High velocity sheet metal forming was the focus of much research approximately 30 years ago, but has become somewhat dormant since then. Initially developed for fabricating big components, their use continued for the manufacture of small components of intricate shapes as well. However, their potential to improve formability was poorly understood.
In the current investigation, electrohydraulic forming was used to generate high workpiece velocity. Electrical energy up to 50 kJ was stored in a bank of capacitors and was discharged rapidly across a pair of electrodes under water. This generated a high intensity shock wave which forced the sheet metal into a die. Comparative conventional forming was done by pressurizing the oil beneath the workpiece with a hydraulic pump.
Formability increase of over 400% was observed on 6061 T4 Aluminum sheets and an increase of over 150% on interstitial free iron and OFHC copper. Limit strains were directly measured on the samples. Forming Limit Diagrams were used to compare low and high rate results. The estimated metal velocities at high rates ranged from 130 m/s to 350 m/s and the strain rates were of the order of 101s-1.
At such energies and time scales, inertial forces act to diffuse the deformation away from the localized areas, thus postponing the failure. Compressive stresses generated by the impact of sheet with the die wall also increase the ductility. However, the strain rate appeared to be still too low to cause adiabatic localization that might have limited the formability. The estimated velocity was also much lower than the von Kármán;n critical velocity for localization. The terminal hardness of finished components differed from the initial hardness within 20%. Thus, the material constitutive behavior did not appear to have changed.
Increased formability makes these high rate processes competitive to traditional superplastic forming. In order to distinguish this effect from superplasticity, the current phenomenon is termed hyperplasticity.