Study On Mechanics And Springback Of Ti6Al4V Involved In Electrically Assisted Forming Process

With the introduction of electric pulses through metals,work hardening can be decreased in low temperature and short period.For Al and Ti alloys,electrically assisted forming process(EAFP)can improve formability and realize the precision deformation of thin sheet.Thus,EAFP has a good prospect in the application of energy,aerospace and biomedicine field.However,metallic plasticity is influenced by thermal effect and electric effect at the same time.The internal mechanism is complicated.Meanwhile,plastic deformation of metals with different element and crystal structures is different under the treatment of electric pulses.Due to the lack of comprehension on electric effect and structural dependence,EAFP can only take thermal effect as the design standard,which is difficult to optimize process parameters in EAFP.In this research,both dislocation motion and microstructure evolution have been studied during plastic deformation under the treatment of electric pulses.Based on electric and thermal effect on dislocation sliding and climbing,dislocation model has been established and analyzes the dislocation motion in Cu,SS304 and Ti6Al4 V treated in EAFP.Nucleation along with grain growth in diffusion transformation has been studied and the constitutive model of Ti6Al4 V has also been proposed.Combined with experimental results,these models have been used to investigate the inhibition of springback in Ti6Al4 V.The detailed information of this research are:1)Experimental analysis on mechanics of metals in EAFPIn this thesis,experimental system has been set up,which aims to distinguish electric effect with thermal effect in EAFP.Based on such system,mechanical properties and microstructure evolution have been observed treated in different current densities and temperature.Both electric effect and thermal effect on flow behavior and ductility have been analyzed numerically.According to experimental results,volume of fiber texture [111] in Cu and martensite phase in SS304 have been restrained and diffusion transformation in Ti6Al4 V has been accelerated by electric effect at the same temperature.2)Microstructure evolution treated in EAFPConsidering the electric effect on dislocation migration energy,dislocation model has been established for describing dislocation sliding in F.C.C crystal structure in EAFP,which can be used to analyze the restraining effect on volume of fiber texture [111] of Cu during deformation.Meanwhile,such model can also calculate such dislocationmultiplication and recovery and explain the decrease of martensite phase volume in SS304.As for Ti6Al4 V with H.C.P crystal structure,accelerated vacancy motion has been introduced to depict dislocation climbing and dislocation climbing model treated in EAFP has also been proposed,which is in accordance with experimental results about stress decrease.3)Constitutive model of Ti6Al4 V treated in EAFPConsidering the enhanced nucleation and growth affected by electric effect,diffusion transformation model of Ti6Al4 V has been established,which is helpful to describing the increase of ? phase at low temperature and short period.Combined with dislocation climbing affected by electric effect,constitutive model of Ti6Al4 V treated in EAFP has been obtained and agrees with experimental observation.Furthermore,stress relaxation of Ti6Al4 V can be explained by taking both electric effect and thermal effect on dislocation recovery into account.4)Experimental investigation on springback of Ti6Al4 V in EAFPCombining accelerated stress relaxation of Ti6Al4 V affected by electric effect,springback prediction model has been established treated in EAFP.Meanwhile,experimental setup has been built to study the change of temperature and strain and observe springback in stretch U-bending deformation assisted by electric pulses.Compared with experimental results,results from prediction model have good consistency.In this research,both electric effect and thermal effect on mechanics and microstructural evolution of metals have been studied both experimentally and theoretically.For different crystal structures,dislocation models have been established to describe dislocation sliding and climbing treated in EAFP.By considering diffusion transformation of Ti6Al4 V treated in EAFP,constitutive model of Ti6Al4 V has been proposed and restraining effect on springback has been studied.It is believed the scientific conclusions and remarks obtained in this thesis can be helpful for the application of EAFP in microstructure fabrication and thin sheet deformation...