Deformation Behavior Of Metal Micro-tube During Hydroforming Process

In recent years,with the increasing demand for implantable drug delivery tubes in shaped shaft,medical equipment and various shapes of hollow micro tubes such as tracheal stents in microelectromechanical systems,a kind of forming technology that can process micro hollow components with complex cross-sections in large quantities is needed urgently.The microtube hydroforming process is a new type of micro-forming process developed based on the traditional tube hydroforming technology.Metal micro tubes can be made into micro-hollow components with complex cross-sections by using the microtube hydroforming process.Due to the size effect in the micro-forming field and unreasonable process parameters,the processed micro metal tubes still have problems such as high failure rate and uniform wall thickness.Due to the size effect in micro-forming,traditional material models cannot be directly applied to the numerical simulation of the microtube hydroforming process.As a result,a metal micro tube material model based on the size effect needs to be established.This study first used the nanoindentation test of metal micro tubes and the corresponding numerical simulation method to determine the size effect parameters in the power-hardening constitutive equation modified by the strain gradient theory based on meso-mechanism.Therefore,the material model of metal micro tubes considering the size effect was obtained.The modified constitutive equation is applied to the numerical simulation model of stainless steel micro tube hydroforming process for cross-shaped components.The accuracy of the model was verified by comparing the maximum expansion height obtained from the test and the simulation.Then this model was used to numerically simulate the crosses-shape microtube hydroforming process with different deformation parameters to analyze the influencing rule of different parameters on the internal high-pressure forming deformation behavior of stainless steel crosses-shaped microtube.Finally,the response surface method was used to optimize the microtube hydroforming process parameters of stainless steel cross-shape microtubes.The main work carried out in this study is as follows:(1)To establish a metal micro tube material model considering the size effects,in this study,the theoretical model of size effects were analyzed,and proposed to apply the power-law constitutive equation modified by the meso-mechanical strain gradient theory to the finite element simulation of the microtube hydroforming process.The uniaxial tensile test and the combination of nanoindentation and finite element method were used to obtain the corresponding material parameters.(2)To verify the rationality of the modified power-law constitutive equation,the FEM model of the cross-shaped micro tube hydroforming were established based on the traditional power-law material constitutive equation and the modified power-law material constitutive equation respectively.The results were compared with the experimental results to verify the accuracy of the modified material constitutive equation.(3)To study the influence of different process parameters on the stainless steel micro tubes cross-shaped micro tube hydroforming deformation behavior,different internal pressure loading paths,axial feeding loading path and friction coefficients were used in the numerical simulations to obtain the changes in wall thickness and the tube heights of the cross-shaped microtubes under different deformation parameters.(4)To optimize the cross-shaped microtube hydroforming forming process parameters of SUS304 stainless steel microtubes,setting the minimum and maximum wall thicknesses after forming as the responsive values.The response surface regression analysis was performed by using the BBD experimental design method.As a result,the quadratic response surface models corresponding to the minimum and maximum wall thicknesses were obtained.The analysis of variance and regression was carried out to prove the reliability of the obtained model.Based on the obtained mathematical model,the response surface analysis was performed with the wall thickness uniformity as the objective.The optimal loading path was obtained.The loading path was brought back to the finite element simulation to prove the reliability of the optimized model.