## 3 Results  
The investigated process conditions were selected according to two criteria: significant distance in fluence values and the highest density (among the samples processed with different processing parameters that provide similar fluence). The laser fluence values ranged from 63 J/mm\({}^{3}\) up to 297 J/mm\({}^{3}\) (see Table 1); these values fit well with the indications achievable from the literature regarding the processability of NiTi SMA powder through the PBPF process [26]. In Figure 2, the evolution of the martensitic transformation (MT) of the initial NiTi powder and the NiTi-built samples, produced at varying laser fluence, is depicted. Each sample was analyzed with three complete heating/cooling cycles in the DSC scans. It can be recognized that all the analyzed samples exhibit the peaks of the martensitic transformation and good overlapping of the DSC signals, indicating good thermal stability of the martensitic transformation. In detail, the initial powder shows a multistage phase transformation upon cooling, which can be associated with chemical inhomogeneities and residual stresses formed during the gas atomization process, as confirmed by BSE micrographs of powders (Figure 1). The corresponding transformation temperatures, measured at the second DSC cycle and plotted in Figure 3, confirm that the NiTi powder is austenitic at room temperature (25 \({}^{\circ}\)C). The sample built at the lowest fluence shows a broad single-stage transformation upon both heating and cooling. By increasing the laser fluence in the building of NiTi samples, it can be observed that the direct martensitic transformation tends to evolve to a better-defined but still broad transformation peak, with two-stage transformation upon cooling and increasing transformation temperatures. The sample built with the highest fluence, conversely to all other specimens, shows a two-stage transformation upon heating, definitely shifted to a temperature higher than room temperature, as depicted in Figure 3.  
In particular, martensite start (Ms) and finish (Mf) temperatures and the austenite start (As) and finish (Af) temperatures are plotted in Figure 3a,b, respectively. It was found that Ms decreased from 10 \({}^{\circ}\)C down to 0 \({}^{\circ}\)C upon increasing the laser fluence from 63 up to 222 J/mm\({}^{3}\), while it increased up to 20 \({}^{\circ}\)C at 297 J/mm\({}^{3}\). The trend of Mf was associated with some oscillations, due also to the sluggish shape of the peaks, that hinder a precise determination of start and finish temperatures. Nether the less, a tendency trend can be marked, suggesting an increase in the Mf from low to high laser fluence up to \(-\)55 \({}^{\circ}\)C. Regarding the reverse martensitic transformation, As was almost constant (\(-\)60 \({}^{\circ}\)C) in the range 63-127 J/mm\({}^{3}\), while it increased significantly up to \(-\)25 \({}^{\circ}\)C; on the contrary, Af decreased from 30 \({}^{\circ}\)C down to 0 \({}^{\circ}\)C in the range 63-160 J/mm\({}^{3}\), then it started to increase up to 50 \({}^{\circ}\)C as well. Figure 4 shows the trend of the transformation enthalpies upon heating and cooling obtained from the DSCs of the fabricated NiTi samples. The enthalpy values of the direct and reverse martensitic transformation of the initial powder were 8 J/g and 7 J/g, respectively. These values are largely lower than the typical ones of the cast and heat-treated NiTi SMAs, reasonably due to chemical inhomogeneity and residual stress left from the atomization process. Regarding the printed NiTi samples, the enthalpies of both the direct and reverse martensitic transformation showed the same trend: they increased from 5 J/g, obtained in correspondence with the lowest laser fluence, up to 13 J/g and 19 J/g, respectively, at the highest fluence value. These latter values are closer to the average values of transformation enthalpies for cast NiTi SMAs [27].