## 3 Results  
### Microstructure and Surface Roughness  
Figure 4 shows two OM micrographs illustrating the microstructure of as-built Ti6 Al4V-ELI 45\({}^{\circ}\)-samples on the perpendicular (Figure 4a) and the parallel (Figure 4b) planes to the build platform. Figure 4a illustrates the presence of different columnar \(\beta\)-grains with the white dotted lines marking their boundaries. Prior columnar \(\beta\)-grains are usually disposed perpendicularly to the molten pool, marked through the yellow dotted line, and totally formed by \(\alpha^{\prime}\)-martensite laths. Figure 4b shows the cross-sections of the columnar \(\beta\)-grains, characterized by a typical rectangular shape. In addition, they are generally contained within the laser scan tracks (marked with dotted yellow lines). Similar results are obtained on the same types of planes of the 0\({}^{\circ}\)XZ-, 0\({}^{\circ}\)XY- and 90\({}^{\circ}\)Z-samples, respectively. Figure 5 shows the microstructure of both the 0\({}^{\circ}\)XZ-sample heat-treated at 704 \({}^{\circ}\)C/120\({}^{\prime}\) (Figure 5a,b) and the 90\({}^{\circ}\)Z-sample treated at 740 \({}^{\circ}\)C/130\({}^{\prime}\) (Figure 5c,d) where the columnar \(\beta\)-grains are always arranged alongside the build direction. The same microstructures are shown by the 0\({}^{\circ}\)XY-, 0\({}^{\circ}\)XZ-, 45\({}^{\circ}\)- and 90\({}^{\circ}\)Z-bars and they are consistent with those reported in [37; 45; 46]. As widely reported in the literature, the \(\alpha^{\prime}\rightarrow\alpha+\beta\) decomposition generates a microstructure where the primary, secondary and tertiary \(\alpha\)-laths form a hierarchical structure such as the former \(\alpha^{\prime}\)-martensite laths. Figures 5b and 5c illustrate the presence of this hierarchical structure where orange, green and light-blue dotted lines indicate the primary, secondary and tertiary \(\alpha\)-laths, respectively. The EDS spectra of Table 3 confirm that the laths are the \(\alpha\)-phase (spectrum A) and lighter zones are the \(\beta\)-phase (spectrum B) [47]. The former is characterized by a higher content of the \(\alpha\)-stabilizer alloying element (Al), while the latter are richer in the \(\beta\)-stabilizing alloy elements (V and Fe) [19]. [PERSON] et al. [32] affirmed that the microstructure shown in Figure 5b,d was strictly related, firstly, to the self-accommodation of the \(\alpha^{\prime}\)-martensite and, secondly, to the Burgers relationships between the \(\alpha\) and \(\beta\) phases. Figure 4: OM micrographs of the microstructure of as-built Ti6 Al4V-ELI 45\({}^{\circ}\)-sample observed: (**a**) on the plane perpendicular to the build platform, and (**b**) and on the plane parallel to the build platform. White and green arrows indicate the boundaries of the columnar \(\beta\)-grains (white dotted lines) and the laths of \(\alpha^{\prime}\)-martensite, respectively. The yellow dotted lines represent the molten pool boundary (**a**), and the laser scan tracks (**b**). In order to characterize the coarsening of the \(\alpha\)-phase, its thickness was measured as a function of the heat-treatment conditions, and it increases from (540 \(\pm\) 60) nm at 704 \({}^{\circ}\)C/120\({}^{\prime}\) to (700 \(\pm\) 10) nm at 740 \({}^{\circ}\)C/130\({}^{\prime}\) due to the different temperatures and the furnace cooling from 740 to 520 \({}^{\circ}\)C in 90\({}^{\prime}\). The same results were obtained in [29; 48; 49]. For example, [PERSON] et al. [29] discussed the same phenomenon during the heat treatments in the low-SSTR (Solid Solution Temperature Region), i.e., at temperatures below 800 \({}^{\circ}\)C.  
Increasing the heat-treatment temperature above the \(\beta\)-transus (T = 1050 \({}^{\circ}\)C), the columnar \(\beta\)-grains recrystallize into equiaxed grains, as shown in Figure 6a,b, where the microstructures of the 90\({}^{\circ}\)Z-sample (Figure 6a,c) and 90\({}^{\circ}\)Z-bar (Figure 6b,d) are shown.