Refining of Cast Aluminum Alloys by Flux Refining and CFF Filtration

The refining of cast aluminum alloys uses compounds for degassing and slag removal. Refining is an important process step in the process of aluminum alloy casting. It can degass, purify and refine aluminum alloy. It can also improve the plasticity and toughness of cast aluminum alloy and prepare for precipitation hardening.

Cast aluminum alloy has relatively active chemical properties. When casting and melting, it is easy to react with water vapor to oxidize and absorb hydrogen. Therefore, defects such as inclusions and pinholes will be formed when molten aluminum solidifies into aluminum castings. These defects will affect the mechanical properties of aluminum castings.

At present, there are two refining methods of cast aluminum alloys

1. Adsorption refining method: mainly including flux refining method, degassing floatation method and CFF filtration method;

2. Non adsorption refining method: vacuum treatment and ultrasonic treatment.

The refining process of casting aluminum alloy can fully dissolve all phases in the alloy, strengthen the refining body, improve the toughness and corrosion resistance, eliminate stress and soften, so as to continue processing or forming.

Refining of Cast Aluminum Alloys

In addition to the inhalation of cast aluminum alloy during melting, it is also inhaled when poured into the mold. With the decrease of temperature, the solubility of the gas decreases, and the excess gas precipitates, some of which cannot escape. Gas is retained in aluminum castings to form holes, commonly referred to as “pinholes.”.

The gas in the casting aluminum alloy is sometimes combined with the shrinkage chamber, and the gas precipitated in the molten aluminum is retained in the shrinkage chamber. If the pressure generated by heating bubbles is large, the surface of the hole is smooth and there is a bright layer around the hole; otherwise, the surface of the hole will become bright. If the pressure produced by the bubble is small, the inner surface of the hole is wrinkled and has the characteristics of shrinkage.

The higher the hydrogen content in aluminum alloy liquid, the more pinholes produced in the casting aluminum alloy.

The pinhole in casting aluminum alloy not only reduces the air tightness and corrosion resistance of the casting, but also reduces the mechanical properties of the alloy. In order to obtain the cast aluminum parts with no or few holes, the key is the melting condition. If a covering agent is added to protect the alloy during melting, the gas absorption of the alloy will be greatly reduced. Therefore, the hydrogen content in molten aluminum can be effectively controlled in the process of aluminum alloy casting.

No matter which refining method is used, the aim is to degass, purify and refine the aluminum alloy, so as to obtain high quality and qualified aluminum casting.