Precipitation hardening is the most important option for increasing the firmness of aluminium alloys since these do not show polymorphic conversion and thus are not made harder by way of martensite formation.
A prominent example of precipitation hardening is duraluminium, an alloy of aluminium, 4 % copper and 1 % magnesium. Solution heat treatment is carried out at between 495 and 505 °C. After quenching the material can be shaped. Unlike steel, duraluminium is initially still soft after quenching. The final firmness is achieved by cold age hardening (at room temperature) or warm age hardening (precipitation annealing). The curing can be delayed by deep cooling (min. -18 °C). This is often used for rivets made with such alloys in order to achieve a longer processing time. Almost all aluminium alloys that may be cured are also very susceptible to corrosion since the alloy elements prevent the formation of a closed oxide layer.