Fatigue damage origin of laser direct energy deposited titanium alloy: Competition and combination of gas pore and microstructure

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.