Microstructure Evolution of a High Zinc Containing Al-Zn-Mg-Cu Alloy during Homogenization

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 derived from a diffusion kinetic model was established to confirm the optimum homogenization parameter. Results indicate that severe segregation exists in the as-cast alloy. The non-equilibrium eutectics consist of α(Al), Mg(Zn, Cu, Al)2, S(Al2CuMg), θ(Al2Cu) and Fe-enriched phases. In the present work, no transformation from Mg(Zn, Cu, Al)2 to S(Al2CuMg) phases occurs during homogenization and Mg(Zn, Cu, Al)2 phases directly dissolve into the matrix. θ(Al2Cu) phases dissolve into the matrix while Fe-enriched phases still exist after homogenization. In addition, the contents of Zn and Mg elements in Fe-enriched phases are reduced or even disappear by prolonging the holding time of homogenization. The proper homogenization parameter is 440 °C/12 h + 468 °C/24 h, which is in consistent with the results of homogenization kinetic analysis.