Research On Formability Of AZ31Magnesium Alloy Sheet In Magnetic Pulse Forming

Magnesium alloy is valued to used in industry, due to it has many specialproperties such as low density, high specific strength, etc. The warm forming wasemployed to form the magnesium alloy in the previous studies and applications. Theformability of magnesium alloys at room temperature is low because they have ahexagonal close-packed crystallographic structure with limited number of slipsystems. Magnetic pulse forming is high speed forming process, which cansignificantly improve the formability of metal material especially for the materialswith poor formability at room temperature. AZ31magnesium alloy commonly usedin industry is invetigated by the magnetic pulse forming in this paper. The forminglimit of AZ31magnesium alloy of magnetic pulse forming, the formability andforming rule of magnetic pulse dynamic driving, micromechanism of AZ31magnesium alloy in magnetic pulse and magnetic pulse dynamic driving and thedeformation behaviours of typical AZ31magnesium alloy shell of magnetic pulseforming are discussed and analysed, respectively.The forming limit diagram of AZ31magnesium alloy sheet of magnetic pulseforming is established by analyzing the three typical strain states namely uniaxialtension, plane strain and equi-biaxial tension. In order to complete uniaxial tensilestrain state, the specimens (same as that of quasi-static uniaxial tensile tests) arebulging by approximate rectangular flat single spiral coil. The uniform pressurefrom uniform coil is acted on the strip workpiece, which is used to achieve the planestrain state. Equi-biaxial tensile strain state is implemented by electromagnetic freebulging the square workpiece with circle flat spiral coil. Compared with quasi-staticforming limit accordingly, the results show that the forming limit of AZ31magnesium alloy sheet of magnetic pulse forming is remarkablely improved.From the systematic investigation of forming limit, it shows that theformability of AZ31magnesium alloy sheet of magnetic pulse forming is enhancedbecause of inertial effect. In order to strengthen the inertial effect, the aluminumalloy or cupper sheets with higher conductivity are put between coil and AZ31sheet.The formability and forming rule of AZ31magnesium alloy sheet in magnetic pulsedynamic driving are deeply researched. Due to the strength of inetial effect, theforming limit of AZ31magnesium alloy sheet is obviously increased compared withthe case without driving. Meanwhile, the changes of velocity, strain rate and stress- strain of typical positions in magnetic pulse dynamic driving are presented.The fracture areas of quasi-static uniaxial tension, quasi-static equi-biaxialtension, magnetic pulse uniaxial tension, magnetic pulse equi-biaxial tension andmagnetic pulse dynamic driving with2mm Al driver sheet were analyzed. Theresults show that the fractured sample for the quasi-static condition clearly displaysbrittle rupture. However, for magnetic pulse forming with the Al driver sheet, thefracture surfaces have plastic dimples that exhibit typical ductile rupture. Thedeformation mechanisms of quasi-static uniaxial tension, magnetic pulse uniaxialtension were analyzed by EBSD method. For quasi-static uniaxial tension, basalslips should be the main deformation mode. However, due to drastic deformation,grain is obviously refined in magnetic pulse uniaxial tension and twinning is morethan the case of quasi-static condition. The deformation mechanisms of quasi-staticequi-biaxial tension, magnetic pulse equi-biaxial tension and magnetic pulsedynamic driving were also analyzed by EBSD method. The results show that basalslips dominate in the quasi-static equi-biaxial tension and due to more energy fordeformation is achieved in magnetic pulse equi-biaxial tension and magnetic pulsedynamic driving, more slips and {10-12} extension twinning (it is the most obviousin magnetic pulse dynamic driving) are activatedThe3D finite element model of typical AZ31magnesium alloy shell ofmagnetic pulse forming is established by using the multi-physics coupling fieldfinite element analysis software ANSYS. The establishment of3D finite elementmodel can overcome the limitation of2D model which was used to analyze the axialsymmetry structure in magnetic pulse forming.3D finite element model can beemployed to simulate the forming of non-axial symmetry structure in magneticpulse forming.The uniform coil is used to deform the shell and the change rules ofmagnetic field, magnetic flux density, magnetic pressure are analyzed anddiscussed.The deformation characteristic of sheet and die collision is investigated inAZ31magnesium alloy shell of magnetic pulse forming.The magnetic pulse forming for AZ31magnesium alloy shell with three typicalbottom corner (R=30mm, R=15mm, R=2mm and R=8mm) is investigatedexperimentally. The effects of process parameters for forming shell are discussed.Due to AZ31magnesium alloy sheet with high velocity impacts with die, thedirection of bottom velocity is opposite. Thus, the hollow on the bottom of shell isoccurred. In order to overcome the defect of hollow, the effect of driver sheet isinvestigated. The effects of thickness of driver sheet, numbers of driving forformimg shell of different bottom corner are analyzed. Due to arcing and swell for uniform coil, the separated uniform coil viz separated the outer channel andprincipal coil is proposed. It can obviously reduce the contact area between outerchannel and sheet, increase the distance between up and down lead and remarkablyimprove the life of uniform coil.