# Laser Cavitation Peening: A Review
###### Abstract
Thus, it is worthwhile to use the impact at cavitation collapse for peening. As mentioned above, as submerged laser peening can use the impact of the laser cavitation collapse, it is a type of cavitation peening that uses laser cavitation [6]. Therefore, it can be referred to as laser cavitation peening. It is thought that the inertial force of water confines the shockwave generated by the plasma, which is caused by laser ablation, and produces local plastic deformation on the material surface. Impact at laser cavitation collapse is also useful to produce local plastic deformation. The collapse of a spherical cavitation bubble near a solid boundary in water was computed by [PERSON] and [PERSON] [26] (see Figure 1), and then the deformation of the bubble was experimentally observed using high-speed photography at a framing rate of 300,000 frames/s [27]. A microjet in a cavitation bubble was also clearly identified by [PERSON] [28]. Microjets and shockwaves have been attractive phenomena for researchers and engineers and are important for understanding bubble dynamics [29] and sonochemistry [30]. The microjet in a cavitation bubble is still an attractive phenomenon for researchers [31; 32; 33; 34]. As shown in Figure 1, the development of a microjet in a spherical cavitation bubble is an interesting phenomenon, and some researchers believe that the microjet during cavitation bubble collapse can be utilized for peening. It has been demonstrated numerically, using fluid/material-coupled numerical simulations, that the pressure produced by the collapse of a hemispherical bubble is much stronger than that of microjet-type collapse, as shown in Figure 2[35]. In Figure 2, the black line shows the shape of the bubble changing with time, and the color contour reveals the pressure in the fluid and the equivalent stress in the material at bubble collapse. Although the microjet during the bubble collapse generates a pressure wave of \(\approx\)100 MPa, the collapse of the hemispherical bubble produces pressure waves several times larger than those of the microjet. Thus, a hemispherical bubble should be used for laser cavitation peening. Note that the pressure induced by a flattened bubble, i.e., a pancake-type bubble, or a longitudinal bubble is weaker than that of a hemispherical bubble [35]. In order to use cavitation peening to enhance the fatigue properties of metallic materials, knowledge of fluid dynamics is required [6], as cavitation is a hydrodynamic phenomenon. Thus, in the present review, to clarify the mechanism of laser cavitation peening, the effectiveness of the laser cavitation impact for peening is explained using published papers, with additional observations with respect to laser cavitation by using a high-speed video camera. The peening intensity was also evaluated by measuring arc height and the introduced compressive residual stress. The effect of the bubble shape is discussed experimentally and numerically, and comparisons are made between a hemispherical bubble and a bubble with a microjet. Key factors with respect to laser cavitation peening are also summarized. Finally, improvements in the fatigue properties of metallic materials, including 3D metals, are demonstrated.