Investigation of Ti-6Al-4V Microstructure Development with Variable Cooling Rates in Laser Powder Bed Fusion

Ti-6Al-4V is an alloy that can develop numerous possible morphologies that are all highly dependent on the temperature history the material experiences. Laser powder-bed fusion (LPBF) can be used to create unique parts with high resolution from Ti-6Al-4V, but also introduces complex thermal histories and very high cooling rates that can create varied microstructure. Prior work has established parameters for fully dense parts in LPBF, however even with fully dense parts, the resulting mechanical properties can be impacted by the unusual microstructure that develops in the process window. This study examined the microstructure of fatigue samples for a broad series of printing parameters to further the analysis of fatigue results conducted by the NASA University Leadership Initiative. Additionally, a novel experimental part was developed by this group to intentional induce an elevated temperature through heat accumulation during the printing process. This part is called the “inverted pyramid“, and extensive analysis of the different conditions for microstructure development has been done with scanning electron microscopy, X-ray diffraction, microhardness, polarized light microscopy, and electron backscatter diffraction. Two inverted pyramids were printed, one of which had additional delay time between layers so that less heat would accumulate. SEM micrographs of both fatigue bar samples and the inverted pyramid samples revealed martensitic α’ morphology in the final parts. Initial SEM micrographs of fatigue bars with a relatively slow laser scan speed showed diffusional α lath structure. In the inverted pyramid that had higher heat accumulation, SEM micrographs and X-ray diffraction showed diffusional α lath structure where the temperature was highest. Polarized light microscopy of the inverted pyramids revealed that the higher temperature pyramid had significant coarsening of grain boundary α. A thermal simulation was also performed for the inverted pyramids to confirm the heat accumulation and provide thermal histories for predicting the morphological development. The temperature histories from the thermal simulation were used as input for a model that calculates Ti-6Al-4V morphological development to predict the expected microstructure. Comparison of the calculated cooling rate from the thermal simulation with the microstructure results suggests that more details besides cooling rate may be needed to predict the morphology fractions. Comparison between the predictions and physical observations help to qualify the microstructure model which can further aid in the development of a digital twin framework for additive manufacturing.