Investigation On The Effect Of Pulsed Current On Deformation Behavior Of AZ31B Magnesium Sheets And Its Mechanism

In recent years,magnesium alloys gradually become the structural metals with the 3rd largest usage because of the extremely high ratio of strength to stiffness,excellent electromagnetic shielding,high conductivity and thermal conductivity,biocompatibility,which are widely applied in many industry,such as aerospace,automotive,electronics and medical.However,the independent slip systems of magnesium alloys are less than 5,and then they have a limited formability.Although,their plasticity can be improved at an elevated temperature,stemming from the activation of prismatic and pyramidal slip systems,hot deformation would cause thermal stress and oxidation,reducing the accuracy of manufacture.Then,a new type of technology is needed to process magnesium alloys,preferably.At the same time,some experts found that electricity can improve the plasticity of metals and reduce the loading,which is called as electroplasticity effect and has been studied by many experts.Meanwhile,some researchers directly made use of such phenomenon,and proposed many electricallyassisted metal processing technologies.In this paper,the influence of pulsed current on the deformation behavior and microstructure evolution of AZ31 B magnesium alloy was investigated,deepening the understanding on the related processing.Moreover,two novel electrically-assisted technologies were proposed.The research contents are shown below:First of all,the specimens with strain of 15% were annealed by pulsed current,and the microstructure evolution and dislocation annihilation were observed.The results show that pulsed current strengthens the migration of grain boundaries,and then the twin grains “spheroidize” to equiaxed grains,separating the coarse grains and refining the microstructure and homogenizes the initial microstructure.Moreover,pulsed current also strengthens the dislocation annihilation,and this effect is enhanced with the increase of peak current density and the decrease of frequency.Then,the deformation behavior and microstructure evolution were investigated by electrically-assisted tension and compression tests of AZ31 B magnesium sheets.It was found that current promotes the activation of prismatic and pyramidal slip systems,accelerates the evolution of dynamic recrystallization,and weakens the work hardening,which induces electroplasticity effect,weakens the twining evolution,and improves the mechanical symmetry.Furthermore,the intensity of electroplasticity effect was measured by the reduction rate of stress caused by current at the same temperature and strain rate,and studied the intensity of electroplasticity effect changes with the Z parameter.When the value of Z is high,the electroplasticity effect is relatively low,and strengthens with the decrease of Z;when Z reduces to a critical value,the electroplasticity effect strengthens dramatically because of the activation of prismatic and pyramidal slip,inducing a new texture;when Z is relatively low,the electroplasticity effect weakens with the decrease of Z.Moreover,the peak current density does not affect the deformation obviously,but the stress decreases with the increase of frequency.Finally,a mathematical model describing the affect of temperature,strain rate,strain and pulsed frequency on electroplasticity effect and the flow stress model for electrically-assisted tension were proposed.Finally,two new types of electrically-assisted forming processes,i.e.electricallyassisted blanking(EAB)and electrically-assisted sheets upset-extruding(EASUE),were designed,and the affect of pulsed parameters on these processes were also investigated.The results show that both of them improve the formability of AZ31 B magnesium sheets,which verifies the superiority of electrically-assisted forming processes and implies that introducing PC to processes has a promising application prospection.