PSI - Issue 17

Radomila Konečná et al. / Procedia Structural Integrity 17 (2019) 138 – 145 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

139

2

produce near fully-dense parts with limited presence of internal defects. Inconel 718 is a nickel – chromium alloy extensively used in gas turbines, rocket motors, space crafts, etc. that is now successfully processed by SLM, Reed (2006), Clark et al. (2018). The microstructure of SLM Inconel 718 is substantially different from that of conventionally manufactured materials because of the typical rapid solidification. The static mechanical properties of SLM Inconel 718 are comparable if not better of their conventionally manufactured counterparts, Popovich et al. (2015). On the other hand, in the presence of fatigue loading, the as-built surfaces of SLM Inconel 718 parts are a source of weakness, Wells (2016). In general, the fatigue properties of SLM metals vary considerably according to the processing parameters because they affect the microstructure, porosity content, residual stresses and relatively rough part surfaces. Quantification and understanding of this fatigue gap is a fundamental step in SLM process qualification, Yadollahi et al. (2017). Part performance in fatigue is therefore significantly lower compared to machined counterparts. In topologically optimized SLM parts, the effect is further enhanced by many local stress concentrations at geometrical notches and cross-sectional variations, Gorelik (2017). The presence of regions of high stress concentration in actual SLM components, contribute to fatigue in ways that may not be predictable based solely on standard smooth specimens. Therefore, notched fatigue specimens should be used to evaluate the influence of stress concentrators. The recent work of Witkin et al. (2018) is noteworthy because the current lack of data on notch fatigue behavior of as-built SLM Inconel 718. Notched specimens designed with three different notch geometry were printed in both the vertical and horizontal direction, so that the notched section was oriented either in-plane (vertical specimens) or vertically (horizontal specimens). Fatigue tests were performed on samples with as-produced notch surfaces and machined notches. The results not only show that the surfaces of SLM metal parts influence the fatigue behavior in the presence of a macroscopic designed notch, but also that the final as-built notch dimensions are dependent on both the notch geometry and specimen orientation. This contribution also investigates the link between surface quality, directional material fabrication and the resulting notch fatigue behavior of SLM Inconel 718. An innovative fatigue test method using miniature notched specimens tested in cyclic plane bending is adopted instead of the thin notched plates in traction of Witkin et al. (2018). Four sets for specimens, each with a different orientation of the notch surface with respect to the build axis, were produced out of Inconel 718 powder by SLM processing and fatigue tested. Since the as-built specimens show the directional nature of the notch fatigue response, a metallographic investigation examines the link between surface quality of the notched specimens and the layer-wise fabrication of SLM process. 2. Experimental details

2.1. Material and SLM process

The material of this study is gas atomized Inconel 718 alloy powder with spherical particles in the diameter range from 15 to 45 µm. The chemical composition was determined by spectrometry and was the following:

Table 1. Chemical composition of IN 718 powder. Element Ni Cr Fe Nb Mo

Co

Ti

Al

Cu

Wt. %

51.56 17.9 18.2 5.23 3.21

0.15

1.14 2.19 0.05

A SLM Solution 280HL system (SLM Solutions, Germany) was used to manufacture four sets of fatigue specimens by service provider BEAM-IT (Fornovo Taro, Italy). A layer thickness of 50 µm was used together with a fluence F = 54.82 J/ mm 3 given by a laser power P = 250 W, hatch spacing h = 0.12 mm and scan speed v = 760 mm/s. The layer-wise powder transformation by selective laser melting was carried out in an Argon atmosphere with built plate temperature maintained at 200 °C. After SLM fabrication the post-processing steps were: heat treatment before specimen removal from the base plate and a two-step heat treatment after removal given by: i) stress relief (solution with heating to 970 °C for 1 hour followed by cooling in Argon atmosphere) ii) age hardening by double aging (heating to 710 °C and holding for 8 hours, further aged at 610 °C for 8 hours and final cooling to room temperature in Argon).