What is Metal Aging?

Aging of metals is one of the most common ways to change the properties of a metal alloy. While the properties of many metals can change through heating and quenching or hardening, some metal alloys are specifically made to age. Aging can change the physical and aesthetic properties of an alloy such that it acquires properties that are quite different from its unaged form.

Aging of metal is a process applied to heat-treated metal alloys that can occur artificially or naturally. Natural ageing occurs during the service life of the metal alloy. During the natural aging process, supersaturated alloying elements in the metal alloy form so-called metal precipitates. These precipitates block dislocations in the metal, increasing the strength and hardness of a metal alloy while decreasing its ductility.

Artificial aging is a process used to accelerate the formation of precipitates in a metal alloy heated in solution to a rate much faster than the natural aging process. The artificial aging process is carried out by increasing the temperature of the metal alloy heated in solution to a point below the recrystallisation temperature, but high enough to accelerate precipitate formation. Once the precipitates of the alloying element have reached the correct size, the metal alloy is rapidly cooled to prevent further change in the metal precipitates.

Which types of metals can be aged?

As long as a metal alloy can undergo heat treatment, it can be aged. The most popular choices are stainless steel, aluminium and copper. All these alloys can become incredibly hard and strong through the aging process. Even copper, which is usually soft and malleable, can be transformed by the metal aging process. It becomes much harder and stronger-but also quite brittle. Magnesium, nickel and titanium can also be aged, as long as these alloys can undergo heat treatment.

Can a metal be overaged?

One concern when aging a metal alloy, naturally or artificially, is something known as over-aging. This occurs when precipitates change in size due to an aging process beyond the point where it is beneficial for application. This often results in reduced strength and hardness.

Two common ways this occurs is by welding or cold working a metal. It should be carefully determined whether a metal that has undergone heat treatment should be artificially aged again after either of these two processes to ensure that the desired mechanical properties are still present.

How do I know if my metal needs to be aged?

In a metal project where the pieces need to be strong, hard and less ductile, then metal aging may be the right process. However, there are other processes that can achieve similar effects. For example, flame and induction hardening, annealing and quenching are all ways in which metallurgists harden metals. The right process depends on the type of metal and what the ultimate goals are.