PSI - Issue 18

Matilde Scurria et al. / Procedia Structural Integrity 18 (2019) 586–593 Matilde Scurria, Benjamin Möller, Rainer Wagener, Thilo Bein/ Structural Integrity Procedia 00 (2019) 000–000

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Keywords: Fatigue, additive manufacturing, Laser Powder Bed Fusion, Inconel ® 718, Inremental Step Test, heat treatment, cyclic behavior, cyclic stress-strain curve, anisotropy

1. Introduction The high-strength, thermally resistant Nickel-based alloy Inconel ® 718 is widely used in the aircraft industry due to its excellent properties in terms of corrosion resistance, fatigue and weldability, combined with a high strength even at elevated temperatures. However, the machinability of this alloy is limited and challenging due to an excessive tool wear and low material removal rates caused by its extreme characteristics of toughness and work hardening ( Rahman et al. (1997)). While the research regarding the manufacture of Inconel ® 718 by means of conventional machining methods is still ongoing, the idea of using non-conventional additive manufacturing (AM) processes for the production of net-shape parts is catching on and therefore the research in this field. One of the additive manufacturing processes that enable the production of net-shape parts from metal powder is Selective Laser Melting (SLM). Selective Laser Melting (SLM) is a specific 3D printing technique, which utilises a high power density laser to fully melt and fuse metallic powders to produce net-shape parts with high density (up to 99.9% relative density). This technology was developed at the Fraunhofer ILT by Meiners (1999) and is also known as Direct Metal Laser Sintering (DMLS). To put a point to these different terms, SLM and DMLS processes have been termed “powder bed fusion” by ASTM International (2012) and will be indicated as Laser Powder Bed Fusion (LPBF) in this work. The static properties of LPBF Inconel ® 718 subjected to different heat treatments, with particular attention to the microstructure, have already been described, for example by Wang et al. (2012) and Deng et al. (2018). However, the cyclic material properties, as a result of the combination of the AM process together with different build directions and subsequent heat treatments, have not yet been investigated. In this work, the cyclic stress-strain behavior of LPBF-processed Inconel ® 718 powder has been investigated by means of variable amplitude strain-controlled fatigue tests called Incremental Step Tests (IST) (see Morrow (1965) and Landgraf et al. (1969)). To state the initial conditions, specimens in different orientations with respect to the powder bed are manufactured using standard process parameters. Adapted support structures, when necessary, are used and subsequently removed before the testing phase. Small-scale flat specimens with a length of 50 mm (Figure 1a), are manufactured by Selective Laser Melting (SLM) of Inconel ® 718, using an EOS M290 system with the EOS standard set of parameters IN718_Performance 1.0 with 40 µm layer thickness. To evaluate the effect of different build orientations, the specimens have been produced with an angle between the down-skin surface and powder bed θ of 90° and 45°, as represented in Figure 1b. Support structures are normally required for surfaces with down-skin angles θ of less than 45° with respect to the build platform, according to the VDI guideline (2015). After removal of the support structures, the surface is irregular and presents several defects, such as pores and geometric notches. Surfaces, previously in contact with these multi-point joints, show a higher value of the roughness R z up to 120 µm (XZ specimens), while, for AM surfaces without support structures, typical values of the roughness R z range from 20 µm to 25 µm. Figure 1c shows an area of contact between the specimen and the support structures etched with V2A etchant and analysed under the light microscope, where several pores can be found. Furthermore, support structures can affect the cooling rate inside the part (see Mishurova et al. (2018)), leading to different microstructures and residual stresses. Regarding this, some studies have already shown how this effect can be reduced by optimisation of the support structures in terms of distance between the joints and contact surface (see Calignano (2014) and Poyraz et al. (2015)). After being manufactured, the specimens are divided into three groups, ‘A’, ‘B’ and ‘C’, depending on the different heat treatments, to which they are subjected. The ‘A’ specimens are heated slowly, for around 9h and through two stages (at 250 °C and 500 °C), up to 650 °C. Then the specimens are kept under isothermal conditions for 2h, cooled to 250 ° C and held for 4h and then cooled again to room temperature, by rapid gas cooling. The ‘B’ specimens are heated to 760°C and kept at this temperature for 2h, then cooled in air. Finally, the ‘C’ specimens are first heated to 2. Experimental campaign 2.1. Material and methods