Cu-induced phase evolution and precipitation behavior in cast Al–Li–Cu–Mg–Si alloy

The precipitation behavior and microstructural evolution of cast Al-Li-Cu-Mg-Si alloys with varying Cu contents were systematically investigated. Cu addition alters the solidification pathway by promoting Cu-containing intermetallics (e.g., Al 2 CuMg and Al 6 CuLi 3 ), suppressing Al 2 MgLi, and, at higher levels, destabilizing Mg 2 Si while inducing AlLiSi formation. During solution treatment, near-complete dissolution of Cu-rich primary phases occurs at Cu ≤ 2 wt%, whereas excessive Cu leads to persistent undissolved phases and reduced solute supersaturation. Aging behavior is predominantly governed by coherent δ'-Al 3 Li precipitates across all compositions, with nucleation and coarsening following diffusion-controlled ripening and showing limited sensitivity to Cu content. Only at prolonged aging times and Cu ≥ 1.5 wt% are minor amounts of plate-like T 1 -Al 2 CuLi and/or θ'-Al 2 Cu observed, with low volume fractions due to kinetic constraints inherent to cast microstructures. Mg 2 Si precipitates exhibit a Cu-dependent morphological transition during long-term aging. These results reveal a simplified, δ'-dominated precipitation pathway in cast Al-Li-Cu alloys, distinct from wrought counterparts, and provide insights for precipitation control in high-Li cast Al-Li-Cu systems.