1.Heat treatment principle of aluminum alloy
The heat treatment of aluminum alloy plate is to select a certain heat treatment specification, control the heating rate to rise to a corresponding temperature, and keep it for a certain time to cool at a certain rate and change the structure of its alloy. Its main purpose is to improve the mechanical properties of the alloy and enhance the corrosion resistance , Improve machining performance and obtain dimensional stability.
2.Characteristics of aluminum alloy heat treatment
It is well known that for steels with higher carbon content, high hardness is obtained immediately after quenching, while plasticity is very low. However, this is not the case for aluminum alloys. After the aluminum alloy is quenched, the strength and hardness do not increase immediately. As for the plasticity, not only does not decrease, but it increases. However, after this quenched alloy is left for a period of time (such as after 4 to 6 days and nights), the strength and hardness will be significantly improved, while the plasticity will be significantly reduced. The phenomenon that the strength and hardness of aluminum alloy after quenching increase significantly with time is called aging. Aging can occur at normal temperature, which is called natural aging, or it can occur within a certain temperature range (such as 100 ~ 200 ° C) above room temperature, which is called artificial aging.
3.Aging principle of aluminum alloy
Aging hardening of aluminum alloy is a quite complicated process. It depends not only on the composition and aging process of the alloy, but also on defects caused by the shrinkage of the alloy during the production process, especially the number and distribution of vacancies and dislocations. At present, it is generally believed that aging hardening is the result of the segregation of solute atoms to form a hardened zone.
When the aluminum alloy is quenched and heated, vacancies are formed in the alloy. During quenching, due to the rapid cooling, these vacancies are too late to be removed and are “fixed” in the crystal. 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 shift to an equilibrium state. The existence of vacancies accelerates the diffusion rate of solute atoms, and thus accelerates the segregation of solute atoms.
The size and number of hardening zones depend on the quenching temperature and quenching cooling rate. The higher the quenching temperature, the greater the vacancy concentration, the greater the number of hardened zones, and the reduced the size of the hardened zones. The greater the quenching cooling rate, the more vacancies are fixed in the solid solution, which is conducive to increasing the number of hardening zones and reducing the size of the hardening zones.
A basic feature of the precipitation hardening alloy system is the equilibrium solid solubility that changes with temperature, that is, the solid solubility increases with increasing temperature. Most heat treatable aluminum alloys meet this condition.
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