Fatigue Failure Mechanism and Crack Growth Behavior of Ti-6Al-4V ELI Titanium Alloy Welded Joints

Titanium alloy welded joints are key parts of deep-sea pressure hulls, which are subjected to fatigue loadings in service. In this study, axial fatigue tests, mode I fatigue crack growth tests, and mixed-mode I–II fatigue crack growth tests were conducted on the Ti-6Al-4V ELI titanium alloy welded joint, and its fatigue failure mechanism and crack growth behavior is investigated and compared with the base material. The results show that the S–N curve of Ti-6Al-4V ELI titanium alloy welded joints has a very similar slope as the base material, but its fatigue performance is lower than the base material. However, the welded joints exhibit a higher resistance in the near-threshold region under mode I loading compared to the base material. Scanning electron microscope observation indicates that the fatigue crack mainly initiates from gas pores during welding for the Ti-6Al-4V ELI titanium alloy welded joints. Under mixed-mode I–II loading, the stress intensity factor range component ΔKI of welded joints is higher than that of the base material, and an equivalent stress intensity factor range model is proposed to describe the crack growth rate under both mode I and mixed-mode I–II loadings. The new model incorporates a parameter dependent on the mode mixity ratio defined by ΔKII/ΔKI in this paper, and it unifies the crack growth data well under mode I and mixed-mode I–II loadings. The paper indicates that the gas pores during welding are an important factor for the poor fatigue performance of Ti-6Al-4V ELI titanium alloy welded joints.