Effect of cooling rate on microstructure of Ti-6AI-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 954degreesC and specimens cut from the forging were annealed at 975degreesC for 30 min and cooled down to room temperature at different cooling rates ranging from 20 K min(-1) to 490 K min-1. The specimens had a duplex microstructure consisting of elongated primary alpha particles and secondary finer a lamellae with some residual beta phase layers between them. The average length and width of primary alpha 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 mum to 0.3 mum when the cooling rate increased from 25 K min(-1) to 490 K min(-1). Lamellar alpha contained 3.0-5.2 wt-%Al and 1.4-2.1 wt-%V, while the beta layers contained 1.4-2.8 wt-%Al, 12-18 wt-%V and 1.5-2.4 wt-%Fe. In the alpha lamellae, the concentration of aluminium increased and the concentration of vanadium went through a minimum with an increase in cooling rate. In the beta layers, the concentration of aluminium increased, while the concentration of vanadium decreased with increasing cooling rate. The volume fraction of the beta 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.