Aerospace-grade alloys such as Inconel 718 are widely used in the airspace industry primarily thanks to their excellent mechanical properties at high temperatures. However, these materials are classified as ‘difficult-to-machine' due to their high shear strength, tendency to work-harden and presence of carbide particles in their microstructure, which lead to rapid tool wear. In addition, low thermal conductivity of Inconel 718 requires appropriate thermal management to prevent temperate-induced tool failure. Post-machining, it is important to assess structural integrity of machined parts. Machining-induced residual stresses in the machined part is an important parameter as it can be used to assess overall structural resilience and the propensity of a part to suffer fatigue failure. To improve structural integrity of machined parts, various non-conventional machining techniques have been introduced over the years. Ultrasonically assisted turning (UAT) is a hybrid machining technique, in which tool-workpiece contact conditions are altered by imposing ultrasonic vibration (typical frequency ~20 kHz) on a cutting tool's movement in a cutting process. Several studies demonstrated successfully the resulting improvements in cutting forces and surface topography. However, a thorough study on UAT-induced residual stresses is missing. In this study experimental results are presented for machining Inconel 718 using both conventional turning (CT) and UAT with different cutting speeds to investigate the effect on cutting forces, surface roughness and residual stresses in the machined parts. Our study indicates that UAT leads to significant cutting-force reductions and improved surface roughness in comparison to CT for cutting speeds below a critical level. The residual stresses in machined workpiece show that UAT generates more compressive stresses and reduces tensile stresses when compared to those in CT. Thus, UAT improves the overall machinability of Inconel 718.
Bai, W., Bisht, A., Roy, A., Suwas, S., Sun, R., & Silberschmidt, V. V. (2018). Effect of hybrid machining on structural integrity of aerospace-grade materials. In Procedia CIRP (Vol. 77, pp. 163–166). Elsevier B.V. https://doi.org/10.1016/j.procir.2018.08.266