An Investigation On Size Effects Of 304 Stainless Steel Foils In Microforming Processes

With the development of modern industry, the demands of micro-parts in various fields have been greatly increased, in metal microforming process, plastic microforming technology is investigated by more and more researchers recently for its advantages including high efficiency, high quality and low cost. Size effect is found during plastic microforming process for the grain size is much closer to the micro-parts size. However, the current research in the metal foil of this area is infrequent, which limits the development of metal foil microforming technology. Therefore, the study of this paper is based on size effect to do research.Firstly, 304 stainless steel foils of various thicknesses are heated at different temperature, and uniaxial tensile tests are investigated. the results show that yield stress increases with the foil thickness decreasing, i.e. a size effect"smaller is stronger", there is a layer of passive film out of the foil surface, the role of this passive film is similar to the enhanced role of the grain boundary, in this paper, the Hall-Petch equation was modified by introducing the influence of foil relative thickness, and applied to calculate yield stress with better agreement with test results; The results also show that limit elongation and tensile stress decreases with the thickness decreasing, i.e. a size effect"smaller is crispier", the reason is that the numbers of the grain through the foil decrease with the foil thickness decreasing, this makes the forming process more unevenly.Secondly,microbending tests of 304 stainless steel foils are investigated, the results show that springback angle and non-dimensi- onal bending moment increase with the foil thickness decreasing, i.e. a size effect"smaller is stronger", this phenomenon can be explained by modified Nix-Gao strain gradient hardening model, and the results agree with the experimental data.At last, micro-deep drawing tests of 304 stainless steel foils are investigated, the results show that limit drawing rations(LDR) decrease with the foil thickness and cylinder diameter decreasing, this can be explained by LDR equation and the role of punch in forming separately. An formula with radial strain gradient is established to forecast the drawing force, and the results agree with the experimental data.