Microstructure Evolution During Variable Channel Angular Shear Extrusion Process Of AZ80 Magnesium Alloy

Magnesium alloy is an important direction of lightweight structural material in aerospace and other fields.The complex and harsh service environment puts forward higher requirements for its mechanical properties.As the most effective way to strengthen and toughen materials,grain refinement can solve the problems of low absolute strength and poor plasticity of magnesium alloys.Domestic and foreign researchers have developed a variety of severe plastic deformation techniques to prepare fine-grained magnesium alloy billets.However,the subsequent hot forming process will lead to the weakening or even disappearance of the fine grain strengthening and toughening effects.Focusing on the problems of complex microstructure evolution mechanisms and difficult microstructure control in the forming process of AZ80 magnesium alloy,a variable channel angular shear extrusion process method combining severe plastic deformation technology and forming process is proposed,which can achieve the forming and manufacturing of fine-grained magnesium alloy sheet components.The research method of experiment,theory and numerical simulation was adopted.The microstructure evolution mechanisms of AZ80 magnesium alloy during hot deformation was analyzed.A viscoplastic constitutive model coupled with microstructure evolution was established.The fine grain uniformity processing window of variable channel angular shear extrusion process was determined,which was applied to the forming of fine grain AZ80 magnesium alloy sheet component.The main work of this paper is as follows:The Gleeble isothermal compression thermal simulation experiments were conducted to study the hot workability and microstructure evolution of AZ80 magnesium alloy under different deformation conditions.Based on the Arrheniustype constitutive model,the flow stress characteristics including work hardening and dynamic softening were analyzed,and the activation energy map at high strain level was established.The hot processing map was constructed by the dynamic material model and the instability criterion.The hot processing window with refined microstructure as well as good workability was obtained combined with microstructure analysis.By considering the viscoplastic flow feature and microstructural mechanism of AZ80 magnesium alloy during hot deformation,a thermoplastic constitutive model coupled with microstructure evolution was established by using the unified internal variable method.The characteristic variables including plastic strain rate,dislocation density rate,recrystallized critical strain,recrystallized volume fraction and grain size was introduced.The model can accurately describe the evolution of flow stress,recrystallized volume fraction and grain size.Based on DEFORM-3D numerical simulation platform,the constitutive model was compiled,and the isothermal compression finite element model was established.The model shows good prediction ability for microstructure heterogeneity and dislocation density evolution during deformation process.The device and scheme of equal channel angular extrusion were designed.A thermal-mechanical-microstructure multi-field coupled finite element model was established.The thermodynamic behavior and microstructural characteristics under the extrusion-shear composite deformation path were analyzed.The extrusion-shear composite processing region of AZ80 magnesium alloy was obtained.The microstructure in different deformation stages shows inhomogeneous distribution characteristics with the effects of three-dimensional compressive stress and shear stress.The grain refinement mechanisms during the extrusion-shear process are dynamic recrystallization and mechanical shear.Considering the influence of extrusion process parameters on recrystallized grain refinement,grain growth and microstructure distribution uniformity,the actual extrusion process should be conducted at the extrusion temperature of 350～385℃ and the extrusion speed of less than 3 mm/s.An extrusion forming process based on variable section and secondary angle was developed to accumulate plastic deformation.Combined with the finite element numerical simulation of extrusion deformation and microstructure evolution characteristics,the quantitative evaluation and the sensitivity analysis of processing parameters were conducted.Based on the optimization of hot working window and extrusion-shear fine grain region,the optimal extrusion parameters were obtained and applied to the extrusion forming experiment of sheet component.The results show the highest equivalent strain and the minimum average grain size after secondary angular shear deformation.The designed extrusion process can significantly improve the deformation and microstructure uniformity of the material.Within the set range of processing parameters,the effect of recrystallization on grain refinement increases,the recrystallization volume fraction increases,the average grain size decreases and the fine grain uniformity of microstructure improves with the increases of extrusion temperature,the decreases of extrusion speed and the increases of friction coefficient.The formed part in the processing unstable region shows obvious heterogeneous characteristics in deformation and microstructure.Metal depletion and crack defects appear in the macro level,and micro crack and flow localization appear in the micro level.The formed part in the processing safe region shows good forming shape and high fine grain uniformity.The average grain size of the plate after secondary shear deformation reaches 8.9μm.The yield strength of the formed sheet is increased from 210 MPa to 259 MPa,the tensile strength is increased from 265 MPa to 338 MPa,and the elongation is increased from 5.8%to 12.9%.The research can provide theoretical guidance for the forming and manufacturing of magnesium alloy sheet components and the regulation of microstructure and properties.