Microstructural features of Ti-6Al-4V manufactured via high power laser directed energy deposition under low-cycle fatigue

Laser additive manufacturing (LAM) technique has unique advantages in producing geometrically complex metallic components. However, the poor low-cycle fatigue property (LCF) of LAM parts restricts its widely used. Here, the microstructural features of a Ti-6Al-4 V alloy manufactured via high power laser directed energy deposition subjected to low-cycle fatigue loading were studied. Before fatigue loading, the microstructure of the as-deposited parts was found to exhibit a non-homogeneous distribution of columnar prior-β grains (200–4000 μm) at various scanning velocities (300–1500 mm/min) and relatively coarse α-laths (1.0–4.5 μm). Under cyclic loading, fatigue microcracks typically initiated within the aligned α phases in the preferred orientation (∼45° to the loading direction) at the surface of the fatigue specimens. Fatigued Ti-6Al-4 V exhibited a single straight dislocation character at low strain amplitudes (<0.65 %) and dislocation dipoles or even tangled dislocations at high strain amplitudes (>1.1 %). In addition, dislocation substructure features, such as dislocation walls, stacking faults, and dislocation networks, were also observed. These findings may provide opportunities to understand the fatigue failure mechanism of additive manufactured titanium parts.