Aging of aluminum

Aging of aluminum is the second stage of precipitation hardening. After solution treatment, very fine particles gradually precipitate from the supersaturated solid solution and hinder deformation of the metal – the casting becomes harder and stronger. Aging occurs spontaneously at ambient temperature (natural aging) or in a furnace, usually at approx. 150–180°C (artificial aging).

In practice, solution treatment and aging of aluminum are carried out as a single heat treatment cycle – only both stages together give the casting a T4 or T6 temper. Aging is applied to precipitation-hardening alloys, in foundry practice mainly aluminum with silicon and magnesium or with silicon and copper.

Solution treatment and aging of aluminum – how it works

1. Solution treatment. The casting is held at approx. 480–545°C (depending on the alloy) until the alloying elements dissolve in the aluminum, and then it is rapidly cooled. This produces an unstable supersaturated solution. Details: aluminum solution treatment.
2. Aging. Dissolved atoms – e.g., magnesium and silicon, or copper – first cluster into fine zones (Guinier-Preston zones) and then into increasingly larger precipitates of strengthening phases. Maximum hardness is reached when the precipitates are very fine and densely distributed.
3. Overaging. Too high a temperature or too long a time causes the precipitates to coarsen – hardness and strength drop, although dimensional and property stability increase. Deliberate overaging produces the T7 temper.

Natural and artificial aging

These are indicative values. The exact aging temperature and time are selected to suit the alloy and the required properties – minimum values for each temper are given in the PN-EN 1706 standard.

Aging aluminum alloys without solution treatment (T5 temper)

Some castings are artificially aged without prior solution treatment – directly after casting and controlled cooling. This is the T5 temper, typical for die castings, among others, which are usually not solution treated because of the risk of blistering. This treatment raises hardness less than a full T6 cycle, but it stabilizes the dimensions and properties of the casting.

What determines the result of aging

• Alloy composition – the magnesium or copper content determines how much strengthening phase can precipitate.
• Quality of solution treatment – if the constituents did not dissolve completely or quenching was too slow, aging cannot make up for the losses.
• Temperature and time – a lower temperature requires longer aging but usually gives a slightly higher peak hardness. Too high a temperature leads to overaging.
• Wall thickness – in thick sections, cooling after solution treatment is slower, so hardness after aging is often slightly lower than in thin walls.

Aging castings in our foundry

We perform heat treatment of castings – solution treatment and aging, as well as annealing – at the customer's request, together with casting and machining of castings. On request, we also perform strength testing. It is worth stating the required casting temper (e.g., T6) in your inquiry together with the alloy grade – this makes it easier to select the technology, e.g. permanent mold casting.

See also: aluminum hardening, aluminum solution treatment, aluminum hardening, heat treatment of castings and aluminum casting alloys.