Imaging-based fatigue mechanism investigation of additively manufactured TI-6AL-4V

The fatigue characterization of additively manufactured Ti-6Al-4V plays a vital role in ensuring the structural safety. This study focuses on image based surface characterization and the fatigue mechanical property investigation of as-built additively manufactured Ti-6Al-4V. Three sets of processing parameters (the absorbed laser power, scan velocity, building orientation) are adopted corresponding to the EOS nominal settings, lack-of-fusion and keyhole regimes. Before the fatigue testing, the specimens are scanned using X-ray micro-computed tomography (microCT) and the complete surface morphology is obtained. During fatigue testing, the specimen is scanned using microCT after certain numbers of loading cycles to capture the fatigue crack initiation locations and trace the crack growth trajectories. After the fatigue testing is completed, the fractured specimen is scanned by both microCT and scanning electron microscope (SEM). Based on the experimental investigation, vertically built specimens have lower average surface roughness than angled specimens along the transverse direction. Along longitudinal direction, the average surface roughness does not very significantly among all specimens. The fatigue crack may initiate from near surface pores or external rough surface. Cracks initiating from different locations at the similar height coalesce while propagating. Fracture surfaces present tortuous or tearing features, which corresponds to shorter and longer fatigue lives under the same fatigue loading, respectively.