On the substitution of vanadium with iron in Ti–6Al–4V: Thermo-Calc simulation and processing map considerations for design of low-cost alloys

Abstract

Two basic cost reducing concepts, manipulation of alloy chemistry and optimisation of the hot working process, were adopted in developing low-cost experimental titanium alloys that may be suitable for land-based applications. Iron, a low-cost beta stabilising element in titanium alloys, was used as a partial and full replacement for vanadium in ASTM grade 5 Ti–6Al–4V. Thermo-Calc simulations were first done on the Ti–6Al-xV-yFe alloy compositions (for y = 4 – x and x = 1–3) to predict the amounts of phases and the β-transus temperatures. Thereafter, the compacted powders of the different experimental alloys were melted and allowed to solidify in the electric arc furnace cold copper hearth. Optical and scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterise the alloys. Also, the hardness of the low-cost alloys was measured and compared to commercial Ti–6Al–4V. The Ti–6Al–1V–3Fe alloy was subjected to hot compression testing at various temperatures (750–950 °C) and strain rates (1-10 s-1) on a Gleeble 3500 thermomechanical simulator. Processing maps and microstructural validation were used to define the most suitable processing conditions. The stress exponent and apparent activation energy were also calculated using a hyperbolic-sine equation. The results show that the low-cost alloys with partial substitution of vanadium with iron contained only α-Ti and β-Ti phases with no TiFe phase. The hardness of the alloys increased with increase in iron addition. The stress exponent “n” value was less than 5 indicating that flow softening was not solely driven by dynamic recovery. The optimum processing conditions for hot working were found at ~900 °C and 0.1 s-1 strain rate. The alloy generally had a large processing window with the softening process controlled by mechanisms such as dynamic recovery, dynamic recrystallisation of prior beta grains, and dynamic alpha lath globularisation. In the region of 750–780 °C and 1.5–10 s−1 an area of unstable deformation was established which should be avoided during processing. This work shows that low-cost α+β titanium alloys containing iron can be manufactured using traditional ingot metallurgy techniques and loss of material due to processing-induced defects can be minimised or avoided during primary conversion process such as forging and rolling.