Research On Multiphase Microstructure Modulation And Properties Of Al-8.54Zn-2.41Mg-xCu Aluminum Alloy

Al-Zn-Mg-Cu series aluminum alloy are widely used in aircraft and spacecraft for main load-structure. For independent development of large aircraft, our country has an urgently demand for Al-Zn-Mg-Cu aluminum alloy with high strength, super high toughness and corrosion resistance properties.Al-8.54Zn-2.41Mg-xCu (x=2.2-0.8) was investigated in the paper. The effect of Cu content on microstructure and peak-aging properties, and multistage aging treatment on precipitates and properties was studied by optical microscopy (OM), electron microstructure analytical methods (SEM, TEM), hardness test, tensile test, conductivity test and corrosion test (SCC, Intergranular corrosion). The results are showed as following:1) With decreasing the Cu content from2.2%to0.8%, residual crystalline phase decreased in cast, homogenization and solution-ageing. The Cu content of crystal phase is also reduced. The residual crystalline phase was disappeared after the solution-aging treatment when the Cu content was below1.5%. For peak aging, with decreasing the Cu content, the strength, hardness increased successively while fracture toughness and exfoliation corrosion first increased and then decreased. SCC resistance was slowly decreased with the Cu content reduced.2) Compared with the T6temper, the fine and minority slightly coarse precipitates precipitates were distributed in matrix and grain boundary precipitates transformed from the continuous distribution to coarse separated shape and the PFZ was formed in RRA temper. Compared with the T6aging, Regression and Re-aging(RRA) significantly improved conductivity and intergranular corrosion resistance of Al-8.54Zn-2.41Mg-xCu alloy. With Cu content decreasing, the hardness, strength and fracture toughness of Al-8.54Zn-2.41Mg-xCu alloy were first increased and then decreased in high regression temperature(180℃); while the hardness and strength increased in low regression temperature (170℃).3) The strength, hardness of the alloys decreased in T76with decreasing Cu content. Compared with the T76temper, new GP zone was formed, the density of matrix precipitates increased and the grain boundaries precipitates remained isolated after T76+T6temper. The strength, hardness, fracture toughness, conductivity and intergranular corrosion resistance of the alloys afer T76+T6temper were increased.