| Recently, with the development of micro-electronics technology, a variety of micro forming processes has been invented. But, with miniaturization of micro parts’ geometry, the flow stress of material has a significant difference with which in macroscale. The size effects have become a research hotspot at home and abroad.In this dissertation, the flexible micro-bending process with laser dynamic loading and quasi-static loading has been developed. The formability of micro-bending under both low and high strain ratio was affected by grain size(d), the ratio of thickness to grain size N(N=t/d) and feature size of micro-dies was studied. Meanwhile, based on ANSYS implicit analysis and LS-DYNA explicit analysis, the numerical simulation of flexible micro-bending processes with quasi-static loading and laser dynamic loading were completed. It shows that the formability of micro-bending processes in quasi-static loading is affected by grain size. The main research work and conclusions are as following:(1) In terms of forming depth, forming homogeneity and micro-hardness, the flexible micro-bending processes with different loading models show a strong size effect. The forming depth is measured by KEYENCE VHX-1000 microscope. It is found that the normalized forming depth increased first and then decreased with the decrease of N value in two different processes. The surface roughness of micro parts was measured by Axio CSM700 confocal scanning microscope. With the increase of grain size, the roughness increases in both processes. According to the measurement of thinning ratio, the thinning ratio increased first and then decreased with the increase of grain size. By the way, with the increase of grain size, the deformation becomes more inhomogeneous. The HV-1000 micro hardness tester was utilized to observe the micro hardness of micro parts. The micro hardness decreased first and then increased with the increase of N value both in shocked and un-shocked materials. The micro hardness was increased significantly after laser dynamic loading. Meanwhile, with the increase plastic strain, the micro hardness increased. The coarse-grained material hardening capacity was higher than the fine-grained material.(2) In the quasi-static loading process, the forming limit depth decreased with the increase of grain size. In the meantime, with the increase of loading force, the ductile fracture was found with different grain size. However, in the laser dynamic loading process, the workpiece can better fit the mold when the laser energy is 2J. The high strain ratio, grain refinement, high dislocation density, dynamic recrystallization in laser dynamic loading process would increase the formability of materials.(3) The flexible micro-bending process with two different loading models was studied by numerical simulation. It was found that the forming depth increased with the feature size of micro-dies. The von mises strain and von mises strain ratio decreased with the increase of feature size of micro dies. The change law in the dynamic loading process was opposite to the quasi-static loading process.(4) Due to the lack of the constitutive model of grain boundary and grain interior in dynamic loading process the deformation behavior affected by grain size and grain boundary strengthening was only discussed in quasi-static loading process. The geometrical model of grain structure was established with Voronoi tessellation. Model based on dislocation density was adopted to describe the flow stress of grain interior and grain boundary. It was found that the forming depth and surface roughness of micro parts increased with the grain size when N>1.Above all, in terms of forming depth, forming homogeneity and micro-hardness, the flexible micro-bending processes with different loading models showed similar size effect. However, the laser dynamic loading preformed a better deformation behavior. The research in this dissertation could provide some theoretical and experimental value for dynamic micro-forming process. |
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