Effects of heat treatment on the microstructure and mechanical properties of extruded 2196 Al-Cu-Li alloy

Abstract

In this study, the effects of heat treatment on the microstructure and mechanical properties of extruded 2196 Al-Cu-Li alloy were examined and the precipitation behavior and strengthening mechanism were investigated. It was found that the solution treatment and artificial aging had a significant strengthening effect on the mechanical properties of the extruded alloy, which was related to the type, size, number density, and distribution of the precipitates. Most of the second phases dissolved back into the matrix after solution treatment. The precipitation sequence of 2196 alloy during aging treatment was identified as: supersaturated solid solution→ GP zone + δ′/β′ (Al3(Li,Zr)) → δ′ + θ′ (Al2Cu) + T1 (Al2CuLi) → θ′ + T1, where T1 phase played a leading role in the precipitation strengthening. The fracture mechanism changed from ductile fracture to intergranular fracture. The pre-stretching deformation can introduce a large number of vacancies and dislocations, which provide conditions for atomic diffusion and precipitation of the phases, and further improve the mechanical properties of the alloy. Besides, pre-stretching can effectively shorten the time required for peak strengthening, which is only 20% of the time needed in single-stage peak-aging (170 °C/36 h). Finally, the optimal heat treatment process of 2196 Al-Cu-Li alloy was determined as: 515 °C/90 min solution treatment+3% pre-stretching+170 °C/16 h artificial aging.