# Laser Cavitation Peening: A Review
## 7 Conclusions  
In order to reveal the importance of the collapse impact of the bubble, which develops after laser ablation during submerged laser peening, the peening method was named \"laser cavitation peening\", and the impact during bubble collapse was compared with the impact during laser ablation using published papers. The types of laser cavitation were reviewed by summarizing experimental data and numerical simulations, with an additional observation of laser cavitation. To show the effect of laser cavitation peening on the improvement of the fatigue properties of metallic materials, including 3D metals, the fatigue life and strength of materials after laser cavitation peening were compared with non-peened samples. The findings of the present review can be summarized as follows:  
1. During submerged laser peening, the impact of laser cavitation, which is generated after laser ablation, can be utilized for mechanical surface treatment. This is referred to as laser cavitation peening. Water quality, such as gas content, is an important factor, as the cavitation bubble collapse impact is affected by the gas content; this is called the cushion effect.  
Figure 26: Laser cavitation generated by Nd:YAG laser without Q-switch: (**a**) images of laser ablation and laser cavitation; (**b**) pressure detected by hydrophone [73].  
2. The impact passing through the target of laser cavitation collapse is greater than that of laser ablation, although the amplitude of the shockwave in water induced by laser ablation is larger than that of laser cavitation collapse.
3. A hemispherical bubble is most effective for laser cavitation peening compared with a spherical bubble, which produces a microjet in the bubble.
4. In the case of laser cavitation peening, the peening intensity is proportional to the volume of laser cavitation when the threshold of the target materials is considered.
5. During laser cavitation peening, the area near the edge can be treated using laser cavitation, which develops and collapses near the edge.
6. Laser cavitation peening improves the fatigue strength of metallic materials, including three-dimensional additively manufactured metals (3D metals). In the case of magnesium alloy AZ31, the fatigue strength at \(N=10^{7}\) evaluated by a plane bending fatigue test was 151 MPa for laser cavitation peening, 112 MPa for shot peening, and 97 MPa for non-peened. In the case of additively manufactured Ti6 Al4V by DMLS, the fatigue strength at \(N=10^{7}\) evaluated by a torsion fatigue test was 361 MPa for laser cavitation peening, 285 MPa for shot peening, and 217 MPa for non-peened.  
H.S.: conceptualization, additional experiment, writing--original draft, visualization, project administration, and funding acquisition. Y.I.: conceptualization, visualization, and writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This research was partly supported by JSPS KAKENHI, grant numbers 23H01292 and 22 KK0050, and the Amada Foundation AF-2021219-B3.  
Not applicable.  
The data presented in this study are available upon request from the authors.  
The authors declare no conflict of interest.