Introduction to Atomization
In metal powder production, different techniques and systems can be used, each of them being dependent on the metal powder configurations, metallurgy and costs. Thanks to the ease to spray a liquid to fine droplets, the prevailing mode of metal powder production is jet atomization of a melt. Obtained products are then divided in gas or water atomized powders.
As processing technologies like additive manufacturing, metal injection molding, thermal spray as well as joining technologies using brazing pastes provided by C.HAFNER & HILDERBRAND require powders of high sphericity, we focus on gas atomization of melts for production of our precious metals powders.
Gas Atomization
From versatile small-scale atomizing system to large atomizers with the most spherical powders, C.HAFNER&HILDERBRAND is capable of atomizing the whole range of precious metal alloys from gold allows to high-melting PGM alloys with Ir and Rh in an extensive range of particle size. This is made possible thanks to the following various gas atomization configurations mastered by C.HAFNER&HILDERBRAND i.e. free-fall, close-coupled (or coaxial) & Nanoval® process atomization.
This latter process is based on a Laval-type nozzle placed concentric below a heated feeder of a melt stream at the opening of a pressure chamber. The nozzle’s converging cross section accelerates the inert atomization gas to the velocity of sound while the melt stream is compressed to a thin filament. The filament finally atomizes at the transition to the diverging cross section of the nozzle. This setup creates very fine droplets with very little convection to provoke the formation of powder particles with undesired satellites.
Sizing (screening)
While powder batches produced by free fall or coaxial nozzles have mean particle diameters [d50] of down to 30 µm to 70 µm, respectively, the process based on the De Laval-nozzle design reduces that value to as small as 15 µm. Thanks to these different gas atomization processes, C.HAFNER&HILDERBRAND covers a wide range of particle size distribution (PSD) for various manufacturing needs and uses.
After atomization, sieving and classification systems ensure the correct sized product depending on the powder use. The sizing or screening process can be done by sieving or air
classification. The limiting factor for selecting either method is not only the cut size, but the density of the powder and a reasonable classification rate. Cuts containing small particle sizes are air classified because fine sieves are limited to a mesh size of ≥25 µm i.e. low classification rates.
Cuts containing larger particle sizes are limited by the basic principle of drag, centrifugal forces and rotation speeds versus the density of the particles. Smaller cuts can be done by air classification while the large PSD cuts are executed by sieving.
Powder Rheology
Size and size distribution are not the only parameters of importance when using precious metal powders. Among these parameters, flowability and external influences like humidity can affect or modify the behavior of atomized powders for direct use in certain manufacturing technologies or conversely as raw materials for paste use for instance.
Similar to rheology methods applied to liquids or soft matter, powder rheology characterizes powders or powder-based products in specific applications. Whether for quality purposes or to predict the behavior of the product during processing, at C.HAFNER&HILDERBRAND we provide the expertise of the behavior adapted to your application.
