High-melting-temperature alloys, such as steel, Inconels, and titanium, present a special challenge. Welding temperatures can be as high as 2000 degrees F, and when welding, the pin tool glows bright red. The elevated temperatures in which the pin tool operates tend to wear the pin tool excessively, and, in thicker material, it’s difficult to complete welds greater than a few feet in length. The pin tool materials are different than those used to weld aluminum alloys. H-13 tool steel and MP-159 are pretty much standard materials to make pin tools for the welding of aluminum. For these other materials (steel, Inconels, titanium, etc.), lanthinated tungsten, tungsten rhenium, and PCBN are used for pin tool materials.
NTB: Speaking of the pin, one of the major problems with friction stir welding that you’ve been credited with solving was the exit hole — also known as the keyhole — that was left in the part when the welding tool was withdrawn. Tell us about that problem and how you solved it.
Ding: Well, early on, back in 1996, when I was making my first welds, that was the first thing that kind of hit me. I thought, well, we’ve got to do something with this hole. So, we thought we needed this pin tool where the pin moves independently in and out of the shoulder. And I remember it was back in, oh, I think maybe it was 1997…it could’ve been 1996…President Clinton shut the Center down. He just closed the government down because he was battling Congress for his budget. Here at Marshall Space Flight Center, as well as all the NASA facilities, they chained the doors shut. So, while MSFC was closed, one of the engineers, Peter Oelgoetz, who worked for Rocketdyne and who was also my partner in crime in developing this process, had to report to an offsite building here in Huntsville. It was during that period – the week or so we were shut down – we put our heads together and decided that this was the design of how we wanted this thing to work. By the way, Pete is on the patent with me – I can’t take full credit because it was the two of us. But I can say, if you asked “Who got the idea to do it?” I said “Let’s do this.” So, we submitted the patent application for the “Auto-Adjustable Pin Tool for Friction Stir Welding.” More simply, it’s called the retractable pin tool, or RPT.
But the two of us put our heads together and figured out how to do it. We did a manual hand crank job, just a hand crank [that moved] the little pin up into the shoulder. We didn’t know if this concept was going to work and we didn’t want to spend a whole lot of money, so we just operated it by hand. It worked great, so we came back with modifications to automate it. The second design iteration was automated but not robust enough, so we came out with the third design change. We made two retractable pin tools, and they were very robust. One was made for my Kearney & Trecker horizontal boring mill and one was made for our vertical weld tool in building 4705.
So now we had a robust mechanical devise that could move the pin in and out of the shoulder under the axial forces associated with the FSW process. One of the benefits of the device is that it provided the mechanical means to weld material that tapered from one thickness to another. Variable thickness, or, tapered weld joints are designed into the Shuttle External Tank where the boosters and orbiter attach. For example, in the liquid hydrogen barrel section, weld joints taper in thickness from .320-in, to .550-in, to 1.00-in, to .650-in, to 1.00-in, to .550-in and finally back to .320-in. So, the pin must be able to automatically extend and retract in and out of the shoulder as it traverses the different weld thicknesses. The tip of the pin must always be positioned within about .030-in from the back side of the weld joint material.
It also allowed us to close out the keyhole in circumferential welds. Once the pin tool traverses the full 360 degree weld joint, it goes past the starting point and slowly retracts the pin into the shoulder, thus, closing out the keyhole.
A third benefit may be the most important in that it provides the mechanical means to operate the self-reacting friction stir weld (SR-FSW) pin tool technology. The SR-FSW is comprised of two shoulders – one rotates on the front side of the weld joint material and the other on the backside. They rotate together in tandem at the same RPM inducing frictional energy into the part from the front and backside surfaces. The conventional FSW pin tool has only one shoulder that rotates and creates frictional energy on the front side of the weld material. The conventional, single shouldered FSW process requires a robust, expensive anvil to “push” against while traversing the weld. The “pushing” forces exerted by the shoulder can approach 14,000 PSI when welding 1.00-in thick aluminum. One can imagine the structural hardware (anvil) required to react against such a force. The SR-FSW technique, on the other hand, does not require the reactive tooling, because there are no “pushing” forces. Instead, the two shoulders, (with the weld joint material sandwiched between them), “squeeze” together, thus, creating an equivalent “pinch” force equal to the “push” force of the conventional FSW pin tool. The Auto-Adjustable Pin Tool for Friction Stir Welding provides the mechanical means to retract one of the shoulders to make it self-react against the other, thus, creating the “pinch force”.
NTB: Since the advent of friction stir welding, two similar stir welding technologies – thermal stir welding and ultrasonic stir welding – have been developed at NASA. How do these two technologies differ from friction stir welding?
Ding: Thermal stir welding was an idea I came up with while developing FSW. In conventional FSW, the shoulder provides much of the frictional energy (heat) as well as the compressive, or forging force. The pin, which is attached to the shoulder, “stirs” the weld joint material together. These three FSW process elements – stirring, forging and heating – work in tandem at a desired RPM and cannot be decoupled from each other. The thermal stir welding (TSW) process de-couples the heating, stirring, and forging elements and allows for individual control of each. This allows for greater process control. I’ll give an example of what I think the benefit is of TSW.