February 2010 - Aircraft jet engine manufacturers are always trying to squeeze more efficiency, along with greater fuel economy, from their engines. When turbine blades and other engine components wear, both efficiency and fuel savings suffer.
Repairing these components is much less costly than replacing them, especially today because some jet engines use blisks that have integral blades that can't be removed--the entire blisk needs to be repaired, which can present challenges.
Electrical power generation companies that use gas turbines also benefit from repairing blades rather than replacing them.
One company that sells the equipment to both manufacture and refurbish turbine blades is Huffman Corp., Clover, S.C. It has a long history of providing cladding and drilling lasers to worldwide flight and industrial gas turbine, medical and material-processing markets.
The company does a lot of R&D work and processes for customers that include GE, Rolls-Royce, Pratt & Whitney, Honeywell and Williams International. Huffman also works with the companies that are first- and second-tier suppliers of these OEMs, along with people who repair turbine blades.
Additionally, it sells to companies that build gas turbines for the power industry, including GE, Siemens, Mitsubishi and Solar.
Huffman started about 50 years ago as a distributor of machine tools and then started manufacturing them about 35 years ago. The owner invented and patented a grinding machine that could make cutting tools from solid stock, which was the first CNC CBN grinder.
Later, Huffman branched out into producing a high-precision motion system that could hold tolerances to 0.0002 in. With this equipment, in 1972, GE Aircraft Engines asked the company to build a precision laser to drill holes in its turbine blades for cooling.
"Aircraft jet turbine engines needed a way to cool the tips of their blades, which would give the turbine engine greater efficiency because it could run hotter," says Roger Hayes, president of Huffman Corp. "Cool air is fed through the blade into a series of bleed holes on its surface. These holes are positioned so that when the hot air from combustion hits the blade, it rides on the cooler film of air that was bled through these holes. Therefore, the hot combustion air doesn't touch the blade. If it did, it would melt it. But these holes had to be drilled precisely, and GE asked if we could use a laser on our precision motion system to drill them."
In the late 1980s, turbine blades had a change in their construction. They eventually went to single-crystal materials and were more expensive. These blades could be repaired, however.
"When the tip of a jet engine's air compressor blade wears, you have air bypassing it, which means the fuel consumption goes up, and the thrust goes down," says Hayes.
Even though the wear might only be a few thousands of an inch, this could be a lot on a blade that might have an edge that's only 0.005 in. thick to begin with. To repair it, the tip of the blade would be cut off and then welded, or cladded, to rebuild the lost metal. Then it's ground and reshaped.
GE aircraft engines invented a method of doing this. It's called laser cladding, and the company licensed Huffman to build these laser cladding machines.
Renewing turbine blades
A jet engine's second and third stages are called the hot section, which is when the combustion and primary thrust take place. Third-stage blades usually have a hard face added to them using stellite, which is melted over the base metal by hand welding--or more efficiently through laser cladding, according to Hayes. Cladding involves using a laser beam focused through a funnel to melt the blade's base material. As the base metal melts, powdered metal is introduced into the beam through the funnel and melted to the blade's surface.
"We do this on the tip or edge of a blade to build up the metal, so it can be remachined and reused," says Hayes.
At one time, Huffman used two laser types to do cladding, either a YAG or CO2 laser. But today, the company uses a fiber laser produced by IPG Photonics Corp., Oxford, Mass.
"We have used both [types of lasers] for years, but then we zeroed in on the IPG fiber laser about three years ago, and we haven't sold any of the other lasers since," says Hayes. "The biggest thing with this laser is its reliability. Our [laser cladding] machines are known as the most reliable ones in the industry. If we have a laser that's unreliable, our customers never say that the laser is down--they say the Huffman is down."
Why a fiber laser?
Hayes also notes that this is a solid-state fiber laser, not a mirror laser. With the CO2 laser, users have to develop costly ways to move the beam to the cladding area. Also, once the CO2 laser's beam is aligned, the operator has to make sure it stays that way, which is an added expense.
In addition to lowering costs, Huffman saved a massive amount of floor space with the fiber laser, according to Hayes. With Huffman's motion equipment, it uses only a third to a quarter of a CO2 machine's footprint.
Beam quality is also important, says Hayes. IPG's fiber laser has a consistent spot size and has a long focal length, which gives Huffman a wide range of power capabilities to control the heat-affected zone with high precision, which is difficult to do with a YAG or CO2 laser, according to Hayes.
"Much of the work that we do is minute with thin parts, which are flight-sensitive ones," he says. "The IPG laser is so precise, we can easily run it down to 1/10 of its power range. So a 2-kW machine we can run at 200 W, and then we can change from a continuous wave to a pulse mode that allows us to run it effectively at 100 W or less. Also, the wall-plug efficiency is much higher.
"Sometimes, a turbine blade can be as thin as 0.005 in., and we have to melt this metal without burning it off, along with adding metal to it," he continues. "So the IPG laser allows us to have incredible control over applying the heat. For very thin blades, we are down to about 60 W of power, which we can achieve with a consistent beam and good focal characteristics. We just could not do this with a CO2 laser."
In building its equipment, Huffman has experienced a lot of benefits using the fiber laser over a CO2 or YAG laser.
For example, mirrors in a CO2 laser require tubes to house the beam and a lot of leveling and aligning. They also have to be cooled and need interlocks. With the fiber laser, there are no mirrors and interrelated components. Additionally, there's no consumption of laser gas, regulators or gas lines, no safety valves and no diode or beam alignment.
"All we have to do is run a fiber into the machine and connect it," says Hayes. "There are really no consumables either."
A customer who buys Huffman's integrated system can get 100,000-hour diode life, and the power is consistent throughout the laser's life, says Hayes.
"It doesn't diminish like it does with the other lasers," he says. "There are no vacuum pumps or blowers that need maintenance. There's no energy loss through mirrors that can become cloudy or cracked. The warm-up time is greatly reduced too. And with all the power variables that a CO2 laser has, we can use a lower-power laser to do the same work. We used to [use] a 2.2-kW CO2 laser to do what a 1-kW [fiber] laser now does." FFJ