Additive manufactured titanium alloys exhibit distinct microstructures and internal defects compared to forged alloys. This has a significant impact on its fatigue failure behavior and fatigue life distribution. Fatigue life dispersity and distribution characteristics of laser direct energy deposited Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy were investigated by a relatively large sample fatigue test, fracture morphology characterization, and statistical analytic method. Then, the intrinsic causes of fatigue life dispersity and distribution characteristics were clarified by correlating with fatigue failure origin modes (FFOMs). The test and analysis results indicate that the existence of internal pores significantly influences FFOMs, fatigue life dispersity, and distribution pattern. The FFOMs of microstructure and interior pores have little effect on fatigue life dispersion. In contrast, FFOMs of surface and subsurface pores present a relatively lower fatigue life and are responsible for fatigue life dispersion. Besides, two competitive effects of FFOMs with different applied stress highly affect fatigue life distribution. A bimodal Weibull model is more suitable for describing fatigue life distribution with multiple FFOMs. However, if there is no fast-computing procedure for the parameter estimation of the Bimodal Weibull model, relatively conservative and simple P-S-N curves based on the Weibull distribution model are recommended in engineering applications.

To accurately predict the fatigue properties of additively manufactured (AM) titanium alloys, it is important to understand the fatigue damage origin behavior. However, this behavior is still ambiguous. Therefore, the effects of internal defects and microstructures on the fatigue damage origin behavior of laser direct energy deposited TC11 (LDED-TC11) alloy were investigated using a fatigue origin criterion. The criterion was proposed to analyze the competing and combining effects by coupling the plasticity-corrected crack driving force, the resistance of short cracks, and the modified Kitagawa-Takahashi diagram. Three scenarios corresponding to the criterion were clarified, representing the damage mechanisms dominated by the microstructure, the combined effect of internal defect and microstructure, and the internal defect. As a result, the fatigue fracture morphology of high-cycle fatigue tests demonstrates two fatigue origin modes, i.e. microstructure and gas pore origin modes. The two fatigue modes belong to Scenario Ⅰ and Scenario Ⅱ, respectively, which indicates that the fatigue damage origin process of this alloy is sensitive to microstructure. Besides, it was found that the width of the primary α phase of this alloy is strongly relevant to intrinsic defect size. Finally, the fatigue origin criterion was verified in three aspects.