Evolution Of Cavities In5A90Al-Li Alloy Sheet During Superplastic Stretching

Superplastic forming is an important method to solve the difficulty in forming of Al-Li alloy at room temperature due to the low plasticity, but the cavitation during superplastic forming reduces the mechanical properties and formability of components. Research of the cavity nucleation and growth mechanism is very important to control the number of cavity. In this thesis, the grain and cavity microstructures of commercial5A90Al-Li alloy sheet were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) through uniaxial tension at different initial strain rates(310-4s-1～110-3s-1) and different temperatures (450～500℃). The main conclusions are as follows:(1) The cavitation at different stages of superplastic deformation corresponds to different growth mechanisms. At the initial stage (ε≤0.45), the cavity growth mechanism is controlled by diffusional process with equally distributed spherical cavities; at the second stage (0.45<ε<1.3), large angle grain boundaries are increased, which leads to the increase of grain boundary sliding. While the sizes of cavities increase, their growth is controlled by superplastic diffusional process; at steady flow stage (ε≥1.3), the superplastic diffusional process and plastic deformation will regulate the cavity growth jointly.(2) The influences of deformation temperature and initial strain rate on the evolution of cavities are showed:the size and volume fraction of cavities increase with the increase of strain; excessively high or low deformation temperature (450℃or500℃) will make the bigger size and more volume fraction of the cavities; in addition, lower initial strain rate (310-4s-1/1475℃) causes the increase of the sizes of cavities as well. And higher initial strain rate (810-4s-1or110-3s-1) might enhance the cavity nucleation rate, which leads to the increase of the cavity volume fraction.(3) The local strain distribution during superplastic deformation of5A90Al-Li alloy sheet and the effect on the evolution of cavities were studied by uniaxial tension at a constant temperature of℃and different initial strain rates(310-4s-1～110-3s-1). The results demonstrate that the strain and the cavities remain non-uniform in the whole sample. The cavity growth is different at different initial strain rates. At high strain rate (110-3s-1), the cavity growth is controlled by plastic deformation; reducing the strain rate to810-4s-1, the cavity growth is controlled by diffusional process. And at the same local strain, transverse direction is controlled by diffusional process with spherical cavities but the same longitudinal direction is controlled by plastic stretching with elongated cavities.