Machining for an increased fatigue life for a Ti-6Al-4V ELI component

Abstract

Abstract Fatigue life is an important design criterion for many structural parts. The surface integrity induced by a typical machining operation may have a marked effect on fatigue life. Literature evidence suggests that the compressive surface residual stress state may change appreciably from the state obtained at the recommended conventional cutting speeds when intermediate cutting speeds are utilized. Titanium alloys typically show an increased compressive residual stress state at intermediate speeds before eventually tending towards tensile stresses at elevated cutting speeds. This paper investigates the effect of an optimised cutting strategy to control the residual stress state of a machined Ti-6Al-4V ELI (extra low interstitial) component to thereby control the fatigue life. An experimental program is conducted whereby stepped shafts are machined at various cutting speeds before being subjected to a full cyclic in-plane bending moment fatigue test. Cutting speed is varied between the typically recommended speed of 40 m/min to an intermediate speed of 110 m/min. Residual stresses are measured by XRD and compared to the fatigue life achieved. Other surface integrity descriptors including surface roughness, sub surface microstructure and selected surface damage effects are presented and discussed in relation to the fatigue performance. In essence the paper reports that fatigue life can be improved by machining at the optimum cutting speeds. These optimum intermediate speeds do however introduce other less desirable effects such as an increase in surface damage including the presence of weldments and ploughing grooves that may have a negative effect on the reliability of the improvements obtained.