Decoupling the effects of internal stresses from heat treatment and machining induced residual stresses, on distortions of Inconel 718 turned parts

Several mechanisms can generate distortions during machining. Among them, residual stresses play a major role. These stresses originate both from the raw material manufacturing process (Internal Residual Stresses, IRS) and from machining itself (machining induced Residual Stresses, MIRS). The goal of this study is thus to distinguish the respective contributions of IRS and MIRS to the distortions of thin turned parts made of Inconel 718 QT. This work details the determination of the stress state in the raw material through a synergistic approach confronting heat treatment simulation and residual stress measurement using the contour method. The results show consistency between the contour method stresses estimation and stresses from heat treatment (HT) simulation, both in terms of stress states and distortions induced by their balancing after machining. In fact, deviations are below 20% over most of the investigated cut surfaces. These findings validate the HT simulation and pave the way for a digital continuity using Forge© software. The simulation then enables the study of IRS balancing during turning. Finally, machining tests are carried out using “fresh” tools, which generate low MIRS, and “worn” tools, which generate high MIRS. The comparison between boolean material removal simulations and machining tests with new and worn tools provides a better understanding of the coupling between distortions due to IRS and MIRS, while also quantifying these effects. Fresh-tool experiments show that internal residual stress balancing mainly generates a front-face concavity, with peak-to-valley distortions up to approximately 0.2 mm. Under worn-tool conditions, machining-induced residual stresses progressively counteract this IRS-driven distortion and can reverse the concavity as the part becomes thinner, with peak-to-valley distortions increasing up to approximately 0.4 mm when IRS and MIRS are combined.