## 5 Conclusions  
In this study, the effect of (\(\alpha+\beta\)) two-phase microstructure on the fatigue life and fatigue crack growth properties of LPBF Ti-6 Al-4V was investigated based on porosity reduction by remelting. The microstructure and morphology, average fatigue life, and CGR curves were obtained, and the fracture surfaces of fatigue specimens and fatigue crack growth specimens of different heat treatment processes were researched. The following are the main conclusions:  
(1). The fatigue life changes of AT and SAT specimens follow the same pattern as heat treatment temperature increases. Heat treatment temperature rises, causing the slatted \(\alpha\) phases to thicken and fatigue performance to deteriorate. The fatigue life of AT and SAT specimens at 850 \({}^{\circ}\)C increased significantly, by 101 times and 63.7 times, over the as-built specimen, respectively.  
(2). By observing the fracture surfaces of fatigue specimens, it was found that the tiny defects inside the part still affected the location of the crack origin. The location of the crack nucleation and the microstructure together control the fatigue life of the fatigue specimen. The farther the nucleus position is from the surface, the higher the fatigue life.  
(3). In terms of Paris CGR, when T\({}_{\alpha}<\) T\({}_{\beta}\), AT and SAT specimens both show less sensitivity to microstructure; When T\({}_{\alpha}>\) T\({}_{\beta}\), the \(\alpha\) colony makes the CGR show excellent performance, which is due to the high deflection of the crack growth and the generation of secondary cracks.  
(4). In the fracture faces of fatigue growth specimens, it was found that all heat-treated specimens were mainly cleavage fractures. Typical fatigue striation features appear on the cleavage steps, and secondary cracks are distributed between the steps. When T\({}_{\alpha}>\) T\({}_{\beta}\), the cleavage step is further widened and the secondary crack deepens.  
(5). Regardless of whether it is AT or SAT, when T\({}_{\alpha}<\) T\({}_{\beta}\), the microstructure is composed of plate-like \(\alpha\) and \(\beta\) phases. T\({}_{\alpha}\) determines the thickness of the \(\alpha\) lath. The higher the T\({}_{\alpha}\), the thicker the \(\alpha\) lath. When T\({}_{\alpha}>\) T\({}_{\beta}\), the microstructure appears as \(\alpha\) colonies. The size of \(\alpha\) colonies depends on the cooling rate. The faster the cooling rate, the smaller the \(\alpha\) colonies.  
Y.Y. and M.Z.: conceptualization, methodology, investigation, formal analysis, writing--original draft; K.Z. and D.X.: methodology, validation, writing--review and editing; H.W., Y.Y. and Y.H.: resources; D.X. and F.L.: formal analysis, writing--review and editing; L.S. and Y.M.: supervision, funding acquisition, writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (No. 51475238), the Key Research and Development Program of Jiangsu Provincial Department of Science and Technology of China (No. BE2019002), the China Post Doctoral Fund (No. 2020M671475).  
Not applicable.  
Data Availability Statement: Data available on request due to restrictions eg privacy or ethical. The data presented in this study are available on request from the corresponding author. The data are not publicly available due to confidentiality.  
**Acknowledgments:** We would like to express our appreciation to Jiangsu 3D Intelligent Manufacturing Research Institute for supporting this research.  
**Conflicts of Interest:** The authors declare no conflict of interest.