Phase composition and fatigue crack growth behavior of Inconel 718 under additive manufacturing

AM samples both in as-deposited and heat-treated states have been subjected to microstructural and phase characterization. Mechanical testing has been performed to heat-treated samples with respect to the build directions. AM samples were made by Direct Laser Deposition under the three different levels of efficient energy from Inconel 718 powder. As-deposited samples contained dispersed particles of Fe2Ti Laves phase, NbC and NbNC as well as starting clusters of gamma ' phase. Although the standard heat treatment at 980 degrees C was found to result in partial dissolution of Laves phase precipitation of 6 phase occurred around it. Depletion in Nb of Ni-based matrix due to formation of NbC, NbNC and Laves phase in as-deposited states causes precipitation of gamma ' phase as the main strengthening phase in heat-treated states. Fatigue crack propagation manners were analyzed with fractography. Fine columnar dendrite structure of samples tested in parallel build direction to the loading is much more resistant to fatigue crack propagation. Fatigue crack bypass dispersed Laves phase which is fixed in matrix by 6 phase.

Automated summary

The microstructural and phase characteristics of AM samples prepared by Direct Laser Deposition with different levels of energy efficiency were studied. In the heat-treated state, gamma' phase was identified as the main strengthening phase, while NbC, NbNC and Laves phase were found in the as-deposited state.