## 4 Conclusions
### Summary and Outlook  
Additively manufactured materials and components usually have manufacturing defects. For this reason, development aims to reduce the number of defects, pores in particular, in components by optimizing the process parameters and to improve pore detection, especially in critical design areas. Moreover, it is necessary to develop or to adapt existing standards for safe application of additively manufactured components. Approval procedures are still hampered by the problems of standardizing the manufacturing process. The results of this study demonstrate that these challenges can be solved  
Figure 28: Calculated growth of a postulated 2 mm deep and 40 mm wide surface crack under cyclic loading (f = 20 Hz, R = 0.1, T = 650 \({}^{\circ}\)C) in various materials (ASME curves for comparison) at elevated temperatures. Figure 27: Calculated growth of a postulated 2 mm deep and 40 mm wide surface crack under cyclic loading (f = 20 Hz, R = 0.1) in various materials (ASME curves for comparison) at ambient temperature. by different approaches. Pores can act as notches in critical design areas and therefore lead to crack initiation, but crack growth is subsequently retarded because the crack must be re-initiated [20]. In sum, this can lead to slower crack growth. Hence, it is recommended to include the fracture mechanics-based damage tolerance concept already in the design and manufacturing phase for strength and durability assessment. Porosity and fracture mechanics-based damage tolerance analysis--two aspects usually considered separately--should be interlinked. The key factor for residual lifetime and in-service inspection interval estimation within a fracture mechanics-based damage tolerance concept is viable crack propagation data. For this purpose, state of the art additively manufactured 316L material was investigated. From commercially available powder, AM-processed 316L provides increased cracking resistance in comparison to conventionally produced material. However, in additively manufactured material, crack propagation is initiated at lower stress intensity than in conventionally produced material. This can be attributed to the fact that small pores in particular can favor the initiation of crack propagation. Grain orientation, depending on build direction and the resulting preferred slip systems, were identified as being responsible for non-horizontal or horizontal cracking. Weak connection of the individual melt pools was eliminated as a possible cause because all the cracks propagated transversatlline, regardless of build direction. Processing defects such as pores, finely distributed SiO\({}_{2}\) particles, and segregation of alloying elements were identified. Different pore distributions, geometries, and sizes were observed on the basis of on build direction. A tendency to crack branching at pores of aspect ratios ranging from 1.3 to 2.8 was observed in the ZY specimen. Evaluation of the experimental results did not indicate a significant impact of small pores on crack propagation behavior. This was congruent to computational analyses [20]. The very small and finely dispersed SiO\({}_{2}\) particles, observed in all the additively manufactured 316L specimens, did not unfold recognizable, negative effects. Segregation of alloying elements may have an indirect impact on crack propagation behavior. Areas, enriched in Ni and Mo, as well as others, depleted in Fe and Mn, were identified. Ni and Mo influenced solid solution hardening and thus the local strength of the material. Strength, in turn, had a direct impact on resistance to crack growth and consequently on crack propagation behavior. High cooling rates, leading to non-equilibrium solidification, were thought to be responsible for the segregation phenomena. 316L, additively manufactured, utilizing process parameters according to the current state of technology, demonstrated significantly improved crack propagation behavior over conventionally produced material. Nonetheless, this study showed a variety of approaches for further improvement.