High-cycle Fatigue Failure Mechanism of Wire Arc Additive Manufacturing (WAAM) Repaired Ti-5Al-2Sn-2Zr-4Cr-4Mo Alloy

Understanding the high-cycle fatigue failure mechanism is essential for the reliable application of wire arc additive manufacturing Ti-5Al-2Sn-2Zr-4Cr-4Mo in blade repair under practical engineering conditions. In this study, its microstructural evolution and fatigue failure behaviors were systematically investigated through microstructural characterization, mechanical tests, and transmission electron microscopy. Pronounced microstructural heterogeneity was observed across the additive zone. The base metal consists of primary α phase and β matrix, while the additive zone is characterized by columnar β grains, which are primarily composed of fine acicular α and a small amount of “slide-wall” structure composed of grain boundary α/α colonies. The synergistic impact of β stabilization and the decrease in α phase content, driven by repeated thermal cycling, renders the additive zone the most mechanically weak region. It is found that fatigue cracks preferentially initiated in the “slide-wall” structure, where long-distance slip of dislocations occurred across α colonies and was blocked by the grain boundary α, resulting in high-density dislocation accumulation and stress concentration, leading to crack initiation. This work contributes to a better understanding of high-cycle fatigue failure in WAAM Ti-5Al-2Sn-2Zr-4Cr-4Mo alloy, indicating that the “slide-wall” structure formed under thermal cycling governs crack initiation and has important implications for microstructural control in repaired blades.