PRODUCTION OF SPHERICAL MONODISPERSE METAL POWDERS BY MEANS OF THE PLATEAU-RAYLEIGH INSTABILITY OF A LIQUID METAL JET

Additive Manufacturing is becoming increasingly important due to its possibilities. Processes such as selective laser melting, electron beam melting or binder jetting are used to manufacture industrial metal parts. The starting material for these techniques is metal powder with a particle size of approx. 30 to 100 microns, depending on the process. Common processes for powder production are e.g. gas or water atomization. Disadvantages of these techniques are the non-spherical particles and the wide size distribution. Today's Additive Manufacturing technologies require a high-quality, spherical powder with a narrow particle size distribution. After powder production, sieving or similar separation methods are required to sort the particles by size and achieve the desired size distribution. Particles that don't have the desired size, have to be discarded. The high consumption of inert gas (argon) for gas atomization and the complex screening process result in high costs for the powder produced. One alternative method for producing monodisperse and spherical particles from molten metal is droplet formation using Plateau-Rayleigh instability. This effect causes a liquid jet to break up into droplets of the same size. This breakup can be stimulated by modulating the jet velocity at a specific frequency. In free fall, the droplet shape becomes spherical due to the surface tension of the melt. In this way a monodisperse powder with spherical particles can be produced. The size of the generated particles depends on the modulation wavelength, while the instability has its greatest effect on a specific wavelength proportional to the jet diameter. By varying the modulation frequency during the process, precisely defined fractions of certain particle sizes can be generated. In this way, powders with tailor-made particle size distribution can be produced for specific applications. In this work a droplet generator for molten solder (Sn 37%, Pb 63%) was designed, built and tested. Due to its temperature stability and mechanical properties, the body of the droplet generator was manufactured from the polymer PEEK (polyether ether ketone). The modulation was performed with a piezo stack actuator controlled by a waveform generator and an amplifier. A sapphire nozzle with a diameter of 200 microns was used for jet forming. The droplet size, jet velocity and modulation frequencies were characterized and compared with theoretical calculations. Jet breakup and drop formation were investigated with a high-speed video camera. The generated particles were examined for size and shape by scanning electron microscopy. As a result, this work shows the potential of using this method to produce high quality powder for metal-based Additive Manufacturing technologies. Further work can be carried out to process metals with higher melting temperatures and to increase the throughput of the system.