February 2010 - When it comes to aerospace parts that use titanium or other nickel-based, heat-resistant alloys, less is really more. NASA would like to use more of these alloys for components because they offer greater strength-to-weight ratios than ultralight alloys, such as aluminum.

The Marshall Space Flight Center at the Redstone Arsenal in Huntsville, Ala., provides multidiscipline engineering expertise for propulsion and transportation systems, such as the space shuttle and the Ares rockets.

Since it was established in 1960, Marshall has used its engineering and scientific expertise to build launch vehicles, spacecraft and scientific instruments that enable the United States to explore space. Today, it’s developing NASA’s next-generation Ares I and Ares V launch vehicles.

Jeff Ding, aerospace welding engineer at Marshall, developed a unique thermal stir welding system that can weld metals like commercially pure titanium and heat-resistant alloys without creating problems.

"This all started with thermal stir welding," says Ding. "I had a prototype system that I used for several years to support work that I was doing through a Space Act agreement with a company in Florida. A Space Act agreement is a mechanism that allows a company to come here to do work. They actually contract work to us. NASA’s policy is not to compete with private industry when we do work with a Space Act agreement. If it’s unique work, and it’s beneficial to NASA, we can do a limited amount of work for a company. For instance, we can develop welding parameters and factory flow of materials to weld a subcomponent of some type. After we determine optimum weld parameters and demonstrate that these parameters can successfully weld whatever the object is for this company, then the work has to be turned over to a private company."

Ding was interested in thermal stir welding because he says that when material is melted through a MIG or TIG welding process, the weld loses much of its mechanical strength, sometimes up to 50 percent.

Therefore, a thicker, heavier material must be used, which is what NASA doesn’t want for a rocket ship’s engine or other parts. Friction or thermal stir welding uses solid-state welding techniques, and the metal isn’t being melted but just plasticized enough for joining. Thus, NASA can use a thinner material with this process because the weld still retains about 75 percent of its tensile strength.

Stir welding types
Conventional friction stir welding uses a pin tool that looks a little like an upside-down Christmas tree with a shoulder on the tool to push down with a force of 5,000 psi to 6,000 psi on the surface of the material. Therefore, the part being welded has to be placed and fixtured to an anvil to absorb this load.

Self-reacting friction stir welding has a shoulder on the top and bottom surfaces of the material being welded. A retractable pin tool is used, which retracts one shoulder against the other, creating a pinch force. Both shoulders on the top and bottom surfaces rotate together. As the material is being stirred, it’s also being squeezed and plasticized because of the two shoulders pinching against the top and the bottom surfaces at about 6,000 psi.

Hybrid friction stir welding preheats the material. Once a preheated temperature is reached, a conventional friction stir welding tool is plunged into the weld joint, and the weld is completed. This process uses a conventional friction stir welding pin tool solely to induce all of the energy needed to plunge into the part and weld it. It’s assisted with an induction coil for part of the heating.

Thermal stir welding heats the material with an induction coil to a higher temperature than hybrid friction stir welding, and then it uses a stir rod to stir the material and process the weld.

Reducing part costs
Ding is experimenting with thermal stir welding that can weld $5-per-lb., commercially pure titanium (also known as low-cost, commercially pure titanium). This material isn’t currently available for purchase by industry. The titanium alloy is relatively inexpensive due to the procedure used to process it. When the cost of this material is compared with traditionally processed, commercially pure titanium at $60 per pound, cheaper parts made from it could give NASA a great deal more latitude for lightweight, heat-resistant parts that are bound for outer space or other ventures.

NASA isn’t using the low-cost, commercially pure titanium right now because impurities trapped in the metal can surface during MIG or TIG welding, creating severe weld defects in the heated weld zone, according to Ding.

"This low-cost titanium has a lot of tramp elements in it, a lot of chlorine from its processing," he says. "Therefore, when it comes to welding it, TIG or MIG welding processes can’t be used because they’re fusion welding processes that melt the material. If you melt the material, there’s so much contamination, we’ll just have bad welds. This is why we’ve used stir welding, which is solid-state, never melting the material.

"Because thermal stir welding decouples the heating, stirring and forging of friction stir welding, I can control each aspect of the process independently," he continues. "Controlling each one gives me a lot more process control over everything, whereas in friction stir welding, you have a pin tool, and everything rotates at some rpm, and you have to live with it. You try and make the process work based on the rotational speed of the pin tool."

Ding also says thermal stir welding allows him to heat independently of the other process elements, including stir-rod rotational speed and forging pressures. Along with heating up titanium to 1,400 F before the part transitions into the stir rod that stirs it, he’s also using the induction coil to give him through-thickness constant temperature.

With friction stir welding, all the heat is generated on the material’s top surface from frictional heat, and there’s no constant temperature profile through the thickness of the material. As a result, stirring is done at different temperatures through the material’s thickness.

This is another big difference why thermal stir welding is preferred for high-temperature-melting or heat-resistant alloys like titanium.

For the prototype system, Ding made a lot of progress on the thermal stir welding of titanium, but he needed a new system. He contacted Don Holman, product manager of friction stir welding at Nova-Tech Engineering LLC, Lynnwood, Wash.

Nova-Tech had an integrated spindle motor for the stir welding process that didn’t use any belts or chains for operation, and Ding was interested in this technology for a new thermal stir welding system.

It was self-contained and compact, and it used a high-torque, low-speed motor. It was also capable of operation under the conditions required for all three welding processes.

"We heard about it, and we were interested in it," says Ding. "We wanted to incorporate this into our thermal stir welding system because we don’t like hydraulic spindles, as they tend to leak. If we’re welding aerospace hardware, we don’t want to contaminate the hardware with hydraulic fluid that has leaked from a spindle motor. Nova-Tech’s motor is electric and water-cooled."

The venture was a group effort among Nova-Tech, NASA and a support engineering contract from Alabama A&M University, according to Ding.

Nova-Tech and NASA co-developed the specifications for the system based on experience from the prototype system and collaborated on the design of the equipment for about five months before Nova-Tech built the system and delivered it in March 2008.

"This is the first machine of its kind in industry that was specifically developed to handle all three welding processes," Holman says. "The machine gives clients the ability to research aspects of all three welding processes with one system, thereby reducing the necessity and cost of multiple systems."

With the new machine on-site, Ding continued with the titanium work. A major advancement was welding an 8-ft.-long, 0.5-in.-thick, commercially pure titanium part using the thermal stir welding process.

With this system, Ding is working on welding a Haynes metal alloy for a nozzle extension of a rocket engine for long-duration spaceflight.

"If we go back to the moon or Mars, our J2X engine will need a nozzle extension to get greater engine performance," he says. "Therefore, we’re investigating various alloys for the nozzle extension application, including Haynes 230 and 282 alloys. We’re going to use Nova-Tech’s system to compare the weldability of the Haynes 230 using thermal stir welding and friction stir welding to see the differences between the two processes."

Additionally, Ding says there will be future parts that can use the various stir-welding processes.

"Hopefully, we will be developing it as a weld process for engine parts and engine subcomponents that we would use on the RS-68 engine for the Ares V vehicle," he says. "Right now, however, we’re concentrating on the Ares I vehicle, which is the smaller of the two that will launch a crew into low-earth orbit." FFJ

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