Study On Strain Gradient Hardening Effects In Microbending Process

Metal plastic microforming is becoming an important manufacturing process for metal micro parts and is investigated by more and more researchers recently. In metal microforming process, some material parameters, such as crystal microstructure, surface roughness etc. are the same as ones in macro scale material forming process, but its macro geometrical dimensions, such as foil thickness, wire diameter and indentation depth, are very small, in finally, the metal plastic forming behaviours are changed. The properties of metal material are different from macro scale forming and related with the macro geometrical dimensions, which is referred to as size effect. The conventional metal forming technology and theory in macro scale cannot be applied directly to metal plastic microforming through scale down the specimen and tooling size due to the lack of the material length scale in their constitutive models. In order to study on the size effects in microforming process, the strain gradient plasticity theory is developed, in which the conventional strain hardening and the strain gradient hardening are included together and the material intrinsic length is present in the constitutive relation. In present dissertation, the plastic stran gradient theories are studied to explain the size effects in metal microforming. The research results are summarized as following:The uniaxial tensile tests are investigated using 25?m~500?m thickness foils for pure aluminium (99.5%) and CuZn37 brass. The results show that there is a size effect“smaller is weaker”, i.e. the yield strength of pure aluminium decreases with the foil thickness decreasing, and which is attribute to the dislocations in the surface grains might slip easily with brittle oxidation film and the share of surface grains in the overall volume increases with decreasing thickness. For the brass foils, the yield strength increases with the foil thickness decreasing, that is to say, there is a size effect“smaller is stronger”. The reason is that there exists the passivated and ductility oxidation film to block the dislocation slip out of the surface of brass and the ratio of the thickness of oxidation film to the brass foil increases with the brass foil thickness decreasing. In the microbending experiments for these two metal materials, the springback angle increases with decreasing foil thickness, indicating obvious size effects of“smaller is stronger”. The hardness distribution in bended area shows that there is an obvious middle layer for fine grain structure samples and no middle layer for coarse structure samples. The size effects in microbending process are investigated with strain gradient theories.The analysis results show that Fleck-Hutchinson's strain gradient plasticity theory with higer-order stress tensor and Nix-Gao's strain gradient plasticity theory based on the Taylor relation can catch these size effects, but the springback angles or the microbending moment based on the modified Nix-Gao strain gradient plasticity theory are in better agreement with the experimental data. The material intrinsic length is one of the important factors in plastic strain gradient plasticity theory and originally proposed for dimensional consistency. In the Nix-Gao's srain gradient theory, the material intrinsic lengths calculated from its equation are different from the ones fitted by the experimental data. In present dissertation, the material intrinsic length is related to material properties and average grain numbers along the characteristic scale direction of part to improve the strain gradient hardening effects. The calculated results of material intrinsic length are closed to the fitting results from experiment data. The less the grain numbers across the foil thickness is, the larger the strain gradient hardening based on the geometrically necessary dislocation is. On the contrary, the more the grain numbers across the foil thickness is, the larger the uniforming strain hardening based on the statistically stored dislocation is, which is in accordance with the Hall-Petch relation.The error of springback angle is larger for thinner foils in microbending experiments, in which there is only one grain across the thinner foils. Therefore, the anisotropic properties of material are more and more highlights together with the size effects phenominon. The anisotropic properties of single crystal together with the shear strain gradient hardening are expressed in the strain gradient crystal plasiticity theory, in which the shear strain gradient hardening is introduced directly into the evolutionary equations of the slip resistence. The new simple numerical algorithm to obtain the shear strain gradient is proposed, in which the concept of the closed neighbour domain of integration points are used to calculated the shear strain gradient between the present integration point and neighbour points in the range of predefined domain. Two examples are provided to illustrate the present theory, including a single crystal subject to microindentation and microbending deforming processes. The FE analysis results of microindentation on a single crystal copper show that the indentation load is closed to the experimental data for the same indentation depth and other results are good agreement with the simulation results from published papers. The accuracy of the strain gradient crystal plasticity theory and simplified numerical algorithm of shear strain gradient is verified. The significant fluctuates of the error for thinner foils in microbending experiments are better attributed to the anisotropic properties of single crystal in microbending simulation of different orientation. The thinner foils are single crystal through the thickness but many grains across the thicker foils; therefore the later is approach to the material isotropic properties.