WHY degas?

The primary purpose for degassing is to remove hydrogen gas from molten aluminum. Degassing also removes other impurities such as inclusions and alkalies.

Hydrogen is a gas present in the atmosphere and is highly soluble in liquid aluminum.

The entrapment of hydrogen in aluminum creates gas voids or porosity which can have negative effects on the desired end product. For example, the aluminum product may not offer acceptable mechanical or physical properties. Tensile strength may be adversely effected. Fatigue cracks may also be attributed to hydrogen entrapment.

The castings, rejections may be the result of pinhole porosity on cast or machined surfaces, or because of leakers where pressure tight castings are required. In ingot fabrication, porosity may result in blisters in forgings, extrusions, sheet and plate. Poor surface finishing characteristics such as bright flakes in forging and blisters in heat treated material are also indicative of the presence of hydrogen.

Hydrogen is approximately 20 times more soluble in liquid than in solid aluminum. Due to these solubility differences, hydrogen tends to come out of solution during solidification, forming molecular hydrogen either by slow diffusion or by rapid formation of bubbles. Bubble formations occur when the hydrogen pressure is greater than the surface tension and hydrostatic pressure.

Thus if the hydrogen content in liquid aluminum is high during solidification, hydrogen will be present as solid solution, porosity or as discontinuities. It is therefore important that hydrogen content be kept low if high quality cast aluminum is required.