Modelling of localised thinning features in the hydroforming of micro-tubes using the crystal-plasticity FE method

The wall thickness of hydro-formed micro-tubes is not uniform due to the low ratio of the wall thickness and the grain size. The features of localised thinning are different from those of the traditional hydroforming process, and this is related directly to the ratio of the wall thickness and the grain size of the material, and also to the amount of deformation. Macro-mechanics finite element (FE) modelling cannot be used to simulate such effects encountered in micro-tube hydroforming processes. In this paper, a simplified plane-strain crystal-plasticity finite element (CPFE)-based modelling technique has been developed and used to capture the localised thinning features in the hydroforming of micro-tubes. The grain structures within the tube workpiece, and their distributions and orientations, are generated automatically using the developed VGRAIN system. A set of crystal-viscoplasticity models is implemented in ABAQUS/Explicit FE code through the user-defined sub-routine, VUMAT. Single-crystal and multi-crystal structures have been studied, and the localised thinning has been analysed for different microstructures of the material using the CPFE modelling technique. It is confirmed from the analysis that the localised thinning in the hydroforming of micro-tubes is affected significantly by the microstructure and grain orientations of the material.