Al-Mg alloy is the heat-treatment and non-reinforceable alloy, the main element in alloy is Mg, it is widely used in the automotive industry, home appliance industry, electronics industry and construction industry, and it has the advantage of high strength, good formability and corrosion resistance, compared with 1xx series alloy and 3 series alloy, it isn’t easy to control and easily cracking during the production. With the improvement of equipment level and operation level, the probability of casting cracks is currently very low, but another phenomenon has occurred during the production. it is completely normal, and then the ingot is heat rolled after multi-milling and soaking. When the rolling thickness reaches around 100mm, cracks start to occur at the edges and ribs of the material, and they continue to be thinned,When the thickness reaches around 60mm, the local crack on the edge of the ingot has reached 200mm, and it is no longer possible to continue rolling. This is the phenomenon of "sodium brittle"
The maximum solubility of Na in aluminium alloy is 0.0025%, trace amounts of Na generally exist in the form of intermetallic compounds(NaAlSi) in aluminium alloys, there is no free Na exist, but when Mg is contained in aluminium alloys, the reaction is as follows:
NaAlSi+2Mg=Mg2Si+Na（Free）+Al, the boiling point of Na is 882.9℃, which is much higher than the temperature of the molten aluminium during the smelting process of the metal, so the free Na will not vaporize and volatilize, but the free Na will be quenched in the subsequent during the process of solidification in the ingot, the ingot will be homogenized and annealed（540/510℃*4h）, and then start hot rolling, the starting rolling temperature is 510℃, the final rolling temperature is 300℃, however, the melting point of Na is lower than this temperature, only 97.8℃, so that during this entire hot rolling process, Na always exists in the free state, and the free Na forms discontinuous molten spherical particles at the grain boundaries. Each particle is equivalent to an original crack core. The increase of the Na content in the solution will inevitably increase the amount of free Na, which is equivalent to the increase of the original crack size, at the same time, the ingot is not creaked during the first pass of the hot rolling process, but has been rolled several times, A certain amount of deformation cracks which indicates that free Na gradually accumulates with the dislocation and the movement of crystals during the plastic deformation of the ingot, at the same time, dislocations and crystals also accumulate at the place where Na accumulates. Results in cleavage fracture of the ingot.
Different Na content leads to different crack initiation in the hot rolling process, so the impact on the rolling process is also different, it can be seen from the rolling process that under6ppm, the rolling of Al-Mg alloy will not be affected, with 6-9ppm Al-Mg alloy has a small rolling effect and it can still be processed to the finial products.
>9ppm alloy will crack during hot rolling and cannot be rolled to the final product. Therefore, the Na content in the alloy molten should be strictly controlled, the content of Na of the sample in the casting furnace should be controlled within 5ppm, so that the products will not be affected by “sodium brittle” during the rolling process, and the performance of final product will be better.