Fundamental understanding of the transient melt pool dynamics, solidification kinetics and build texture in spot-melt additive manufacturing of Ti-6Al-4V

The overarching goal of this dissertation is to better understand the underlying process-structure relationships in play during the implementation of a spot melt strategy for metal additive manufacturing, which has become a popular alternative to the conventional raster melt strategy for site-specific microstructure control. In the first part of this dissertation, the effect of a spot melt strategy on the solidification texture, variant selection, phase fraction, and their variations along the build height of an E-PBF Ti-6Al-4V is investigated in comparison to a conventional linear melt strategy using high-energy synchrotron x-ray diffraction. In spite of the thermal excursions involved, the a [alpha] phase exhibit a Burgers orientation relationship (BOR) with the parent b [beta] phase for both melt strategies. Overall, the novel spot melt strategy produces a more homogeneous microstructure in terms of both the phase fraction and texture across the build height.

Motivated by these findings, the second part of this dissertation tries to understand the fundamentals of microstructure formation in laser spot melts (on Ti-6Al-4V alloy, as a function of laser power) by probing the transient evolution of melt pool dimensions and solid-liquid interface velocity using a rapid (mm-ms resolution) in-situ, dynamic synchrotron x-ray radiography (DXR) and post-mortem EBSD. Solidification kinetics was observed to follow three stages (with a prominent steady state) in both the conduction and keyhole mode. Further, the steady-state solidification velocity was found to decrease with increasing laser power. Finally, physical processes operational during the melting and solidification events are ascertained using dimensional analysis and their bearing on solidification microstructure is discussed.

In the last part of this dissertation, the conversion of radiography images of a spot melting event to density maps (using Beer-Lambert’s law as the physical basis) and further, to transient sub-surface temperature evolution is demonstrated. In summary, a deeper understanding of the fundamental processes behind the spot melt strategy has been achieved in this work through experimental means such as in-situ radiography, ex-situ diffraction, and post-mortem EBSD. This understanding to serve as a basis to validate existing and improve spot melt pool models and help AM process design.