Effects of surface condition on fatigue in nickel-based superalloys for aero-engine applications

Resistance against fatigue failure is a major requirement for critical rotating parts, which could be sensitive to surface condition after machining such as roughness, modification of microstructure and residual stress. The speed, tool material and other machining parameters can also have a significant effect on the surface quality of the parts. The effects of baseline and damage machining on the surface integrity of Alloy 720Li have been characterised comprehensively using metallography, EDX, surface roughness, micro hardness, nano-indentation, electron backscattered diffraction, differential scanning calorimetry and transmission electron microscopy. It was found that machining damage imparted a work-hardened layer, poorer surface roughness and surface features containing recrystallised material. A test matrix was constructed to establish whether machining induced changes of surface integrity would impact the fatigue performance of the material. Baseline samples and samples damaged by machining both with and without shot peening, and at different Kt factors were tested. Low cycle fatigue tests were conducted. Some damage machined samples exhibited a lower fatigue life. This was attributed to brittle surface features, caused by high temperatures and forces of machining, which resulted in early crack initiation