Study On Microstructure Evolution And Mechanical Property Of Mg-6Al-Zn Alloy Treated By Electrical Pulse Treatment

Magnesium alloys have the advantages of high specific strength,low density,easy recycling and abundant resourcces and has been widly used in automobile,aerospace,weapons and equipment,3C industry and other felds.However,magnesium has a close-packed hexagonal structure and strong basal texture after deformation,resulting in poor room temperature formability,which a challenge that must be overcome in industrial applications.As a common means in the industrial production of magnesium hot extrusion can not only refine the grain size,but also improve the mochanical properties of the materials.Gencrally,long time aging treatmentis needed to further improve the proproperties,which greatly increases the production cost.EIectrical pulse treatment(EPT)is a new processing technology,which can improve the microsturcture and mechanical properties of the alloy by applying pulse current to the deformed material,and is very suitable for the subsequent treatment of magnesium alloy sheet or bar.In this paper,the static recrystallization,second phase and microstructure evolution of extruded(EX-AZ61),cold-rolled(CR-AZ61)and aged(T5-AZ61)AZ61 magnesiumalloys were studied by EPT with different parameters,and the mechanical properties were tested,the thermodynamic and dynamic processes of electric pulses acting on magnesium alloys were analyzed,and the matrix structure and second phase precipitation of AZ61 magnesium alloys and the evolution mechanism discussed.The main conclusions are as follows:(I)After EPT,the average grain size of EX-AZ61 magnesium alloy is reduced from 15μm to to 11μm,and that of CR-AZ61 magnesium alloy form 7 μm to 5.3 μm.EPT promotes rapid recrystallization of EX-AZ61 magnesium alloy,suppresses Joule heat effect,effectively refines grain size,and optimizes the mierostructure of AZ61 magnesium alloy.(2)EPT has&significant effect on the microstructure of EX-AZ61 magnesium alloy strip.which greatly promotes the spheroidization and dissolution of β-Mg17Al12 phase in a short time,and obtains a spherical second phase with a particle size of about 1 μm;Compared with EXAZ61 strip,spheroidized β phase is beneficial to maintain good elongation and increases the tensile strength from 270 MPa to 319 MPa,significantly improveing the tensile strength of the alloy.At the same time,the mechanism of rapid spheroidization and dissolution of β phase in EPT process is as follows:EPT promotes the dissolution of β phase by lowering the thermodynamic barrier,that is,β→α phase transition;Due to the coupling of thermal effect and non-thermal effect of EPT,EPT significantly increase the diffusion flux of Al atoms,changes the concentration gradient,and further induces the spheroidization and dispersion distribution of βphase in EX-AZ61 magnesium alloy.With the increase of electric pulse parameters,the thermal effect increases,and the β phase gradually dissolves into the matrix.(3)EPT promotes the rapid increase of high-angle grain boundary fraction of CR-AZ61 magnesium alloy and accelerates its recrystallization behavior.EPT increases the nucleation rate of recrystallization in the alloy,reduces the energy barrier of recrystallization and increases the atomic diffusion flux.Under the non-thermal effect of electric pulse,recrystallization occurred inside the alloy;The kinetics of recrystallization depends on the nucleation rate and growth rate of recrystallization nuclei:EPT accelerates the exchange of vacancies and single atoms,thus promoting the migration of low-high angle grain boundaries,which not only increases the nucleation rate,but also reduces the growth rate of most recrystallization grains.(4)EPT reduces the base texture strength of CR-AZ61 magnesium alloy from 22.5 to 10.3,which weakens the base texture,reduces the anisotropy of CR-AZ61 magnesium alloy rolled sheet and improves its elongation at break.EPT promotes the recrystallization behavior of CRAZ61 magnesium alloy,and the recrystallization grains have random orientation,thus reducing the basal texture strength.(5)EPT can regulate β phase precipitation and re-dissolution of T5-AZ61 magnesium alloy,thus improving the mechanical properties of AZ61 magnesium alloy.