Evading the strength-ductility trade-off dilemma in AA2024 alloy by short-term natural re-aging after T351 temper

In this research, the influence of short-term natural re-aging after T351 temper on the microstructural evolution and mechanical behavior of AA2024 aluminum alloy was investigated. Grain growth occurred in the microstructures of the natural re-aged sample and a large number of Al7Cu2Fe particles were located inside the alpha grains. At the re-aging time of 1440 min, the peaks of XRD were shifted strongly to the right due to the formation of theta '', S '', theta ', and S '. The results revealed that the precipitation rate was high in the AA2024 alloy during natural aging. With increasing the re-aging time, texture parameters remained almost unchanged. The hardness increased slowly within the first 60 min, then enhanced rapidly between 60 and 2880 min, and finally became stable at around 139 HV between 2880 and 11520 min. When the natural re-aging time increased from 240 to 2880 min, the strengthening trended speed up, viz, the yield strength increased from 226.6 to 357.3 MPa, and the ultimate tensile strength enhanced from 452.2 to 535.5 MPa. Compared to the as-received sample (T351 temper), the ultimate tensile strength of the re-aged sheet improved from 455.5 to 535.5 MPa, the ductility remained unchanged, and the hardness increased from 128.8 to 138.2 HV, which was owing to the acceleration of the precipitation caused by the presence of high-content Al7Cu2Fe particles in the interior of the alphaaluminum grains in the natural re-aged sample. It was found that the Portevin-Le Chatelier instability of AA2024 alloy was effectively postponed after natural re-aging. With increasing the natural re-aging time, the strain hardening rate of the AA2024 sheet increased. The strengthening of the natural re-aged sample for 1440 and 2880 min was a result of a synergistic effect of precipitation hardening due to the formation of theta '', S '', theta ', and S ' phases, elimination of Portevin LeChatelier instability, and highly efficient load transfer from alpha-aluminum to Al7Cu2Fe. Finally, to use the A2024 alloy produced by natural re-aging for 1440 or 2880 min, two methods were proposed.