## 5 Conclusions  
This work shows that the microstructure of fully dense AM EB-PBF Ti-5553 specimens can be adjusted in order to create tailored mechanical properties. By using an STA heat treatment, very high mechanical strength and good elongation can be achieved. When a BASCA heat treatment is applied, a microstructure that results in very ductile and good mechanical strength can be achieved. Other researchers have shown [15; 42] that the distribution of the two Ti phases found in Ti-5553 has a significant influence on the mechanical properties. The correlation of the mechanical strength with the achieved microstructure shows that the shape, size and distribution of \(\alpha\) precipitations have a considerably large impact. Generally, large \(\alpha_{\mathrm{p}}\) precipitations result in an increased elongation at break, whereas small needle-shaped \(\alpha_{\mathrm{s}}\) precipitations lead to very high mechanical strength and low elongation at break. If the precipitates are very large, more \(\beta\)-matrices can be found, which leads to very ductile and good fatigue properties. One main focus of this presented work was the implementation of tensile testing using a Deben CT5000 in an industrial \(\mu\)-CT YXLON ff35 CT. For this purpose, an appropriate specimen geometry was developed. In order to evaluate, analyse and monitor the failure-critical defects of the tested parts on the tensile stage, the scan parameters were optimised. Furthermore, strategies were developed and standardised for the optimisation of the grey scale and brightness of 3D reconstructed volumes, in order to improve the subsequent analysis. If new specimens are investigated, it must be mentioned that the same specimen geometries as well as \(\mu\)-CT parameters should be used, in order to be able to compare the results. In situ tests and analyses were made for specimens processed with each heat treatment. For each sample the failure-critical defect(s) was found. Conclusively it can be said, for conventionally tested specimens, that the defects have a minor influence on the static mechanical properties. Regarding the small specimens used for the in situ tensile test, the defects can have a major influence on the mechanical properties. The main difference when comparing conventional and in situ tensile test results is that the defect-to-test-volume ratio is much worse for the in situ specimens. This effect is even worse when the defects are located at/close to the surface, and thus great stress concentration occurs. Moreover, it should be mentioned that the cuboid tensile specimen cross-section used in the in situ tensile test can also have a significant influence on the mechanical properties obtained. Furthermore, it was possible to monitor defects during every load level for BASCA heat-treated specimens. In contrast to the almost constant volume growth of the defects, it was observed that just before failure the failure-critical defect showed a major growth in the direction of the z-axis (in this setup, the direction of deformation). It has been found that this effect is used as an indicator for the failure-critical evaluation of the respective defect. It should be noted that for materials which have more ductile properties the defect monitoring is much better. Materials that fail in a ductile manner show a detectable defect growth compared to materials that fail in a brittle manner. Conclusively, it can be said that when the area of stress distribution is small, a more critical evaluation of defects regarding their influence on the mechanical strength must be made. If materials fail in a brittle manner, that effect is enhanced. For further investigations, it will be necessary to develop a new tensile specimen geometry and also change the cutting technology due to the rough surface of the specimens. Due to the AM of the specimens using the EB-PBF process, it can be assumed that the defects inherent in the process are evenly distributed in the components.