Abstract
Aerospace-grade Ni-based alloys such as Inconel 718 and 625 are widely used in the airspace industry thanks to their excellent mechanical properties at high temperatures. However, these materials are classified as ‘difficult-to-machine’ because of their high shear strength, low thermal conductivity, tendency to work-harden and presence of carbide particles in their microstructure, which lead to rapid tool wear. Machining-induced residual stresses in a machined part is an important parameter which is assessed since it can be used to evaluate overall structural resilience of the component and its propensity to fatigue failure in-service. 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 tool’s movement in a cutting process. Several studies demonstrated successfully the resulting improvements in cutting forces and surface topography. However, a thorough study of UAT-induced residual stresses is missing. In this study, experimental results are presented for machining Inconel 718 and 625 using both conventional turning (CT) and UAT with different machining parameters to investigate the effect on cutting forces, surface roughness and residual stresses in the machined parts. The 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 when compared to those in CT. Thus, UAT demonstrates an overall improvement in machinability of Inconel alloys.
Highlights
As heat-resistant superalloys, Inconel alloys are extensively used in the aerospace industry for components exposed to harsh environments, such as in critical parts of gas turbine engines, nuclear power plants and pressure vessels
Outeiro et al [4] and Wardany et al [5] reported that thermal loads promoted tensile stresses, with mechanical loads being responsible for compressive residual stresses
The aim of this paper is to study the feasibility of improving the machinability of nickelbased alloys by employing a hybrid machining process— Ultrasonically assisted turning (UAT), and compare its effect on the machinability of two nickel-based alloys under different machining parameters together with resultant residual stresses
Summary
As heat-resistant superalloys, Inconel alloys are extensively used in the aerospace industry for components exposed to harsh environments, such as in critical parts of gas turbine engines, nuclear power plants and pressure vessels. Assessing structural integrity of parts is essential for reliability analysis. Machining studies showed that the residualstress profile was typically tensile in nature at a machined workpiece surface [3]. Outeiro et al [4] and Wardany et al [5] reported that thermal loads promoted tensile stresses, with mechanical loads being responsible for compressive residual stresses. Compressive residual stresses are beneficial to service life of components, whereas tensile ones are detrimental [6].
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