Research On The Strengthening And Toughening Mechanism Of High-density Electropulsing On The Alloy Steels

The development of civilization and industry claim more requirements for alloy steels.Thanks to modern high-tech,new alloy steels with excellent physical and mechanical properties emerged(e.g.nano-precipitation high-strength electrical steels,high-frequency low-iron-loss high-silicon steels,and twinning-induced plasticity steels).The processing technology of these new alloy steels faces new challenges of improving its formability and microstructure modification technology.Specifically,the challenges include reducing deformation resistance,lowtemperature deformation and microstructure modification,and improving efficiency.Applying high-density electropulsing to materials is a new assisted processing technology.The electroplastic manufacturing process could reduce the deformation resistance,increase the die life,increase the surface quality and yield ratio.Moreover,the annealing and acid pickling during the conventional process could be omitted,significantly improving production efficiency and reducing production costs.The static electropulsing treatment(EPT)could improve the micro structure at a lower temperature and a shorter time.Moreover,the EPT shows advantages of increasing texture strength,reducing internal stress,and crack healing.The present thesis focuses on the strengthening and toughening of high-density electropulsing on the alloy steels.Fe-4.0%Si-2.0%Cu,Fe-6.5%Si,and Fe-18%Mn0.6%C(mass fraction,wt.%)were selected as the model materials,and the effects of high-density electropulsing on point defects(atom diffusion),line defects(dislocation slip)and plane defects(deformation twinning)were studied.The effect of static electropulsing treatment on the aging strengthening and Cu precipitation in Fe-4.0%Si-2.0%Cu alloy was systematically studied.The aging hardness curves showed that,under electropulsing,the peaks of hardness were achieved at 520～540℃ × 0.5 min,470～490℃ × 4 min,and 420～440℃ × 80 min,which were five times faster than those under the conventional aging heat treatment.The TEM and 3 DAP results showed that the Cu-rich zones were more uniform in size,and the Cu-rich zones had a higher degree of segregation after the electropulsing treatment.Lower apparent activation energies were obtained under electropulsing,indicating that the Cu precipitation was effectively promoted under electropulsing.The effect of electropulsing on the plasticity improvement,dislocation slip,and microstructure in Fe-6.5wt%Si alloy was systematically studied.Compared with conventional tensile tests,the plasticity of Fe-6.5wt%Si was significantly improved under electropulsing.The maximum elongation increased by 489%,and the yield strength decreased by 19.4%.Under electropulsing,as the elongations of Fe-6.5wt%Si alloy increased,the degree of microstructure deformation increased,the work hardening occurred,the work hardening index n increased,and volume fractions of ordered phases B2 and D03 decreased.Based on the dissimilarity of dislocation distribution at various crystal planes in Fe-6.5wt%Si alloy under electropulsing,the anisotropy effect of electroplasticity was proposed.The effect of electropulsing on the deformation twinning and microstructure in Fe-18%Mn-0.6%C alloy was systematically analyzed.Under electropulsing,at 310℃ and above,a new sparse-B serration appeared.The peak values of the number of stress drops,the average amplitude of stress drop,and the critical strains were 190,450,and 450℃,respectively,which were higher than those under conventional tensions.High temperature suppressed the serration,so the enhanced serrations established the non-thermal effect of electropulsing.Under electropulsing,the volume fraction of twins decreased by 25%,indicating weakened prerequisites for twinning,reduced generation of twins,reduced local internal stress,and reduced pile-up of dislocations,which proved that the electropulsing enhances the dislocation mobility.The effect of Joule heating on alloy steels during electropulsing was systematically studied.The electropulsing tension of Fe-6.5%Si alloy showed that air-cooling could significantly reduce the temperature rise.Moreover,through temperature gradient and rise calculations,the effect of Joule heat on mechanical properties was minimized.It established the non-thermal effect of electropulsing.The electropulsing tension of Fe-18%Mn-0.6%C under the same electropulsing parameters and different temperatures suggested that the strengths,the elongations,the dislocation density,and the volume fraction of twins decreased more at higher temperatures,establishing the coupling effect of the thermal and non-thermal effect.The static electropulsing treatment of the Fe-4.0%Si-2.0%Cu alloy indicated the electropulsing could achieve the effect higher than its actual measured temperature,so the non-uniform heating effect of high-density electropulsing is proposed.In short,the effect of electropulsing on the alloy steels showed noticeable non-thermal effect.The thermal effect and the non-thermal effect are coupled to achieve the high-efficiency and energy-saving electroplastic effect.Based on the above research results,this thesis systematically studied the strengthening and toughening of alloy steel under high-density electropulsing.It revealed that the electroplastic effect came from the enhancement of dislocation mobility by electropulsing,which was characterized by the decrease of dislocation density and pile-up.It proposed that electroplasticity manifested an anisotropy effect on dislocations in various crystal planes.It found that the mechanism of the electropulsing on the diffusion-controlled second-phase precipitation was the coupling effect of electric effect and Joule heating effect(the electric effect enhanced the diffusion of atoms and reduced the activation energy,and the Joule heating effect was non-uniform heating).It clarified that the mechanism of electropulsing suppressing the twinning deformation and enhancing the serration effect was that electroplasticity enhanced the dislocation mobility.This paper could provide a theoretical basis and scientific guidance for the industrial application of high-density electropulsing in the processing and structure modifications of new alloy steels.