PSI - Issue 52

Ivo Šulák et al. / Procedia Structural Integrity 52 (2024) 143–153 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

147

5

mechanism of fatigue crack propagation at different temperatures. At temperatures up to 550 °C, fatigue crack propagation is transgranular via a striation mechanism (Fig. 4a-d and Fig. 5a-d). In this case, the finer-grained structure typical of LPBF Alloy 400 is preferable for the mechanical properties as there is a significant increase in strength characteristics (Armstrong, 1970). Fine-grained materials generally exhibit considerably better fatigue properties (Fintová et al., 2021, 2020; Fintová and Kunz, 2015) . In this case, however, the positive effect of grain size is counterbalanced by the detrimental impacts of LPBF defects like keyholes, gas porosity of lack of fusion (Du Plessis et al., 2020), as can be seen from fracture surfaces in Fig. 5. The fatigue crack propagation mechanism changes with increasing temperature. At 650 °C and 750 °C, fatigue crack propagates predominantly along grain boundaries (intergranularly), which leads to faster fatigue crack propagation and thus to a reduction in fatigue life (Eckmann and Schweizer, 2017; Šulák et al., 2022) . Since the number of grain boundaries is inversely proportional to the grain size, it can be assumed, that the fatigue crack growth rate is higher for fine-grained material (LPBF Alloy 400) than for coarse-grained material (bulk) (Fintová et al., 2017) . Therefore, in conjunction with a number of LPBF defects, a significant decrease in the lifetime of LPBF Alloy 400 can be observed with increasing temperature. The association between fatigue crack propagation direction and printing strategy (orientation of keyhole defects) was not observed.

Fig. 3. S-N curves of bulk and LPBF alloy 400 tested in the temperature range of 400- 750 °C (full symbols and lines – bulk; empty symbols and dash lines – LPBF).

Fig. 4. Representative fracture surface of bulk Alloy 400 fatigued at a), b) 400 °C; c), d) 550 °C; e), f) 650 °C; g), h) 750 °C.