Date of Award

8-2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Grujicic, Mica

Committee Member

Li , Gang

Committee Member

Ochterbeck , Jay

Abstract

In order to satisfy the need for better ballistic performance against lethal threats, new grades of Titanium (e.g. Ti-6Al-4V) and Aluminum (e.g. AA5083) alloys are being employed in the design of blast survivable structures. These better performing alloys are not readily amenable to conventional welding process or result in inferior welds when joined using conventional welding process. On the other hand, Friction Stir Welding (FSW), a relatively new welding process, has been found to be successful in producing good quality welds in these alloys. FSW also offers better weld performance in comparison with the conventional welding process. But the methodology for employing FSW to weld blast survivable structures remains unexplored. Therefore a robust and cost-efficient three-step process to Friction-Stir-Weld blast survivable structures is introduced in the present work.
The first step in the proposed three-step methodology is to identify the FSW process parameters and tool design parameters that results in best quality welds and maximum productivity of the process. Since a purely experimental investigation of FSW process is expensive, computational Finite-Element-Analysis (FEA) procedures are incorporated in the methodology to reduce the amount of experimental investigation required. A fully-coupled thermo-mechanical FEA procedure is employed to investigate the spatial distribution and temporal evolution of material properties/microstructure with the FSW joints of Aluminum (AA5083) and Titanium (Ti-6Al-4V) work-pieces. In case of Ti-6Al-4V, the thermal history result from the computational analysis is used to determine the temporal evolution of the material microstructure in the weakest Heat-Affected-Zone (HAZ) region. Based on the well-established property vs. microstructure relationship for Ti-6Al-4V, and the temporal evolution of material microstructure for HAZ region, the overall structural performance of the weld is predicted. The computational results are compared with their corresponding experimental results found in open literature, and are found to be agreeable.
In the second step, the optimal weld joint designs used in different regions of the blast survivable structures are identified.
In the third step, problems regarding sub-scale modeling of blast survivable vehicle test structures are analyzed. The results obtained are used to analyze the potential of the current approach in enhancing blast survivability of military structures.

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