Effects Of The Main Alloying Composition On Microstructures And Properties Of Super-strength Al-Zn-Mg-Cu Aluminum Alloy

Abstract:7B50super-high strength aluminum alloys are used in the super large aircraft wing panels. Because of high degree of alloying, the formed residual crystalline phases significantly lower the toughness and corrosion resistance, and limit its applications. The main alloying elements are in a large scale in7B50alloys, thus it is very important to carry out the optimization of the main component systematically.The phase equilibrium of Al-Zn-Mg-Cu alloys and effects of the main alloying composition (Mg, Cu, Zn) on the super-strength Al-Zn-Mg-Cu aluminum alloys were studied by using thermodynamic calculations, machanical tensile test, exfoliation corrosion testing, slow tensile stress corrosion test, U-type stress corrosion testing, differential thermal analysis (DSC), and combining with optical microscopy (OM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS). The main research and conclusions are as follows:(1) The isothermal section phase diagram in Al-rich corner of Al-Zn-Mg-Cu system at480℃is established by calucation and experimental verification. With the increasing of Zn and Cu content, the a(Al) phase field narrows and (a(Al)+S(Al2CuMg)) phase field expands.(2) Effects of Mg content on the microstructures and properties of Al-6.2Zn-xMg-2.0Cu alloys were studied. With the increasing of Mg content from1.9wt.%to2.6wt.%, the residual consistutent phases (S+T) increase. And the strength increase first and then decrease by a little, which reach the maximum at Mg=2.3wt.%. The fracture toughness, resistance to exfoliation corrosion and stress corrosion cracking (SCC) resistance decreases moderately and then decreases dramatically with the increasing of Mg content. The optimization of the element Mg content is about2.3wt.%.(3) Effects of Cu content on the microstructures and properties of Al-6.2Zn-xMg-2.0Cu alloys were investigated. With the increasing of Cu content from1.3wt.%to2.6wt.%, the quantity of residual consistutent phases (S-phase) increases. The tensile strength, fracture toughness and stress corrosion cracking (SCC) resistance first increase and then decrease with the increasing of Cu content. The resistance to exfoliation corrosion is shown to first decrease moderately and then decreases dramatically with increasing Cu content. When copper content is in the range of (1.6-1.9) wt.%, the alloys perform the best.(4) Effects of Zn content on the microstructures and properties of Al-6.2Zn-xMg-2.0Cu alloys were investigated. The amount of coarse residual consistutent phase (T phase) and matrix precipitates are show to increase with the increasing of Zn content from6.2wt.%to10wt.%in the alloys. And the strength first increases and then decreases moderately. The elongation of the alloys decreases. Fracture toughness, conductivity, resistance to exfoliation corrosion and stress corrosion cracking (SCC) resistance decrease moderately and then decreases dramatically with the increasing of Zn content. The optimization of the element Zn content is about (6.2-6.7) wt.%.