Melting Characteristics of Different Aluminum Alloys

Release time:

Jul 30,2026

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Melting Characteristics of Different Aluminum Alloys

The melting processes of different aluminum alloys are basically similar, but each alloy has its own detailed process requirements. Based on the melting characteristics of several aluminum alloys commonly remelted from solid aluminum in aluminum processing plants, the main points are as follows.

Industrial Pure Aluminum

When melting industrial pure aluminum, its purity must be maintained. Primary aluminum ingots should generally be selected according to the purity requirements and processing performance of the final product.

During pure aluminum melting, the melting temperature and melt holding time have a very clear influence on the grain size of the ingot. The melting temperature should generally not exceed 745°C. The melt holding time should not exceed 2 hours, especially in flame furnaces, where stricter control is required.

In terms of chemical composition, attention should be paid to the iron-silicon ratio and the content of grain refiners. When producing high-purity aluminum products above 99.90%, strict requirements should be placed on the furnace lining material. The SiO2 content in the lining should be kept as low as possible; otherwise, it may increase the silicon content in the aluminum and reduce purity. Magnesia bricks are preferred.

Al-Mn Series Alloys

Al-Mn aluminum alloys contain a relatively high amount of manganese. Because manganese has very low solubility in aluminum, a master alloy containing 10% Mn dissolves very slowly at normal melting temperatures.

For this reason, the Al-Mn master alloy should be evenly distributed in the upper layer of the furnace charge. When the melt temperature reaches 720°C, the melt should be stirred slowly and properly to accelerate manganese dissolution. Another method is to raise the melting temperature by about 50°C, to approximately 780°C, and then slowly stir the melt to speed up manganese dissolution.

In electrolytic aluminum plants, it is beneficial to use high-temperature liquid primary aluminum to melt Al-Mn series alloys.

When 3A21 (LF21) alloy sheet is annealed, coarse grain structure is likely to form. Therefore, during melting, the Fe impurity content may be appropriately increased, and a suitable amount of Ti may be added. However, if the Fe content is too high, primary crystal compounds such as (MnFe)Al6 may form, reducing the mechanical properties of the alloy.

In general, the Fe content should be controlled within 0.4%-0.6%, and w(Fe + Mn) should be <= 1.8%. To reduce the tendency of hot cracking, it is generally better to keep the Fe content above 0.2% and add a small amount of Ti.

Al-Mg Series Alloys

Al-Mg aluminum alloys contain a relatively high amount of magnesium. As the magnesium content increases, the density of the oxide film becomes poorer, and oxidation resistance decreases. This problem becomes especially obvious after melting.

The main effects are as follows:

The oxide film loses its protective function, alloy burning loss becomes serious, and magnesium is more easily burned off.
The oxide film becomes less dense, increasing the gas absorption tendency of the alloy.
Oxide inclusions are easily formed, reducing ingot quality. Oxide inclusions on the ingot surface may cause stress concentration and increase the tendency of ingot cracking.

When high-temperature liquid primary aluminum is used to produce this type of alloy, this problem exists throughout the entire melting process and must be handled carefully.

Increasing magnesium content brings certain difficulties to the melting and casting process. Therefore, for high-magnesium aluminum alloys, except 5A03 alloy, 0.002% Be should be added. For 5A06 (LF6) alloy, 0.004% Be should be added to modify the oxide film and improve oxidation resistance.

In high-magnesium alloys, sodium brittleness becomes more obvious as the sodium content increases. This is because sodium has a low melting point and is insoluble in aluminum and magnesium. During alloy solidification, sodium forms a low-melting adsorption layer at the grain boundary, reducing grain boundary strength.

However, sodium brittleness occurs only when sodium is in a free state. In high-magnesium aluminum alloys, Mg and Si first form the Mg2Si phase, causing free sodium to precipitate:

NaAlSi + 2Mg -> Mg2Si + Na (free) + Al

When the magnesium content exceeds 2%, sodium precipitation may occur. When the magnesium content is low, the effect of precipitated free sodium is not significant.

During melting, impurity sodium mainly comes from fluxes such as Na3AlF6, NaCl and NaF. Therefore, sodium-containing fluxes must not be used as refining agents or covering agents for high-magnesium aluminum alloys. Sodium-free fluxes should be used instead, such as carnallite-based refining and covering fluxes. Liquid primary aluminum produced from acidic electrolyte aluminum electrolysis cells may also be used.

Al-Cu-Mg Series Alloys

Al-Cu-Mg aluminum alloys have relatively high mechanical properties and are widely used. These alloys contain a relatively high amount of copper. To save master alloy consumption and reduce production cost, pure copper plates may be directly added into the alloy.

In this case, the copper plates should be added evenly into the furnace. They must not be exposed above the liquid surface, and the melt should be stirred uniformly to ensure even chemical composition. After that, samples should be taken for chemical composition analysis.

The Fe and Si contents have a major influence on the casting performance of this type of aluminum alloy. To prevent cracking in 2A12 (LY12) round ingots, it is better to keep the Fe content slightly higher than the Si content. However, years of production experience show that the Fe-Si ratio has no obvious effect on 2A12 flat ingots.

To reduce cold brittleness, w(Fe + Si) should be controlled at <= 0.5%. For 2A11 (LY11) alloy, the situation is different. Controlling Si within 0.485%-0.6% can reduce the tendency of hot cracking. Ingots with a diameter below 190 mm may not require this control.

2A12 alloy contains slightly more Mg than 2A11 alloy. In the liquid state, its oxide film is less dense, which increases gas content in the alloy. At the same time, 2A12 alloy has a wider crystallization interval. Therefore, 2A12 alloy has a greater tendency to form looseness than 2A11 alloy.

During melting, special attention should be paid to preventing gas absorption in 2A12 alloy. Covering should be carried out at the proper time, and refining and degassing should be strengthened. In terms of process control, melting temperature and melting time have a great influence, so the melting temperature must be strictly controlled and should not exceed 745°C.

Al-Cu-Mg-Si and Al-Cu-Mg-Fe-Ni Series Alloys

Most products made from these alloys are used for important components. During melting, the cleanliness of the metal must be maintained, and melt refining should be strengthened.

During operation, gas and metallic inclusions must be prevented from entering the melt.

In 2A70 (LD7), 2A80 (LD8) and 2A90 (LD9) alloys, Fe and Ni often precipitate from the melt and segregate, forming intermetallic compounds. Therefore, the Fe and Ni contents should not be too high and should be controlled near the middle or lower limits.

Al-Cu-Mg-Zn Series Alloys

Al-Cu-Mg-Zn alloys have complex compositions and a relatively high total content of alloying elements. The density difference among the elements is also large. To make the composition uniform, melt stirring should be strengthened to prevent Zn precipitation.

During melting, covering agents should be used properly to prevent gas absorption and oxide inclusion formation. The raw materials used should be kept as clean as possible.

During charge calculation, the Si content should be controlled near the lower limit, while Mg and Zn should be kept near the upper limit. If the Si content is too high, more Mg2Si may form, limiting the entry of Mg and Zn into the solid solution.

7A04 (LC4) alloy has a relatively strong tendency to crack. For flat ingots, Mg is generally controlled near the upper limit, Cu and Mn near the lower limit, and Fe at around 0.4%.

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Aluminum-Magnesium Alloy Composition

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Aluminum-Magnesium Alloy Composition