## 3 Mechanical Properties
Indeed, the growth of alpha laths at high temperature during HIP implies a decrease in the tensile strength of the part and an increase in its elongation. By understanding the effect of the thermal history on microstructure and mechanical properties of the DED samples, it is possible to limit their anisotropy, for instance by using different scanning strategies. In fact, [PERSON] and [PERSON] measured the tensile properties of Ti-6 Al-4V samples produced by using 90\({}^{\circ}\) rotation between the layers. Under these conditions, the tensile properties of DED components in the X- and Y-direction are almost equal (Figure 14) [65]. [PERSON] et al. used gas atomized pre-alloyed Ti-6 Al-4V powder. In order to evaluate the effect of various building systems on microstructure evolution during the DED process a low power Nd:YAG based system and a high power Co\({}_{2}\) system was used. The influence of power and scan speed on the microstructure and consequently the mechanical properties of the DED component was undertaken by employing \(\pm\)20% variation from the manufacturer's recommended setting. On the basis of these results, the anisotropy in mechanical properties of DED components may be attributed to the columnar grain morphology and also to the limited grain growth that generally takes place in the direction perpendicular to the columnar grain [65]. Furthermore, [PERSON] et al. demonstrated that UTS and YS of DED Ti-6 Al-4V samples which are built at the optimum parameters (1360 W as laser power, 800 mm/min as a laser scanning speed, 7 g/min as a powder feeding rate and 0.84 mm as a Z step) are high while their elongations are low (Table 6). Indeed in this work, a 6.5-axis TRUMPF blown powder system with a 4 kW disc laser were used to deposit the parts and the gas atomized powder was delivered into the melt pool by means of a coaxial nozzle. The fine microstructure and residual stress remained within the specimen were indicated as the main factors responsible for the strength of the DED samples. In addition, the evaluation of the fracture surfaces suggested that the main source of failure is the lack of fusion pores which are very harmful for mechanical properties [88]. The fracture surface of as-DED samples built in the vertical direction is completely different from that of the horizontal specimens (Figure 15). In the vertical direction it is possible to recognize a few \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Condition** & **YS (MPa)** & **UTS (MPa)** & \(\varepsilon\) **(\%)** \\ \hline As-DED Vertical & 950 & 1025 & 5 \\ As-DED Horizontal & 950 & 1025 & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Tensile properties of as-DED Ti-6 Al-4V samples [88]. Figure 14: Tensile properties of DEDed Ti-6 Al-4V in different building orientations after heat treatment and HIP process (reproduced with permission from [18] Copyright Elsevier, 2015). large opened-up voids with smooth surfaces that consist of some un-melted or partially melted powder particles. This type of fracture surface implies that the fractured layer was not completely remelted during the consecutive deposition. On the other hand, the fracture surface of horizontal samples consists of closed pores or seams that are typical features of fracture surfaces. It was concluded that this significant discrepancy in the fracture surfaces could be attributed to the relation between the direction of loading during the tensile tests and the orientation of pores usually placed at the inter layer boundaries. Several studies have shown that, in the case of DEDed horizontal samples, since loading is nearly parallel to the flat pores, the pores are sealed and closed during the loading whereas, in the case of vertical ones, the pores start to be torn apart along the loading direction [88; 104].