Microstructure and fatigue crack growth behavior of heat-treated electron beam melted Ti-6Al-4V alloy

The effect of post-deposition heat treatment on microstructure and fatigue crack growth has been analyzed for electron beam melted Ti-6Al-4V plates. Samples have been heat-treated at temperatures of 950 degrees C and 1050 degrees C, and subsequently cooled at different cooling rates in the furnace and the water. The as-built sample possesses columnar prior beta grains filled with exceptionally fine alpha+beta beta Widmansta<spacing diaeresis>tten patterns and epitaxially grows in the vertical direction. Heat treatment with a slow cooling rate increases the alpha lath thickness, whereas fast cooling results in multiple needle-shaped alpha/alpha ' ' phases inside the prior beta grains. The yield strength and ultimate tensile strength in as-built conditions are greater than the extruded mill annealed sample. The as-built samples show better crack growth resistance during the fatigue crack growth rate test than the heat-treated and mill-annealed samples. The lower plastic deformation of the as-built sample than the mill-annealed sample is attributed to the existence of fine alpha laths that restrict the motion of dislocation.