PSI - Issue 43

Jiří Man et al. / Procedia Structural Integrity 43 (2023) 203 – 208 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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Brno while horizontal 316L-TUK specimens manufactured by turning from preprinted bars were fatigued under load-control mode at TU Kaiserslautern. Two microscopic techniques have been adopted to assess the microstructure corresponding to the virgin state and microstructural changes generated in the bulk of both SLMed 316L steels by cyclic straining. A SEM (Tescan LYRA 3 XMU SEM) equipped with electron backscattered diffraction (EBSD) and focused ion beam (FIB) technique was used for characterization of microstructure, grain morphology and crystallographic texture. For this purpose, the gauge parts of fatigued specimens were sectioned longitudinally, i.e. parallel to the stress axis and the cuts were mechanically and electrolytically polished. To visualize characteristic fine features of solidification SLM microstructure and to reveal DIM a special colour etching technique (modified Beraha II) was applied after electropolishing. High-resolution characterization of the dislocation arrangements and DIM was performed using transmission electron microscopy (TEM) with a JEOL JEM-2100F operating at 200 kV. Thin foils were prepared either by standard twin jet electropolishing technique or locally using FIB lift-out technique. The volume fraction of DIM was assessed quantitatively by a ferritescope (Fischer Feritscope MP30). 3. Results and discussion Irrespective of the different SLM building conditions the microstructural characterization of both SLMed 316L steels showed, generally, very similar microstructural features typical for as built SLM state (Kong et al. (2021)). Since the results of detail characterization of the virgin microstructure of SLMed 316L-IPM steel has been reported recently elsewhere (Man et al. (2022)), in the present work thus only the characteristic microstructure features of the 316L-TUK steel as revealed at different scales by various microscopic techniques are presented – see Fig. 1. The EBSD orientation maps presented in inverse pole figure (IPF) color code with respect to the x, y and z axis reveal irregularly shaped grains which are often considerably elongated in the building direction and thus running trough Fig. 1. Characteristic features of virgin, as-built microstructure of SLMed 316L-TUK steel. (a – c) EBSD orientation maps in inverse pole figure (IPF) coloring and corresponding IPFs with respect to the three directions x, y and z (see the schematic drawing), (d) solidification microstructure as revealed by colour etching (SEM) and (e) SLM process-induced dislocation arrangement reflecting solidification cells (cf. the schematic drawing in Fig. 1d) (BF-TEM). S.A. = stress axis, BD = building direction, MPB = melt pool boundary.