Degradation of titanium 6Al-4V fatigue strength due to electrical discharge machining

Electrical discharge machining (EDM) has been known for many years to induce degradation of fatigue strength. Outdated or coarse EDM processing methods can reduce the fatigue strength of titanium alloy Ti-6Al-4V by as much as factors of 2-5, but a paucity of fatigue data exists for this important aerospace alloy processed with modern EDM techniques. An experimental study is presented in which the degradation of fatigue strength of Ti-6Al-4V due to improved EDM processing is measured in axial tension at a load ratio of R = 0.1. It is shown that, relative to specimens finely milled, state-of-the-art EDM processing causes a reduction of fatigue strength by 15-30%. This strength degradation is found to correlate directly to the thickness and roughness of recast layers created by the solidification of melt material formed during EDM. Post-processing with either electrochemical polishing or bead blasting is demonstrated to remove the deleterious effects of EDM, such that specimens possessed intrinsic fatigue behavior as indicated by crack initiation at interior locations.