Degassing of Aluminum Alloy Casting

The degassing of aluminum alloy casting is very important for obtaining high-quality aluminum products.

After the homogenization heat treatment of the aluminum alloy ingot, blister surface bubbles are sometimes formed on the surface. The surface of the aluminum alloy casting appears as scattered protrusions, and the protrusions are cavities. Through magnification, the inner wall of the cavity has a shiny metallic luster.

The dendritic structure remaining after the loose and uniform annealing will be near the bubble cavity, and the dendritic structure at the corresponding position on the inner wall of the bubble has a corresponding relationship. Observed with an electron microscope, the inner wall of the bubbles has a trapezoidal pattern, and the surface bubbles are loose. After casting, there are no bubbles on the surface of the aluminum alloy ingot, and only after the casting is uniformly annealed. It does not seem to be a metallurgical defect, but it is due to the high hydrogen content in the ingot casting process.

In the melting process of aluminum alloy, due to incomplete degassing, excess gas (mainly hydrogen) remaining in the melt is retained in the aluminum ingot. If the hydrogen content is too high, bubbles will form in the aluminum ingot.

Water vapor and aluminum alloy surfaces produce atomic hydrogen. The hydrogen atom has a small radius and diffuses into the metal layer along the grain boundary and lattice. When the temperature in the furnace is lowered, the hydrogen concentration in the furnace is very low, and hydrogen is precipitated from the solid solution. When the pressure reaches multiple atmospheric pressures, the surface metal swells to form bubbles.

In order to prevent bubbles from appearing on the surface of aluminum ingots and aluminum products, the degassing of aluminum alloy casting is important. We can choose a suitable aluminum liquid degassing system and filtering equipment to minimize the hydrogen content of aluminum ingots.