## 3 Mechanical Properties
### Building Direction  
The correlation between the building direction and tensile properties is a crucial aspect in the determination of the mechanical properties of AM samples. The anisotropy of the microstructure and the presence of some defects affect the stress-strain curve [49]. The main factors related to the building direction that have an effect on the tensile properties can be listed as follows:  
* Grain morphology
* Texture
* Elongated dendrites
* Lack of fusion defects  
[PERSON] et al. investigated the impact of the building direction of the mechanical properties and showed that the highest mechanical properties (YS, US, and \(\varepsilon\)) were observed in the H samples (Table 4). The authors attributed this difference to the superior metallurgical bonding of these samples along the tensile direction. Horizontal samples have at least one layer in which the deposition direction is parallel to the tensile direction. The consolidated scan tracks deposited along the direction parallel to the tensile direction act, therefore, as fibers that reinforce the materials during the tensile tests [70].  
On the contrary, the V samples might contain some critical elongated defects perpendicular to the tensile direction due to a lack of fusion between different layers, which can be detrimental during the tensile test.  
Similar results were obtained by [PERSON] et al. and [PERSON] et al., who confirmed that the highest YS and US were achieved in the H direction. These authors, however, observed higher elongation values in the V samples (Table 4) [35; 83]. The higher elongation of vertical samples was attributed by [PERSON] et al. to the improved ductility due to the dendrites along the growth direction [83]. [PERSON] et al. also analyzed the tensile properties of DED AISI 316L samples (both vertical and horizontal) extracted from a more complex geometry and also obtained lower YS and US in the vertical samples [84]. It is important to underline that these data showed a large scattering, probably because of the different distribution of defects due to the complex geometry from which samples were extracted.  
In this case, the vertical samples had a drastically lower elongation value, which was attributed to the delamination phenomenon that was clearly detectable by the fracture surface analyses (Figure 13). The fracture surface analysis indicates that the V and H samples showed different deformation characteristics. The vertical samples had a very reduced necking and displayed a fracture surface characterized by some features, such as unmelted particles and smooth areas, that indicate an incomplete fusion during the building process. The horizontal samples, on the contrary, have a strong necking phenomenon and display the typical ductile fracture surface characterized by fine dimples with a size comparable to the PCAS.  
In general, based on these results, it can be concluded that the highest YS and US are generally achieved in the horizontally built samples, and the effect of building direction on the elongation value is still controversial. This discrepancy can be probably related to the other building conditions, as well as the porosity and inclusion content, together with the effect of other strengthening phenomena.  
In the case of horizontal samples, however, further studies are needed to investigate the effect of the distance from the substrate on the tensile properties. [PERSON] et al. demonstrated that YS and US of DED 304L stainless steel increase as the distance from the substrate decreases due to the different microstructures that solidify based on the cooling rate [31].  
Figure 13: SEM micrographs of the tensile fracture surface: (**a**,**c**) vertical; (**b**,**d**) horizontal [84].