Study On The Effect Of Rolling Routes On The Microstructure And Mechanical Properties Of AZ31B Magnesium Alloy Sheets

Magnesium(Mg)and its alloys are attractive as the light-weight structural materials and widely used in many industial fields,such as the aerospace,automotive and communication due to their high specific strength,high specific rigidity and select electromagnetic shielding capacity.At the same time,it is regared as “green metal structure materials in 21 th century”.However,Mg alloys have a closed-pack hexagonal structure leading to the insufficient indepedent slip systems to be activated.Thus,the Mg alloys sheets processed by rolling exhibite a strong(0001)basal texture,which leads to the limition of the secondary processing performance of Mg alloy sheets at room temperature.In studies,the mechanical properties of Mg alloy sheets is related to the microstructure characterizes,such as the grain size,grain orientation and basal texture,while,the rolling process parameters have an important relation with the evolution of its structure characteristics as well.Thus,improving rolling process and weakening the(0001)basal texture is the key to ameliorate Mg alloy sheets ductility and formability at ambient temperature.As studies reported,the introduction of shear strain in rolling can effectively weaken the basal texture and optimize the grain orientation,such as asynchronous rolling,equal-channel rolling,etc.,however,those processes have to face the problem that they cannot attain the requirements of industrial production.For this reason,this paper uses self-developed differential temperature rolling equipment to systematically study the relationship between roll temperature difference and shear strain.The influence of the roller's temperature difference on the mechanical properties of the Mg alloy sheets is analyzed via the uniaxial tensile test method.The optimic microscope(OM)and X-ray diffraction technology(XRD)are used to invesgiate the influence of corresponding roller's temperature difference on the evolution of microstructure and texture of sheets.In addition,considering that the change of the rolling route will affect the angle of shear stress,we further investigate the changing of shear stress based on conventional rolling and differential temperature rolling.And,the mechanical properties and microstructure changes of sheets rolled by conventional rolling and differential temperature rolling under different rolling routes are compared and studied.The conclusions are as follows:1.The differential temperature rolling process with the upper roller's temperature keeping constant and the lower roller's temperature decreased gradually is carried out on the commercial as-cast AZ31 B magnesium alloy.It was found that: compared with conventional rolling,with the decreasing of the temperature of the lower roller's,i.e,the increasing temperature difference between the upper and lower rollers',the enlongnation of the as-rolled samples becomes lower and lower,and the number of twins becomes more and more.However,after annealing,there are few numbers of twins can be observed in the low temperature rolled sheets.Moreover,the twins disappear in the high-temperature-rolled sheets and the grain size becomes fine and uniform.Its elongation is slightly higher than that of the conventional rolled sample,which is increasing from 22.6% to 26.5%.But,its yield strength and tensile strength are reduced to 166.0and 279.1 MPa,respectively.2.The finding of the investigation of rolling routes:1)Two rolling routes,forward rolling(A route)and turned reversed rolling(D route),are used for commercial annealed AZ31 B magnesium alloy sheets via conventional rolling.The elongation of the turned-reversed rolled sheets(35.7%)is about 10% higher than that of the conventional rolled sheets(26.0%),while the strength of the turned-reversed rolled sheets almost unchanged.It is because of that,during rolling deformation,the degree of dynamic recrystallization of the turned-reversed rolled sheets is about 1.5 times as large as that of the conventional rolled sheets,which is increasing from 59.2% to 85.9%.In addition,the basal texture of the turned-reversed rolled sheets is weaker than that of the conventional rolled sheets.2)Four different rolling routes,forward rolling(A route),reversed rolling(B route),turned rolling(C route)and turned reversed rolling(D route)are carried out on the commercial as-cast AZ31 B magnesium alloy sheets under differential temperature rolling.Our results show that the elongnation of the annealed sheet via changing rolling routes under high temperature differential rolling is significantly improved.Compared with the original sheets,the elongation of the sample rolled under the C rolling route is increased from 14% to 33.5%.It is because of that C rolling route can achieve a more homogeneous cumulative shear strain compared with other rolling route,which contributes to that the more twins systes are activated and the more twin-twin boundaries are obtained.Thus,it provides more nucleation positions for the subsequent stastic annealing process and promotes the transformation of grain orientation,leading to a more homogeneou structure,weakened basal texture and the improvement of plasticity of the sheet.