## 2 Microstructure
The Cr\({}_{\text{eq}}\) and Ni\({}_{\text{eq}}\) of their DED AISI 316L suggest that the solidification of their material falls within the upper range of the austenitic-ferritic mode in such a way that an austenitic steel forms a predominantly austenitic microstructure with columnar \(\delta\)-ferrite along the solidification direction (Figure 11) [41]. The \(\delta\)-ferrite phase can be recognized by the higher Cr and Mo content. [PERSON] et al. studied the microstructure of DED AISI 316L and revealed that Cr and Ni equivalent contents lie in the range of 5-10% \(\delta\)-ferrite [71]. [PERSON] et al. also investigated DED of AISI 316L and found that intercellular \(\delta\)-ferrite formed at sub-grain boundaries as a consequence of its enrichment of Cr and Mo, and depletion of Ni [35]. Figure 11: Element mapping for deposition body area: (**a**) layered element image, (**b**) molybdenum and target body, boundary element composition points, (**c**) chromium, (**d**) silicon, (**e**) iron, (**f**) nickel, (**g**) carbon, (**h**) manganese [41]. Figure 10: (**a**) Schaeffler constitution diagram showing the location of the composition of the AISI 316L stainless steel powders, (**b**) pseudo-binary phase diagram [37]. Formation of a duplex microstructure during DED of 316L was also revealed by [PERSON] et al. [37]. Indeed, they showed that the interdendritic arms are enriched in molybdenum and chromium, which are \(\delta\)-stabilizer elements. This elemental distribution can also be explained by the cooling rate of the process in which, during rapid solidification, austenite-promoting elements such as Ni and C are consumed to solidify the austenite phase and then the residual liquid phase is enriched in \(\delta\)-stabilizer elements in the interdendritic regions. Inclusion formation, such as of oxides rich in Si or Si and Mn, is an undesirable feature reported in DED of 316L [72]. These oxide structures are typically found during the ladle practice of high-content Mn/Si steels [73; 74]. It is found that, due to their very high reactivity with oxygen, the formation of these oxides, even in secondary steelmaking, is difficult to avoid. Thus, in spite of using protective shielding gas to protect the melt pool, finding these kinds of oxides is not surprising. However, it should be noted that their detrimental effect on the components produced via DED with respect to the conventional steelmaking processes is relatively low, mainly owing to their final reduced size and spherical shape [37]. [PERSON] et al. studied the oxide inclusion in laser AM of AISI 316L and reported their detrimental effect on the toughness and stress corrosion cracking behavior [75]. In fact, in their work, intergranular Si-rich and Si/Mn-rich oxides were found in as-built AISI 316L components (Figure 12). In another study, [PERSON] et al. investigated the corrosion behavior of AISI 316L produced via DED [76]. They found that the presence of these oxides in the final microstructure weakens the corrosion resistance of the AISI 316L parts. The size and the composition of the oxides found in AM AISI 316L samples, together with their effect on the material properties, are reported in Table 3. All these findings prove that in order to achieve the full potential of AISI 316L alloy in different applications, a more reliable deposition atmosphere should be used to comprehensively protect the melt pool and consequently reduce the oxide content of the alloy. Figure 12: Oxide inclusions in the fully-recrystallized additive manufacturing (AM) AISI 316L stainless steel: (**a**) low magnification back-scattered electron image; (**b1**) high magnification back-scattered electron image; (**b2**) high magnification secondary electron image [75]. However, it should be highlighted that since the size and quantity of these inclusions are normally lower than the resolution of the X-ray diffraction (XRD) analysis, they have mainly been analyzed via SEM and image analysis.