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.
As a former Design Engineer at Newport News Shipbuilding I worked extensively with titanium alloys in the design of submarine components. Titanium is an excellent material for a number of reasons and is especially well suited to a marine environment because of it's corrososion resistance and strength.
Some time back the Russian navy produced a class of subs with a titanium hull. They were welded in a large inert gas filled chamber with welders in moon suit like gear and oxygen supplied through hoses. The issue at the time as far as my understanding goes was that in order to prevent contamination of the weld material the hot weld must be shielded with argon gas until the weld is below around 650 degrees F.
@Ivan: Friction stir welding is a solid-state welding process and doesn't require either fill metal or a shielding gas. So you don't need to wear a space suit in order to do it. It also results in some beneficial microstructural changes which improve material properties. (Sometimes the process is applied to a surface specifically to induce these microstructural changes, rather than to weld one surface to another). It's a very promising process, and it's great to see this collaboration between industry, government, and academia to develop it.
Ivan, the Soviets were willing to take chances with people's lives in all kinds of situations. It is good to see that a safer process is being developed.
The issue, as the article states, is with the tools. They have to operate in very adverse conditions.
As for the cost, most military systems that would use this technology and material have a very long life time. Just look at the B-52. My father worked on the design of it around the time I was born (and you can see that was a while ago). They are still flying and projected to be around for a long time.
Sorry, but we evloved from using the old heat it up and smash it togther process of welding many decades ago. The current most popular processes use an electrical arc to create a metal puddle, if you will, to melt two metal pieces together. There are several forms of this, Arc welding, TIG and MIG just to name a few. The new process you mentioned in the article is still a very impressive development, however, if it tests out that it indeed has the long term structural integrity needed for ship building.
From the article, I didn't get a sense if the weld joint strength was stronger than/weaker than/or same as the surrounding material. There are obvious manufacturing advantages to this new friction welding, but I wonder how does the resulting material properties of this new process compare to the more traditional methods?
The Soviets followed up with a robotic system that was able to weld the 4 in thick hull in a single pass. No doubt making the next variant in Titanium a "no brainer".
I believe I saw soemthing on TV about the Airbus A380 using some stir welding processes as well. I don't recall if it was on titanium pieces or Aluminum but it was interesting. The video of the actual process was impressive in how little it affected the workpieces being joined.
It would be interesting to note the limitations of the process as applied now and also to see what is being undertaken to make it more versatile. stir welding a 4 inch thick piece of Titanium hull section would certainly be an impressive feat.
Does anyone have details on this process? What I saw in the video on Airbuss looked like a spinning tool being forced into the intersection of two sheets of materials. As the tool progressed along the interface the rapid spinning created friction to plasticise the metals and it closed up around the tool on the trailing side. It made a very neat and clean looking bond. Presumably full strength.
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