Free deformation mechanism and change of forming mode in tube inversion under conical die

Free deformation and the change in forming mode have an important effect on the realization of tube inversion with a conical die, particularly on the size and degree of precision of the formed part. Based on a mechanical model, the mechanism of free deformation and characteristic behavior of forming mode are investigated by an FE system combined with analytical analysis, which permits the condition to realize change in forming mode and inversion to be determined. The results show that for the given tube blank there exists a critical die conical angle αcr, above which the tube will curl to form a double-walled tube; αcr is determined mainly by the tube material parameters, while the geometric parameters have very little effect; the larger the material hardening exponent the smaller the αcr value.