D.H. Lammlein PhD Dissertation - Vanderbilt University

applications for its high strength and low density. Titanium partial sphere sections can be
made by spin forming, blow forming, or machining. The titanium pieces are then joined
by Tungsten Inert Gas (TIG) welding or solid-state diffusion bonding [14]. Although
aluminum has a lower yield strength than titanium, it is also used extensively in
aerospace due to its low density and moderate cost.
Carbon fiber reinforced polymer vessels have a high tensile strength to weight
ratio but limited impact resistance, durability, and compressive strength. Compressive
strength is required in deep sea applications [15]. Additionally, the shape of a carbon
fiber vessel is limited by the ability of a fiber winding machine to accommodate the
chosen geometry. Some radial and axial asymmetries can be accommodated but
concavities in the part cannot. The process is further limited by the necessity of an inner
supporting boss for winding.
Often, a layered approach is taken in the design of pressure vessels in order to
meet all design requirements [16,17]. One example is winding a kevlar reinforced
polymer outer layer for tensile strength and low weight around an aluminum inner sphere
for stiffness and leakage reduction. Leakage via material voids and diffusion is lowest in
metal vessels or vessels with some metallic layer. Aluminum spheres are low leakage,
lightweight, stiff, durable, and moderate cost vessels with applicability to external and
internal compression applications. The results reported here demonstrate that Friction
Stir Welding is an efficient and effective method for manufacturing spherical aluminum
pressure vessels. The techniques developed here can also be applied to manufacturing
steel, titanium, and other metallic spheres.
Experimental Approach
In friction stir welding, a large magnitude force is directed vertically downward
through the tool, the workpiece, and into an anvil which traditionally opposes the tool
axial force. The anvil is necessary in full penetration welds to support the region around
the weld root and prevent material expulsion and failure on the underside of the weld. In
the case of a highly curved surface with dual curvature, the interior of the workpiece is
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