Repairing turbines with supersonic spray

Scientists at the GE Global Research labs (GRC), New York, have developed a way of repairing parts in need of service by blowing metal powder, at four times the speed of sound.

"The tiny bits of material fly so fast that they essentially fuse together when they hit the target," said Gregorio Dimagli, materials scientist from Avio Aero. "Unlike welding, you don't need to apply heat to make them stick. The bond happens on the atomic level. That's why we are so excited."

The method, called cold spray, will allow Avio Aero and its parent, GE Aviation, to repair turbine and compressor blades without changing their complex underlying crystal structure.

"Manufacturers spend a lot of time to make the part just right," Dimagli continued. "But when you heat up metal and then cool it again, it changes in the same way powder snow can become a sheet of ice after a warm spell."

Dimagli and his team have partnered with the Polytechnic University of Bari, Italy, to perfect the applications of cold spray, sometimes called '3D painting', as well as laser deposition and other additive manufacturing techniques.

Anteneh Kebbede, manager of the Coatings and Surface Lab at the GRC that helped developed cold spray, says the technology is "like a fountain of youth for machine parts."

3D painting deposits metal powder flying at velocities of up to Mach 4 on precise models to produce and repair jet engine blades, rotors and other components without resorting to machining or welding. The method can also be used to create whole new parts with walls an inch or more thick.

"For manufacturers the potential benefits are enormous," Kebbede said. "Imagine being able to restore an aging part to its original condition with a tool that looks like spray gun."

The 3D painting gun uses pressurised carrier gas shot through a de Laval nozzle to accelerate powder particles as small as 5µm to supersonic velocities. The speed causes localised high energy collisions when the particles hit the surface forming a diffusion bond with the part.

Cold spray operators use a computer-controlled robot to manipulate the gun. Like 3D printers, the computer works with a 3D image of the part. Engineers program the robot so that it moves in an optimal way to deposit the powder.

"All the hard work is in the details," Kebbede said. "The powder selection, the conditions the powder experiences in the gun, the speed of the gun, the gun distance from the part and its angle relative to the part are just some of the inputs that lead to a good bond. That's the trick. The same process that can cause build up can also cause erosion."

Possible applications for cold spraying range from heavy-duty gear boxes for oil and gas machinery, to gas turbine rotors and jet engine blades.

"These methods are the future," explained Dimagli. "Compared to what we are using now, you get better quality for less money and you are also done faster."