Effects of Wire Electrical Discharge Machining and shot peening on the axial High Cycle Fatigue strength of Inconel 718

Nickel-based superalloys are widely used in structural components subjected to fatigue loads at high temperatures. Inconel 718 is probably one of the most used materials, especially for the production of turbine discs. The disc-blade connection is one of the most critical zones due to the simultaneous presence of severe stress cycles, elevated temperature, and high-temperature gradient. Thanks to its low cost, Wire Electrical Discharge Machining (WEDM) can be considered the most effective technology for manufacturing fir-tree slots on turbine discs. However, it is known that WEDM has a detrimental effect on the material fatigue strength. Shot peening is usually used for enhancing fatigue performances. The paper deals with the effects of the WEDM and shot peening treatment on the fatigue strength of Inconel 718 in the range comprehended between 105 and 107 cycles. The investigation has been carried out by determining the Wöhler curve (S-N curve) under pulsed axial loading condition using specifically designed hourglass specimens. Tests were conducted at room temperature and at 1000°F. In order to identify the issues of the fatigue damage process, fractographic analysis has been conducted for each broken specimen using Optical (OM) and Scanning Electron Microscopy (SEM). No significant variation of the S-N curve has been found in the tested conditions, excepted for the reduction of the scatter of the data: shoot peening was seen to reduce the scatter of the fatigue endurance at low-stress amplitudes. The results of the fractographic analysis suggest that this effect can be correlated to a different initial phase of the crack propagation. Moreover, the two tested temperatures were found not to have significant effects on the Wöhler curve, both for the as-built and shot-peened specimens.