Research On High Strain Rate Rolling And Its Microstructure And Properties Of AZ31 Magnesium Alloy

Magnesium alloy has many advantages such as low density,high specific strength and high specific rigidity.Rolling is widely used to prepare large-size magnesium alloy sheets,but conventional rolling has the problems of low production efficiency and relatively high cost.For the problem of conventional rolling,the high-strain rate rolling,which is high efficient and short process,has been proposed to produce the fine-grained magnesium alloys.High-strian rate rolling can achieve large plastic deformation by single pass.The efficiency of rolling is obviously improved.In this paper,the macro and micro model of high strain rate rolling of AZ31 magnesium alloy is constructed and the macro and micro simulation is carried out.The effect of microstructure on edge crack was studied,the formation mechanism of edge crack was revealed,and the relationship between microstructure and properties in high strain rate rolling was analyzed.The main conclusions of the paper are as follows:(1)The macro finite element model and micro recrystallization model are built.The macro element historical loading and micro recrystallization model are coupled with Cellular Automaton(CA)to simulate macro microcosmic simulation of high strain rate rolling.The macro damage distribution,the equivalent stress difference between the edge and center of the rolled plate,and dynamic recrystallization are accurately simulated.The high-strain rate rolling process is guided by the macro and micro multi-scale simulation results.(2)During high-strain rate rolling,due to the influence of the combined factors of temperature rise and strain rate,the grain size fluctuates.With increasing average strain rate,the recrystallization is gradually complete.The texture of high strain rate rolling is weaker than the typical rolling basal texture of magnesium.The maximum polar density values of the base plane of the plates are 8.04,12.94,7.60 and 8.75 respectively at the temperature of400?with average strain rate of 10s^-1,15s^-1,20s^-1and 29s^-1.(3)With the increase of the average strain rate,the edge cracking of the rolled plate is suppressed.The reason is that with the average strain rate increasing,the recrystallization tends to be complete,followed by the consumption of energy for crack propagation.This finally causes that the crack propagation is suppressed.As the average strain rate increases,the equivalent stress difference between the edge and core of the rolled plate decreases.At high-strain rate rolling,sharp crack tip emission dislocation,these emitted dislocations accelerate the formation of subgrains,and then subgrains with low misorientation angles transformed into recrystallized grains with high angle grain boundaries.This causes the grain refinement in front of the edge crack tips as well as the onset of crack blunting.For the edge of the blunted crack,stress concentration appearing at the grain boundary,especially at the grain junction causes the voids nucleation.The growth and connection of voids leads to further crack propagation.The crack linking to the blunted edge crack tip finally causes the generation and growth of the secondary crack.(4)The existence of grain elongated along the rolling direction leads to obvious anisotropy.The trend for the differences between(CRSS/m)ratios of 2nd-order pyramidal glide for the rolled sheets of 400?is the same as the trend for the differences of yield stress for the rolled sheets of 400?.Accordingly,the anisotropy of yield stress is associated with2nd-order pyramidal glide.The anisotropy of tensile strength is associated with the tilt angle of the c-axis towards the RD.The differences between the tensile strengths of RD and TD increase with increasing the tilt angle.(5)A large number of dimples and tearing edges are observed in the fracture of rolled sheet with high strain rate,indicating that the fracture mode is mainly ductile fracture.The cupping value of the rolled plate is related to the uniformity of the microstructure.If the microstructure is uniform,the cupping value is high.However,when the average strain rate is10s^-1,the cupping value of the plate is abnormal for the formation of shear bands.