Melt Damage and Prevention of Gas Nozzle Tip in Close-Coupled Gas Atomization

Gas atomization is one method for producing fine metal powder. In close-coupled gas atomization, a high-speed gas jet is ejected near the molten metal, and the molten metal is further broken down in the shear layer at the outer edge of the jet, producing fine metal powder of several micrometers to several tens of micrometers. By the way, in close-coupled gas atomization, if the protrusion length of the molten metal nozzle is short, a backflow occurs that goes around the melt delivery nozzle tip and reaches the gas nozzle tip, and the small droplets of molten metal that are atomized at the exit of the melt delivery nozzle are carried by this backflow to the gas nozzle tip, causing it to erode. In this study, we experimentally clarified the existence of the backflow for the first time through measurements of velocity distribution, then the flow state of the gas flow inside the gas atomizer was visualized approximately using the atomized water flow, and the existence of a backflow was confirmed. It was shown that microdroplets of water are carried by the backflow and reach the gas nozzle tip. This was also clarified through numerical analysis results for the air flow. Furthermore, the protrusion length of the melt delivery nozzle at which backflow does not occur was determined, and this was verified in actual gas atomization experiments using molten copper. In addition, the length of the melt delivery nozzle at which backflow does not occur, i.e., the gas nozzle tip does not melt, was found. Furthermore, molten-copper experiments were conducted using this gas atomizer to evaluate its performance.