# Laser Cavitation Peening: A Review
###### Abstract  
During submerged laser peening using a pulsed laser, a bubble that behaves like cavitation is generated after laser ablation (LA). The bubble is referred to as laser cavitation (LC). The amplitude of the shockwave in water generated by LA is larger than that of LC; however, the impact passing through the target metal during LC is larger than that of LA. LC impact can be utilized for peening at optimized conditions. Thus, submerged laser peening is referred to as \"laser cavitation peening\", as the peening method using the cavitation impact is known as \" cavitation peening\". The impact induced by a hemispherical bubble is more aggressive than that of a spherical bubble with a microjet. Laser cavitation peening can improve the fatigue strength of metallic materials by producing work-hardening and introducing compressive residual stress. Three-dimensional additively manufactured metals (3D metals) such as titanium alloy are attractive materials for aviation components and medical implants; however, the fatigue strength of as-built components is nearly half of that of bulk metals, and this is an obstacle for the applications of 3D metals. In the present study, published research papers are reviewed to identify the key factors of laser cavitation peening, with additional visualization of LC and data. Then, improvements in the fatigue strength of metallic materials, including 3D metals treated by laser cavitation peening, are summarized. pulsed laser; cavitation; mechanical surface treatment; cavitation peening; additively manufactured metal; fatigue strength; titanium alloy +
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types of laser peening, water or a confinement layer [12] is required. In the third method, dry laser peening is carried out without a confinement layer using a femtosecond pulsed laser [13; 14]. For all three laser-peening methods, including submerged laser peening, it has been hypothesized that the peening mechanism that introduces local plastic deformation on the metal surface is caused by laser ablation [15]. Namely, it is thought that the inertial force caused by the shockwave is contained by the water, and this produces plastic deformation in metallic materials [4; 15]. In the case of submerged laser peening, a bubble that behaves like a cavitation bubble is generated after laser ablation, and this is known as laser cavitation [16; 17]. In previous studies, when the amplitude of the pressure wave in the water was measured using a shockwave sensor that was placed in the water, the amplitude of laser ablation was larger than that during laser cavitation collapse [18; 19; 20]. However, in another study, when the impact force passing through the target metal was measured using an impact sensor with a polyvinylidene fluoride PVDF film [21], the impact during laser cavitation collapse was larger than that during laser ablation [16; 19; 20]. Cavitation bubble oscillation during submerged laser peening has also been investigated, and it was concluded that the first bubble oscillation was effective for the submerged laser peening [22]. The introduction of compressive residual stress and the work-hardening of aluminum alloys has also been demonstrated using laser cavitation [23]. These results reveal that laser cavitation impact can be utilized for peening. Cavitation peening, in which cavitation bubble collapse impacts are used for peening, has been developed [6], and it has been applied for mitigation of the stress corrosion cracking of nuclear power plants [24]; there are also reports that show improvements in the fatigue strength of metallic materials, including 3D metals [6]. In the case of conventional cavitation peening, cavitation is generated by injecting a high-speed water jet into water, i.e., a cavitating jet [6]. The peening mechanism using a cavitating jet is different from that of water jet peening, in which water-column impacts are used for peening [25]. The peening intensity using a cavitating jet under optimum conditions was shown to be more effective than that of a water jet [6].