# Laser Cavitation Peening: A Review
## 5 Improvement of Fatigue Properties of Metals via Laser Cavitation Peening  
In order to reveal the improvement of the fatigue properties of 3D metals via laser cavitation peening, Figure 22 shows the \(S\)-\(N\) curves of additive-manufactured titanium alloy Ti6 Al4V via direct metal laser sintering (DMLS) treated using laser cavitation peening compared with a non-peened (as-built) specimen [60], as 3D metals are attractive materials [61]. As shown in Figure 22, the fatigue life was improved by laser cavitation peening (CP by laser). Figure 23 shows images of additively manufactured titanium alloy Ti6 Al4V produced using DMLS: (a) as-built specimen and (b) laser-cavitation-peened specimen [10]. Many unmelted particles were observed on the surface of the as-built specimen, and the maximum height of roughness \(Rz\) was about 140 \(\upmu\)m. After laser cavitation peening, \(Rz\) decreased to about 100 \(\upmu\)m, and compressive residual stress was introduced [10]. The hardness was also increased. Thus, the reduction in surface roughness and the introduction of compressive residual stress and work-hardening are the reasons why laser cavitation peening improves the fatigue properties of 3D metals. Table 1 shows the fatigue strength at \(10^{7}\) of non-peened metallic materials and materials that underwent laser cavitation peening compared with shot peening. The fatigue strength at \(10^{7}\) was calculated using [PERSON]'s method [62]. Figure 24 illustrates the relationship between the fatigue strength of the non-peened specimen, \(\sigma_{N}\), \(\tau_{N}\), and the fatigue  
Figure 23: Images of surface of additive-manufactured titanium alloy Ti6 Al4V via direct metal laser sintering: (**a**) as-built; (**b**) laser cavitation peening [10]. Figure 22: Improvement of fatigue life of titanium alloy Ti6 Al4V additively manufactured via direct metal laser sintering using laser cavitation peening (CP by laser) compared with shot peening and cavitation peening using a jet. Reprinted from [60] with permission from Elsevier, License Number 5532491421394.  
strength of the peened specimen, \(\sigma_{P}\), \(\tau_{P}\). As shown in Table 1 and Figure 24, both laser cavitation peening and shot peening improve the fatigue strength compared with the non-peened specimen. When the proportional constant is considered, laser cavitation peening and shot peening can be described as in Equations (5) and (6). \[\sigma_{P}\text{, }\tau_{P}=1.52\pm 0.15\,\sigma_{P\!N}\text{, }\tau_{P\!N} \tag{5}\]  
\[\sigma_{P}\text{, }\tau_{P\!P}=1.42\pm 0.15\,\sigma_{P\!N}\text{, }\tau_{P\!N} \tag{6}\]  
Thus, laser cavitation peening can improve fatigue strength by 10% over shot peening. In order to consider the difference in the improvement of fatigue strength between laser cavitation peening and shot peening, Figure 25 reveals an EBSD analysis of the cross-section after (a) cavitation peening, CP, using a cavitating jet; (b) shot peening, SP; and (c) cavitation peening using a laser, i.e., laser cavitation peening [63]. During the experiment, the stainless-steel plate was treated so that the arc height was equivalent in all three cases. As shown in Figure 25, the dislocation density of cavitation peening using both jet and laser was less than that of shot peening. As the dislocations might become a source of crack initiation, the fatigue properties of shot peening would be weaker than those of cavitation peening. As is known, the compressive residual stress introduced by the surface treatments is released, and the relaxation of the compressive residual stress depends on