Plastic Deformation And Microstructure Evolution Investigation Of AZ31Magnesium Alloy During ECAP Process

As the lightest metal structure material, magnesium and magnesium alloys has a broad application prospect in aerospace, transportation and3C products manufactur-ing, but its plastic deformation ability is poor because the hexagonal close packed structure. Constrained by poor deformation ability has became a major obstacle for magnesium alloy to wider application. So, it has became a hot spot in the study of magnesium alloys that how to improve the magnesium alloy plastic deformation ca-pacity. The material’s strength and plasticity can be increased at the same time when its grain size refine to nanometer range. As one of servere plastic deformation method for effective produce ultrafine grain and even nanocrystal structure materials, equal channel angular pressing has been widely used in making ultra-fine grained material for many kinds of metals. The grain refining effect of magnesium alloy is closely related to the field distribution during ECAP process, such as stress, strain, tempera-ture and etc. These fields distribution depends on the Settings of ECAP die structure and deformation process parameter. It will be of great importance to comprehensive study the rule of different parameters influence on the extrusion effect in making ul-tra-fine grained magnesium alloy by ECAP process.ECAP deformation process of magnesium alloy was investigated by Numerical simulation and experimental analysis together in this paper. ECAP deformation process was simulated by the finite element software, DEFORM-3D under different conditions at the first. The influence of different parameters on the extrusion process, such as the die geometry, process parameters, deformation routs and etc was analyzed generally. It was acquired that the deformation mechanism and appropriate process parameters about the ECAP process of magnesium alloy. Four processing routes are analyzed in detail by using finite element method with spatial switching method through rotating three-dimensional model in multi-pass pressing. The accumulated effective strain distribution of the work-piece processed by ECAP for four pressing routes are obtained respectively through finite element simulation. On the basis of the simulations, the ECAP experiments are carried out by the optimized design of mould geometry that suite for magnesium alloy. And the finally pressed workpieces are ob-tained in the experiments. The microstructure evolutions of the grain refinement me-chanisms and micro-structural characters for different multi-pass ECAP processing routes are verified by using OM (optical microscope), SEM and XRD analysis. The mechanical properties was obtained room temperature tensile experiment and micro-hardness test.The homogeneous deformation is the main factor that influences the material performance during ECAP process. The die structure is an important factor that af-fecting homogeneous deformation of samples. The strain size and distribution of AZ31magnesium deformed by ECAP for different die corner angle Ψ and inner cor-ner radius r were investigated through the finite element simulation. When the die out corner angle is equal to20°and inner corner radius is equal to2mm, the results show that the effective strain distribution in the samples is more homogeneous than those of others. The effect of the stain distribution in pressed workpiece on the micro-structure and mechanical properties of the pressed workpiece was explored by using optical observation and micro-hardness testing. The result show that the microstruc-ture is significantly refined and the mechanical property is improved, but the distribu-tion of the grain size and micro-hardness in the cross-section of the pressed samples is inhomogeneous. The inhomogeneous distribution of the effective strain is one of the main factors that lead to the uneven microstructure and property.The temperature is an important factor that affects the deformation of magne-sium alloy. A thermomechanical coupled finite element model was established and the temperature distribution of the magnesium work-piece during Equal Channel Angular Pressing at different extrusion speed and friction condition was analysed in this paper. The result showed that the distributing of temperature is not uniform during equal channel angular pressing, there exists obvious temperature gradient and the tempera-ture increased significantly in the mold Angle shear part. The effect of temperature on deformation of magnesium alloy was acquired by XRD analysised and microscopic examination. XRD analysis and microscopic examination shows that the pyramidal face diffraction intensity enhanced significantly, the recrystallization rate of magne-sium alloy increases obviously with the rise of deformation temperature. The optimal deformation temperature of250℃was obtained according to the temperature distri-bution.The impact on average equivalent strain and extrusion load is limited during multi-pass ECAP process, but there are certain influence on the strain distribution. The equivalent strain distribution at cross section is similarity after multi-pass ECAP deformation through different route, and there is bigger difference at longitudinal sec-tion. The equivalent strain distribution at cross section inherited the first pass ECAP deformation characteristics for the most part, but different at the longitudinal section through different route. There is obvious gradient distribution at main deformation area by A and Ba routes, the distribution is well when deformation by Be and C routes. From the non-uniform deformation coefficient at the main deformation zone cross section, strain accumulation obtained by Be route is the most uniform.The tensile strength and elongation under room temperature were improved ob-viously for AZ31magnesium alloy processed by ECAP four passes through different routes. A higher tensile strength can be obtained through A or Ba routes, elongation is higer when processed through Be or C routes. It can be obtained from SEM fracto-graphic analysis that the as-received magnesium alloy shows brittle cleavage fracture while the alloy processed by ECAP exhibits dimple-like fracture characteristics. The better processing route for AZ31magnesium ECAP processed is acquired through comparing the results of Simulation and experimental study. Finite element simulation and experimental analysis results show that small and uniform microstructure can be obtained Through the appropriate deformation condition. Mechanics performance of magnesium is preferably when strain accumulation uniform.