Very-high-cycle-fatigue behaviour of ultrasonic-assisted TIG welded TC4 joints: Microcrack initiation and life prediction influenced by dislocations and oxides under shear stress

Abstract

Titanium alloy welded structures are commonly applied in aircraft industry manufacturing. Therefore, the behaviour of Ti-6Al-4 V (TC4) welded joints was investigated at very-high-cycle-fatigue (VHCF) conditions using ultrasonic-assisted tungsten inert gas (TIG) welding, aims to reveal the mechanism of crack initiation and fine granular area (FGA) formation, as well as to develop a model for predicting fatigue life. In particular, the data suggest that S-N curves are single-linear at both stress ratios, while three failure modes and three defect types were found. Transmission electron microscopy (TEM) analysis combined with image processing method revealed the presence of dislocation structures in FGA, indicating that stress concentration effect led to the generation of high-density dislocations, the produces lattice distortion and crystal slip at maximum ά martensitic Schmidt factor plane, concludes that the dislocation piles up together with oxides produced by oxygen aggregation promotes the microcrack initiation. Meanwhile, the cracks near fatal defect were driven by tensile and shear stresses with the propagation mode of Mode I + Mode II, and the region was observed to consist of discontinuous nano fine grain layers and broken martensitic laths. Additionally, the S-N curves with higher point-to-line convergence are reconstructed based on the defect size and defect depth. Finally, based on failure mechanism and linear elastic deformation theory, a VHCF life prediction model is developed by considering the modified maximum shear stress, mean stress and stress intensity factors (SIF) value with the effect of Schmidt factor mean value and defect features, which can improve the safety of structures.