Thermomechanical processing of a high strength metastable beta titanium alloy powder, consolidated using the low-cost FAST-forge process

摘要

The high strength titanium alloy Ti-5553 has been fully consolidated and thermomechanically processed from powder using the FAST-forge process, in only three steps, both at the small and the pilot scale. Titanium alloy components are conventionally produced using a time-consuming process, which involves carbo-chlorination extraction of TiO2, triple vacuum arc re-melting, and multiple thermomechanical and heat treatment steps, before machining. The proposed FAST-forge processing route for titanium alloy components cuts out or significantly reduces these stages, and uses field-assisted sintering technology (FAST) to consolidate powder. This paper assesses the effectiveness of the process for a conventionally used high-strength beta titanium alloy, Ti-5553. Ti-5553 has been fully consolidated by the FAST process at two different dwell temperatures, 850 and 1000 degrees C, and for a 30 min dwell time. Small-scale upset forging of cylinders machined from each FAST condition has been performed at forging temperatures 785, 810 and 835 degrees C, and strain rates 0.01, 0.1, 1 and 5 s(-1), in order to examine the flow stress behaviour. The flow behaviour of both FAST-produced Ti-5553 specimens was found to be very similar to conventionally produced Ti-5553, and the forged microstructures were also comparable. Large-scale forging has been performed on three double truncated cones machined from a FAST specimen produced at a temperature of 1000 degrees C with a 30 min dwell time. The cones were forged at 785, 810 and 835 degrees C, and at a strain rate of approximately 3 s(-1). Each forged specimen was heat treated for 4 h at a proprietary temperature between 750 and 850 degrees C, and aged for 8 h at a proprietary temperature below 650 degrees C in an inert atmosphere, before air cooling to room temperature. The forged microstructures and heat treated microstructures were found to be comparable to that of conventionally produced Ti-5553. Microhardness measurements of the heat treated specimens, with averages of between 410 and 417 Hv, were higher than that of conventionally produced Ti-5553, and were very consistent despite variations in strain and forging temperature. Successful scale-up of the process for a metastable beta titanium alloy indicates its potential to be utilised at an industrial scale.