Study On Tensile Properties And Forming Behavior Of Titanium Foil Based On Size Effec

With the progress of science and technology,the mechanical industry technology is gradually developing in the direction of lightweight,miniaturization and intelligence,and the demand and quality requirements for micro parts are also increasing.When the size of the material in the two-dimensional direction reaches the millimeter or micron level,the forming performance and process are different from the macroscopic size due to the size effect.In this thesis,TA2 pure titanium foils with thicknesses of 10μm,20μm,40μm,80μm and 120μm were selected as test materials.Through experimental analysis,theoretical model construction and numerical simulation,the tensile size effect,cupping forming performance and forming limit of pure titanium foil were studied.Firstly,the average grain diameter d and t/d ratio(the ratio of material thickness to average grain diameter)at five thicknesses were obtained by testing the microstructure of pure titanium foil.EBSD tests were performed on titanium foils with thicknesses of 40μm,80μm,and 120μm,respectively,to study the fiber structure.The grain preferred orientation was in the // ND direction and showed a bimodal basal texture in the ND direction.The tensile stress-strain curve was obtained by uniaxial tensile test at room temperature,and the influence of size effect on the tensile properties of titanium foil was studied.The results show that the yield strength gradually increases with the decrease of t/d ratio,showing the size effect of “the smaller the stronger”.Due to the existence of passivation layer on the surface of pure titanium foil,the surface strengthening effect is more obvious when the t/d ratio is smaller.As the proportion of internal grains increases,the uniform deformation ability of the material increases.The tensile strength shows a trend of “decrease-increase-decrease”with the increase of t/d ratio,while the elongation of the material increases with the increase of t/d ratio.The fracture morphology after tensile test was observed by SEM test.The fracture of pure titanium foil with thickness of 10μm and 20μm was 90° with the tensile direction and there was almost no dimple structure on the tensile fracture section,showing typical brittle fracture.The fracture of pure titanium foil with the thickness of 40μm,80μm and 120μm is45°～55° to the tensile direction.There are a large number of dimples and “ river-like ” tearing edges on the tensile fracture section,which shows ductile fracture.Based on Hall-Petch equation,Schmid’s law and surface layer model,the micro flow stress model of pure titanium foil material is studied by combining the macroscopic geometric size,grain size and the proportion of surface grain.According to the tensile test data of TA2 pure titanium foils with different thicknesses,the linear change of the plastic stage of the material was fitted,and the constitutive equations of pure titanium foils with two thicknesses were obtained and verified with the test data.According to the national standard GB/T 4156-2020,the cupping test mold was designed,and the cupping test of five kinds of thickness pure titanium foil was carried out at room temperature to study the influence of size effect on the cupping formability of pure titanium foil.The experimental results show that the cupping value of pure titanium foil increases with the increase of t/d ratio.When the width of the specimen becomes narrower,the forming state of the specimen gradually changes from ’tension-tension’ to ’tension-compression’,the forming force gradually decreases,and the fracture position shifts from the middle to the edge.Titanium foils with a thickness of 40μm,80μm,and 120μm show an ’increase-decrease’ trend as the width of the specimen narrows.The finite element simulation of pure titanium foil cupping forming process was carried out by using ABAQUS software.The cupping test simulation model was established,and the established titanium foil constitutive model was introduced.The flexible damage fracture criterion was used to simulate and analyze the specimens with different thickness and width.The results show that the location of the fracture is consistent with the fracture location of the actual cupping test,and the forming force and the value of the cupping are also close to the experimental data,which verifies the rationality of the finite element simulation of the cupping forming process.Finally,the forming limit diagram of pure titanium foil with different thicknesses is drawn by the primary and secondary strains at the forming limit under different strain paths.