Study On Microstructure And Mechanical Properties Of AZ Series Deformed Magnesium Alloys Processed By High Density Electrical Pulse Treatment

Magnesium and its alloy,as the lightest metal structural material,have gained great interest for their potential use in the fields of aerospace,automobile,transportation,electronics and biomedicine.However,compared with steel and aluminum alloy,the industrial application of magnesium alloy is limited due to the insufficient strength and poor forming ability.Although conventional plastic deformations can improve its mechanical properties to some extent,there still exist some problems such as strong basal textures,imbalance of strength and plastic property,etc.In this dissertation,from the perspective of grain refinement,different plastic deformation methods combined with electropulsing treatment（EPT）are adopted to refine and regulate the microstructure of deformed magnesium alloy in order to improve its comprehensive mechanical properties.The microstructure and texture evolution under different deformation conditions and electropulsing treatment were characterized using metallographic analysis,electron microscopy（SEM/TEM）,electron backscatter diffraction（EBSD）and X-ray diffraction（XRD）.Meanwhile,mechanical properties of magnesium alloy sheets were investigated with the tensile tests.Based on these experimental data,the influence mechanism of EPT on recrystallization behavior,dislocation evolution and phase precipitation of magnesium alloy was discussed.The main research contents and results are as follows:Firstly,AZ61 alloy with high dislocation density was prepared by equal-channel angular pressing（ECAP）with 160°channel angle.Through comparative analysis,it is found that a higher dislocation density could be accumulated in magnesium alloy by multi-pass ECAP at variable temperatures.The maximum value of dislocation density is32.5×10¹⁴ m^-2 in AZ61 alloy with a microstructure composed of homogeneous fine grains and large amount of deformation twins.Secondly,the microstructure and mechanical properties of the ECAPed AZ61 alloy treated by high density pulse current were studied.The results show that the completely recrystallized structure of AZ61alloy is obtained after EPT with pulse width of 25μs and processing time of 10 min.The average grain size was approximately 0.7μm,and the basal texture strength with maxima texture index was effectively weakened to 3.58.Furthermore,EPT can promote the spheroidization and dispersion of the precipitates in AZ61 alloys.The best comprehensive mechanical properties were obtained by the ECAP coupled with EPT method.And the yield strength,tensile strength and elongation were 330MPa,448 MPa and 15.5%,respectively.Furthermore,the microstructure evolution and mechanical properties of AZ31,AZ61 and AZ91 alloys with different aluminum content during ECAP and EPT processing were investigated.The solid solution atoms and the precipitated Mg₁₇Al₁₂ phase in the grains and grain boundaries play important roles in pinning dislocations,which is conducive to the accumulation of higher dislocation density.However,when the content of aluminum is too high,the deformation of magnesium alloy becomes difficult and should be carried out at higher temperatures,which will result in the occurring of dynamic recrystallization.Therefore,compared with AZ31 and AZ91,AZ61 alloy with medium aluminum content has accumulated the highest dislocation density after ECAP.In addition,AZ91 alloy with bimodal microstructure was prepared by low-temperature slow rate extrusion combined with EPT,and the corresponding yield strength,tensile strength and elongation are 463 MPa,573 MPa and 5%,respectively.Finally,by studying the micro mechanism of dislocation movements and the electromigration mechanism of atoms and vacancies,the thermodynamic and dynamic processes of high-density pulse current exerting on the deformed magnesium alloy were analyzed.The influence mechanism of EPT on static recrystallization behavior and recrystallization nucleation rate in magnesium alloy was elucidated.To promote dislocation movements and induce static recrystallization,the driving force of pulse current acting on dislocations must be greater than the resistance caused by dislocation stacking and entanglement in deformed alloy.In addition,the electromigration induced by pulse current greatly promotes the recrystallization process of wrought magnesium alloy.The static recrystallization of magnesium alloys was induced by both thermal and athermal effects during EPT processing.However,regarding the high density pulse current used in this study,the athermal effect has a greater contribution to static recrystallization.