Are the days numbered for great steel ships?
It’s possible: The U.S. Office of Naval Research has announced that it is producing a full-size ship hull section made entirely of marine-grade titanium using a welding innovation that could help bring titanium into future Navy ship construction.
The section may be completed as soon as this summer, using a metal joining process known as friction stir welding. The FSW welds on the new hull are more than 17 feet long, says ONR, which coordinates, executes and promotes the science and technology programs of the Navy and Marine Corps.
Creative Commons / VZLU Plc.
|Friction stir welding is used to join sheets of USIBOR 1500 steel.|
The contractor team building the section recently completed the industry’s longest friction-stir titanium alloy welds to join the titanium alloy plates for the 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.
Light Weight, Corrosion Resistant
Titanium metal and its alloys are desirable materials for ship hulls and other structures because of their high strength, light weight and corrosion resistance, ONR noted in a recent article on the project.
Navy ships built of titanium would have lighter weight for the same size, allowing for a bigger payload, and virtually no corrosion—a problem that costs the Navy billions of dollars each year.
|NASA used friction stir welding at its Michoud Assembly Facility to join the bulkhead and nosecone of the Orion spacecraft.|
But because titanium costs up to nine times more than steel and is technically difficult and expensive to manufacture into marine vessel hulls, the shipbuilding industry has avoided its use.
Now, however, 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, according to ONR.
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.
Friction Stir Welding
Friction stir welding was invented at The Welding Institute, a UK research and technology organization, in 1991. Unlike conventional welding, which joins metals by melting them under extreme heat, FSW is a solid-state joining process used for applications where the original metal characteristics must remain as unchanged as possible.
Creative Commons / Anandwiki
|A schematic diagram shows the friction stir welding (FSW) process. Figure A shows two discrete metal work pieces butted together, along with the tool (with a probe). Figure B shows the progress of the tool through the joint, the weld zone and the region affected by the tool shoulder.|
The process uses the heat of friction produced by a spinning pin tool pressed down on both pieces of metal at their common joint. Heat from the high-speed rotation softens the metal to a plastic-like condition but does not melt it.
As the tool passes down the common joint line, the heated, plasticized metal from both pieces is kneaded together like clay in its wake, forming the weld.
The process has been primarily used on aluminum, most often on large pieces and in a wide variety of industrial applications. Using it on titanium has been more difficult, because of the high temperatures required and because the pin tool materials erode and react with titanium, weakening the weld, officials say.
Creative Commons / Haruno Akiha
|Friction stir welding was used to prefabricate the aluminum panels of the super liner Ogasawara at Mitsui Engineering and Shipbuilding in 2008.|
Now, however, researchers have overcome that problem with new titanium friction stir welding methods developed by Florida-based Keystone Synergistic Enterprises Inc.
The processes, developed with funding from ONR and the Air Force, were scaled up and transferred to the National Center for Advanced Manufacturing (NCAM), which is a partnership of the University of New Orleans, NASA and the state of Louisiana.
70 Feet of Welds
To fabricate the Navy’s new 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, says ONR. The welding was made at a high linear speed, showed excellent weld penetration, and had no distortion of the titanium adjoining the weld, according to ONR.
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 as part of the Sea Base Enabler Innovative Naval Prototype program.