## 2 Background of the Study  
Because of the distinct manufacturing differences between casting, which implies the complete melting and overheating of the alloy, the milling of a prefabricated metal block, and the PBF of a fine metallic powder, large differences in the microstructural characteristics of the substructure are to be anticipated [19]. SLM leads to a different microstructural evolution compared to that formed in casting and milling. SLM alloy microstructures show finer grains when compared to casting alloys [18; 20], as well as to milled ones [21]; they offer better mechanical properties and similar ceramic bond strength compared to casting alloys [22; 23]. The microstructure of SLM Co-Cr prosthetic restorations greatly influences their mechanical properties [16; 24], revealing variations due to the beam parameters and scan strategies [25; 26]. In the case of crowns and bridges, the subsequent ceramic veneering of the metallic substructure necessitates the repeated firing of the different ceramic layers at temperatures over 900 \({}^{\circ}\)C [27]. Comparative analysis of the specific properties of metallic substructures obtained by SLM is still limited; thus, the effect of the SLM technique on the mechanical, electrochemical, microstructural, and other properties of the resulting products is still not extensively documented [19; 24; 28]. Most of the available studies concerning the influence of repeated firings on porcelain-fused-to-metal SLM Co-Cr restorations are mainly focused on the evaluation of their marginal and/or internal fit [7; 29; 30], or on testing the metal-ceramic bond strength [31; 32; 33; 34; 35; 36; 37]. [PERSON] et al. reported that repeated firing has a negative effect on the passive fit of metallic substructures that had been obtained via laser sintering [38], while other studies showed no significant influence of repeated firings on the marginal accuracy of SLM copings [39] or SLM crowns, exhibiting a marginal and internal accuracy that was comparable to those created using conventional production procedures [29]. The results of previously published studies that aimed to assess the alloy's microstructure following simulated ceramic firing are also quite controversial. [PERSON] et al. aimed to compare the microstructure of cast, milled, and 3D-printed alloys that were subjected to porcelain firing from the corrosion point of view, and concluded that SLM alloys exhibited the highest corrosion resistance due to the microstructural homogeneity of the oxide layers and the fact that the specimens retained their structure [40]. [PERSON] et al. aimed to assess a Co-Cr-Mo alloy, fabricated by SLM, from the standpoint of its corrosion behavior before and after ceramic firing. The ceramic firing altered the mi crostructure, microhardness, and X-ray photoelectron spectroscopy (XPS) results, although it had no significant influence on the corrosion behavior of the SLM alloy [41]. [PERSON] et al. examined the microstructure and surface composition of an SLM Co-Cr dental alloy before and after ceramic firing, using metallographic microscopy, X-ray diffraction (XRD), and XPS. Although the ceramic firing resulted in an alteration of the alloy microstructure, the samples still exhibited a compact and homogeneous structure, and no significant differences were detected in the surface composition of the specimens after firing [42]. [PERSON] et al. compared the microstructure, porosity, mechanical properties, ion release, and tarnish resistance of Co-Cr alloys developed via conventional casting, milling, SLM, and the milling of soft metal, concluding that the porcelain firing did not affect their microstructural characteristics [43]. [PERSON] et al., after an investigation of the ceramic bond strength of Co-Cr alloys processed by casting, milling, and SLM, concluded that their microstructures were not affected by ceramic firing [34]. [PERSON] et al. tested two types of alloys: a high Cr-Mo alloy without Be and a low Cr-Mo alloy with Be, examined via XPS and XRD.