Research On The Microstructure Evolution And Simulation Of AZ31 Magnesium Alloy During Hot Deformation

Magnesium and magnesium alloys are the lightest metal structural materials which used in industrial applications.They have good potential for application in aerospace,automobile,3C products,chemical and medical instruments.It is called the title of "twenty-first Century green basic engineering materials".However,it has poor formability and limited ductility at room temperature,which rooted in the intrinsic hexagonal closed-packed crystal structure.The application of deformed magnesium alloys is limited.Therefore,it is important to study the microstructure evolution of magnesium alloys during hot deformation to improve their properties.In this paper,AZ31 magnesium alloy is researched,a finite element model suitable for simulating microstructure evolution of magnesium alloy during hot deformation is proposed.The Equal Channel Angular Pressing(ECAP)technology is optimized to improve the microstructure and mechanical properties of magnesium alloys by combination of numerical simulation and experiments.The increment forms of microstructure evolution models to simulate multi pass deformation are put forward.The thermal-mechanical-microstructure coupled finite element model is established to simulate the process of thermal compression and ECAP for magnesium alloy.By designing a simple back pressure device,an ECAP experiment at a lower temperature is realized,and the ECAP process is further optimized.The main research contents are as follows:(1)In order to solve the non-linear microstructure evolution under unsteady-state deformation,the incremental forms of microstructure evolution models include DRX,MDRX,SRX,Grain Growth for AZ31 magnesium alloy undergoing multi-pass hot deformation were developed.Then they were integrated into the FE software by FORTRAN language to achieve a temperature-deformation-microstructure coupled finite element model.The distributions of deformation and microstructure for AZ31 cylindrical sample during single-pass and double-pass hot compressions were quantitatively calculated and compared with the metallographic observation.The distribution of average grain size is non-uniform whether under single-pass or double-pass hot compression.The mean value of average grain size and its standard deviation under double-pass compression are slightly smaller than those under single-pass compression.The good agreement between the simulation results and the experimental results validates the feasibility of the developed FE model integrating incremental forms of microstructure evolution.(2)The high temperature flow stress curve of magnesium alloy was obtained by hot compression under the condition of constant temperature and constant strain rate.The thermal-mechanical-microstructural coupled finite element model for magnesium alloy is established to simulate the distribution of stress,strain and microstructure evolution in the process of ECAP.The simulation results show that AZ31 magnesium alloy will undergo severe shear plastic deformation and obvious dynamic recrystallization when passing the mold Angle part.There are obvious strain gradient and grain size gradient.The ECAP deformation process of AZ31 magnesium alloy is sensitive to temperature,the lower the temperature,the smaller the grain size and the recrystallization fraction.However,the effect of extrusion speed on the ECAP deformation is less.When the extrusion speed is in low level,the grain size increases with the decrease of speed,and the uniformity of grain size is better.When the extrusion speed is in high level,the grain size distribution is more uneven and recrystallization fraction decreases with the increase of speed.(3)The influence of ECAP process parameters on the microstructures and mechanical properties of AZ31 alloy was analyzed by multipass ECAP experiment under middle and low temperature using a self-designed backpressure device.In terms of microstructure,the equiaxed structure with grain size of about 3 ?m was obtained by four-pass extrusion at 200?,the grain size distribution is fairly uniform.In terms of mechanical properties,compared with the as-extruded Mg alloy,the elongation of magnesium alloy increased after ECAP,but the tensile strength was different under different deformation conditions.The results show that the strength of magnesium alloy obtained at 150? has a significant increase compared with the as-extruded one,reach 315MPa,but the increase of the elongation is not much.The magnesium alloy abtained at 200? has the highest elongation,elongation rate reaches 48%,and the strength of the alloy reached 240MPa,slightly lower than as-extruded Mg alloy.Based on the above analysis,the best ECAP process to improve microstructure and mechanical properties of magnesium alloy under current back pressure conditions is as follows:200?.four pass,1.5mm/s,back pressure.