Jeff Ding developed Ultrasonic Stir Welding (USW) to join large pieces of very high‑strength metals, such as titanium and Inconel. The solid-state weld process improves the current Thermal Stir Welding process by adding high-power ultrasonic (HPU) energy at 20 kHz frequency.
NASA Tech Briefs: What is Ultrasonic Stir Welding?
Jeff Ding: Ultrasonic Stir Welding is a solid‑state weld process, which means the material being welded does not melt. The material is heated into a plastic state which is between solid and liquid. (Weld properties resulting from a solid state weld process are superior as compared to those properties resulting from fusion weld processes that melt the weld material). Finally, ultrasonic energy is integrated into the stir rod and a non-rotating containment plate.
NTB: What are the advantages of this type of welding?
Ding: I've integrated ultrasonics through the stir pin, as well as a non‑rotating containment plate. The ultrasonics operate at 20 kHz, about 4 and a half kilowatts of power. The advantages of integrating ultrasonics are an overall reduction in plunge, frictional and shear forces as compared to Friction Stir Welding. Faster travel rates can also be realized.
NTB: What are commercial applications for this type of welding?
Ding: I've just started the development of the process. I'm hoping to see an increase in travel rate. I believe we can see up to twenty inches a minute, maybe even faster. This would put the process in the same class as other weld processes in the commercial sector, where an increase in travel rate going as fast as you can means dollars to a company. By being able to take a solid‑state weld process and increased travel to be competitive with other fusion weld processes, such as tig or mig electron beam, now this could be a value to the commercial sector. Also, by reducing the overall loads of the process, it is feasible to integrate the process with relatively inexpensive, off‑the‑shelf robots for robotic applications.
NTB: Could it even play a role in manufacturing in space?
Ding: Absolutely. The agency has to consider making things, in real time, onboard the space capsule or in space. Right now we have the 3D printer on the Space Station, demonstrating the capability to make parts. Let's say we do have the 3D printer producing some parts of subcomponents in the event of a failure of some kind. There has to be a process to take subcomponents and join them into an overall part. Welding certainly has to be considered. Ultrasonic welding would be a safe process for the operator, as there are no high‑energy beams, and no spatter as you would find in electron beam welding, which has been considered for welding in space. This would produce a safe process for the operator and get the same benefits as other fusion weld processes.
NTB: Is there an ease of use advantage with this type of process?
Ding: It would be easier in respect to protecting the operator from any possible harm from using the process, compared to, say, electron beam, if that were selected. I'm sure in the development of in‑space applications, you would have to consider all the safety and overcome all of those issues. The same thing with ultrasonics welding in space. You'd have to overcome safety issues, but as I mentioned, the safety issues would not be as severe as those experience with the fusion weld processes.
NTB: Why is ultrasonic stir welding such an exciting technology?
Ding: It has never been done before. It is certainly leading‑edge. Early data from 2008 and 2010 and 2011 that I've generated indicates that it is feasible, and it is doable, so we'll see after the development effort goes on.
NTB: What is your day‑to‑day work now with the technology?
Ding: I've just completed some initial welds to look at, and based upon the results, I will change the development effort, and I will eventually get into a design of experiments to find out what the best parameters are for different materials: aluminums and your heat‑resistant alloys, such as your titaniums and your steels. One thing I have not done yet in the early development effort: I have not pulsed the ultrasonics on and off during a welding, and I think this will be beneficial to the quality of the weld, as compared with welds done with the ultrasonics just constantly on through the weld.
NTB: And why is that?
Ding:One of the experiments that I did in 2008: I inserted a steel rod about half an inch diameter up into the tool holder, the spindle holder. Without this little half‑inch‑diameter rod rotating at all, just being static, I turned the ultrasonics on. If you slide your finger over the steel rod, you'll find that it feels like it is totally lubricated. Turn the ultrasonics off: and then you'll feel the friction between your skin and the rod. What this tells me is that, in a Stir Weld process, we want to move the plasticized material in the weld nugget. We're moving it from the advancing side to the retreating side of the weld nugget (relative to stir rod rotation). And you have to wonder: Is this material movement even possible by having the ultrasonics on all the time? Are we slipping through that plastic nugget as we're trying to move material from the advancing to the retreating side of the nugget? It's just a theory right now, and I have to do the work and start pulsing it on and off to see what the effect is of having the ultrasonics on and ultrasonics off. When you pulse it off, through a stir pin, it will act as a mechanical device to move the plastic material, similar to Friction Stir Welding. When it is on, it will act as a device to increase travel speed and reduce the sheer forces on the pin as it's traveling through the weld, and that's what I feel is going to happen when it's on. When it's off, it will be a mechanical device.
NTB: You won our 2012 Create the Future Design Contest with your Thermal Stir Weld process. How is Ultrasonic Stir Welding different?
Ding: Thermal Stir is similar to Ultrasonic Stir Welding, in that I have non‑rotating containment plates, a stir rod, and an induction coil for heating. [Similarly,] I have decoupled the heating, stirring, and the forging of Friction Stir Welding so I can control each element independently. Thermal Stir Welding doesn't have any ultrasonics, and Thermal Stir Welding is designed for heat‑resistant alloys, such as your thick‑sectioned titanium. I was very successful in welding half‑inch‑thick titanium — commercially pure and Ti 6‑4 ELI. Right now the ultrasonic stir welding prototype system is only designed for a quarter‑inch material and less. It's just a testbed to characterize the process and show the benefits of it. I do hope to get ultrasonics integrated into the Thermal Stir process so I can realize these benefits in the thick‑sectioned alloys, by using ultrasonics. When it's all said and done, Ultrasonic Stir Welding and Thermal Stir Welding will be very, very similar in nature. In order to pursue the technology, of course, it comes down to funding. And in these austere times, it's very difficult to get the level of funding to advance the technology to the next higher level.
So it's in the prototype phase. What's next for Ultrasonic Stir Welding?
Ding: After we prove out the benefits, the next level would be to build the necessary equipment to demonstrate this for a commercial application. We do have another weld system at Marshall. It's called High‑Speed Friction Stir Welding, and it would be relatively easy to remove the main spindle stack, integrate ultrasonics in it and then reattach the spindle stack to the machine. This machine would have the capability of integrated motion. It would be able to do circles and complex paths, not necessarily just linear welds. It could do a number of things, and this would be beneficial in demonstrating the process for an application in the commercial sector. To learn more about Ultrasonic Stir Welding, read a full transcript, or listen to a downloadable podcast, visit www.techbriefs.com.
Want to learn more?
Jeff Ding will host a live webcast on November 13. To register for the free Ultrasonic Stir Welding presentation, go to http://www.techbriefs.com/webinar250.