Plastic Deformation Behaviors Of Dp780under Compound Strain Rate Loadings

With the rapid development of automobile industry and the increasing global environment pressure,in recent years,the automobile lightening is becoming the inevitable developing trend of automobile industry.In the automobile manufacturing field,the use of high strength steel has become one of the main ways to ensure the strength of the car and reduce it’s weight.However,high strength steel can be hard formed into complex-shap parts and large rebound,wrinkle,and crack can be easily happening under the conventional technology.Electromagnetic forming can improve the plasticity of high strength steel at room temperature.Electromagnetically assisted sheet metal stamping(EMAS),which integrates the high-speed forming advantages of EMF into the conventional quasi-static stamping process,has provide a new way for room-temperature precision processing of complex-shape parts of high strength steel.The plastic deformation behaviors of DP780 sheet sample was studied at the typical strain rate range of 1000s-1-4000s-1 under EMF processing conditions through the Split-Hopkinson Tension Bar(SHTB),which will lay the foundations for the in-depth understanding of the dynamic deformation mechanisms of DP780 and the promotion of engineering applications of EMAS.According to the sequential deformation characteristics of quasi-static straining and dynamic re-deformation of sheets during EMAS,the dynamic stress-strain curves with different pre-straining and strain rates were obtained by SHTB and the effect of pre-straining and strain rates on the mechanical property of DP780 were analyzed.Subsequently,the modified Johnson-Cook and Zerilli-Armstrong dynamic constitutive models of DP780 with different pre-strainings were established.Results show that,under high strain rate loadings,the strain rate sensitivity of DP780 sheets exists.The yield strength and ultimate tensile strength increases obviously when the strain rate increases to 1900s-1 from 1150s-1.However,it does not change clearly any more when strain rate more than 1900s-1 and there even is a trend of dropping about the ultimate tensile strength.Analysis suggests that the adiabatic temperature rising led to the trend,and quasi-static pre-straining make an higher flow stress.Experimental data indicates that both of the constitutive models can well describe its high strain rate materials responses.Comparing with5% pre-straining,the strain hardening of DP780 with 10% pre-straining is higher and the strain rate hardening is lower.The fracture and dynamic plastic deformation mechanisms of pre-strained DP780 sheets were investigated by theoretical analysis,microhardness,and microanalysis methods of SEM,TEM etc.Results show that,the microhardness near the facture has not an obvious change when the strain rate is more than 1900s-1,which is also the effecte from thermal softening effect.The SEM of fractures show that,all of the fracture modes of DP780 samples with different strain conditions are ductile fracture.The TEM of area near fractures show that,with the increasing strain rates,the dislocation density in the crystal increases and dislocation tangle become more apparent.There were twin crystallites discovered in fracture region when the strain rate was 1900s-1,which illustrated that main plastic deformation way of DP780 is dislocation motion,however,with the increasing strain rate,the plastic deformation way becomes dislocation motion and twinning.And,different from the single-slip pattern of dislocations of quasi-static deformation,the dislocation system of dynamic deformation tends to more slips,large areas showing clear cross-slip structures.