  The elasto-plastic material behavior, material strength and failure modes of
metals fabricated by additive manufacturing technologies are significantly
determined by the underlying process-specific microstructure evolution. In this
work a novel physics-based and data-supported phenomenological microstructure
model for Ti-6Al-4V is proposed that is suitable for the part-scale simulation
of selective laser melting processes. The model predicts spatially homogenized
phase fractions of the most relevant microstructural species, namely the stable
$\beta$-phase, the stable $\alpha_{\text{s}}$-phase as well as the metastable
Martensite $\alpha_{\text{m}}$-phase, in a physically consistent manner. In
particular, the modeled microstructure evolution, in form of diffusion-based
and non-diffusional transformations, is a pure consequence of energy and
mobility competitions among the different species, without the need for
heuristic transformation criteria as often applied in existing models. The
mathematically consistent formulation of the evolution equations in rate form
renders the model suitable for the practically relevant scenario of
temperature- or time-dependent diffusion coefficients, arbitrary temperature
profiles, and multiple coexisting phases. Due to its physically motivated
foundation, the proposed model requires only a minimal number of free
parameters, which are determined in an inverse identification process
considering a broad experimental data basis in form of time-temperature
transformation diagrams. Subsequently, the predictive ability of the model is
demonstrated by means of continuous cooling transformation diagrams, showing
that experimentally observed characteristics such as critical cooling rates
emerge naturally from the proposed microstructure model, instead of being
enforced as heuristic transformation criteria.
