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 316 L stainless steel. We find that the use of ultrasound favours the columnar-to-equiaxed transition, which promotes 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.

通过基于融合的增材制造（AM）或3D打印制造的金属和合金会经历复杂的熔化和固化过程，这对有效控制晶粒结构提出了挑战。本文中，我们报道了使用高强度超声控制316 L不锈钢在增材制造过程中的凝固过程。我们发现超声的使用有利于柱状到等轴的转变，这促进了具有随机晶体织构的细等轴晶粒的形成。此外，超声波的使用将晶粒的数量密度从305 mm ^-2增加到2748 mm ^-2尽管在AM期间熔池中冷却速率和温度梯度会相应降低。我们对晶粒尺寸与冷却速率之间关系的评估表明，超声处理可增强AM过程中微晶的形成。此外，超声波的使用通过降低熔池整体中的温度梯度，增加了凝固过程中的组织过冷量，从而创造了有利于晶粒成核，生长和存活的环境。这种新的理解为更好地利用超声波控制AM制造金属产品中的晶粒结构提供了机会。