Research On Microstructure Evolution Of Pre-rolled Mg-6Al-1Zn Magnesium Alloy During Electric Pulse Treatment

Magnesium alloy is considered as the lightest metal structural material for industrial application because of its high specific strength and stiffness,and it has great application potential in many manufacturing fields,such as transportation,aerospace.But,due to HCP crystal structure,its plastic processing ability at room temperature is poor.Obtaining high-performance magnesium alloy structural parts requires complex processing technology and high cost,which leads to the fact that magnesium alloy can’t be widely used in industry at present.According to research reports,reasonable use of electrical pulse treatment of magnesium alloy can effectively optimize the microstructure,improve and enhance its plastic processing ability at room temperature.Compared with traditional magnesium alloy heat treatment,this method is more efficient and energy-saving,which makes it possible to replace traditional heat treatment with electric pulse assisted metal processing.However,at present,there is no systematic theoretical system for the influence law and mechanism of electrical pulse treatment on magnesium alloy microstructure.Especially,the key scientific problems such as the recrystallization behavior and characteristics of twins under the action of pulse current are still unclear,which limits the application and development of electrical pulse-assisted metal processing technology,and it is urgent to carry out in-depth systematic research to provide theoretical support for the regulation of magnesium alloy microstructure.In view of the above problems,this paper researched the microstructure evolution of pre-rolled Mg-6Al-1Zn magnesium alloy during electric pulse treatment.In which,the evolution process of recrystallization and precipitation phase of hot-rolled AZ61 magnesium alloy during electric pulse treatment and isothermal heat treatment to different time was explored,and the synergistic effect of non-thermal effect and thermal effect of electric pulse treatment on recrystallization evolution and precipitation phase evolution of hot-rolled AZ61 magnesium alloy was revealed.And the relationship between recrystallization evolution and precipitated phase evolution under electric pulse treatment was studied,and the mechanism of electric pulse treatment on microstructure evolution of hot rolled AZ61 magnesium alloy was further revealed.Furthermore,the static recrystallization process of twin in AZ61 magnesium alloy rolled at low temperature under quasi-in-situ electric pulse treatment was studied,and the static recrystallization mechanism of characteristic twin and interaction of characteristic twin under electric pulse treatment was explored to reveal the static recrystallization characteristics of twin under electric pulse treatment.The main conclusions are as follows:(1)Electric pulse treatment(voltage as 1.2 V,current density as 85 A/mm2,frequency as10 k Hz)can improve the grain growth rate of hot rolled AZ61 magnesium alloy.Under the same temperature condition of 300°C for 300 s,the average grain size of the sample after electric pulse treatment and isothermal heat treatment is 17.1μm and 6.1μm,respectively,and the grain growth rate under electric pulse treatment is more than twice that under heat treatment.It is revealed that the fundamental reason for the difference in grain growth rate between the two treatments is the difference in precipitated phase fraction,not the difference in recrystallization mechanism.(2)The electrical pulse treatment(parameters such as(1))promoted the dissolution of Mg17Al12 phase which precipitated during hot rolling process in AZ61 magnesium alloy.Under the same temperature condition of 300°C,the Mg17Al12 phase in the microstructure is dissolved in a large area after being treated by electric pulse for 300 s,while the Mg17Al12 phase fraction in the sample after isothermal heat treatment for 900 s is basically unchanged.By analyzing the diffusion kinetics of the two treatments,it is revealed that the total diffusion flux J in the tissue is much larger than that of the heat treatment by increasing the non-thermal effect flux Ja and enhancing the thermal diffusion flux Jt at the same time,which is the fundamental reason for the obvious evolution difference of precipitated phases.(3)Under electric pulse treatment(voltage as 1.2 V,current density as 54 A/mm2,frequency as 10 k Hz),{10-12}-{10-11} secondary twin and {10-11} compression twin are effective recrystallization nucleation points in AZ61 magnesium alloy rolled at low temperature,and recrystallization will occur preferentially in the early stage of electric pulse treatment.{10-12} tensile twins will not undergo recrystallization and nucleation,but will be consumed and disappeared by the surrounding grains with growth advantages,but the interaction interface is a favorable nucleation site for recrystallization.The difference of grain boundary distortion is the fundamental reason for the difference of nucleation position of twin recrystallization.(4)By analyzing the growth process of twin characteristic twin and recrystallization grain at the interaction between characteristic twin and twin in AZ61 magnesium alloy after low temperature rolling under quasi-in-situ electric pulse treatment(parameters such as(3)),it is revealed that the newly nucleated recrystallization grain has obvious directionality during growth.The newly nucleated recrystallized grains will preferentially grow along the twin boundary direction of {10-12}-{10-11} secondary twin and {10-11} compression twin,and will turn to the {10-12} tensile twin direction only after completely replacing the secondary twin and compression twin.This is because the electric pulse treatment accelerates the dislocation movement,which makes the grain boundary of the grown grain migrate to the high strain energy region,that is,the high KAM region.