The aim of this paper is to characterize the evolution of fine and spheroidal microstructure of Al-Zn-Mg-Cu alloy by strain-induced melting activation(SIMA)and recrystallization and partial remelting(RAP) routes.The phase transformation and microstructural evolution were investigated during the isothermal treatment in a wide semisolid temperature range from 550 to 610.C. Effects of isothermal temperatures and processing routes on coarsening rate constant were also analyzed. The results show that during the semisolid isothermal treatment, recrystallization process occurred and intermetallic particles were gradually dissolved into the liquid matrix once the heating temperature was above the solidus. Further extension of soaking time led to the coarsening and spheroidization of the solid grains. The increase in the soaking temperatures raised the liquid fraction and accelerated the spheroidization process. Smaller spheroidal grains and better spheroidization effect were obtained in SIMA microstructure in comparison to RAP microstructure. The coarsening rate constants were, respectively, 319 and 454 μm^(3)·s^(-1) in SIMA samples after soaked at 600 and 610.C. The coarsening rate constants of 349 and 522 μm^(3)·s^(-1) were achieved in RAP microstructure.
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