Non-contact ultrasound to assist laser additive manufacturing.

In ultrasound-aided laser melting processes such as additive manufacturing, it is generally believed that acoustic cavitation is essential for grain refinement during solidification while acoustic streaming plays a negligible role. We propose a non-contact ultrasound approach to provide low-intensity ultrasound, i.e., below the melt cavitation threshold, ensuring a pure acoustic streaming regime. Without cavitation, it is found that fine equiaxed grains still can be achieved. This is attributed to the combined effects of acoustic streaming and Marangoni force, which create a high-frequency-shaking type melt flow in the melt pool, leading to fatigue fracture of dendrites and thus grain refinement. Moreover, low-intensity ultrasound can offer stable melt pool modulation throughout layer-by-layer processing, enabling uniform grain refinement in large-scale samples, which is a challenge for the current direct-contact ultrasound approach.