商用AA7055(Al-Zn-Mg-Cu) 擠形鋁合金，在擠形時擠件之表部和芯 部加工量不同，導致熱處理後合金呈現不同形貌之微結構。而不同形貌之 微結構會導致析出結果差異，使合金擁有不同之腐蝕性質。本研究透過 OM、EBSD、STEM 和剝落腐蝕試驗來探討冷抽（cold drawing）對AA7055-T6 合金的微結構和合金腐蝕性質之影響。 結果顯示熱擠後並進行熱處理之AA7055（Al-Zn-Mg-Cu）合金，表面擁 有細小之變形晶粒，其晶界上析出物為不連續析出，因此抗剝落腐蝕性較佳。冷抽（30％）後進行固溶、時效熱處理（T6），會使AA7055-T6 合金表面產生粗大的再結晶晶粒，在粗大晶粒中的晶界析出物為連續析出，因此對剝落腐蝕抵抗性較差。不論有無冷抽之AA7055-T6 合金，在熱處理後其芯部(center)處皆呈現細小變形晶粒、與粗大之回復晶粒交錯，使中間剝落腐蝕形貌呈現層狀交錯腐蝕。其中冷抽（30％）後再加以時效，合金之表面再結晶之晶粒抗腐蝕性最差，因此冷抽後、並時效之AA7055-T6 合金之抗剝落腐蝕能力較低。;Commercial AA7055 extruded aluminum alloy has different cold working ratios on the surface and center of the extrusion section, which cause different morphologies of microstructure after heat treatment. However different morphologies of microstructure will lead to different results of precipitation, making different corrosion properties of the alloys. In this study, OM, EBSD, STEM and exfoliation corrosion tests (EXCO) were used to investigate the effect of microstructure on the corrosion properties of alloys with different cold working ratios (0%/30%) after subsequent heat treatment.
The results show that the fine grains have better resistance to exfoliation corrosion, and the alloys with 0%/30% cold drawing ratio both show coarse grains at the center, so the resistance to exfoliation corrosion is poor. However, heat treatment after cold drawing will cause coarse recrystallized grains on the surface of the Al- Zn-Mg-Cu alloy, which will damage the corrosion resistance. Therefore, the alloy without cold drawing after hot extrusion has better resistance to exfoliation corrosion.
