I. Technical Background: Why Has Ultrasonic Atomization Become the Preferred Method for Powder Production?

At present, the mainstream methods for producing metal powders used in additive manufacturing are gas atomization and water atomization. However, these processes suffer from inherent limitations: poor powder‑property consistency can lead to unstable material rheology and energy absorption during printing, ultimately resulting in defects such as closed porosity, non‑uniform grain structure, and fluctuating mechanical properties in the final part.

Ultrasonic-assisted melt atomization As a novel powder‑making technology, it relies on ultrasonic vibration to shatter molten metal into tiny droplets; upon rapid solidification, these droplets yield metallic powders. Compared with conventional processes, it offers significant advantages:

• It boasts a broad compatibility and can be used to produce various micron- and nanoscale metal powders, including aluminum-based, magnesium-based, stainless steel, and titanium alloy powders.
• The process is highly controllable, and the powder particle size can be tuned by adjusting the ultrasonic parameters.
• The resulting powder exhibits high sphericity and low impurity levels, making it an ideal powder‑production technology for additive manufacturing.

Previously, the academic community has proposed three major hypotheses regarding the ultrasonic atomization mechanism: the capillary-wave‑dominated theory, the cavitation‑dominated theory, and the combined capillary‑cavitation theory. For a long time, these hypotheses lacked direct experimental evidence, which has also hindered the optimization and advancement of this process. In this study, water and glycerol were used as model media, and physical experiments were conducted using pure aluminum melt, providing, for the first time, direct experimental confirmation. Cavitation effects synergize with capillary waves, with cavitation playing the dominant role. The synergistic mechanism.

II. Experimental System: Multi‑method in situ observations to reconstruct the entire atomization process

To elucidate the dynamic mechanisms of ultrasonic atomization, the research team constructed multiple sets of observation and experimental apparatuses, conducting layered investigations into the underlying mechanisms and validating the associated process parameters.

1. Imaging Observation Device

• High-speed optical imaging system : A 20–24 kHz ultrasonic transducer is employed, paired with a carbon‑fiber plate as the atomization platform. The carbon‑fiber plate exhibits excellent high‑temperature resistance and fatigue strength, and it demonstrates favorable wettability with molten aluminum, making it an ideal substrate for metal atomization. High‑speed cameras operating at frame rates of 10–50 kHz are used to capture the macroscopic atomization morphology of droplets and liquid films.
• Ultrafast Synchrotron Radiation X-ray Imaging System By leveraging a synchrotron radiation source, we achieve nanosecond‑level temporal resolution and micrometer‑level spatial resolution, enabling penetration through the liquid surface to observe microscopic dynamics such as the growth and collapse of internal cavitation bubbles, thereby addressing the observational blind spots of optical imaging.

2. Metal Atomization Experimental Apparatus

A high-temperature atomization system was designed for molten pure aluminum: induction heating melts the aluminum at 800°C, while argon gas is introduced into the atomization chamber to create a low-oxygen, inert atmosphere, thereby preventing oxidation of the liquid metal. Under the influence of a pressure differential, the molten aluminum flows onto a 60 kHz carbon-fiber ultrasonic vibrating plate, where atomization and powder production are accomplished. Finally, the powder morphology and particle size are characterized using scanning electron microscopy and laser diffraction analysis.

III. Core Mechanism: The Complete Dynamic Process of Ultrasonic Atomization (Key Interpretation)

The study divided the experimental subjects into Stationary droplets / liquid films, impacting droplets Two major categories, combined with media of varying viscosities (water, glycerin), fully replicate the entire ultrasonic atomization process, clearly… Cavitation is the core driving force behind atomization. 。

1. Atomization in a quiescent system: Cavitation generates capillary waves, which progressively break up into droplets.

The process is divided into eight stages and represents the most fundamental mechanism of ultrasonic milling:

• Initial state : Natural microscopic cavities exist within the liquid, keeping the liquid surface stable;
• Cavitation activation : After ultrasonic vibration is initiated, the local pressure in the liquid decreases, and the pre-existing cavitation bubbles begin to oscillate and grow.
• Bubbles form clusters. : A large number of cavitation bubbles aggregate to form a bubble cloud, and the cavitation effect progressively intensifies;
• Cavitation collapse : The bubble cloud undergoes violent implosion, generating shock waves and high-speed liquid jets;
• Capillary wave generation : The shock waves generated by cavitation propagate to the gas–liquid interface, inducing the formation of regular capillary waves (Faraday waves) on the liquid surface;
• Interface distortion : Intense cavitation continuously perturbs the interface, causing capillary waves to become distorted and unstable;
• Droplet detachment : The distorted wave crest fractures and contracts, forming fine droplets and completing the primary atomization;
• Process terminated : When the liquid film thickness drops below the critical value, atomization ceases.

The experiments also revealed a notable phenomenon: the thickness of the liquid film directly influences both droplet size and jet velocity. As the liquid film becomes thinner, cavitation intensifies, leading to smaller droplet diameters. In high-viscosity liquids such as glycerol, cavitation dominates the formation of slender liquid lobes; upon rupture, these lobes give rise to spray droplets, while the role of capillary waves is significantly diminished.

In addition, the experiment also observed Rotating Bubble Typhoon Phenomenon: Bubble clusters within the liquid film rapidly move and coalesce under acoustic streaming, while droplets are continuously ejected at the edge of the vibrating plate—this constitutes an important intermediate stage in the atomization process.

2. Impact‑induced droplet atomization: a combination of impact and ultrasound, accelerating the atomization process.

When a droplet freely impacts the surface of an ultrasonic vibrating plate, it generates a composite atomization effect characterized by “edge splashing plus surface splashing”:

• After a droplet impacts a solid surface, it spreads rapidly, with capillary waves first emerging at the contact region.
• Cavitation oscillation and collapse within the liquid film enhance interfacial turbulence.
• Splashing preferentially occurs at the edge of the liquid film, generating small droplets, after which the entire spreading layer gradually atomizes.

Ultrasonic amplitude is a key determinant of the atomization rate: higher amplitudes result in stronger momentum transfer and faster initiation of atomization; under high-amplitude operating conditions, droplets exhibit virtually no visible deformation upon impact, and atomization is completed instantaneously.

3. Key Conclusion: The Synergistic Relationship Between Cavitation and Capillary Waves

• Cavitation effect is Root Cause Driver : Cavitation bubble pulsation, collapse, and the generation of shock waves are prerequisites for the formation of capillary waves;
• Synergistic fragmentation: cavitation undermines interfacial stability, while capillary waves further amplify the instability, jointly driving droplet formation.
• The subharmonic oscillations generated by cavitation give rise to high-speed jets, significantly increasing the droplet ejection velocity.

IV. Process Validation: Ultrasonic Atomization for the Preparation of Aluminum Powder Performance and Process Principles

Aluminum and aluminum alloys are among the most widely used materials in additive manufacturing; however, aluminum melts are prone to oxidation and have low density, making continuous ultrasonic atomization challenging. In this study, we conducted direct ultrasonic atomization experiments on pure aluminum melts, demonstrating the technology’s industrial viability and elucidating its key process principles.

1. Powder Morphology and Quality

According to scanning electron microscopy observations, aluminum powders prepared under different ultrasonic amplitudes. Excellent sphericity , with no elongated particles, agglomerates, or satellite particles; the particle cross-sections exhibit no closed porosity, and the powder demonstrates excellent flowability and bulk density, making it ideally suited for mainstream additive manufacturing processes such as laser powder bed fusion.

2. Effect of Ultrasonic Amplitude on Powder Particle Size

The ultrasonic frequency was fixed at 60 kHz, and the powder particle size distribution was compared under three input power levels—60%, 75%, and 100%—corresponding to different vibration amplitudes.

• The smaller the amplitude, the finer the powder particle size and the narrower the particle size distribution. : At 60% low-power conditions, the powder’s D10–D90 range is only 21.8 μm, yielding optimal particle-size uniformity; at 100% high power, the particle-size distribution widens to 67.6 μm, with an increased proportion of larger particles.
• Power threshold effect: At 60% and 75% power levels, the powder particle size exhibits negligible differences; further reduction in amplitude yields no significant improvement in powder properties, making these values useful reference thresholds for process optimization.
• Particle size distribution characteristics: Ultrasonic‑atomized aluminum powder exhibits a positively skewed distribution, with a higher proportion of fine particles, which is conducive to increasing the powder’s bulk density.

Overall, Reduce the ultrasonic vibration amplitude. It is the core process strategy for preparing aluminum powder with fine particle size and high uniformity, enabling precise control of the particle size distribution while maintaining excellent sphericity.

V. Summary and Industry Outlook

This study, employing multi-scale in situ imaging techniques, has resolved the long-standing controversy surrounding the mechanism of ultrasonic atomization, thereby clarifying… Cavitation effect dominates; cavitation and capillary waves act in synergy. the core mechanism, while also fully validating Ultrasonic Atomization for the Preparation of Metal Powders Feasibility and process control logic.

From research to application, the value of this achievement is reflected in two aspects:

• Theoretical level : It has supplemented the experimental evidence for the microscopic mechanism of ultrasonic atomization, providing data support for simulation and process modeling.
• At the industry level : The control strategies for key parameters such as ultrasonic amplitude, liquid‑film thickness, and medium properties have been clarified, paving the way for the transition of ultrasonic milling technology from the laboratory to industrial-scale production.

For the additive manufacturing industry, Ultrasonic atomization technology It holds the potential to overcome the limitations of conventional powder‑making processes, enabling the mass production of high‑quality metal powders with excellent sphericity, a narrow particle size distribution, and no porosity, thereby supporting the advancement of additive manufacturing technologies in high‑end sectors such as aerospace, advanced equipment, and medical implants. Looking ahead, further optimization of ultrasonic frequency, atomization atmosphere, and melt delivery methods will also emerge as key research priorities in this field.

Image and Text Guide

Figure 1. Schematic diagram showing the experimental setup for high-speed optical imaging of the ultrasonic atomization process.

Figure 2. Schematic diagram illustrating the ultrafast synchrotron X-ray imaging experimental setup for the ultrasonic atomization process at the Advanced Photon Source’s 32-ID-B beamline.

Figure 3. The schematic diagram illustrates the configuration of the ultrasonic atomization apparatus used for melting and atomizing a pure aluminum melt.

Figure 4. High-speed image sequences showing (a) and (b) the complete stages and temporal evolution of the atomization process of a single stationary water droplet on a plate‑shaped ultrasonic transducer, acquired at a sampling frequency of 20 kHz. See Supplementary Video 1 for details.

Figure 5. Histograms show the distributions of ultrasonic atomization droplet sizes (a, b, and c) and velocities (d, e, and f) obtained under different dome thickness conditions.

Figure 6. A sequence of high-speed images illustrating the complete atomization process of a glycerol droplet resting on an ultrasonic transducer, recorded at a sampling rate of 40 kHz. See Supplementary Video 2 for details.

Figure 7. High-speed image sequences of the initial atomization process of a liquid film on a flat plate, captured at a sampling rate of 35 kHz. See Supplementary Videos 3 and 4 for details.

Figure 8. High-speed image sequences of cavitation activity and the resulting capillary waves in the antinodal region on the surface of a thick film, captured at a sampling rate of 40 kHz. See Supplementary Videos 7 and 8.

Figure 9. A high-speed image sequence acquired at a sampling rate of 10 kHz, illustrating the atomization effect induced by vortex‑bubble storms at the side edge of a flat plate. See Supplementary Video 9 for details.

Figure 10. High-speed image sequences of a single water droplet impacting a vibrating plate, recorded at a sampling rate of 50 kHz. See Supplementary Videos 10 and 11.

Figure 11. High-speed image sequence of the atomization process during the impact of a glycerol droplet on a horn-shaped nozzle, recorded at an acquisition rate of 35 kHz. See Supplementary Video 12 for details.

Figure 12. (a) Ultrafast synchrotron X-ray images showing the atomization sequence of a water droplet impacting an angled surface, with an imaging acquisition rate of 50 kHz. See Supplementary Videos 13 and 14. (b) Estimated diameters and velocities of the atomized droplets, as determined from Supplementary Video 15.

Figure 13. The schematic illustrates the sequential stages of ultrasonic atomization of a droplet or liquid film resting on a vibrating plate (from left to right).

Figure 14. The schematic illustrates the evolution sequence of the ultrasonic atomization process when a droplet impacts a vibrating horn/plate.

Figure 15. Particle size distribution of aluminum (99.9%) subjected to ultrasonic atomization at different amplitudes (100%, 75%, and 60%).

Figure 16. Scanning electron microscope images of aluminum (99.9%) samples subjected to ultrasonic atomization, at different amplitudes: (a) 100%, (b) 75%, (c) 60%, and (d). In (d), a schematic illustration of a typical cross-section of the as‑grown particles, showing no trapped pores.

Figure A1. Amplitude of the vibrating plate, measured with a vibrometer along (a) the length direction and (b) the width direction, from zero to peak value.

Figure A2. High-speed images show a sequence of the atomization process captured at a 10 kHz frame rate from a top-down view of the carrier‑film surface. In panels (b) and (c), the red dashed circles illustrate the evolution of Faraday ripples that initiate at the edge of the CF carrier; in panel (d), the arrow and the small red circle indicate, respectively, a fully developed antinode on the carrier and the earliest signs of droplet ejection. See Supplementary Videos 5 and 6 for further details.

Table A1. Transducer vibration amplitude for liquid atomization at different ultrasonic input powers.

Table A2: Experimental and Theoretical Estimates of the Frequency of Distorted Capillary Waves in Water under Different Atomization Conditions.

Table A3 Particle size distribution of aluminum powder prepared by the ultrasonic atomization method.