Which of the following is an example of a heat treatable aluminium alloy?

The selected answer is correct because the 2xxx series of aluminum alloys, which primarily incorporate copper as the main alloying element, are known for their heat treatability. These alloys can undergo a process of solution heat treatment followed by aging, which significantly enhances their mechanical properties, such as strength and hardness.

The heat treatment process involves heating the alloy to a temperature at which the solute elements dissolve into the aluminum matrix. Upon rapid cooling, the alloy achieves a supersaturated solid solution. Subsequently, aging (natural or artificial) allows for the precipitation of fine particles, which impede dislocation movement and increase strength.

In contrast, the other series are less associated with heat treatment. The 3xxx series, primarily alloyed with manganese, is categorized as work-hardenable and benefits more from cold working rather than heat treatment. The 5xxx series, alloyed with magnesium, also typically relies on work hardening and does not exhibit the heat treatability characteristic necessary for significant strength improvements via thermal means. The 8xxx series, often used for applications involving lithium and other elements, may contain variations in heat treatability but generally don’t provide the same degree of enhancement in mechanical properties through heat treatment as the 2xxx series does.