Long fatigue crack growth behavior of Ti–6Al–4V produced via high-power laser directed energy deposition

The fatigue crack growth (FCG) behavior of long cracks was studied in Ti–6Al–4V alloy fabricated by high-power laser directed energy deposition. The results show the deposited parts under heat treated conditions have superior FCG resistance than that of the other reported additive manufactured Ti–6Al–4V, owing to the specimens with coarse prior-β grains and interior thick α-laths. Using linear elastic fracture mechanics and two (von Mises and Tresca) yielding criterions, we calculated the monotonic and cyclic plastic zone sizes, and found the FCG rates as a function of cyclic plastic zone sizes ahead of the crack tip during FCG region. A method was proposed to determine the typical microstructural characteristic unit of the additive manufactured Ti–6Al–4V parts with basket-weave structure, which shows the same length-scale as plastic zone sizes at the transition point in FCG curves.