## 3 Mechanical Properties  
Characterization of DED parts in terms of mechanical properties plays a relevant role in the exploitation of this innovative process in industrial applications of complex shaped and high performing metallic components. Tensile, hardness, fracture toughness, creep, and fatigue are the common tests performed to evaluate the mechanical characteristics of DED components. Since a huge number of failures is related to the cyclic fracture or excessive deformation, tensile and fatigue tests are widely employed to characterize the behavior of DED parts. In general, owing to high cooling rates and consequently finer microstructures, DED components are characterized by tensile properties and hardness equal or higher than the components produced by conventional methods like casting. As highlighted in Table 4 for annealed Ti-6 Al-4V components produced by DED and conventional processes, the ultimate tensile (UTS) and yield (YS) strengths of DED parts is higher than the conventionally processed ones [42]. Although, in the case of DED parts the elongation at break (\(\varepsilon\)) is lower than that of wrought materials, because of the presence of microporosity and oxide inclusions inside the component [41; 42]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Ultimate Tensile Strength (MPa)** & **Yield Strength (MPa)** & \(\varepsilon\) **(\%)** \\ \hline DLD & 931 & 855 & 10 \\ Wrought & 900 & 850 & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison between the tensile properties of Ti-6 Al-4V produced by DED and conventional processes [41; 42; 102; 103]. Figure 12: Different deposition patterns: (**a**) raster, (**b**) bi-directional, (**c**) offset-out, and (**d**) fractal. Several studies have shown that the tensile properties of a component strongly depend on the orientation of the building of the component [18; 85; 104]. For instance, [PERSON] et al. produced tensile samples in different directions (Figure 13), building in the direction along the length of the tensile sample (X-direction) or depositing perpendicular to the length of tensile samples (Y- or Z-direction) [18]. They observed that the tensile strength of samples produced in the X-direction is higher than those fabricated in other directions, as underlined by the comparison of data listed in Table 5.  
This anisotropic behavior of DED Ti-6 Al-4V is attributed to the presence of different interfacial layers in different directions, implying the easy formation of shear bands during the testing of the corresponding samples [65; 105]. These results provide further support for the hypothesis that various cooling rates in two different directions lead to the formation of different microstructures and consequently different mechanical properties. For example, in the X-direction the time between consecutive layers is longer than in other directions because of the presence of a long deposition path that results in higher cooling rates and consequently a finer microstructure and higher tensile strength [31]. Figure 13: Schematic view of DED Ti-6 Al-4V specimens, (**a**) as-deposited surface in the X-direction (horizontally oriented), (**b**) as-deposited surface in the Y-direction (latrally oriented), and (**c**) as-deposited surface in the Z-direction (vertically oriented) (reproduced with permission from [18] Copyright Elsevier, 2015). \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **UTS (MPa)** & **YS (MPa)** & \(\epsilon\) **(\%)** \\ \hline X-direction & 1073 & 973 & 10 \\ Y-direction & 1065 & 941 & 11.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison between the tensile properties of DED Ti-6 Al-4V in different directions [33]. The effect of post treatment on the tensile properties of Ti-6 Al-4V parts was investigated by [PERSON] et al. [65]. They demonstrated that by HIPing the components at 900 \({}^{\circ}\)C and 100 MPa for 2 h it would be possible to densify the DED components and improve the ductility and elongation at break of the part.