Size Effects And Electroplastic Effects On The Mechanical Behavior Of Copper Sheets In Uniaxial Tension

With the rapid development of precision machinery and electronics industry,micro forming technology has become the main method for the mass production of micro parts because of its advantages in production efficiency,material utilization and performance.However,with the miniaturization of the metal parts,the deformation behavior of the material is different from that of the macro forming and obvious size effects can be observed.The research of size effect on the deformation behavior of metal has important academic value in enriching and perfecting the theory of micro forming and controlling the micro forming process.In this paper,T2 copper sheets with different thickness and different grain sizes were used to study the influence of size effects on the mechanical properties(namely,the yield strength,the tensile strength,the maximum uniform strain and the fracture strain),flow stress and damage evolution.Different models such as Hall-Petch based model,the dislocation density model and the slip length model were adopted to predict the flow stress during uniaxial tension.Finally,the electrically-assisted uniaxial tension tests were conducted to reduce the size effects on micro forming.Research on size effects of mechanical properties of T2 copper sheets shows: The fine grain strengthening is the main mechanism for the strength and deformation of the sheets with the same thickness;with the sheets of same grain size,obvious strength difference for the sheets with different thickness can be observed both in yield strength and tensile strength,and the maximum uniform strain and fracture strain decrease with the decrease of the sheet thickness.With the same thickness,the yield strength has an approximate liner relationship with the ratio of the sheet thickness to the grain size(N).While,the tensile strength,maximum uniform strain and fracture strain varies with N exponentially.The modified Hall-Petch model and the dislocation density model are used to predict the flow stress.The crossing phenomenon between the flow stress curves can not be described with these two models and there is a large deviation when N ? 10.While,the slip length model predicts both the crossing phenomenon and flow stress curve of thin specimen accurately.Research on size effects of the damage evolution of T2 copper sheets shows: The damage evolution of T2 copper sheets is closely related with the N value.When 1 ? N < 3,there are no obvious relationship between the initial damage position and N value,and the damage value is very small and the slopes of the damage curves almost remain constant.When N > 5,the N value almost has no influence on the initial damage positions and the slopes of the dmage curves.The fracture mechanism may change from the ductile fracture which the nucleation and growth of micro-cavities dominates to shear dominated fracture when N < 3.The experiments of electrically-assisted uniaxial tension show: By controlling the frequency,duty cycle and current density of the pulse current,obvious electroplastic effects can be obversed and the size effects can be reduced.The stress-strain curve decreases with the increase of pulse frequency,duty cycle and current density.The effect of grain size on the mechanical properties of the T2 copper sheet is the same as that of the conventional tensile test.When the current density is constant,the tensile strength,the maximum uniform strain and the fracture strain increase with the increase of N;When N increases from 2.8 to 22.2,the amplitude of the increase of the maximum uniform strain and fracture strain decreases with the increase of current density.