Solid Solution And Quenching Process Of 7050 Aluminum Alloy Thick Plate Fracture Toughness

Aluminum alloy7XXX has been widely used in the aerospace and aeronautics industry. Aluminum alloy7050has a combination of low squench sensitivity, high fracture toughness, high fatigue and stress corrosion resistance, for thick plate and heavy castings. In recent years, rapid development in the aerospace industry has urgently demanded the large structure of components of high proformance. However the aluminum alloy7050thick plates are behind to the same products abroad, because of the research of aluminum alloy7050thick plate was too late. Focused on the excellent combined properties of this alloy, the influence of solution treatment and quenching were investigated by means of ambient temperature tensile testing, fracture toughness testing, optical microscopy (OM),scanning electron microscopy (SEM), transmission electron microsocopy (TEM) and differential scanning calorimetry (DSC).The main conclusions are as follows:(1) The optimal single-stage solution is473℃/2.5h. When solution time is the same,with the increasing of single-stage solution temperature, the volume fracion of the residual phase decreases, while the volume fracion of the recrystallized grains increase. Thus, the strength and fracture toughness of the single-stage solution treated samples increase first and then decrease. When solution temperature is the same,with the increasing of single-stage solution time, the volume fracion of the residual phase decreases firstly and then keep invariant, while the volume fracion of the recrystallized grains increase. Thus, the optimal single-stage solution time is2.5h.(2) The recovery is prolonged then the racrystallization is inhibited when the samples are held by stage-solution treatment. Moreover, the residual phase of stage by stage enhanced solution-treated samples is much less than that in the single-stage solution-treated samples, because its solution temperature can beyond the multi-phase eutecic temperature. The optimal stage by stage enhanced solution is (200℃,11h)+(1h/in furnace)+(300℃,11h)+(1h/in furnace)+(400℃,5h)+(1h/in furnace)+ (450℃,4h)+(1h/in furnace)+(493℃,2.5h). The tensile strength (σ0.2),yield strength (σb) and fracture toughness (KIC) are500.6MPa and534.0MPa and37.4MPa·m1/2respectively.(3) The relationship of the strength, fracture toughness (KIC) and quenching medium (cooling rate) are:σ0.2=445+7.21n(QT); σb=471+7.1ln(QT); KIC=29.3+0.81n(QT), where the QT is the cooling rate. During the slow quench cooling rate (such as quenching in air), many coarse equilibrium η phases will precipitate primarily on the incoherent Al3Zr dispersoids and at high angle grain boundaries, which depletes the solutes in the matrix and reduced hardening precipitates during subsequent aging. With the equilibrium η phases at high angle grain boundaries became coarse and PFZ became broadening, the fracture toughness (KIC) of Aluminum alloy7050thick plate decleased.(4) The influence of the quench transfer time as to the microstructures and properties was investigated. When the quench transfer time was less than30second, the microstructures and properties of aluminum alloy7050thick plate were stable. With quench transfer time was more than30seconds, the equilibrium η phases at high angle grain boundaries became coarser quickly,the PFZ became broadening and the phases on the grains became coarser quickly too, the tensile strength (σ0.2),yield strength (σb) and fracture toughness (KIC) decreased fastly.