Abstract
The effects of surface roughness parameters and residual stresses induced by finish turning on fatigue life and crack propagation behavior of Inconel 718 super-alloy are investigated. A criterion serving as a good indicator of surface quality is also discovered. This is carried out for some specific turning conditions in order to control surface characteristics including surface roughness and the bi-axial state of surface residual stresses and to investigate the effects of these characteristics on fatigue behavior. Rotating bending fatigue (RBF) tests are performed on the machined specimens at room temperature. Characterization techniques including X-ray diffraction (XRD), laser confocal microscopy, and opto-digital microscopy are used to investigate surface residual stresses in axial (ARS) and hoop (HRS) directions, surface roughness, and secondary fatigue cracks, respectively. The huge dispersion in fatigue life of smooth turned Inconel 718 with very low arithmetic average roughness (Ra) values indicates that surface finishing is not the only reliable criterion for quality control of machined components. Multivariate statistical analyses show that axial surface residual stress is the most important surface characteristic affecting the fatigue life of machined Inconel 718 for the conditions tested. Changing ARS from compressive to tensile increases the number of secondary cracks and also decreases the fatigue life of turned specimens.
Highlights
In today’s aerospace industry, the main concerns include achieving an optimum, reliable, and reproducible manufacturing process with high predictability of component service life at the lowest production cost
Polishing successfully decreased the level of residual stresses and produced a free-stress surface resulting in a target fatigue life of 106 cycles
The current study focuses on the effects of residual stresses and surface roughness on fatigue life and crack propagation behavior of turned Inconel 718 super-alloy
Summary
In today’s aerospace industry, the main concerns include achieving an optimum, reliable, and reproducible manufacturing process with high predictability of component service life at the lowest production cost. Some critical parts in engine hot compartments such as gas turbine disks are made of Inconel 718 super-alloy, which is arguably the most popular precipitationhardened alloy in this class [2] The manufacturing of these components involves various machining processes, namely milling, turning, and drilling, and each may have multiple steps, including roughing, semi-finishing, and finishing. The low thermal conductivity and high strain rate sensitivity of Inconel 718 favor the generation of high-magnitude tensile residual stresses at the surface and deteriorate fatigue life considerably [13, 20] It appears that, it is the main machining process used in the manufacturing of rotating parts such as turbine disks, the effects of turning-induced surface integrity on the fatigue life of Inconel 718 have not been investigated and understood thoroughly.
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