| dc.description.abstract | In this study, the microstructures of Al-8.0Zn-2.1Mg-2.3Cu-0.16Zr (AA7055) high-strength aluminum alloys are analyzed using optical microscopy (OM), field-emission-electronic-micro-probe (FE-EPMA), conductivity measurement instrument, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Hardness and tensile tests are conducted to study the effects of aging process on the microstructure and mechanical properties of the alloy. Corrosion test is carried out to investigate its resistance to stress corrosion. The hot extrusion formability of AA7055 alloy is also investigated by the change of processing parameters (extrusion ratio, extrusion speed, temperature, etc.).
The results show that the as-cast microstructure contains η (Mg(ZnCu)2), T (Al2Mg2Zn3Cu), S (Al2CuMg-Zn), and θ (Al2Cu) phases which are eutectic with aluminum. The dominant phase is η (Mg(Zn-Cu)2). Homogenization treatment can effectively eliminate (η+T) phases as only trace S-phase and iron-rich phase (Al7Cu2Fe) are retained. Two-stage homogenization treatment is able to elevate the homogenization temperature of the as-cast alloy. When the extrusion ratio is larger and the extrusion speed is higher, the safe extrusion region of the hot extrusion is reduced, resulting in over-burning of the extruded part.
In addition, AA7055 has the best mechanical strength in the T651 temper and the best corrosion resistance in the T7351 temper. Both T7651 and T7751 (RRA) tempers show qualified electrical conductivity and tensile properties to meet the specifications of Aerospace AMS 4337 Standard, and exfoliation corrosion resistance to meet the ASTM Specification of G34-13.
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