Hot working of Ti-6Al-4V with a complex initial microstructure

Abstract

The hot deformation behaviour of wrought Ti-6Al-4V, with an initial microstructure of equiaxed and elongated α phase and intergranular β, was investigated. Isothermal hot compression testing was performed using a Gleeble 3500 thermomechanical simulator at strain rates of 0.01–10 s⁻¹, up to a total strain of 0.6, and at temperatures of 750–950 °C, i.e. in the α + β phase region. The stress-strain data were used to develop constitutive equations and processing maps so that the stress exponent, activation energy and the most advantageous processing conditions for deforming the alloy could be determined. Microstructural examination for validation of the processing maps was carried out by optical microscopy and scanning electron microscopy. The average activation energy (Q) and stress exponent (n) at all strains were typical of dynamic recrystallisation values reported for α + β titanium alloys. The processing maps showed different features at different strains. There was no domain of instability when samples were deformed to a total strain of 0.2 but regions of instability were observed at strains of 0.5 and 0.6. The optimum processing conditions were identified at ~900 °C/0.05 s⁻¹ and 940 °C/1.7 s⁻¹(0.2 strain); 900 °C/0.02 s⁻¹ and 945 °C/1.5 s⁻¹ (0.5 strain); and 800 °C/0.01 s⁻¹ and 940 °C/1.2 s⁻¹ (0.6 strain). Power dissipation efficiency values and microstructural features confirmed that the main deformation mechanism corresponded to dynamic globularisation of the α phase. Increased transformation of α-Ti to β-Ti also enhanced flow softening at higher deformation temperatures.