As we all know, for steel with high carbon content, high hardness is obtained immediately after quenching, while plasticity is very low. However, it is not the case for imported aluminum plate and aluminum alloy. After the aluminum alloy is quenched, the strength and hardness do not increase immediately. As for the plasticity, it does not decrease, but increases. However, the strength and hardness of this quenched alloy will be significantly improved after a period of time, while the plasticity will be significantly reduced. After quenching, the strength and hardness of aluminum alloy increase significantly with time, which is called aging. Aging can occur at room temperature, called natural aging, or within a certain temperature range (such as 100 ~ 200℃) higher than room temperature, called artificial aging.

Vacancies were formed in 2024 aluminum alloy during quenching and heating. During quenching, these vacancies were "fixed" in the crystal because of the rapid cooling. Most of these vacancies in supersaturated solid solution are combined with solute atoms. Because the supersaturated solid solution is in an unstable state, it will inevitably change to an equilibrium state. The existence of vacancies accelerates the diffusion speed of solute atoms, thus accelerating the segregation of solute atoms. The size and number of hardening zones depend on quenching temperature and quenching cooling rate. The higher the quenching temperature, the greater the vacancy concentration, the more the number of hardened zones, and the smaller the size of hardened zones. 

The greater the quenching cooling rate, the more vacancies fixed in solid solution, which is beneficial to increase the number of hardened zones and reduce the size of hardened zones. A basic feature of precipitation hardening alloy system is the equilibrium solid solubility that changes with temperature, that is, the solid solubility increases with the increase of temperature, and most aluminum alloys that can be strengthened by heat treatment meet this condition. The solubility-temperature relationship required by precipitation hardening can be explained by Al-4Cu alloy of Al-Cu system. For the binary phase diagram of the aluminum-rich part of Al-Cu system, eutectic transformation L→ α+θ (Al2cu) was carried out at 548℃. The limit solubility of copper in α phase is 5.65% (548℃), and the solid solubility decreases sharply with the decrease of temperature, and it is about 0.05% at room temperature. [3] 2024 alloy belongs to Al-Cu-Mg series high-strength hard aluminum alloy. Due to the Z-excellent quenching process of alloy plate and strip, the purpose of improving alloy properties, controlling its special quenching deformation with high strength, good heat resistance, excellent formability and damage resistance, and improving product quality is achieved. [4] The solid solution degree of high-purity and high-strength aluminum alloy has a strong influence on its properties, and increasing the solid solution degree as much as possible is an effective way to improve the comprehensive properties of this kind of aluminum alloy. [5] However, with the increase of temperature, the performance changes have certain characteristics, and it is critical to control the speed of heating, mainly because the eutectic temperature (504.98℃) of 2024 aluminum alloy should be considered, and it will change when it is higher than the eutectic temperature. The cooling time of aluminum alloy workpiece after heating must be very short, so as to avoid the local or overall temperature drop of the workpiece before solid solution treatment. The interval between the discharge of the workpiece and its entry into the solid solution treatment tank should be strictly controlled.

 If the delay time is too long, the temperature of the workpiece will drop, part of the solid solution will decompose, and coarse and loose phases will be precipitated, resulting in structural segregation, thus reducing the aging strengthening effect. [6] The heating time of solution heat treatment is first related to the alloy properties and original state. Because of the similar composition of various aluminum alloys, there is no need for special consideration, so the key consideration is the original microstructure. When the strengthening phase is fine, the heating time can be shortened because of the faster solid solution. For example, the heating time of cold-rolled sheet is shorter than that of hot-rolled sheet, and repeated quenching is shorter, while the general annealing state should have a longer holding time because of coarse strengthening. In addition, the heating time is also directly related to the heating medium, the size and quantity of parts.

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