Hydroforming of anisotropic aluminum tubes: Part II analysis

Part I presented an experimental investigation of hydroforming of Al-6260-T4 tubes and a simple two-dimensional model of the process. Relatively long, extruded circular tubes were formed against a square die with rounded corners, with simultaneous application of axial feeding. Localized wall thinning was reported to occur at mid-span which, accentuated by friction, led to burst. Part II presents fully 3D models of the process that include friction as well as more advanced constitutive models shown in previous studies to be essential for simulation of burst in free hydroforming of aluminum alloy tubes. The models are used to simulate several of the experiments of Part I, emphasizing the prediction of all aspects of the forming process, including wall thinning and its localization that lead to rupture. A shell element model is shown to capture the majority of the structural features of the process very successfully. However, even with the implementation of advanced constitutive models, it fails to reproduce correctly the localization of wall thinning. It is demonstrated that switching to solid elements coupled to non-quadratic yield functions results in accurate predictions of all aspects of the problem, including the onset of rupture. Apparently, slow growing depressions that develop at the interface between the flattened part of the cross section that is in contact with the die and the rounded part that is not, have a complex three dimensional stress state requiring accurate modeling offered by solid elements. Furthermore, the evolution of these depressions is only reproduced with accuracy when in addition non-quadratic yield functions are adopted.