The heat treatment of aluminum guardrail castings involves controlling the heating rate to a certain temperature for a certain period of time and cooling at a certain rate to change the structure of the alloy. The ultimate goal is to improve the mechanical properties of the alloy, enhance corrosion resistance, improve machining performance, and achieve dimensional stability.
Dealing with steel with high carbon content, high hardness is immediately obtained after quenching, while plasticity is very low. However, this is not the case for aluminum alloys. After quenching, the strength and hardness of aluminum alloy do not immediately increase, and the plasticity not only does not decrease, but also increases. But for this quenched alloy, after a period of time, its strength and hardness will significantly increase, while its plasticity will noticeably decrease. The phenomenon that the strength and hardness of aluminum alloy significantly increase with time after quenching is called aging. Aging can occur at room temperature, known as natural aging, or at a temperature range higher than room temperature, known as artificial aging.
The age hardening of aluminum alloys is a considerable process, which not only depends on the composition and aging process of the alloy, but also on the defects caused by the shrinkage of the alloy during the production process, especially the number and dispersion of voids and dislocations. It is widely believed that age hardening is the result of solute atom segregation forming a hardened zone.
During quenching heating, voids are formed in the alloy. Due to rapid cooling, these voids cannot be removed in time and are "firmly" trapped within the crystal. These voids in supersaturated solid solutions are mostly associated with solute atoms. Due to the unstable state of supersaturated solid solution, it is bound to change towards equilibrium state. The existence of voids accelerates the diffusion rate of solute atoms, thus accelerating the segregation of solute atoms.