  Metals and alloys fabricated by fusion-based additive manufacturing (AM), or
3D printing, undergo complex dynamics of melting and solidification, presenting
challenges to the effective control of grain structure. Herein, we report on
the use of high-intensity ultrasound that controls the process of
solidification during AM of 316L stainless steel. We find that the use of
ultrasound favours the columnar-to-equiaxed transition, promoting the formation
of fine equiaxed grains with random crystallographic texture. Moreover, the use
of ultrasound increases the number density of grains from 305 mm-2 to 2748 mm-2
despite an associated decrease in cooling rate and temperature gradient in the
melt pool during AM. Our assessment of the relationship between grain size and
cooling rate indicates that the formation of crystallites during AM is enhanced
by ultrasound. Furthermore, the use of ultrasound increases the amount of
constitutional supercooling during solidification by lowering the temperature
gradient in the bulk of the melt pool, thus creating an environment that
favours nucleation, growth, and survival of grains. This new understanding
provides opportunities to better exploit ultrasound to control grain structure
in AM-fabricated metal products.
