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Ultrasonic atomization principle

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Ultrasonic atomization historical background

According to literature reports, the ultrasonic atomization process was first described in 1927. However, the development of this method for producing metal powders was initiated in 1964 by Stamm’s work on the atomization of metals with melting points up to 700°C using vibration frequencies as high as 0.8 MHz. However, fatigue strength issues with the sonotrode (the active vibrating part) material were a barrier that greatly limited the use of this method in favor of plasma or gas atomization.

How ultrasonic atomization works

Ultrasonic atomization is a liquid to a solid type of process, however contrary to the gas atomization techniques instead of high-velocity gas, the ultrasonic vibrations are being used to create powders.

The main principle of the technology is based on amplitude and surface wettability. When the threshold value of the vibration amplitude in the liquid layer wetted to the sonotrode is exceeded, the phenomenon of the formation of standing capillary waves occurs (Lierke et al., 1967). A further increase in amplitude is accompanied by a bursting of the internal forces of the liquid and the small droplets are ejected from the melt. This phenomenon occurs with the thin film of liquid spread on the surface of the sonotrode. With the use of thicker films of liquid material, there is additionally a cavitation effect, during which collapsing voids eject the liquid material from the surface of the liquid. The size of the particles produced depends mainly on the frequency, but also on the physical properties of the liquid material. Lang in 1962 further developed Rayleigh’s equation from his work in 1945 to predict the droplet size ejected from standing capillary waves more accurately (1).

Peskin et. al, 1963 continued the research on this topic and noticed that also the amplitude of the vibrations and thickness of the liquid layer has an influence on the droplet sizes. The equation of droplet size was also predicted (Rajan et. al. 2001) with additional factors of liquid viscosity, liquid flow rate, and amplitude of ultrasonics (2).

 

Understanding Ultrasonic Atomization

In simple words, ultrasonic atomization is a process where a liquid, in contact with a surface vibrating at ultrasonic frequencies, forms standing waves, leading to the ejection of tiny droplets. This process is exemplified in air humidifiers, where ultrasonic vibrations create a fine water mist.

Visualizing with Water Atomization

High-speed videos (42,000 fps) capture the essence of ultrasonic atomization using deionized water (DI water) on a sonotrode, a key component in this process:

  • 1 video: Layer of DI water showing formation of standing waves in the material under ultrasonic agitation.
 

 

  • 2 video: When the critical value of amplitude is reached (specific for each liquid) the surface tension of the liquid breaks and the atomization begins. Small droplets of liquid are ejected from the standing waves.

 

 

  • 3 video: Good wetability of the sonotrode (also specific for each liquid) is crucial in the process to create a proper area for the liquid to be atomized.

 

Ultrasonic Metal Atomization

Ultrasonic atomization can be used for metal powder production, especially on a small scale. Unit manufacturing processes dedicated to specific implementations, or prototyping nowadays carried out with 3D printing technology, require small quantities of powder that can be produced using only ultrasonic atomization. The technology also has another significant advantage over other methods of manufacturing alloy powders. The small amount of batch materials allows, under laboratory conditions, rapid validation of the chemical composition and phase structure of newly designed alloys with specific, strictly dedicated performance properties. The ability to control particle size distribution with frequency allows the manufactured powders to be adapted to most additive technologies from Direct Energy Deposition processes, Electron Beam Melting to Laser Powder Bed Fusion. That way the ultrasonic atomization fits perfectly with the latest trends of designing and manufacturing a wide range of alloy and composite powders for use in the progressively developing 3D printing technology.

The rePOWDER ultrasonic metal atomizer by AMAZEMET

The small size of ultrasonic atomizers allows for much freedom in their design. A great example of such is rePowder – ultrasonic atomizer and alloy prototyping platform created by AMAZEMET company. The system allows to perform recycling of materials, parts, failed printouts, samples after mechanical testing, leftover powder etc. which is highly beneficial, especially in the case of expensive materials that contain high volumes of rare earth elements.

Exploring all the advantages of ultrasonic atomization, rePowder is focused on low-cost, eco-friendly pulverization of metallic materials with tailored chemical composition for the purpose of researching new materials suitable for Additive Manufacturing, Plasma Spraying and Powder Metallurgy. It is capable of atomizing nearly all alloying systems and due to the fact of narrow particle size distribution nearly 80% of output material is suitable for selected process. The device can be equipped with both arc/plasma and induction melting modules.

The arc/plasma melting unit itself can be equipped with 20 and 40 kHz while 60 kHz unit is being developed for this particular case and is available for induction system at this moment.

Enhancing Additive Manufacturing and Material Research

Moreover, the technology is designed not only for atomization. The rePowder platform can also be used for alloying, homogenizing, and casting metallic materials of any composition. Thanks to its modular design and a range of developed feeders, the feedstock can come in any form – from wire or rod to powder, failed prints, or production waste, which can thus be recycled. Working together with other metal 3D printing equipment, the device allows for a closed production loop.

rePowder is a research and development platform for new materials development that uses patented ultrasonic technology to create AM-grade powder. The machine is designed to alloy, cast, and atomize metallic material within one device starting with an arbitrary material form.

Frequency and Particle Size Distribution (PSD) Correlation

The narrow particle size distribution unlike the regular gas atomization process allows to use of up to 80% of manufactured powder for dedicated technology. The ultrasonic frequency and amplitude as stated before is the main factor that influences the manufactured PSD of the powder and thus:

  • 20 kHz is most suitable for Electron Beam Melting (EBM) and Direct Energy Deposition (DED), the PSD of powder manufactured with this frequency has a d50 of 60-100 µm depending on the atomized material. 
  • 40 kHz is most suitable for Laser Powder Bed Fusion (LPBF) technologies, the PSD of powder manufactured with this frequency has a d50 of 45-60 µm depending on the atomized material. 
  • 60 kHz is most suitable for Binder Jetting (BJ) and LPBF technologies with finer PSD than the 40 kHz module, the PSD of powder manufactured with this frequency has a d50 of 32-38 µm for the last trials on silver alloys by AMAZEMET. 

Conclusion

Ultrasonic atomization, as demonstrated by AMAZEMET’s rePOWDER, represents a significant advancement in material science, particularly in the production of fine metal powders for 3D printing and other high-tech applications. Understanding the fundamentals of this technology, from water atomization to the sophisticated processes involved in metal atomization, is crucial for researchers and professionals in this field.

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What's more
Metal Powders for Additive Manufacturing
Creating high-quality metal powder for metal additive manufacturing is a sophisticated process. Manufacturing metal powders involves transforming materials in their solid state into fine particles through atomization - a key powder manufacturing process. While gas-atomized metal powders have dominated the field, the process isn't without its limitations, particularly in R&D scenarios where gas atomizers are not flexible enough for material development.
Metal Powders for Additive Manufacturing
Creating high-quality metal powder for metal additive manufacturing is a sophisticated process. Manufacturing metal powders involves transforming materials in their solid state into fine particles through atomization - a key powder manufacturing process. While gas-atomized metal powders have dominated the field, the process isn't without its limitations, particularly in R&D scenarios where gas atomizers are not flexible enough for material development.

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