###### Abstract  
With the development of additive manufacturing (AM), the Ti-6 Al-4V alloy manufactured by laser powder bed fusion (LPBF) is becoming more widely studied. Fatigue fracture is the main failure mode of such components. During LPBF processing, porosity defects are unavoidable, which hinders the exploration of the relationship between fatigue performance and microstructure. In this study, a laser remelting method was used to reduce porosity defects inside the Ti-6 Al-4V alloy. Three annealing treatments (AT) and three solution-aging treatments (SAT) were used to study the effect of the two-phase zone (\(\alpha+\beta\)) microstructure on fatigue life and fatigue crack growth behavior. Fatigue life and crack growth rate (CGR) curves were obtained, and fatigue fracture surface and crack growth fracture surface were analyzed. The results show that microstructure influences fatigue life but has little effect on CGR. Compared with the as-built specimen, the fatigue life of the AT and SAT specimens increased significantly at 850 degC by 101 and 63.7 times, respectively. The thickness of the \(\alpha\) lath and the location of crack nucleation together affect the fatigue life. In the stable growth stage, the layered microstructure of \(\alpha\) colonies is the most resistant to crack growth. laser powder bed fusion; heat treatment; Ti-6 Al-4V; fatigue performance Article  
by Laser Powder Bed Fusion after Post-Heat Treatment  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], L. Microstructure and Fatigue Performance of Ti6 AlAV Produced by Laser Powder Bed Fusion after Post-Heat Treatment. _Applid_  
1  
[PERSON], [PERSON], [PERSON], [PERSON], Y.  
and \(\beta\)[10; 11]. Therefore, the use of heat treatment to regulate the non-equilibrium phase is a necessary process after LPBF processing. Aircraft engines are subjected to cyclical and complicated loads, so the material's properties must match the operating load requirements. Many things influence fatigue properties, including defects, rough surfaces, residual stress, and microstructure [7; 12; 13; 14; 15; 16]. [PERSON] et al. [16; 17] showed that defects such as gas porosity and lack of fusion (LOF) are the main reasons for the lower fatigue life of LPBF parts compared to traditional parts. [PERSON] et al. [7; 18; 19] evaluated the impact of surface finishing on the fatigue performance of LPBF Ti-6 Al-4V, including turning, grinding, sandblasting, and polishing. It was discovered that reducing surface roughness significantly improved fatigue properties, with the polishing process having the best effect. [PERSON] et al. [8; 20] found that heat treatment has the potential to reduce residual stresses and improve fatigue life. [PERSON] et al. [9; 21; 22] found that higher porosity (0.25-4%) makes it difficult to examine the effect of microstructure on fatigue performance due to excessive defects. The impact of microstructure on mechanical characteristics gradually becomes more apparent when defects are reduced. The influence of microstructure on the fatigue performance of LPBF parts is often difficult to accurately determine due to the presence of defects, so it is necessary to reduce the number of defects. Numerous studies have shown that optimizing process parameters [13; 19] and hot isostatic pressing (HIP) [15; 16] can reduce porosity and thus improve fatigue performance. However, defects in LPBF parts cannot be completely eliminated. Currently, HIP is considered to be the most effective method to eliminate defects [15; 16]. However, on the one hand, HIP will change the original microstructure, thus making the microstructure impossible to be effectively analyzed, while, on the other hand, the high cost also limits its application [7]. As an economical and feasible method, laser remelting can effectively reduce the porosity of parts and achieve ultra-low porosity (\(<\)0.01%) [21; 23]. Remelting is critical in the reduction of porosity, and the underlying mechanism has been extensively studied and validated [23; 24; 25].