Fatigue notch sensitivity of laser direct energy deposited versus wrought Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy

The notch, such as hole and chamfer, is unavoidable for aircraft structural components. Stress concentration induced by notch is the leading cause of fatigue failure. It is significant for structural integrity and safety assessment to clarify the intrinsic mechanism causing the difference in fatigue notch sensitivity. The notch fatigue fracture behavior of TC11 samples, fabricated by laser direct energy deposited (LDED) and wrought processes, is studied by high cycle fatigue (HCF) tests, fracture morphology, systematic microstructural characterizations, and theoretical analysis. HCF test results show that the notch fatigue performance of LDED samples reported in this study is superior to that of the wrought ones. The modified critical radius calculated from the equivalent material concept and average strain energy density criterion correlates well with fatigue notch sensitivity q and notch fatigue performance. The thick lamellar α phase produces more crack resistance and higher local plasticity around the notch root, causing lower q of LDED samples. Besides, the difference in q caused by build orientations is attributed to the variations in the angle between the c-axis of the fiber texture and load direction, the dominant slip system, the boundaries between soft and hard grains, and the presence of prior β grain boundaries.