| Tube inversion is a special metal forming process in which a tube, when subjected to axial compression, will undergo external inversion or internal inversion to form double-walled tubular components that are difficult to produce by any other manufacturing process. These components are often used as impacting energy absorbers in transport vehicles like aircrafts and automobiles.However, transient behaviour was neglected in free inversion, and there were some buckling failures. Because the deformation mechanism is not sufficiently well understood, as a result, tube inversion cannot get an industrial application well. In this dissertation, a systematical and theoretical investigation on tube inversion has been carried out by using FE simulation combined with theoretical analysis and experiment. The new achievements are as follows:(1) The forming process was investigated by mechanical analysis on the basis of the membrane theory of axis symmetric shell. The characteristics of loads and deformation during transient and steady forming process were obtained. The research showed that the effects of inverting radius was crucial to the feasility of tube inversion and forming load, there was a favorable inverting radius to get the minimum inverting load. At the beginning of the process, there were the change of deformation modes, which was mainly determined by the circumferential deformation.(2) The axial symmetry buckling distortions during tube inversion process were investigated, there was a simple linearly proportional relation between the critical buckling load and the average buckling load of the back fluctuation process, and the calculation formula of the critical buckling load of tube inversion was established. Besides, introducing the coefficient of tube inversion and the method of forming limit diagram, the forming scopes were predicted intuitively and expediently. The results showed that the coefficient of external inversion at stabilized external inversion should be 1.18-1.42 , whose opposite fillet radius should be 2～6mm for 3A21 aluminum alloy tube with a diameter of 42mm and a thickness of 1mm, and the coefficient of internal inversion should be 0.82～0.53 , whose opposite fillet radius should be 2～4mm.(3) The simulations on external inversion and internal inversion of 3A21aluminum alloy tube were thoroughly studied by using self-developed FASTAMP software and commercial Dynaform5.2 software, the mechanism of tube inversion was opened out. The research of the numerical simulation indicated the following facts: 1) The result of numerical simulation was consistent with experiment one. In the deforming area the radiuses of the bending edge were not always the same. The smaller the radius of fillet die was, the larger the load-peak was. On the other hand, when the radius of fillet die became larger, the load-peak didn't occur. 2) In the simulation R3 was the optimal radius of fillet die, which extend towards both sides. In the same deforming degree, the inverting load of 3A21 aluminum alloy tube on fillet die was larger than the one on conical die with the same mode, and the one with internal inversion was larger than the one with external inversion. 3) After external inversion, the thickness of the new wall was nearly no change, while it turned to be thicker obviously afer internal inversion .To 3A21 aluminum alloy tube, the thickness of new wall was as 1.38 times as the original one.(4) Experiments on tube inversion of 3A21 aluminum alloy tube and 1Cr18Ni9Ti stainless steel tube were carried out, and the effects of processing parameters and tube geometry size on inversion were also investigated. The results showed that the steady inversion of 3A21 aluminum alloy tube can be acquired with a suitable fillet die or conical die under axial compression, on the other side, it was impossible for 1Cr18Ni9Ti stainless steel tube with a diameter of 40mm and a thickness of 1mm at room temperature , only if employing a strectching inversion method.
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