Electrically-assisted Micro Deformation Behaviour In TA2 Pure Titanium

Microforming parts have a broad application prospect in the filed o f micro-electro-mechanical system and biomedicine,etc because of the advantages its high material utilization,excellent mechanical properties and low cost. However,the existence of size effect of microforming pr ocess severely restrict the rapid development of microforming technology.Metallic materials under current excitation conditions show a special mechanical properties.This paper introduces electrically-assisted forming technology into micro-plastic deformation process, which is greatly important for the development of electrically-assisted microforming technology.For the study of the effect of pulse current on the micro plastic deformation o f TA2 pure titanium foils, the electrically-assisted uniaxial micro-tension equipment was developed.The device consists of uniaxial micro-tension system,pulsed current and its parameter acquisition system and temperature collect and control system.Thermoelectric coupling finite element analysis was conducted in order to acquire the temperature field of micro-tension sample with different jig structrue using Ansys workbench. Eventually the jig structure in the form of "line + face" was determined.From the perspective of both static and dynamic,the effect of pulsed current o n micro-tension deformation of TA2 pure titanium foils with different grain and specimen size was studied. Under the same current density, the smaller the grain size is, the thermal effect is more obvious.In the EPT process of 8% pre-deformed sample, plasticity recovery becomes increasingly remarkable with the decrease of grain size above recovery temperature,w hich disappears below recovery temperature. The experiment result of EAT shows that:(1)the yieldstrength and tensile stress of pure titanium decrease significantly with the increase of thickness of foils,the introduce of pulsed current leads to the disappearance of sample size effect.The flow stress reduction△σ of medium grain size pure titanium is largest,the fine and corse grain case is in the same level. when grain size exceeds 59.8μm,on the contrary,the yield strength of pure titanium increases with the increase of grain size.The effect of pulsed current on grain size effect is weak.(3)By decoupling the thermal and athermal effec,the contribution of thermal effect on flow stress drop is dominant;(4) Microstructure analysis showed that in the EAT process the fraction of low angle grain boundaries sharply decrease with the deformation temperature. Dynamic recrystallization was observed in the EAT process of pure Ti foils without aircooling when grain size is 70.4μmFinally, the effect of pulsed current on micro-compression deformation o f TA2 pure titaniumbar with different grain size was studied. Findings are as follows:(1) the sample temperature drops rapidly as the compressive deformation process, eventually flatten out;(2)at low temperature(< 180 ℃), the flow stress of pure titanium decline obviously(3) under the same current density, the flow stress reduction value increase with the grain refinement and the grain size effect is weakened and even disappear;(4) There may be a threshold current density making the material softening, its value represents the threshold energy which promotes dislocations pass obstacles. In the corse grain size, the typical electroplastic effect flow stress drop steps like "zigzag" was observed, with the increase of Nominal current density, the steps shifted to the right.