Study On The Microstructure And Properties Of AZ31 Magnesium Alloy By Pre-die Forging Extrusion Deformation

Due to the low density,high specific strength,and easy recycling,magnesium alloy has become one of the most competitive traditional materials in the 21st century,and has been widely used in aerospace,transportation,electronics and other fields.However,there are fewer slip systems that can be activated at room temperature due to the hexagonal close-packed(hcp)structure of magnesium alloys,and it is difficult to coordinate deformation during plastic deformation,resulting in poor plasticity and limited application.In order to overcome the above problems,this article took commercial AZ31 magnesium alloy as the research object,adopted closed forging and traditional extrusion combined deformation,and changed the deformation conditions to control the microstructure of the alloy,thereby achieving purpose of the overall performance improvement of magnesium.In traditional extrusion,the billet undergoes a upsetting process before extrusion,similar to forging.This process will cause a certain strain of the cast slab,but it is usually ignored due to the extremely short time and small strain.If the strain of the upsetting process is used,it will help to improve the structure of traditional extruded alloys.In this paper,based on the conventional extrusion process,using a self-made closed die,the casting billet was closed pre-die forging under high pressure in the extrusion barrel before extrusion,and then direct extrusion was emploied immediately after replaceing the conventional extrusion mold.The characteristic of this process is that the sample is pre-deformed by the traditional extruder,which can accumulate the strain generated by the rough process of the extrusion,which caused the appearance of dislocation,twinning and other structural defects in the ingot,thus provided nucleation sites and storage energy for recrystallization in the subsequent extrusion process,and finally refined the structure of the extruded alloy.In this work,the law of structure evolution during pre-die forging extrusion was systematically studied,and the influence of pre-die forging time and extrusion process parameters on the structure and properties of AZ31 magnesium alloy was studied.Using optical metallographic microanalysis(OM)and EBSD analysis methods,the microstructure and grain size of the sample after the extrusion process and the complete extrusion were observed,counted,calculated and analyzed,and the influence of different deformation processes on the structure of AZ31 was discussed.The tensile and compression mechanical properties of samples were tested at room temperature,the effect of pre-die forging extrusion process on the mechanical properties of AZ31magnesium alloy was studied,and the pre-die forging extrusion process parameters were optimized.The main results are as follows:(1)Under the pre-die forging extrusion process,the pre-die forging treatment of magnesium alloy has a finer microstructure,and the compression yield,plasticity,and tension-compression asymmetry have been significantly improved.With the extension of the pre-die forging time,the grain size of the alloy gradually decreases,and the overall performance is also significantly improved.(2)After pre-die forging at 300?for 60s,the sample processed by direct extrusion has obtained fine grain structure and excellent mechanical properties.The TYS,UTS,CYS,elongation and yield asymmetry?_CYS/?_TYS of the alloy are 305 MPa,337 MPa,295 MPa,27%and 0.97,respectively.(3)The texture strength is mainly affected by the unrecrystallized area and the grain size.The more unrecrystallized regions are,the larger the crystal grain is,and the more uneven distribution is,the stronger the texture of the material is.Under the influence of these factors,The texture of the extruded sample for pre-die forging 60s at 400? is the strongest,which is 22.73;while the texture of the sample extruded for pre-die forging 60s at 280?is the weakest,which is 4.13.(4)The degree of recrystallization of the material is affected by the pre-forging time and deformation temperature.The longer the pre-die forging time and the higher the deformation temperature is,the greater the degree of recrystallization is.At 400?,the sample basically obtains a complete recrystallized structure,and the recrystallized degree reaches 93%.