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Title: Failure analysis of titanium tailor-welded blanks under multi-stage forming process
Authors: Lai, Chi Ping
Degree: Ph.D.
Issue Date: 2014
Abstract: The use of an alternative material to manufacture lightweight and functional improved components is a trend of modern automobile industry. Titanium and its alloys seem to be a marvelous choice as they are light in weight and provide outstanding corrosion resistance, low density and high strength. However, due to the high cost of raw material and poor machinability, the applications of titanium alloy sheets in the automobile or aircraft industry seem to be limited. To tackle these problems, by combining the advantages of titanium alloy and tailor-welded blank technology, forming titanium tailor-welded blanks (Ti-TWBs) at elevated temperatures is expected to be one of the solutions. The primary objective of this project is to develop a method of failure analysis for titanium tailor-welded blanks under multi-stage forming process. This project aims to develop a reliable failure prediction model based on damage mechanics together with the experimental validation and material analyses that should enable the acquisition and validation of the formability for Ti-TWBs at elevated temperatures under the multi-stage forming process. With the aid of the experimental results (i.e. structural deformation and mechanical properties of Ti-TWBs under complex loading paths at elevated temperatures) and theoretical analyses, a damage-based failure model is then developed to examine the deformation behaviors for Ti-TWBs under the multi-stage forming process. It was found that a temperature of around 550°C did improve the elongation of Ti-TWBs significantly. An experimental measurement system developed by Chow et al. was then modified and systematically employed to acquire the distinctive material properties, mechanical data and damage variables for each material region of a TWB, i.e. base metal, heat affected zone and weldment. Such data were then implemented with other material properties which measured experimentally into the forming simulations of Ti-TWB in a general-purpose finite element package.
The formability analysis and prediction of Ti-TWBs under multi-stage forming process was then predicted by using the damage model. The model is based on an isotropic damage mechanics with temperature dependent effect. The thermal-dependent damage parameters and mechanical properties of tested materials under various temperatures (i.e. stress-strain curves, effective Young's modulus and effective Poisson's ratio) were employed to simulate the damage evolution and deformation behavior of the captioned materials during the thermal process. A damage criterion for localized necking was proposed not only for proportional loading but also for non-proportional loading conditions. This model was not only to simulate the whole forming process of sheet metal, but also to predict the occurrence of localized necking and final fracture of the formed component. In order to conduct a more accurate formability diagram, as well as to verify the developed prediction model, both theoretical and experimental investigations were then conducted, and the obtained data and findings were systematically analyzed using statistical methodology. The predicted failure locations, FLDs and the stress-strain relationship of Ti-TWBs were determined accordingly after simulations. Validation of the results obtained from the simulations and experiments were then compared for the evaluation of reliability and accuracy. Quite a satisfactory agreement was found between the simulated and experimental results. The significant contribution of the study is to develop a reliable failure prediction model that should enable the acquisition and validation of the formability for Ti-TWBs at elevated temperatures under the multi-stage forming process. This workable approach will certainly facilitate engineers to predict and virtually optimize the main technological parameters for forming Ti-TWBs before its physical realization.
Subjects: Light metals -- Metallurgy
Titanium alloys -- Welding
Hong Kong Polytechnic University -- Dissertations
Pages: xxvi, 186 pages : color illustrations
Appears in Collections:Thesis

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