Research On Microstructures And Properties Of High Strain Rate Rolled Mg-6Zn-1Mn Magnesium Alloy Sheets With Pre-rolling

Mg-Zn-Mn alloy has advantages of low cost,good age hardening effect and excellent corrosion resistance,which occupies an important position in wrought magnesium alloy.In recent years,the high strain rate rolling(HSRR)technology has been proposed by many researchers as an effective way to manufacture magnesium sheet since it can induce strong dynamic recrystallization and dynamic precipitation,thus achieve grain refinement and nanometer precipitates to enhance the mechanical properties of magnesium sheet.On this basis,the pre-rolling process is imposed on ZM61 magnesium alloy prior to the HSRR.The effect of pre-rolling on the original microstructure of ZM61 and finally on the dynamic recrystallization and dynamic precipitation during HSRR are investigated,in order to fabricate magnesium sheet with advanced comprehensive mechanical properties by pre-rolling and HSRR technology.In this study,the original microstructure is studied with the pre-rolling strain of 20%,30%and the pre-rolling temperature from 300? to 420?.The results show that the original microstructure of pre-rolled ZM61 alloy are composed of twin and fine grains,the proportion of this two structure fluctuate as the pre-rolling condition changes,the twin ratio raises when decreasing the pre-rolling temperature or increasing the pre-rolling strain and vice versa.Dynamic precipitation is occurred during pre-rolling,this procedure is prior to dynamic recrystallization but after twin.The ZM61 magnesium sheets with or without pre-rolling are high strain rate rolled at 300 ?,the effect of different original microstructure on dynamic recrystallization and dynamic precipitation is investigated,and the interactions between dynamic recrystallization grain and dynamic precipitates is clarified.The results indicate that dynamic recrystallization is significantly promoted by pro-rolling,the original twin and fine grain boundaries provide the preferrential nucleation sites for dynamic recrystallization grain,increase the nucleation rate.The average grain size of dynamic recrystallization grain is refined to 0.5?m of the pre-rolled samples from 1.3?m of the unpre-rolled specimens,the pre-rolling temperature exhibits stronger promotion effect than pre-rolling strain.Besides,the density of dynamic precipitates is increased by pre-rolling,for the pre-rolling plus the HSRR can broke the Mn particles to a higher extent,thus provide more nucleation sites for dynamic precipitates,the twin intersections inhibit dislocation movement,which also provide more driving force for the nucleation of dynamic precipitates.The high density of precipitates pinning the dynamic recrystallization grain boundaries and restrict the migration of grain boundary and the growth of dynamic recrystallization grain.The ZM61 alloy sheets fabricated under 20%pre-rolling strain at 350? and high strain rate rolled at 300? exhibits the best mechanical properties with the ultimate tensile strength,yield tensile strength and elongation of 397MPa,289MPa and 15%respectively.The advanced mechanical properties are contributed to the combined effect of grain refinement strengthening and nanometer precipitation hardening.