Heat treatment of wrought aluminium alloys

Constitution of the different types of alloy:

The main alloying elements in aluminium engineering alloys are manganese (Mn), silicon (Si), zinc (Zn), magnesium (Mg) and copper (Cu). In accordance with the international classification system and EN 573, the different types of alloy are divided into groups with a four-figure designation:

  • 1XXX (Al 99,99 … Al 99,5)
  • 2XXX (AlCu)
  • 3XXX (AlMn)
  • 4XXX (AlSi)
  • 5XXX (AlMg(Mn))
  • 6XXX (AlMgSi)
  • 7XXX (AlZnMg(Cu))
  • 8XXX (Al(Fe,Li))

The combination of the different alloy constituents results in two main groups of aluminium alloys:

  • non-age-hardenable (non-heat-treatable) alloys
  • age-hardenable alloys

The main difference between these two groups is in their hardening mechanisms, which are responsible for their strength properties.

The term wrought alloys is derived from the manner in which the alloys are processed. They are mainly processed by hot and cold rolling, extrustion, drawing and forging, in other words using a combination of working and heat treatment.

There are basically four different ways in which the strength of a material can be increased by influencing the microstructure of wrought alloys:

  • introducing alloying atoms into the aluminium atomic lattice (solid-solution hardening)
  • increasing the dislocation density by plastic deformation (work hardening)
  • producing extremely fine precipitates with a different composition and structure (precipitation hardening or age hardening)
  • achieving a fine-grained microstructure (grain-boundary hardening).