  Understanding the spatio-temporal evolution of thermal gradient (G) at and
velocity (R) of the solid-liquid and liquid-vapor interfaces is critical for
the control of site-specific microstructures in additive manufacturing. In-situ
dynamic x-ray radiography (DXR) has been used in recent years to probe the
evolution of R with high spatial and temporal resolutions. However, the current
methods used to measure the temperature (and therefore, G) are inadequate for
sub-melt-pool surface measurement (e.g. thermography) or have lower resolution
or limited field-of-view (e.g. x-ray diffraction). In this study, we
demonstrate a novel approach to estimate the sub-surface temperature
distribution and its time evolution with significantly higher resolutions using
DXR data. This methodology uses the Beer-Lambert's law as a physical basis and
is demonstrated using an in-situ laser spot-melting experiment on Ti-6Al-4V
alloy.
