Abstract:The microstructural evolution of a Mg, Ag and Zn micro-alloyed Al-3.8Cu-1.28Li (mass fraction, %) alloy ingot during two-step homogenization was examined in detail by optical microscopy (OM), differential scanning calorimetry (DSC), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) methods. The results show that severe dendritic segregation exists in the as-cast ingot. There are many secondary phases, including T_B (Al₇Cu₄Li), θ(Al₂Cu), R(Al₅CuLi₃) and S(Al₂CuMg) phases, and a small amount of (Mg+Ag+Zn)-containing and AlCuFeMn phases. The fractions of intermetallic phases decrease sharply after 2 h of second-step homogenization. By prolonging the second-step homogenization time, the T_B,θ, R, S and (Mg+Ag+Zn)-containing phases completely dissolve into the matrix. The dendritic segregation is eliminated, and the homogenization kinetics can be described by a constitutive equation in exponential function. However, it seems that the AlCuFeMn phase is separated into Al₇Cu₂Fe and AlCuMn phases, and the size of Al₇Cu₂Fe phase exhibits nearly no change when the second-step homogenization time is longer than 2 h.

Key words: Al-Cu-Li alloy; homogenization; microstructural evolution; Al₇Cu₂Fe; AlCuMn