The Effect Of Enhanced Solid-solution Treatment On Mechanical Properties Of 7A04 Aluminum Alloy

Al-Zn-Mg-Cu aluminum alloy has been widely employed in aerospace and transportation industries for its excellent properties,such as low density and high strength as well as superior corrosion resistance.The material of this study is hot extruded rod of 7A04 aluminum alloy.By comparing the microstructure,mechanical properties and corrosion behavior of the samples treated with different heat treatment processes,the optimum heat treatment temperature was found.Aiming to study the anisotropic fatigue behavior of extruded aluminum alloy after heat treatment,the high frequency fatigue test was performed.The effects of the second phase and stress on the fatigue properties of 7A04 aluminum alloy were discussed in this thesis.The main research results are as follows.(1)The optimal multi-stage solution temperature for the single-step solution is 450?,and the two-stage solid-solution is 480?.And the yield strength(Rp),tensile strength(Rm)and hardness reach the peak value 612.5MPa,682.2MPa and 186HBW respectively.Compared with the single-step solution,the 7A04 alloy with double-stage solution and triple-stage solid-solution treatment,has less coarse second phases and higher stress corrosion resistance.With the solid solution stages increases to the triple,the depth of intergranular corrosion was very shallow,the exfoliation susceptibility of the alloy was reduced and the scale of the exfoliation corrosion resistance improved from EC to EA-.(2)The extuded 7A04 alloy is a pancake structure and the fatigue performance is anisotropic.Compared with the specimens taken from the extrusion direction show the highest fatigue strength,which is 302.7MPa,the fatigue strength of specimens from 45° decreased by 13.3%and those from 90° decreased by 33.5%.It was found that the fracture surface of 7A04 alloy was related to fatigue stress level,the higher stress leads to the lager area of the fatigue final rupture.By observation of fracture surface,it was known that secondary phase had a significant impact on fatigue behavior.The second phase particles containing Si,Fe,S in the alloy,which was easy to form the fatigue crack initiation site near the surface of the sample,and also reduced its fatigue strength.