How to effectively remove impurities and gases from aluminum melt during aluminum casting process?

In aluminum casting production, impurities and gases in the aluminum liquid are the core hidden dangers that affect the quality of aluminum ingots and subsequent aluminum alloy products. Impurities mainly include oxide inclusions, refractory debris, unmelted metal particles, etc. The gas coefficient is mainly hydrogen gas. These substances can cause defects such as slag inclusion, porosity, and looseness in aluminum ingots, reducing product density and mechanical properties. In severe cases, they can cause product scrap and increase production costs for enterprises. With the transformation of the aluminum processing industry towards precision and high-end, downstream industries such as automotive and aerospace have continuously increased their requirements for the purity of aluminum alloy products. How to effectively remove impurities and gases from aluminum liquid and achieve deep purification of aluminum liquid has become a key issue for aluminum melting and casting enterprises to improve quality and efficiency, and enhance core competitiveness.

According to research data from the Aluminum Processing Branch of the China Foundry Association, over 45% of defects in aluminum ingots are caused by excessive impurities and gases in the liquid, with the highest proportion of porosity defects caused by excessive hydrogen gas, followed by slag inclusion defects caused by oxidation inclusions. Industry experts point out that impurities and gases in aluminum liquid mainly come from raw materials, production environment, and process operations. The removal of impurities and gases in aluminum liquid needs to follow the principle of "source control+process purification+end control". Combined with the characteristics of aluminum melting and casting process, targeted scientific methods are adopted to achieve efficient removal of impurities and gases, ensuring that the purity of aluminum liquid meets the standard.

Source control is the foundation for effectively removing impurities and gases from aluminum liquid, with the core being strict control of raw material quality and production environment, reducing the introduction of impurities and gases from the source. In terms of raw materials, aluminum ingots, scrap aluminum, and other raw materials used in aluminum melting and casting must be selected from products with qualified purity. Before feeding, strict cleaning must be carried out to remove impurities such as oil stains, rust, and mud attached to the surface, to avoid impurities from entering the furnace with the raw materials; For damp raw materials, it is necessary to perform drying treatment in advance to reduce the moisture content in the raw materials and minimize the source of hydrogen gas production - the reaction between moisture and high-temperature aluminum liquid to generate hydrogen gas, which is one of the main sources of hydrogen gas in aluminum liquid.

Production environment control is equally important. The aluminum melting and casting workshop needs to be kept dry and clean to avoid humid air entering the furnace. It is recommended to control the relative humidity of the workshop below 60% to reduce the contact between moisture in the air and aluminum liquid; At the same time, regularly clean equipment such as furnaces, runners, and gates to remove refractory debris, oxide slag, and other impurities attached to the inner walls, avoiding impurities from mixing into the aluminum liquid and reducing the risk of aluminum liquid pollution from the source. In addition, during the feeding process, it is necessary to handle with care to avoid the collision of raw materials and the generation of debris. At the same time, the contact time between aluminum liquid and air should be reduced to decrease the amount of oxide inclusions.

Process purification is the core step in removing impurities and gases from aluminum liquid, mainly achieved through refining processes. Currently, the commonly used refining methods in the aluminum casting industry are chemical refining and physical refining. Enterprises can flexibly choose according to their own production line scale and product needs, or adopt a combination of "chemical refining+physical refining" to improve the purification effect. Chemical refining mainly relies on the reaction between refining agents and impurities and gases in aluminum liquid to achieve impurity separation and gas removal, which is a commonly used refining method in small and medium-sized aluminum melting and casting enterprises.

The key to chemical refining lies in the rational selection of refining agents and control of refining parameters. Refining agents should be environmentally friendly and free of secondary pollution products, with priority given to composite refining agents that can simultaneously achieve impurity and gas removal functions. Their main components are salts, rare earth compounds, etc., which can react with oxidized inclusions in aluminum liquid to generate slag with a density lower than that of aluminum liquid, making it easy to float and separate; At the same time, the gas produced by the decomposition of refining agents can combine with hydrogen gas in the aluminum liquid to form bubbles and be discharged from the aluminum liquid. The amount of refining agent needs to be flexibly adjusted based on the volume, impurities, and gas content of the aluminum liquid. If the amount is too small, the ideal purification effect cannot be achieved. If the amount is too large, new impurities may be introduced. Generally, the ratio of refining agent amount to aluminum liquid quality should be controlled within a reasonable range.

Physical refining mainly achieves the removal of impurities and gases through physical methods, including inert gas injection refining, vacuum refining, etc., which are suitable for large-scale and high-end aluminum casting production lines. Inert gas injection refining is currently the most widely used physical refining method. Inert gases such as argon and nitrogen are injected into the aluminum liquid through a hollow rotating rod. The inert gas forms a large number of small bubbles in the aluminum liquid. During the rising process, the bubbles adsorb hydrogen gas and small oxide inclusions in the aluminum liquid, and are discharged from the surface of the aluminum liquid together with the bubbles, achieving a dual effect of impurity and gas removal.

When using inert gas injection refining, it is necessary to control the gas flow rate, injection time, and refining temperature. The gas flow rate should be uniform and stable to avoid splashing of aluminum liquid due to excessive flow rate or affecting the adsorption effect due to insufficient flow rate; The spraying time is generally controlled at 15-30 minutes to ensure sufficient contact between the bubbles and the aluminum liquid; The refining temperature is controlled between 700 ℃~750 ℃, which can ensure the adsorption effect of inert gas and avoid excessive oxidation of aluminum liquid. Vacuum refining is achieved by placing aluminum liquid in a vacuum environment, reducing the solubility of hydrogen gas in the aluminum liquid, allowing hydrogen gas to quickly precipitate from the aluminum liquid, while promoting impurity flotation. The purification effect is outstanding and suitable for the production of high-end aluminum alloy products with high purity requirements for aluminum liquid.

End of pipe inspection is the last line of defense to ensure the purification effect of aluminum liquid, mainly achieved through filtration technology. The core is to intercept small impurities remaining after refining, further improving the purity of aluminum liquid. At present, the most widely used filter equipment in the aluminum melting and casting industry is the foam ceramic filter, which has a three-dimensional connected porous structure. Products with appropriate pore size can be selected according to the needs of aluminum purification. Aluminum oxide based foam ceramic filters of 20PPI~25PPI can be selected for the production of ordinary aluminum ingots, and zirconia based foam ceramic filters of 30PPI can be selected for the production of high-end aluminum alloy products.

During the filtration process, attention should be paid to the preheating and installation specifications of the filter to avoid cracking due to thermal shock, which may cause impurities to enter the aluminum liquid; At the same time, a graded filtration mode of "coarse filtration+fine filtration" can be adopted, first intercepting large impurities through a large aperture filter, and then intercepting small impurities through a small aperture filter, which not only avoids filtration blockage but also ensures filtration effect. In addition, the filtered aluminum liquid needs to be left in the holding furnace for a certain period of time to allow residual small impurities and bubbles to fully float up, further improving the purity of the aluminum liquid.

Industry experts remind that in the process of aluminum melting and casting, the removal of impurities and gases in the aluminum liquid requires the establishment of a full process purification system, which combines source control, process refining, and end filtration organically, and cannot rely solely on any one link. Enterprises need to develop targeted purification plans based on their own production reality, specifying operating standards such as refining agent dosage, refining time, gas flow rate, filtration parameters, etc; Strengthen the training of operators, enhance the professionalism of practical operations, standardize the operation of each link, and reduce human errors; Equipped with high-precision detection equipment, real-time monitoring of the purity of aluminum liquid, timely adjustment of purification parameters, to ensure stable quality of aluminum liquid.

In practice, many aluminum melting and casting enterprises have effectively improved the purity of aluminum liquid by optimizing purification processes. For example, a certain aluminum processing enterprise adopts a full process purification scheme of "raw material drying+composite refining agent chemical refining+argon gas injection physical refining+graded filtration", which controls the hydrogen content in the aluminum liquid within a reasonable range, increases the impurity removal rate by more than 40%, and reduces the incidence of porosity, slag inclusion and defect in aluminum ingots by 35%; Another enterprise has introduced intelligent refining and filtering equipment to achieve automated monitoring and regulation of the purification process, further improving the efficiency and stability of impurity and gas removal.

With the continuous upgrading of aluminum melting and casting technology, impurity and gas removal technology is also being continuously optimized. Related equipment companies continuously develop efficient refining agents, high-precision filtration equipment, and intelligent refining systems to help improve the purification level of aluminum liquid; Aluminum casting enterprises achieve precise removal of impurities and gases through technological innovation and optimization of purification process combinations. In the future, with the application of technologies such as 3D printing and big data, aluminum liquid purification will develop towards intelligence and efficiency, helping the aluminum melting and casting industry produce higher quality aluminum alloy products and providing solid support for downstream industries.