AT first glance, it would seem that bombarding a metal part with an intense stream of tiny metal or ceramic balls might not be the best approach for making that part more resistant to cracking and corrosion. And yet, shot peening is a tried-and-true technique for strengthening metals. Now a team of Lawrence Livermore researchers, in tandem with colleagues at New Jersey-based Metal Improvement Co. Inc., have replaced the tiny balls with short-lived, repetitive blasts of light from a reliable, high-powered laser.
The new technology, called the LasershotSM Peening System, is designed to extend the service lifetime of critical metal parts, from aircraft engine fan blades (Figure 1) to hip joints, by a factor of three to five times over conventional peening treatments. The process also holds the promise of lighter, stronger products of entirely new designs.
In traditional shot-peening procedures, each metal or ceramic ball acts as a minuscule ball-peen hammer, imparting on a metal surface a small indentation or dimple. This process produces, below the dimple, a hemisphere of highly shocked and compressed material. In time, overlapping dimples provide a very thin (about 0.25 millimeter), uniform layer that is extremely resistant to cracks, corrosion, and fatigue. Because of these benefits, the springs and transmission components of almost every automobile are shot peened for longer life, as are aircraft structural components.
With the invention of the laser, researchers quickly recognized that peening could be achieved using high-energy lasers with pulse lengths in the tens of nanoseconds (billionths of a second), short enough to generate a rapid yet energetic shock. Prototype laser peening machines were developed in the 1970s, but they and subsequent versions over the past two decades were not cost effective because the lasers lacked the high repetition rate required for treating parts rapidly.

Borrowed Technology
Working under a Department of Energy Cooperative Research and Development Agreement, the team of researchers from Livermore (Figure 2) and Metal Improvement Co. developed a machine that for the first time makes laser peening a cost-effective option by providing a tenfold increase in metal part treatment rate over all other laser systems. The LasershotSM Peening System features a 600-watt, high-energy (100-joule), neodymium-doped glass laser capable of firing 6 pulses per second continuously and 10 pulses per second in bursts.
In use, the system fires a 20-nanosecond laser pulse that is focused to an energy density of about 200 joules per square centimeter at the surface of a metal coated with dark paint and a thin overlay of transparent material, like water. The laser light passes through the water and is absorbed by the dark paint. The interaction creates a pressure shock wave in the range of 10,000 atmospheres (150,000 pounds per square inch) that in turn creates a deep compressive stress layer directly underneath the focused pulse. (The water layer acts like a lid on a pot to help contain the shock.)
The system borrows from major advances made in the past decade by Livermore's laser fusion program. Like Livermore's giant Nova laser, Lasershot employs neodymium-doped glass, a master oscillator for originating and then amplifying the laser pulse, a much stronger amplifier optically pumped by flashlamps, and more recently, an active laser wavefront correction that allows the high-energy laser to be operated at high repetition rates. Indeed, says Livermore physicist Lloyd Hackel, a laser system based on neodymium-doped glass is the only identified technology that can realistically achieve the required energy output with acceptable pulse duration.
According to Hackel, the most significant feature of the system is the laser's high repetition rate, which enables the system to treat parts at about the same rate as conventional peening and at a substantially faster rate than existing laser peening methods. "Until now, the lack of a fast enough laser repetition rate has held back laser peening from significant adoption by industry," he says.
The new system does not have the kinetic energy limitations of metal or ceramic shot. As a result, it can induce a compressive stress layer more than 1 millimeter (0.04 inch) thick, some four times deeper than that obtainable with shot. The increased depth effectively extends the service lifetime of parts some three to five times over that provided by conventional treatments, an increase essential for preventing cracking on blades, rotors, and gears. In fact, tests on deliberately nicked (and hence weakened) turbine fan blades show that laser peening will actually render these parts stronger than new, unflawed--but not laser-peened--blades.
A unique advantage of Lasershot's laser pulse is its square profile. A typical laser's round output beam requires overlapping spots on a metal surface in an inefficient manner, but the new system allows full coverage of each square spot directly adjacent to the next. What's more, the system automatically maintains the laser-pulse wavefront near the physically allowable limit, enabling higher power without worry of laser glass damage or damage to the treated part.






Strength for Future Products
The first commercial use of the technology is expected in late 1999. With Livermore support, Metal Improvement Co., the nation's leading provider of metal-peening services, has been introducing the new product to the manufacturing and user community and has received very high interest. Hackel anticipates that many companies will send parts to Metal Improvement Co. for treatment, engineering evaluation, and production. An option for large firms like aircraft and automobile parts manufacturers is purchasing and operating their own Lasershot system.
Hackel says the new process will probably not replace conventional shot peening but instead will be used in areas where greater depths of compressive stress are desired, such as jet engine rotors, disks, blades, gears, and shafts. In these cases, component cost is high, and resistance to metal fatigue is extremely important. Recent studies showed that engine blades costing $30,000 to $40,000 apiece last three to five times longer when treated with the laser-peening process. Lasershot is also expected to increase the resistance of turbine blades to objects such as birds, ice, and stones that can damage the edge of a blade and trigger small cracks.
Other industrial applications include oil tools (drill collars, bits, and mud pumps), marine and rocket engine parts, golf equipment, and even hip joints. "We're going after high-value parts," says Hackel. "If we can triple the lifetime of a $30,000 part, the cost of peening becomes insignificant." Likewise, he says, doubling the lifetime of a hip implant becomes a virtually priceless advantage for doctor and patient.
Looking to the future, Hackel notes that laser-peened parts, because of their strength, can be made thinner and thus lighter. The new-found strength in turn will free engineers to think differently about how they can design products. One potential result, says Hackel, will likely be products that need less energy to operate.
The technology underlying the laser system was originally supported by the Defense Advanced Research Projects Agency (DARPA) of the Department of Defense for a far different purpose than working metal parts. DARPA was searching for a method to convert infrared light to high-power, short-wavelength x rays as a light source for printing advanced-generation integrated circuits.
More recently, funding was provided by the U.S. Navy and the U.S. Air Force. The military branches are interested in obtaining a light source for long-range and highly coherent illumination of missiles and space objects. One of the lasers has been in service since 1995 at a Navy facility at the Kennedy Space Center in Florida. A second, more powerful and longer pulse (600-nanosecond) version was recently delivered to the Air Force Research Laboratory in New Mexico.
Clearly, the future is bright for Lasershot and high-repetition, high-powered lasers.
--Arnie Heller

Key Words: high-energy laser, high-repetition-rate laser, laser peening, LasershotSM Peening System.

For further information contact Lloyd Hackel (925) 422-9009 (hackel1@llnl.gov).


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