Research On Electropulsing Assisted Single Incremental Forming Of Ti-6Al-4V Titanium Alloy Sheet

Titanium alloy has been widely applied in aerospace,transportation and energy equipment due to its high specific strength,excellent corrosion resistance and thermal stability.However,the poor plasticity of titanium alloy with close-packed hexagonal structure at room temperature limits its wide application.The electropulsing assisted single point incremental forming technology combines the electropulsing assisted forming technology with the incremental forming process,which can significantly improve the sheet formability.The electropulsing assisted single point incremental forming is a fast non-equilibrium process with the coupling effect of electrical energy,thermal energy and deformation energy.This technology can result in the complex deformation behavior,affecting the forming process and forming quality.In this study,based on experiment investigation and theoretical analysis,the electropulsing assisted single point incremental forming of Ti-6Al-4V alloy sheet was studied.The main result conclusions are as follows:The electropulsing assisted tensile,electropulsing assisted bending and electropulsing treatment of Ti-6A1-4V sheet were carried out.And the effects of electropulsing on the plastic deformation behavior and microstructure evolution were studied.The effect of the RMS current density and peak current density of the electropulsing on the tensile strength,fracture elongation,bending stress and springback of the sheet were analyzed.The thermal effect and athermal effect of electropulsing on the dynamic recovery,dynamic recrystallization and phase transition mechanism were explored.The results show that the fracture elongation increases,the deformation resistance and the springback decrease with the enhancement of the RMS current density.When the RMS current density keeps approximate,the electropulsing with the high peak current density can lead to the more significant electro-plastic effect.Based on the effect of electropulsing on the dislocation mobility,the microscopic mechanism of electropulisng on the formability enhancement of the Ti-6A1-4V sheet was investigated.The results show that the thermal activation effect and athermal effect of electropulsing play an important role in dislocations movement.The electric wind force induced by athermal effect promotes dislocation motion.Compared with the influence of the thermal effect on the dislocation,the dislocation motion can be further promoted under the influence of athermal effect,contributing to the plasticity improvement of Ti-6A1-4V sheet.Electropulsing assisted single point incremental forming experiment was carried out.Based on the finite element simulation,the electric current field,electric field and temperature field distribution during electropulsing assisted single point incremental forming were obtained.The effects of frequency and peak current density on formability,forming force,friction coefficient,strain distribution,strain path,fracture mechanism and microstructure evolution were studied.The relationship between strain distribution and sheet fracture failure during incremental forming was investigated.The results show that the strain state at both groove ends is approximately biaxial tensile,and the strain state in the middle deformation region is plane strain.The major strain and thickness strain increase gradually with the increase of frequency and peak current density.The frequency and peak current density enhancement can significantly improve sheet formability.The mainly deformation mechanism of Ti-6A1-4V sheet during electropulsing assisted single point incremental forming is dynamic recovery below 400 ?.The elongation(3 grain can be observed under the combined effect of the electro-thermal effect and deformation.The fracture mechanisms of Ti-6Al-4V sheet during increment forming are brittle fracture at room temperature and ductile fracture under electropulsing.Electropulsing assisted single incremental forming and direct current assisted incremental single forming experiments were carried out to reveal the influence of thermal effect and athermal effect on sheet formability.It was found that when RMS current density was the same,electropulsing assisted single point incremental forming can obtain the higher formability and the relatively smaller forming force than direct current assisted incremental forming.It is validated the existence of athermal effect,and thermal effect plays a major role in the formability improvement of Ti-6Al-4V alloy sheet.The anisotropic behavior of electropulsing assisted single point incremental forming of Ti-6A1-4V sheet was studied.The strain distribution,texture evolution,crack growth behavior,fracture mechanism and microstructure evolution in different directions during electropulsing assisted single point incremental forming were investigated.The results show that the thichness strain differences reduce between the transverse direction and the rolling direction under electropulsing.It can lead to the similar thickness reduction along the two directions.The dynamic recrystallization occurred under electropulsing.The different deformation microstructures in each direction tend to be uniform.The texture results show that the electropulsing can weaken the(0001)base texture and change the(10-10)(11-20)non-base texture.The anisotropy is weakened under electropulsing,which leads to the formability improvement.The effect of texture on the crack propagation behavior of single point incremental forming was analyzed.The electropulsing decrease the texture strength along the transverse direction-strong texture,which delays the crack growth along the rolling direction.It causes the crack along the rolling direction at room temperature to the crack along the rolling direction and the small crack along transverse direction under electropulsing.The forming accuracy during electropulsing assisted single-point incremental forming of Ti-6A1-4V plate was studied.The effects of the water-cooling tool and the rigid tool on forming temperature,surface quality,dimensional accuracy,forming force,friction coefficient,forming height and microstructure were investigated.It was found that the water-cooling tool can reduce the friction,decrease the surface roughness and improve the surface quality of the workpiece significantly.The water-cooling tool has the same formability and the forming accuracy of the Ti-6A1-4V sheet as the rigid tool.Microstructure result shows that the softening mechanisms of the two process conditions are dynamic recovery and dynamic recrystallization,and the average grain size,grain distribution and grain morphology of the specimens formed by the water-cooling tool and the rigid tool are basically the same.It is explained that the formability of the Ti-6Al-4V sheet formed by the water-cooling tool and rigid tool are the same.Above all,the water-cooling tool can significant enhance the formability,improve the surface quality and dimensional precision during electropulsing assisted incremental forming.