High stability and high strength β-titanium alloys for additive manufacturing

Abstract

Metastable β-Ti alloys are attractive for additive manufacturing as they offer high strength and toughness, with the ability to readily tailor properties through control of the phase fraction and morphology of precipitate phases. However, during additive manufacturing processes thermal cycling associated with repetitive layer-by-layer deposition can potentially promote unintended β-phase decomposition and in-situ precipitation of other phases. This work investigates the stability of the β-phase during high heat input (1648 J/mm) Wire Arc Additive Manufacturing (WAAM) of Ti–3Al–8V–6Cr–4Mo–4Zr (Beta-C). Thermocouples and pyrometers to first used to measure the thermal environment during WAAM, and SEM, XRD and TEM characterisation techniques are used to investigate the microstructure of the as-built alloy. Despite observing large thermal excursions during WAAM well above the temperatures necessary to precipitate α-phase, no evidence of β-phase decomposition was found during WAAM. A Time-Temperature-Transformation (TTT) diagram is used to show that the cooling rates are too fast and the cumulative thermal exposures are too short to decompose the β-phase during WAAM. Consequently, the alloy retains the bcc β-phase which gives the alloy moderate strength (tensile strength ~748 MPa) and good ductility (~20%) during WAAM.