Aluminum alloy smelting is the first process in  aluminum sheet&product manufacturing process.

In this guide, I will explain everything you should know about aluminum alloy smelting  basic knowledge.

If you want to know how to protect aluminum alloy melts in produce process, read this guide to the end.

Aluminum alloy smelting is a process of melting aluminum and other metal elements together to manufacture alloy materials with specific properties and uses.

Aluminum is mixed with other metal elements such as copper, zinc, magnesium, etc., and the physical state of the metals is changed by heating, so that aluminum metal and alloyed metals are melted in the required proportions to form a homogeneous melt, which meets the process requirements for the purity of various internal elements, casting temperature, and other specific requirements.

The quality of the melt has a decisive impact on the processing performance and final utilization performance of aluminum materials. If the quality of the melt is inherently insufficient, it may pose potential risks to the use of the products. Therefore, smelting is a key process that governs the final quality of processed products.

Aluminum alloy smelting has the characteristics of high energy consumption, easy oxidation, hydrogen absorption, and easy absorption of impurities.

Although the melting point of aluminum is low, it has a high latent heat of fusion, high specific heat, and low density. When compared to other commonly used metals such as iron and copper, aluminum consumes more energy during the melting process.

The amount of oxygen dissolved in aluminum and aluminum alloys is very small. Most of the oxygen exists in the aluminum melt in the form of oxides. Unlike the reaction between hydrogen and aluminum, which results in hydrogen dissolution, the reaction between aluminum and oxygen results in the formation of aluminum oxide. Their chemical reaction equations are as follows:

                                                     4Al+3O2= 2Al2O3

The conditions for oxidation reactions are related to the decomposition pressure of aluminum metal and the oxygen partial pressure in the furnace gas during smelting. If, under smelting conditions, the decomposition pressure of aluminum oxide is lower than the oxygen partial pressure of aluminum gas under those conditions, the oxidation reaction may occur. Conversely, aluminum oxide cannot be formed.

The heat of formation of aluminum oxide is high, and the decomposition pressure is low, which means that aluminum has a strong affinity for oxygen. At the same time, the formed aluminum oxide is very stable, so aluminum is highly prone to oxidation.

Although the aluminum oxide film formed on the surface of the melt has a protective effect, once it is damaged, it is difficult to remove the oxide film that enters the solution. On one hand, Al2O3 is not easily reduced, and on the other hand, its density is similar to that of the solution. It remains suspended in the melt and, as the melt enters the ingot, it adversely affects the quality of the processed material. More seriously, aluminum oxide is a good carrier for various gases, and its presence leads to the absorption of a large amount of hydrogen by the aluminum melt. Therefore, oxidation should be minimized during the aluminum alloy smelting process.

The easy oxidizability of aluminum can lead to the following phenomena: during the smelting process of aluminum alloys, it is often found that solid non-metallic impurities and unwanted metal impurities gradually increase in the aluminum melt. Known solid non-metallic impurities that may exist in the aluminum melt include oxides, carbides, nitrides, hydrides, etc., but the main impurity is oxide. The oxides in the aluminum melt mainly include Al2O3, MgO, and spinel (other metal oxides in the solid solution of Al2O3). In addition to the reaction of oxygen and water vapor in the melt and furnace gas during the smelting process, the source of oxides is mainly formed by displacement reaction of oxides in the melt and furnace lining, including the introduction of furnace materials, tools, etc. In addition to impurities brought in by raw materials and artificial factors such as incomplete furnace cleaning, mixing of materials, and addition of electric heating materials, the main impurities in the aluminum melt are due to various physical and chemical reactions that occur when the aluminum melt comes into contact with aluminum lining, tools, various refining agents, and additives.

Aluminum and aluminum alloys have strong gas absorption capabilities, especially in furnace gas containing water vapor, they have a strong hydrogen absorption capacity. Although the amount of hydrogen absorbed by aluminum and aluminum alloys is not large, in the temperature range of melting, aluminum and hydrogen do not actually form chemical compounds, but the dissolution of hydrogen in aluminum is very significant. However, there is a significant difference in the solubility of hydrogen in the solid and liquid phases at the melting point, which leads to a tendency for porosity and looseness during ingot solidification. Therefore, it is necessary to minimize hydrogen absorption during the aluminum alloy melting process.

Some alloying elements in aluminum and aluminum alloys have high chemical reactivity. They can not only absorb iron directly dissolved from iron crucibles and tools, but also displace metals such as iron, silicon, and zinc from many oxide linings and chloride salts of fluxes. Once these metal impurities enter the aluminum melt, they cannot be removed. As the number of smelting increases, the impurity content also increases, resulting in a greater impact on the alloy’s properties. In severe cases, it can lead to a degradation in the taste of pure aluminum and cause alloy compositions to exceed the standards and be scrapped. Therefore, preventing contamination from metal impurities is also an important issue in aluminum alloy smelting.

We already know that the contact between gas and metal, leading to diffusion and absorption, originates from active adsorption, which is the beginning of chemical reactions. Therefore, in order to solve the problem of gas and non-metallic impurities in molten ingots, a set of casting methods that reduce the reactivity of chemical reactions must be established. The current approach is based on the principle of “prevention, elimination, melting, and prevention as the main focus.

• Ensure maximum cleanliness of all furnace materials
• Store raw materials under conditions that prevent contact with moisture whenever possible
• Smelt in furnaces that contain the least amount of water vapor
• Use chemically stable refractory materials
• Implement protection on the surface of the melt

• Carry out dedicated refining and purification of the aluminum liquid during the melting process

• Establish appropriate ingot crystallization conditions so that hydrogen that cannot be eliminated from the aluminum liquid can partially or even completely dissolve into the ingot during solidification, without causing the formation of gas porosity defects

There are two methods for protecting aluminum melts: one is to cover the surface of the melt with a flux to form a continuous cover layer. The other method, which is less commonly used, is the protective alloy method. Both methods are based on establishing an inert surface to prevent gas and melt from interacting with each other or significantly reduce the reactivity of chemical reactions to achieve protection.

This method is suitable for all alloys. It not only prevents the oxidation and hydrogen absorption of the melt but also has a dehydrogenation effect. This is because the melting point of the covering solvent is usually lower than the melt temperature, and its density is smaller than that of the melt. It also has good wetting properties, allowing a continuous liquid covering film to form on the surface of the melt, separating the solution from the furnace gas.
In general, oxygen and water vapor cannot or rarely react with the melt through the cover layer, while hydrogen atoms dissolved in the melt, due to their small radius, can penetrate the cover layer and escape.
The coverage of aluminum liquid surface refers to the ability of the liquid flux to automatically spread and form a continuous covering layer on the surface of the molten metal.

The process and operating procedures of removing gases, impurities, and harmful elements from the molten metal to obtain high-quality aluminum melt are called refining, also known as purification. There are many methods for refining aluminum alloys, which can be divided into two basic types based on their principles: adsorption refining and non-adsorption refining.

Gas filtration is a commonly used method for purifying aluminum melt. It removes solid impurities and gas bubbles from the aluminum melt using filters to improve the purity and cleanliness of the aluminum melt. Common filters include ceramic filters and foam ceramic filters. The schematic diagram of a typical ceramic filtration equipment is shown in the following figure.

These filters have small pores and high filtration efficiency, which can effectively remove solid particles and bubbles from the aluminum melt. Gas filtration can reduce the impurity content in the aluminum melt and improve the quality and performance of aluminum alloy.

Foam ceramic filters are filtration devices that use foam ceramics as the medium. They are constructed by installing foam ceramics in the molten metal filtration basin between the holding furnace and the casting table. The schematic diagram is shown below.

Gas removal is another commonly used method in aluminum liquid purification. In the gas removal process, the aluminum liquid is placed in a vacuum environment, and the gas is removed from the aluminum liquid using the buoyancy principle of bubbles. By performing gas removal under vacuum conditions, the hydrogen content can be effectively reduced, and the content of sodium, potassium, and calcium can be controlled, thereby improving the purity and cleanliness of the aluminum liquid and ensuring the quality of aluminum alloy products. One typical method is the Gas Bubbling Filtration (GBF) method, which uses high rotation (700r/min) to produce small and uniform bubbles (diameter 1~4mm) and purifies the gas using Ar with a small gas volume (0.2m^3/tAl) to achieve purification effect. As below it shows the the GBF equipment schema picture.

As below is the table of GBF method degassing effect