Evolution Of Microstructure And Mechanial Property Of7050Aluminium Alloy During TMT

In this study,7050aluminum alloy was processed by thermomechanical treatment(TMT)technology.TMT contains solution heat treatment, cold rolling, annealing andaging,which can control microstructure and improve mechanical property efficiently.The evolution of grain structure and precipitation behavior were researched by OM,TEM, DSC. Mechanical properties of samples in the corresponding state were tested.Cold rolling was applied after solution heat treatment. When the cold deformationwas lower, deformed structure was extremely non-uniform. With the increasing ofdeformation, deformed structure was less non-uniform, occurring more grain shearband.Dislocation within shear band tangled severely. The ultimate cold-rollingdeformation of solution-treated7050aluminum alloy was about70%.When annealed in the temperature range of250~350℃，recovery occurred. Withtemperature increasing, dislocation cells and subgrain of smaller misorientation formed.When annealed in the temperature range of380~400℃, incomplete recrystallizationoccurred and microstructure was larger misorientation subgrains and recrystallizedgrains. With annealing temperature increasing, the size of the precipitated phaseincreased and then decreased and its shape became from short rod to elongated and thenbecame to spherical, strength decreased and then increased, the minimum strength wasshowed when annealed at350℃, elongation increased gradually.When annealed in a low temperature, the recovery degree, the sizes of the subgrainand the misorientation increased with time growing, The density of dislocation insubgrain decreased, the original bulky blocky second phase balled and reduced in sizeaccompanied by continuously precipitated smaller second phase. Compared in the hightemperature, when the time increased, the degree of recrystallization increased and thegrain size grew up with the second phase reducing through melting. Strength decreasedand elongation increased with time growing.The degree of supersaturation of the annealed sample increased with thetemperature increasing. The residual supersaturation is higher in the sample when thetemperature is not lower than380℃. When annealed in low temperature the degree ofsupersaturation reduced with time growing, compared with that in high temperatureincreased. Higher density and smaller sized η phase formed after the aging treatment of the annealed sample which can greatly increase the strength and maintain goodplasticity in the same time. The yield strength was440MPa and elongation was19.1%after aging treatment. Compared with that before the aging treatment, the yield strengthincreased by179MPa, elongation decreased by2.1%.The solid solution samples were cold-rolled, and then proceed to the hightemperature annealing for a short time. The average grain size of annealed sample isabout25μm and elongation is21%.Compared with the solid solution state samples, theannealed samples showed smaller grain size and better plasticity. Therefore, the grainsize can be refined and the the plasticity can be improved by cold deformation with hightemperature short-time annealing process. Annealed samples and solid solution samplesaged at120℃for16hours. Yield strength of annealing-aging samples is435MPa,elongation is19.8%. Compared with solution-aging state samples, the annealing-agingsamples have better plasticiy, with similar intensity. Therefore, the plasticity ofmaterials can be significantly improved by TMT process without reducing the strength.