05 May 2004
Laser-peened fan blades are enabling aircraft to fly for longer periods between engine maintenance. Oliver Graydon reports on the commercialization of the technique.
From Opto & Laser Europe May 2004.
Ever since the idea of strengthening metal parts by laser shock-peening was developed at the Battelle Laboratory, US, in 1965, its commercialization has been hindered by the lack of a reliable, high-repetition-rate, high-power laser.
Now, thanks to advances in the output power and repetition rate of solid-state sources, laser peening has finally made the transition from laboratory to production line. The process, which increases the strength of metal parts by bombarding them with intense light pulses, is now a commercial service being offered by Metal Improvement Company (MIC) in the US and UK.
Laser shock-peening relies on creating a shock wave which penetrates deep inside the metal part, compressing it and making it stronger. Although the enhancement in fatigue strength is similar to that delivered by conventional shot peening, using a laser offers several advantages over firing metal shot at the part.
However, the big attraction of the laser technique is that the strength improvement is trapped up to five times deeper within the material than with conventional shot peening, providing much longer service life.
"Put simply, this means that the residual stress you put in to prevent failure stays longer, a lot longer," says Peter O'Hara, vice-president of MIC. "If you're four to five times deeper then your part has four or five times the lifespan."
"Today, the parts inside the engines of airplanes and cars are being stretched to their limits in terms of performance by using lighter, stronger materials," explains O'Hara. "For decades the parts have been shot-peened, but the residual stress level that we can introduce has reached a limit, and we need to go deeper to slow down the propagation of any cracks and withstand operational issues."
Peening takes flight The recent commercialization of laser peening has been fuelled by a keen interest from the aerospace industry, which is always searching for ways to improve the performance and lifespan of parts. In December 2001, MIC won a contract from an original equipment manufacturer (OEM) to laser peen titanium components inside a commercial turbine engine.
Within four months of starting production, the facility was operating 24 hours a day, five days a week and making 100,000 laser firings each day.
Following the success of the facility in California, MIC has now established a second production facility in Earby, UK. Each facility is equipped with two lasers and is capable of handling parts up to 100 kg and 2 m in length. To date, more than 5000 fan blades for the engines inside various aircraft have been laser-peened at the Livermore and Earby facilities.
At the heart of the peening process is Nd:Glass solid-state laser technology developed by LLNL. Using similar technology to that going into the National Ignition Facility (NIF), a giant laser fusion experiment at Livermore, LLNL has developed a flashlamp-pumped Nd:Glass laser that delivers high-energy (up to 25 J) nanosecond (10-100 ns) pulses with a repetition rate of up to 5 Hz. In contrast, the best commercial lasers with similar pulse parameters can only manage a laser shot once every 4 s or so - too slow for a production environment.
Before the part is illuminated with the laser pulses, it is covered with a thin light-absorbing material and a layer of running water. When intense laser pulses (5-15 GW/cm2) strike the light-absorbing layer they ablate it, creating a high-pressure plasma on the surface of the part. The plasma creates a powerful shock wave which reflects off the water layer and travels inside the metal part, compressing it.
"The water is not there as a coolant but as a confinement for the shock wave," explains O'Hara. "Being virtually incompressible, it acts like a solid wall or mirror for the shock wave, forcing it to travel into the base material of the part."
The process is repeated over the entire part by carefully manipulating it so that all of the surface is exposed to the laser pulses. Two robots are required in the process, one to position the part and the other to apply a stream of water onto the target area.
At present the laser fires three pulses per second, each 10-30 ns in duration and containing around 20 J of energy. Each pulse processes an area of about 3 mm2.
After treatment the light-absorbing layer, which serves as both an ablative layer and a protection from the heat of the plasma, is removed, leaving the part beneath. MIC says that the maximum temperature that the base material experiences is 70-80 ºC.
"So far, we have developed processes for all kinds of ferrous materials including stainless steel and aluminium alloys. It's our belief that any metal that can be shot-peened can be laser-peened. You just need to adjust the parameters of the laser pulse," comments O'Hara. "However, today we have only one material that we are processing in production. That's titanium-6/4, where we are putting in residual compressive stresses up to 1.25 mm deep."
"If you've flown in a plane in the past six months then it is likely that it had laser-peened parts," says O'Hara. "Today we rely on aerospace but there is potential in the marine sector such as submarines and aircraft carriers, as well as the medical industry and power plants - to mention just a few."
One application that is currently a focus of attention is the design of spent-fuel nuclear-waste storage canisters that are capable of surviving thousands of years inside a storage facility (Yucca Mountain, US). Because the cannisters will be sealed by welding, there are concerns that tensile-weld residual stress may occur which may ultimately lead to a rupture. Laser peening is being considered as a way to combat the onset of this stress by improving the welds.
Practical considerations Aside from price, several other factors are limiting the type of parts that can be laser-peened today. One is that the current MIC system is not portable and uses a fixed laser beam line that requires that part to be moved. Many potential large-scale applications require the process to take place on site with a laser beam that can be scanned over the piece. However, the design of a portable system is in the works.
"We expect that we will have a portable system by early next year," explains O'Hara. "The challenge is that the laser pulses are so powerful that they would destroy traditional mirror or fibre-optic beam-guiding systems, so we are developing our own method."
Development issues aside, O'Hara believes that laser peening will become increasingly popular outside aerospace applications. "In five to ten years we are of the belief that this could match shot peening and perhaps in 15 years' time it will be cheaper," claims O'Hara. "In a few months' time this technology is likely to enter the automotive system via Formula One cars, and perhaps the family car is only five years away."