(1) The fatigue strength of the unpeened specimens of Ti-6Al-4V alloy was found to be 490MPa, 430MPa, 410MPa and 440MPa at 20°C, 250°C, 350°C and 450°C, respectively. The respective values for the shot peened specimens were 620MPa, 520MPa, 440MPa and 260MPa. The fatigue strength was definitely increased by shot peening at 20°C, 250°C and 350°C, and the increments were about 27%, 21% and 7%, respectively. The fatigue strength, however, was remarkably decreased by shot peening at 450°C, and its decrement reached about 41%.
(2) The fatigue strength of the specimen shot-peened and then electro-polished was 470MPa at 450°C. The fatigue strength greatly increased from 260MPa to 470MPa after polishing. The value of 470MPa is comparable to 460MPa of the fatigue strength of the unpeened specimen.
The decrease in fatigue strength due to shot peening at 450°C may be explained as follows. The compressive residual stress in the near-surface region induced by shot peening disappear during fatigue cycling at 450°C, but the great surface roughness due to shot peening still remains, which reduce the fatigue strength of the shot peened specimen.