaped specimens, contrary to the milled samples, where the cracks originated from bulk defects. The dependence of fatigue properties on surface condition and defect population, commonly observed within AlSi10Mg AM specimens, was shown in this work. Further studies need to be performed to understand the implications of the AM parameters setup in the microstructure and mechanical properties of the specimens. In addition, the performed tests strongly indicate that the contour parameter, which is widely applied throughout the metal powder bed based AM industry, can create sub-surface defects, ultimately lowering the fatigue performance of such parts. Further investigation about the contour’s influence regarding microstructure, residual stresses or porosity is recommended to determine the cause of the reduced fatigue life. References Orme, M., Gschweitl, M.; Ferrari, M.; Vernon, R., Yancey, R., Mouriaux, F.; Madera, I., 2017a. Holistic Process-Flow from Concept to Validation for Additive Manufacturing of Light-Weight, Optimized, Metallic Components Suitable for Space Flight, 58th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference. Grapevine, Texas, U.S.A., paper #AIAA 2017-1540. Wohlers Report: 3d printing and additive manufacturing state of the industry annual worldwide progress report, Wohlers Associates, Apr. 2017. Orme, M., Gschweitl, M.; Ferrari, M., Madera, I. Mouriaux, F., 2017b. Designing for Additive Manufacturing: Lightweighting Through Topology Optimization Enables Lunar Spacecraft. Journal of Mechanical Design 139, 100905-100905-6. Powder Metallurgy Review: RUAG looks to Additive Manufacturing for space satellite applications, Inovar Communications Ldt, Vol.3 No. 4 Winter, Oct. 2014. Hubscher, B., 2014. Open for business: 3-D printer creates first object in space on International Space Station. [Accessed 10 Jul. 2017] http://www.nasa.gov/content/open-for-business-3-d-printer-createsfirst-object-in-space-on-international-space-station/ Pambaguian, L., Makaya, A., Lafont, U., 2016. Utilization of In-Situ Resources and Transported Materials for Infrastructure and Hardware Manufacturing on the Moon – Ongoing Developments by ESA Materials Scientists, presented in the Annual Meeting of the Lunar Exploration Analysis Group. Columbia, U.S.A, November 1–3. Meurisse, A., Cowley, A., Cristoforetti, S., Makaya, A., Pambaguian, L., Sperl, M., 2016. Solar 3D Printing of Lunar Regolith, European Lunar Symposium, Amsterdam, The Netherlands, Abstract #023. Ghidini, T.; Pambaguian, L.; Blair, S., 2015. Joining the Third Industrial Revolution: 3D Printing for Space. European Space Agency Bulletin; European Space Agency: Noordwijk, The Netherlands; Volume 163, pp. 24–33