Peening With Light: New Laser, New Benefits

With newly gained performance and efficiency, laser peening could change how metal structures are designed and manufactured, says Dave Francis, executive vice president, Metal Improvement Co., Paramus, N.J. He's referring to new laser technology from Lawrence Livermore National Laboratory, Livermore, Calif., that sheds laser peening's reputation for being a slow, expensive way of enhancing metal properties. In effect, the process enables metal structures to be more easily strengthened where vulnerable without having to "beef up" the entire configuration. All users gain weight-free strengthening and lower maintenance characteristics -- issues especially important to fuel conscious aerospace and automotive applications. Medical implants also are an emerging application. Another use of laser peening involves metal shaping. Treating just one side of a metal piece will make it naturally bend, which places both peened and unpeened sides under compression and makes the part more resistant to failure. Metal Improvement uses an LLNL-developed Neodymium-doped glass laser that produces 1 billion watts of peak power, about the output of a large commercial power plant, in 20-billionths of a second bursts. With 125 watts of average power, the LLNL laser can fire five pulses per second compared with one pulse every four seconds for the best commercially available lasers. The Livermore laser allows compressive stress to be imparted to a depth of 4/100ths of an inch beneath the surface, about four times deeper than conventional shot peening. This added depth is important as a means of inhibiting crack initiation and propagation and can extend the life of parts three to five times over that provided by conventional treatments. "This is 21st Century technology that will enable engineers to design aircraft parts that are safer, lighter, perform better and are more economical," says Lloyd Hackel, leader of LLNL's Laser Science and Technology Program and initial developer of laser peening at LLNL. Rolls-Royce plc has used Metal Improvement's services to laser peen critical fan blade components (above left) installed in more than 250 Rolls-Royce aircraft engines. Due to the deeper compressive residual stress imparted by the laser peening process, components are more resistant to fatigue stress, improving the cost effectiveness of the operation of the component in terms of increased life and reduced maintenance costs. Other applications are under development for Rolls-Royce. The possibilities include upgrading other commercial aircraft components, such as discs, landing gears, spars, bulkheads and drive gears.