Microstructure Evolution During Laser Additive Manufacturing of Ti6Al4V Alloys

Laser welding based additive manufacturing has become an enabling joining process for making one-of-a-kind parts, as well as, repairing of aerospace components. Although the process has been established for more than a decade, optimization of the process is still performed by trial and error experimentation. At the same time, deployment of integrated process-microstructure models has remained as a challenge due to some of the reasons listed below. (1) Lack of good process models to consider the laser-material interactions, (2) Inability to capture all the heat transfer boundary conditions, (3) thermo-physical-mechanical properties, and (4) robust material model. This work pertains to the development of robust material model for predicting microstructure evolution as a function of arbitrary thermal cycles (multiple heating and cooling cycles) that can be integrated into SimLAM process model. To validate the model, experimental builds were made by changing the processing, as well as, boundary conditions. Spatial variation of microstructural morphologies including grain boundary, Widmanstätten, basket weave, colony, and martensitic alpha were measured. The data show, due to extensive spatial variation of beta grain size, significant variations in microstructure morphologies was observed. For example, fine beta grain size and large fraction of grain boundary alpha was found near the substrate. However, near the top of the builds, extensive basket weave alpha was observed. Material models based on simultaneous transformation kinetics (STK) theory to explain these microstructure evolutions are being explored.