[China Aluminum Industry Network] The process of changing the state of metal from a solid state to a liquid state is called melting. The melting of metal is during the heating process. As the thermal motion of atoms increases, the bonded parts of the atoms are destroyed, and the original crystal grains gradually disintegrate into small atomic groups, transforming the metal from a solid state with a fixed shape to a liquid state with a flowing ability. the result of. Due to the high atomic energy at the grain boundary, the energy required to cause the atoms to jump and transfer when heating is small, so the melting of the metal always starts from the grain boundaries.
When one component of an alloy dissolves in another component, heat is released or absorbed due to the formation of a certain compound, alloying, or delamination. This is called heat of solution. The amount of the amount varies depending on the composition of the alloy. For example, when aluminum and copper are alloyed, an intermetallic compound CuAl2 is generated, and a large amount of heat is emitted, so that the local temperature can be 200 to 300° C. higher than the original temperature. In addition, an exothermic reaction also occurs when aluminum forms a eutectic with silicon and when chromium is added to aluminum. Therefore, great attention should be paid to the production. It should be pointed out that high magnesium alloys can also cause natural heating and overheating when too much magnesium is added or the furnace temperature is high, but this is not caused by the heat of solution, but is caused by oxidation and heat generation.
The phenomenon of gasification on the surface of the melt is called evaporation. At a certain temperature, the saturated vapor pressure at which a metal melt evaporates and the metal vapor condenses in a closed space is called the vapor pressure of the metal at that temperature. The greater the vapor pressure, the smaller the gas solubility in the melt, but the greater the evaporation loss. The evaporative loss of metal generally increases with the increase of melt temperature, furnace pressure, smelting time, molten pool surface area, and increase of oxide film porosity on the surface of the melt, which is related to the composition and content of the alloy. Under the same smelting conditions, the elements with high vapor pressure are generally more volatile, and the evaporation heat is small, the boiling point is low (see Table 2-1-16), and the elements that are insoluble or rarely dissolved in the alloy and the content is high are generally more volatile. . Under normal smelting conditions, the vapor pressure of the aluminum melt is very small (1mmHg at 1537°C) and the evaporation loss can be neglected. However, in vacuum melting, the evaporation loss of volatile elements such as sodium, magnesium, zinc, and lithium in the melt should be taken into consideration.
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