# Laser Cavitation Peening: A Review
## 4 Key Factors of Laser Cavitation Peening
### Bubble Shape  
In this section, we investigate the effect of the bubble shape. Figure 12 shows images of laser cavitation. In Figure 12, the specimen was set closer than the optimal \(s_{w}\) in Figure 5. When \(s_{w}\) was large enough, the laser cavitation developed as a hemispherical bubble, as shown in Figure 6a. However, when \(s_{w}\) was shorter than the optimal \(s_{w}\), the bubble developed into a flattened hemispherical bubble, i.e., a pancake-shaped bubble, \(t=0.04\)-\(0.22\) ms, as shown in Figure 12. During the shrinking of the bubble, it became a longitudinal bubble, as shown at \(t=0.70\) ms, and then collapsed. Figure 10: Relation between developing time and diameter of bubble [16]. Figure 13 shows the bubble that developed at the edge. When the pulsed laser was irradiated at the edge, the bubble developed at the edge, and the outer shape was nearly a spherical bubble, i.e., \(t=0.09\)-\(0.43\) ms, as shown in Figure 13. During the shrinking of the bubble, it became a kind of horn-shape, then collapsed. After the first collapse, the other two bubbles, which are indicated by blue arrows at \(t=0.87\) ms in Figure 13, were generated and then collapsed. One of the secondary bubbles became considerably large and then collapsed. In order to illustrate the effect of the bubble that developed at the corner, Figure 14 shows the distribution of surface residual stress measured using a collimator of 0.146 mm in diameter via the 2D method [57]. The target material of Figure 14 was titanium alloy Ti6 Al4V. After laser cavitation peening at 30 and 40 pulse/mm\({}^{2}\), compressive residual stress of about 100 MPa was introduced, and the compressive residual stress introduced near the corner was almost equivalent to the value at the center of the specimen. Namely, a bubble developed at the corner that can introduce compressive residual stress into titanium alloy Ti6 Al4V. Next, we compare experimentally microjet-type bubbles, as shown in Figure 2a, and hemispherical bubbles, as shown in Figure 2b. Figure 15 shows the behavior of a bubble that was developed near the target surface by focusing on the water [58]. The bubble was initiated in the water near the target and developed, then collapsed in the water. After that, the bubble was developed on the target, then collapsed on the target. In order to compare the peening intensity of the bubble shown in Figure 15 and the hemispherical bubble, Figure 16 illustrates the residual stress as a function of the distance from the target surface. The blue line shows the residual stress of stainless steel treated at 10 pulse/mm\({}^{2}\) by the bubble shown in Figure 15 [59], and the violet line shows treatment at 4 pulse/mm\({}^{2}\) by the hemispherical bubble [6]. The red line reveals the residual stress  
Figure 11: Effect of laser cavitation volume on peening intensity [16]. Figure 12: Images of laser cavitation on the wall near the glass chamber. without peening [59]. For both treatments, a Q-switched Nd:YAG laser of 0.35 J and 1064 nm was used. As shown in Figure 16, the hemispherical bubble at 4 pulse/mm\({}^{2}\) introduced large compressive residual stress into a deeper region compared with the bubble shown in Figure 15 at 10 pulse/mm\({}^{2}\). Namely, the collapse of the hemispherical bubble produced a more intense impact compared with the microjet. Now, we investigate the effect of the bubble shape using numerical simulation. [PERSON] and [PERSON] carried out a fluid/material-coupled numerical simulation of the first stage of the non-spherical collapse of a bubble near a wall, including the stress state inside the elastic material of the wall [35]. Figure 17 shows the results of typical cases (a) \(\gamma=-0.3\), (b) \(\gamma=0\), and (c) \(\gamma=1.2\)[35].