Research On Size Effects In Microtension And Microbending Of Pure Titanium Thin Sheet

In recent years,micro electro mechanical systems has been rapidly developed in many new fields due to its excellent performance.The large demand for micro parts has made the industry have a higher expectation and more stringent technical requirements for metal materials,parts performance and processing technology.Titanium has the advantages of small density,high specific strength,excellent chemical stability and good biocompatibility。It has broad application prospects in the manufacture of micro parts and light structural parts.At present,with the deepening of the application of micro-electromechanical systems,the research on the material properties of titanium at the mesoscale has been deepened.Titanium has a very broad potential application value in the fields of MEMS sensors and micro fuel cells.The CP-Ti has complex microstructure evolution behavior and the huge difference between the performance of the surface grains and the core grains make it hard to predict the flow stress and fracture behavior,when the size reduces to meso scale.Therefore,it is necessary to observe and study the meso scale of pure titanium sheet.In this paper,commercial pure titanium(CP-Ti)sheets with a series of gradient thickness and grain size are used as experimental materials to conduct unidirectional micro-tension experiments to study the scale effect of their mechanical properties and fracture behavior.It is found that when t/d is less than 15,the flow stress shows the size effect ‘smaller is weaker’,and when t/d decreases under 2,the size effect‘smaller is stronger’ occurs.The fracture behavior of the material gradually changed from ductile fracture to obvious brittle fracture characteristics with the decrease of t/d.In addition,it is found that twins affect the fourth stage of work-hardening rate of the matel,thereby affecting the fracture elongation.The microstructure evolution behavior of the CP-Ti sheet during micro-tension was studied.It shows that the main types of twins produced in this experiment were{1122} compressional twins and {1012} tensile twins.Twins increases in a large number along with the the plastic deformation,and the number of twins shows a big difference between the core grains and the surface grains.Based on the surface layer model and the evolution of twins during the plastic deformation process,the constitutive of pure titanium thin-plate micro-tensile material at mesoscopic scale was constructed.The springback of meso-scale micro-bending of CP-Ti sheet was studied with a series of gradient thickness and grain size.The research shows that the bending angle can affect the length of the elastic deformation zone,The greater the bending angle,the greater the rebound angle of the material.The grain size of the material can affect the number of grains in each bending deformation zone,thereby affecting the stability of rebound angle.Since the material is affected by the strain gradient at the mesoscopic scale,the smaller the thickness,the greater the springback angle of the material.When the thickness is t=50μm,the hardening effect of the strain gradient is limited due to the lower yield point of the material,so the springback angle of the material decreases.The bending springback prediction model is established based on the difference between the constraint of the core grain and the die contact grain.