Understanding aluminum degassing.

To meet the demands of today's aluminum casting market, foundries have focused on improving molten metal quality by developing processes to produce clean metal. These processes focus on eliminating a variety of impurities (i.e. inclusions) that pose serious problems in the production of quality castings. At the forefront of these impurities is hydrogen, the only gas soluble in molten aluminum, because it creates porosity in castings.

This article discusses hydrogen absorption during aluminum melting and casting and assesses methods to evaluate and remove hydrogen.

Role of Solidification

The solubility of hydrogen in molten aluminum increases with temperature (Fig. 1). A degree of 'alloy specificity' also exists--at a given molten metal temperature, less hydrogen is soluble in 319 alloy than in 356 alloy, for example. However, both exhibit substantially reduced solubility in the solid state.

Solidifying metal must reject the hydrogen or resultant castings will suffer from porosity. Here, solidification plays a role. For a given hydrogen concentration in the liquid state, the faster the solidification rate, the fewer problems with resultant porosity. Because not all sections of the casting solidify at the same rate, ample opportunity exists for hydrogen buildup in the remaining liquid metal during solidification. If the hydrogen partial pressure in this remaining liquid exceeds a critical level, a gas molecule forms from the association of hydrogen atoms, creating porosity.

Shrinkage Porosity vs. Gas Porosity

Not all porosity can be blamed on hydrogen. Figure 2 portrays a classical distinction between shrinkage porosity and gas porosity. Shrinkage porosity occurs as the metal solidifies; a 6% volume change occurs when an aluminum alloy transforms from a liquid to a solid. This change is accommodated in normal casting practice through the use of risers. Shrinkage porosity indicates a lack of feed metal reaching the place where it is needed--the last portion of the casting to solidify. Consequently, the resultant porosity follows the solidification path and the dendrite boundaries, and shrinkage porosity displays a ragged, irregular edge.

By contrast, gas porosity shows a more distinctive regular contour, often round. Casting porosity is often a combination of the two. Porosity also may be due to a reaction from molding materials and coatings or simply air entrapment due to inadequate venting.