| This paper explored the forming for a composite structure of AZ31/AA5083bilayer cylinder. The temperature range of magnesium alloy and aluminum alloy for superplastic forming overlaps and their thermal expansion coefficients are different. The cylinder is formed by gas blow forming during heating and the holding force of bilayer cylinder is provided by cooling contraction combination during cooling, which achieves the connection of bilayer cylindrical composite structure.A finite element analysis software MSC.MARC with shell element was used to simulate the superplastic forming process of bilayer cylinder. The results showed that the wall thickness distribution of cylinders is nonuniform and the thickness of rounded corners on the bottom is the thinnest and this location is the most vulnerable to rupture. When initial pressure and the maximum pressure are constant, the loading paths and the friction coefficients between different sheets have no effect on the wall thickness distribution. The smaller the friction coefficient between the sheet and the mould is, the more uniform the wall thickness distribution is. The force between the internal sheet and the external sheet is equivalent to the gas pressure, and the equivalent von mises stress distribution is consistent.Experiments for gas blow forming/cooling contraction combination of bilayer cylinder were carried out under the different temperatures and different loading paths with1.0mm and1.5mm thick rolled sheet. The results showed that if the temperature is too high, the bottom of internal cylinder occur instability. On the contrary, the temperure is too low, the corner radius of cylinder bottom is too large. At450℃, the gas pressure increased0.4MPa every600s from1.6MPa to4.4MPa, then holding8100s, a perfect cylinder with80mm in diameter and24mm in height was formed, which the ratio of height to diameter is0.3. The numerical and experimental results of wall thickness distribution are fit well, and the difference between corresponding thickness thinning ratio is less than5%. The suitable forming temperature range for AZ31/AA5083bilayer cylinders is425~450℃, guided by taking advantage of the plasticity of two dissimilar alloys. The location of sheets has a greater influence on its thickness distribution. The wall thickness distribution of internal sheet is more uniform than that of external sheet. The microstructure evolution of AZ31magnesium alloy during the forming process was investigated by metallurgical microscope. The results showed that the grain size decreases with the increase of the thickness thinning ratio and the grain at the bottom corner is the smallest. When the thickness thinning rate reduces from67.6%to0%, grain size increases from9μm to20μm. The grain size increases with the increase of forming temperature. When the forming temperature increase from400℃to450℃, the grain at the bottom corner increases from5μm to9μm.The holding force of bilayer cylindrical component was calculated theoretically. The results showed the contact pressure between internal and external cylinder is about63.45MPa, assuming that the interference fit between bilayer sheets occurs due to cooling contraction. The stress and instability analysis during gas blow forming/cooling contraction combination process of bilayer cylinder were performed. Gas blow forming/cooling contraction combination process can cause protuberance of internal sheet and fracture. The protuberance at the bottom of internal sheet is caused by cooling contraction force. Increasing forming temperature and the corner radiu and cooling with the gas pressure can avoid the instability. Fracture occurs easily at the location with the thinnest wall thickness, and the position of fracture at internal and external layer is consistent. |
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