Meaning
Aerodynamic shear forces exerted by high-velocity gas streams onto the surface of a molten metal or liquid stream to induce deformation and stretching. Liquid film shear acts as the primary driver for surface instability and subsequent ligament formation during gas atomization. This mechanical action occurs at the boundary where the gas jet and the liquid metal stream meet.
Shear Mechanism
High gas velocities create a high-speed boundary layer that transfers kinetic energy to the liquid interface. As this energy is transferred, the surface of the liquid stream develops waves that grow rapidly in amplitude. The liquid is drawn out into thin sheets of diminishing thickness as the gas shears along the boundary.
Eventually, the surface tension of the liquid cannot sustain the thin sheet structure and it breaks up into unstable filaments.
Atomization Outcome
Disruption of the liquid film produces a spray of droplets with a size distribution that depends on the intensity of the shear force. Higher gas velocities increase the shear rate, which produces thinner liquid films and consequently coarser powder particles. This process is highly sensitive to the viscosity and surface tension of the molten alloy.
If the metal has high viscosity, it resists shear deformation, which leads to coarser droplets and irregular shapes.
Control Metric
Gas pressure and nozzle geometry are the primary parameters used to regulate this interfacial force. Increasing the gas pressure increases the kinetic energy at the shear boundary, which enhances the efficiency of the breakup. This relationship is quantified by the Weber number, which balances the destabilizing aerodynamic forces against the stabilizing surface tension of the liquid.
