Calculation of Geometrical Parameters of the Laval Nozzle and Gas Dynamic Spraying Conditions in the Production of Fine Tool Steel Powders

The production of powders with predetermined particle sizes is an important task in various branches of powder metallurgy and is especially relevant in additive manufacturing, where powders with an equivalent particle diameter smaller than 50 μm are used. The following parameters were calculated in the paper: theoretical gas flow speed to produce particles of required size by gas atomization of superheated fluid metal; specific flow rate of the metal flowing out of the metal tundish, and atomization nozzle parameters (such as critical and outlet cross-sectional areas and their ratio). Gas dynamics methods, being widespread in aviation engineering, were used to calculate the nozzle. The supersonic Laval nozzle parameters and gas dynamic parameters for atomization of the molten 10R6M5 tool steel were calculated at gauge gas pressures ranging from 0.5 to 2.0 MPa, allowing fine powders to be produced, including those with a particle size smaller than 50 μm. Graphical dependences were plotted to illustrate the theoretical speed at which particles of required size formed and the gas speed calculated as a function of the gas pressure before the atomization nozzle. A graphical method for determining the cross-sectional areas of the Laval nozzle and the inert gas flow speed for a given gauge pressure in the studied range was proposed. The following parameters for the production of 10R6M5 tool steel powders with a particle size smaller than 50 μm by gas atomization were established: gas flow speed at the nozzle outlet of 525 m/sec, temperature of –140°C, and pressure higher than 16.8 MPa. The calculated critical and outlet cross-sectional areas of the Laval nozzle were 110 and 290 mm² and their ratio was 0.379.