Non-metallic inclusions in aluminum
The presence of non-metallic inclusions in aluminum ingots for pressing is common. Non-metallic inclusions can be found in any ingot, regardless of the technological methods used for melting and casting.
Usually any undissolved debris, which are present in cast metal, called inclusions. Non-metallic inclusions in cast ingots-pillars negatively affect their processability during pressing, final mechanical properties and appearance. Therefore, the reduction in the number of inclusions in cast columns - billets for pressing - is an important part of quality control in their production.
Where do nonmetallic inclusions come from?
Several types of non-metallic inclusions in aluminum poles are known., such as aluminum oxides, magnesium oxides, furnace lining particles, spinels and carbides. The main influence on the quality of cast metal, including, bullion-pillars for pressing, exert its microstructure, uniformity, chemical composition and degree of contamination or, if you like, degree of purity.
Factors, which determine purity and, Consequently, the quality of molten aluminum are:
- the content of impurities of alkali metals;
- solid impurities (inclusions);
- dissolved hydrogen.
Shape and dimensions of inclusions
Inclusions in aluminum melt include various forms, such as films, individual particles and agglomerating inclusions of oxides, nitrides, borides, carbides and other combinations of elements, for example, chlorides and CaS.
The size of these inclusions can vary from a few microns to half a millimeter. Number of Inclusions, which can be in one kilogram of aluminum melt can be from hundreds to several thousand. These inclusions can cause various defects., reduce the mechanical properties of products and the quality of their surface.
Harm of inclusions in aluminum
If these inclusions are not removed from the melt, they will appear after hardening and will give many defects in the final product, such as the:
- “punctures” in foil,
- surface defects of rolled products,
- cracked ingots,
- poor machinability,
- molding defects,
- increased wear of press dies, etc.. .
Types of non-metallic inclusions in aluminum
In the molten aluminum there are various types of inclusions - metallic and non-metallic. They are usually visible under an optical microscope.. Typical non-metallic inclusions are:
- oxides (Al2O3);
- carbides (SiC, Al4C3);
- spinel (Al2MgO4, MgO);
- borides (TiB2).
Metal inclusions in aluminum alloys 6060/6063
In alloys of type 6060 and 6063 metal inclusions are also present, such as Al-Si-Mg and Al-Si-Fe. These secretions may take the form of needles., plates, dendrides, rods.
Sources of non-metallic inclusions in aluminum
Sources of inclusion in molten aluminum at various stages of production are:
- bauxite in an electrolysis cell;
- aluminum scrap during its remelting;
- refractory materials for furnaces and other melting equipment;
- atmospheric air during alloying and melt flow to the casting machine;
- introduction of a bar for grinding grain;
- melt mixing.
This type of inclusion is the most common in all aluminum alloys.. There are two types of alumina structure - alpha and gamma, as well as its amorphous form. They can be distinguished under an x-ray microscope..
Alumina is most often found in the form of a film or group of films with a thickness of less than 1 microns and diameter from 10 to 1000 m. In these films, other elements are often found under an electron microscope., such as magnesium, zinc, manganese and iron.
There are three main sources of alumina in molten aluminum:
- from aluminum ores during the electrolysis of aluminum - in the form of thin films or thick agglomerates;
- from the contact of the aluminum melt with the atmosphere;
- from the reaction of the melt with the lining of the furnace and other melting equipment - reddish or brown oxides.
Spinels include Al2MgO4 и MgO. There are many of them special in 5xxx series aluminum-magnesium alloys. They have the appearance of black or brown thick films with a thickness of 10 to 500 m, as well as elongated large particles. These inclusions increase in size., when the melt oxidizes at high temperature. Moreover, spinels are known as harmful inclusions due to their high hardness and large size - they damage the press matrix.
Borides are small inclusions, which are usually found in the form of clusters of rectangular or hexagonal shape in size from 1 to 50 m. The number of borides increases when a modifying rod is introduced into the aluminum melt before casting. Should be considered, what's molten from aluminum scrap, for example, aluminum profiles, some borides are already present, which increases with each new scrap remelting.
There are several types of borides - solid inclusions - such as TiB2, VB2 and zrb2, which are difficult to distinguish from each other.