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 the Al-Cu3-Si-Mg alloy after aging treatment are θ(Al2Cu), θ′(Al3.6Cu2), γ′(Al0.63Mg0.37), 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. After aging treatment at 165–185 °C for 4 h, chain-like θ(Al2Cu) precipitates are discontinuously distributed at the α-Al grain boundaries, and disc-shaped θ′(Al3.6Cu2) 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, and the fraction increase from 1.5% to 15.2%. The amount of the θ(Al2Cu) phase in the grains increases from 2 to 6, and the size of θ′(Al3.6Cu2) decreases obviously. After aging treatment at 185 °C for 5–6 h, the chain-like θ(Al2Cu) precipitates become more continuous, and the fraction continues to increase from 32.1% to 52.6%. The effect of chain-like precipitates at grain boundaries on the mechanical properties of the matrix is opposite to the strengthening contribution of dispersed intragranular precipitates. When the aging condition exceeds 185 °C × 5 h, the excessive formation of chain-like grain boundary precipitates causes both the strength and hardness of the alloy to show a decreasing trend.