Alkali metals in aluminum melt mainly include elements Na, Li, K. Among the alkali metal elements, lithium can be used as a strengthening element for aluminum alloys. Its function is to reduce the density of aluminum alloys and improve the elastic modulus of aluminum alloys. In other cases, the alkali metal elements are in the form of impurities in aluminum. Existing in the melt, these impurity alkali metals will change the structure and phase composition of the material through physical and chemical interactions with other impurity elements, thereby affecting a series of properties such as the mechanics, processing and surface treatment of aluminum alloy.
Among the aluminum melt alkali metals, the most harmful is sodium element (Na). The harm of sodium to aluminum alloy casting mainly includes two aspects:
- Increase the viscosity of aluminum alloy during casting, increase the tendency of ingot to crack during casting, and destroy the surface quality of ingot.
- Cause the “sodium brittleness” phenomenon. When there is sodium in aluminum alloy, sodium will be free and finally adsorbed on dendrites or grain boundaries, which will increase the brittleness of aluminum alloy, leading to cracks during casting, or cracking during downstream deep processing processes such as hot rolling. The phenomenon of “sodium embrittlement” is particularly harmful to high-magnesium alloys. In order to prevent the occurrence of Na freeing, sodium salt flux is not allowed in actual production.
Alkali Metals in Aluminum Melt Removal Technology
Since sodium is the main alkali metal hazardous element, the effect of removing alkali from aluminum body is mainly determined by the change of sodium content. Generally speaking, after the aluminum alloy melt is purified, the content of alkali metal sodium should be controlled within 5ppm. For Al-Mg alloys with higher Mg content, the Na limit is more stringent.In the 5182 alloy, sodium may cause hot rolling cracks.
Alkali metals in aluminum melt removal technology can be divided into three types: alkali removal technology before furnace, furnace alkali removal technology and after furnace alkali removal technology. In the production, it is possible to select a variety or a combination of alkali removal technologies according to the alloy varieties produced to ensure the control of the alkali metal content in the melt.