Numerical modeling of thermally driven phase transformation and α lath width evolution during laser directed energy deposition of Ti6Al4V alloy

As an important technology in additive manufacturing (AM), laser directed energy deposition (LDED) has gained increasing applications due to its high deposition rate, great volume density of formed parts, and simple system of manufacturing apparatus. Nowadays, an increasing number of aerospace components made of Ti6Al4V are being processed using LDED techniques to enhance material utilization and augment the freeform fabrication capability. However, as a crucial factor in determining the mechanical properties, predicting and controlling the microstructure morphologies of the AM processed Ti6Al4V components are still challenging. In this article, an integrated process-microstructure numerical model for phase transformation kinetics and alpha lath width evolution kinetics during the LDED process of Ti6Al4V components is proposed and validated. Firstly, the heat transfer model of the LDED process and its spatial multiscale considerations are introduced. Then, an integrated microstructural evolution model including the formation and dissolution of grain boundary alpha, Widmanst & auml;tten colony/basketweave alpha, martensite alpha' and beta phases, and the coarsening kinetics of Widmanst & auml;tten colony/basketweave alpha lath is proposed, validated, and compared with the previous models through numerous experimental data. Finally, the thermal model is verified and then coupled with the integrated microstructural evolution model to investigate the microstructural evolution during the LDED process of a Ti6Al4V block. The simulated volumetric phase fractions and alpha lath width distribution closely match experimental observed phenomenon, including layer band distribution, interlayer microstructure characteristics, and martensite features on the sample cross-section. Therefore, the proposed integrated process-microstructure numerical model could be useful for engineers to understand and control the part-scale or interlayer-scale process-microstructure relationships in LDED-processed Ti6Al4V parts.