So, I came back to the weld group in November 1995 and I pursued friction stir welding. The first thing I needed was a friction stir welding machine, but there were none available on the market since the technology was so new. What people did back in the 1990’s was use a CNC milling machine with “beefed up” bearings so that the machine could sustain the high forces associated with the technology. For the .300-in thick aluminum I spoke about earlier, you’re pushing down with probably 5,000 or 6,000 psi, so you need that structural, robust tooling – the anvil – to support the workpiece and react the load. Not many machines can sustain forces like that during operation.
So, I needed a milling machine, and I’d just left my position in the SSME Chief Engineer’s Office, where, for seven years, I interfaced with the SSME prime contractor, Rocketdyne, located in Canoga Park, CA. I knew Rocketdyne, was going through this big factory equipment divestment program because much of the SSME engine hardware had already been made and delivered to NASA. I visited Rocketdyne and found the perfect machine. It was a 14-ton Kearney & Trecker CNC horizontal boring mill, and it belonged to Rocketdyne. I offered to buy it, but I guess buying a contractor’s piece of equipment would’ve taken years in red tape, so I said, “Well, why don’t you guys just give that to me?” I was kind of being facetious, but they said, “Well, Mr. Ding, we’ll look into that.” The next day I got a call and they said they’d noticed that the book value on the thing was only worth $500 to them, so they said, “It’s yours.”
NTB: So it was easier for them to give it to you than it would’ve been for them to sell it to you?
Ding: Yes, much easier. After I knew the milling machine was mine, I visited my previous boss, Otto Goetz, SSME Chief Engineer. I knew there was a set of instrumented ducts coming to MSFC from Rocketdyne for our SSME testbed engine. I asked Otto if it would be possible to ship my machine along with the instrumented ducts. He said, “Well sure. We’ll do that.” So a rigger disassembled, boxed up, and put this big 14-ton Kearney & Trecker horizontal boring mill on a flatbed truck for me, and for $8,000 I had that thing sitting on the doorstep. The local Rocketdyne guys hooked it up for me, and I was in business and made my first weld, probably I think, in November 1996.
NTB: Being the first, that made you “the expert,” right?
Ding: Well, there weren’t a whole heck of a lot of people looking at the process. Back then this was just a lab curiosity, nothing more. It showed promise, and what I did was I participated in TWI’s Group Sponsored Projects. There were three Phases. The “Group” consisted of about 20 companies, worldwide. We each contributed around $50,000 (US) for the Phase I study that took FSW from a lab curiosity into a very low technical readiness level.
Phases II and III follow-on efforts continued for several more years. By the end of Phase III this [technology] was mature enough that people could take all of the data that TWI generated and pursue the technology for their own applications. By participating in those group-sponsored projects, NASA has a significantly reduced licensing fee for the life of that patent.
NTB: Why is friction stir welding preferred over more conventional fusion types of welding in certain applications?
Ding: Well, the number one reason is the fact that you aren’t melting the material, so you get much higher weld mechanical properties from the weld joint as compared to the fusion weld processes. That’s the primary reason.
Another reason is that it’s a highly automated process and very repeatable. If there were ever a weld process where you just push the button and let her go, this is it. There are four process variables - the pin rotational speed (RPM), travel rate, the force that the shoulder is pushing into the part, and the position of the shoulder below the surface of the weld piece. They are all highly computer controlled. Once you develop a parameter window and find the optimal parameters, you will perform the same weld over and over. The reliability and repeatability of manufacturing processes is very important when fabricating man-rated space hardware.
NTB: I assume you don’t have to heat treat it afterwards?
Ding: No. In some instances you might want to heat treat the whole part, at which point you could heat treat the weld itself. But for our applications here, for the aluminum that we’re welding for Ares I and for the external tank that we’ve used this for, it’s just as-welded. So we get better properties out of the weld joint. The design people and structural people love it because it helps them with their margins of safety. They appreciate that.
NTB: Are there certain materials or types of parts that are better suited to being joined by friction stir welding than other processes, or will the technique work just as well on anything?
Ding: Well, it was originally designed for aluminum, and it works very well with aluminum. It works well with joining the same alloy of aluminum, and it works well with joining dissimilar alloys of aluminum – say a 2195 to a 2219.
It works very well for joining those alloys considered unweldable, like your 7000 series aluminum alloys. You really can’t fusion weld them – the resulting weld properties are terrible. The aircraft industry uses a lot of 7000 series aluminum and, until recently, the material has always been riveted, not welded. The new Eclipse jet, however, is welding 7000 series aluminum. But with friction stir welding you get acceptable properties. And you can weld 7000 series to 2000 series, so it works well for aluminum.