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

Abstract

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 α′ martensite. The α′ martensite in the Ti–6Al–4V repaired by in-air DED decomposed into α+β 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-β 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-β boundaries in the Ti–6Al–4V repaired by in-air DED was not obvious and the local colony α+β played a dominant role in the resistance of short fatigue crack propagation.