Microstructural modelling of above β-transus heat treatment of additively manufactured Ti-6Al-4V using cellular automata

A heat treatment is an essential part of the metal additive manufacturing process chain. If an additively manufactured part, made of Ti-6Al-4 V, is heated above its β transus temperature, the columnar prior-β grains will become equiaxed β grains. This work quantitatively models this transition and the subsequent cooling down to room temperature by using the well-established cellular automata (CA) technique.

Using this microstructural model allows visualisation of the local variation in the microstructure. The final microstructure consists of both the equilibrium phase α and β, organised in laths. This paper shows that the developed CA is capable of modelling the microstructural evolution during the entire above-β transus heat treatment.

In order to get an accurate simulation of the microstructural change during such a heat treatment, the nucleation and grain growth functions are dependent on temperature. Since there exists a thermal gradient throughout the simulated cube, the local values of these functions will vary, leading to spatial differences in the nucleation frequency and growth velocity of new β grains.

The model is verified by comparing the transformed volume fraction with a typical Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation for isothermal grain growth. However, the JMAK equation insufficiently describes the grain growth during the initial stage of the heat treatment, namely while heating up to above the β transus temperature.

Finally, the simulations of the second half of the heat treatment show that there are underexplored mechanisms during the growth of α laths when cooling down to room temperature. The simulations show, that it is not a requirement to nucleate α in the centre of the former β grains to form basketweave α.

Moreover, the basketweave morphology in the simulated microstructures is a result of the difference between the viewing plane of the microstructure and the plane in which the laths grow, with a pure Widmanstätten morphology only appearing when the planes are parallel.