Evaluation of the optimal working conditions for the warm sheet HydroForming taking into account the yielding condition

The present work aims at determining the optimal working conditions to manufacture an aluminium component by warm sheet HydroForming. The following parameters were considered: temperature, blank holder force, oil pressure; while, as output variables, die filling and maximum oil pressure. The optimization procedure was preliminary based on numerical data. A finite element model was created and great attention was paid to the material modelling: an extensive characterization for evaluating the mechanical and deformative behaviour at different temperatures was conducted; in addition, the effect of modelling different plastic behaviours was considered through the implementation of two yield criteria (the anisotropic Barlat '89 criterion and the isotropic Von Mises one). Results from the optimization procedure indicated the room temperature as the optimal working condition when implementing the Barlat '89 criterion, while the optimal temperature was higher (about 110. °C) when considering the isotropic formulation. It is relevant that, irrespective to the yield criterion, the tuning of both the numerical models was possible, but HydroForming tests confirmed the validity only of the anisotropic criterion. In fact, results from the optimization procedure based on experimental data indicated the room temperature as the optimal condition, highlighting the key role played by the yield criterion in the process modelling.