## 4 Discussion  
The increase in the laser fluence can dramatically change the microstructural features of the laser-melted NiTi SMA. As depicted in Figure 6, the evolution of the microstructure suggested a change in the solidification conditions, as can be observed by the passage from rather equiaxial small grains to bigger columnar grain growth oriented along the building direction. The marked difference in texture can be ascribed to geometrical constraints and changes in solidification structures. In detail, the equiaxial grains are associated with very quick cooling rates; in fact, under irradiation at limited laser fluence, the dimension of the liquid pool is small; therefore, the limited amount of liquid metal can experience fast cooling rates and consequent dendritic or columnar/dendritic grow. Fast cooling favors nucleation overgrowth from existing grains. On the contrary, high laser fluence can induce a larger amount of liquid material, requiring a longer time to be cooled and hence reduced, even if still high, cooling rates. As a result, in the latter process condition, the microstructure would be characterized by columnar grains that preferably grow epitaxially from existing grains along the direction of the thermal gradient. The latter can be assumed, as the first approximation, perpendicular to the melt pool boundary: at low fluence, melt pools are semicircular; hence, more growth direction can be experienced by the grains. At higher fluence, only the central part of the melt pools survives to subsequent melting passes; and they are aligned mostly parallel to the building plane. Consequently, grains with different growth directions are in less amount, as they were melted during subsequent steps. The different degrees of texture evidenced by the samples can be ascribed to the combination of different extend of epitaxial growth, combined with rotation at each pass of the laser scanning direction and different shape/remelted portions of melt pools. The MT exhibits a modification of its transformation temperatures and enthalpies under the described increase in laser fluence, as depicted in Figures 2-4. In this case, an almost continuous decrease in the Ni content is directly associated with the increase in the energy irradiated to the powder bed during the LPBF process; higher overheating temperatures are reasonably reached in the melt pool for longer times, promoting selective evaporation of Ni. This effect has already been observed for a similar process applied to NiTi SMAs [24; 29]. Not only higher overheating of the melt occurs at higher fluence: the increased amount of transferred heat propagates to the previously built layers and promotes recovery and annealing phenomena in the heat-affected zones; this effect is witnessed by the increase in enthalpy at increasing fluence. In fact, it is known that an excess laser fluence can typically induce porosity due to gas entrapment and potential local vaporization of elements. In particular, the presence of these defects can be explained by the energy required by the material to allow its melting and its partial vaporization, according to the following equation:  
\[\mathrm{E}=\mathrm{A}\cdot\mathrm{\rho}\cdot\left\{\mathrm{C}_{\mathrm{P}} \cdot\left[\mathrm{T}_{\mathrm{M}}-\mathrm{T}_{\mathrm{i}}\right]+\mathrm{L} _{\mathrm{M}}+\mathrm{\ m}\prime\cdot\mathrm{L}_{\mathrm{V}}\right\}, \tag{2}\]  
where A is the absorption coefficient; \(\mathrm{\rho}\) is the density; \(\mathrm{C}_{\mathrm{P}}\) is the thermal capacity; \(\mathrm{T}_{\mathrm{M}}\) and \(\mathrm{T}_{\mathrm{i}}\) are the melting and initial temperatures, respectively; \(\mathrm{L}_{\mathrm{M}}\) and \(\mathrm{L}_{\mathrm{V}}\) are the latent heat of melting and vaporization, respectively; and m' represents the percentage of liquid material that is subjected to vaporization, if present during the laser irradiation. In the case of laser fluence, just sufficient for inducing a proper melting of the powder bed, the term m' is equal to zero.