Microstructural Modeling of Thermally-Driven β Grain Growth, Lamellae &Martensite in Ti-6Al-4V

The microstructural kinetics of β grain growth in the β field of a Ti-6Al-4V alloy was studied by a series of controlled heat treatments at constant temperature rates. Heating rates of 5°C/s, 50°C/s and 500°C/s were considered, stopping at different peak temperatures. The thickness evolution of martensitic needles and lamellar α laths, formed on cooling, was also investigated, by soaking the material above its β-transus temperature and cooling down at 5°C/s, 50°C/s, 100°C/s and 300°C/s till ambient temperature. Quantitative microstructural analyses were used to measure the particle dimensions. The β grain growth kinetics was reasonably well described by a modified Avrami equation. The thickness of α lamellae was a function of the cooling rate and the β grain dimension in which they nucleated. The martensite needle thickness was shown to be a function of the cooling rate to which the material was subjected.

A Study of the Deformation Derivatives for a Ti-6Al-4V Inertia Friction Weld

The velocity–versus-time rundown curves from two experimental Ti-6Al-4V inertia friction welds were analysed and differentiated several times, to produce rotational acceleration, jerk, jounce (or snap), crackle and pop versus-times curves for each weld. Titanium alloys and their mechanical properties are known to be highly sensitive to strain rate as the material is deformed, though nothing has ever been considered in terms of the higher-order time-derivatives of position. These curves have been studied and analysed further, for a more complete understanding of the derivative trends. Rotational acceleration and jerk traces both display behavior patterns across the two welds as the part rotates under action from the flywheel. The rotational snap also displays a pattern in this derivative during the final approximately 0.5 s of welding, as the energy dissipates. Evidence of a distinct oscillatory pattern in the rotational crackle and pop terms was noted for one weld when differentiating over a larger time-base, though could not be replicated in the 2nd weld. The higher derivative curves allow distinction of different process regimes, indicating that inertial energy mostly influences the time-base of dynamically steady-state phase. Qualitative differences between initial energies are evident in higher derivatives.