均匀化处理及冷却方式对7075铝合金铸锭显微组织的影响

Influence of homogenization treatment and cooling modes on microstructure of 7075 aluminum alloy ingot

首先对7075合金铸锭分别进行一级均匀化与二级均匀化处理,并将热处理后的试样以不同方式进行冷却。利用光学显微镜(OM)、差示扫描量热仪(DSC)、扫描电镜(SEM)和透射电镜(TEM)等研究了7075合金均匀化处理前后的组织演变及均匀化冷却速度对其微观组织的影响,并比较了小试样实验炉与大铸锭工业炉均匀化处理后的组织差异。结果表明:铸态7075合金的显微组织主要由α(Al)+η-Mg(Zn,Cu,Al)2非平衡共晶相组成,还含有少量的Mg2 Si和含Fe相。在468℃下进行一级均匀化时,未出现过烧组织,而在474和480℃时出现过烧现象,一级均匀化后仍存在枝晶偏析。在476℃下进行二级均匀化时,未出现过烧组织,在482和488℃时出现过烧现象。经过468℃×8 h+476℃×24 h均匀化处理后,7075合金中的枝晶偏析消除。在均匀化热处理前,合金晶粒内组织较为纯净,在晶界处存在成分偏析。经过均匀化处理后,晶内存在较多细小的板条状η相,且大铸锭均匀化后的η相尺寸最大。均匀化后不同冷却方式的组织差异较大,二级均匀化的小试样经水冷或空冷降温后,在晶界附近形成了无沉淀析出带(PFZ),且小试样炉冷与大铸锭空冷的组织相似。

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 ofα(Al)+η-Mg(Zn,Cu,Al)2 non-equilibrium eutectic phases and a small amount of Mg2 Si 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 likeηphase within the grains,and the size ofηphase 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.