# Laser Cavitation Peening: A Review
## 6 Future Work  
One of the major issues to address in laser cavitation peening is the drastic improvement in efficiency, as it takes time to enhance the fatigue properties of metallic materials. A big problem of present laser cavitation peening is that the speed of the process is too slow and the slow processing speed leads to high costs. During conventional laser cavitation peening, an Nd:YAG laser with a Q-switch is used, as the impact induced by laser ablation is also utilized. As mentioned in this review paper, when suitable conditions such as the feeding of degassed water to enhance the laser cavitation collapse impact are applied, the laser cavitation impact can treat metallic materials. If the laser cavitation impact without the laser ablation impact can treat the metallic materials, a short laser pulse, whose pulse width is a few ns to generate laser ablation, is not required. It was reported that an Er:YAG laser, whose pulse width is 300 \(\upmu\)s, can produce laser cavitation [48]. When a laser pulse of several hundred \(\upmu\)s in pulse width can be used for laser cavitation peening, the processing efficiency of laser cavitation peening can be drastically improved. Other types of laser systems, such as fiber lasers, whose repetition frequency is 50 kHz, can be used for laser cavitation peening. Currently, the repetition frequency range of a conventional Nd:YAG laser with a Q-switch is dozens of Hz to hundreds of Hz. Thus, one of the future research directions with respect to laser cavitation peening is the research and development of laser cavitation peening using a pulse laser with a longer pulse width to increase the repetition frequency.  
In order to reveal the possibility of laser cavitation peening using a long laser pulse, Figure 26 shows (a) images of laser cavitation and (b) the pressure detected by a hydrophone [73]. The pulse laser used was an Nd:YAG laser without a Q-switch, i.e., with normal oscillation, whose pulse width was about 200 \(\upmu\)s. As a Q-switch was not used, the high-speed observation and the recording of the output signal from the hydrophone were not synchronized. As shown in Figure 26a, a relatively long laser pulse such as 200 \(\upmu\)s  
Figure 25: EBSD analysis of cross-section after (**a**) CP via jet, (**b**) SP, and (**c**) CP via laser (laser cavitation peening) surface treatments showing inverse pole figure maps (top) relative to the normal of the cross-section and grain reference orientation deviation (GROD) maps (bottom). Reprinted from [63] with permission from Elsevier, License Number 5532500298391.  
can generate a bubble, and the bubble develops and then collapses in the same way as in Figure 7a. During laser ablation, i.e., \(t=0\) ms, a small pressure pulse was detected, as shown in Figure 26b. A large signal was observed during the first collapse of the laser cavitation. However, during the second collapse of the laser cavitation, the amplitude of the signal was larger than that of the laser ablation. Thus, a relatively long laser pulse such as \(200\) us can generate laser cavitation, and this can be applicable for laser cavitation peening. Thus, other types of lasers, such as fiber lasers, can be utilized for laser cavitation peening to drastically increase efficiency. In order to optimize laser cavitation peening using a long laser pulse, the effect of parameters such as the laser energy, repetition frequency, standoff distance, and focal distance of the lens should be investigated, as laser cavitation peening utilizes a subcooling phenomenon, as shown in Figure 9.