Interactive effect of microstructure and cavity dimension on filling behavior in micro coining of pure nickel

Scientific Reports, Apr 2016

In this study, interactive effects of microstructure and cavity dimension on the filling behaviors in micro coining were investigated. The results indicate that the filling ability is dependent on both the cavity width t and the ratio of cavity width to grain size t/d strongly. The critical ratio t/d for the worst filling ability increases with cavity width t and tends to disappear when the cavity width t increases to 300 μm. A polycrystalline filling model considering the friction size effect, effect of constrained grains by the tools, grain size, cavity width and ratio of cavity width to grain size is proposed to reveal the filling size effect in micro coining. A quasi in-situ Electron Backscatter Diffraction (EBSD) method is proposed to investigate filling mechanism in micro coining. When several grains across the cavity width, each grain deforms heterogeneously to ordinate the deformation compatibility. When there is only one grain across the cavity width, the grain is fragmented into several smaller grains with certain prolongation along the extrusion direction to coordinate the deformation in the cavity. This is different from the understandings before. Then the filling deformation mechanism is revealed by a proposed model considering the plastic flow in micro coining.

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Interactive effect of microstructure and cavity dimension on filling behavior in micro coining of pure nickel

Abstract In this study, interactive effects of microstructure and cavity dimension on the filling behaviors in micro coining were investigated. The results indicate that the filling ability is dependent on both the cavity width t and the ratio of cavity width to grain size t/d strongly. The critical ratio t/d for the worst filling ability increases with cavity width t and tends to disappear when the cavity width t increases to 300 μm. A polycrystalline filling model considering the friction size effect, effect of constrained grains by the tools, grain size, cavity width and ratio of cavity width to grain size is proposed to reveal the filling size effect in micro coining. A quasi in-situ Electron Backscatter Diffraction (EBSD) method is proposed to investigate filling mechanism in micro coining. When several grains across the cavity width, each grain deforms heterogeneously to ordinate the deformation compatibility. When there is only one grain across the cavity width, the grain is fragmented into several smaller grains with certain prolongation along the extrusion direction to coordinate the deformation in the cavity. This is different from the understandings before. Then the filling deformation mechanism is revealed by a proposed model considering the plastic flow in micro coining. Introduction Micro metal parts are widely applied in automotive, biomedical, consumer electronics and with the rapid development of micro electro-mechanical-systems (MEMS) and micro system technology (MST)1,2,3,4. In the last two decades, micro forming as a new micro manufacturing technology plays an important role on manufacturing micro metal parts. When the dimensions of metal parts down scale to micro scale, size effects occur and restrict the rapid development of micro forming5. It is needed to investigate the deformation behaviors in micro forming in-depth. Fu et al.6 found that the flow stress decreases and its scatter increases with the increase of grain size or decrease of specimen diameter by micro compression tests of pure cooper cylinders. The flow stress reduction with miniaturization can be interpreted by the surface models and modified surface models1,7,8,9,10,11 based on the softening effect of surface grain with free surfaces. Wang et al.12 revealed the flow stress scatter in micro compression through the proposed model considering orientation distribution of the surface grain. Wang et al.13,14 found that the flow stress increases when there are less than 3-4 grain across the specimen diameter in micro compression and revealed the mechanism through the proposed model considering the effect of surface, inner and constrained grains. Chan et al.15 found that the degree of inhomogeneous deformation increases with the increase of grain size in micro extrusion process of pure copper. Cao et al.16 found that the extruded micro pins curves when using the coarse grained materials. Lin et al.17 proposed a model based on crystal plasticity theory to reveal the curvature in micro extrusion of coarse grained materials. Meng et al.18 manufactured a multi-level flanged part is produced via progressive micro extrusion and blanking and investigated the effect of grain size on the microstructure evolution and fracture behaviors in progressive micro forming. Meng et al.19 also investigated the microstructure evolution of commercially pure titanium in thermal-aided meso forming of a dental abutment. The surface grains on the square extrudate generate an equiaxed structure because of severe deformation, reflecting that meso forming at elevated temperature facilitates the homogenization of material flow without coarsening grain size. Kim et al.20 manufactured micro gear shaft with good quality through ECAP process. Wang et al.21 developed a feature-based method for defect-free cold-forging process to manufacture a non axisymmetrical micro part. Yang et al.22 studied the effect of high-energy assistance on the micro deep drawing and micro forging processes. The formability and the surface roughness were improved. Wang et al.23 manufactured a micro turbine by isothermal micro forging process. A micro turbine with a higher micro blade is manufactured when using the circular ring preform compared to that using the circular one. To reveal the deformation mechanism in-depth, Wang et al.24 investigated the effect of the ratio of cavity width to grain size on the filling behavior through micro coining process. It indicated that the filling behavior is the worst when where are only about 2 grains across the cavity width at elevated temperature. Wang et al.25 found the similar results in micro coining at room temperature. The similar filling size effect was also found. Ast et al.26 investigated the microstructure evolution of three different grained materials in nano coining process by electron back scatter diffraction (EBSD). The results indicated that strong orientation gradients occurred below the cavities for single crystal, a sub-g (...truncated)


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Chuanjie Wang, Chunju Wang, Jie Xu, Peng Zhang, Debin Shan, Bin Guo. Interactive effect of microstructure and cavity dimension on filling behavior in micro coining of pure nickel, Scientific Reports, 2016, Issue: 6, DOI: 10.1038/srep23895