Study on microstructure evolution of a high zinc containing Al-Zn-Mg-Cu alloy during homogenization

Abstract:

The microstructural evolution of a high Zn-containing Al-Zn-Mg-Cu alloy during homogenization was investigated by optical microscopy, differential scanning calorimetry, scanning electron microscope and X-ray diffraction. A homogenization kinetic equation deriving from a diffusion kinetic model was established to confirm the optimum homogenization parameter. The results showed that severe segregation exists in the as-cast alloy. The non-equilibrium eutectics consisted of α(Al), Mg(Zn,Cu,Al)₂, S(Al₂CuMg), θ(Al₂Cu) and Fe-enriched phases. In present work, no transformation from Mg(Zn,Cu,Al)₂ to S(Al₂CuMg) phase occurred during homogenization and Mg(Zn,Cu,Al)₂ phase directly dissolved into the matrix. θ(Al₂Cu) phase dissolves into the matrix over homogenization. Fe-enriched phases still existed after homogenization, Zn and Mg elements in Fe-enriched phases were reduced or even disappeared by prolonging the holding time. In consistent with the results of homogenization kinetic analysis, the proper homogenization parameter was 440 oC×12 h 468 oC×24 h.