Effect of Variants of Thermomechanical Working and Annealing Treatment on Titanium Alloy Ti6Al4V Closed Die Forgings

Performance of titanium alloy Ti6Al4V pressure vessels made of closed die forged domes of route 'B' (multiple step forged and mill annealed) is reported to be better than route 'A' (single/two step forged and mill annealed). Analysis revealed that forgings processed through route 'B' have uniformity in microstructure and yield strength at various locations within the forging, as compared to that of route 'A.' It is attributed to in-process recrystallization (dynamic as well as static) of route 'B' forgings as compared to limited recrystallization of route 'A' forgings. Further, post-forging recrystallization annealing (RA) effect is found to be more significant for route 'A' forgings in achieving uniform microstructure and mechanical properties, since route 'B' forgings have already undergone similar phenomenon during the thermomechanical working process itself. Considering prime importance of yield strength, statistical scatter in yield strength values within the forgings have been evaluated for forgings of both the routes. Standard deviation in the yield strength of route 'B' forgings was lower (< 10 MPa) as compared to route 'A' (> 15 MPa), which later became lower (similar to 10 MPa) after RA with a minor decrease in yield strength. The present work discusses these variants of thermomechanical processing along with annealing to achieve better uniformity in properties and microstructure.