Pre-Aging Prior to Delayed Artificial Aging Modulates Precipitation Behavior and Strengthening Response in an Al-Mg-Si-Cu Alloy

Room-temperature storage after solution treatment and quenching is often unavoidable during the industrial processing of Al-Mg-Si-Cu alloys, but its interaction with pre-aging and the subsequent artificial aging response remains insufficiently quantified. Here, we demonstrate a pre-aging-assisted delayed artificial aging strategy to regulate β″/L precipitation and improve the strengthening response of an Al-Mg-Si-Cu alloy after short-term room-temperature storage. After solution treatment and quenching, the alloy was stored at room temperature for 6–48 h with or without prior pre-aging at 120 °C, followed by artificial aging at 170 °C. It is found that both the delayed artificial aging (DA) and pre-aging-assisted delayed artificial aging (PDA) routes show limited sensitivity to room-temperature storage time within 6–48 h, whereas PDA consistently increases the yield strength and ultimate tensile strength by approximately 10 MPa compared with the corresponding DA route, with a slight reduction in elongation. Based on Avrami–Johnson–Mehl analysis, the activation energy for β″ precipitation decreases from 102.74 kJ/mol in the DA-pre state and 107.13 kJ/mol in the solution-treated state to 70.63 kJ/mol in the PDA-pre state, indicating that pre-aging modifies the initial microstructural state in a manner that facilitates subsequent β″ precipitation. Quantitative TEM characterization reveals that PDA has little influence on the average sizes of β″ and L precipitates but increases their number densities. In particular, the number density of β″ precipitates increases by approximately 15% relative to the corresponding DA samples. Strengthening-model calculations further show that β″ precipitates make the dominant contribution to precipitation strengthening, and that the strength improvement under the PDA route mainly originates from the increased β″ number density rather than changes in precipitate size. These results establish a quantitative link between pre-aging, precipitation kinetics, β″/L precipitate population, and strengthening response, providing guidance for optimizing heat-treatment schedules of Al-Mg-Si-Cu extrusion alloys that experience unavoidable room-temperature storage before final artificial aging.