|Title:||Experimental and numerical studies on the macro- and micro-scale metal-deformed product design and development|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Mechanical Engineering|
|Pages:||xv, 225 leaves : ill. ; 30 cm.|
|Abstract:||In the traditional metal-deformed product design and development paradigm, product, process and tooling design is generally conducted based on heuristic know-how and experience, which are basically acquired through many years of practice. This kind of product development paradigm is of more trial-and-error than in-depth scientific calculation and analysis. Late design changes are always needed. It is thus costly, time-consuming and error-prone. The optimal design in this design paradigm would be very difficult, if not impossible, as there are many design parameters related to forming process, tooling, preform, deformed part geometry, and the equipment used, which interact and interplay. On the other hand, this traditional manufacturing process is facing challenges since the competition in this industry cluster is becoming very severe and the profit is very marginal. Therefore, how to improve product quality, cut production cost and shorten product development lead-time are critical to keep the companies' cutting edge in the marketplace. To address these challenges, "design right the first time" and "optimal design" are critical. To achieve these goals, the understanding of material deformation behaviors is the first step to reduce the trial-and-error in workshop. The issues to be addressed are in two different domains, viz., macro- and micro-scale. This dissertation is therefore focused on addressing these two-domain issues and thus divided into two parts. The first part is to develop the methodologies to support the design and development of macro metal-deformed part in terms of product quality assurance and die life improvement, while the second part is to investigate and model the size effect in micro-scale plastic deformation. For the first part, the focus is on product quality assurance via the research on folding defect in macro-sized axially-symmetrical flanged part. Folding defects commonly exist in metal-deformed parts and create a significant material discontinuity. This kind of defect significantly degrades the quality of the deformed parts. The defective region always causes stress concentration and leads to the early or sudden failure of parts in service. The folding defect formation in the forging process of axially-symmetrical flanged parts and its avoidance mechanisms were systematically investigated in this research. Different geometrical parameters affecting folding defect occurrence and the parameter variation characteristics were extensively examined. The design of die geometries for defect avoidance was conducted. In addition to the product quality assurance issue, the method for die fatigue life improvement is the other focus in the first part. Tooling usually takes up a considerable amount of cost in product development. It is subjected to dynamic stress in metal forming process. The dynamic stress is repeated for each production shot. Fatigue is a major failure mode, especially in cold forging process. Improper design is a common issue which leads to the early failure of tool. In this dissertation, an integrated finite element method (FEM) and artificial neural network (ANN) approach for optimizing part and tooling design for prolonging die fatigue life is proposed. In the proposed approach, the FEM simulation is employed to generate the training cases for ANN. The well-trained ANN is used to predict the performance of design solutions and identify the desired design parameter configuration. The presented methodology helps optimize forming system design in up-front design stage and reduces the number of simulation scenarios.|
In the second part of this dissertation, the research on material size effect in micro-scaled plastic deformation is presented. The demand on micro-part is increasing due to product miniaturization. The micro-part fabricated by micro-forming is defined as the plastic deformed part with at least two dimensions in sub-millimeter range. When the material size is decreased to micro-scale, the so-called size effect occurs and the mechanical behavior of material changes. The conventional know-how on macro-forming process is not applicable when the material size is scaled down to micro-scale. It is thus difficult to get the material deformation under control since there are many unknowns about size effect. To design the micro-forming process, and tooling, and further control the product quality of micro-parts, the deformation behavior and mechanics in micro-scale including material flow stress, anisotropy, ductility and formability, etc., need to be extensively studied and explored. This research investigated the size effect on micro-scale plastic deformation and frictional phenomena of bulk and sheet metals. Through the extensive experiments of tensile test, micro-cylinder compression test, and micro-ring compression test, it is found that the flow stress decreases, the amount of springback increases, the interfacial friction increases and the material tends to flow inhomogeneously when the ratio of specimen size to grain size is decreased in micro-scale plastic deformation. The analytical and FE-based models for describing the size effect related phenomena, such as the change of flow stress, friction and deformation behavior, are proposed. It is further revealed that the flow stress curve obtained from micro-compression test is not applicable in modeling of micro-extrusion process. It is thus believed that the flow pattern, the material surface constraint and the material deformation mode are critical in determination of material flow stress curve. The identified deformation and mechanics behaviors presented in the second part of this dissertation provide a basis for further exploration of the material deformation behavior in micro-scale plastic deformation and the development of micro-scale products via micro-forming.
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