Influence of welded pores on fatigue behavior of TC17 titanium alloy welded joints subjected to gigacycle regime at room and high temperatures

Welded structures in aero-engines commonly operate in high-temperature environments, making them susceptible to reduced fatigue life and premature failure due to welding defects within the structure. Thus, the gigacycle fatigue behaviors of titanium alloy welded joints at both room temperature (RT) and 400 °C were investigated, aiming to uncover the mechanism behind the formation of fine granular area (FGA) surrounding welded pores. The research findings demonstrate that the S-N curves of TC17 titanium alloy electron beam welded joint undergo a transition from a single linear decline at RT to a bilinear decline at 400 °C. However, the fatigue failure mode remains unaffected by temperatures, and crack initiation is attributed to welded pores. By utilizing the Chapetti model curve to modify the Kitagawa-Takahashi (K-T) diagram, the lower threshold stress amplitude is introduced, enabling the determination of a safe size for welded pores at 400 °C, which is calculated to be 11.3 µm. Additionally, the Gumbel probability distribution function is employed to assess the maximum size of welded pores. Finally, based on dislocation interactions, the formation mechanism of the FGA consisting of discontinuous nanograins with high-density dislocations is elucidated.