Investigation of short-term creep properties of a coarse-grained Inconel 718 fabricated by directed energy deposition compared to traditional Inconel 718

Abstract

Short-term creep performance of a coarse-grained Inconel 718 alloy fabricated by Directed energy deposition (DED) technique has been investigated at 650 °C in comparison to a conventional wrought material. In order to study the effect of heat treatment and sample orientation on the creep behaviour, as-deposited specimens oriented parallel and perpendicular to the building direction have been treated by double aging (DA), solution annealing + double aging (SA) and homogenization + solution annealing + double aging (HSA) conditions. Microstructural changes along with their impacts on high temperature mechanical properties have been examined with respect to grain size, crystallographic orientation, precipitates, and dislocation substructure. Results show that, superior creep strength and creep ductility of DED-manufactured DA specimen can be achieved compared to wrought specimen. The process-induced dislocation substructure and the irregular columnar grains with the random network of grain boundaries primarily contribute to the superior creep behaviour. However, when the unique columnar grain features formed during DED process are replaced by recrystallized, equiaxed grain structure in the HSA case, the worst creep performance is observed due to the coarse grains with regular-shaped grain boundaries facilitating the crack propagation. In addition, anisotropy of creep performance with respect to sample orientation observed for the heat-treated DED specimens can be significantly alleviated by a higher temperature of solution treatment.