The physical and chemical properties of aluminum melt are highly sensitive to temperature, and temperature changes can alter the quality of the melt from multiple dimensions such as fluidity, oxidation behavior, gas absorption capacity, and elemental stability, ultimately leading to the microstructure and properties of the ingot.

Liquidity is the core indicator for measuring the filling ability of aluminum liquid, referring to the flow ability of the melt itself, which directly affects the filling effect of the melt and the surface quality of the ingot during the casting process. The fluidity of liquid metal is negatively correlated with its viscosity, which is a function of temperature.

For aluminum alloys, in the temperature range above the liquidus line, the viscosity of the melt decreases exponentially with increasing temperature, and the flowability significantly increases. Taking pure aluminum as an example, around the melting point of 660 ℃, the viscosity of the aluminum liquid is about 1.2mPa · s. When the temperature rises to 750 ℃, the viscosity drops to about 0.8mPa · s and the fluidity increases by about 50%; The temperature continued to rise to 800 ℃, and the viscosity further decreased to about 0.7mPa · s, but the increase in fluidity significantly narrowed, and the marginal effect significantly decreased.

In production, appropriately increasing the melting and casting temperatures can help reduce the difficulty of filling thin-walled castings and complex cavities, and reduce defects such as cold shut and insufficient pouring. However, it should be clarified that fluidity is not solely determined by temperature: oxide inclusions, hydrogen content, solid particles, and other factors in the melt can significantly reduce fluidity. Many on-site issues of poor fluidity and blockage are essentially caused by insufficient cleanliness of the melt and excessive slag inclusion, rather than insufficient temperature. Blindly raising the temperature at this point not only fails to fundamentally solve the problem, but also exacerbates oxidation, forming more inclusions and falling into a vicious cycle of "the more blocked, the hotter, and the more slag is trapped".

Aluminum is a highly chemically active metal with a high affinity for oxygen. It can react with oxygen in the air at room temperature, forming a dense layer of aluminum oxide film on the surface to prevent further oxidation of the internal metal. However, under high-temperature melting conditions, this protective effect will gradually fail as the temperature increases, and the oxidation reaction rate will increase geometrically.

According to the principle of chemical reaction kinetics, for every 100 ℃ increase in temperature, the oxidation reaction rate of aluminum can be increased by 3-5 times. When the melt temperature is below 700 ℃, the oxidation reaction is relatively smooth, and the resulting γ - Al ₂ O Ⅲ film structure is dense, which can to some extent hinder the continuation of oxidation. At this time, the oxidation loss is at a low level; When the temperature exceeds 782 ℃, the oxidation rate accelerates sharply, and the structure of the oxide film begins to transform into a loose and porous α - Al ₂ O3, losing its protective effect. Oxygen can continue to react with the internal aluminum liquid through the oxide film, resulting in a significant increase in burn loss; When the temperature exceeds 850 ℃, aluminum liquid will undergo severe oxidation or even combustion, producing a large amount of powdery oxide slag, and aluminum loss will increase exponentially.

Oxidation burn is the main component of aluminum melting loss. Taking ordinary 6-series aluminum alloy as an example, when the melting temperature is controlled within 740 ℃, the loss of alumina in a single furnace is usually controlled between 0.5% and 1.0%; If the melting temperature is raised to above 780 ℃ for a long time, the aluminum loss can increase to 1.5%~2.5%, and the metal loss per ton of ingot can increase by 10-20kg. For a melting and casting workshop with an annual output of tens of thousands of tons, the cost of aluminum loss caused by temperature control alone can reach millions of yuan per year.

In addition, a large amount of alumina inclusions generated at high temperatures will be suspended in the melt, which is difficult to completely remove through refining, and eventually enter the ingot to form slag inclusion defects, reducing the mechanical properties and surface processing quality of the material.

More than 90% of the porosity defects in aluminum alloys are caused by hydrogen evolution. Hydrogen is the only gas that can dissolve in liquid aluminum in large quantities, and its solubility in liquid aluminum is directly related to temperature and follows the square root law.

At standard atmospheric pressure, the solubility of hydrogen in solid aluminum is only 0.036mL/100g, while in liquid aluminum with a melting point of 660 ℃, the solubility suddenly increases to 0.65mL/100g, a difference of nearly 18 times. As the temperature continues to rise, the solubility of hydrogen will further increase rapidly: about 0.9mL/100g at 700 ℃, about 1.25mL/100g at 750 ℃, and up to 1.8mL/100g at 800 ℃, almost three times the melting point.

Meanwhile, high temperatures significantly accelerate the reaction between aluminum liquid and water vapor in the air

The atomic hydrogen generated by the reaction of 2Al+3H2O=Al2O3+6 [H] will continue to dissolve into the melt. The higher the temperature, the faster the reaction rate and the faster the hydrogen absorption rate of the melt. In humid environments, the hydrogen content of high-temperature melted melt can increase by 0.3~0.5mL/100g within 1 hour, far exceeding the removal capacity of refining processes.

The high hydrogen base brought by high-temperature melting will significantly increase the difficulty of subsequent refining and degassing. Even after the same refining process, the final hydrogen content of superheated melt will still be significantly higher than that of normal temperature melt, and more hydrogen gas will be released during the solidification process, forming pores, pinholes, and loose defects. For die-casting products and high-end aluminum foil products that require high airtightness, excessive hydrogen content can directly lead to product scrap.

Aluminum alloys contain various alloying elements such as magnesium, zinc, copper, silicon, manganese, etc. The oxidation activity and saturated vapor pressure of different elements vary significantly. Increasing temperature will generally exacerbate the burning and volatilization of elements, leading to compositional fluctuations and even non-compliance.

Among them, alkali metals and alkaline earth metal elements (sodium, calcium, strontium) as well as magnesium, zinc and other elements are most sensitive to temperature. Taking magnesium element in 5-series aluminum magnesium alloy as an example, the oxidation activity of magnesium is much higher than that of aluminum, and the vapor pressure increases sharply with temperature: the burning rate of magnesium is low at 700 ℃, while after the temperature exceeds 750 ℃, the oxidation and volatilization burning of magnesium will increase linearly. For every 20 ℃ increase, the amount of magnesium burned per unit time can be doubled. If the melting temperature is controlled above 780 ℃ for a long time, the magnesium content in 5083 alloy may decrease by 0.2%~0.4% after 1 hour of insulation, exceeding the composition tolerance range. Additional magnesium ingots must be added, which not only increases costs but also easily causes uneven composition and magnesium oxide inclusions.

For 2-series and 7-series high-strength aluminum alloys, high temperature overheating may also lead to premature melting of low melting point phases at grain boundaries, resulting in a tendency towards overburning. Once the melt is overheated, the grain boundary structure will undergo irreversible deterioration, resulting in a significant decrease in the mechanical and crack resistance properties of the final ingot, which cannot be repaired through subsequent heat treatment. The entire furnace material can only be downgraded or scrapped.