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

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-6Al-
2Sn-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.