Quarter Century Development of Laser Peening without Coating

Abstract

This article summarizes the development of laser peening without coating (LPwC) during the recent quarter century. In the mid-1990s, the study of LPwC was initiated in Japan. The objective at that time was to mitigate stress corrosion cracking (SCC) of structural components in operating nuclear power reactors (NPRs) by inducing compressive residual stresses (RSs) on the surface of susceptible components. Since the components in NPRs are radioactive and cooled underwater, full-remote operation must be attained by using lasers of water-penetrable wavelength without any surface preparation. Compressive RS was obtained on the top-surface by reducing pulse energy less than 300 mJ and pulse duration less than 10 ns, and increasing pulse density (number of pulses irradiated on unit area). Since 1999, LPwC has been applied in NPRs as preventive maintenance against SCC using frequency-doubled Q-switched Nd:YAG lasers (λ = 532 nm). To extend the applicability, fiber-delivery of intense laser pulses was developed in parallel and has been used in NPRs since 2002. Early first decade of the 2000s, the effect extending fatigue life was demonstrated even if LPwC increased surface roughness of the components. Several years ago, it was confirmed that 10 to 20 mJ pulse energy is enough to enhance fatigue properties of weld joints of a structural steel. Considering such advances, the development of 20 mJ-class palmtop-sized handheld lasers was initiated in 2014 in a five-year national program, ImPACT under the cabinet office of the Japanese government. Such efforts would pave further applications of LPwC, for example maintenance of infrastructure in the field, beyond the horizons of the present laser systems.

Laser Shock Processing and Related Phenomena

“…As a first section, two review articles are included, representing, on one side, the previous history of developments of LSP technology [1] and, on the other, an analysis based on such developments of the prospects for the industrial implementation of the LSP technique in critical reliability applications [2]. It is needless to say that these two review papers were written by two of the most renowned experts in the LSP field-i.e., Dr. Clauer was one of the original inventors of the LSP technique at Batelle Columbus Labs.…”

Fatigue performance of laser shock peened Ti6Al4V and Al6061‐T6 alloys

“…Compared with conventional peening methods such as shot peening, LSP is a highly precise process that introduces very high strain rates (>10 6 s −1 ) generating deep compressive residual stresses 2 . The technique has been successfully implemented in aircraft turbine fan blades to improve their resistance against foreign object damage 3 as well as in nuclear plants to increase stress corrosion cracking resistance of weldments 4 . Apart from this, it has also found applications in improving fatigue life of automotive parts 5 and as postprocessing method in additive manufacturing 6 …”

“…2 The technique has been successfully implemented in aircraft turbine fan blades to improve their resistance against foreign object damage 3 as well as in nuclear plants to increase stress corrosion cracking resistance of weldments. 4 Apart from this, it has also found applications in improving fatigue life of automotive parts 5 and as postprocessing method in additive manufacturing. 6 Ever since the invention of LSP in 1960s, 7 a number of studies have been dedicated to understand its mechanism and potential for industrial applications.…”

Effect of Laser Peening on the Mechanical Properties of Aluminum Alloys Probed by Synchrotron Radiation and X-Ray Free Electron Laser

“…Some researchers are applying LSP to new areas such as engineering ceramics [13] and medical implant [14]. Meanwhile, the authors previously described a laser peening process using low-energy laser pulses (0.1 J or less) [15][16][17][18][19]; no surface preparation (pasting of a "sacrificial overlay") was required. We initially termed this process laser peening (LP) [19] and later laser peening without coating (LPwC) [20,21] to emphasize that no sacrificial overlay or coating was necessary.…”