## 2 Materials and Methods  
The parts were all produced by NCL-M150 additive manufacturing equipment (Nanjing Chamilion Laser Technology Co., Ltd., Nanjing, China). The gas atomization method was used to produce Ti-6 Al-4V powder. Table 1 shows the powder's main chemical composition. Figure 1a displays the morphology and particle size distribution of the powder, which is between 10 \(\upmu\)m and 90 \(\upmu\)m, with an average of 42 \(\upmu\)m. The parts were manufactured by remelting with the laser scanning speed and power tuned to 1800 mm/s and 160 W, respectively. The scanning strategy selected the serpentine path and 57\({}^{\circ}\)/67\({}^{\circ}\) rotation scanning. The remelted layer's scan path was perpendicular to the initial, and the laser scanning speed and power were adjusted to 1600 mm/s and 180W, respectively. The diagram of the remelting operation is shown in Figure 1b, where the blue line was the initial scanning path and the red line was the remelting scanning path.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**H** & **C** & **V** & **Al** & **Fe** & **O** & **Ti** \\ \hline
0.0018 & 0.011 & 3.91 & 6.28 & 0.04 & 0.086 & Bal. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of Ti-6 Al-4V alloy powder (wt.%).  
The remelting process can obtain nearly fully dense, high-quality Ti-6 Al-4V specimens. Internal defects in LPBF parts are identified and counted using a 3D X-ray computed tomography (\(\mu\)-CT) system. This approach ensures that these specimens have low and consistent porosity, thus guaranteeing the validity of microstructure analysis. Figure 2a,b are 3D defect diagrams of the fabricated specimens before and after the remelting process, respectively. The density of the specimen in Figure 2a is 99.762%, while in Figure 2b it is 99.998%. After remelting, the pores are greatly reduced, and the density is significantly improved.  
The heat treatment was performed in a vertical tube furnace with a vacuum degree of greater than 10\({}^{-6}\) MPa, and the temperature was increased at around 10 \({}^{\circ}\)C/min. The specific heat treatment protocols and markings are described in Table 2. Some of the fabricated specimens were used as the reference group, marked as \"as-built\"; others were subjected to AT and marked as 850 FC, 950 FC, and 1050 FC, respectively; the remaining specimens were subjected to SAT, marked as 850 WAC, 950 WAC, and 1050 WAC, respectively. In addition, scanning electron microscopy (SEM) and optical microscopy (OM) were used to examine the microstructure of Ti-6 Al-4V.  
Figure 1: Morphology and particle size distribution of the powder (**a**) and schematic diagram of printing and scanning (**b**).  
Figure 2: Three-dimensional pore diagram of the fabricated specimens before (**a**) and after (**b**) the remelting process.  
Fatique crack growth CT specimens and fatigue specimens were prepared according to the national standard specimen size, and the specific dimensions are shown in Figure 3. The initial length of prefabricated cracks in CT specimens is 2.6 mm. Before the test, specimens were polished and ground to reduce the influence of surface roughness on fatigue. The fatigue test at room temperature was carried out on the QBG-50 high-frequency fatigue testing machine.  
The fatigue test adopted uniaxial sinusoidal cyclic loading. The maximum stress (\(\sigma_{\max}\)) was 500 MPa, the stress ratio (R) was \(-1\), and the vibration frequency (f) was 110 Hz. This load is similar to the operating conditions of the turbine blades investigated. Three replicated tests were conducted and averaged. The room-temperature fatigue crack growth test was carried out using an Instron 8801 low-frequency fatigue testing machine. The test adopted cyclic sine wave loading, and the crack length was detected by the potential method, and \(\sigma_{\max}\) was 2.5 kN, R was 0.1, and f was 10 Hz.