Melting Characteristics of Different Aluminum Alloys

The melting processes of different aluminum alloys are generally similar, but each alloy has its own specific requirements. The following points summarize the melting characteristics of several common aluminum alloys remelted from solid aluminum in aluminum processing plants.

Industrial Pure Aluminum

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

The melting temperature and melt holding time have a significant influence on ingot grain size. In general, the melting temperature should not exceed 745°C, and the melt holding time should not exceed 2 hours. This should be controlled especially strictly in flame furnaces.

Attention should also be paid to the Fe/Si ratio and the amount of grain refiner. When producing high-purity aluminum products above 99.90%, the furnace lining material must meet strict requirements. The SiO2 content in the lining should be kept as low as possible; otherwise, silicon may enter the aluminum melt and reduce purity. Magnesia bricks are preferred.

Al-Mn Series Alloys

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

Therefore, 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 to accelerate manganese dissolution. Another method is to raise the melting temperature by about 50°C, to approximately 780°C, and then stir slowly to improve dissolution.

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

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

Generally, Fe should be controlled within 0.4%-0.6%, and w(Fe + Mn) should be <= 1.8%. To reduce hot cracking tendency, Fe is usually kept above 0.2%, with a small amount of Ti added.

Al-Mg Series Alloys

Al-Mg alloys contain relatively high magnesium. As magnesium content increases, the oxide film becomes less dense, oxidation resistance decreases, and this becomes more obvious after melting.

Main effects include:

1. The oxide film loses its protective function, causing serious burning loss, especially magnesium loss.
2. The oxide film becomes less dense, increasing gas absorption.
3. Oxide inclusions form easily, reducing ingot quality.
4. Oxide inclusions on the ingot surface may cause stress concentration and increase cracking tendency.

For high-magnesium aluminum alloys, except 5A03 alloy, 0.002% Be should be added. For 5A06 (LF6), 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 sodium content increases. Sodium has a low melting point and is insoluble in aluminum and magnesium. During solidification, sodium forms a low-melting adsorption layer at grain boundaries, reducing grain boundary strength.

In high-magnesium aluminum alloys, Mg and Si first form Mg2Si, causing free sodium to precipitate:

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

When Mg content exceeds 2%, sodium may precipitate. When Mg is low, the effect of free sodium is not significant.

During melting, sodium impurities mainly come from fluxes such as Na3AlF6, NaCl and NaF. Therefore, sodium-containing fluxes should not be used as refining or covering agents for high-magnesium aluminum alloys. Sodium-free fluxes should be used instead.

Al-Cu-Mg Series Alloys

Al-Cu-Mg alloys have high mechanical properties and are widely used. Because these alloys contain a relatively high amount of copper, pure copper plates may be directly added to save master alloy and reduce cost.

Copper plates should be added evenly into the furnace, kept below the liquid surface, and the melt should be stirred uniformly to ensure even chemical composition. Samples should then be taken for chemical analysis.

Fe and Si have a strong influence on the casting performance of these alloys. To prevent cracking in 2A12 (LY12) round ingots, Fe content is preferably slightly higher than Si. However, production experience shows 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), controlling Si within 0.485%-0.6% can reduce hot cracking tendency. Ingots with diameters below 190 mm may not require this control.

2A12 contains more Mg than 2A11. Its oxide film is less dense in the liquid state, increasing gas content. Meanwhile, 2A12 has a wider crystallization range, so its looseness tendency is greater than that of 2A11.

During melting, gas absorption must be prevented, covering should be done at the right time, and refining and degassing should be strengthened. 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, metal cleanliness must be maintained and melt refining should be strengthened.

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

In 2A70 (LD7), 2A80 (LD8) and 2A90 (LD9) alloys, Fe and Ni often precipitate and segregate from the melt, forming intermetallic compounds. Therefore, 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, high total alloying-element content and large density differences between elements. 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 inclusions. Raw materials should be kept clean.

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

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