Study On Size Effects And Simulation Techniques Of Micro-forming

The production of miniature parts is becoming more and more important due to the trend of miniaturization, which is increasingly determining the development of product ranging from mobile phones, computers and micro digital camera to medical products and so on. The application of conventional manufacturing processes for the production of such micro-parts is possible, but there are some problems that result from the small dimensions. During micro-forming materials will behave some characters that are different from normal size processing. Up to now, the systematical research for micro-forming have not come into being. In conventional forming processing, a systematical processing technology has been developed, but in the micro range, the processing parameters have changed greatly. Many experiments have shown that materials display strong size effects when the characteristic length scale associated with non-uniform plastic deformation is in the order of microns. Two kinds of size effects on micro deep drawing are investigated, which can be explained by surface layer model and strain gradient model. With the increase of the thickness ratio of surface grain layer to the whole sheet, the bending moment of the sheet decreases, leading to decrease of feasible zone in the feasibility diagram. This paper also systematically studies the processing parameters and the size effect of micro-plastic-forming. The formulation concerning cylindrical deep drawing parameters is rewritten in a dimensionless form. A diagram, in which a feasible zone is drawn to bound both the maximal allowable tension and compression stress during the deep drawing process, is established. Since it is presented in a dimensionless form, it may be applied for both conventional and micro deep drawing. In this paper, cylindrical cup deep drawing is taken as an example to show the dimensionless process design method. Numerical simulations are carried out to compare the strain distribution with or without consideration of size effect. It is found that in micro deep drawing, surface layer softening may have dual effects on micro formability: it decreases bending moment and the wrinkling resistance capability, but strain distribution tends to be more uniform, which is advantageous for preventing fracture. Strain gradient hardening may lead to heavier strain concentration and worse micro formability. The feasibility and practicability of the proposed method is qualitatively verified by comparison with exsisting experimental results and finite element simulation.