Corrosion-Fatigue of L-PBF Ti-6Al-4V Lattice Struts: Role of Geometrical Imperfections under Simulated Physiological Conditions

The corrosion-fatigue behaviour of laser powder bed fusion (L-PBF) Ti-6Al-4 V lattice struts was investigated with particular emphasis on the role of manufacturing-induced geometrical imperfections. Thin strut specimens representing strut-based lattice sub-unit elements were built at a 60° orientation and tested under tension–tension loading (R = 0.1) in laboratory air and in phosphate-buffered saline (PBS) at 37 °C. Micro-computed tomography was employed to quantify surface roughness, geometrical deviations, and the interaction between surface valleys and near-surface porosity. Quasi-static tensile tests showed limited scatter in strength and ductility, indicating that monotonic behaviour is governed by global geometry. In contrast, fatigue performance was strongly defect-sensitive. While comparable fatigue strength was observed in air and PBS in the low-cycle regime, exposure to PBS led to a marked reduction in high-cycle fatigue strength, reaching approximately 25% at 10⁶ cycles. Fractographic and EDXS analyses revealed that fatigue cracks initiated at surface-connected valleys in both environments, whereas the physiological environment primarily accelerated crack propagation through corrosion-assisted mechanisms and suppressed crack branching. A micro-CT-based deepest-valley analysis showed good agreement between predicted critical defects and experimental failure locations. The results highlight the dominant role of extreme surface geometrical imperfections and environment-assisted crack growth in the fatigue behaviour of L-PBF Ti-6Al-4 V lattice structures.