| Author: | Hu, Dien |
| Title: | Investigation on size effect in meso/micro-scaled plastic deformation via compound forming and multi-stage deep drawing techniques |
| Degree: | M.Sc. |
| Year: | 2022 |
| Subject: | Microfabrication Manufacturing processes Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xiii, 74 pages : color illustrations |
| Language: | English |
| Abstract: | Meso/microforming has gained much more attention in the last decades and is widely used as a reliable method to fabricate meso/micro-scaled metallic components. In this dissertation, a compound meso/microforming system which combines deep drawing, punching and blanking operations was developed to fabricate multiscale central-punched cups by using brass sheets. The parts with three scales were produced by using the brass sheets with various thicknesses and grain sizes to investigate geometrical and grain size effects on the deformation behaviors, dimensional accuracy, and material flow behaviors in the forming process. Through physical experiments and finite element simulations, it is revealed that the ultimate deformation load in the drawing-punching stage is smaller than that in the single deep drawing stage under microscale, but the results in the meso-scaled scenarios are opposite. In addition, the thickness variation is increased with grain size, but the variation of the normalized thickness variation does not show an obvious tendency with different size scales. In the bending area, the material flow is tangential to the thickness direction, leading to the formation of thinning area. In addition, the material flow is almost opposite to the punching direction in the punching area, avoiding the expanding deformation of the hole. Thus, the punching operation barely affects the dimensional accuracy including the thickness and hole diameter of the formed parts. Furthermore, the micro-scaled cups with finer grains have a better surface quality. These findings enhance the understanding of size effect in compound meso/microforming with the combined deep drawing and punching operations. Nowadays, pogo-pin is a kind of micro-scaled component wildly used in many fields. The central-punched cup needs to be further study to fabricate barrel-shaped cup with a large limit drawing ration which one of parts can be assembled to form a pogo-pin. A three-stage deep drawing microforming system was developed to fabricate the barrel-shaped cup directly using brass sheets. The cups with various grain sizes were produced to investigate how grain size affects the material evolution including deformation behavior, dimensional accuracy, microhardness and surface quality in this process. The deformation load in each operation is increased with the decrease of grain size through experiments and finite element (FE) simulations, though there are some deviations between them. Besides, there is a thinning area of the cups caused by the various material flows. The diameter of the upper hole and the thickness variation of the cross-section of the components also increase when grain size decreases. However, the inner diameter is not affected by grain size. The dimensions have the potential ability to be assembled with the current bulk and plunger. In addition, the microhardness value of coarse grain is lower and the variation of the value is corresponding to that of effective strain in FE simulations. Wrinkle occurs in the inner wall of the cup. Bugle, micro crack, facture surface and uneven surface also appear in the cup. The cups with fine grains have the best surface quality. These findings promote the understanding of material evolution and grain size effect in multi-stage deep drawing process. In this dissertation, scale size effect and grain size effect in various process, especially deep drawing process, were further studied. Besides, a potential method of the fabrication of barrel-shaped cups was developed. |
| Rights: | All rights reserved |
| Access: | restricted access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| 6876.pdf | For All Users (off-campus access for PolyU Staff & Students only) | 15.2 MB | Adobe PDF | View/Open |
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