## 3 Mechanical Properties  
Mechanical properties can be considered one of the main indicators of the quality of an AM process. Hardness and tensile properties are, in fact, often used as key performance indicators (KPI) of AM components. From an industrial point of view, indeed, tensile samples are generally built together with AM components in order to validate the building process. Because of these reasons, mechanical properties of AM materials have been deeply investigated, and several tensile and hardness analyses of as-built DED AISI 316L samples obtained using different building or post-processing conditions are available in the literature. Table 4 reports mechanical properties (i.e., Vickers hardness, Hv; yield strength, YS; ultimate strength, US; elongation, (\(\epsilon\))) of DED AISI 316L steel samples produced in different conditions (i.e., with several power (P) and scan speed (V) values, built along a perpendicular (V) or parallel (H) direction with respect to the building platform, with new powder or re-used powder, with Ar or N\({}_{2}\) shielding gas (SG) or a build chamber (BC) as protective atmosphere). These are compared with the tensile properties of AISI 316L samples obtained by conventional technologies (CT). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & **P** & **V** & **Direction** & **Gas** & **Hv** & **YS** & **US** & **(MPa)** & **\(\epsilon\) (\%)** & **Hc** & **Ref. ** \\ \hline \multirow{3}{*}{CT} & \multicolumn{4}{c}{Hot rolled} & \multicolumn{2}{c}{-} & 360 & 625 & 69 & 0.74 & [81] \\ \cline{2-10}  & \multicolumn{4}{c}{Cast} & \multicolumn{2}{c}{170} & 310 & 620 & 45 & 1.00 & [82] \\ \hline \multirow{6}{*}{Building parameters} & 1600 & 28 & - & Ar BC & 250 & 430 & 650 & 43 & 0.51 & [49] \\  & 3400 & 10 & - & Ar BC & 210 & 370 & 590 & 36 & 0.59 & [49] \\  & 4600 & 5 & - & Ar BC & 190 & 300 & 560 &  
From the comparison of the data, it is evident that the tensile strength of DED AISI 316L samples is generally higher than that of conventionally manufactured steels. The reason for these peculiar mechanical properties of as-built AISI 316L parts can be found in their unique microstructure. The main factors that allow the achievements of high mechanical properties are the reduced grains and dendrite size, the presence of residual \(\delta\) ferrite, and the presence of a dense dislocation network. These factors can also explain the low ductility values of the deposited parts. As previously discussed, these microstructural features are strictly correlated to the high cooling rate which the material undergoes while being processed. The strengthening effect of the refined microstructure can be correlated to the well-known Hall-Petch equation that associates the material grain size and the YS as follows:  
\[YS=YS_{0}+\frac{k}{\sqrt{d}} \tag{1}\]  
where \(YS_{0}\) is the frictional stress resisting the motion of gliding dislocations in the absence of grain boundaries, \(d\) is the grain size, and \(k\) is a material constant. [PERSON] et al. suggested that for AM AISI 316L parts, there is a Hall-Petch type strengthening effect that correlates the YS to the cell size rather than to the grain size [73]. [PERSON] et al. claimed that metastable \(\delta\)-ferrite also plays a vital role in the determination of the mechanical properties of DED AISI 316L samples as it is harder than the austenitic matrix [78]. [PERSON] et al. also attributed the higher YS and US values obtained in their work to the presence of the high-temperature \(\delta\)-ferrite phase. The ferritic phase causes a refinement of the microstructure and a consequent reduction of crack propagation [37]. The higher tensile properties associated with the presence of \(\delta\)-ferrite are also correlated to the internal strain hardening that arises during the AM process caused by the different coefficient of thermal expansion of the two phases. The high mechanical properties of AM AISI 316L were also ascribed to the high dislocation density of the as-built material [85]. [PERSON] et al.