Thermomechanical behavior of laser metal deposited Inconel 718 superalloy over a wide range of temperature and strain rate: Testing and constitutive modeling

Abstract

The lack of comprehensive insight and accurate description on the dynamic thermomechanical behaviors in high-temperature and high-strain-rate loading environment could be one of the major obstacles to extend the laser metal deposited (LMD) Inconel 718 to engineering applications in aeroengine. In this study, to obtain in-depth understanding of plastic flow behavior of the LMD Inconel 718, uniaxial compressive experiments were conducted over a wide range of temperature (298–1273 K) and strain rate (0.001–5300/s). Based on a great number of experimental results, not only the anisotropy of compressive property, but the dependence of the plastic flow stress on temperature and strain rate were investigated systematically. The flow stress of the as-deposited alloy in laser scanning direction is higher than that in depositing direction, which is attributed to the columnar grains epitaxially growing along the build direction and featuring the high length-diameter ratio within the as-deposited sample. Since the small-size precipitates in the under-aged (as-deposited) Inconel 718 alloy could be cut by the moving dislocations directly during plastic deformation, they have various effect on the interaction between moving dislocations and interstitial elements at different strain rate. The third-type of strain aging (3rd SA) effect, exhibiting an anomalous bell-shaped flow stress vs. temperature relation, was observed more apparently than that in the post-aged Inconel 718 reported by other literatures. As for the strain-rate effect, the flow stress of the alloy exhibits inconspicuous strain-rate sensitivity over the range of strain rate below ∼103/s, while increases sharply with the strain rate once it exceeds ∼103 /s. Finally, taking into account the mechanical anisotropy, as well as the anomalous temperature and strain rate sensitivities of the flow stress, we developed a constitutive model based on a physical frame. In this model, an anisotropy component was introduced to consider the effective size of columnar grains in the corresponding loading direction, the strain rate hardening was enhanced as the strain rate gets close to a certain level, and a normal distribution with temperature was used to describe the 3rd SA effect. The developed model was shown to be able to accurately reproduce the plastic flow behavior of the LMD Inconel 718 over a wide range of temperature and strain rate.