Effects of heat treatment and pre-stretching on the mechanical properties and microstructure evolution of extruded 2050 Al–Cu–Li alloy

Abstract

Heat treatment process is one of the key methods to improve the mechanical properties of age-hardening Al–Cu–Li alloy, and pre-stretching can further increase nucleation sites of precipitates by introducing dislocations. In this study, the effects of solution treatment, artificial ageing, and 4% pre-stretching on the microstructure evolution, mechanical properties, and fracture mechanism of extruded 2050 Al–Cu–Li alloy were investigated. The results indicated that the residual secondary phase, including Al2Cu and Al2CuLi, precipitated in hot extrusion were dissolved and a non-recrystallization characteristic for the 2050 alloy was presented after solution treated at 530 °C for 90 min. During artificial ageing at 150 °C, the precipitation sequence of the alloy without pre-stretching was: SSS → GP zone + δ′/β'→ θ' + T1. Pre-stretching led to denser dislocations thus a higher nucleation rate of T1 phase in compared with the alloy without pre-stretching. Additionally, the segregations of non-Cu atom at dislocations inhibited the formation of GP zones in the early ageing state, thereby providing sufficient Cu atoms for accelerating the growth of T1 phases. After pre-stretched by 4% and aged at 150 °C for 12 h, a good combination of strength (YS = 671 MPa, UTS = 682 MPa) and ductility (EL = 10.3%) was obtained. The fracture mode changed from transgranular fracture to intergranular fracture with increasing ageing time due to the dislocation density and the distribution, size, and number density of the precipitates.