## 4 Discussion
Focusing on the error bars (Figure 14), which are obtained considering the tensile results of each build orientation (Figure 11), the samples heat-treated at 704 \({}^{\circ}\)C are characterized by a more pronounced anisotropy (\(\Delta\)UTS = 4.75%, \(\Delta\)YS = 7.40%) than those heat-treated at 740 \({}^{\circ}\)C (\(\Delta\)UTS = 3.86%, \(\Delta\)YS = 4.30%). These variations are calculated considering the maximum and minimum values of all build directions for each temperature. [PERSON] et al. [46] reported \(\Delta\)UTS = 4.40% and \(\Delta\)YS = 6.74% after 730 \({}^{\circ}\)C/2 h with air cooling, while [PERSON] et al. [63] did not detect any differences after the same heat treatment. As a matter of fact, the anisotropy depends significantly on the correlation between the load direction and both the lack-of-fusion (LOF) defects and the columnar \(\beta\)-grains [16; 63; 64; 64]. The presence of \(\alpha_{2}\) and/or TiAl\({}_{3}\) precipitates can be another possible contribution to the anisotropy, as will be later discussed. Therefore, future works related to the influence of LOF, grain boundaries and \(\alpha\) phases may be necessary for the heat treatments analyzed in the present manuscript. The only influence of \(\alpha\)-phase coarsening is observable on the HV\({}_{0.5}\) microhardness values shown in Table 5.  
During the SHT at 1050 \({}^{\circ}\)C for 60\({}^{\prime}\), the recrystallization process of the columnar \(\beta\)-grains in equiaxed grains takes place (Figures 5 and 6), as also reported by [12; 45; 65; 12]. The same results were obtained by [PERSON] et al. [32] who suggested that the equiaxed grains form from the split of the progenitor columnar \(\beta\)-grains. [PERSON] et al. [66], however, showed the recrystallization process already after 1000 \({}^{\circ}\)C/40\({}^{\prime}\), followed by water quenching. Due to this microstructural change, together with the \(\alpha\)-laths in a basketweave structure (Figure 6), the UTS and YS values decrease (Figure 9) up to 918 \(\pm\) 7 MPa and 817 \(\pm\) 6 MPa, respectively; meanwhile, the elongation increases only up to 12.75 \(\pm\) 1.01% due to both the \(\alpha\)-laths rather than \(\alpha\)-colonies in a basketweave structure (Figure 6a,c) and the possible detrimental effect of the \(\alpha\)-case [12; 20; 49; 67]. The same tensile properties are confirmed by [19; 32; 68; 69]. In this scenario, the mechanical properties of all heat-treated samples satisfy the minimum requirements dictated by the ASTM F3001 (UTS of 860 MPa, YS of 795 MPa and \(\varepsilon\) of 8%). However, in relation to the samples heat-treated at 1050 \({}^{\circ}\)C/60\({}^{\prime}\), the ASTM F3001-13a is not satisfied due to the presence of the \(\alpha\)-case layer. In fact, post-processing operations are required to remove the \(\alpha\)-case due to its worsening effects on fatigue life and ductility in aerospace components [2; 49]. At the same time, the samples heat-treated at 704 and 740 \({}^{\circ}\)C, which are characterized by higher tensile properties than the solution heat-treated samples, also satisfy the F2924-14 standard specification (UTS of 895 MPa, YS of 825 MPa and \(\varepsilon\) of 10%, [70]); this, however, is related to the Ti6 Al4V alloy. In relation to the cooling pathway, the Argon gas and the samples' thickness do not influence the microstructure of the samples heat-treated at 704 and 740 \({}^{\circ}\)C, as also reported in [29; 33; 68]. As a matter of fact, the cooling methods (water, air or gas, and furnace) have different effects in relation to the heat-treatment temperatures. In fact, the microstructure is not influenced by the cooling pathway if the cooling starts below \(\sim\)800 \({}^{\circ}\)C [12; 33; 68]. By contrast, the \(\beta\rightarrow\alpha^{\prime}\)-martsine transformation can take place when cooling starts from above 800 \({}^{\circ}\)C, as the sample is converted to \(\beta\)-phase and cooled at a high cooling rate. [PERSON] et al.