PSI - Issue 38

Michaela Zeißig et al. / Procedia Structural Integrity 38 (2022) 60–69 Zeißig, Jablonski / Structural Integrity Procedia 00 (2021) 000–000

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1. Introduction Additive manufacturing (AM) processes such as the laser powder bed fusion (LPBF) process of selective laser melting (SLM) are already applied across various industries. The near net-shape geometries are produced via a repetitive process where a layer of metal powder is melted by a laser, a new powder layer is applied and again melted according to the previously defined geometry. More detailed descriptions of the process and its steps may be found e.g. in Zhang et al. (2020). Apart from advantages in the field of small-scale series production, AM processes also offer great geometrical freedom which enables the realization of complex geometries not executable with conventional manufacturing techniques such as cooling channels as described in Snyder et al. (2015). However, due to the multi physics and multi scale nature of the process combined with a multitude of adjustable process parameters, prediction of the resulting material and specimen properties is a complicated task, especially as a simulation from the stochastic powder bed to final part in full detail is not feasible with the current computational power available, see Zhang et al. (2020). While the experimental data base for static properties of various materials used for AM is growing and incorporates the influence of building parameters such as building direction, layer height or scanning strategy, the equivalent experimental data for fatigue properties is not as extensive. A closer look on the main fatigue influencing factors in AM materials takes place in the next section. This information is then used for the choice and application of different fatigue models which are described and applied in section 4. A discussion and comparison of the results is given in section 5. 2. High cycle fatigue failure in AM specimens Comparing the main material-related driving parameters of fatigue, as given e.g. in Radaj and Vormwald (2007), with the characteristics of LPBF specimens and the experimentally identified origins of fatigue failure, four main aspects can be identified, as was e.g. stated by Edwards and Ramulu (2014) for Ti-6-Al-4V. They attribute the inferior fatigue performance to the high surface roughness of the unprocessed state, the so-called as-built state, to the porosity, both internal and sub-surface, to the tensile residual stresses and to the microstructure. Porosity, other defects and the surface roughness may be summarized under the term stress raisers. Surface roughness values for 316L are reported in the range of Ra approximately 5 to 25 µm depending on the manufacturing parameters by Wang et al. (2016). Furthermore, pores and other defects are found in the sub-surface region and in the volumetric part of the specimen, although the overall volumetric porosity is usually well below 0.5% as shown by Wang et al. (2016). While pores are rather spherical in shape and thus have limited stress concentration factors (SCF), defects can take on almost crack-like shapes and therefore may have much larger SCFs. In Nadot et al. (2020), defects in AlSi10Mg smaller than 100 µm are identified to be rather spherical in shape, while larger defects become significantly less spherical. In the context of stress raisers, the stress gradient, determining the highly-stressed volume, and the stress triaxiality, governing the formation and coalition of pores, are also of importance. The residual stresses form during the building process due to repeated rapid heating and cooling and thus melting and solidification process, as described e.g. in Mercelis and Kruth (2006). This leads to distortion and tensile residual stresses which are detrimental to the fatigue life. As described by Thijs (2014), the microstructure is a result of the building process with its high thermal gradients. Vrancken (2016) stated that different microstructure types are developed depending on the material. For example, for Ti-6-Al-4V a columnar microstructure is obtained after the SLM process, while for most materials, including 316L, the microstructure type is cellular. As the origin of all these characteristics is found in the SLM process itself and the chosen process parameters, this leads to a large range of possible final material properties. Moreover, some authors report anisotropic stiffness and