  The governing mechanistic behaviour of Directed Energy Deposition Additive
Manufacturing (DED-AM) is revealed by a combined in situ and operando
synchrotron X-ray imaging and diffraction study of a nickel-base superalloy,
IN718. Using a unique process replicator, real-space phase-contrast imaging
enables quantification of the melt-pool boundary and flow dynamics during
solidification. This imaging knowledge informed precise diffraction
measurements of temporally resolved microstructural phases during
transformation and stress development with a spatial resolution of 100 $\mu$m.
The diffraction quantified thermal gradient enabled a dendritic solidification
microstructure to be predicted and coupled to the stress orientation and
magnitude. The fast cooling rate entirely suppressed the formation of secondary
phases or recrystallisation in the solid-state. Upon solidification, the
stresses rapidly increase to the yield strength during cooling. This insight,
combined with IN718 $'$s large solidification range suggests that the
accumulated plasticity exhausts the alloy$'$s ductility, causing liquation
cracking. This study has revealed additional fundamental mechanisms governing
the formation of highly non-equilibrium microstructures during DED-AM.
