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Laser Bursts Strengthen Metal Parts

Photonics Spectra
Feb 1998
David Lytle

LIVERMORE, Calif. -- A new high-energy "laser peening" process can improve metal parts by sending a shock wave through the metal, inducing compressive stresses that reduce metal fatigue and boost resistance to corrosion.
Manufacturers have been treating engine components such as blades, rotors and gears with shot peening, a process that bombards the part with small metal grains to create compressive stress to a depth of about 0.01 in. Researchers have long known that lasers could accomplish the task with deeper penetration (useful for stopping stress cracking), but creating a commercial high-energy, high-repetition-rate laser has been difficult.
Lawrence Livermore National Laboratory physicists Lloyd Hackel and Brent Dane have developed an Nd:glass slab laser that will do the job, providing 600 W of average power and 3 GW of peak power with a 10-Hz repetition rate.

A high-energy laser pulse incudes compressive stress in a jet-engine fan blade, strengthening the part and increasing its lifetime. Lawrence Livermore National Laboratory researchers have developed an Nd:glass laser to commercialize this process, called laser peening.
"There is definitely a need for this process, but until now there's never been a laser available to make it economically justifiable," said Jim Daly, senior vice president for Metal Improvement Co. The New Jersey-based firm is the exclusive licensee for the technology, which is expected to begin producing finished laser-peened components for the commercial marketplace in about two years.
Producing high energy from a solid-state glass laser is not a problem, but increasing the repetition rate is difficult because of thermal loading, according to papers presented by Hackel, Dane, Daly and Metal Improvement's James Harrison at meetings of the Materials Research Society in Boston and at the US Air Force Aircraft Structural Integrity Conference in San Antonio.
The researchers developed several features to reduce thermal buildup and minimize wavefront distortions generated by thermal loading. The laser's gain medium is thin, providing more efficient heat extraction. Also, laser light propagates through the slab in a zigzag pattern to average wavefront distortions.
One of the first expected applications is avionics, where researchers have been experimenting with laser peening jet engine components for more than a decade. Industry studies have shown that laser peening could extend the lifetime of an aircraft engine turbine blade by three to five times. The new laser has shown four times the penetration of shot peening.

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