PSI - Issue 43

Jiří Man et al. / Procedia Structural Integrity 43 (2023) 203 – 208

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Author name / Structural Integrity Procedia 00 (2022) 000 – 000

Table 2. Volume fraction of DIM in Fe-% as measured by a Feritscope at various locations in fatigued specimens of SLMed 316L steels. Steel sign. Fatigue test condition Sp ec imen ga u ge p a r t † Fracture surface† Surface‡ Longitudinal cross- section§ 316L-IPM  a = 0.4% 0.10 – 0.13 (0.12) 0.00 – 0.10 (0.06) 0.24 – 0.41 (0.33)  a = 0.5% 0.00 – 0.18 (0.04) 0.00 – 0.05 (0.02) 0.28 – 0.52 (0.38)  a = 0.8% 0.00 – 0.15 (0.05) 0.00 – 0.06 (0.03) 0.27 – 0.47 (0.37) 316L-TUK  a = 360 MPa 0.13 – 0.23 (0.17) 0.12 – 0.15 (0.14) 0.30 – 0.37 (0.31)  a = 440 MPa 0.09 – 0.12 (0.11) 0.09 – 0.12 (0.10) 0.17 – 0.21 (0.19) † Value in round brackets represents the average value from at least five measurements. ‡ Specimen surface states of are different in SLMed steels – 316L-IPM: rough as built surface, 316L-TUK: machined and polished surface. § Surface of longitudinal cross -section was polished mechanically and then electrolytically.

several melt pool boundaries (cf. Figs 1a1 – c1 and 1d). In addition, individual grains are often subdivided into sub grains bounded by the low-angle grain boundary (LAGB) with the misorientation angle of several degrees (see the presence of orientation gradients within numerous individual grains in Fig. 1a – c). The 316L-TUK steel exhibits similarly to 316L-IPM steel (Man et al. (2022)) a weak-to-moderate <101> texture along the building direction (Fig. 1c2) while this steel is nearly texture free with respect to the loading direction – see Fig. 1b2. The most unique feature inherent to SLM process represents the fine cellular solidification structure corresponding to the ultrafast solidification (cooling rates ~10 6 K/s (Kong et al. (2021))) under non-equilibrium conditions. The solidification cells, resembling in 3D honeycomb arrangement (see the inset in Fig. 1d), are generally accompanied by interdendritic micro-segregation which is overlapped by dislocation microstructure originating from constraints surrounding the melt pool and thermal cycling during SLM process (Bertsch et al. (2020), Voisin (2021)). Both features are clearly apparent from Figs 1d and 1e. Note that due to 3D character of cellular structure this structure in 2D representation may appear as equiaxed or elongated cells and/or columnar features. The average cell size in individual grains as evaluated from both SEM and TEM micrographs yielded the similar values in the range of 400 – 700 nm for both SLMed 316L steels are consistent with other studies (Bertsch et al. (2020), Godec et al. (2020), Voisin (2021), Wang et al. (2021)). The cyclic stress-response in the form of cyclic hardening/softening curves is shown for both SLMed 316L steels in Fig. 2. Cyclic straining of 316L-IPM steel with three constant total strain amplitudes of 0.4%, 0.5% and 0.8% (see Fig. 2a,b) resulted in a very short initial mild hardening followed by permanent cyclic softening until the end of fatigue life the intensity of which decreased with decreasing applied total strain amplitude. The true origin of the observed cyclic softening, namely the cyclic strain localization into PSBs (persistent slip band) has been evidenced recently by Man et al. (2022). The strain response of 316L-TUK steel fatigued under stress-control was dependent on the applied stress amplitude – see Fig. 2c. While the cyclic loading with stress amplitude of 440 MPa resulted in permanent cyclic softening, an initial period of mild cyclic softening followed by a stable cyclic deformation response for the majority of fatigue life is apparent for lower applied stress amplitude of 360 MPa. (c) Fig. 2. Cyclic hardening/softening curves of SLMed 316L steels produced using two different AM systems and fatigued under (a,b) total strain- and (c) load-control. (a) (b)