Effect of the Over-ageing Treatment on the Mechanical Properties of AA2024 Aluminum Alloy

The evolution of the hardness of the over-ageing AA2024 alloy scale was followed by measurements of Vickers hardness. The nanoindentation is adapted to the determination of elastoplastic properties (hardness and Young's modulus) of the matrix and also of coarse intermetallic precipitates. Influence of the artificial over-ageing time to hardness and to mechanical properties as the local scale was investigated. The aluminum alloys of the 2000 series, AA2024-T351 alloy, are used in the aerospace industry for the wings and fuselage skins of the aircraft, due to the high strength to weight ratio associated with good fracture toughness. A combination of alloying elements, heat treatment and ageing induce the improvement of mechanical properties, as well as good damage tolerance and resistance to fatigue (1-6). The phenomenon of precipitation hardening and the influence of heat treatment on the mechanical properties of AA2024 alloy have been extensively studied (7, 8). The microstructure has an important role in corrosion resistance, and some intermetallic particles can be the cause of pitting and intergranular corrosion. The phenomenon of precipitation hardening in aluminum alloys takes place at relatively low temperature and induces the precipitation of intermetallic particles composed of the main alloying elements i.e., copper and magnesium. The fundamental stage of the age-hardening process consists in the acceleration of the decomposition phenomenon of the supersaturated solid solution, resulting in the coarse intermetallic particle precipitation (4, 9, 19), stage where the mechanical properties reaches the maximum values, but at the cost of a low corrosion resistance. The precipitation of the equilibrium phase, S -Al2CuMg phases and the phases S" and S' is the main responsible for final hardening. Most authors considered that the dislocations serve as the heterogeneous nucleation sites for the S (Al2CuMg) phase in Al-Cu-Mg alloy during artificial ageing (11-19). The mechanical properties are improved due to hardening precipitation that lead to peak-ageing condition and to the over-ageing condition with increasing ageing. During ageing treatment, the change in mechanical properties function of the treatment time can be plotted as a belle-shape curve, with a maximum value. The evolution of mechanical properties can be described by three successive stages: under-aged temper, aged temper and over-aged temper. In the figure 1 are represented the evolution of hardness and mechanical strength of the aluminum alloy during ageing treatment (20, 21). The over-ageing treatment (T7) is supposed to stabilize the microstructure and the mechanical properties to improve the corrosion resistance. The present work was aimed to investigate effects of over-ageing treatment on the mechanical properties of the