What Does Natural Aging Mean?
Natural aging is the spontaneous aging of a supersaturated solid solution at room temperature. This process is important for strengthening heat treatment of alloys containing aluminum, copper, magnesium and nickel. Natural aging is contrasted with artificial aging, which is done at elevated temperatures.
Natural aging is also known as low-temperature aging, while artificial aging is also known as high-temperature aging.
Corrosionpedia Explains Natural Aging
Natural aging is a step in the heat treatment of aluminum alloys in which the metal is removed from the quench bath and allowed to gain its full strength at room temperature. In artificial aging, the metal is held at an elevated temperature for it to gain its full strength in a shorter period of time.
At relatively low temperatures, the decomposition of solid solutions often stops at the stage in which Guinier-Preston (GP) zones are formed. The formation of zones is characteristic of natural aging, which proceeds at room temperatures in the case of aluminum alloys, as well as of low-carbon steel or iron, in which there is a solid solution (ferrite) supersaturated with carbon or nitrogen. In some cases, the zones may be considered embryos of a separation phase.
In some aluminum alloys, the precipitation hardening that results from natural aging alone produces useful tempers (T3 and T4 types) that are characterized by high ratios of tensile-to-yield strength, high fracture toughness and resistance to fatigue. For the alloys that are used in these tempers, the relatively high supersaturation of atoms and vacancies retained by rapid quenching causes rapid formation of Guinier-Preston zones, and strength increases rapidly, attaining nearly maximum stable values in four or five days.
The adverse effects of aging may be reduced considerably by special alloying and heat treatment. In some applications, naturally aging alloys may be stored in a freezer to prevent hardening until after further operations — assembly of rivets, for example, may be easier with a softer part.