Underwater laser ablation with nanoseconds lasers generates high-pressure plasma exceeding GPa, and can be used as a hammer to forge the surface of most metals. This technology is known as laser peening (LP) and has been used in aeronautical and nuclear industries since the late 1990s. Most recently, we have developed a novel LP process without water by using a femtosecond laser, which extends the application to integrated systems with mechanics and electronics incompatible with aqueous environment, and even to components in space. Various applications would be realized by enhancing usability through miniaturization and simplification. In this context, we have developed ultra-compact handheld microchip lasers with passive Q-switch generating sub-nanosecond pulses, which paves the way to a wide range of new applications beyond the horizons posed by current laser systems.
Laser peening without coating (LPwC) is an innovative surface enhancement technology to mitigate fatigue and stress
corrosion of metallic materials by imparting a compressive residual stress. Toshiba has established a process without
coating, whereas the coating is inevitably required in conventional process of laser peening to protect the surface from
melting. Since the energy of laser pulses in LPwC is significantly small compared to that in the conventional process, a
commercially available Nd:YAG laser can be used, and moreover, an optical fiber can be utilized to deliver the laser
pulses. Compressive residual stress nearly equal to the yield strength of the materials was introduced on the surface after
LPwC. The depth of the compressive residual stress reaches 1 mm or more from the surface. High-cycle fatigue tests
proved that LPwC significantly prolonged the fatigue lives despite the increase in surface roughness due to ablative
interaction of laser pulses with material surface. Accelerating stress corrosion cracking (SCC) tests showed that LPwC
completely prevents SCC of sensitized austenitic stainless steels, nickel-base alloys and their weld metals. LPwC has
been used since 1999 to prevent SCC of core shrouds or nozzle welds of ten nuclear power reactors of both boiling water
reactor (BWR) and pressurized water reactor (PWR) types, already covering nearly one fifth of the existing nuclear
power plants (NPPs) in Japan.
The authors have developed a new process of laser-induced shock compression to introduce a residual compressive stress on material surface, which is effective for prevention of stress corrosion cracking (SCC) and enhancement of fatigue strength of metal materials. The process developed is unique and beneficial. It requires no pre-conditioning for the surface, whereas the conventional process requires that the so-called sacrificial layer is made to protect the surface from damage. The new process can be freely applied to water- immersed components, since it uses water-penetrable green light of a frequency-doubled Nd:YAG laser. The process developed has the potential to open up new high-power laser applications in manufacturing and maintenance technologies. The laser-induced shock compression process (LSP) can be used to improve a residual stress field from tensile to compressive. In order to understand the physics and optimize the process, the propagation of a shock wave generated by the impulse of laser irradiation and the dynamic response of the material were analyzed by time-dependent elasto-plastic calculations with a finite element program using laser-induced plasma pressure as an external load. The analysis shows that a permanent strain and a residual compressive stress remain after the passage of the shock wave with amplitude exceeding the yield strength of the material. A practical system materializing the LSP was designed, manufactured, and tested to confirm the applicability to core components of light water reactors (LWRs). The system accesses the target component and remotely irradiates laser pulses to the heat affected zone (HAZ) along weld lines. Various functional tests were conducted using a full-scale mockup facility, in which remote maintenance work in a reactor vessel could be simulated. The results showed that the system remotely accessed the target weld lines and successfully introduced a residual compressive stress. After sufficient training for operational personnel, the system was applied to the core shroud of an existing nuclear power plant.
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