|Title:||Experimental and numerical studies on multi-scaled progressive and compound forming of bulk parts and size effects on process performance and product quality|
|Advisors:||Fu, Mingwang (ME)|
|Subject:||Production engineering |
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Mechanical Engineering|
|Pages:||xi, 166 pages : color illustrations|
|Abstract:||Progressive and compound meso/microforming by directly using sheet metals is a promising approach to realizing mass production of complex and meso-/micro-scaled bulk parts and structures with high productivity and material utilization. However, due to the size-related extrinsic and intrinsic parameters of materials and forming systems, the emergence of size effect induces different mechanical responses and deformation behaviors in differently size-scaled domains. Investigation of the size-dependent process performance and product quality from the aspects of forming load, material flow, dimensional accuracy, defects, surface quality, and fracture is necessary to promote the application of this technology. In this research, various progressive and compound forming systems for making different meso-/micro-scaled parts were developed and their forming processes and products were comprehensively explored through physical experiments and numerical simulations. The results revealed that the formation mechanism and characteristics of shear bands and dead metal zones are related to velocity gradient and strain accumulation. For the punching/blanking operation, the grain size effect results in the deviation of punch stroke and the variation of part dimensions. When the punch-die clearance equals the grain size, the maximum ultimate shear stress of blanking and the highest burr are obtained. The larger grain size and punch-die clearance increase the material loss and reduce the bulge diameter of the produced parts. Moreover, various surface defects including microcracks, micro pits, uneven surface, longitudinal surface texture, sunken area, and surface damage were found on different features.|
To quantitively explore the size-dependent meso-/micro-scaled deformation behaviors in the progressive and compound meso/microforming, constitutive modeling considering grain orientations and the grain boundary-interior difference was proposed. The orientations and anisotropies of the individual grains become a non-trivial issue in prediction of the mechanical responses and deformation behaviors of the down-scaling materials. Meanwhile, the grain boundary is generally considered to be isotropic and to impede deformation. The feasibility of this modeling was verified through experiments and simulations with different specimens and grain sizes, and geometrical and grain size effects on the flow stress were reflected. The modeling combining GTN criterion revealed that cracks initiate and elongate near grain boundaries. More and slighter cracks appear with the fine-grained material while coarser grains promote larger and serious cracks on the heading-formed parts. Compared with crystal plasticity finite element (CPFE) model in meso-compression tests, the modeling loses a bit of accuracy in flow stress and free-surface formation but has advantages of efficiency. The modeling can also predict a zig-zag distribution of shear bands and similar geometries as the compressed specimens with different grain sizes. This modeling thus considers both precision and efficiency in meso-/micro-scaled deformation and forming problems of polycrystalline metallic materials.
In addition, the quality of defect-free parts and the efficiency of processes are crucial in metal forming, and size effects induce many unknown deformation behaviors in product miniaturization. In this research, a numerical design-based method associated with different tooling designs and processes was employed to investigate the size-dependent mechanism of folding defects and their avoidance to improve material flow. Four designs were proposed with different forming sequences of the part features, and the best of them was selected by the analysis of material flow during each forming process and the comparison of energy consumption, folding avoidance, and geometrical accuracy among different designs. The experiments and morphological observations were conducted with three size scales and the results were consistent with the simulations. It was found that the macro-scaled folding defect was serious and regular but the meso-/micro-scaled one was slight and irregular.
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