Effect of ageing on precipitation kinetics, tensile and work hardening behavior of Al-Cu-Mg (2024) alloy

The effect of artificial ageing conditions on precipitation kinetics, tensile and work hardening mechanical behavior of aluminum alloy 2024 was investigated. Tensile specimens were artificially aged in three different temperatures and for various times so as to bring the material in all possible ageing conditions. These included under-ageing (UA), peak-ageing (PA) and over-ageing (OA) and then the specimens were tensile tested. Yield stress was found to increase up to the peak-ageing condition with a simultaneously decrease in elongation at fracture due to the precipitation of the S-type particles. It was confirmed that precipitation kinetics of this alloy is in between the diffusivity rate of solute Cu and Mg atoms in the aluminum matrix. Peak yield stress was found to be absolutely dependent on the iso-thermal ageing temperature and it was experimentally noticed that a 6 MPa yield stress drop exists per increasing ageing temperature by 10 °C. At the OA condition, non-uniform elongation attains values exceeding 40% of total elongation, and therefore the damage tolerance capability of the material at this ageing condition is further reduced. Work hardening Stage I dealing with pile-up of the dislocation debris of dipoles is almost double in PA when compared with the respective in UA condition. Duration of Stage II increases up to 40% of uniform elongation within the PA condition, where the precipitation transition of coherent S′′ to semi-coherent S′ type takes place. Stage II is essentially decreased in the OA condition to approximate 30% of uniform elongation, while dislocation cross slipping (Stage III) exceeds 50% of uniform elongation in the OA condition. Duration of Stages II and III for different artificial ageing conditions have a similar trend with yield stress and elongation at fracture values, respectively, thus giving proof that work hardening stages are artificially aged sensitive.