| The wide application of MEMS has promoted the rapid development of micro-components.Plastic forming technology has become the most important preparation method of micro-components for its high material utilization rate,high productivity,low cost,excellent mechanical properties and dimensional accuracy.Since the size and feature size of the micro-component are in the order of submillimeter or micrometer,the plastic deformation ability of the material in the mesoscopic scale is the core factor affecting the formability and dimensional accuracy of the product.Therefore,material with good plastic deformation ability（such as pure copper）is favored in the plastic forming research of micro-components.Titanium alloy has a broader application prospect and can be used in more harsh environments because of its high specific strength,good corrosion resistance,excellent high and low temperature performance and good biocompatibility.However,titanium alloy is difficult to deform due to high deformation resistance and low plasticity.In addition,the existence of size effect further reduces the micro-forming scale limit,and then seriously affects the filling integrity of titanium alloy micro-components.In this paper,α+βdual-phase TC4 titanium alloy micro-gear was taken as the research object.Based on the good plastic forming ability ofβphase,the tensile and compressive plastic deformation behavior and microstructure evolution of TC4titanium alloy inβsingle-phase region were studied systematically,which laid a theoretical foundation for the plastic forming process of micro-gear and its numerical simulation.Secondly,the continuous extrusion of micro-gear was realized through mold design and process exploration,and the structure,precision and performance of the formed micro-gear were analyzed and evaluated.Then,based on the optimal forming parameters of the micro-gear,the dynamic evolution mechanism of the micro-gear filling cavity inβsingle-phase region was revealed by numerical simulation and step extrusion experiment.Finally,the influence of size effect on the forming,microstructure and properties of micro-gear was investigated from the aspects of grain size and geometric size.The main research work is as follows:（1）The tensile and compressive plastic deformation behavior of TC4 titanium alloy inβphase zone were studied systematically.The constitutive equation of TC4titanium alloy inβphase region was established based on Arrhenius model,and the accuracy of the constitutive equation was verified by comparing the calculated and experimental values.Theβsingle-phase thermal working diagram constructed by the dynamic material model showed that the plastic working of the material was unfavorable when the temperature was higher than 1075℃and the strain rate was higher than 1 s-1.The Zener-Hollomon parameter ln Z was introduced and the high temperature phaseβwas reconstructed.The results showed that with the continuous increased of ln Z,the dominant softening mechanism ofβphase changed from dynamic recovery to continuous dynamic recrystallization,and further transition to geometric dynamic recrystallization and continuous dynamic recrystallization.In situ tensile process ofβsingle-phase region,the deformation behavior was regulated by a synergistic mechanism of grain boundary sliding-grain boundary migration-intrachrystalline slip-grain refinement,and the surface of the sample had an obvious"orange peel effect".（2）The influence of mold material,structure and process parameters on the formability of micro-gear was studied,the microstructure,precision and performance of the formed micro-gear were analyzed and evaluated.The results showed that the extrusion forming of TC4 titanium alloy micro-gear was achieved by the traditional metal mold at the lowest temperature of 830℃,but the temperature induced plastic deformation of the mold itself,which lead to uneven surface quality and poor precision of the extruded parts,and difficult to disassemble the mold.Graphite mold had high temperature stability,self-lubrication and wear resistance,but it was easy to brittle crack under high external load.By increasing the clearance between the graphite punch and the container,it was helpful to reduce the extrusion load,and the optimum clearance value was 80μm.The extrusion stroke did not increase with the continuous increase of temperature and the continuous decrease of extrusion speed,but with the coupling effect of heat,force and grain size.Only under the extrusion parameter of 1200℃and 0.01 mm/s,the continuous extrusion of punch stroke of 2mm was realized.At room temperature,αmicrostructure continuously evolved along the longitudinal and transverse directions for the formed micro-gear rod,Ti C grains appeared in local areas.The overall hardness of the micro-gear was higher than that of the original billet,but the hardness of different regions at the same section and the same region along the extrusion direction was uneven.The prepared titanium alloy micro-gear had outstanding wear resistance,the wear amount of tooth edge microstructure was only 24%of the original billet.The titanium alloy micro-gear was assembled in a delayed deployment gear train,the test results met the core parameter requirements of the deployment mechanism.For the size measurement of the formed micro-gear,a special high precision image measurement system with the detection accuracy of 0.6μm was established.The average measurement size deviation of root circle,tip circle,tooth thickness and tooth groove from the design size was 2.3μm,3.2μm,10.2μm and 10.7μm,respectively.（3）The dynamic microstructure evolution mechanism of TC4 titanium alloy micro-gear during continuous filling inβphase region was revealed.The results showed that there were temperatures gradient in the billet both longitudinally and laterally.The strain rate,strain and flow rate were unevenly distributed.In addition,there were carburized and non-carburized regions in the billet,andβand Ti C grains were coarsened continuously during the extrusion process.From the beginning of extrusion to the end of extrusion,the deformation degree and deformation behavior at different feature structures were different.Therefore,the microstructure of the tooth shape,root and center of the micro-gear at the same section was different,and the microstructure of the same feature area along the extrusion direction was constantly evolving.During the whole filling process,the outer region of the billet was subjected to shear action,but there was no plastic flow behavior.（4）The influence of size effect on filling forming of TC4 titanium alloy micro-gear inβphase region and its mechanism were investigated.The results showed that the extrusion pressure increased with the increase ofβgrain size,showing grain size effect.The grain size effect affected the size and distortion ofβgrains in gear tooth,but had no obvious effect on dimensional accuracy and hardness.Based on the yield stress constitutive model related to grain size and the size factor expression related to the feature structure of the tooth profile,the Boolein equation related to the grain size effect was established.The filling stroke of micro-gears with the same tooth number and different modules indicated that there was a geometric scale effect in micro-gear forming,which was not"the smaller the stronger"or"the smaller the weaker",but an extreme point.The geometric size effect had an effect on theαandβmicrostructure of micro-gears with different modules.With the increase of the modulus,the roughness at the tooth root increased and the overall hardness decreased,but the geometrical size effect did not affect the forming accuracy. |
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