D.H. Lammlein PhD Dissertation - Vanderbilt University
D.H. Lammlein PhD Dissertation - Vanderbilt University
D.H. Lammlein PhD Dissertation - Vanderbilt University
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INTRODUCTION<br />
Friction Stir Welding, FSW, is a materials joining process with great potential as<br />
joint quality is exceptionally high and the process is very repeatable. Additionally, the<br />
process does not use fillers and, like fusion welding, eliminates the need for fasteners,<br />
which add weight to a structure. These attributes combined with its particular<br />
effectiveness on low-melting point alloys (which happen to be low-density) like<br />
aluminum and magnesium make FSW very applicable to vehicular applications where<br />
costs can easily be justified by increases in strength versus weight and high joint quality.<br />
One current limitation of the process is its range of application with respect to joint<br />
geometry.<br />
In this work, the FSW process is applied to butted hemispherical joints and butted<br />
pipe joints of small diameter (approximately 4 inches). These joints are nonstandard and<br />
the application of FSW to these geometries, with such small diameters, has not been<br />
presented in the literature. These cases present complications to the standard FSW<br />
process and equipment. These complications arise from the surface curvature of these<br />
geometries and from the circular nature of the weld path required to join them. The<br />
circular nature of the weld path means that either the tool (and the bulk of a spindle motor<br />
and control instrumentation) must orbit the work or that the work itself must be rotated<br />
on its axis. Either approach presents difficulties. An orbital tool requires a complicated<br />
and bulky orbital apparatus. The apparatus must be bulky enough to provide the large<br />
down-force required for weld consolidation and material containment in friction stir<br />
welding. In addition, this apparatus must be made to orbit smoothly around the pipe or<br />
sphere.<br />
Rotating the work presents its own difficulties. The primary difficulty is that the<br />
entirety of the work must be rotated about an axis. In the case of a pipe, the length of the<br />
sections to be joined is indefinite. In addition, the bulk of the work must be rotated<br />
smoothly about its axis, with the eccentricity of this rotation limited within the ability of a<br />
force feedback control system to compensate for changes in the height of the work<br />
surface with respect to the tool. In the case of an experimental testbed, the choice of a<br />
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