Investigation On Influence Factors Of Surface Roughness Of Micro Bulk-forming Parts And Construction Of Prediction Model

With the rapid development of Micro System Technology(MST)and Micro Electro Mechanical System(MEMS),the forming and manufacturing technology of micro-parts has attracted great attention.Because of the size effect,the traditional macro-plastic forming theories such as material constitutive model and friction model can not be directly applied to the analysis of micro-plastic forming process.In addition,different from the traditional plastic forming,micro forming technology not only has small size and high dimensional accuracy,but also has high requirements on the surface quality of formed parts because of its the high-end application.The surface quality of metal plastic forming is affected by many factors such as the material state,the forming process conditions,the mold structure,the friction and the lubrication conditions.The surface quality of macro-plastic forming products is generally guaranteed by the subsequent machining,etc.Therefore,the research on surface roughness control theory and method in forming process has been neglected in the field of metal plastic forming.The surface roughness of the part will not decrease with the decrease of part size,and the surface quality of micro parts is hardly to be improved because the difficulty of clamping and operation with size miniaturization.Therefore,it is of great significance for improving the surface quality of micro formed parts and promoting the application of micro plastic forming technology through the investigation on size effect of friction and surface morphology in micro forming and the prediction and control of surface quality for micro forming parts.In this paper,the friction size effect has been studied in micro metal forming process and a method for calculating the correlation coefficients in friction model suitable for micro forming was proposed.Based on the micro scale feature extrusion experiment and the finite element simulation,the influencing factors and rules on surface quality in micro forming process were systematically studied.By introducing scale parameters,a surface roughness prediction model for micro metal forming was established considering the size effect,and the corresponding surface quality control criteria and methods were proposed.The research results are as follows:The effects of specimen size,average grain size and surface roughness of die on friction size effect were studied by upsetting experiments of pure copper at meso/micro scale under dry friction conditions.Based on Wanheim/Bay friction model,the relationship between the real contact ratio and the normal pressure and the shear film strength coefficient on the contact surface was established.The transformation between Wanheim/Bay friction model and general friction model was carried out,and the application of Wanheim/Bay friction model in finite element simulation was realized.The method for calculating the shear film strength coefficient in Wanheim/Bay friction model under dry friction condition was determined by combining the finite element simulation with experiment.The simulation results were in good agreement with the experimental ones,which verifies the rationality of the solution method.The influence of different process parameters on the surface roughness of micro feature was analyzed through the micro groove extrusion process.Through the finite element simulation,the effects of the surface enlargement,the normal pressure on contact surface,the relative sliding distance between the workpiece and the die,and the scale miniaturization on the surface roughness of micro scale features were systematically studied by using the coulomb friction model transformed by the Wanheim/Bay friction model established in this paper.Based on W-M fractal function,the finite element analysis model with rough surface was established,and the influence of the main factors such as normal pressure on the contact surface,initial roughness of blank,relative sliding distance between the workpiece and the die,and the surface enlargement on the surface roughness of workpiece during plastic deformation were analyzed and quantified in detail,then the surface roughness prediction model for metal bulk forming considering multi-factors were established.On the basis of the Voronoi diagram method and the secondary development of ABAQUS/Python,a cylindrical upsetting finite element model with random grain size and orientation of pure copper polycrystals with two-dimensional rough configuration on the end surface was established.The influence of size effect on the roughness and its distribution uniformity on contact surface during upsetting process was analyzed.By introducing the ratio of grain number in surface layer to total grain number as the scale parameter,the size effect on the contact normal pressure and its distribution uniformity was studied.Based on the prediction model of surface roughness for metal forming parts and the introduced scale parameter,the influence of size effect on surface roughness was quantified,and the micro metal forming prediction model of surface roughness was established reflecting the normal pressure on contact surface,initial surface roughness of blank,relative sliding distance between the workpiece and the die,the surface enlargement and size effect.The accuracy of the surface roughness prediction model was verified by comparing the experimental and the predicted results.Aiming at the average roughness of the surface nodes and the uniformity of their distribution,a criterion for evaluating the surface quality of micro forming parts considering the uniformity of surface roughness distribution was established by using the linear weighting method in the weighted function method.Based on the surface roughness prediction model and evaluation criteria established in this paper,the influence of grain size and die cavity shape on surface roughness distribution and the application of surface quality evaluation criteria were studied by means of finite element numerical simulation.Suggestions were put forward to improve the surface quality of micro-forming parts with grain size and die cavity shape as control objects The reliability of the surface roughness model and the feasibility of the surface quality evaluation criteria were verified by meso/micro extrusion experiments under the corresponding process parameters.