Evolution Behavior of Precipitated Phases During Aging Treatment of Al-Cu3-Si-Mg Alloy by MMDF

In this paper, the supersaturated solid solution of Al-Cu3-Si-Mg alloy prepared by molten metal die forging (MMDF) was used as the research object. The formation and evolution of precipitates during aging treatment were investigated through experiments at different temperatures and times, and the precipitation mechanisms and sequences of various precipitates were analyzed. The main precipitated phases formed in the supersaturated solid solution of Al-Cu3-Si-Mg alloy after aging treatment are θ(Al₂Cu), θ'(Al_3.6Cu₂), γ'(Al_0.63Mg_0.37) and η'(Cu, Si). Based on XRD and TEM analysis under different aging treatment conditions, the precipitation sequence is determined as follows: SSS → GP₀ → GP₀+γ'→GP₀+(γ'+γ)+θ''+η'→(γ'+γ)+(θ''+θ')+(η'+η)→(γ'+γ)+(θ+θ')+(η'+η)→(γ'+γ)+(θ+θ')+η→γ+θ+η. With increasing aging temperature and time, precipitates tend to accumulate at the α-Al grain boundaries. After aging treatment at 165-185 °C for 4 h, chain-like θ(Al₂Cu) precipitates are discontinuously distributed at the α-Al grain boundaries, disk‑shaped θ'(Al_3.6Cu₂) and θ''(Al₂Cu) phases mainly precipitate within the grains. When the temperature exceeds 185 °C, the chain-like θ(Al₂Cu) precipitates at the grain boundaries gradually become continuous, the amount of θ(Al₂Cu) phase in the grains increases significantly, θ''(Al₂Cu) disappears completely, and the size of θ'(Al_3.6Cu₂) decreases obviously. After aging treatment at 185 °C for 5-6 h, the chain-like θ(Al₂Cu) precipitates at the grain boundaries become more continuous, and their length fraction continues to increase with prolonged aging time.