Effects of Homogenization on Second Phase and Grain Size of Al-Zn-Mg-Cu Alloy
The microstructure evolution of Al-Zn-Mg-Cu alloy during homogenization was studied by optical microscopy(OM),scanning electron microscopy(SEM)and differential scanning calorimetry(DSC)characterization.The results show that a large amount of Al7Cu2Fe insoluble phases and Mg(Zn,Cu,Al)2 nonequilibrium eutectic phases,Al2CuMg phases exist in the crystal boundary of the cast state alloy.After the single-stage homogenization treatment at 420℃/8 h,some of the refractory and non-equilibrium eutectic phases are back-dissolved into the matrix,but a large amount of dendrite segregation still exist,and the non-equilibrium eutectic microstructure and dendrite segregation are even completely eliminated after the second-stage homogenization at 470℃/36 h is continued.The average grain size of the alloy shows a tendency to increase with time in the two stages of homogenization,and the area fraction of residual phase and eutectic microstructure in the matrix decrease with the increase of holding time,and the area fraction of phase is only 0.34％ when the second-stage homogenization reaches 36 h,indicating that the homogenization is very adequate.After the homogenization kinetic analysis,it is concluded that at the second homogenization temperature of 470℃,the holding time required for the second phase to fully diffuse back into solution is about 37 h,which is consistent with the homogenization experimental results.
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