| Recent years, micro products such as micro-channel reactor, micro-channel heat exchanger, heat pipe and heat sink, are widely used in the field of energy, chemical engineering, mechanical engineering, electronics and bio-medicine, due to the reduced size and the enhanced performance. Increasing attention has been paid on the large-scale high-efficiency machining process for the micro products manufacturing. Electric Assisted embossing is a promised technology for the efficient manufacturing of micro products. In electric Assisted embossing, the yield stress of the metallic material is reduced and the plastic deformation is increased. Heavy electric current not only raises the temperature of the workpiece by the Joule effect, reduces the shear modulus and the yield stress, but also lowers the yield stress by the electroplastic effect and the effects on the variation of the micro structure during the embossing process. Only the effect of the workpiece temperature on the yield stress is included in the traditional theory on the embossing process. Because of the lack of modeling theory on the plastic deformation under the stimulation of electric current, the advanced electric assisted embossing process cannot be predicted precisely. As a result, it is difficult to optimize electric assisted embossing process.In this research, the numerical modeling of electric assisted embossing process was studied and then applied to the electric assisted micro embossing process of stainless steel 316 L. Firstly, the electric assisted tensile tests were employed to investigate the influence of electric current on the yield stress of stainless steel 316 L. Secondly, the constitutive model of stainless steel 316 L was established, in which both the influences of temperature and electric current were taken into consideration. Based on this constitutive model, the numerical model of the electric assisted embossing process was built to analyze the influences of process parameters on the feature size of workpiece. Finally, the experiment platform was set up, and the numerical model of the embossing process was verified by the process experiments performed on the experiment platform. The research works were as follow. 1) Experimental research in the effect of heavy electric current on the yield stress of stainless steel 316LBecause of the coexistence of the thermal effect and the athermal effect in the electric assisted embossing process, the electric assisted tensile test system was designed for metallic material. The test system consists of a heavy electric current power source and a tensile test system. The experiment methods for the yield stress and the micro structure of metallic material were proposed, in which the thermal effect and the athermal effect were separately quantified. The yield stress of stainless steel 316 L in electric assisted tensile test was investigated. According to the experiment result, both the thermal effect and the athermal effect reduce the yield stress of stainless steel 316 L by stimulating the thermal activated sliding of the dislocation, delaying and restraining the strain-induced martensitic transformation 2) Constitutive model of stainless steel 316 L taking into consideration the effect of electric currentBased on the theories of dislocation sliding, solid solution hardening, dislocation density variation and strain-induced transformation, the constitutive model of stainless steel 316 L was constructed for electric assisted embossing process. The material parameters of the constitutive model were obtained by fitting the experiment result. The variations of the material parameters showed that both the heavy electric current and the elevated temperature stimulate the dislocation sliding, enhance the storage and the recovery of the dislocation, delaying and restraining the strain-induced martensitic transformation. The constitutive model predicts the thermal effect and the athermal effect on the yield stress quantitatively. It is the basis of the numerical analysis of electric assisted embossing process. 3) Multi-field coupling model of electric assisted embossing process considering both the thermal effect and the athermal effect of electric currentSince the athermal effect coexist with the thermal effect in the electric assisted embossing process, an ‘electro-thermal-structural’ multi-field coupling model was established based on Abaqus. In this model, a Fortran subroutine was developed to export the current density in the workpiece to the user defined field variables, and then the yield stress was updated according to the current density, the temperature and the strain. With the help of this model, the embossing process of micro channel features was analyzed. Both the heavy electric current and the elevated temperature reduce the effective stress and the phase transformation hardenin, increase the plastic deformation. However, the influence of the heavy electric current is different from that of the elevated temperature. 4) Experiment study in electric assisted embossing processThe experiment platform consisting of the heavy electric current power source and the embossing system has been designed and built for the electric assisted embossing process experiment. A series of embossing process experiments of various process parameters have been carried out. The comparison between the experiment results and the simulation results on the channel depth and the microstructure has verified the proposed simulation model. The comparison also shows the necessary of taking into consideration the athermal effect of the heavy electric current in the numerical analysis of electric assisted embossing process.In this research, the constitutive model of stainless steel for the electric assisted embossing process was established, in which the instant effects and the accumulated effects of the electric current and the temperature can be analyzed quantitatively. The ‘electro-thermal-structural’ multi-field coupling simulation of electric assisted embossing process was proposed. The influences of electric current parameters on the embossing process were studied. The process experiments have been carried out to validate the electric assisted embossing process and to verify the simulation model. |
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