“Also, the exceptionally high corrosion resistance of
titanium as compared to steel and aluminum can afford
major savings in maintenance costs over a ship’s lifetime.
Continued research investments toward reducing the cost
of titanium base materials [e.g., plate, extrusions, etc.] and
fabrication processes [e.g., welding and shielding process-es] for ship construction have potential for enabling the
use of titanium in large ship structure,” Mitchell said.
Titanium has a low magnetic signature, which obviates the need for heavy and power-hungry degaussing
coils to protect against magnetic-influence mines. It
also has temperature resistance, so it’s safer for structures like gas turbine exhaust systems.
Yet another advantage for titanium is its excellent
fatigue characteristics. Most ship structure is subjected
to cyclic loading, and for aluminum and steel that frequently limits the allowable stresses in the hull material to well below the nominal yield strength. With titanium, allowable stresses can be a lot closer to the
nominal yield strength, and repair of fatigue cracks
will be far less common.
Although the titanium suitable for ships is a very
mature material and well understood, changes in typical ship design rules and associated validated ship
design processes would be required, Mitchell said.
Also, there are issues associated with coupling titanium to other materials in some cases, such as protecting
non-titanium systems like piping and the propeller and
shafting from accelerated corrosion if a hull were made
out of titanium. Since most repair activities do not have
the expertise to fabricate titanium, new capabilities would
be required for executing repairs over the life of a ship.
“All of these issues could be readily overcome with
proper planning, development and oversight,” Mit -
chell said.
Aerospace-grade titanium used in aircraft is about
nine times more expensive than steel. But industry
experts predict that marine grades of titanium for ship
hulls could be made less costly by changing the processing and finishing requirements.
Ship hull construction usually requires miles of welds,
according to Pingsha Dong, a professor at University of
New Orleans (UNO) School of Naval Architecture and
Marine Engineering, and director of UNO’s Welded
Structures Laboratory.
“More research is needed to develop high-productivity
Ti [titanium] welding processes while maintaining a high
degree of consistency in quality for ship hull construction,” Dong said. “We are getting there. In the ongoing
ONR-funded program at UNO on a full-scale titanium
midship section, we recently have achieved a welding
speed that is comparable with welding steel ship panels.”
Colen Kennell, a naval architect at the Center for
Innovation in Ship Design at Naval Surface Warfare Center
GREG DOBSON
As part of the University of New Orleans’ College of Engineering, the National Center for Advanced Manufacturing at
NASA’s Michoud Assembly Facility in Louisiana develops
new processes for friction stir welding for titanium structures. Titanium metal inert gas weld training is shown here.
(NSWC) Carderock, Md., said titanium offers a potentially
ship-scale cost-effective alternative to steel structures.
“For faster ships, lightweight hulls are a critical
enabling technology,” he said.
“It all boils down to material and fabrication costs.
Until the program offices can look beyond the acquisition
costs associated with using titanium components and see
that the long-term benefits outweigh the acquisition
costs, there will still be hurdles to using titanium,” said
Kim Tran, a materials engineer at NSWC Carderock,
where she is the non-ferrous welding lead.
Raymond M. Walker, vice president with Keystone
Synergistic Enterprises, Port St. Lucie, Fla., said the aerospace industry understands life-cycle ownership cost and
has justified gas turbine engine investments to achieve
thrust and fuel efficiency improvements.
“Increased thrust relates to increased payload or
passenger capacity. Fuel efficiency benefits rapidly
multiply into very large life-cycle cost savings for both
military and commercial aircraft,” Walker said. “A sim-
ilar case exists for a titanium vessel in terms of main-
tenance, corrosion management, reduced painting and
coating requirements, lower vessel weight, fewer ship-
yard visits, longer service life and so on.”
“In terms of ship design and production, mainte-
nance, protection and survivability, we’re trying to build
things that last longer. And we’re looking at new tools
that have a high return on investment, that can do a bet-
ter or more efficient job for a ship’s crew, shipyard or con-
tractor, because that will ultimately save the Navy money
and increase our availability,” Schuette said. ;
