Degree

Doctor of Philosophy (PhD)

Department

Mechanical and Industrial Enginnering

Document Type

Dissertation

Abstract

Friction- stir- welding (FSW) is comparatively a new welding process invented by The Welding Institute in 1991. Since no melting or fusion occurs during the welding process, FSW is free of high heat input and solidification defects. Two main objectives have been set forth in this work. The first object of this thesis is to develop thermomechanical models based on experimental knowledge and understanding the FSW process at a fundamental level. The quality of friction-stir-welding (FSW) joints depends on many critical weld process parameters. The main challenge for the FSW is in the selection of these critical process parameters that would produce a defect-free weld joint. For a particular pin tool, spindle rotational speed, welding speed, plunge rate and vertical plunge force are considered as key factors to generate heat during welding process. In this work, a temperature dependent friction coefficient is employed that takes into account both sticking and sliding friction conditions. Furthermore, both strain rate independent and strain rate dependent plasticity model was applied to develop FSW. Moreover, heat generation as a process in itself is modeled by accounting for friction heat and plastic deformation between tool/boundary conditions. To demonstrate the validity of the model, the model is applied to different weld schedules of Aluminum AA2219 alloys. Finally, the developed model is used to carry out parametric studies on the effect of process parameters such as rotational speed, welding speed, plunge rate and plunge force on heat generation during FSW. This parametric study helps to give an insight into creating defect free weld joints. Also the effect of process parameters on the quality of predictions using Coulomb and modified Coulomb models of FSW has been analyzed in this current work. A second objective of the research is to improve fatigue life of defect free AA2219 Friction welded joints by Post Weld Heat Treatment (PWHT). Later fatigue life was experimentally compared with base material, as welded specimen and post weld heat treated specimen. Post weld heat treated specimens have higher fatigue strength compare to as welded specimen.

Date

8-28-2018

Committee Chair

Wahab, Muhammad

Available for download on Monday, August 16, 2021

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