Improved HCF Performance and FOD Tolerance of Surface Treated Ti-6-2-4-6 Compressor Blades

Abstract

High cycle fatigue (HCF) strength and the resistance to foreign object damage (FOD) can be improved by the use of mechanical surface treatments like shot peening and low plasticity burnishing (LPB) to introduce beneficial surface layers of compressive residual stress. In this paper, results from an extensive study of the relative effects of these two surface treatments on the residual stress, cold work distributions, HCF performance, and FOD tolerance of alloy Ti-6Al2Sn-4Zr-6Mo (Ti-6-2-4-6) are presented. The compressive layer produced by LPB is shown to be stable even after thermal exposure to 371C. Blade-edge bending fatigue specimens were designed to simulate the leading edge of an integral bladed rotor (IBR) compressor blade. FOD was simulated by controlled size notches introduced on the specimens using electrical discharge machining (EDM). Both disk and blade simulation specimens with 0.5 mm (0.020 in) deep FOD had HCF strengths after LPB over 4-times higher than 8A shot peening. The HCF performance after LPB was relatively unaffected by FOD up to 0.75 mm (0.030 in) deep. FOD up to 2.5 mm (0.10 in) in depth after LPB decreased the fatigue strength only nominally. If the traditional design criterion of Kt (notch sensitivity factor) of 3 were to be used, LPB effectively mitigated FOD damage up to 2.5 mm (0.10 in) deep.