Hardening of aluminum alloys

Plastic deformation of a crystalline material modifies its properties by influencing its internal structure. These changes in properties and microstructure, known as work-hardening, play a very important role in the material's mechanical properties. Indeed, work-hardening is widely used to enhance the mechanical properties of many metal alloys. More generally, work-hardening is what gives metal alloys their essential properties of toughness and (relative) ease of forming. In the first case, work-hardening takes place very locally – at the bottom of a crack in a damaged material, for example – and helps absorb the energy of mechanical stress. In the second case, the entire material undergoing the large-scale plastic forming operation is susceptible to work-hardening.

Hardening depends on the material, the amount of deformation applied and the deformation conditions (temperature, rate and mode of deformation). In particular, a distinction is made between cold deformation (deformation temperature below approximately 1/3 of the absolute melting temperature) and hot deformation (T _def >T _f /3). Schematically, we can say that increasing temperature facilitates deformation. The relationships between applied strain and stress are known as strain-hardening, flow or behavior laws. They are closely linked to the fundamental mechanisms of plastic deformation and to the evolution of the microstructure within the grains. Forming processes are largely directional: properties evolve differently depending on the direction of stress (e.g. in spinning: the direction of spinning). As a result, the microstructure acquires a preferential orientation, called texture, which becomes increasingly important as deformation proceeds.

Work hardening creates numerous crystalline defects, sources of non-equilibrium stored internal energy, which can eventually be annihilated by high-temperature heat treatments to restore initial properties. This process is usually divided into two stages:

• restoration, which softens the material by rearranging and annihilating, usually partially, crystalline defects;

• recrystallization, during which defects are eliminated by migration of grain boundaries over relatively long distances. Recrystallization gives rise to a rapid and profound evolution of the granular structure and largely controls the grain size of the material. In general, recrystallization depends on the same parameters as the preceding strain-hardening, as it is governed by the same deformation microstructures.

If deformation takes place at a sufficiently high temperature, the restoration...