Experimental and numerical investigation of free-bulge formation during hydroforming of aluminum extrusions

Free-bulge hydroforming experiments were conducted to investigate the biaxial behavior of extruded aluminum tubes. A special fixture, capable of providing only internal pressure and free sliding of tube-ends, was successfully designed to conduct these free-bulge experiments. Forming limits obtained under these conditions were found to be significantly lower than those obtained with superposed axial feed. A finite element model was constructed to simulate the hydroforming process and assess the influence of friction between the die walls and the tube, tube material properties and tube anisotropy. A power law constitutive model based on stress–strain curves obtained for various test geometries was used in these simulations. It was found that the parameters obtained based on the tensile test data of the curved tube sections better correlated to the experimental bulge height measurements. The analysis also revealed that the material hardening coefficient had the most significant influence on the formability characteristics during hydroforming.