Grain refinement of additively manufactured stainless steel by ultrasound

Metallic 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 application of high-intensity ultrasound that controls the solidification process during AM of 316L stainless steel. We find that the application of ultrasound favours the columnar-to-equiaxed transition, which can promote the formation of fine equiaxed grains with random crystallographic texture. The grain number density increases from 305 mm-2 to 2748 mm-2 by using ultrasound 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, ultrasound enables increased constitutional supercooling by lowering the temperature gradient in the bulk of the melt, thus creating a solidification environment that favours grain nucleation, growth and survival. This new understanding provides opportunities to better exploit ultrasound for the control of grain structure in AM-fabricated metallic materials.