Kinetics of Precipitation Hardening Phases in Recycled 2017A Aluminum Alloy

This study investigated the effect of the recycling process on the microstructure, hardness, and precipitation kinetics of strengthening phases in the 2017A aluminum alloy. Light microscopy (LM) and X-ray diffraction (XRD) analyses revealed that the as-cast microstructure of the recycled 2017A alloy contained intermetallic phases, including θ-Al₂Cu, β-Mg₂Si, Al₇Cu₂Fe, Q-Al₄Cu₂Mg₈Si₇, and α-Al₁₅(FeMn)₃(SiCu)₂, and was comparable to that of the primary alloy, confirming its potential for high-performance applications. During solution heat treatment, most of the primary intermetallic precipitates, such as Al₂Cu, Mg₂Si, and Q-Al₄Cu₂Mg₈Si₇, dissolved into the solid Al matrix. DSC analysis of the solution-treated alloy established the precipitation sequence as follows: α-ss → GP/GPB zones → θ″ → θ'/Q' → θ-Al₂Cu/Q-Al₄Cu₂Mg₈Si₇. The combined results from XRD, LM, TEM, and DSC confirmed that both θ and Q phases contributed to strengthening, with θ″ and θ' phases playing a dominant role. Brinell hardness measurements during natural and artificial aging revealed that hardness increased with aging time, reaching a maximum value of 150.5 HB after ~22 h of artificial aging at 175 °C. The precipitation kinetics of the recycled 2017A alloy was studied via DSC measurements over a temperature range of ~25 to 550 °C, at heating rates of 5, 10, 15, 20, and 25 °C/min. The peak temperatures of clusters, GP zones, and hardening phases (θ', θ″, θ, and Q) were analyzed to calculate the activation energy using mathematical models (Kissinger, Ozawa, and Boswell). The obtained values of activation energies of discontinuous precipitation were comparable across methods, with values for the θ″ phase of 89.94 kJ·mol^-1 (Kissinger), 98.7 kJ·mol^-1 (Ozawa), and 94.33 kJ·mol^-1 (Boswell), while for the θ' phase, they were 72.5 kJ·mol^-1 (Kissinger), 81.9 kJ·mol^-1 (Ozawa), and 77.2 kJ·mol^-1 (Boswell). These findings highlighted the feasibility of using recycled 2017A aluminum alloy for structural applications requiring high strength and durability.