## 3 Results
Figure 4: Evolution of the transformation enthalpies of the direct and reverse martensitic transformation of the NiTi powder and the NiTi samples, produced at varying the laser fluence. Figure 3: Evolution of the transformation temperatures of the direct (**a**) and reverse (**b**) martensitic transformation of the initial NiTi powder and the NiTi samples, produced at varying the laser fluence. EDXS measurements were performed on XZ sections of the as-built samples; nine area measurements of about 0.5 \(\times\) 0.5 mm\({}^{2}\) were acquired for each specimen, spread over a total area of about 2.5 \(\times\) 2 mm. Figure 5 shows the semiquantitative trend of Ni and Ti content, expressed in atomic percentage (at. %), at varying laser fluence. No significant O content was observed, without any variation among the position in the specimens; consequently, it was disregarded from calculations. In the graph, two regions can be clearly observed; at low laser fluence values up to 63-160 J/mm\({}^{3}\), the compositions of the specimens are quite constant and in good agreement with the nominal composition of the starting powder, i.e., Ni content of about 51.0 \(\pm\) 0.3 at. %, quite close to the nominal 50.8%at. By increasing further the laser fluence, up to 297 J/mm\({}^{3}\), Ni content decreased down to 50.4 \(\pm\) 0.2 at. %, with a consequent increase in Ti content, was detected from 49.0 \(\pm\) 0.3 at. % up to 49.6 \(\pm\) 0.2 at. %. The latter values are well in agreement with the observed increase in the transformation temperature in DSC analyses. EBSD analyses were carried out in as-built conditions at room temperature. Figure 6 shows the evolution of the orientation imaging microscopy (OIM), inverse pole figures (IPF), and calculated (100) pole figures of the samples sectioned along the Z direction, i.e., the building direction (orthogonal to the platform). It can be seen that the PW laser emission mode can induce a significant variation of the microstructure and crystallographic texture at varying laser fluence. The samples, processed with laser fluence up to 160 J/mm\({}^{3}\), exhibit a full austenitic phase, as shown in Figure 6a-c; at 222 J/mm\({}^{3}\), the sample shows small areas not recognized by EBSD as austenite; therefore, traces of martensite can be indirectly identified (see Figure 6d). At the highest value of laser fluence (297 J/mm\({}^{3}\)), during the first analysis, most of the analyzed area could not be recognized by EBSD and exhibited very low pattern quality. After a thermal cycle (heating with hot water, then cooling again to room temperature), EBSD analysis was repeated (Figure 6e); still, some areas were nonindexed as observable, but most of the specimen was indexed as austenite; the nonindexed regions in the first analyses were clearly related to the region of the samples in which martensite could be easily formed upon cooling during specimen preparation, by washing with too cold tap water, that easily transformed back upon moderate heating. Indeed DSC analyses show that Martensite formation started at about 20 \({}^{\circ}\)C. The other nonindexed regions can be ascribed hence at chemical inhomogeneity or residual stresses-retained martensite [28]. Change in the microstructure of the specimen can be clearly seen from OIM: at low fluence values, a rather homogenous microstructure is observed, characterized by small grains, while at increasing fluence, a coarser microstructure can be observed, characterized by bands parallel to the Z direction, composed by columnar grains. Figure 5: Compositional analysis, carried out with EDS, of the NiTi built samples processed at different laser fluence values. In Figure 7, the pattern quality maps of small areas of the analyzed surfaces are reported; in order to highlight melt pool shape and size, red lines were added to representative melt pool boundaries to guide the eyes. At the lowest value of laser fluence (63 J/mm\({}^{3}\)), rather small semi-spherical liquid pools are generated.