Study On Microstructures And Properties Of7475Aluminum Alloy By Single Curvature Creep Age Forming

Creep age forming is a newly forming technology for integral wing panel of large aircraft, namely by utilizing viscosity, transforming elastic deformation of material into permanent plastic deformation, and changing the mechanical properties and microstructure of material in the stage of age forming. In this paper, basic single-stage and double-stage experiments of7475aluminum alloy were studied by means of self-made four-point bending creep age forming equipment. Meanwhile, to calculate the creep activation energy and stress index, creep tensile test under double-stage system was conducted. The evolution process of microstructure and properties was investigated by using optical microscopy microstructure (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other kinds of testing techniques, and the following conclusions were obtained:(1) The developing regulation of springback, microstructure and mechanical properties of creep age forming process of7475aluminum alloy under different temperature, aging time and radius of curvature were revealed. With the increasing of aging temperature and aging time and decreasing of curvature radius, springback declined gradually to the steady state. For single-stage age forming, Incorporating springback and properties, the appropriate forming cycle was150℃/32h/1000mm among all forming conditions, There is little difference between metallographic structure after creep age forming and those after artificial aging, where both were flat grain structure. But the intracrystalline precipitation phase of creep aging samples was much finer, compacter and more well-distributed than those of artificial aging. The yield strength was523MPa, and conductivity was36.1%IACS which was5.2%and5.6%higher than that of artificial aging, the tearing fracture morphology under that aging system was intergranular and transgranular mixed mode.(2) The springback, microstructure and mechanical properties of7475aluminum alloy during two-stage aging forming process were revealed. Double-stage aging system can reduce springback and improve the forming rate. Considering springback and mechanical properties, shape and properties can be coordinated by adopting two-stage system121℃/6h+180℃/6h. The yield strength reached493MPa and the springback was57.94%. Forming large components1000mm×500mm×2mm by means of this technology, forming joint was less than0.5mm. Property of large components:yield strength is520MPa, elongation is10%, and conductivity is40%IACS.(3) The influence of double-stage aging process on creep tensile experiment of7475aluminum alloy was studied. Double-stage creep tensile curve has two platforms, which are corresponding to two steady states. Meanwhile, creep rate is increased obviously with high temperature in double-stage. Creep tensile stress helps refine matrix precipitated phase and the grain boundary precipitation becomes coarser. In double-stage creep tensile creep activation energy Q is155.8kJ/mol and stress index n is6.85. From preliminary study, it’s concluded that dislocation climb mechanism is creep mechanism in double-stage aging process. There are a total50figures,4tables and64references in this paper.