In-situ SEM investigation on the fatigue behavior of Ti-6Al-4V ELI fabricated by the powder-blown underwater directed energy deposition technique

Underwater directed energy deposition (UDED) is a promising technology for on-site maintenance and repair of Ti-6Al-4V alloy. The microstructurally short fatigue crack initiation and growth behaviors of Ti-6Al-4V repaired by UDED were investigated through in-situ SEM fatigue testing. The results were compared with those of the Ti-6Al-4V repaired by in-air directed energy deposition (DED) and a substrate. The experimental results show that the microstructure of the Ti-6Al-4V repaired by UDED was dominated by alpha' martensite. The alpha' martensite in the Ti-6Al-4V repaired by in-air DED decomposed into alpha+beta due to the intrinsic heat treatment. Both the strength and microhardness of the Ti-6Al-4V repaired by UDED were higher than those of the Ti-6Al-4V repaired by in air DED. Compared with the Ti-6Al-4V repaired by in-air DED, the Ti-6Al-4V repaired by UDED presented a short micro crack initiation time and a poor fatigue crack propagation resistance. The short fatigue crack initiation and propagation behaviors of the as-deposited Ti-6Al-4V were primarily controlled by the structures and morphologies of the microstructures rather than the residual stresses. In addition, the prior-beta boundaries in the Ti-6Al-4V repaired by UDED had a resistance effect on the fatigue crack growth rate. However, the resistant effect of prior-beta boundaries in the Ti-6Al-4V repaired by in-air DED was not obvious and the local colony alpha+beta played a dominant role in the resistance of short fatigue crack propagation.