## 3 Results  
### Impact of the Heat Treatment on Microstructure  
Numerous studies have discussed the mechanical performance and microstructure of as-built specimens in detail [11; 14; 18; 26]. Therefore, they will be described briefly in this paper. This study uses six different heat treatment schedules to give Ti-6 Al-4V a broad (\(\alpha+\beta\)) microstructure. Figures 4 and 5 show their effects on the microstructure. \begin{table}
\begin{tabular}{c c} \hline \hline
**Specimen Marking** & **Heat Treatment Protocol** \\ \hline as-built & 600 \({}^{\circ}\)C/2 h/FC \\
850 FC & 850 \({}^{\circ}\)C/2 h/FC \\
950 FC & 950 \({}^{\circ}\)C/2 h/FC \\
1050 FC & 1050 \({}^{\circ}\)C/2 h/FC \\
850 WAC & 850 \({}^{\circ}\)C/2 h/WC + 540 \({}^{\circ}\)C/4 h/AC \\
950 WAC & 950 \({}^{\circ}\)C/2 h/WC + 540 \({}^{\circ}\)C/4 h/AC \\
1050 WAC & 1050 \({}^{\circ}\)C/2 h/WC + 540 \({}^{\circ}\)C/4 h/AC \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The specific heat treatment protocols and markings. Figure 3: The dimensions of CT specimens (**a**) and fatigue specimens (**b**). During the LPBF process, a complete acicular \(\alpha^{\prime}\) martensite structure was formed, with equiaxed \(\beta\) grains of various sizes in the cross-section and long primary \(\beta\) columnar grains in the longitudinal section. Representative 3D microstructures of the 850 FC and 950 FC specimens are displayed in Figures 4a and 4b, respectively. The transformation point temperature (T\({}_{\beta}\)) of Ti-6 Al-4V is 995\({}^{\circ}\)C. Annealing below T\({}_{\beta}\) doesn't alter the structural characteristics of the primary \(\beta\) columnar grains. Compared with 850 FC, the heat treatment temperature (T\({}_{\alpha}\)) of 950 FC is closer to T\({}_{\beta}\), resulting in a thickening of the \(\alpha\) and \(\beta\) phases. The high magnification image in Figure 5a shows that the fine needle \(\alpha\)-laths in 850 FC have \(\beta\) phase structure outside the grain boundaries. The grain boundaries between \(\alpha\) and \(\beta\) phases in Figure 5b are clearer. A similar microstructure was also found by [PERSON] et al. [27] after heat treatment at temperatures below T\({}_{\beta}\). When 1050 FC is heat treated at a temperature higher than T\({}_{\beta}\) (Figure 4c), the complete \(\beta\)-phase microstructure formed an (\(\alpha+\beta\)) layered structure during the slow cooling process. Since the size of equiaxed \(\beta\) grains limits the size of \(\alpha\) colonies, at high temperatures, \(\beta\) grains grow rapidly and \(\alpha\) colonies grow accordingly. [PERSON] [28] showed that \(\alpha\) colony size was a determinant of mechanical properties. From the above, temperature determines the final microstructure, and increasing temperature promotes the growth of a large number of grains. According to the Burgers relationship [29], the \(\alpha\) phase grows preferentially parallel to the \(\beta\) phase crystallographic plane family. These planes grow relatively fast, and the \(\alpha\) phase forms into plates and layers. Figure 4: Representative 3D microstructures (OM) of Ti6 Al4V following heat treatment schedules: (**a**) 850 FC, (**b**) 950 FC, (**c**) 1050 FC, (**d**) 850 WAC, (**e**) 950 WAC, (**f**) 1050 WAC. The \(\beta\) phase in the two-phase zone is too late to undergo a diffusion-type transformation during the quenching process of SAT specimens, and a metastable martensite \(\alpha^{\prime}\) phase is formed. Followed by the aging strengthening operation at 540 \({}^{\circ}\)C, the long holding time promotes the decomposition of acicular martensite \(\alpha^{\prime}\), and the dissolved secondary \(\alpha\) and \(\beta\) phases are dispersed around the primary \(\alpha\) phase, resulting in a stable (\(\alpha+\beta\)) structure. The formation mechanism of similar microstructures has also been confirmed in other studies [30]. Figure 4d,e display representative 3D microstructures of 850 WAC and 950 WAC, respectively. Compared with 850 FC and 950 FC, the needle-like \(\alpha\) phase is coarsened and deformed.