Uncoventional Welding
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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JET ENGINE COMPONENTS need to operate at higher temperatures to burn fuel more efficiently and therefore
lessen the burden that burning such fuel has on the natural environment. |
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SUPERALLOYS USED in modern jet engines can be joined only by using unconventional welding techniques
such as inertia welding.
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INERTIA WELDING involves bringing a rotating component into contact with a stationary one in
such a way that the heat generated by friction fuses the components together. |
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Joining dissimilar superalloys requires inertia welding to avoid micro-cracking.
By Dean Jobb
Nickel Magazine, July 2004 -- If the aerospace industry is to reach its goals of reducing emissions and lessening its burden on the environment, the next generation of jet engines will need to burn fuel more efficiently and at higher temperatures. But building engine components capable of doing so poses a technological challenge: while nickel-based superalloys offer the heat resistance needed, they are difficult to join using conventional welding techniques.
Engineers at the University of Manchester and Rolls-Royce plc, one of the world's largest manufacturers of gas turbines for aircraft, have teamed up to tackle this problem by applying inertia welding techniques to the production of compressor drums, turbine discs and shafts for jet engines. "Inertia welding has been around for some time," notes Prof. Philip Withers of the university's Materials Science Centre. "What's new is inertia welding as applied to aero-engines."
Inertia welding uses the heat generated by friction to fuse metal components together. A workpiece is spun at high speed on a flywheel and brought into contact with a stationary component. Within seconds, the pieces reach forging temperature at the point of contact and are bonded together without melting or the addition of liquid metal.
"Because of the large flywheel, you have a lot of stored kinetic energy," Withers explains, "and that kinetic energy will gradually dissipate as those surfaces rub. Heat is generated, which softens the metal, and you essentially hot-forge the two parts together." The speed of rotation and the pressure exerted as the pieces are brought into contact are strictly controlled to ensure a solid weld and that melting does not occur.
Withers' Manchester colleague, Dr. Michael Preuss, and Rolls-Royce metallurgist Gavin Baxter used inertia welding to join tube structures made of RR1000, a superalloy Rolls-Royce developed specifically for use in turbines. RR1000 consists of 50-60% nickel, 14-15% chromium, 14-19% cobalt, 4-5% molybdenum, 3% aluminum, and about 4% titanium. The powder-processed alloy resists cracking, corrosion and oxidation when subjected to extreme heat but is prone to micro-cracking as the metal solidifies after undergoing conventional welding.
The Manchester team studied the microstructure of the test welds and found that inertia welding produced better properties than fusion techniques using liquid metal. "You get an increased hardness in the weld zone," Withers reports. "It has the advantage that it also cleans out the surfaces so that you have a weld made from high quality metal." Post-weld heat-treating of the joint at a temperature of 50°C above the normal heat-treating temperature was found to reduce the residual stresses produced during welding.
The researchers also discovered that inertia welding can be used to join RR1000 to two other superalloys used in aerospace engines - N07001, which can withstand temperatures of up to 870°C and N07720.
"They have different properties, and it is often difficult to join dissimilar metals while maintaining joint integrity," Withers notes. "With this joining technology, certain parts of the disc assembly may be made of one alloy and certain parts may be made of another."
Researchers are also exploring aerospace applications of linear friction welding, a related welding process in which components are rubbed together back and forth until enough heat is generated to join them. "That technology is being developed for blade-to-disc applications," says Withers. "You would essentially join a blade to a disc by a similar kind of weld, dispensing with the conventional dovetail joints used to attach turbine blades."
The research is part of a £4.7 million research program known as ADAM (for Advanced Aero-engine Materials), spearheaded by Rolls-Royce, that brings together scientists at Manchester and five other British universities. The goal is to develop lightweight, high-temperature materials and new manufacturing techniques for aerospace applications. A demonstration engine may be completed as early as 2008.
Rolls-Royce is committed to cutting nitrous oxide emissions from engines used in civil aircraft by 50%. Also, by 2010, the company expects new engines to be using 10% less fuel than a comparable model produced in 1998. Inertia welding of nickel based engine components appears to hold one of the keys to reaching these lofty goals.
"Aerospace has to move towards improved emissions," notes Withers. "The way to do that is by being more
efficient, and you get more efficient [and] you get a big environmental benefit if you can run your engine
hotter."
Dean Jobb is a Wolfville, Nova Scotia-based freelance writer.
PHOTOS: Rolls-Royce and University of Manchester
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Manchester Materials Science Centre
Rolls-Royce International Limited |
