Steel
may have met its match: An Office of Naval Research (ONR)-funded
project will produce a full-size ship hull section made entirely with
marine grade titanium using a welding innovation that could help bring
titanium into future Navy ship construction, officials announced April
3.
The
contractor team building this section recently completed the industry’s
longest friction-stir titanium alloy welds and aims to complete the
ship hull section this summer. Friction stir welds more than 17 feet
long joined the titanium alloy plates for the section’s deck.
“This
fast, effective friction stir weld technique is now an affordable
manufacturing process that takes advantage of titanium’s properties,”
said Kelly Cooper, the program officer managing the project for ONR’s
Sea Warfare and Weapons Department.
What it means for the Navy
Titanium
metal and its alloys are desirable materials for ship hulls and other
structures because of their high strength, light weight and
corrosion-resistance. If constructed in titanium, Navy ships would have
lighter weight for the same size—allowing for a bigger payload—and
virtually no corrosion. But because titanium costs up to nine times more
than steel and is technically difficult and expensive to manufacture
into marine vessel hulls, it has been avoided by the shipbuilding
industry. But perhaps not for much longer.
Researchers
at the University of New Orleans School of Naval Architecture and
Textron Marine and Land Systems are demonstrating the feasibility of
manufacturing titanium ship hull structures. Using lower cost marine
grades of titanium, they fabricated a 20-foot-long main deck
panel—composed of six titanium plates, joined together by friction stir
welding—as part of technology studies for an experimental naval vessel
called Transformable Craft, or T-Craft.
Since
antiquity, blacksmiths have joined iron or steel parts together by
heating them in a forge, placing them on an anvil and striking the two
pieces repeatedly with a heavy hammer. After several repetitions of
heating and striking, the two pieces were “hammer forged” or “forge
welded” together.
Friction
stir welding joins metals using the heat of friction produced by a
spinning pin tool pressed down on both pieces of metal at their common
joint. Friction heating produced by the high-speed rotation causes both
metal pieces to heat up to a “plastic” condition, but not to melt. As
the tool passes down the common joint line, the heated, plasticized
metal from both pieces is kneaded together in the rotating tool’s wake,
forming the weld between them.
How it was accomplished
Friction
stir welding works well for most aluminum alloys. Titanium, however, is
difficult to join by the same process because of the high temperatures
required, and pin tool materials that erode and react with titanium,
weakening the weld.
The
researchers overcame that problem by using new titanium friction stir
welding methods developed by Florida-based Keystone Synergistic
Enterprises Inc. with funding from both ONR and the Air Force. The
processes were scaled up and transferred to the National Center for
Advanced Manufacturing (NCAM), which is a partnership between the
University of New Orleans, NASA and the state of Louisiana.
To
fabricate the ship hull structure, more than 70 feet of welded linear
joints were made—the longest known welds in titanium made with the
friction stir process. This friction stir welding achievement showed a
noticeable improvement from previous similar processes. It was made at a
high linear speed—indicating reduced manufacturing time; showed
excellent weld penetration—indicating a secure connection; and had no
distortion of the titanium adjoining the weld.
Experts
attribute the success to an effective design of the pin tool, process
parameters that emphasized pin tool life and exact duplication of the
process steps from facility to facility and machine to machine.
ONR
funds collaborative projects investigating novel shipbuilding materials
and improved processes for titanium friction stir welding—especially
its affordability—as part of the Sea Base Enabler Innovative Naval
Prototype program.
Source: Office of Naval Research
