Cavitation Peening Using a Submerged Water Jet anda Pulsed Laser Comparing with Shot Peening
H. Soyama1
1 Department of Finemechanics, Tohoku University, JapanAbstract
Improvement of fatigue strength treated by cavitation peening, in which cavitation impact atbubble collapse is utilized for peening, was demonstrated by using a plane bending fatigue
test comparing with shot peening and non-peened. In the present paper, a submerged water
jet and a pulsed laser were used for cavitation peening. Stainless steel SUS316L, aluminumalloy A2024-T3 and magnesium alloy AZ31B were treated by cavitation peening and shotpeening. It was revealed that the best improvement of the fatigue strength comparing with
non-peened was 25% for SUS316L by cavitation peening using the jet, 42% for A2024-T3 bycavitation peening using the laser and 55% for AZ31B by cavitation peening using the laser.
Keywords cavitation peening, submerged jet, pulsed laser, fatigue strength.
Introduction
Cavitation normally causes severe damage in hydraulic machineries such as pumps andvalves because of intense impacts at bubble collapse. However, the impact at bubble collapse
can be utilized for mechanical surface treatment in the same way of shot peening. A peening
method using cavitation impact is named as “cavitation peening” [1-3].
At conventional cavitation peening, cavitation is produced by injecting a high-speed water jet
into a water filled chamber through a nozzle, and cavitation is generated in a shear layer
around the jet. The submerged high-speed jet with cavitation is called as “cavitating jet” [4].
Note that peening mechanism of cavitation peening is different from that of water jet peening,in which water column impacts in jet center are used. It was reported that efficiency ofoptimized cavitation peening was 3.2 times better than that of water jet peening [5]. In thecase of cavitation peening using the submerged water jet, the optimum injection pressure is
40 MPa [5], then an expensive plunger pump is not required.
In the case of submerged laser peening, a bubble, which behaves like a cavitation bubble, isgenerated after laser ablation [6-8]. In the present paper, the bubble after the laser ablation
was named as “laser cavitation”. When amplitude of pressure wave in water was measured
by a submerged shock wave sensor, the amplitude at laser ablation was larger than that oflaser cavitation collapse [6-8]. On the other hand, when impact which propagated in the target
metal was measured by a PVDF sensor, the impact of laser cavitation collapse was larger
than that of laser ablation [7,8]. Namely, the submerged laser peening is a kind of cavitation
peening using the pulsed laser.
When the fatigue poperies of stainless steel treated by cavitation peening were compared withthat of shot peening, the fatigue strength of cavitation peening was larger than that of shotpeening, and the relief of compressive residual stress introduced by cavitation peening during
the fatigue test was smaller than that of shot peening [9]. At the equivalent peening intensity
condition for stainless steel, the summation of compressive residual stress was nearly
equivalent for cavitation peening and shot peening, however, the dislocation density of thesub-surface treated by cavitation peening was smaller than that of shot peening [10]. These
differences might be caused by the impact characteristics such as strain speed and appliedstress distribution of the impact [3]. Then, these effects would cause the differences on thefatigue properties.

In the present paper, in order to make clear the difference on improvement of fatigue
properties by cavitation peening comparing with shot peening, stainless steel JapaneseIndustrial Standards JIS SUS316L, aluminum alloy JIS A2024-T3 and magnesium alloy JIS
AZ31B were treated by cavitation peening using the submerged water jet and the pulsed laserand shot peening, then tested by a displacement-controlled plane bending fatigue test.
Experimental Methods
Figure 1 illustrates geometry of specimen. The thickness of specimen was 2 mm for SUS316L,
3 mm for A2024-T3 and 4 mm for AZ31B, respectively.
Figure 2 shows a schematic diagram of cavitation peening system using the submerged waterjet. The water pressurized by a plunger pump was injected to the specimen, which was set in
the recess, through a nozzle. The nozzle throat diameter d was 2 mm and it had an optimized
outlet bore whose diameter D was 16 mm and the length L was 16 mm. It also had a cavitator
whose diameter dc was 3 mm and a guide pipe to enhance aggressive intensity of the cavitation
impacts. The injection pressure was 30 MPa and the standoff distance was 222 mm.
Figure 3 reveals a schematic diagram of cavitation peening system using a pulsed laser. The
used pulse laser source was a Q-switched Nd:YAG laser. The wave length was 1,064 nm, as
laser cavitation impact was used for the peening at the present test to utilize heat effect causedby the pulse laser, as the laser cavitation is a kind of sub-cool boiling. Note that conventional
submerged laser peening uses 2nd harmonics of Nd:YAG laser, i.e., 532 nm, to mitigate
attenuation of the pulse energy due to water, and 40 % of source energy was lost at thewavelength conversion from 1,064 nm to 532 nm. The used pulsed laser energy was 0.35 J,
the pulse width was 6ns, the beam diameter was 6 mm, the repetition frequency was 10 Hz.
The pulsed laser was reflected by the mirrors, and expanded by a concave lens, then focusedon the specimen, which was placed in a water filled glass chamber, by a convex lens to avoid
the damage of the glass chamber. The standoff distances in air sa and water sw were optimized
by measuring the peening intensity. The specimen was placed on the stage which wascontrolled by stepping motors.
Figure 4 shows the schematic diagram of shot peening system. In the preset experiment, arecirculating shot peening system accelerated by a water jet was used. Stainless steel shots,
whose diameter were 3.2 mm were installed in the chamber. The number of shots was 500.
R45 90 mm
30 mm20 mm
65 mm

Fig. 1 Geometry of specimen
Stainless tank
Cavitator
Nozzle plate
Guide pipeStandoffdistanceSpecimen
RecessPressurized water jet
Waterdcdd

DLStage
Water

Specimen
Glass chamber
Laser cavitation
Convex lensLaser

Ablation

Cloud
cavitation
Ring vortex
cavitation

Fig. 2 Schematic diagram of cavitation
peening system using submergedwater jetsasw

Fig. 3 Schematic diagram of cavitation
peening system using pulsed
laser

Pressurized water jet

SpecimenNozzleShot

Standoff distance

Water flow

Scanning

Chamber

Loadcell~:~i::,•;t;::~ngcodDischarged water
RecessSpecimen

Fig. 4 Schematic diagram of shot peening
system accelerated by water jet

Fig. 5 Schematic diagram of a Schenk-type
displacement-controlled plate bending
fatigue tester

The standoff distance from the nozzle to the specimen surface was 50 mm. The water jet wasinjected into the chamber through three holes with a diameter of 0.8 mm. The injection
pressure was 12-15 MPa. At the present condition, the water jet without shots did not introduce
compressive residual stress into metallic surface.
Figure 5 illustrates a schematic diagram of a Schenk-type displacement-controlled plate
bending fatigue tester. The fatigue properties of tested materials were evaluated at stress ratio
R =-1. The test frequency was 12 Hz. In order to investigate mechanical properties of peened
surface, the residual stress on the surface was evaluated by 2D method using X-ray diffraction[11]. The surface roughness and the surface hardness were also measured.
Experimental Results
In order to reveal effect of impact at laser cavitation collapse comparing with laser ablation,Fig. 6 shows the aspect of laser ablation and laser cavitation observed by a high-speed video,and Fig. 7 reveals signal from the PVDF sensor and the submerged shock wave sensor [7].
After the laser ablation, laser cavitation was developed and it was collapsed at t = 1 ms. The
amplitude of laser ablation was larger than that of laser cavitation, when the amplitude ofpressure was measured by the submerged shock wave sensor. On the other hand, when theimpact passing through the target metal was measured by the PVDF sensor, the impact atLaser ablationt = 0 ms0.060.12Laser cavitation

0.21

0.50

0.80

0.94

0.97

1.00

1.06

1.27

1.48

1.53

1.59

Fig. 6 Aspect of laser ablation and laser cavitation
Laser ablation

Laser cavitation
15
Amplitude ofsignal Vshock

Amplitude ofsignal VPVDF V2

Laser cavitation

10

10-1

Laser ablation-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8Time after irradiated pulse laser t ms(a) Signal from PVDF sensor

50-5-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8Time after irradiated pulse laser t ms(b) Signal from submerged shockwave sensor

Fig. 7 Signals of laser ablation and laser cavitation

the laser cavitation collapse was larger than that of the laser ablation. Namely, the laser
cavitation impact can be utilized for the peening.
In order to find optimum coverage for cavitation peening and shot peening, Fig. 8 shows thenumber of cycles to failure at constant amplitude of bending stress as a function of coverage
Cov for (a) SUS 316L, (b) A2024-T3 and (c) AZ31B. The used pulse density or processing
time per unit length to calculate coverage for each materials and peening method are shownin Table 1. As shown in Fig. 8, the number of failures at constant applied stress Nf wasincreased with Cov, and then saturated, except AZ31B treated by CP by laser. Although
AZ31B was relatively soft material, the fatigue life was still increasing at 14 pulse/mm2.
In order to demonstrate the improvement of fatigue properties of metallic materials bycavitation peening using the laser and the jet comparing with shot peening, Figs. 9 -11 reveal
the S-N curves for SUS316L, A2024-T3 and AZ31B obtained by the plane bending fatigue
test. Table 2 shows hardness of non-peed one and fatigue strength obtained by Little’s method[12]. In Figs.9-11, the amplitude of bending stress was normalized by the fatigue strength ofnon-peened which was shown in Table 2.
As shown in Fig. 9, at bending stress σa = 400 MPa, the fatigue life of shot peening was better
than that of cavitation peening using the jet. However, the fatigue strength of cavitation
peening using the jet was larger than that of shot peening. In the case of AZ31B, σa ≈ 140
MPa, the fatigue life of shot peening was better than that of cavitation peening using the jet,and the fatigue strength of cavitation peening using the jet was slightly larger than that of shotpeening, as shown in Fig. 11. In the case of A2024-T3, the fatigue life of shot peening at σa ≈280 MPa was better than cavitation peening using the laser, but the fatigue strength ofcavitation peening using the laser was larger than that of shot peening. These might be causedby the difference on the increase of surface roughness by the peening methods.
In the case of the fatigue strength of SUS316L, cavitation peening using the jet was best, andit improved 25% comparing with non-peened. In the case of A2024-T3 and AZ31B, theimprovement by cavitation peening using the laser was best, and it improved 42% for A2024T3 and 55% for AZ31B.
Table 1. Used Pulse Density or Processing Time per Unit Length to Calculate Coverage.

1061051040··~~.
..•\o. .
♦ ••♦..

Shot peening

Cavitation peeningby jet000Cavitation peening
by laser
Water jet peening(a) SUS316L

3••••••♦•• ,105'\

1
2
Coverage Cov

106

1040

•

~

1P

Number of cycles to failure Nf

Number of cycles to failure Nf

SUS316L
A2024-T3
AZ31B

Peening method
Cavitation peening
Water jet peening
by jet
8 s/mm
8 s/mm
8 s/mm—16 s/mm—

Cavitation peening
by laser
4 pulse/mm2
10 pulse/mm2
14 pulse/mm2

Number of cycles to failure Nf

Material

Shot peening
Cavitation peening
by jet123
Coverage Cov

40.88 s/mm
0.9 s/mm
1 s/mm107/106

105

Cavitation peening
by laser

Shot peening

1040(b) A2024-T3
Fig. 8 Fatigue life as a function of coverage••Cavitation peening
by laser
Cavitation peening
by jet-~✓
♦ ♦♦•

.·,♦♦

♦

♦

Shot peening

123
Coverage Cov(c) AZ31B

4

Normalized applied of bending stress σa ’

1.8

Cavitation peening by jet
Cavitation peening by laser

1.6

Shot peening
1.4
1.2
Non-peened
1.0
0.8

Water jet peening
104105107

106

Number of cycles to failure Nf cycle

Normalized applied of bending stress σa ’Fig. 9 S-N curves (SUS316L)
1.8

Cavitation peening
by laser

1.6
1.4

Cavitation peening
by jet1.2
Non-peened

1.00.8

Shot peening

104

105

106

107

Number of cycles to failure Nf cycle

Normalized amplitude of bending stress σa’

Fig. 10 S-N curves (A2024-T3)
1.8

Cavitation peening by laser

1.6
Cavitation peening by jet

1.4

Shot peening

1.2
Non-peened

1.00.8

104

105

106

Number of cycles to failure Nf cycles
Fig. 11 S-N curves (AZ31B)

107

Table 2. Improvement of Fatigue Strength of Tested Materials.
Material
AZ31B
A2024-T3
SUS316L

Yield
strength[MPa]311
216

Hardness
HR15N

HR15T

38.6±0.663.3±0.363.2±0.774.3±0.4
87.0±0.2
86.6±0.2

Fatigue strength [MPa]
CPCPNP
SPby laser
by jet
97±3
151±2
115±2
112±3
175±5
248±3
193±3
202±6
279±3
303±5
348±5
325±5

Conclusions
In order to investigate the fatigue properties of metallic materials treated by cavitation peening
using the pulsed laser and the submerged water jet comparing with shot peening, stainlesssteel SUS316L, aluminum alloy A2024-T3 and magnesium alloy AZ31B were treated, andtested by the plane bending fatigue test. It was revealed that the fatigue strength of AZ31B
and A2024-T3 treated by cavitation peening by the laser was better than shot peened one,and that of SUS316L treated by the jet was better than shot peened one.
Acknowledgments
This work was partly supported by JSPS KAKENHI, grant numbers 20H02021 and The Amada
Foundation AF-2021219-B3.
References[1] H. Soyama, Key factors and applications of cavitation peening, International Journal ofPeening Science and Technology, Vol. 1 (2017), pp 3-60.
[2] H. Soyama, Cavitation Peening: A Review, Metals, Vol. 10 (2020), article No. 270.
[3] H. Soyama and A.M. Korsunsky, A critical comparative review of cavitation peening andother surface peening methods, Journal of Materials Processing Technology, Vol. 305(2022), article No. 117586.
[4] H. Soyama, Cavitating Jet: A Review, Applied Sciences, Vol. 10 (2020), article No. 7280.
[5] H. Kamisaka and H. Soyama, Effect of injection pressure on mechanical surface
treatment using a submerged water jet, Journal of Jet Flow Engineering, Vol. 33, No. 2(2018), pp 4-10.
[6] A. Sasoh, K. Watanabe, Y. Sano and N. Mukai, Behavior of bubbles induced by theinteraction of a laser pulse with a metal plate in water, Applied Physics, Vol. A 80 (2005),
pp 1497-1500.
[7] H. Soyama, Comparison between the improvements made to the fatigue strength ofstainless steel by cavitation peening, water jet peening, shot peening and laser peening,
Journal of Materials Processing Technology, Vol. 269 (2019), pp 65-78.
[8] H. Soyama, Laser cavitation peening and its application for improving the fatigue strengthof welded parts, Metals, Vol. 11 (2021), article No. 531.
[9] H. Soyama, Christopher R. Chighizola and Michael R. Hill, Effect of compressive residualstress introduced by cavitation peening and shot peening on the improvement of fatigue
strength of stainless steel, Journal of Materials Processing Technology, Vol. 288 (2021),
article No. 116877.
[10] M. Kumagai, M.E. Curd, H. Soyama, T. Ungar, G. Ribarik and P.J. Withers, Depthprofiling of residual stress and microstructure for austenitic stainless steel surface treatedby cavitation, shot and laser peening, Materials Science and Engineering A, Vol. 813(2021), article No. 141037.
[11] H. Soyama, C. Kuji, T. Kuriyagawa, C.R. Chighizola and M.R. Hill, 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 (2021), article No.
2772.
[12] 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.