Investigation of rejuvenation of fatigue damage in Ti--6Al--4V. Interim report, 1 January 1976--15 March 1977

The feasibility of restoring the fatigue properties of fatigue-damaged Ti--6Al--4V by using hot isostatic processing (HIP) was investigated. Ti--6Al--4V specimens in the beta solution treated and over-aged condition were fatigue damaged to two levels of damage: just before crack initiation and after crack initiation. The fatigue specimens were coated with a Ti--6Al--4V coating after damaging to isolate the surface cracks from the HIP atmosphere and cause them to be closed during HIP. Metallographic examination showed the cracks were not all bridged by the coating and these unbridged cracks did not close-during HIP. Evidence was obtained to indicate the fatigue cracks which were bridged by the coating could be closed and bonded under appropriate conditions. The results of the fatigue damage, coating and HIP experiments indicated that the total fatigue life of fatigue damaged specimens could be extended by coating and HIP, including specimens having surface fatigue cracks. Although the feasibility of the process has been demonstrated, further work to explore other coating procedures and more extensive fatigue testing is required to define the full potential and limitations of the process.