Formability and deformation behavior of DP600 steel sheets during a hybrid quasi-static/dynamic forming process

Abstract

The purpose of this work was to investigate the formability and deformation behavior of DP600 steel sheets during a hybrid forming process composed of quasi-static forming and dynamic high strain rate electrohydraulic forming (EHF) sequence. The results showed that the ultimate limit major strains under the uniaxial tensile strain path were 40–47%, and those under plane strain and biaxial tensile strain paths were 28–31% and 43–57%, respectively. Compared with quasi-static data, the hybrid data under these three strain paths were improved by 30%, 70%, and 36%, respectively, demonstrating a hyperplasticity characteristic. The quasi-static prestrain had little effect on the ultimate formability, and the hyperplasticity effect of the quasi-statically prestrained specimens mainly depended on the inertial effect and the change in constitutive behavior in the high strain rate EHF loading step. Furthermore, the optimized Johnson-Cook constitutive model of the quasi-statically prestrained sheet during high strain rate condition was obtained by means of dynamic Hopkinson tensile tests, and the numerical simulation results based on this material constitutive were in good agreement with that obtained by experiments, showing an error of less than 5%. Then, the numerical simulation reproduced the propagation and evolution process of the pressure wave from ellipsoid to sphere, and quantified the dynamic deformation behavior of the quasi-statically prestrained specimen during an EHF process. The maximum velocity and strain rate of the prestrained specimen during EHF step were 270 m/s and 3700/s, respectively, and the ultimate height of the deformed specimen was 43 mm.