# Development of High-Power Ultrasonic System Dedicated to Metal Powder Atomization
## 2 Ultrasonic Atomization Phenomena
### Materials and Methods  
#### 3.1.1 Ultrasonic Atomization Set-Up Idea  
The target atomization system consists of: melting system and ultrasonic system closed in a working chamber but also control system, ultrasonic generator, cooling systems, diagnostics, etc. However, the design phase began with two key components, i.e., the melting system and the ultrasonic atomization system. The idea of combining both components was presented schematically in Figure 2.  
The most important part and the 'hearth' of the ultrasonic atomization system was the ultrasonic waveguide. Two UT systems working with different frequencies were assumed here, 20 kHz and 70 kHz, due to the significant impact of this parameter on the size of the resulting droplet of atomized metal [15], which was also discussed in the previous chapter. In addition, it should be noted that with an increase in frequency, it is more difficult to achieve a high amplitude of vibrations and an even distribution of amplitude on the surface of the working plate. For this reason, 70 kHz was adopted as the maximum frequency. The design process took into account the mechanical characteristics of the sonotrode materials, considering its shape-dimensional relationships and possible temperature changes during the process (temperature increase induced by the molten metal). As a power transducer, the use of a Langevin type transducer is assumed to obtain the required amplitude of vibrations on the surface of the sonotrode's working plate. The second parameter was the size of the vibrating plate, which is supposed to have direct contact with the stream of the molten metal. The Twin-Wire Arc Spraying (TWAS) \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Medium**} & **Density** & **Surface** & **Dynamic** & \(A_{m}\) & \(d\) & \(A_{m}\) & \(d\) \\  & \(\rho\) & **Tension** & **Viscosity** & **(Frequency** & **(Frequency** & **(Frequency** & **(Frequency** \\  & **[kg/m\({}^{3}\)]** & \(\sigma\) & \(\mu\) & **20 kHz)** & **20 kHz)** & **70 kHz)** & **70 kHz)** \\  & **[N/m]** & **[mPa*s]** & **[\(\mu\)m]** & **[\(\mu\)m]** & **[\(\mu\)m]** & **[\(\mu\)m]** & **[\(\mu\)m]** \\ \hline Water & 1000 & 0.07 & 1.0016 & 1.223 & 55.70 & 0.805 & 24.16 \\ \hline Liquid metal & 7220 & 1.872 & 5.443 & 0.595 & 86.15 & 0.392 & 37.37 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties [21] and calculated values of amplitude threshold and droplet diameter. Figure 2: The idea of an ultrasonic atomization set-up for metal powders production. process was considered here as a method of melting the wire feedstock materials by an electric arc. The melting process has mainly been studied in terms of controlling the geometry and direction of the liquid material stream, based on the preselection of TWAS process parameters, i.e., working gas pressure [bar], wire feed speed [m/min], intensity [A] and arc voltage [V], and others. The melting process was also investigated and verified in terms of in-light temperature and in-flight velocity of the melted material as a function of the stand-off distance between the torch head and the potential working plate surface. The molten stream of particles should hit the surface of the working plate coupled to the ultrasonic power transducer. To determine the diameter of the plate, the distance at which the working plate should be mounted relative to the melting system is extremely important. It is affected primarily by the angle of convergence of the stream, the velocity of the particles in the stream, and the temperature of the particles over different distances, and has been validated to be between 100 mm and 300 mm (Figure 3). It was assumed that the diameter of the working plate should cover as much of the melted material stream as possible to improve the process efficiency. The working plate should at the same time maintain a high amplitude and uniform distribution of vibrations, so the homogeneous powders may be produced. Finally, the range of the considered diameters of the vibrating plate was 60 to 190 mm.