PSI - Issue 68

J. Köckritz et al. / Procedia Structural Integrity 68 (2025) 962–968

967

6

J. Köckritz et al. / Structural Integrity Procedia 00 (2025) 000–000

will likely still not achieve a good fit, as the size of defects with undercuts might be underestimated by optical roughness measurement. Computer tomography measurements as described in du Plessis and Beretta (2022) improve defect detection, but are often too costly for practical application in component manufacturing.

Fig. 4: Crack initiating defects with (a) SEM imaging of a crack initiating defect in an AB FWA, (b) size and (c) distance to highest stress concentration of the crack initiating defects for AB and TW components

Fractography revealed the defects where the crack initiation occurred, exemplary shown in Fig. 4 (a). The defects are void like, with sharp edges, several undercuts and baked-on powder particles and are located upskin (inside) and downskin (outside) alike. EDX imaging revealed 20x higher levels of oxygen than the surrounding bulk material, suggesting that an aluminium oxide skin is the most likely cause of these defects. Possibly, the required high VED for Al2319-AM during the PBF-LB/M process in combination with the thin wall, few support structures and the resulting heat accumulation led to a higher occurrence of this kind of defects throughout the whole component. When comparing defect size after Murakami (2005) and distance to the maximum stress in Fig. 4 (b), it can be observed that for the AB components smaller defects in close proximity to the maximum stress concentration led to failure. In contrast, for the TW components, the crack initiated at larger defects further from the highest stress areas. This observation explains both the greater scatter and the higher fatigue exponent for TW in Fig. 3, because widely different local stresses have been acting at the crack initiating defects. Finally, both AB and TW show similar fatigue lives overall, because smaller local stresses at the defects of TW were counteracted by the larger defects acting as stress raisers and Al2139-AM likely having a high defect sensitivity tied to its low fracture elongation, see Table 1.

Fig. 5.: Crack paths and crack initiating defect location over main stress distribution for as built (AB) and trowalised (TW) components

For the AB components, small defects around the lower component edge causing crack initiation very close to the stress concentration at the lower component edge can sometimes be observed, see Fig. 5. Apparently, the TW did remove the smaller defects process-related predominantly at the edges of the component, where the abrasive particles slide and grind along with more prevalence and force. Only larger defects remained, which, distributed statistically, were not always located in the area of highest stress. If the component did not include large defects, for example through careful print parameter optimization or choosing a less challenging alloy, the TW might be more effective.