A Study Of Free Deformation Law Under Dieless Constraint And Tube Axial Compressive Precision Forming Process

Tube axial compressive forming, with which can form a complicated part with simple die that is difficult to be manufactured by any other technique, can be developed into an advanced plastic forming technology with low consumption, good flexibility, high efficiency, and high forming precision. The free deformation law under dieless constraint, the determining and optimization of forming parameter, and tearing and buckling failure during the process are the key problems urgently to be solved for the research and development of the forming process. In this dissertation, a systematical and thorough investigation on the tube axial compressive forming process has been carried out by using rigid-plastic FEM simulation combined with theoretical analysis and experiment. The new achievements made are as follows:(1) The forming process is investigated and the FEM model of forming process oriented to fillet die and conical die is set up. The key technology problems related to the rigid-plastic FEM simulation are solved, then the FEM simulation system which can be used for the forming process study and failure prediction is developed. The experiments carried out show that the system developed is reliable. So it provides an effective approach for further and systematical study of the forming process.(2) The tube axial compressive forming process under radius die and conical die is thoroughly studied, the mechanism of deforming mode changing and free deformation is investigated and effects of forming parameters on the free deformation and the whole forming process are obtained. The results show that during the forming process there exists the change of deformation mode, which is mainly determined by geometrical parameter of die; the larger the half die angle, the earlier the unbending forming of tube beginning and reaching stable forming stage, meanwhile the gap of double-walled tube formed increases with the decrease of relative thickness of tube blank, and the increase of fillet die radius or the decrease of conical die angle; the tube material property and friction condition have little influence on the double-walled tube, while a significant influence on the forming load.(3) The mechanism of tearing and buckling failure in the process is studied, the criteria of failure are established; and then a method for the failure prediction is proposed based on the failure criteria combined with the finite element code, with which the effects of forming parameters on the tearing and buckling failure are analyzed. The results show that tearing and buckling are possible to occur with the decrease of tube relative thickness and strain-hardening exponent; for the process under fillet die there exists a maximum die radius which makes no tearing happen, and exists a range die radius value within which no buckling happens; for the process under conical die there exists a range of maximum and minimum half die angles, within which no tearing or buckling happens.(4) With the FEM simulation system developed the feasibility of stable tube axial compressive forming process is systemically studied and the range of forming parameters for it is obtained; and the relationship between forming result and parameters is established by using regression analysis, based on which the forming parameter for the process can be determined rapidly and exactly by FEM. Used in the forming process under fillet die the result shows that the method proposed is reliable and practical, and can serve as a significant guide to the design and optimization of the process.The achievements of this study are important for the development of tube plastic forming theory and tube axial compressive precision forming process. Furthermore, it offers a new idea to the research and development of the advanced plastic processing technology.