Influence of process parameters and heat treatments on the microstructures and dynamic mechanical behaviors of Inconel 718 superalloy manufactured by laser metal deposition

Abstract

To achieve high-performance nickel-based superalloys by laser metal deposition (LMD) technology for applications in aeroengines, we prepared Inconel 718 superalloys by LMD with three groups of process parameters and then heat treated them by two different protocols. We carried out compressive experiments for these Inconel 718 samples over a wide range of strain rate (0.001–5000/s) to evaluate the effects of process parameters and heat treatments on their microstructures and dynamic mechanical properties. We observed both the initial microstructures and the failure characteristics of the samples using the optical microscope and the scanning electron microscope. We found that a higher energy input density during laser additive manufacturing led to a wider range of primary dendrite spacing. The plastic flow stress of the alloy decreased near-linearly with increase in primary dendrite spacing. The anisotropy of the compressive properties of the sample resulted from the anisotropy of the as-deposited and the direct aged structures, while the microstructural and mechanical anisotropy almost vanished after full heat treatment. We further carried out compressive experiments over a wide range of strain rate (0.001–5300/s) and temperature (298–1193 K) to understand the mechanical properties of Inconel 718 by LMD in an extremely high-strain rate and high-temperature loading environment. We noticed an anomalous high-temperature peak in the flow stress, in the flow stress vs. temperature relation, under different strain rates, and we proved that it is attributed to the third type of strain aging effect. Finally, by observing the compressive failure characteristics, we found that the propagation path of a crack is dependent on the loading direction. The compressive fractography morphology could reflect the effect of heat treatment on the ductility of the samples. Furthermore, it was evident that the initial defects (gas and shrinkage porosities) in Inconel 718 samples caused by LMD can contribute to the generation, deflection, and branching of cracks.