# Development of High-Power Ultrasonic System Dedicated to Metal Powder Atomization
## 1 Introduction  
Ultrasonic (UT) atomization is a phenomenon involving the disintegration of droplets or a stream of liquid and the formation of an aerosol under the influence of ultrasonic waves, including high-power ones. Ultrasonic atomization, like other atomization methods, uses the energy supplied by an external medium to disintegrate a liquid as a result of bringing it out of a stable state [1]. In this characteristic atomization method, energy is carried by ultrasonic waves, which cause an increase in pressure in the droplets of liquid located on the surface of the sonotrode [2]. When the critical value is exceeded, the liquid breaks down into very fine droplets. Ultrasonic atomization has found its applicationin medicine as a method of administering inhaled drugs (inhalation support) and the production of medicines and chemistry--for the separation of fractions with different properties and mechanics--for example for the production of protective coatings [2; 3; 4]. It should be emphasized that the phenomenon of ultrasonic atomization of liquids is well understood. However, its practical application is limited to typical liquids such as aqueous solutions, alcohols or fats.  
Recently, ultrasonic atomizers have also been considered as a novel approach for metal and metal alloy powder production. The idea assumes that the ultrasonic vibrations are used to disintegrate the molten metal and create fine metal or metal alloy droplets, which later, after the solidification, form fine and spherical powder particles. Such materials are usually produced by other atomization processes, with gas, water and plasma atomization considered as state-of-art technologies. Ultrasonic atomization cannot replace typical industrial, large-volume atomization processes, but can be an interesting alternative in the case of the development, laboratory-scale or on-demand production, especially of rare or high-quality powder materials. Ultrasonically induced atomization process has a few significant advantages, mainly stable and predictable dependences between the operating parameters and vibration characteristics, atomization rate or droplet diameter [5; 6]. Furthermore, UT atomization is known to produce very narrow droplet size distribution, so this may allow producing powders tailored for specific manufacturing processes and limit the material losses. In terms of metal powder production, it is also important that the process can be operated in a small chamber, consuming much less energy, gas or other media when compared to other atomization methods. It was demonstrated in the literature that the ultrasonic atomization was already laboratory-tested for the production of various advanced metal alloys or even composite materials, including: aluminum matrix composites (AMCs) [7]; Zr-based metallic glass [8]; refractory Mo-Si-Ti alloys [9]; stainless steel [10; 11]; lightweight and low-melting-point alloys, like Al-Cu- [12], Al-Si- [13] or Bi-based alloys [14].  
This work presents the first stage of the ultrasonic atomization technology development, dedicated to metal powder production. The designing and experimental verification of two high-power ultrasonic set-ups, operating at 20 kHz and 70 kHz, are shown here. The UT atomization capabilities of both set-ups were measured by a precise displacement sensor and then observed with a high-speed camera when the disks were loaded with water. The distribution of atomized water droplets, depending on the operating frequency, was also analyzed. The ultrasonic waveguides were designed to have atomizing disks with the diameters of 140 mm, so they can effectively work later with different melting set-ups and the atomization may be done effectively over a large surface. Up to the knowledge of authors, such UT systems were not tested yet in the context of metal powder production.