Author: Wang, Jilai
Title: Investigation on defect formation and dimensional accuracy in micro-forming process using experiments and numerical simulations
Advisors: Fu, Mingwang (ME)
Degree: Ph.D.
Year: 2016
Subject: Microtechnology.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xi, 158 pages : color illustrations
Language: English
Abstract: In macro-scaled plastic deformation processes the knowledge related to material deformation behavior, formability, and fracture behavior have been well-developed, which the industrial community has well used to produce qualified products. However, the existing theory and knowledge in macro-forming domain may not be exactly accurate and cannot directly be applied in micro-scaled deformation processes, as the emergence of the so-called size effects impedes the transfer of knowledge from macro-scale to micro-scale. To realize defect-free production by using microforming technology, the material flow behaviors, defect formation mechanism, and the factors which affect the dimensional accuracy of the micro-formed products, need to be systematically studied and investigated. In tandem with this, the two critical issues in microforming, viz., flow-induced defect and dimensional accuracy in micro-deformed parts, are the focus in this research. They are systematically explored from a theoretical and rationale perspective by using physical experiments and numerical simulations. Flow-induced defects are generated by the irrational material flow in a forming process and have a significant effect on the quality of micro-formed parts. In design of microformed part and microforming processes, this type of defects need to be analyzed and the formation mechanism identified, such that the defects can be predicted and avoided via the rational design of micro-formed parts and microforming process. To address this issue, size effect affected material flow and deformation behaviors are firstly investigated and how the size effect affecting the flow-induced defects is then studied. Through experiment, the effects of geometry and grain sizes on formation of flow-induced defects in the microforming process of pure copper is extensively explored. In addition, microstructure evolution and flow pattern in micro-scaled extrusion of the parts with complicated geometries have also been studied. The experimental results show that the formation of folding defects is mainly affected by geometric size. The defect-free deformation occurs in the cases with coarse grains. Therefore, it is believed the parts with coarse grains do not have flow-induced defects, but some grains are broken and become a potential insecurity source. Furthermore, there is a balanced relationship between grain and geometry sizes with which the flow-induced defects and grain breaking can be successfully and simultaneously avoided.
In the micro-bending process of sheet metals, springback caused by the considerable elastic recovery during unloading has a significant effect on the dimensional accuracy of micro-bent sheet metal parts. To explore the unloading springback of sheet metals after microbending, experimental studies on the interactive influence of a few significant parameters - including sheet thickness, grain size, and punch radius - on the springback in micro U-bending process of copper alloy sheets were conducted. An appropriate constitutive model based on the surface layer model, which is widely used in micro-scaled forming processes, is proposed, in order to predict the amount of springback, and the relationship among these parameters is further established. The research shows that the springback angle generally increases with decreasing sheet thickness, and for different punch radii the springback angle has different variation trends for sheet metals with different grain sizes. The findings on how size effect affects the springback behaviors in micro-bending process are validated and corroborated by the experimental results and finite element simulation. In micro-scale plastic deformation, material deformation and ductile fracture are quite different from those in macro-scale deformation domain, due to the existence of size effects, leading to change of the mechanical behaviour of materials. To explore the interactive geometry and grain size effect and stress condition on material fracture behaviour in meso/micro-scaled plastic deformation, multi-scaled uniaxial tensile tests and compression tests of pure copper with different geometrical sizes and microstructures were performed. The experimental results reveal that microvoids in the compressed samples are due to localization of shear band instead of macro fracture, while microvoids on fracture surfaces get smaller with the decreasing surface grain ratio {466} in uniaxial tensile samples. The FE simulation was conducted by using the size-dependent surface layer constitutive model to study the interaction of size effects and stress condition on material fracture behavior in multi-scaled deformation. It is found that the stress triaxiality T generally increases with the ratio of surface grains {466} in compression condition. Fracture strain and energy are much smaller with positive T than negative T, regardless of feature and grain sizes. This research provides an in-depth understanding of fracture in micro-scaled plastic deformation.
Rights: All rights reserved
Access: open access

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