**1. Introduction**

Progress in laser science and technology has realized advanced processes and applications in industries. Development of laser peening without coating (LPwC) is a landmark to deploy high-power lasers for maintenance work of infrastructure in the field. LPwC has advantage because of inertia-less process over mechanical treatment in operating nuclear facilities [1,2]. LPwC introduces compressive residual stresses (RSs) on metallic materials by simply irradiating successive laser pulses to the bare surface of components covered with water [3]. A remote processing system of LPwC was developed and has been applied to components of existing nuclear power reactors (NPRs) to mitigate stress corrosion cracking (SCC) since 1999 [1].

In the earliest system of LPwC for NPRs, laser pulses travel 50 m from laser units to the reactor components through waterproof guide pipes with reflecting mirrors at corners [1]. A technology for delivering 20 MW (100 mJ, 5 ns) laser pulses using optical fiber was also developed to increase the flexibility and extend the applicability of LPwC [4,5]. A miniaturized optical head with a diameter of 10 mm was developed with a fast-responding focusing function [6,7] that controls the focal point just on the surface within an accuracy required for fiber-delivered LPwC, namely less than ±0.5 mm. By integrating these technologies, fiber-delivery has been utilized in NPRs since 2002 [2].

Regarding fatigue issues, LPwC has positive effects to improve mechanical properties of various materials including ceramics [8,9]. LPwC significantly enhanced the fatigue strength and prolonged the fatigue life of steels [10–12], aluminum alloys [13] titanium alloys [14,15], etc. Recently, Sakino et al. confirmed the effect enhancing fatigue properties of HT780 (780 MPa grade high-strength steel) by low-energy LPwC with pulse energies down to 20 and 10 mJ [16]. Considering these advances, the Japanese governmen<sup>t</sup> launched a five-year national program, ImPACT (Impulsing PAradigm Change through Disruptive Technologies) in 2014 to develop compact high-power pulsed lasers including 20 mJ-class palmtop-sized handheld lasers [17], which brings about further applications beyond the horizons of the present LPwC by realizing a portable system with the handheld lasers, for example applications to infrastructure in the field such as bridges, windmills, etc.

In this article, the development of LPwC in the recent quarter century is reviewed including the perspective brought by palmtop-sized handheld lasers.

### **2. Fundamental Process of LPwC**

The fundamental process of LPwC is illustrated in Figure 1a [18]. When the high-power laser pulse with a duration of several nanoseconds is focused on the material, the top surface immediately transforms into plasma through ablative interaction with the laser pulse. If the surface of the material is covered with water, the pressure of the plasma significantly increases because the inertia of the water prevents expansion of the plasma. Under certain conditions, the peak pressure becomes 10 to 100 times higher than that in air, reaching several GPa which exceeds the yield strength of most metals. A shock wave is generated by this sudden pressure rise, propagates toward inside the material and attenuates to induce plastic deformation of the material. After passage of the shock wave, compressive RS generates in the surface layer due to elastic constraint from the surrounding part.

**Figure 1.** Fundamental process: (**a**) Laser peening without coating (LPwC); (**b**) Laser peening with coating (sacrificial overlay).

LPwC usually employs Q-switched Nd:YAG lasers. In our development, the wavelength was halved to water-penetrable visible light (λ = 532 nm) to apply to water-immersed objects. Surface RSs become compressive by increasing the number of pulses irradiated in unit area (pulses/m2) [18], in spite of intense heat input due to the direct interaction of laser pulses with the bare surface of the objects. To make the heat input negligible the interaction time was reduced, i.e., the laser pulse duration was decreased to several nanoseconds from tens of nanoseconds in the laser peening with coating [19–22]. The pulse energy was also reduced to around 200 mJ from several tens of Joules.

In the mid-1990s, we attained surface compression by LPwC for the first time in the world [18]. This achievement is a landmark for the maintenance of NPRs because LPwC doesn't require drainage of cooling water used for radiation shielding but only irradiates laser pulses to bare components underwater without any preparation on the surface of the components.

In case of laser peening with coating, sacrificial overlay (coating) is pasted on material [19–22], which controls laser energy absorption and prevent the surface from melting. This scheme of laser peening uses high energy Nd:glass lasers with near infrared wavelength (λ = 1.05 μm) and black polymer tape or metal foil as the coating which is pasted prior to laser irradiation and removed after the treatment. The details of the process described elsewhere [22].
