Study On The Effect Of Pulse Current And Its Direction On Superplastic Deformation Of AZ31 Magnesium Alloy

Magnesium alloys are widely used in electronics,aerospace,automotive,and military industries due to their high specific strength and stiffness.However,their poor room temperature is difficult to form because of their unique close-packed hexagonal structure.Therefore,the superplasticity and formability of magnesium alloys at high temperatures have attracted extensive attention and research among scholars worldwide.To further enhance their superplastic properties,researchers have developed various auxiliary forming processes,among which pulse current-assisted superplastic forming is one of the most effective methods.However,the mechanism by which pulse current improves the superplasticity of materials is not yet fully understood,and the working principles are still unclear.Therefore,based on the traditional single-directional current-assisted mode,this thesis conducts a study on the superplastic forming properties of AZ31 magnesium alloy assisted by pulse current under various current directions.The goal is to further clarify the effect of pulse current on the superplastic deformation of materials and enrich the ideas for expanding the forming process.Accordingly,relevant experiments were designed,mechanism research were conducted,and carried out a preliminary process exploration.This thesis designed high-temperature tensile tests of AZ31 magnesium alloy under different current intensities and directions,obtaining stress-strain curves under various conditions such as current density and electron movement direction.The results show that the flow stress of the material decreases with increasing peak current intensity,while the elongation rate increases with increasing current intensity.Regarding current direction,when the direction of electron movement is consistent with the direction of material deformation,the effect of current on the superplasticity of the material is most significant,while the effect is weaker when the current direction is perpendicular to the deformation direction.High-temperature swelling tests of AZ31 magnesium alloy were conducted under two-dimensional current field conditions.By changing the arrangement of electrodes to match the current flow pattern with the material deformation,better forming results were obtained.Compared with no current,the high-to-diameter ratio of the AZ31 magnesium alloy swelled part increased from 0.45 to 0.56,and AZ31 magnesium alloy parts of different shapes were formed using this method with good forming quality.Microstructure analysis of AZ31 magnesium alloy samples under different deformation conditions was carried out using methods such as SEM,EBSD,and TEM.The results show that pulse current,especially the pulse current with a favorable orientation,not only enhances the motion ability of dislocations at grain boundaries,strengthens the slip effect of grain boundaries,but also promotes grain rotation.Moreover,it can promote recrystallization and to some extent refine grains.In addition,the pulse current with a favorable orientation also reduces the adjacent radius of cavity nucleation,reducing the tendency of material fracture caused by cavity aggregation.Based on the experimental data of superplastic tensile tests of AZ31 magnesium alloy under different current conditions,a constitutive equation including parameters such as current density and direction was constructed.The equation was applied to finite element numerical simulation of tensile deformation to verify its correctness and effectiveness.