Research On Flow Stress Size Effects And Filling Behavior In Microforming Of Pure Nickel

Comparing to other micromanufacturing technologies, microforming offers many attractive advantages, e.g. high production rates, high material utilization, low manufacturing costs, excellent performance of forming parts, high dimensional accuracy and so on. It will play an important role in promoting the rapid development of the miniaturization of productions. However, microforming is not just scaled down the processes to microscale simply from macroforming. The plastic deformation behaviors are different from those in macroforming, when the feature sizes or the dimensions of the metal parts scale down to the micro scale. The plastic deformation behaviors are dependent on the part dimensions strongly, so so-called size effects occur. Size effects are the most basic problems in microforming, also the hot and difficult researching points in microforming all over the world. It is urgent to carry out basic research related to size effects.In this dissertation, the deformation behaviors including flow stress size effects and inhomogeneous deformation behaviors are investigated experimentally. A size effect model based on Taylor polycrystals in microscale is proposed to reveal the mechanism of flow stress size effects in uniaxial micro compression. The deformation behaviors of polycrystals in micro coining process are investigated experimentally and the mechanism of filling is revealed. The microforging processes of the micro turbine are studied experimentally. The crucial technology for intergral forming of the complicated micropart has been solved.A micro compression device is developed based on the characteristics of micro compression. The micro compression tests of cylinders are carried out using the developed micro compression device. It indicates that the degree of inhomogeneous deformation tends to increase with the increase of grain size or decrease of specimen size. The flow stress decreases with the increase of grain size. However, the flow stress tends to increase when there are only3to4grains across the thickness. Further analysis indicates that the flow stress grain size effect occurs when the ratio of specimen size to grain size is larger than9. Or the flow stress specimen size effect occurs.A flow stress size effects model based on the Hall-Petch law in macroforming is developed considering the effect of boundary conditions of grains on the Taylor factors and grain boundary densities. The effect of specimen size, grain size and mould constraint on the fractions of surface layer grains, internal layer grains, mould constrainted grains and grain boundary size factors is analyzed. The functional relationship between flow stress and these parameters is established. A flow stress size effects model in microfroming of polycrystals is established. Analytical results indicate that flow stress specimen size effect occurs when the critical specimen size to grain size ratio scales down to6. For micro tension, the critical specimen size to grain size ratio is4.The experimental results in micro coining indicate that the formability is related to the cavity width t and the ratio of cavity width to grain size t/d strongly. The worst formability occurs when t is50m and t/d is1.04. The critical ratio of cavity width to grain size t/d increases with the increase of the cavity width t. The critical ratio of cavity width to grain size t/d tends to disappear when the cavity width t increases to300m, because of the transformation of the deformation patterns. The analytical results of Electron Backscattered Diffraction (EBSD) indicate that each grain deforms inhomogeneously, when there are several grains across the cavity width in micro coining. The grains approaching the mould corners deform heavily, the inner grains take the second place and the surface grains have a minimum deformation. The single grain undergoes severe plastic deformation and fragmentates into smaller grains, when there is only one grain across the cavity width in micro coining. The fragmentated grains with certain prolongation distribute along the extrusion direction.Based on the above research, the forging processes of micro turbine of pure nickel and7075Al alloys are carried out experimentally. A device with an ejector of a screw is developed to remove the formed micropart easily. The experimental results of forging the micro turbine of pure nickel at the room temperature indicate that it is hardly to form micro turbine with good quality no matter using coarse or fine grain materials. The effect of processing parameters on the forming processes of isothermal micro forging of micro turbine using7075Al alloys is investigated experimentally. A micro turbine with good quality is manufactured under the conditions of forming temperature450℃and forming load9kN using the circular ring preform with2mm hole. A device is proposed to measure the shearing property of the micro blade and applied to evaluate the mechanical property of micro turbine by the shear strength of the micro blade. The shear strength of the micro blade is enhanced by solid solution and aging treatments. Expeimental results indicate that the proposed processes can be used to manufacture the micro turbine with good quality.