###### Abstract
As a matter of fact, there is \(\alpha^{\prime}\rightarrow\alpha\) decomposition and a subsequent loss in mechanical properties of the as-built sample if the build platform is kept at high temperature (T \(>\) 400 \({}^{\circ}\)C, the temperature at which the martensite decomposes [26; 27; 28]) and/or due to the different values of energy density used \(\left(E=P/vht,\ \left(J/\text{mm}^{3}\right)\right)\). At the same time, the ductility is often lower than 9% and, consequently, the minimum tensile properties required by the F3001-14 standard specification are not satisfied [20]. In this scenario, optimized heat treatments can be necessary to obtain an improved balance in mechanical properties. Due to the chemical composition of the Ti6 Al4V-ELI alloy and the phase transformations that can take place during different cooling pathways, its mechanical properties can be improved and/or tailored through different heat treatments performed below or above the \(\beta\)-transus temperature [12; 29; 30; 31]. In particular, the as-built UTS and YS always decrease for each heat treatment performed on the as-built samples [12]. As a matter of fact, also considering a fully martensitic microstructure obtained in solution heat-treated and water-quenched samples, the UTS and YS are lower than 1.0 GPa and 950 MPa, respectively, due to the presence of recrystallized equiaxed \(\beta\)-grains [32; 33]. At the same time, the elongation values do not show continuous growth with increasing heat-treating temperatures and time due to the following reasons: firstly, the presence of columnar \(\beta\)-grains, which significantly influence the fracture mechanisms, remains even after the heat treatments performed below the \(\beta\)-transus; secondly, defects generated during the L-PBF process play a fundamental role in the premature failure of samples [12; 35]. On the other hand, the best balance between tensile strength and good elongation values (\(16\div 18\%\), [12]) was obtained through the bimodal microstructure achieved after cyclic annealing heat treatments [21; 36]. As previously discussed, the Ti6 Al4V-ELI alloy is characterized by lower contents of oxygen (max 0.13%), nitrogen, carbon and iron than the similar Ti6 Al4V alloy. This difference in chemical composition confers improved values of ductility and fracture toughness,which are required in the aerospace sector. At the same time, the surface finishing of the Ti6 Al4V-ELI sample is often correlated with air/gas flow problems correlated with the aerospace applications, but there are other reasons why aerospace engineering components require suitably low roughness. In fact, this is correlated with improved corrosion and wear resistance and with reduced friction during sliding contacts; moreover, it allows the uniform coverage of a component with a thin-film coating [37; 38; 39]. Surface roughness also has a huge impact on the fatigue life of a component, which is another important factor within the space and aerospace fields [40; 41]. [PERSON] et al. [42] correlated the surface roughness with fracture toughness, but no research concerning the effects on mechanical properties of Ti6 Al4V-ELI samples was found in the literature. The present paper aims to evaluate the microstructural effects induced by different heat treatments and the resulting mechanical properties, which are compared to the current standard specifications. Firstly, it analyzes the effects of two heat treatments performed below the \(\beta\)-transus temperature at 704 and 740 \({}^{\circ}\)C following the aeronautical standard specification (AMS 2801B); secondly, it analyzes the effects of the solution heat treatment at 1050 \({}^{\circ}\)C, which will be the starting point of future aging heat treatments. In addition, the present work studies the impact of the different heat treatments on samples characterized by different thicknesses. As a matter of fact, distinct aerospace components can possess widely different thicknesses, which can have an effect on their microstructures.