Influence of homogenization treatment and cooling modes on microstructure of 7075 aluminum alloy ingot
Firstly,the 7075 alloy ingot was subjected to one-stage and two-stage homogenization treatment,and the heat-treated specimens were cooled in different ways.Microstructure evolution of the 7075 alloy before and after homogenization treatment and the effect of homogenization cooling rate on its microstructure were studied using optical microscope(OM),differential scanning calorimetry(DSC),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The differences in microstructure between small sample experimental furnaces and large ingot industrial furnaces after homogenization treatment were compared.The results show that the microstructure of as cast 7075 alloy is mainly composed of α(Al)+η-Mg(Zn,Cu,Al)2 non-equilibrium eutectic phases and a small amount of Mg2Si and Fe containing phases.When conducting one-stage homogenization at 468℃,there is no overburned structure,while overburning occurs at 474℃and 480 ℃,and dendrite segregation still exists after one-stage homogenization.When conducting two-stage homogenization at 476 ℃,there is no overburned structure,but overburning occurs at 482 ℃ and 488 ℃.After 468℃×8 h+476 ℃ ×24 h of two-stage homogenization treatment,the dendritic segregation in the 7075 alloy is eliminated.Before homogenization heat treatment,the microstructure inside the alloy grains is relatively pure,and there is component segregation at the grain boundaries.After homogenization treatment,there are many fine lath like η phase within the grains,and the size of η phase after homogenization of large ingots is the largest.After homogenization,there are significant differences in the microstructure of different cooling methods.The small samples with two-stage homogenization treatment form a precipitation-free zone(PFZ)near the grain boundaries after water cooling or air cooling,and the microstructure of the small samples cooled by furnace is similar to that of the large ingot air-cooled.
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