How to Remove Hydrogen in the Aluminum Alloy Die Casting?​

In the production process, in order to avoid the shortcoming of pores in aluminum alloy die casting, it is necessary to adopt effective measures to reduce the moisture of the raw materials as much as possible, strengthen the quality management of smelting, and improve the exhaust capacity of the mould. The details are as follows:

The main reason for the porosity in the casting is that the die cast alloy contains excessive H. The hydrogen content accounts for 0.02% to 0.03% of the total gas content, and the rest is N, O, CO, etc. The H is derived from the heat-differentiated moisture in the atmosphere and various metal raw materials, fluxes and coatings. Under high temperature conditions, the HO=H + O- reaction occurs, which is a reversible reaction. The differentiated oxygen can easily form AlO with a higher melting point with the molten metal. The reaction equation is: Al + O -=AlO, which promotes the high-temperature differentiation of water vapor, and hydrogen ions are continuously dispersed in the alloy liquid.

Hydrogen exists in the molten aluminum in two ways:

The first is to differentiate into the atomic state and dissolve in the molten aluminum, which is called the dissolved type, accounting for about 60%;

The second kind of hydrogen is absorbed in the appearance or the gap of the inclusions in the form of molecular bubbles, which is called the adsorption type. Since the solubility of hydrogen in the aluminum alloy die casting liquid increases with the temperature's rise, the alloy liquid will absorb a lot of H during the smelting process. In the process of crystallization and condensation, due to the decrease in temperature, the surface layer of the alloy liquid firstly condenses and the viscosity of the alloy increases. Although the solubility of hydrogen decreases and needs to be separated from the molten metal, it is now very difficult to achieve it. And then it stays in the alloy liquid and thus gives rise to the shortcoming of pores. The longer the time of melting and heat preservation, the higher the hydrogen content.

The solubility of hydrogen in the alloy liquid is not only proportional to the temperature, but also proportional to the pressure and the humidity of the air, that is, the partial pressure of hydrogen.

Thealuminum alloy die casting element and its content also have a certain effect on the solubility. The solubility of hydrogen decreases when the content of silicon and copper is added, and the solubility of hydrogen increases when the content of magnesium is added. The composition of aluminum alloy die casting is different, and the critical content of hydrogen in the alloy liquid is also different. ZL aluminum alloy has the largest hydrogen absorption capacity when it is a hypoeutectic aluminum-silicon alloy.