  Fusion-based metal additive manufacturing (AM) is a disruptive technology
that can be employed to fabricate metallic component of near-net-shape with an
unprecedented combination of superior properties. However, the
interrelationship between AM processing and the resulting microstructures is
still not well understood. This poses a grand challenge in controlling the
development of microstructures during AM to achieve desired properties. Here we
study the microstructure development of a single melt pool, the building block
of AM-fabricated metallic component, using a phase-field model specifically
developed for the rapid solidification of AM. It is found that during the rapid
solidification of the melt pool, the solid-liquid interface is initially
controlled by solute diffusion followed by a thermal diffusion-controlled stage
with an undercooling larger than the freezing range. This switching of
controlling mechanisms leads to the sudden changes in interfacial velocity,
solute concentration, and temperature, which perfectly explains the formation
of various heterogeneous microstructures observed in AM. By manipulating the
processing conditions, the switching of controlling mechanisms can be
controlled to form refined microstructures or layered structures for improved
mechanical properties and resistance to cracking.
