| In this work,high strain rateforming mechanism of AZ31 magnesium alloy is analyzed.The microstructure evolution during high strain rate multidirectional forging and high strain rate rolling is explored,the effects of forging temperature and pass deformation on the microstructure and properties of forged alloy are studied,the effects of rolling temperature and initial grain orientation on the microstructure and properties of rolled plate are also explored.The main conclusions are as follows:(1)In theprocess of high strain rate plastic deformation,the grains of alloy are refined rapidly due to the dynamic recrystallization induced by twin.The main twin mechanismsare{101 2} tensile twin and {101 1}-{101 2} secondary twin.(2)During high strain rate forming,due to the slight influence of temperature on the twin,the uniform and fine recrystallization structure can be obtained between 250-200 ℃,and the average grain size increases slightly with forging temperature,which is in the range of7.07-9.99 μm.The intensity of the base texture decreases with the increase of forging temperature.then the forging temperature is lower than 300 ℃,the polar axis deviates from the center of the polar graph.then the forging temperature is higher than 350 ℃,the texture is found in the form of double peak base texture.The strength of alloy decreases with the increase of forging temperature,and the elongation increases first and then decreases with the increase of temperature,but the strength and elongation of forging alloy are higher than 300 MPa and 20% respectively.It is proved that the high strain rate multi-directional forging can produce magnesium alloy with excellent performance in a wide range of temperature.(3)Pass strain and cumulative strain have significant effects on high strain rate multi-directional forging.then the pass strain is too small(<0.1),the twin density is low and the twin lamella is thick,the recrystallization grain size is large.then the pass strain is more than 0.16,the twin density is high and the twin lamella is thin,the grain is remarkably refined.then the cumulative strain is less than 0.96,the recrystallization of the alloy is incomplete.then the cumulative strain is between 0.96 and 1.2,uniform and fine recrystallization structure is obtained.then the cumulative strain is beyond 1.2,the recrystallized grains grow abnormally.The intensity of the basal texture increases with the increasing pass strain.then the pass strain is small(<0.16),the bimodalbasal texture is observed.then the pass strain is large(>0.22),the polar axis deviates from the center of the polar graph.In order to obtain excellent comprehensive mechanical properties,the pass strain is suggested to be controlled in the range of 0.16-0.35,and the cumulative strain should be controlled in the range of 0.96-1.2.(2)The average grain size of the rolled plate increases with the increase of rolling temperature,due to the smallinitial grain size of the sheet,thegrain size increasesslightly with the increase of temperature.The average grain size of plate rolled at 250-200 is in the range ℃of 6.97-8.13μm.The rolled plate presents characteristics of typical sheet texture.The intensity of the basal texture first increases and then decreases with the increase of temperature,but the fluctuation of the extreme value is not obvious.The strength of the plate decreases with the increase of rolling temperature,and the elongation first increases and then decreases with the increase of rolling temperature.However,the plate has obtained excellent comprehensive mechanical properties in a wide range of rolling temperature,the tensile strength and elongation are higher than 315 MPa and 25% respectively,indicating that high strain rate rolling can be used to prepare magnesium alloy plates with stable mechanical properties in a wide range of temperature.(5)Due to the special forming mechanism of high strain rate rolling,plates with different orientations can form strong basal texture under small deformation,and the initial grain orientation has little effect on the grain size of rolled plates.However,the effect of initial grain orientation on the texture of rolled plate is obvious.The basal texture of the plate with hard oriented becomes stronger after rolling,the plate with soft oriented plate obtains weak basal texture after rolling.In order to ensure the subsequent forming property of the plate,the soft oriented sheet should be selected for high strain rate rolling. |
