Introduction 

There are two main methods of laser peening using pulsed lasers. One technique is to place a target in water (submerged laser peening), and the other technique is to form a water film on the target surface (water film laser peening) as shown in Fig. 1. 

In water film peening, water splashes out after laser ablation, whereas in submerged laser peening, a bubble, which behaves like a cavitation bubble, is generated. Impacts of cavitation collapse are utilized to enhance metallic materials properties such as fatigue strength [1], and it is referred to as “cavitation peening”. In this column, the bubble induced by the pulsed laser is named “laser cavitation”, and a peening method using the impact of laser cavitation collapse is called “laser cavitation peening”. When the main parameters to enhance the impact of laser cavitation collapse are considered, laser cavitation can be utilized to improve the fatigue properties of metallic materials.  

In the present column, mechanism and key factors of laser cavitation peening were explained and the effects of laser cavitation peening were introduced. 

Laser cavitation  

In the research area of bubble dynamics, a bubble near a solid boundary has been studied for over 50 years, as change of bubble shape, which produces a microjet in the bubble (see Fig. 2 (a)), is a very interesting phenomenon. However, from viewpoint of the impact intensity at the bubble collapse, the collapse of the hemisphere produces a more intense impact (see Fig. 2 (b)) [2] [3]. Then, the generation of hemispherical bubble by the pulsed laser is the most important for laser cavitation peening. The other important factor of peening intensity is the air content of water, as air bubbles in water reduce the impact intensity of cavitation collapse, which is called the cushion effect. Thus, the degassed feed water is filled into the vinicity of the peening area (Fig. 3). Standoff distance in air sa and water sw were optimized to evaluate peening intensity changing with sa and water sw. 

In order to compare the impact intensity of laser ablation and laser cavitation, Fig. 4 shows (a) an aspect of laser ablation and laser cavitation, (b) signal from a submerged shock wave sensor, and (c) signal from a PVDF sensor [4]. In the experiment, a Nd:YAG laser with Q-switch was used, and the wavelength was 1,064 nm, as a fundamental wave length is useful for generating cavitation. When the pressure wave in water was measured by the submerged shock wave sensor, the amplitude of laser ablation was larger than that of laser cavitation collapse. However, the impact passing through a target metal was evaluated by the PVDF sensor, the impact induced by laser cavitation collapse was larger than that of laser ablation. Note that the impact energy was proportional to the square of the amplitude, then the impact energy of laser cavitation collapse was nearly twice that of laser ablation.

The Nd:YAG laser with Q-switch is very useful to generate laser cavitation. Both the wavelengths, 532 nm  and 1,064 nm, are applicable for laser cavitation peening. However, the repetition frequency is too slow for peening because the typical repetition frequency is tens of Hz. It has been proofed that a Nd:YAG laser without Q-switch, whose wave width was about 200 μs, can be utilized for laser cavitation peening [5]. Therefore, a fiber laser with maximum repletion frequency of 50 kHz and maximum power of 500 W was used to generate laser cavitation. Figure 5 shows the aspect of laser ablation and laser cavitation, and the signal from a hydrophone which was placed in a water-filled chamber. In Fig. 5, the pulse width was 60 μs. As shown in Fig. 5 (a), a laser cavitation was generated after laser ablation. Under the present condition, the maximum diameter of the laser cavitation was about 2.5 mm at 0.13 ms. As the pulse width, i.e., 60 μs, was considerably large comparing with that of the Nd:YAG laser with Q-switch, i.e., about 6 ns, the amplitude of the sound pressure at laser ablation was weak, and the amplitude at the laser cavitation collapse was remarkably larger than that of laser ablation, as shown in Fig. 5 (b). It has been reported that the combination process of laser cavitation peening using the fiber laser and the Nd:YAG laser with Q-switch can improve the fatigue strength of powder bed fusion (PBF) laser sintering (LS)/aluminum alloy AlSi10Mg [6].     

As mentioned later, the peening intensity of laser cavitation peening is proportional to the maximum volume of the laser cavitation, Fig. 6 shows the relationship between the energy of the pulsed laser and the volume of laser cavitation. As laser cavitation is initiated just after the laser ablation, develops and shrinks, then collapses, the maximum volume represents the maximum size of the laser cavitation. It was calculated from the maximum diameter of bubble dmax which was evaluated by the developing time of bubble tD using the following equation (1).

dmax [mm]= 10.3 tD [ms]                       (1)

Eq. (1) can be explained by the Rayleigh equation [7]. Figure 6 was obtained by the Nd:YAG laser with Q-switch at 532 nm wavelength. As shown in Fig. 6, the maximum volume of laser cavitation was roughly proportional to the pulsed energy.

Effect of laser cavitation peening 

In order to show the effect of laser cavitation peening in comparison with shot peening, Fig. 7 reveals an electron backscatter diffraction EBSD map of a cross-section of stainless steel SUS316L [8]. In Fig. 7, the peening intensity, i.e., the arc height of the specimen, was equivalent, and the introduced compressive residual stress was nearly equivalent. As shown in Fig. 7, the increase of the introduced dislocation density of laser cavitation was less than that of shot peening. This tendency was confirmed by X-ray diffraction analysis [8]. As is well known, the compressive residual stress introduced by peening methods is relieved during fatigue. It has been reported that the relief of compressive residual stress introduced by cavitation peening was less than that of shot peening [9], and the increase in dislocation by cavitation peening was less than that of shot peening [8]. Therefore, the tendency shown in Fig. 7 might be a good effect for the improvement of fatigue properties.      

In order to show the effect of laser cavitation size on peening intensity, Fig. 8 illustrates peening intensity changing with developing time of laser cavitation [10]. The peening intensity was evaluated by the arc height. Almen strip (N-strip), stainless steel and duralumin plates were treated by laser cavitation peening, which changed with the energy of the pulsed laser, and the arc height was measured. As shown in Eq. (1), the maximum diameter of laser cavitation dmax is proportional to the developing time of laser cavitation tD. When the threshold was considered for each metal plate, the arc height was proportional to the cube of the developing time of laser cavitation. Namely, the peening intensity of the laser cavitation peening is proportional to the volume of laser cavitation. As shown in Fig. 8, a certain bubble size was required for the metallic materials.  

In order to demonstrate the improvement of fatigue strength by laser cavitation peening compared with shot peening, Table 1 reveals the fatigue strength at N = 107 obtained by Little’s method [11]. As shown in Table 1, the fatigue strength of metallic materials can be improved by laser cavitation peening. The main mechanisms of improvement of fatigue strength by laser cavitation peening were the introduction of compressive residual stress and work hardening [1]. In the case of additive manufactured (AM) metals, the fatigue strength was improved by laser cavitation peening drastically. The other key factor of the improvement of fatigue properties of AM metals by laser cavitation peening is smoothing by laser ablation.

Evaluation of the surface treated by laser cavitation peening 

As mentioned above, one of the main factors of the improvement of fatigue strength by laser cavitation peening is the introduction of compressive residual stress, Fig. 9 shows the residual stress of stainless steel SUS316L treated by cavitation peening evaluated by sin2ψ method and 2D method, compared with slitting method [15]. When the diameter of collimator dcol was small, e.g., dcol = 0.146 mm, the averaged value obtained by sin2ψ method was different from the value of slitting method and the scatter band was too large. On the other hand, the averaged value and the scatter band of the 2D method were reasonable. When the residual stress of the surface of stainless steel SUS316L treated by laser cavitation peening was evaluated by the 2D method with dcol = 0.146 mm, the cyclic pattern of distribution of residual stress was obtained [15]. The 2D method is also useful for evaluating the residual stress of milled components [16]. Fig. 10 illustrates comparison of residual stress evaluated by hole drilling, sin2ψ method, 2D method and cos α method [16].

Conclusions 

In the present column, the mechanism and key factors of laser cavitation peening using the Nd:YAG laser and the fiber laser were explained, and the effects of laser cavitation peening were introduced. It is revealed that a certain pulsed energy is required for the treatment of relatively hard metallic materials.

References

[1] H. Soyama and A. M. Korsunsky, “A critical comparative review of cavitation peening and other surface peening methods”, Journal of Materials Processing Technology, vol. 305, 117586, 2022.

[2] Y. Iga and H. Sasaki, “Relationship between a non-spherical collapse of a bubble and a stress state inside a wall”, Physics of Fluids, vol. 35, 023312, 2023.

[3] H. Soyama and Y. Iga, “Laser cavitation peening: A review”, Applied Sciences, vol. 13, 6702, 2023.

[4] H. Soyama, “Comparison between the improvements made to the fatigue strength of stainless steel by cavitation peening, water jet peening, shot peening and laser peening”, Journal of Materials Processing Technology, vol. 269, pp. 65-78, 2019.

[5] H. Soyama, “Development of laser cavitation peening using a normal-oscillation Nd:YAG laser”, Coatings, vol. 13, no. 8, 1395, 2023.

[6] H. Soyama, “Improvement of fatigue strength of additive manufactured aluminum alloy AlSi10Mg by laser cavitation peening”, Proceedings of Annual Meeting of Society of Materials Science, Japan, vol. 73, 107, 2024.

[7] H. Soyama, et al., “Development of a cavitation generator mimicking pistol shrimp”, Biomimetics, vol. 9, no. 1, 47, 2024.

[8] M. Kumagai, et al., “Depth-profiling of residual stress and microstructure for austenitic stainless steel surface treated by cavitation, shot and laser peening”, Materials Science and Engineering A, vol. 813, 141037, 2021.

[9] H. Soyama, et al., “Effect of compressive residual stress introduced by cavitation peening and shot peening on the improvement of fatigue strength of stainless steel”, Journal of Materials Processing Technology, vol. 288, 116877, 2021.

[10] H. Soyama, “Laser cavitation peening and its application for improving the fatigue strength of welded parts”, Metals, vol. 11, no. 4, 531, 2021.

[11] R. E. Little, “Estimating the median fatigue limit for very small up-and-down quantal response tests and for S-N data with runouts”, ASTM STP, vol. 511, pp. 29-42, 1972.

[12] H. Soyama, et al., “The effects of submerged laser peening, cavitation peening, and shot peening on the improvement of the torsional fatigue strength of powder bed fused Ti6Al4V produced through laser sintering”, International Journal of Fatigue, vol. 185, 108348, 2024.

[13] H. Soyama and F. Takeo, “Effect of various peening methods on the fatigue properties of Titanium alloy Ti6Al4V manufactured by direct metal laser sintering and electron beam melting”, Materials, vol. 13, no. 10, 2216, 2020.

[14] H. Soyama, et al., “Comparison of the effects of submerged laser peening, cavitation peening and shot peening on the improvement of the fatigue strength of magnesium alloy AZ31”, Journal of Magnesium and Alloys, vol. 11, no. 5, pp. 1592-1607, 2023.

[15] H. Soyama, et al., “Optimization of residual stress measurement conditions for a 2D method using X-ray diffraction and its application for stainless steel treated by laser cavitation peening”, Materials, vol. 14, no. 11, 17, 2021.

[16] C. Kuji, et al., “Experimental study on the effect of the milling condition of an aluminum alloy on subsurface residual stress”, International Journal of Advanced Manufacturing Technology, vol. 127, pp. 5487-5501, 2023.

Department of Finemechanics, 

Tohoku University 

6-6-01 Aoba, Aramaki, 

Aoba-ku, Sendai, 980-8579, Japan

E-mail: soyama@mm.mech.tohoku.ac.jp