Research On Size Effects Of Mechanical Property And Micro Deep Drawing Of Copper Foil

In recent years, metal foil is wide applied in electronics industry, micro electromechanical systems, medical and new energy fields. Plastic microforming technology becomes the first choice of mass production of miniature parts for its advantages including high efficiency, high quality and low cost. However, the mechanical properties and fracture mechanism of metal foil differ significantly from sheet in macroscopic size, and the current research in this area is infrequent, which limits the development of metal foil microforming technology. Therefore, the study of size effects of metal foil during plastic deformation is necessary.Tensile tests of pure copper foil and brass foil with various thickness and grain size were performed at room temperature to investigate the size effects on yield strength, tensile strength and elongation. A set of non-contact video extensometer based on image measuring technique was developed to improve the strain measurement accuracy of the tensile test. A reliable tensile test method for metal foil was established.The results showed that the influence of the sample width on the mechanical properties of copper foil is inconspicuous. The yield strength affected by the thickness and grain size, but with no obvious proportional relations between them, its variation was different from the "smaller is weaker" of the metal sheet, but also from the "smaller is stronger" of the metal film, presented the complicated size effect. The size effect mechanism of yield strength was analyzed from the dislocation theory, the "smaller is weaker" phenomenon of yield strength was caused by the surface layer, and the "smaller is stronger" was the result of the strengthening effect of strain gradient, which rapidly increased with decreasing thickness. As the thickness of foil was intervenient between sheet and film, so the yield strength was sensitive to the change of thickness, which also showed the variation of "transition". The experimental rules of two kinds of copper foil were almost identical: the value about 10 times of the characteristic length was a critical point for the thickness. For the copper foil, the variation of yield strength would exhibit the "smaller is weaker" if the thickness was greater than 40μm, but otherwise it showed as "smaller is stronger". Integrated the influences of the surface layer and strain gradient, a modified equation of yield strength contained the thickness and the ratio of surface layer grain based on Hall-Petch equation was established. This relationship can describe the yield strength size effect of foil properly.The strain hardening exponent of copper foil was proportional to the grain size and thickness. The tensile strength was proportional to the thickness, and inversely proportional to the grain size. Elongation increased with the increasing grain size, and decreased sharply with the decreasing thickness. The variation of copper foil in elongation was entirely different from sheet metal. The calculated results based on spherical cavity model and strain gradient constitutive equation showed that the growing rate of hole is significantly restrained by the strain gradient. The fracture surface analysis of tensile specimen revealed the size effect mechanism of elongation. The elongation of copper foil which thickness less than 40μm was lower because its fracture mechanics was intergranual crack. As the fracture mechanics was dimple fracture, and the holes postponed the fracture, the elongation of the thicker copper foil was well.The simulation of micro deep drawing was performed by finite element software DYNAFORM. Multiple sets of micro deep drawing dies were designed and fabricated. Micro deep drawing experiment with billets under different conditions was carried out to study the size effects in microforming. The results showed that the nondimensional drawing load and limit drawing ratio were decreased with the reduction in the size of deep drawing parts, but at the same time, the impact of friction escalated. Influenced by the increasing force of friction, the LDR of 1mm drawing parts with larger grain size was inversely proportional to the elongation. The surface quality of the micro deep drawing parts depended on the grain size. Finally, drawing parts of 1mm to 8mm in diameter with good surface quality were drawn by blanking-deep drawing compound dies.