## 3 Results and Discussion
### Manufacturing Defects and Their Impact on Fatigue Crack Propagation Behavior  
Parallel to the vessel examinations of Section 3.1, we investigated the fatigue crack growth behavior of conventionally and additively manufactured specimen material. Manufacturing defects were identified and the possible effects on crack propagation behavior were analyzed. The distribution, number density, and morphology of pores depending on build direction were investigated. Thereby, the lowest amount of circular pores (maximum diameter: 80 \(\upmu\)m) was observed in the ZX specimen (Figure 8c), which furthermore exhibited the lowest da/dN (Figure 7). In the ZY specimen (Figure 8b), circular and larger, elongated pores (aspect ratio \(\approx\)1.3-10) were observed, while in the XY (Figure 8a) sample, predominantly small, circular pores (maximum diameter: 60 \(\upmu\)m) occurred.  
Nevertheless, several larger and elongated pores were found. Both ZY and XY specimens exhibited higher da/dN than the ZX sample (Figure 7). Accordingly, the measured crack length increased considerably faster with increasing number of load cycles (Figure 15).  
However, because both the curves of the ZY and XY specimens increased monotonically, it cannot be concluded that the pores were decisive for the higher crack propagation rate when compared to the  
Figure 14: Crack branching at oblonged pores (aspect ratio range of 1.3 to 2.8) in the additively manufactured 316L in the ZY specimen.  
Figure 15: Crack length based on number of loading cycles of additively manufactured material with regard to build direction.  
almost pore-free ZX sample. Furthermore, a tendency of crack branching at pores of an aspect ratio of \(\sim\)1.3-2.8 (Figure 14) was found in the ZY specimen. Material characterization thus confirmed the results of computational analysis of small pores, with a limited effect on crack growth (nevertheless, pores can act as crack initiators) [20].  
Crack branching generally leads to a decrease in the crack propagation rate--a further indication that pores may not constitute the decisive factor on differences in crack propagation rates.  
Further processing-related issues, such as finely distributed SiO\({}_{2}\) particles, may constitute another influencing factor for crack propagation. These were observed in all LPBF-manufactured specimens (Figure 16). On the other hand, such process-related, small, and finely distributed particles may have a strengthening effect, which was not further addressed in this study.  
Moreover, traces of segregation (Figure 17a) were found. EDX analyses (Figure 17b) in the vicinity of the cracks confirmed enrichment in Ni and Mo along with depletion in Fe and Mn contents. The concentrations of the alloying elements Ni and Mo had a direct impact on solid solution hardening (and carbide precipitation) and consequently the strength of the material, which directly influenced resistance to crack growth and therefore crack propagation behavior. A plausible reason for segregation would be excessive cooling rate, resulting in non-equilibrium solidification in the LPBF process. Process optimization towards avoidance of segregation effects could offer further potential for the improvement of crack propagation resistance.  
Figure 16: Finely distributed SiO2 particles in the additively manufactured 316L specimens indicated by white arrows.