| Titanium alloy micro-complex components have a wide application prospect in the fields of aerospace,weapons and marine equipment due to the high specific strength,good corrosion resistance,excellent high and low temperature performance.However,titanium alloy has low thermal conductivity and large deformation springback,so it is difficult to guarantee the machining accuracy.In addition,titanium alloy micro-complex components must be formed at high temperature due to the poor plasticity at room temperature,which puts forward strict requirements on micro-forming die materials,lubricant and forming process.Thus,it is difficult to be achieved by conventional micro-forming methods.Electrically-assisted(EA)micro-forming applies a continuous or impulse current to workpieces,which is the plastic deformation process of materials under the action of multiple physical fields such as force field,temperature field and electric field.EA micro-forming can realize the integrated manufacturing of parts forming shape and performance,and it has the advantages of low forming temperature,small residual stress and excellent microstructure.Therefore,EA micro-forming is particularly suited to the high quality forming of titanium alloy and other difficult-to-deform materials.However,the physical mechanisms of coupling effect between size effect and electroplastic effect is not clear,the basic theory is not mature,and there are few researches on EA micro-forming process.This thesis aims to research the quantification and microscopic mechanism of electroplastic effect of EA micro-forming,and to provide a new technology for manufacturing TC4 titanium alloy micro-gear components by utilizing various effects and mechanisms of current interacting with materials.The influence of multi-field coupling effect of electric current treatment on mechanical behavior of TC4 titanium alloy was studied.It was found that the elongation and tensile strength of the electric current treatment sample compared with the untreated sample increased 19.4%and 22.1%,respectively.Based on the electromigration theory and simulation analysis,it was clear that local potential gradient,local defect hot spot and drifting electron scattering lead to the accelerated phase transformation rate and reduced phase transformation temperature.The microstructure characterization showed that shorter electric current treatment time and faster cooling speed promoted the formation of nano-martensite and the increase ofβphase fractions.Under the combined action of both,the TC4 titanium alloy sample after electric current treatment showed enhancement and toughening.An EA micro-tension test platform was established,and the strain localization behaviors of TC4 titanium alloy samples in EA micro-tension were studied by digital image correlation technology.It was found that the original single high strain band of the micro-tensile specimen suddenly changed into two intersecting higher strain bands when a single pulse was applied.And the high strain band gradually accumulated toward the center and developed into intersecting localized flow zone with the increase of strain.The flow stress drop model in EA micro-tension was established based on thermal expansion,thermal softening,strain hardening and the non-thermal electroplastic effect.The results showed that the contribution of non-thermal electroplastic effect to stress drop was about 11%,and no less than 89%was due to Joule thermal softening and thermal expansion,indicating that the main mechanism of electroplastic effect was Joule heat effect.Experiments and finite element methods were adopted to analyze the electroplastic behaviors during EA micro-upsetting of TC4 titanium alloy.It turned out that the plastic strain at the center of the sample was accelerated compared to the rest of the test section at higher current density and larger strain rate.The electric current induced microstructure evolution was investigated by comparing the results of EBSD and TEM.The results show that the EA micro-upsetting showed higher stress drop than isothermal micro-upsetting due to the activation of non-base plane sliping induced by electric current and Joule heat local softening.Finally,the local deformation mechanism induced by Joule heat was discussed based on experimental observation.Experiments and finite element methods were conducted to analyze the electroplastic behaviors during EA micro-scale shear-compression of TC4 titanium alloy.It was found that the flow curves displayed strain softening for widmanstatten microstructure,while hardening for equiaxed microstructure.SEM and TEM were used to comparatively investigate the shear bands evolution and microcracks nucleation of two samples.The results showed that the local fluctuations can contribute to the initiation of shear bands,and the microcracks were nucleated in the grain boundaries for widmanstatten microstructure.While the formation of elongated dislocation cells,formation of elongated subgrains and break-up of elongated subgrains were the evolutionary process of shear bands for equiaxed microstructure,and the microvoids were nucleated at the breakpoint of subgrains.The effect of sample size and electric current parameters on friction coefficient in the EA micro-forming process was studied.It was found that the friction size effect was reduced during EA micro-forming,and this trend increased with the increase of electric current density,which was because the electric current induced interfacial Joule heat effect promoted the fluidity of lubricant and softened the convex peak on the surface of the sample.The friction size effect model was successfully established based on the open-closed pockets friction model by introducing the scale factor and current density during EA micro-forming,which could effectively predict the friction coefficient and explain the mechanism of friction size effect.The friction coefficients of TC4 titanium alloy in EA micro-ring compression process under four different lubrication conditions were tested.It was proved that copper conductive grease was more suitable as a lubricant for EA micro-forming process.The effect of electric current density and load parameters on micro-filling behavior was analyzed by using the micro-cavity filling experiment.It was found that the filling height of micro-cavity increased quickly first and slowly afterwards with the increase of load and electric current density,and the filling integrity of the top of the micro-cavity was better when the electric current density was more than21.7 A/mm~2and the load was more than 25 k N.On this basis,a new EA micro-die-forming process of titanium alloy micro-gear by compound heating with die resistance and electric current was presented.The results showed that all tooth shapes of micro-gear can be filled completely under the conditions of 500°C die preheating,36 A/mm~2electric current density and 25 k N load.The geometric precision,surface quality and microstructure of micro-gear parts were analyzed under the optimal process parameters.The profile precision of micro-gear part was better than 49.8μm,the tooth profile precision was better than 22.8μm,the surface roughness was less than SRa 68.4 nm,the hardness was comparable to that of the original material,and the metal flow line was distributed along the tooth profile. |
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