Degassing Refining of Molten Metal

In the process of heating and melting aluminum alloy, both solid and liquid metals have the ability to absorb H2, O2, N2 and other gases. This property of metal is called gettering, which is one of the important melting characteristics of metals. Practice shows that the gas existing in metal in the state of adsorption, dissolution and combination has adverse effects on the properties of metal and alloy and the quality of ingot. Hydrogen dissolved in the alloy is the main cause of porosity, porosity, blistering and delamination of the ingot, and even hydrogen embrittlement. The inclusions of oxygen, nitrogen and their compounds in materials will deteriorate the process and mechanical properties of materials. Therefore, another main task of smelting is to remove the gas dissolved in the molten metal. Understanding and mastering the source of gas, the dissolution process of gas in metal, the factors affecting the gas content of metal, and the degassing refining principle and method of molten metal are the key to reduce the gas absorption and degassing process of metal. It is of great significance to improve the quality of aluminum melt and obtain qualified ingots.

There are three forms of gas in ingot: solid solution, compound and pore. Most gases dissolve in metal lattice in atomic state, forming solid solution. The gas that exceeds the solubility and insoluble gas is adsorbed on the solid slag as gas molecules or exists in the form of pores. If the chemical affinity between the gas and a certain element in the metal is greater than that between the gas atoms, the gas can form a compound with the element, such as Al2O3, MgO and other slag inclusions.

In the melting process, the most harmful compound is water vapor, which reacts with metal to produce hydrogen and oxygen which is easy to be absorbed by metal.

The results show that the gas dissolved in the molten metal is easy to form pores when the ingot solidifies. According to the analysis, the main gas in these pores is hydrogen, so the so-called metal suction, mainly refers to hydrogen absorption. The gas content in the metal can also be approximately regarded as the hydrogen content. Therefore, degassing refining of molten metal mainly refers to the removal of hydrogen from the melt.