Hydrogen is the only gas that dissolves in significant amounts in molten aluminium. It is far more soluble in the liquid than in the solid metal, so during solidification the excess hydrogen comes out as bubbles and causes porosity. Degassing removes this hydrogen before casting.
Sources of hydrogen
- Moisture in the air, especially in humid seasons
- Damp or contaminated tools, crucibles and ladles
- Oily or wet scrap and damp fluxes
- Excessive melt temperatures, which increase gas pick-up
Degassing methods
Rotary degassing: the most common industrial method. A graphite rotor disperses inert gas (argon or nitrogen) through the melt as very fine bubbles; hydrogen diffuses into the bubbles and leaves the surface with them. The bubbles also carry oxide particles to the surface.
Degassing tablets: used in small foundries, but they offer less control and consistency.
Cleaning: flux and filters
Salt fluxes help separate oxides from the melt and form dross. Ceramic foam filters in the pouring path trap the remaining inclusions.
Melt quality control
The reduced pressure test (RPT) and the density index are a simple, widely used way to measure the gas content of the melt: one sample solidifies under vacuum and another at normal pressure, and the difference in their densities shows how prone the melt is to porosity.
Good degassing goes hand in hand with good ingot. Read more about casting defects, or contact Almas specialists for advice.