Naval researchers are exploring the use of titanium for future ship design by using a new welding technique that makes the metal and its alloys more cost-effective and easier to manufacture.
A project out of the Office of Naval Research (ONR) will build a full-sized ship hull section made with marine-grade titanium using what's called "friction stir welding," a process that uses friction heat produced by a spinning pin tool pressed down on pieces of metal where they join together.
To prove the technique can work, researchers at the University of New Orleans School of Naval Architecture and Textron Marine and Land Systems already fabricated a 20-foot-long main deck ship panel. They used six titanium plates joined by friction stir welding for the Navy's experimental naval vessel called Transformable Craft, or T-Cr, according to the ONR. The team plans to complete the ship hull section this summer with ONR funds as part of its Sea Base Enabler Innovative Naval Prototype program.
Titanium is notoriously hard to work with. However, titanium and its alloys have a number of benefits for use in building ship hulls and other structures because they are stronger, weigh less, and are more resistant to corrosion than steel. Titanium also has military aerospace applications. One of the main drawbacks to titanium, however, is its high price tag. The metal costs nine times more than steel.
Traditional steel welding uses a process of heating the metal and then striking it repeatedly to bond two pieces together. The friction stir welding process is quite different. The rotation of a spinning pin tool causes the metal pieces to heat up into a state like plastic but not melt, according to the ONR. The movement of the tool passing down the common joint line forms a weld between the heated, plasticized, larger pieces of metal.
Researchers made more than 70 feet of welded linear joints for the section of hull that's been completed so far -- the longest known welds in titanium to be made with the friction stir process. The work shows an improvement over similar processes previously used to weld together titanium, according to the ONR. For example, it was made at a high linear speed, showing a reduction in manufacturing time, and showed strong penetration of the weld, indicating the connection between the joints is secure.
Researchers believe the key to the success of using the process for welding titanium lies in effective design of the pin tool, process parameters that emphasized the life of the tool, and exact duplication of process steps from facility to facility and machine to machine.
Applying the process to titanium was not without its challenges, however, according to the ONR. Titanium can be difficult to join by the process because the high temperatures required erode the materials of the pin tool.
To overcome the challenges, researchers used new titanium friction stir welding methods developed by Florida-based Keystone Synergistic Enterprises with funding from both the ONR and the Air Force. The processes used 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.
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