D.H. Lammlein PhD Dissertation - Vanderbilt University

often an inaccessible area (e.g. spherical pressure vessel) or an area where setup and
fixturing is difficult (e.g. nosecone). In these cases the insertion or removal of interior
support is complicated. In this work, cases with and without an anvil are tested to
accommodate both applications where an anvil can be used and those situations where an
anvil cannot be used. It is shown in this work that FSW of spheres can be accomplished
at partial penetration without an interior supporting apparatus. This sort of weld
capability can also be used in making full penetration welds with minimal interior
support. The latter case would differ slightly in geometry from the case presented here, as
it would be a butted and lapped weld with two butted sections welded over and joined to
a separate sheet of underlying material (see Appendix, Figure 137). In airframes, this
configuration is used when an outer skin is joined to an interior skin or a supporting
member (e.g. rib).
Partial penetration type welds of butted joints, where some unwelded interface is
left below the weld nugget, are useful in situations where the tensile loading is
significantly less than the compressive loading [18-21]. The particular experimental
geometry in this work; that of a two hemispherical shells joined by a partial penetration
weld (50%), is useful in external compression applications (e.g. submersibles [Yano, et
al.]). Research has shown that in external compression applications the effectiveness of
the vessel is negligibly reduced by decreases in weld penetration [22].
The flat shoulder geometry of a traditional FSW tool is not ideal for spherical
applications and in this work two alternative tool geometries are tested: one consisting of
a cupped shoulder and threaded probe for supported welds, and the other consisting of
only a conical probe with a snubbed, cupped nose (see Appendix, Figure 135) for
unsupported welds [23-25]. For the experiment, sections of 0.20” thickness, 4.5”
diameter, Al-6061-T6 pipe were machined into rings with the external surface curvature
of a sphere. These samples, depicted in Figure 97, are intended to represent a sphere (or
when halved, two hemispheres).
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