Numerical/Experimental Investigation of Plunge Stage and Effect of Donor Material in Friction Stir Welding

Friction Stir Welding (FSW) was developed in 1991 as a robust solid-state joining technique that uses a specially shaped rotating tool to generate heat and plasticize material around the tool. The tool then mixes plasticized material along the joint line to produce the weld. Over the last decade, FSW has become increasingly popular for welding aluminum. The combination of attractive properties of the weld and cost-efficiency has led researchers to investigate the feasibility of using FSW for steel. One of the major impediments for using friction stir welding for harder materials such as steel is tool wear. It is well-documented that a large part of this wear occurs during the initial plunge phase. This dissertation focuses on developing a better understanding of the plunge stage of FSW and also proposes a novel concept to mitigate tool wear during plunge. The commercial FEA code, ABAQUS is used to simulate the plunge phase and the results are compared with experimental results obtained from literature. Plunge experiments on AA 2024 were also conducted during the course of this research and the axial load and temperature were measured. The ‘donor material’ concept was proposed for the reduction of tool wear at the plunge by providing localized pre-heating at the plunge area using a softer material as a ‘donor.’ This process creates heat in a relatively soft ‘donor’ material, which is transferred to the much harder workpiece material by conduction. The expected advantage of this localized process is that it reduces the chances of altering the microstructure of the base material due to excessive heat, which is a possibility in conventional pre-heating methods. This research includes several numerical simulations of the donor material concept with different donor materials and plain carbon steel as the workpiece. It was observed in the case of using a donor material that the axial load during the plunge decreased by approximately 80%. Additionally, the contact stresses at the tool workpiece interface also decreased by approximately 75% when a donor material was used in the plunge area. Decrease in both the axial force and contact stress should contribute to the decrease of tool wear. Proof of concept experiments are also demonstrated with copper as the donor and AA 2024 as the base material

Using Eye Movement Data Visualization to Enhance Training of Air Traffic Controllers: A Dynamic Network Approach

The Federal Aviation Administration (FAA) forecasted substantial increase in the US air traffic volume creating a high demand in Air Traffic Control Specialists (ATCSs). Training times and passing rates for ATCSs might be improved if expert ATCSs’ eye movement (EM) characteristics can be utilized to support effective training. However, effective EM visualization is difficult for a dynamic task (e.g. aircraft conflict detection and mitigation) that includes interrogating multi-element targets that are dynamically moving, appearing, disappearing, and overlapping within a display. To address the issues, a dynamic network-based approach is introduced that integrates adapted visualizations (i.e. time-frame networks and normalized dot/bar plots) with measures used in network science (i.e. indegree, closeness, and betweenness) to provide in-depth EM analysis. The proposed approach was applied in an aircraft conflict task using a high-fidelity simulator; employing the use of veteran ATCSs and pseudo pilots. Results show that, ATCSs’ visual attention to multi-element dynamic targets can be effectively interpreted and supported through multiple evidences obtained from the various visualization and associated measures. In addition, we discovered that fewer eye fixation numbers or shorter eye fixation durations on a target may not necessarily indicate the target is less important when analyzing the flow of visual attention within a network. The results show promise in cohesively analyzing and visualizing various eye movement characteristics to better support training. 

Interacting fermions in two dimension in simultaneous presence of disorder and magnetic field

We have studied the revival of Hofstadter butterfly due to the competition between disorder and electronic interaction using mean field approximation of unrestricted Hartree Fock method at zero temperature for two dimensional square and honeycomb lattices. Interplay of disorder and electronic correlation to nullify each other is corroborated by the fact that honeycomb lattice needs more strength of electronic correlation owing to its less co-ordination number which enhances the effect of disorder. The extent of revival of the butterfly is better in square than honeycomb lattice due to higher coordination number. The effect of disorder and interaction is also investigated to study entanglement entropy and entanglement spectrum. It has been observed that for the square lattice, area law of entanglement entropy is violated for intermediate strength magnetic and magnitude of such departure from area law depends on disorder and interaction as well. However such departure from area law is absence for honeycomb lattice. Moreover the entanglement spectrum for square lattice does have the symmetry of original Hofstadter butterfly and this symmetry is destroyed in the presence of disorder. The interaction opens up a gap in the entanglement spectrum as well. For the honeycomb lattice, the entanglement spectrum forms a continuous band without any symmetry and its feature is mostly unchanged in the presence of disorder as well as interaction