Modeling of material deformation behavior in micro-forming under consideration of individual grain heterogeneity

This study aims to develop a model to characterize the inhomogeneous material deformation behavior in micro-forming.First,the influence of individual grain heterogeneity on the deformation behavior of CuZn20 foils was investigated via tensile and micro-hardness tests.The results showed that different from thick sheets,the hardening behavior of grains in the deformation area of thin foils is not uniform.The flow stress of thin foils actually only reflects the average hardening behavior of several easy-deformation-grains,which is the reason that thinner foils own smaller flow stress.Then,a composite modeling method under consideration of individual grain heterogeneity was developed,where the effects of grain orientation and shape are quantitatively represented by the method of flow stress classification and Voronoi tessellation,respectively.This model provides an accurate and effective method to analyze the influence of individual grain heterogeneity on the deformation behavior of the micro-sized material.

Deformation behavior of materials in micro-forming with consideration of intragranular heterogeneities

To describe the relationship between the whole material deformation behavior and each grain deformation behavior inmicro-forming,experimental and numerical modelling methods were employed.Tensile test results reveal that contrary to the valueof flow stress,the scatter of flow stress decreases with the increase of thickness-to-grain diameter(T/d)ratio.Microhardnessevaluation results show that each grain owns unique deformation behavior and randomly distributes in each specimen.The specimenwith less number of grains would be more likely to form an easy deformation zone and produce the concentration of plasticdeformation.Based on the experiment results,a size-dependent model considering the effects of grain size,geometry size,and thedeformation behavior of each grain was developed.And the effectiveness and practicability of the size-dependent model wereverified by experimental results.

Hybrid Micro-forming Processes and Quality Evaluation of Micro-double Gear

Micro-gear is an important actuating component used widely in the micro electro mechanical systems(MEMS) devices.The technologies of micro-forming and precision assembly are urgently developed to manufacture the micro-double gear with central shaft.In the paper,a novel hy-brid-forming process with two kinds of piercing method have been proposed to manufacture the micro-double gear using micro forming technology.The tests of hybrid forming process were carried out with two steps and the micro-double gear was successfully manufactured with good surface quality.The results also show that the hybrid micro-forming process with central piercing method can improve the defects of inclining shaft generated by double-ended piercing method.The quality evaluation of micro-double gear was conducted with surface roughness,micro-hardness and impact tests.The results show that the micro-double gear with good mechanical properties can meet the requirements of application for milli-machines.

Establishment of size-dependant constitutive model of micro-sheet metal materials

The inherent mechanism of size effect in micro-sheet material behavior of plastic forming was explained by the surface layer model and theory of metal crystal plasticity. A size-dependant constitutive model based on the surface layer model was established by introducing the scale parameters and modifying the classical Hall-Petch equation. The influence of the geometric dimensions and the grain size on the flow behavior of the material was discussed using the new material constitutive model. The results show that, the flow stress decreases while the sheet metal thickness decreases when the grain size keeps constant, and the micro-sheet metal with a larger grain size is more easily to be influenced by the size effects. The material constitutive model established is validated by the stress-strain curve of the micro-sheet metal with different thicknesses derived from the tensile experiments. The rationality of the material model is verified by the fact that the calculation results are consistent with the experimental results.

Size effects on plastic deformation behavior in micro radial compression of pure copper

Micro radial compression tests were carried out on cylindrical specimens of pure copper polycrystals with different grain sizes. Experimental results indicated that phenomena of decreasing forming force, increasing scatter of forming force and more irregular surface topography occurred with the increase of grain size. A modified surface model based on dislocations pile-up in surface layer grains, and a flow stress scattering formulation based on standard deviation and grain size distribution were proposed to analyze size effects on forming force in micro compression. The inhomogeneous deformation of surface layer grains was discussed by the main deformation manner of rotation. A good agreement with the experimental results was achieved.

Crystal Plasticity Finite Element Process Modeling for Hydro-forming Micro-tubular Components

Micro-tubes manufactured by hydro-forming techniques have now been widely used in medical and microelectronics applica- tions. One of the difficulties in forming such parts is the control of localized necking in the initial stages of the deformation/forming process. A lack of microstructural information causes conventional macro-mechanics finite element（FE） tools to break down when used to investigate the localized microstructure evolution and necking encountered in micro-forming. An effort has been made to create an integrated crystal plasticity finite element（CPFE） system that enables micro-forming process simulations to be carried out easily, with the important features in forming micro-parts captured by the model. Based on Voronoi tessellation and probability theory, a virtual GRAIN（VGRAIN） system is created for generating grains and grain boundaries for micro-materials. Numerical procedures are devel- oped to link the physical parameters of a material to the control variables in a Gamma distribution. A script interface is developed so that the virtual microstructure can be input to the commercial FE code, ABAQUS, for mesh generation. A simplified plane strain CPFE modeling technique is developed and used to capture localized thinning and failure features for hydro-forming of micro-tubes. Grains within the tube workpiece, their distributions and orientations are generated automatically by using the VGRAIN system. A set of crystal viscoplasticity constitutive equations are implemented in ABAQUS/Explicit by using the user-defined material subroutine, VUMAT. Lo- calized thinning is analyzed for different microstructures and deformation conditions of the material using the CPFE modeling technique. The research results show that locations of thinning in forming micro-tubes can be random, which are related to microstructure and grain orientations of the material. The proposed CPFE technique can be used to predict the locations of thinning in forming micro-tubes.

Effects of specimen size on flow stress of micro rod specimen

With the miniaturization of parts,size effects occur in the micro-forming processes.To investigate the effects of the specimen size on the flow stress,a series of upsetting deformation experiments were carried out at room temperature for specimens with different diameters.And the grain size of billets was changed by anneal processes to analyze the grain size effects on the size dependence of flow stress.The deviation of stress was observed.The results show that the flow stress decreases with decreasing billet dimensions.As the dislocation accumulation in free surface layer is slight,the reduction degree of flow stress becomes larger when the plastic deformation goes on.The flow stress is enlarged by grain size,which can be analyzed by the grain boundary length per area.The deviation increases with decreasing specimen size.This can be explained by the effects of grain orientation stochastic distribution according to the Schmid law.As a result,the micro-forming process must be considered from the viewpoint of polycrystalline structure,and the single grains of micro-billet dominate the deformation.

Effect of Relative Grain Size on the Flow Stress of Polycrystalline FCC Sheets

A dynamic explicit finite element code and rate-dependent elastic-viscoplastic polycrystalline model were used to simulate the simple tension test of annealing FCC polycrystal and 6111-T4 aluminum alloy sheet metal. The variability of flow stress was investigated, which was induced by various grain boundary constraints when the ratio of thickness-to-grain size was changed. The simulated results show that, when the relative grain size increases, the constraint of grain boundary will increase accordingly, which results in the increase of the flow stress of polycrystal. The results agree with experiments.

Effect of thickness and grain size on material behavior in micro-bending

Micro-bending tests were performed to investigate effects of thickness and grain size on material behavior in sheet metal forming.The rolling brass C2680 foil was selected as the experimental material,and it was annealed to eliminate the work-hardening and get different grain sizes.A device was specially designed for three-point bending with two load sensors.The results show that the bending force increases with increasing the punch displacement.With the foil of same thickness,a smaller punch radius leads to a larger bending force.When the grain size increases,the bending force becomes smaller.Size effects are observed obviously.These results have been analyzed by work-hardening,Hall-Petch equation and free surface effect.

Micro-hydromechanical deep drawing of metal cups with hydraulic pressure effects

Micro-metal products have recently enjoyed high demand. In addition, metal microforming has drawn increasing attention due to its net-forming capability, batch manufacturing potential, high product quality, and rela- tively low equipment cost. Micro-hydromechanical deep drawing （MHDD）, a typical microforming method, has been developed to take advantage of hydraulic force. With reduced dimensions, the hydraulic pressure development changes; accordingly, the lubrication condition changes from the macroscale to the microscale. A Voronoi-based finite element model is proposed in this paper to consider the change in lubrication in MHDD according to open and closed lubricant pocket theory. Simulation results agree with experimental results concerning drawing force. Changes in friction significantly affect the drawing process and the drawn cups. Moreover, defined wrinkle indexes have been shown to have a complex relationship with hydraulic pressure. High hydraulic pressure can increase the maximum drawing ratio （drawn cup height）, whereas the surface finish represented by the wear is not linearly dependent on the hydraulic pressure due to the wrinkles.