Due to the repeated thermal cycling that occurs with the processing of each subsequent layer, the microstructure of additively manufactured parts undergoes complex changes throughout the deposition process. Understanding and modeling this evolution poses a greater challenge than for single-cycle heat treatments. Following the work of Kelly and Charles, a Ti-6Al-4V microstructural model has been developed which calculates the phase fractions, morphology, and alpha lath width given a measured or modeled thermal history. Dissolution of the alpha phase is modeled as 1D plate growth of the beta phase, while alpha growth is modeled by the technique of Johnson–Mehl–Avrami (JMA). The alpha phase is divided into colony and basketweave morphologies based on an intragranular nucleation temperature. Evolution of alpha lath width is calculated using an Arrhenius equation. Key parameters of the combined Kelly–Charles model developed here are optimized using the Nelder–Mead simplex algorithm. For the deposition of two L-shaped geometries with different processing parameters, the optimized model gives a mean error over 24 different locations of 37% relative to experimentally measured lath widths, compared to 106% for the original Kelly–Charles model.