Severe Plastic Deformation Of Metal And Mesoscopic Constitutive Model Including Size Effect

Microstructure alteration of material subjected to severe plastic deformation brings on great changes of mechanical property. However, the evolution of intrinsic mechanism and model description on which still haven’t been well resolved until now. In order to research mechanical property changes of material subjected to severe plastic deformation, ductile material were researched by the experiment of equal channel angular pressing (ECAP) and theoretical analysis of polycrystalline plasticity.Moulds of ECAP were designed and complied. Extrusion experiments of copper and aluminum were completed in these moulds. Research indicates that there are dramatic increase in the yield strength of materials subjected to extrusion of ECAP and small scale increase in ultimate strength of that. Non-homogeneity of materials subjected to extrusion with different routes of ECAP was researched. Distribution regularities of pressure head’s force during the process of ECAP were studied using experiment and numerical simulation. User material subroutine of polycrystalline plasticity model was compiled which can reflect the physical mechanisms of crystal lattice slipping. In order to describe mechanical property of materials subjected to cyclic loading, the back stress was introduced in polycrystalline plasticity model. Furthermore, size effect was considered through slide resistance evolution function. Distribution regularities of local stress and strain were analyzed using polycrystalline plasticity model during the process of ECAP.The main results of the paper are as below:1. The ECAP moulds including90degree and120degree angle were designed and compiled. Round bar specimens with90mm length were successfully extruded through these moulds.2. Completed tensile test of materials subjected to extrusion of ECAP or not. Experiments indicate that:(1) The increase proportion of the yield strength of materials subjected to different routes of ECAP is up to312.5%and that of ultimate strength is26.3%;(2) Increase of yield stress of pass-one-extrusion is the most significant and the contribution percentage is up to70%during the multi-pass extrusion;(3) Plastic properties of materials subjected to process of ECAP are decreased sharply. Unit extension of non-extrusion materials is30%and that of materials subjected to pass-one-extrusion is7%;(4) Plastic properties of materials during the process of subsequent extrusion are increased slightly.3. Completed the mechanical properties experiment of test piece interception from different location of the same specimen of T3copper subjected to extrusion of route A, Ba, Bc, C and45-degree-pass-eight. Experiments indicate that:(1) Regular non-homogeneity can be found from tensile test nothing to do with extrusion routes;(2) The maximum of non-uniformity coefficient of section strength during the four test pieces in different location of the same specimen is24.6%among the above routes.4. Distribution regularities of pressure head’s force were researched under different routes and extrusion times during the ECAP experiment. There is remarkable difference for pressure head’s force during different routes and extrusion times.5. Analytical model of polycrystalline plasticity was researched and user material subroutine (VUMAT) interface with ABAQUS was compiled in which constitutive relation were described by rate type of grain slide. Evolution of the back stress was taken in the model and material element was endued with grain characteristic size information which could describe grain size effect during the process of materials’yield and flow. Especially, it could describe grain size effect under the condition of strain cycling.6. Adopting numerical simulation, distribution regularities of pressure head’s force were relative to the factors of mould parameters, continuous extrusion of multi-specimen, material properties, length of specimen and fraction coefficients etc during the process of ECAP. The estimated formula of maximum pressure head’s force was presented based on experiment and numerical simulation during the process of ECAP.7. Influence range of head-effect in the specimen subjected to ECAP treatment was researched. Regarding strain invariant as analysis parameters, deformation state and non-homogeneity compression regularity in the specimen were researched during the ECAP processes. Non-homogeneity state was quantitatively analyzed by the definition of non-homogeneity coefficients of the section during the ECAP processes, which were in accord with experiment results.8. By the analysis of tensile stress components during the ECAP process, the phenomenon of presenting transverse crack along ordinate axis of specimen in the experiment was reasonably interpreted.9. Stress distribution during the ECAP process was researched using polycrystalline plasticity model, in which stress in material element was average effect of a certain amount of anisotropic grain. Discrepancy of stress among the grains with different orientations in material element was prominent, which have important effect on damage evolution and rupture in material.