Hyperplasticity mechanism in DP600 sheets during electrohydraulic free forming

This work discussed the multi-scale mechanisms of hyperplasticity during the high strain rate electrohydraulic forming (EHF) process by exploring the formability of DP600 sheets in a state of uniaxial tensile stress. The experimental results showed that the limit strains and limit dome heights of the deformed specimens obtained by EHF were improved by 15 %–27 % and 22.54 %, respectively, as compared with quasi-static specimens, showing a hyperplasticity characteristic. The inertial effects that occurred during the EHF process were responsible for the macro-scale enhancement in terms of formability, which could generate an additional principal stress along the direction of stretching that slowed the velocity gradient of the necked elements to restrain uneven deformation, resulting in a 60 % broadening of the action zone of maximum Y-displacement. The proportion of inertial effects that contributed to the plastic deformation of the deformed specimens was 87.1 %, indicating that the vast majority of the deformation in the EHF process occurred as a result of inertial effects after the electrical energy was completely discharged. A larger dislocation density and a more uniform dislocation distribution were observed in the EHF specimens, which were regarded as the micro-scale causes of the hyperplasticity in the EHF process. Multiplication and entanglement of dislocations caused by the significant shear stress, together with the extensive nucleation of new dislocations caused by the high strain rates, demonstrated the micro-scale mechanism of hyperplasticity during EHF.