| Aluminum alloys are widely used to meet the need for lightweight structural metals in aerospace industries because of their benefits such as good corrosion resistance, high strength-to-weight ratio and advantageous structural performance, especially for the aluminum-lithium alloys with lighter weight and higher strength. However, further application is limited by their poor formability at room temperature and thermal distortion in solution heat treatment. In order to solve these problems, integrated process of forming-quenching with cold-hot dies is proposed in this paper, combining hot forming and heat treatment together in one operation. Good formability can be obtained because the forming process is finished at the temperature close to solution temperature. The thermal distortion is avoided by cooling within forming dies. In this paper, the deformation behavior and strengthening mechanism for the integrated process of 2195 Al-Li alloy sheet will be investigated. The obtained results are supposed to provide theoretical guidance and technical support for forming actual structural components.The effects of solution temperature and strain rate on deformation behaviors were investigated by hot tensile test. The evolution of flowing stress during tensile deformation were analyzed. The higher solution temperature or lower strain rate, the smaller flowing stress. Enhanced formability was obtained at the solution temperature and higher strain rates. The elongation could arrive at the value of 145% when the solution temperature was 520℃. The deformation behaviors subjected to rapid hot gas forming after enough solution heat treatment were analyzed by free bulging testing at different pressurizing rates(0.0088-2.37MPa/s). The profile and thickness of the central section of the sheets were measured in different stages to describe the hot deformation behavior. Compared to slow pressurizing, enough formability could be obtained under rapid pressurizing with 0.74 in effective strain although the bursting time reduced to 1.59 s. Meanwhile, the thickness distribution became more uniform because of the increasing strain and strain-rate hardening effect.Microstructure evolutions at different solution temperatures and strain rates were observed to reveal the deformation mechanism by using SEM, EBSD and TEM methods. The grains were elongated along the tensile direction, but held equiaxed under biaxial stress state. Dislocation sliding was the main deformation mechanism. Dynamic recovery was the main softening mechanism, which depended on the deformation degrees and rates. When the deformation rate was relatively slow, the further deformation could accelerate the occurrence of dynamic recrystallization, accompanying with grain fragmentation and decreasing grain size. The fracture mechanism was mainly determined by the solution temperature. When the solution temperature was 460 ℃, the nucleation, growth, and coalescence of microvoids contributed to the final fracture. As the temperature increased, the fracture mechanism became the mixed ductile and intergranular fracture.The influences of hot tensile and bulging deformations during solution heat treatment(SHT) on precipitation hardening were investigated. The strain obtained at the solution temperature would increase the final hardness, since large deformation also accelerated the concurrent softening at high temperature. The promoting effect mainly depended on the deformation degree, and the influence of the deformation rate was relatively small. Meanwhile, the role of hot deformation played in aging hardening was much less than that of room temperature deformation due to the occurrence of dynamic restoration. Precipitate distribution was observed by TEM to reveal the strengthening mechanism. The θ’, δ’ and T1 phases were the main precipitates. A high density of dislocations and low angle grain boundaries were produced in the deformed area, which accelerated extreme and fine precipitates precipitating by providing large number of nucleation sites. The density of T1 phase increased with increasing deformation. Precipitation strengthening was the main strengthening mechanism, which would be enhanced by the precipitation of fine and dispersed T1 phase.Special device was developed to investigate the effect of cold-hot composite forming dies on springback and strengthening evolutions in the integrated process of forming-quenching. The springback of U-shaped and double curvature sectional specimens was described by 3D measure. Effect of thermal stress on the shape of formed parts reduces obviously, dimensional shaped accuracy can be guaranteed in the integrated process of forming-quenching. When the temperatures of upper and lower dies were the same, the cooling within forming dies at a lower temperature could realize the effective quenching, accompanying with the strengthening level at the value of T6 condition. As the temperature of forming dies increased, no obvious decrease in strength appeared until the forming-dies temperature reached 250℃. When the lower die was heated and upper die was water cooled, the upper die could still ensure the effective quenching.In order to reflect comparatively the cooling ability of upper cold dies, three additional cooling mediums(RT. water, boiling water and air) were utilized to quench the part after being formed in cold-hot dies. The effective quenching was finished during hot forming in cold-hot dies when the temperature of lower die was 250℃. When the temperature rose to 450℃, the strength decreased while being removed for air cooling. In order to obtain enough strengthening effect, the temperature of isothermal forming dies should not be higher than 200℃. The temperature of heated lower die could be elevated to 450℃ for avoiding the rapid decrease of forming temperature, but cooling within upper cold die for realizing the rapid quenching and avoiding the thermal distortion was necessary. |
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