The requirements for the purity of the melt vary slightly with different alloy types and uses. Generally, the hydrogen content is required to be less than 0.15cm^3/100gAl, and the mass fraction of sodium is generally required to be controlled below 5/10^6. In order to obtain a pure aluminum alloy melt, it is necessary to add a refining agent to the aluminum alloy melt, through physical action or chemical reaction, to achieve the purpose of removing oxide inclusions and gases. The gas refining methods commonly used in furnace processing are inert gas, chlorine gas or their mixed gas, and these refined gases are injected into the melt through a gas blowing system.
The system that blows the refined gas into the melt in the furnace and achieves the refining effect is called the blowing refining system. Due to the different activity of the refined gas, the refining principle is also different.
Inert Gas Refining
Inert gas refers to a gas that does not chemically react with molten aluminum and dissolved hydrogen, and does not dissolve in aluminum. The commonly used inert gases are nitrogen and argon, and the principle of degassing is to take out [H] and inclusions in the aluminum liquid through physical methods. The temperature of the molten aluminum with nitrogen should be controlled at 710-720℃ to avoid the reaction of nitrogen and molten aluminum to form aluminum nitride. The degassing effect of argon is better than that of nitrogen, but the cost of argon is high. Generally, argon alone is not used for degassing.
Active Gas Refining
For aluminum, the main reactive gas is chlorine. Chlorine itself does not dissolve in molten aluminum, but chlorine and aluminum and hydrogen dissolved in molten aluminum can react quickly. Can play a refining role. Therefore, the purification effect of chlorine is much better than that of nitrogen, and the effect of removing sodium is also significant. However, chlorine pollutes the environment, is harmful to the human body, easily corrodes equipment and heating elements, and easily makes the crystal structure of the ingot coarse.
Mixed Gas Refining
Refining with nitrogen alone is less effective, and chlorine is harmful to the environment and equipment, so nitrogen-chlorine mixed gas refining is often used to improve the refining effect and reduce its harmful effects.