Aluminum alloy heat treatment technology
[China Aluminum Industry Network] 1. Aluminum alloy heat treatment principle
Aluminum alloy casting heat treatment is to use a heat treatment specification, control the heating rate to a certain temperature and heat for a certain period of time to cool at a certain speed, change the structure of the alloy, its main purpose is to improve the mechanical properties of the alloy, and enhance corrosion resistance Performance, improved processing performance, and dimensional stability.
2, aluminum alloy heat treatment characteristics
It is known that for steels with high carbon content, high hardness is obtained immediately after quenching, while plasticity is very low. However, it is not true for aluminum alloys. After the aluminum alloy has just quenched, the strength and hardness do not increase immediately. As for the plasticity, it has not decreased but has increased. However, after the quenched alloy is left for a period of time (for example, 4 to 6 days and nights), the strength and hardness are significantly increased, while the plasticity is significantly reduced. The phenomenon that the strength and hardness of the aluminum alloy after quenching increases significantly over time is called aging. Aging can occur at room temperature, called natural aging, and can also occur within a certain temperature range above room temperature (such as 100 ~ 200 °C), said artificial aging.
3, aluminum alloy aging strengthening principle
The age hardening of aluminum alloy is a very complicated process. It is not only determined by the composition and aging process of the alloy, but also depends on the defects caused by alloy shrinkage in the production process, especially the number and distribution of vacancies and dislocations. It is currently believed that age hardening is the result of segregation of solute atoms to form a hardened zone.
When the aluminum alloy is heated by quenching, vacancies are formed in the alloy. During quenching, these vacancies are too late to remove and are “fixed†in the crystal. These vacancies in supersaturated solid solutions are mostly associated with solute atoms. Because the supersaturated solid solution is in an unstable state, it will inevitably shift to an equilibrium state. The presence of vacancies accelerates the diffusion rate of the solute atoms, thus accelerating the segregation of solute atoms.
The size and number of hardened zones depends on the quenching temperature and quenching cooling rate. The higher the quenching temperature, the greater the concentration of vacancies, the greater the number of hardened zones and the reduced size of the hardened zone. The greater the cooling rate of quenching, the more vacancies fixed in the solid solution will help increase the number of hardened zones and reduce the size of hardened zones.
A basic feature of precipitation hardened alloys is their equilibrium solubility as a function of temperature, ie, increasing solid solubility with increasing temperature, most heat treatable aluminum alloys meet this requirement.
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