[China Aluminum Industry Network] 1. Principle of Aluminum Alloy Heat Treatment
Heat treatment of aluminum alloy castings involves applying a specific thermal process, where the material is heated at a controlled rate to a certain temperature and held for a defined period before being cooled at a specified speed. This process modifies the microstructure of the alloy, aiming to enhance its mechanical properties, improve corrosion resistance, optimize machinability, and ensure dimensional stability.
2. Characteristics of Aluminum Alloy Heat Treatment
Unlike high-carbon steels, which become very hard immediately after quenching but lose plasticity, aluminum alloys behave differently. After quenching, their strength and hardness do not increase right away, while their ductility actually improves. However, after a certain period—typically 4 to 6 days—the strength and hardness rise significantly, while ductility decreases. This gradual strengthening over time is known as aging. Aging can occur naturally at room temperature or be accelerated by heating, known as artificial aging, typically between 100°C and 200°C.
3. Mechanism of Age Hardening in Aluminum Alloys
The age hardening process in aluminum alloys is complex and influenced by both the alloy’s composition and the manufacturing process. It also depends on defects such as vacancies and dislocations that form during solidification. Currently, it is widely accepted that age hardening results from the segregation of solute atoms into hardened zones.
During quenching, the rapid cooling creates vacancies within the alloy structure. These vacancies are trapped in the crystal lattice because they cannot diffuse out quickly enough. These vacancies interact with solute atoms, accelerating their diffusion and promoting the formation of precipitates. The size and distribution of these precipitates depend on the quenching temperature and cooling rate. Higher quenching temperatures create more vacancies, leading to a greater number of smaller hardened zones. Faster cooling also increases the number of fixed vacancies, further enhancing the precipitation effect.
A key feature of precipitation-hardened alloys is the temperature-dependent solubility of the alloying elements. As temperature increases, the solubility of the alloying elements in the matrix also increases, which is essential for effective heat treatment. Most heat-treatable aluminum alloys follow this principle, making them suitable for age hardening processes.
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