Effect of surface integrity generated by machining on isothermal low cycle fatigue performance of Inconel 718

Abstract

Critical aero-engine components such as turbine discs must withstand severe cyclic stresses, which can eventually lead to low cycle fatigue (LCF) failures. These components are made of difficult-to-cut alloys and machining conditions that are employed in the last stage of the manufacturing chain must be correctly defined to avoid the generation of an adverse surface condition (tensile residual stresses, excessive surface roughness or microstructural defects) that will accelerate fatigue failure. The analysis presented in this paper is aimed at understanding the isothermal LCF behaviour of turned Inconel 718 workpieces and finding quantitative correlations with the surface integrity. For this purpose, two Inconel 718 forged discs were face turned at cutting conditions employed in the aero-engine manufacturing industry using two different tools (1.2 mm and 4 mm nose radius respectively). The surface integrity produced by the face turning process was characterised in both discs: surface topography, residual stresses and surface layer anomalies. Specimens extracted from both discs were tested in load control at 450 ◦C to obtain LCF fatigue behaviour. Importantly, interrupted isothermal LCF fatigue tests were conducted and residual stresses were measured by the X-ray diffraction technique at the surface of the tested specimens to study the role of residual stresses in the LCF fatigue behaviour. For the tested conditions, the specimens machined with 4 mm nose radius showed 1.3–1.4 times longer fatigue lives than the specimens turned with 1.2 mm nose radius. These results are in agreement with the better surface integrity generated with the 4 mm nose radius, mainly because it induced lower tensile residual stresses. A novel local approach was implemented to understand the influence of surface integrity (surface roughness, surface residual stresses and altered stress-strain properties of the surface layer) on the isothermal LCF fatigue behaviour of both machined discs. Interestingly, a satisfactory correlation was found between the maximum applied stress in the surface layer and the isothermal LCF fatigue life employing the local approach.