Study On Quench Sensitivity Of 7050 Alloy And Its Mechanism

7000 series aluminum alloys with low quench sensitivity are critical to fabricate integral or monolithic structures of the aircrafts. The invention of 7085,7081 and 7B85 alloys meets the demand for thick plates and forgings with a maximum thickness of 300 mm. In recent years, numerous experiments and reports focused on the observation of the quench induced precipitates and evaluating the quench sensitivity in this field. However, investigations on the mechanism of the quenching sensitivity are rather limited. It is necessary to carry out research on the precipitation and mechanism of quench induced precipitates as well as their effects on the microstucture and properties of the aged alloys.By using TEM, XRD, SAXS, hardness test, electrical conductivity test and tensile test, the variation law of the microstructure, properties of the as-quenched and as-aged 7050 alloy and lattice parameters of solid solution with different component were researched. Investigation was done on microstructure and properties of the commercial 7050-T7451 thick plates. The main conclusions were drawn as follows:Using Jominy end quench method, the quench depth value (at 10% reduction of hardness in T6 condition is defined as quench depth value) of AA 7050 can reach 45～50 mm from the quenching end. The electrical conductivity of the as-quenched Jominy bar increased rapidly during the first 75 mm from the quenched end, and then rose slowly. With the evaluation of temperature-time-property (TTP) curve, the nose temperature and the corresponding incubation period of 7050 alloy are about 320℃and 1.7 s, respectively. And the quench sensitive temperature range of 7050 alloy is 230～410℃.The quench induced precipitates appeared at grain boundary, subgrain boundary and intragranular Al3Zr dispersoids at 5 mm,10 mm and 15 mm from the quenched end, respectively. With the increasing of the distance from the quenched end, the size and volume of the quench induced precipitates increased. The priority of heterogeneous nucleation of quenched-induced precipitates can be summarized as:grain boundary, subgrain boundary, Al3Zr dispersoids. Residual phases like Al7Cu2Fe and Al2CuMg phases are not found to be heterogeneous nucleation of the quench induced precipitates.The addition of main alloying elements Cu, Mg, Zn changes the lattice parameters of the Al-based solid solution. With the increasing of Cu, Mg, Zn content in Al matrix, the lattice parameters of the Al-Cu, Al-Mg, Al-Zn based binary alloys solid solution decreased dramatically, rising rapidly and declined slightly, respectively. Among main elements the Cu element has the largest influence on the lattice parameter of the 7000 series aluminum alloy solid solution, then Mg, finally Zn. The lattice distortion of the AA7B04, AA7050 and AA7B85 solid solution successively decreased, which is obvious that the stability of the solid solution were enhanced.The aging treatment has little influence on the grain boundary precipitates with low quenching rate. After aging treatment, remarkably coarser and discontinuously distributed grain boundary precipitates present near the quenched end. The similar subgrain boundary precipitates formed during the aging treatment. With the increasing of the distance from the quenched end and isothermal holding time, the size, aspect ratio, volume of the intragranular quench induced precipitates as well as width of PFZs in the as-aged alloys increased, while the volume of age hardening phases declined, which giving rise to the reduction of the hardness.The electrical conductivity increased from 23.1 MS-m-1 at the surface to 24.2 MS-m-1 at center part of the 180 mm thick 7050-T7451 alloy plate. The plate shows inhomogeneous properties, with the UTS, TYS and elongation values being 35 MPa,40 MPa and 5.5% lower than that of the surface part. The proportion of recrystallization decreased from surface to center part of the plate, with a growing quench induced precipitates as well as width of PFZs. Intragranular small quench induced precipitates nucleared on Al3Zr dispersoids at 1/8 thickness of the plate. The failure morphology is dimpled fractures with a higher proportion of brittle fracture, which results in a decreased elongation from the surface to center layer.