Abstract
The creep aging behaviors of retrogressed Al-Zn-Mg-Cu alloy were studied by uniaxial tensile creep tests at 140 °C. The effects of creep aging time and applied stress on microstructures and properties of the studied alloy were investigated by using transmission electron microscope (TEM), hardness, and corrosion resistance tests. Results show that the effects of the creep aging process on microstructures and properties are significant. The size of matrix precipitate (MPt), distance between MPts, width of precipitate-free zone (PFZ), and distance between grain boundary precipitates (GBPs) increase with the increase of creep aging time and applied stress. With the increase of creep aging time and applied stress, the corrosion resistance of the studied alloy improved. After creep aging for 20 h, the electrical conductivity varied with different applied stress from 35.99% to 37.24% International Annealed Copper Standard (IACS), and the exfoliation corrosion (EXCO) resistance increased to the corrosion rating of “EB”, which express slight surface corrosion. Compared with the traditional retrogression and re-aging process (RRA), the retrogression and creep aging process (RCA) can increase the MPt size, widen the precipitates distribution, narrow the PFZ width, and enhance the corrosion resistance while offering the hardness comparable to that of the RRA process.
Highlights
As a relatively new method for metal forming, creep age forming (CAF) is advantageous for manufacturing large, integral, and stiffened lightweight structures in light alloys for the aircraft and aerospace industries [1,2]
Annealed Copper Standard (IACS), and the exfoliation corrosion (EXCO) resistance increased to the corrosion rating of “EB”, which express slight surface corrosion
A high strength Al-Zn-Mg-Cu alloy of 7B04 is taken as the case material in this paper, and this study aims to investigate the effects of retrogression and creep aging (RCA) treatments on the precipitate microstructures and properties of 7B04-T651 aluminum alloy by using the uniaxial tensile creep tests under different CAF conditions
Summary
As a relatively new method for metal forming, creep age forming (CAF) is advantageous for manufacturing large, integral, and stiffened lightweight structures in light alloys for the aircraft and aerospace industries [1,2]. CAF simultaneously strengthens the part and changes its shape, resulting in a one-step forming and heat treatment process that can largely increase the mechanical properties. Controlling material performance is very difficult due to the presence of heavy interaction between creep and aging in the CAF process. The dislocation multiplication from creep deformation can provide more nucleation sites to promote aging precipitation. It is well-known that the heat-treatable high strength aluminum alloys have many heat treatment systems (such as T6, T73, and T74), and these systems may be used before CAF to obtain desired initial temper for controlling subsequent creep and aging process
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