Fragmentation and solidification of fusible alloy melt by water spray

To elucidate the fragmentation and solidification processes that occur during the production of metal powders from a thermo-fluid perspective, we conducted experiments to visualize the water atomization of molten 42Sn–58Bi fusible alloy and made measurements of the circularity of the resulting metal powder. The fragmentation process inside a fully developed fast water spray with a pressure up to 5 MPa was clearly captured. The time-resolved images revealed fragmentation patterns involving extending molten ligaments resulting from splashing, vapor explosions, and splitting. The time variation of the thinning of the neck diameter of the ligaments revealed that the fragmentation is rate-controlled by the capillarity of the molten drop itself, and solidification and viscosity have no influence on the pinching-point dynamics. The produced metal powder displayed size-dependent circularity, and larger particles tended to be less spherical than small particles. The mechanism is attributed to non-uniform solidification, which is enhanced at the pinching part of a droplet, consistent well with the images obtained.