Deformation Behavior And Hyperplasticity Mechanism Of DP600 Steel Sheets During Quasi-Static-dynamic Hybrid Forming

Advanced High Strength Steel/AHSS is a new type of steel developed to adapt to the development trend of body weight reduction,and has become the main material of body lightweight in recent years.However,the room temperature formability of AHSS is obviously reduced compared with that of ordinary steel.The quasi-static-dynamic hybrid forming combines the dynamic electrohydraulic forming(EHF)with the quasi-static stamping process,providing an effective way for the accurate forming of AHSS sheet at room temperature.Since the process is still in the stage of experimental exploration,its deformation mechanism and basic problems of application have not been studied yet.This paper revealed the deformation behavior and plasticity enhancement mechanism of DP600 steel sheets during the quasi-static-dynamic hybrid forming by combining theoretical analysis,experiments,numerical simulation and microstructure evolution.The waveforms of current,voltage,deposition energy and deposition energy density in the phase transformation process of wire electrical explosion were explored by model analysis,which provided a theoretical basis for the selection of preset wire in experimental program.According to the propagation mechanism of underwater shock wave,the motion model of shock wave caused by wire electric explosion was established,and the normalized propagation characteristics of shock wave,such as the position,velocity and pressure,were obtained.The dynamic EHF limit test program was established and FLD was drawn to evaluate the dynamic formability of DP600 steel sheets.The results showed that the limit major strain under uniaxial tensile,plane strain and biaxial tensile strain paths was 45.0%,25.6% and 48.7%,respectively.Compraed with quasi-static FLC,the limit major strain was improved by 36%,70% and 17.5%,respectively,in these three strain paths.The quasi-static-dynamic hybrid forming limit tests under typical strain paths were studied.The results showed that under different pre-deformation levels,the limit major strains were in the range of 40-47%,28-31% and 43-57%,respectively,under uniaxial tensile,plane strain and biaxial tensile strain paths.Compraed with quasi-static FLC,the limit major strain was improved by 36%,90% and 26%,respectively,in these three strain paths.High prestrain levels can enhance the hyperplasticity effect of dynamic EHF.Based on the multi-physical field finite element software ANSYS/LS-DYNA,the dynamic deformation behavior of the sheet was reproduced,and the hyperplasticity mechanism and proportion of inertial effect during EHF process were quantified.The results showed that the maximum velocity was 350 m/s,and the maximum equivalent strain rate was 1740/s,showing a high strain rate forming process.The inertial effect reduced the velocity gradient of the mass particle along the tensile direction by generating additional tensile stress,so as to achieve the effect of stabilizing and coordinating deformation,and its contribution to plastic deformation was as high as 87.4%.By analyzing the dynamic re-deformation behavior of quasi-static prestrained microelement and establishing the motion model,the macro plasticity enhancement mechanism of quasi-static-dynamic hybrid forming was revealed.The change of dynamic material constitutive behavior(extra stress increment Λσ)caused by quasi-static pre-deformation promoted inertial effect to play a more active role in coordinating the further dynamic deformation of the prestrained microelement after the onset of plastic instability,which was the main macroscopical factor that caused the hybrid plasticity to be slightly superior to the pure dynamic plasticity.By observing and analyzing the microstructure,the micro plasticity enhancement mechanism of quasi-static-dynamic hybrid forming was revealed.In the EHF process,the nucleation of existing dislocations significantly increased the dislocation density in ferrite,which offset the easy stacking and uneven distribution of the dislocations caused by quasistatic pre-deformation;the [0 1-1]MT deformation twins induced in the martensite enhanced the plastic compatibility of the ferrite and martensite phases,adjusting the strain gradient and reducing the risk of debonding and void nucleation.The quasi-static-dynamic hybrid forming test scheme of 90° bending specimens was established,achieving the the accurate forming of bending specimens with different final fillet radii of R10 mm,R8 mm,R5 mm,R3 mm and R0 mm.Numerical simulation revealed the mechanism of reducing springback in hybrid forming: under the combined action of the special load form,the pressure holding characteristic of liquid medium water and inertial effect,the tangential stress distribution in the deformation zone of the bending specimen presented a fluctuating state,thus reducing the springback and improving the forming accuracy;and then quantified the dynamic flow behavior and stress-strain evolution law of materials caused by the impact effect of die during the quasi-staticdynamic hybrid forming process.