Study On Plastic Behavior And Microstructure Evolution Of AZ91 Magnesium Alloy In Rolling Process With Heated Roller

Magnesium alloy has poor plastic deformation at room temperature,and the sheets and strips are prone to crack during rolling,which greatly affect the application prospect of magnesium alloy.Preheating of the magnesium alloy before rolling and preheating of the roller to maintain the deformation temperature are essential for improving the rolling deformation ability of the magnesium alloy sheets.In this thesis,AZ91 magnesium alloy is taken as the research object.The high-frequency induction heating technology is used to heat the roller in real time,and the deformation temperature of the strip is ensured by the heat output of the roller.Based on the tests of the microstructure and mechanical properties of the strip after rolling,explore the microstructure evolution and plastic behavior of AZ91 magnesium alloy after rolling at different roller surface temperatures,passes and reductions.The main research contents and conclusions are as follows:1.The AZ91 magnesium alloy heated roller rolling experiment was carried out under different roller surface temperatures of 200-350?,different initial conditions and rolling passes,and the room temperature tensile test of cast-rolled and as-extruded AZ91 magnesium alloy strips rolling at different roller surface temperatures and passes was carried out.When the temperature of the roller surface is low,a large amount of twins appear in the structure.As the temperature of the roller surface increases,significant dynamic recrystallization occurs in the structure.As the rolling pass increases,the increase of plastic deformation further promotes the occurrence of dynamic recrystallization;when the roller surface temperature is 350?,the cast-rolled and as-extruded AZ91 magnesium alloy undergoes five passes and a total reduction of 80%,the strip undergoes sufficient dynamic recrystallization.The microstructure is a finely distributed equiaxed crystal,and the average grain size is refined from the original microstructure of 43.7 ?m and 97.3 ?m to 3.4 ?m and 3.2 ?m,respectively.2.With the increase of the roller surface temperature,the(0002)base texture strength of as-extruded alloy strip structure is gradually decreased.When the roller surface temperature is the same,the increase of the rolling pass causes the(0002)base texture strength to increase.After five passes at the roller surface temperature of 350?,the maximum polar density of the(0002)base texture in the as-extruded structure was reduced from 14.97 of the original sample to 5.7.A composite process of extrusion+ rolling can obtain a magnesium alloy sheets and strips with a weak base texture.After rolling at a roller surface temperature of 350? with five passes of a total deformation about 80%,the strip has excellent comprehensive mechanical properties.The yield strength,tensile strength and elongation of the cast-rolled and as-extruded AZ91 magnesium alloys are respectively 185 MPa,333 MPa,29.5%and 193 MPa,331 MPa,31.3%.3.A heated roller rolling experiment was carried out with AZ91 magnesium alloy in a single pass with different reductions,the roller surface temperature was 350?,and the initial states were cast-rolled and as-extruded.The room temperature tensile properties were carried out.When the reduction is 30%,the deformation is mainly basal and non-base slips;when the reduction is 40%-50%,a large amount deformed twins occur in the structure,twins coordinated deformation;when the single pass reduction is further increased to about 70%,the structure undergoes complete dynamic recrystallization,the grains are remarkably refined and evenly distributed,and the average grain size of the cast-rolled state is about 4.9 ?m,the as-extruded is about 6.1 ?m.After the roller surface temperature is 350? with large reduction of 72%under a single pass rolling,the cast-rolled and as-extruded AZ91 magnesium alloy both exhibit a weak(0002)basal texture with a maximum polar density of 4.76 and 5.11.4.When the roller surface temperature is 350?,with the increase of the reduction,the mechanical properties of the strip after rolling have been significantly improved.After about 72%rolling under a single pass,the cast-rolled and as-extruded AZ91 magnesium alloys exhibit good comprehensive mechanical properties,the yield strength,tensile strength and elongation are 172MPa,319 MPa,21.2%and 177 MPa,317MPa,21.9%,respectively.In short,the heated roller rolling process effectively guarantees the deformation temperature during the rolling process of magnesium alloy strip and improves the plastic deformation ability of the magnesium alloy,through the setting of different rolling process parameters,it is possible to obtain a thin sheet or strip with excellent comprehensive mechanical properties.