Effect Of Initial Texture On Room Temperature Forming Property And Microstructure Evolution Of AZ31 Magnesium Alloy Sheet

As the lightest commercial metal structure materials,deformed magnesium alloy has a series of excellent properties,such as light specific gravity,high specific strength and specific stiffness,good machining performance,strong vibration damping effect and strong electromagnetic shielding ability.Therefore,it has a wide range of applications in the automotive field,aerospace and 3C products.However,the hexagonal close-packed crystal structure of deformed magnesium alloy sheet leads to the limited independent slip systems at room temperature,and it is easy to form a strong basal texture in the processing,which leads to the poor deformation property at room temperature and restricts the large-scale application.Therefore,weakening the basal texture becomes an important way to improve the deformation property at room temperature.Based on our preliminary experimental results,the texture type of AZ31 magnesium can be changed by equal channel angular rolling-continuous bending deformation and recrystallization annealing process.Previous basal texture is transformed to bimodal non-basal texture deflecting from ND to RD around 45°,which makes its formability significantly improve at room temperature.However,the microstructure evolution and plastic deformation mechanism of the AZ31 magnesium alloy sheet with the non-basal texture in the subsequent forming processes such as room temperature Erichsen testing and drawing are unknown.Therefore,this paper chooses the different texture(strong basal texture and bimodal non-basal texture)of magnesium alloy sheet to systematically study different microstructure and texture evolution in the processing of Erichsen testing and deep drawing at room temperature by metallographic test and EBSD.Meanwhile,we analyze the influence of different stress state to sliding and twin activation,and aim to clarify the plastic deformation mechanism differences and its reason between basal texture and bimodal non-basal texture.Those will promote the wide application of magnesium alloy sheets with high formability.The main experimental conclusions are as follows:(1)The IE values of basal and non-basal texture sheets are 4.8mm and 7.6mm,respectively.For the basal texture sheet,at the PS of the Erichsen process(PS=1mm),there are no obvious twin in the upper and middle regions,while many twins appear on the lower surface.When PS increases to 2mm,twins begin to appear in the upper and middle regions,but more twins disappear on the lower surface,showing an obvious detwinning phenomenon.Detwinning phenomenon is mainly caused by the stress state of "three-way compression" on the lower surface at PS=1,and the stress state of "two pulls and one pressure" at PS=2,and the change of loading direction leads to detwinning.Continuing to form,the number of twins in each region gradually increase.For non-basal texture sheet,twins form in the upper,middle and lower regions at PS=1mm,and there were relatively few tensile twin variants.When PS increases to 2mm,the tensile twin variants activated in some grains become complex.Compared to basal sheet,the twins on the lower surface do not decrease.It is because most of the grains in the non-basal texture sheet deviate from the ND,and it is conducive to the formation of twins before and after the stress state changing,no detwinning phenomenon was observed.With the increasing of PS,the number of twins increases gradually,and the grain orientation gradually turns to TD.Finally,TD texture is formed in all regions.(2)Deep drawing tests are carried out on basal texture and non-basal texture sheets at room temperature,and the limiting drawing ratios are 1.3mm and 1.6mm,respectively.The study on the stress state and microstructure evolution of different regions of the drawing parts shows that:(1)the bottom of the drawing sample is basically not affected by external forces,so the microstructure of the basal texture and non-basal texture sheets have no obvious change.(2)In the shoulder area of the drawing samples,a small number of twins appear on the upper and lower surface of the basal texture sheet,while more twins appeared on the upper surface of the non-basal texture sheet,and the area and number of twins were significantly larger than that of the basal texture sheet.The difference of twins are mainly because the stress state of "two pulsations and one pressure" of the grains with the basal orientation is not conducive to the activation of twins,while the grains with the non-basal orientation deviate from ND direction ±40°,which is conducive to the activation of twinning.Due to the large number of twins in the non-basal texture sheet,the texture changes from the original bimodal non-basal texture to the one deflecting to TD.(3)In the wall area of the deep drawing,firstly,the sheet in the blank holder area bears the stress state of “ two pressure and one pull”,which is easy to activate a large number of twins.Then,the sheet bears the stress state of "a pull a pressure" and bending deformation.The stress state in the wall area is similar to uniaxial tensile stress.Detwinning occurs in part of the twins,and parts of grain orientation change from the original RD-split bimodal non-basal orientation into base orientation with c-axis//ND.