Time-resolved MHz imaging of molten-droplet gas atomization inside and outside a supersonic jet

High-quality metal powders with controlled particle size distributions are critical for achieving consistent performance in powder-bed fusion and other metal additive manufacturing (AM) processes. Gas atomization is one of the most common method for the production of metal powder, in which a supersonic under-expanded gas jet accompanies a strong shock wave structure. Due to lack of direct visual evidence, the mechanism of powder generation including the effects of shock-droplet interactions on powder characteristics remains poorly understood. Here, we employ ultra-fast imaging at one million Hz to directly observe the atomization dynamics of 2.5 mm molten tin droplets at 400 C subjected to a single under-expanded nitrogen jet at mean Mach number of 1.5. We identify two primary atomization locations: (i) inside the jet core, where intense aerodynamic forces induce Rayleigh–Taylor instabilities that generate fine ( m) high-velocity fragments optimal for AM, and where direct collision with reflected shock waves further enhances breakup through re-acceleration behind the shock wave, increasing the proportion of AM-preferred fine particles; and (ii) above the shear layer, where capillary-driven breakup of a horseshoe sheet yields coarser particles. These findings link flow-field instabilities to tunable gas atomization parameters, offering a mechanistic pathway to optimize powder yield and reduce post-process sieving in AM powder production.