Alloy Stoichiometry and Gas Entrapment Control in Atomized Powders
Controlling melt superheat and nozzle pressure limits vapor pressure elemental loss while helium purge or vacuum degassing eliminates micro-cavity gas voids.
Hollow Particles
Meaning
Spherical powders containing internal voids or gas pockets occur during the gas atomization process for metal materials. These hollow particles reduce the density of the final component.
Formation Mechanism
Rapid cooling of molten droplets often traps inert gas at the center of the solidifying mass. When the cooling rate exceeds the rate of gas diffusion, hollow particles develop as permanent features of the powder lot. This phenomenon is frequently observed in nickel based superalloys or aluminum powders where high pressure argon is used for processing.
Mechanical Consequence
Structural integrity suffers when these defective grains are incorporated into a sintered or printed part. Because hollow particles possess lower mass than solid ones, they can lead to unexpected porosity and reduced fatigue life in aerospace components. The presence of such voids lowers the effective cross sectional area available to carry loads.
Detection Method
Pycnometry measures the average density of a powder sample to identify deviations from theoretical values. While helium pycnometry reveals total void volume, hollow particles are specifically isolated through cross sectional imaging or advanced x-ray techniques. Monitoring the gas to metal ratio during atomization helps limit their frequency.
The removal of these defects is necessary for structural integrity in turbine components.