Aluminum is the most common metal in the Earth's crust. At room temperature, a thin film called aluminum oxide forms on its surface. It protects the metal from corrosion and oxidation. The oxidation time of the metal increases rapidly as the temperature rises, so in the process of melting aluminum or its alloys, the surface of the molten material is quickly covered with a layer of oxide.
Melting point of aluminum
Aluminum is extremely common in nature. It ranks fourth among all elements and first among metals (8.8% of the mass of the Earth's crust), but it does not occur in pure form. It is extracted mainly from bauxite, although several hundred aluminum minerals (aluminosilicates, allites, etc.) are known, the vast majority of which are not suitable for the production of this metal.
The metal's basic physical properties allow it to be used in various branches of industrial production, and its ability to form permanent compounds with other chemical elements greatly expands its range of applications. A separate issue remains - how to melt aluminum.
Aluminum is melted by supplying it with heat energy from outside or directly into its volume (induction heating). The average melting point of aluminum is 660 degrees Celsius or 993.5 degrees Kelvin. The melting point depends on the chemical purity of the material, pressure and other factors.
- The melting point of 99.996% pure aluminum is 660.37 degrees Celsius.At an aluminum content of 99.5%, melting begins at 657 degrees Celsius.
- At an aluminum content of 99.0%, melting begins at 643°C.
During melting, the volume of the metal increases. The sharp increase in volume occurs under the influence of a process that is called latent heat of fusion. Atoms lose their dense and ordered original crystal structure. This process is reversible - it works both when heated and when cooled.
Initial and final melting temperatures
To understand how to melt aluminum, we need to understand what the initial and final melting temperatures are. The specific heat of melting of aluminum determines its physical property - the transition from one state to another. The temperature at which aluminum and its alloys begin to melt is called the solidus point. The solidus temperature means that the alloy is still in a solid state. The final melting point is the liquidus point. The temperature range means that the compound is in an intermediate state - semi-liquid, semi-solid, mushy.
Aluminum melting - the effect of impurities and alloying additives on the process
Adding additives, including alloying additives, to a chemically pure metal (primary aluminum) lowers the initial melting point. For example, melting aluminum in an alloy containing magnesium (Mg) and silicon (Si) is reduced to almost 500°C. Aluminum casting in alloy form has virtually no specific melting temperature. The melting and solidification process of aluminum alloy occurs within a certain temperature range. The temperatures of liquidus and solidus, the most popular industrial aluminum alloys for forming, vary depending on the specific volume of pure metal.
In some compounds of aluminum with other chemical elements, there is no gap between the temperature indicators of the transition from the solid to the molten state. Such alloys are called eutectic. For example, casting aluminum containing 12.5% silicon is characterized by a constant temperature of 577°C. As the silicon content increases, the liquidus gradient decreases from the maximum value characteristic of pure metal. Among other alloying additives, magnesium reduces the temperature gradient to 450°C. When combined with copper, the eutectic temperature is 548°C, and with manganese only 658°C. Most aluminum alloys are not even binary, but triple or quadruple. So with the combined influence of several alloying elements, solidus-liquidus ratios can be even lower.
- 13 February 2024
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