Effect of cooling rate on microstructure of Ti-6Al-4V forging

The objectives of this work were to study the microstructural changes and tensile properties that resulted from controlled cooling of a Ti-6Al-4V alloy that had been pancake forged. The alloy was forged at 954°C and specimens cut from the forging were annealed at 975°C for 30 min and cooled down to room temperature at different cooling rates ranging from 20 K min-1to 490 K min-1. The specimens had a duplex microstructure consisting of elongated primary a particles and secondary finer a lamellae with some residual ß phase layers between them. The average length and width of primary a particles decreased and the aspect ratio and volume fraction of particles increased when the cooling rate was increased. The average lamellar spacing decreased from about 1 µm to 0.3 µm when the cooling rate increased from 25 K min-1to 490 K min-1. Lamellar a contained 3.0 - 5.2 wt-%Al and 1.4 - 2.1 wt-%V, while the ß layers contained 1.4 - 2.8 wt-%Al, 12 - 18 wt-%V and 1.5 - 2.4 wt-%Fe. In the a lamellae, the concentration of aluminium increased and the concentration of vanadium went through a minimum with an increase in cooling rate. In the ß layers, the concentration of aluminium increased, while the concentration of vanadium decreased with increasing cooling rate. The volume fraction of the ß phase decreased from 10 - 12% to 4 -5% as the cooling rate increased from 25 to 490 K min-1. The tensile strength increased by approximately 7% when increasing the cooling rate from 25 to 370 K min-1, while the reduction in area achieved a 29% peak increase when the cooling rate was increased from 25 to approximately 325 K min-1. A discussion of the effects of cooling rate on microstructure and properties is also given.