Research On Anisotropy Plastic Behavior And Cold Rolling Deformation Theory Of Textured Pure Titanium Plate Strip

With the rapid development of micro-manufacturing fields such as electronics,new energy,and biomedical,the demand for technology-intensive high-quality metal thin strip has also increased significantly,among which titanium has become an indispensable part of thin strip products due to its high specific strength,strong corrosion resistance and non-magnetic advantages.The plastic behavior of rolled parts and the elastic flattening of rolls have significant effects on the formulation of pure titanium thin strip rolling process and the theoretical calculation accuracy of deformation zone.The former is embodied in the significant mechanical anisotropy caused by the hexagonal close packed low-symmetry crystal structure and preferential orientation of the rolling texture.The latter will cause the rolling difficulty to increase with the decrease of the thickness of the rolled part.The coupling of the two causes a lot of process waste,and also bringing great challenges to the accurate prediction of contact characteristics in the deformation zone.However,the traditional ideal isotropic thin strip rolling deformation zone theory does not consider the material properties and the anisotropy problem introduced during subsequent processing,and the establishment assumptions and application scope of each theory are also different.Therefore,the anisotropy mechanism and deformation zone theory of rolled pure titanium thin sheet strip are studied.In Chapter 2,in order to study the mechanism of texture on the anisotropy of the plastic behavior of pure titanium thin plate,the combination of mechanical test and microstructure observation was used to analyze and identify the anisotropy of macroscopic mechanical behavior under different loading conditions and the corresponding mesoscopic deformation characteristics.The variation law of each deformation mode with the orientation angle of the crystal was calculated by Schmid’s law,and the slip mechanism of dominant yield was determined by the critical shear stress.The control mechanism of strain hardening/softening behavior of twinning was analyzed by combining strain hardening rate curves,texture reorientation and microstructure characteristics.In Chapter 3,in order to deeply explore the correlation between initial texture type,loading conditions,mechanical properties,plastic deformation mechanism,and texture redirection,a viscoplastic crystal self-consistent model was introduced,and the crystal plasticity parameters were calibrated and verified by Schmid’s law,literature statistics and virtual orthogonal experiments.The anisotropic evolution process and formation mechanism of pure titanium yield hardening behavior at room temperature were elucidated,and the heritability of pure titanium thin strip texture and the non-negligibility of anisotropy caused by it under the single-direction rolling process were explored,which provided a theoretical basis for considering anisotropy in the rolling of pure titanium thin strip.In Chapter 4,in order to explore the variation of the contact characteristics of the deformation zone with the rolling process during the process of rolling from thick to thin,the validity of the plane strain assumption and the cross-section uniformity assumption during thin strip rolling was verified based on the elastoplastic finite element model.According to the contact profile and equivalent stress,the deformation zone was divided into sections,and the main reasons why the thin strip was becoming more and more difficult to roll were clarified.The limitations of the rigid roll model and the Hitchcock arc flattening and enlargement model in predicting the contact characteristics of thin strip deformation zone are compared,which provides the necessary theoretical basis and data support for the further improvement of thin strip rolling theory in the subsequent chapters.In Chapter 5,in order to further understand the establishment basis and limitations of the existing deformation zone theory and expand it,the total slip Stone theory and the total elastic hypothesis Johnson theory were compared,and based on the latter,the extreme point of the equivalent force and the limit rolling force per unit width were proposed to distinguish the elastoplastic state transformation and the limit rollable thickness of the rolled parts.On the basis of the first two theories,a combined thin strip rolling model was established,and the contact pressure of the deformation zone was characterized by different unit pressure equations,and the scope of application of each deformation zone theory was divided by the actual decompression rate.In Chapter 6,in order to establish the deformation zone theory suitable for the prediction of the rolling contact characteristics of anisotropic thin strip,the influence of yield stress anisotropy and yield function difference on the contact characteristics of the deformation zone was studied.The anisotropy coefficient in the Hill48 yield criterion was used to establish the proportional relationship between the anisotropic yield stress and RDTY,the anisotropic yield criterion and the Tresca isotropic criterion,and the unit pressure equation applicable to anisotropic thin strip with RDTY as reference was derived,and the research results could greatly improve the application scope and prediction accuracy of the existing deformation zone contact theory.