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Manually shaking or vibrating molten metal using ultrasonic waves helps reduce air bubbles, cracks and grain sizes in a finished metal part. Metal 3D printing researchers hypothesized that vibrations were the key to increasing quality, but until now, the mechanisms were not well understood.

Using high-energy X-ray imaging, a team of researchers led by Christopher Kube, associate professor of engineering science and of acoustics in the Penn State College of Engineering, captured footage of a cross-section of liquid metal as it cooled.

Their results confirmed longstanding hypotheses in the field that through local pressure changes, ultrasonic vibrations encourage air bubbles to increase in number, enlarge, migrate to the surface of a melt pool and pop.

Sonication, or vibration by ultrasound, also increases the speed that metal cools, which helps suppress additional bubbles from forming. The team published their findings in Communications Materials.

"Metal additive manufacturing has inherent constraints on part quality due to the process," Kube said. "Our work aims to alleviate these constraints by utilizing external forces like ultrasound to afford better control of the process, leading to higher quality and better performing parts."

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