Study On Grain Refinement And Microstructure Uniformity Of AZ31 Magnesium Alloy By ECAP

Equal Channel Angular Pressing(ECAP)is the most effective severe plastic deformation(SPD)process to improve the mechanical properties of magnesium alloys by grain refinement.In this paper,as-extruded AZ31 magnesium alloy was chosen for the study.Firstly,ECAP deformation process of AZ31 magnesium alloy was simulated by the finite element method at different temperatures(225℃,250℃,275℃ and 300℃)and different extrusion passes(from1 to 4 passes).Meanwhile,the flow of materials,extrusion load,distribution of temperature,stress and strain status and the homogeneous deformation was investigated during ECAP.And then,the bulk grain-refining AZ31 magnesium alloy samples were prepared by the mold.Then the microstructure and the crystal orientation of the samples were examined by using optical microscope(OM)and XRD,and the microstructure evolution during ECAP was analyzed.Furthermore,the coupling relationship between the deformation conditions,the equivalent strain and the grain refinement was discussed.The results show that:(1)The value of equivalent strain through finite element simulation and the theoretical results had the high goodness of fit,and the effectiveness of finite element simulation was verified.(2)The flow of material is non-homogeneous during ECAP.And the flow rate at inner corner is lower than that at out corner at the main deformation area.(3)The material nearing inner corner is under tensile stress status from compressive stress status when passed the main deformation area,the lower the temperature,the greater the tensile stress of material,and the easier material causes defects.The AZ31 magnesium alloy will occurs jagged fracture when deformation temperature below 200℃.(4)The shear deformation of the samples occurs in the main deformation area(in the mold angle corner part).The value of equivalent effective strain at inner corner is larger than that at out corner.The distribution of grain refinement in the different area of cross section of samples is agree with the distribution of the equivalent effective strain obtained by finite element analysis.This is why the grain size at inner corner is obviously smaller than that at out corner,and the distribution of grain size at inner corner is more homogeneous.The grain refining effect is directly affected by the equivalent effective strain of material.(5)To increase the extrusion passes is conducive to grain refinement as well as homogeneous distribution.However,due to the combined effects of dislocation increasing and annihilation,the refinement of grain size by increasing extrusion passes will gradually weaken.(6)The lower temperature is more conducive to grain size refinement,and the higher temperature is more conducive to homogeneous distribution of fine grain.(7)When the homogeneity as the main indicator of refinement,the extrusion temperature should be raised properly and the extrusion passes should be reduced.When the grain size as the main indicator of refinement,the extrusion temperature should be decreased properly and the extrusion passes should be reduced.The fine grains and homogeneous distribution for AZ31 magnesium alloy can be obtained by ECAP 4 or more passes at 250℃.