Experimental and model-based analysis of machining-induced residual stresses and part distortion when turning Inconel 718

• Applicants: Professor Dr.-Ing. Thomas Bergs; Professor Dr.-Ing. Christoph Broeckmann
• Subject Area: Metal-Cutting and Abrasive Manufacturing Engineering
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 494882739

Functionality, operational behavior and service life of highly stressed, safety-critical jet engine components, such as turbine disks, rings, and blades that are made out of the nickel-based alloy NiCr19Fe19Nb5Mo3 (Inconel 718) are largely determined by the dimensional accuracy and by the properties of the machined surface. Therefore, manufactured turbine parts have to meet the highest demands concerning part distortion, residual stresses and microstructure modifications in the machined surface. The adjustment of these component properties is usually achieved by the final machining step of forged and subsequently heat-treated semi-finished products. During the machining process, the thermo-mechanical load spectrum induces internal stresses near the machined surface, which superimpose on the inherent internal stresses. This superposition of residual stresses leads to a distortion of the component, which is difficult to control, especially in turning operations where large volumes of material are being removed. The distortion of the component can result in time-consuming and costly reworking or, in the worst case, in scrap parts. The avoidance of the machining-induced component distortion a priori has not yet been achieved in its entirety and its compensation required time-consuming and cost-intensive iterative design cycles.The main goal of this research project is the fundamental investigation of the influencing factors and mechanisms during the generation of machining-induced residual stresses and part distortion when turning Inconel 718. Based on this, a multiscale numerical modeling approach will be developed, which allows an efficient prognosis and adjustment of residual stress-induced part distortion. The potential of the modeling approach for the description of the thermo-mechanical loads and the adjustment of resulting residual stresses as well as part distortion will be analyzed by means of analogy machining tests on rectangular samples of Inconel 718. Thereafter, the modeling approach will be transferred to longitudinal turning of a demonstration ring that is made out of Inconel 718 by means of a model-based description of the specific cutting force and heat flux. The initial residual stresses in the workpiece will be computed by the numerical simulation of the heat treatment and will be considered as initial stress state in the discretized workpiece model for the virtual turning operation. For a successful processing of this challenging research project, an interdisciplinary collaboration between researchers from the field of material science and machining technology is required.

• DFG Programme: Research Grants