Date of Award

2015

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Anthony P. Reynolds

Abstract

In the friction stir welding (FSW) process, tool stirring and synchronized movement of the weld materials along a pre-existing seam line causes thermal gradients and severe plastic deformation resulting in the bonding of the adjacent materials. For a given set of welding parameters, tool pin geometries (dimensions, shape) and vertical/helical features that dictate the material motion also have significant effects on process response variables during FSW. Among the primary process controlling parameters in FSW, tool pin geometry and feature vary multifariously in terms of shape, dimensions, feature insertion technique depending on the weld material and application of joint. The current state-of-the-art of FSW tool design is evolved with instinctive perceptions which are typically based on empirical knowledge. It is important to understand the behavior of FSW process response variables such as, in-plane reaction forces, torque, weld power, stir zone temperature and material transport phenomena with the variation of pin features and geometries in order for the process to flourish over a range of manufacturing applications. This dissertation seeks to systematically quantify and establish the relationships among the tool geometric parameters, welding parameters and FSW process response variables to a reasonable extent.

In this work, the friction stir weldability of different aluminum alloys in similar/dissimilar joints as well as other aspects of the process including: condition of defect free welds, material flow, process temperature, forces, etc. are examined. The feasibility of using different pin features (thread forms/flats/flutes) coupled with a conventional scrolled shoulder configuration was investigated. Results revealed that joint quality, tool reaction forces, temperature and weld power are highly affected by complex geometric features of the pin. Thread form was found to be an essential component in pin design criteria for effective downward material movement during welding. Completely defect free welds as well as lower in-plane forces were produced in both similar and dissimilar butt joint arrangement while using a mildly tapered conical coarse threaded pin having three shallow flats. The placement of the stronger alloy on the advancing side during bi-material welds also resulted in an effective material flow to produce defect free welds as well as involved with less in-plane forces. In order to prevent premature failure of the pin during the welding process, the stress condition and stress concentration of the pin were also estimated using finite element method (FEM). Moreover, the structural analysis using FEM also provided an optimum dimension of pin features. Finally, welding was performed using a stationary shoulder configuration to demonstrate the effectiveness of a coarse threaded conical pin with three flats in producing defect free welds. Taken together, the studies encompassed in this dissertation provided the basis for a systematic evaluation of tool design criteria as well as the basis to optimize processing window as FSW technology continues to evolve.

Rights

© 2015, Md. Reza-E-Rabby

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